Symposium Organizers
Thomas Fischer, University of Cologne
Fabio Di Fonzo, Istituto Italiano di Tecnologia
Rita Toth, Swiss Federal Laboratories for Materials Science and Technology (EMPA)
Mmantsae Diale, University of Pretoria
Symposium Support
Kenosistec
Nature Catalysis | Springer Nature
Sustainable Energy &
Fuels | The Royal Society of Chemistry
ES02.01: Devices and Application of Solar Fuels
Session Chairs
Monday PM, November 27, 2017
Hynes, Level 3, Room 306
8:30 AM - *ES02.01.01
A Short and Golden Age of Solar Fuel Pioneering
Artur Braun 1
1 , Empa, Swiss Federal Laboratories for Materials Science and Technology, Duebendorf Switzerland
Show AbstractThe first MRS Symposium on "Photoelectrochemistry for Solar Hydrogen" was planned for Spring 2009 in San Francisco; that was 10 years ago 1. Since, the solar water splitting community has grown worldwide and it had a steady presence on the MRS Meetings since 2. I will showcase how basic science studies and component and device engineering developed over this short and golden age of solar fuel pioneering. Notwithstanding that artificial photosynthesis is a 100 year old dream 3 and we newcomers are standing on the shoulders of giants. Specifically will I focus on the iron oxide photoanode system, which is a well-studied system and which keeps many of us busy still at this time 4. My presentation will include very advanced analytical studies at synchrotron facilities 5, 6 and fill also the gap between inorganic semiconductor photoelectrochemistry and the bio-based artificial photosynthesis – two fields for solar fuel production which have developed independently from each other 7.
1. Braun A, Alivisatos AP, Figgemeier E, Je J, Turner JA. Call for Papers - MRS Symposium S: Materials in Photocatalysis and Photoelectrochemistry for Environmental Applications and H2 Generation. In: Materials Research Society Spring Meeting. Materials Research Society (2009).
2. Braun A, Gaillard N, Miller EL, Wang H. Introduction. Journal of Materials Research 31, 1545-1546 (2016).
3. Ciamician G. The Photochemistry of the Future. Science 36, 385-394 (1912).
4. Bora DK, Braun A, Constable EC. “In rust we trust”. Hematite – the prospective inorganic backbone for artificial photosynthesis. Energy Environ Sci 6, 407-425 (2013).
5. Bora DK, et al. Evolution of an oxygen near-edge x-ray absorption fine structure transition in the upper hubbard band in α-Fe2O3 upon electrochemical oxidation. Journal of Physical Chemistry C 115, 5619-5625 (2011).
6. Braun A, et al. The electronic, chemical and electrocatalytic processes and intermediates on iron oxide surfaces during photoelectrochemical water splitting. Catalysis Today 260, 72-81 (2016).
7. Braun A, et al. Biological Components and Bioelectronic Interfaces of Water Splitting Photoelectrodes for Solar Hydrogen Production. Chemistry-a European Journal 21, 4188-4199 (2015).
9:00 AM - ES02.01.02
Preliminary Results in the PECSYS Project—Demonstration of a Solar Driven Electrochemical Hydrogen Generation System with an Area > 10 sq. m.
Sonya Calnan 1 , Rutger Schlatmann 1 , Marika Edoff 3 , Tomas Edvinsson 3 , Peter Neretnieks 4 , Lars Stolt 4 , Stefan Haas 5 , Manuel Langemann 5 , Martin Mueller 5 , Bugra Turan 5 , Anna Battaglia 6 , Salvatore Lombardo 2
1 , Helmholtz- Zentrum Berlin, Berlin Germany, 3 , Uppsala Universitet, Uppsala Sweden, 4 , Solibro Research AB, Uppsala Sweden, 5 , Forschungszentrum Juelich GmbH, Juelich Germany, 6 , SUN SRL, Catania Italy, 2 , Consiglio Nazionale Delle Ricerche CNR-IMM, Catania Italy
Show AbstractWe present preliminary results of the PECSYS project that is aimed at demonstrating an operational photovoltaic (PV) powered electrolyser (EC) system with an exposed surface area of at least 10 sq.m. This system size would be suitable for photovoltaics driven stand-alone water electrolysis for hydrogen production to provide both electricity and heating to residential and small commercial consumers.The demonstrator is targeted to achieve a solar to hydrogen efficiency (ηSTH) of at least 6 % supporting a hydrogen production of at least 16 g/h at a levelised cost of €5/kg. The system is also expected to operate continuously under outdoor conditions with a ηSTH loss of less than 10% relative after six months. The consortium shall use a stage gate elimination process to select the more promising laboratory scale PV-EC devices based on thin film silicon, silicon heterojunction and CuInGa(Se,S)2 PV absorbers that shall be scaled to prototype size of at least 100 sq. cm. The prototype devices shall be used to refine the modular concept that shall eventually be used in the ultimate 10 sq.m demonstrator. First results show that a 2.4 sq.cm CuInGa(Se,S)2 based PV mini-module can supply sufficient operating voltage (~1.8V) to drive an electrolyser using catalysts consisting of abundant materials (Ni, Mo, Fe) leading to ηSTH of about 10.6%. Also, a wireless integrated device using on a 64 sq.cm thin film silicon mini-module achieved a value of ηSTH of 3.9 % using bare nickel foam as a catalyst. The energy conversion efficiency is expected to increase with the use of catalysts with lower over-potentials. Lastly, a comparison between N117 membrane and NR212 membrane revealed that the latter could achieve a current density of 500 mA at 1.7 V with a platinum loading factor reduced by a factor of 6.6 compared to the former. Future activities include using silicon wafer based solar cells as the PV component, investigating the feasibility of alkaline versus anion exchange membranes as the electrolyte in electrolysers and maximizing the overall energy efficiency and long term stability of the devices. The project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking under grant agreement No 735218. This Joint Undertaking receives support from the European Union’s Horizon 2020 Research and Innovation programme and Hydrogen Europe and N.ERGHY.The project started on the 1st of January 2017 and has a duration of 48 months.
9:15 AM - ES02.01.03
Implications for Photoelectrochemical Hydrogen Production Based on Scale-up Results from PEM Water Electrolysis
Luke Dalton 1 , Andrew Roemer 1 , Katherine Ayers 1
1 , Proton OnSite, Wallingford, Connecticut, United States
Show AbstractFor photoelectrochemical hydrogen production to be effective as envisioned, the cells need to be scaled to a substantial cell area. Over the last 20 years, non-light-driven proton exchange membrane (PEM) water electrolysis has been scaled up in the commercial product environment of single cells by over two orders of magnitude, and of stack total active area by over three orders of magnitude. The design experience gained through this related product development provides insight into the challenges and can accelerate PEC cell development. The findings regarding reactant water flow and pressure drop, reactant distribution across a large active area, distribution among different cells fed by common manifolds, two-phase flow caused by product gas evolution, and the impacts of insufficient flow will be summarized.
Many competing design requirements need to be optimized to implement a durable commercial electrolysis product. Feeding sufficient water for good intra-cell distribution of reactant against the product gas evolution is essential to prevent hot spots and local dry-out of the membrane. Proton has evaluated a variety of cell active area shapes and flow field patterns including round and square cells with symmetric and asymmetric feed and return port locations such that the implications can be extended to larger active area platforms. While the impacts of acute water starvation might result in immediate cell damage, there are also longer term, slower acting impacts of chronic marginal water supply that lead to higher voltage degradation rates and premature cell failure or loss of efficiency. Examples of controlled experiments demonstrating the initiation of increased degradation and subsequently stopping that degradation through control of water flow rates alone will be described.
In addition to intra-cell reactant distribution and dry-out, the bulk water distribution in a large electrolysis platform has overall system implications. It is generally economical for the feed water to be far in excess of the required water for the reaction rate so that the reactant water also serves as the cooling fluid for the cell internals. This eliminates the need for a separate cooling loop and it locates the cooling right where it is needed within the cell. In addition, a common water circulation pump can be used to support many cells and groups of cells. As a result, the bulk implications of water pressure drop, pump sizing, and successive branches of fluid manifolding to arrays of cells, groups of cells, and individual cells must be carefully designed to avoid reactant starvation. Finally, the implications of PEM water electrolysis scale-up on water flow requirements will be extended to the photoelectrochemical cell case with design experience and design guidance for areas to study regarding flow distribution for large arrays of cells.
9:30 AM - ES02.01.04
Development of Catalysts for An Integrated Photovoltaic-biased Electrosynthetic Device for Solar Water Splitting on Large Area
Katharina Welter 1 , Niloofar Hamzelui 1 , Jan-Philipp Becker 1 , Vladimir Smirnov 1 , Wolfram Jaegermann 2 , Friedhelm Finger 1
1 Institute of Energy and Climate Research - 5 Photovoltaic, Forschungszentrum Juelich GmbH, Juelich Germany, 2 Institute of Materials Science, TU Darmstadt, Darmstadt Germany
Show AbstractSolar water splitting is a promising way to sustainably produce hydrogen as a clean and storable fuel. We recently reported the application of thin-film silicon multi-junction photocathodes with a solar-to-hydrogen (STH) efficiency of 9.5% employing noble metal catalysts.[1] As practical applications critically rely on approaches that are scalable to large areas, we investigated different approaches on the upscaling of multi-junction photocathodes. We demonstrated their successful implementation in a photovoltaic-biased electrosynthetic (PV-EC) device. A long-term stable STH efficiency of approximately 5% was achieved for an active area of 64 cm2, employing platinum as well as iridium oxide as respective gas evolution reaction catalysts.[2] However, regarding the cost-effectiveness of this technology, especially on large areas, noble metal catalysts should be replaced by low cost, well working, earth abundant alternatives. It is the topic of the present study to develop such catalysts for the application on appropriate electrode size and to implement them into our up scaled PV-EC device.
For the oxygen evolution reaction (OER), NiFeOX is reported to be a well working catalyst in alkaline media with an overpotential that is competitive to iridium oxide.[3] The catalysts reported in literature are usually optimized on a laboratory scale device area of less than 1 cm2. We investigated electro deposition both in steady state as well as in pulsed operation mode as synthesizing method for NiFeOX with variation of the Ni/Fe ratio on Ni substrates for electrode areas of up to 50 cm2. Those compounds showed excellent catalytic activity superior to the iridium oxide reference material.
For the hydrogen evolution reaction (HER), catalysts such as Nickel chalcogens (e.g. Ni2S3[4]) or nickel-metal compounds (e.g. NiMo[3]) have shown promising overpotential in alkaline media. Therefore, we up scaled the synthesis of Ni2S3 deposited on high surface area nickel foam. We successfully preserved the 3D substrate structure and achieved a homogenous coverage with Ni2S3. Furthermore, we investigated the electro deposition of NiMo on nickel sheet substrates on areas of approximately 50 cm2. The fabricated catalysts are suitable for the implementation in our up scaled PV-EC device.
We will report on the application of these OER and HER catalysts based on earth-abundant materials in our large area stand-alone integrated solar water-splitting module and will present results on its performance and stability.
[1] F. Urbain et al., Energy Environ. Sci. 2016, 9, 145.
[2] J.-P. Becker et al., J. Mater. Chem. A 2017, 5, 4818.
[3] C. C. L. McCrory et al., J. Am. Chem. Soc. 2015, 137, 4347–4357.
[4] L.-L. Feng et al., J. Am. Chem. Soc. 2015, 137, 14023–14026.
9:45 AM - ES02.01.05
Monolithic Photoelectrochemical Device for Water Splitting with 19% Efficiency
Wen-Hui Cheng 1 , Matthias H. Richter 1 , Matthias May 2 , Jens Ohlmann 3 , David Lackner 3 , Frank Dimroth 3 , Thomas Hannappel 4 , Harry Atwater 1 , Hans-Joachim Lewerenz 1
1 , California Institute of Technology, Pasadena, California, United States, 2 , University of Cambridge, Cambridge United Kingdom, 3 , Fraunhofer Institute for Solar Energy Systems ISE, Freiburg Germany, 4 , Technische Universität Ilmenau, Ilmenau Germany
Show AbstractDirect solar hydrogen generation provides storable renewable energy of the intermittent solar irradiation by mimicking the basic architectural component features of a leaf. Efficient artificial systems use dual junction photovoltaic tandem structures that comprise corrosion protection layers and hydrogen- as well as oxygen-evolution heterogeneous catalysts. Realization of near limiting photocurrents at the thermodynamic potential for water splitting demands optimized adjustment of the optical and electronic properties of the surface films exposed to the electrolyte.
We report on the performance of a monolithic tandem dual junction water-splitting device prototype that exhibits reduced surface reflectivity in conjunction with metallic Rh nanoparticle catalyst layers that minimize parasitic light absorption. Additionally, the anatase TiO2 protection layer on the photocathode creates a favorable internal band alignment for hydrogen evolution. An initial solar-to-hydrogen efficiency of 19.3 % is obtained in acid electrolytes and an efficiency of 18.5 % is achieved at neutral pH conditions (under simulated sunlight).
Our device approaches 85% of the theoretical limit of realistic water splitting for the light absorber bandgap combination employed in our device. The achievement of high solar-to-hydrogen efficiencies, relative to limiting efficiencies, derives from the ability to tailor light absorption and electron transport in a protective TiO2 coating that serves as the reactive catalyst support as well as the electrolyte interface. High photocurrent densities require the combination of the antireflection properties of the anatase TiO2 layer, with the use of an optically transparent Rh nanoparticle surface layer. In addition, conduction band alignment through the surface layers across AlInP / AlInPOx / TiO2 / Rh /electrolyte, that promotes the transport of the excess electrons and inhibits voltage drops, is necessary.
This improvement in integrated PEC device efficiency constitutes a significant step towards achievement of a target 25% solar-to-hydrogen efficiency needed to fulfill techno-economic goals for hydrogen production from photoelectrochemical water splitting that are scalable to widespread application in the transportation sector.
ES02.02: New Concepts in Solar Fuel Production I
Session Chairs
Monday PM, November 27, 2017
Hynes, Level 3, Room 306
10:30 AM - *ES02.02.01
Photoelectrochemical Water Splitting for Solar Energy Conversion and Storage
Avner Rothschild 1
1 , Technion–Israel Institute of Technology, Haifa Israel
Show AbstractPhotoelectrochemical (PEC) water splitting is a promising route for solar energy conversion to hydrogen. It produces clean hydrogen that can be used for refueling fuel cell electric vehicles or serve as a feedstock for the production of drop-in liquid fuels by CO2 hydrogenation or ammonia via the Haber–Bosch process. The greatest challenges towards PEC solar water splitting technology lay in the selection and optimization of stable photocatalytic materials for water photo-oxidation, and the design of scalable PEC devices that produce hydrogen at a competitive cost. Iron oxide (α-Fe2O3, hematite) is one of few materials meeting the basic selection criteria for stable photoanodes, but its poor charge transport properties and fast recombination present challenges for efficient charge separation and collection. We explore innovative solutions to these challenges using ultrathin (20-30 nm) films on specular back reflectors. This optical design traps the light in otherwise nearly translucent ultrathin films, amplifying the intensity close to the surface wherein photogenerated charge carriers can reach the surface and split water before recombination takes place.1 This is the enabling key towards the development of high-efficiency epilayers whose properties can be tailored by material design at the atomic scale.2 Our recent efforts to uncover the design rules of these photoanodes will be presented. On the other end of the spectrum we explore innovative device architectures and operation schemes for scalable and competitive PEC solar water splitting technology. These include power and optical management schemes for optimizing the hydrogen and power outputs of PEC – PV tandem cells,3 and separating the hydrogen production from the oxygen production for safe operation and on-site hydrogen production.4
References:
1. Dotan et al., Nature Materials 12, 158-164 (2013).
2. Grave et al., The Journal of Physical Chemistry C 120, 28961–28970 (2016).
3. Rothschild et al., ACS Energy Letters 2, (2017) 45-51.
4. Landman et al., Nature Materials (2017, DOI: 10.1038/nmat4876).
11:00 AM - ES02.02.02
Enhanced Carrier Lifetimes in Metal Oxide Photoelectrodes through Mild Hydrogen Treatment
Fatwa Abdi 1 , Ji-Wook Jang 1 , Dennis Friedrich 1 , Soenke Mueller 1 , Sheikha Lardhi 2 , Zhen Cao 2 , Moussab Harb 2 , Luigi Cavallo 2 , Rainer Eichberger 1 , Roel Van de Krol 1
1 Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin Germany, 2 , King Abdullah University of Science and Technology, Saudia Arabia (KAUST), Thuwal Saudi Arabia
Show AbstractIn recent years, hydrogen treatment has emerged as an effective method to improve the photoelectrochemical (PEC) performance of metal oxides. While initially reported only for TiO2 [1], this simple post-synthesis annealing treatment under H2 atmosphere is also applicable to other metal oxides such as WO3, Fe2O3, and BiVO4 [2-4]. Although the actual mechanism of the improvement in photoactivity upon hydrogen treatment is not yet fully understood, it is likely related to a change in charge transport and/or recombination properties. Here, we report on the influence of performing a mild hydrogen treatment (300 °C in 2.4% H2/Ar atm. for 10 minutes) on the carrier dynamics of BiVO4, which is one of the most promising metal oxide photoelectrodes. Time-resolved microwave conductivity (TRMC) measurements reveal a significant increase of the carrier lifetime in BiVO4 photoanodes upon hydrogen treatment, without significantly affecting the carrier mobility. The longer lifetime is attributed to the passivation of deep trap states, as evident from the intensity-dependent carrier mobility. Density functional theory (DFT) calculations identified vanadium interstitials to be the likely candidates for these trap states. As a result of the hydrogen-induced passivation, the photocurrent plateau of BiVO4 increases by ~25% and the onset potential shifts cathodically by ~100 mV. In addition, nuclear reaction analysis shows direct evidence for the presence of ~0.7 at% hydrogen in the lattice of hydrogen-treated BiVO4. The incorporated hydrogen forms O-H bonds and distorts the lattice of BiVO4, resulting in a shift of the Raman external vibration peak. This additional distortion, however, does not affect the hole polaron formation, as shown by time-resolved THz conductivity (TRTC) measurements. This means that the passivated trap states are located energetically deeper than the hole polaron states. These results advance our understanding of the influence of hydrogen on the carrier transport and recombination dynamics in metal oxides and may help establish additional pathways towards highly efficient photoelectrodes.
[1] X. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science 2011, 331,746-750.
[2] G. Wang, Y. Ling, Y. Li, Nanoscale 2012, 4, 6682-6691.
[3] G. Wang et al. J. Phys. Chem. C 2013, 117, 10957–10964.
[4] J. K. Cooper et al. Chem. Mater. 2016, 28, 5761-5771.
11:15 AM - ES02.02.03
Dynamic Equilibrium of the Methylammonium Lead Iodide Photocatalyst in Aqueous Solution for Hydrogen Evolution via HI Splitting
Sunghak Park 1 , Woo Je Chang 1 , Ki Tae Nam 1
1 , Seoul National University, Seoul Korea (the Republic of)
Show AbstractThe solar-driven splitting of hydrohalic acids (HX) is an important and fast growing research direction for H2 production. In addition to the hydrogen, the resulting chemicals such as I2/I3-, Br2/Br3-, or Cl2 are themselves value-added products, which have a variety of uses in the energy and hygiene industries. In an effort to develop a cost-effective and easily scalable process, we suggest a new strategy for photocatalytic HI splitting using methylammonium lead iodide (MAPbI3). Although the MAPbI3 is known for its unstablity in humid and aqueous environment, MAPbI3 powder was stably existing in aqueous HI solution by the principle of dynamic equilibrium. Considering that MAPbI3 is a water-soluble ionic crystal, we pay attention to the dynamic equilibrium between the precipitation and dissolution of the ionic crystal in saturated solutions. The I- and H+ concentrations of the aqueous solution were determined to be the critical parameters for the stabilization of the tetragonal MAPbI3 phase. Stable and efficient H2 production under visible light irradiation was demonstrated. Stoichiometric production of H2 and I3- was verified using GC and UV/Vis spectroscopy measurement, indicating that main photocatalysis by MAPbI3 was HI splitting reaction without any side reactions. Indeed, the photocatalytic performance of MAPbI3 can be kept constant even after 160h of continuous illumination of light. Higher photocatalytic H2 generation rate was achieved through Pt cocatalyst deposition and thermal annealing of MAPbI3 in polar solvent atmosphere such as dimethylformamide or dimethyl sulfoxide. The solar HI splitting efficiency of MAPbI3 was 0.81 %, when using Pt as a cocatalyst.
11:30 AM - *ES02.02.04
Development and Integration of Photo-Electro Catalysts to the Semiconducting Metal Oxide Based Systems for Enhanced Solar Energy Conversion
Renata Solarska 1 , Krzysztof Bienkowski 1 , Monika Arasimowicz 1
1 , Centre of New Technologies University of Warsaw, Warsaw Poland
Show AbstractPhoto-electro activation of semiconducting metal oxides is continuously gaining a considerable interest in view of enhanced production of renewable energy and overcoming intrinsic limitations of well-known and recognized towards specific action semiconducting oxides such as iron oxide, titanium oxide, tungsten oxide or copper oxide. Given the band edges positions are not always suitable for efficient water splitting, of primary importance is to enhance their intrinsic properties and couple it with the electro-catalytic modification of the surface. Therefore, efforts including incorporation of metal nanostructures or creation of low level sub- stoichiometry are continuously devoted to minimization of the required bias voltage, improvement of light capture or charge collection, respectively. Recent advances regarding tungsten oxide, owing the band gap of 2.5 eV, consist of combination of enhanced intrinsic activity reached by a moderate doping of WO3 with sodium ions and identification of new molecular oxygen evolution reaction (OER) catalyst, active in acidic media [1]. These two approaches, allowed to enhance visible light absorption of tungsten trioxide and, consequently, its photocurrent conversion efficiency. The stable and reproducible water splitting photocurrents reached 4.5 mA cm-2 and were attained at standard conditions. Incorporated in small amount polyoxometalates act as highly effective molecular OER catalysts leading to very large enhancement of water oxidation photocurrents at the WO3 photo-anode. Dissolved in the electrolyte, the POMs are transparent to visible light and can easily penetrate in-depth over extended internal photo-active surface area, thus are particularly well suited for application with nanoporous photo-electrodes. These findings have also been tested in other semiconducting system employing either titanium oxide or strontium titanate. Another example of very effective photoelectrochemical system relying onto engineered low substoichiometric disorder, includes copper (I) oxide enhanced and stabilized by the presence of titanium oxide [2]. A special attention will be paid to its stabilization, intentional modification, as well as to optimization of experimental conditions permitting control and diversification of the CO2 reduction products.
[1] M. Sarnowska, K. Bienkowski, P. J. Barczuk, R. Solarska, J. Augustynski, Adv. Energy Mater. (2016), 1600526
[2] E. Szaniawska, K. Bienkowski, I. Rutkowska, P. Kulesza, R. Solarska, Cat. Today (2017) doi.org/10.1016/j.cattod.2017.05.099
ES02.03: New Concepts in Solar Fuel Production II
Session Chairs
Monday PM, November 27, 2017
Hynes, Level 3, Room 306
1:30 PM - ES02.03.01
Aluminum-Water Reactions for Hydrogen Production Using Liquid and Supercritical Water—200°C–400°C
Keena Trowell 1 , Sam Goroshin 1 , David Frost 1 , Jeffrey Bergthorson 1
1 , McGill University, Montreal, Quebec, Canada
Show AbstractIt is widely recognized that anthropogenic climate change must be halted and that curtailing the use hydrocarbon fuels is essential to meeting this goal. Hydrogen has been put forth as a possible replacement for fossil fuels; however, the hydrogen economy has remained elusive due to persisting challenges related to the production, storage and transportation of this clean fuel. Broader use of hydrogen fuel would be possible if in-situ, on-demand hydrogen production was realized. Aluminum-water reactions are one avenue of hydrogen production that could achieve this. Both aluminum and water are relatively easy and safe to store and transport. However, the challenge of producing hydrogen at a rate that is useful for real-world applications remains.
A high-temperature, high-pressure metal-water experimental apparatus, capable of creating supercritical water conditions, was designed and tested. In this work, high-temperature aluminum-water reactions are studied using commercially available aluminum powders. The powders are placed in the reaction vessel along with an excess of water. The vessel is then heated to the desired temperature and held at temperature until the pressure reading stabilizes. To determine the effect of particle size and correlated available surface area, the powders used in this work range in nominal size from 12μm to 108μm. To determine the effect of temperature on reaction onset, yield, penetration thickness and reaction rate, experimental temperatures ranging from 200°C to 400°C are used.
It was found that full yields were achievable with smaller particles at lower temperatures or, conversely, that higher temperatures are required to achieve full yield in larger particles. Full yield with the largest available powder (108μm) was achieved in water with temperatures above 350°C. Full yield for the 12μm powder was achieved in 200°C water. Reaction onset temperature increased with particle size. It was also observed that reactions rates increased with temperature. When compared to results from earlier works, an exponential trend, consistent with the predictions of the Arrhenius equation, emerges.
1:45 PM - ES02.03.02
Oxide Bilayers as High Efficiency Water Oxidation Catalysts through Electronically Coupled Phase Boundaries
Jennifer Leduc 1 , Yakup Gonullu 1 , Thomas Fischer 1 , Sanjay Mathur 1
1 , University of Cologne, Cologne Germany
Show AbstractNew semiconductor metal oxides capable of driving water-splitting reactions by solar irradiation alone are required for sustainable hydrogen production. Whereas most metal oxides only marginally deliver the photochemical energy to split water molecules, uranium oxides are efficient photoelectrocatalysts due to their absorption properties (Eg ~ 2.0 - 2.6 eV) and easy valence switching among uranium centers that additionally augment the photocatalytic efficiency. Although considered a scarce resource, the abundance of uranium compounds in the environment is manifested in the huge quantities of stored UF6 gas, produced as waste streams in the nuclear fuel enrichment process. Here we demonstrate that thin films of depleted uranium oxide (U3O8) and their bilayers with hematite (a-Fe2O3) are high activity water oxidation catalysts due to electronically coupled phase boundaries. The electronic structure of uranium oxides showed an optimal band edge alignment in U3O8//Fe2O3 bilayers (DFT calculations) resulting in improved charge-transfer at the heterojunction as supported by TAS and XAS measurements. The enhanced photocurrent density of the heterostructures with respect to well-known hematite offers unexplored potential of uranium oxide in artificial photosynthesis.
2:00 PM - *ES02.03.03
Designing High-Performance Nanoscale Catalysts for Small Molecule Reactions—Probing Size and Composition-Dependent Electrocatalytic Behavior in Noble Metal-Based Nanowires
Stanislaus Wong 1
1 , SUNY Stony Brook, Stony Brook, New York, United States
Show AbstractThe inherently finite quantity of fossil fuels has triggered a need for developing alternative renewable energy sources. In recent years, we have expended significant effort in probing and understanding the use of one-dimensional (1D) noble metal-based nanostructures for a number of energy-relevant, small-molecule reactions. In this talk, we highlight recent theoretical and experimental progress aimed at precisely deducing the nature of the complex interplay and correlation amongst parameters such as motif, size, and chemical composition, in determining electrocatalytic performance in classes of crystalline, well-defined, and high-quality elemental, binary, and ternary 1D noble metal-based nanowire systems. Significant enhancements in both activity and durability can be achieved by rationally tuning both wire size and chemical composition. The fundamental insights acquired are then utilized to discuss future and potentially new directions towards the continuous improvement and optimization of these 1D catalysts.
2:30 PM - ES02.03.04
Plasmonic Heterostructures for Solar-to-Fuel Conversion
Yang Yang 1
1 , University of Central Florida, Orlando, Florida, United States
Show AbstractIn this study, we report a nonmetal plasmonic MoS2@TiO2 heterostructure for highly efficient photocatalytic H2 generation. Large area laminated Z-scheme MoS2 in conjunction with TiO2 nanocavity arrays are achieved via carefully controlled anodization, physical vapor deposition, and chemical vapor deposition processes. Broad spectral response ranging from ultraviolet (UV)-visible (vis) to near-infrared (NIR) wavelengths and finite element frequency-domain simulation suggest that this MoS2@TiO2 heterostructured photocatalyst possesses an enhanced activity for H+ reduction. A high H2 yield rate of 580 mmol h-1 g-1 is achieved using a low catalyst loading mass of 10.2 μg. The spatially uniform heterostructure, correlated to plasmon-resonance through conformal coating MoS2 that effectively regulated charge transfer pathways, is proven to be vitally important for the unique solar energy harvesting and photocatalytic H2 production. As an innovative exploration, our study demonstrates that the photocatalytic activities of nonmetal, earth-abundant materials can be enhanced with plasmonic effects, which may serve as an excellent catalytic agent for solar energy conversion to chemical fuel.
2:45 PM - ES02.03.05
High Performance Materials for Solar Fuel Generation and Pathways to Utilization of IR-Photons
Tomas Edvinsson 1
1 Dept of Engineering Sciences - Solid State Physics, Uppsala University, Uppsala Sweden
Show AbstractAbout half of the energy in the solar spectrum is in the IR region and results in a mismatch between the solar energy distribution and the energetic requirements for the water splitting reaction leading to a fundamental efficiency problem for a single band gap device. Very modest efficiencies have so far been achieved in one band gap devices where we discuss the likely reasons for this and how this relates to the charge transfer limitations of electrons and holes in low dimensional semiconductors at the semiconductor/liquid interface. Combinations of photoactive materials with multiple band gaps have instead shown to be more promising routes where lower energy level spacing also allows absorption of IR photons. Tandem approaches have here reached high efficiencies while Z-schemes, photon upconversion, and intermediate band gap photovoltaics still have some way to go. Applications of these approaches have so far been limited by the low conversion efficiencies or the high cost of the tandem approach. Utilization of serial interconnected photo-absorber with a buried junction approach is an alternative and cost-effective solution to the spectral mismatch problem. Taking losses due to charge carrier separation and overpotential for catalysis into account, the maximum STH-efficiency for a series interconnected solar splitting device is 24.6 %, compared to 32.0 % for an optimum double junction tandem device at 1 sun (Air Mass 1.5, 1000 W/m2). We present water splitting devices based on Cu-In-Ga-Se2 spanning all the way from classical photoelectrochemical cells (PECs) immersed in water to PV/PEC hybrid devices, and also the corresponding PV-electrolysis where the systems can utilize IR photons up to 1100 nm in the solar spectrum and convert this energy into solar fuel with over 11% solar-to-hydrogen (STH) efficiency. The device architecture is also applied to hybrid perovskite solar cell materials giving close to 13% STH efficiency using earth-abundant catalysts. The approach is a promising alternative to tandem devices and is not sensitive to spectral matching and also opens the way for many other photo-absorbers that previously have been disregarded due to their ill-placed band edge positions or a too low band gap.
3:30 PM - *ES02.03.06
Composite Nanostructures for High-Efficiency Sunlight Conversion
Alberto Vomiero 1
1 Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology, Luleå Sweden
Show AbstractIn several types of excitonic solar cells and solar fuels, nanomaterials can play a critical role in boosting photoconversion efficiency by ameliorating the processes of charge photogeneration, exciton dissociation and charge transport. Several strategies can be pursued, including broadening of light absorbance to reduce solar light losses, fastening exciton dissociation and charge injection from the photoactive medium to the charge transporting materials, reducing charge recombination during charge transport and collection at the electrodes. In this lecture, a few examples of application of nanocomposites will be thoroughly discussed in two specific categories of excitonic solar cells, namely dye- and quantum dot-sensitized solar cells and for photochemical water splitting. Emphasis will be given to the investigation of both the photoactive medium (including composite and core-shell quantum dots) and the charge transporting scaffold (including metal oxide hierarchical structures, nanowires, nanorods and carbon-based hybrids) towards a materials-by-design approach.
4:00 PM - ES02.03.07
Pushing the Boundaries of Cu2O Photocathodes for Solar Water Splitting
Linfeng Pan 1 , Jingshan Luo 1 , Matthew Mayer 1 , Min-Kyu Son 1 , Anders Hagfeldt 1 , Michael Graetzel 1
1 , EPFL, Lausanne Switzerland
Show AbstractAs a scalable and sustainable technology for carbon-neutral production of hydrogen, solar-driven water splitting provides a means to address major concerns that have been raised over the security of our energy future. Even for the most technologically advanced solar-fuel systems, it is still challenging to simultaneously fulfill the requirements of being efficient, robust and scalable. Here, we will discuss the recent development of one of the most promising oxide-based semiconducting photoelectrodes – cuprous oxide (Cu2O) photocathode. We highlight the key steps that take Cu2O photoelectrode towards delivering a fully functional solar fuels generator, which have pushed the boundaries of this field in our group. Using advanced thin film deposition techniques, conformal semiconductor layers are applied on nanostructured photo absorbers, which allows efficient charge separation, light harvesting and robust protection. In the latest progress gallium oxide that has suitable conduction band alignment with cuprous oxide has been applied as electron selective layer and achieved unprecedented overall performance. Finally, an all earth-abundant photocathode was demonstrated in alkaline electrolyte. Current and future research on regulating electronic properties of electron selective layers and new materials development for hole selective layers will also be presented.
4:30 PM - ES02.03.09
A Novel CeO2 – xSnO2 / Ce2Sn2O7 Pyrochlore Cycle for Enhanced Solar Thermochemical Water Splitting
Chongyan Ruan 1 2 , Lin Li 1 , Junhu Wang 1 , Yuan Tan 1 2 , Xiaoyan Liu 1 , Xiaodong Wang 1
1 State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Dalian, Liaoning, China, 2 , University of Chinese Academy of Sciences, Beijing China
Show AbstractSolar thermochemical water splitting (STWS) directly converting solar energy to renewable H2 is highly desirable for reducing greenhouse gas emissions, while ensuring a sustainable energy future. Developing an efficient redox material for thermochemical cycles is crucially important for enabling practical implementation of the solar hydrogen production. In this study, a novel CeO2 – xSnO2 / Ce2Sn2O7 pyrochlore stoichiometric redox cycle with superior H2 production capacities is identified and unequivocally corroborated for two-step STWS. During the first thermal reduction step (1400 oC), a reaction between CeO2 and SnO2 ocurred for all the CeO2 – xSnO2 (x=0.05-0.20) solid compounds, forming thermodynamically stable Ce2Sn2O7 pyrochlore rather than metastable CeO2-δ. Consequently, substantially higher reduction extents were achieved owing to the complete reduction of Ce4+ to Ce3+. Moreover, in the subsequent reoxidation with H2O (800 oC), H2 production capacities increased by a factor of 3.8 as compared to the current benchmark material ceria when x=0.15, with the regeneration of CeO2 and SnO2 and the concomitant reoxidation of Ce3+ to Ce4+. The H2O-splitting performance for CeO2 – 0.15SnO2 was reproducible over 7 consecutive redox cycles, indicating the material was also robust. We anticipate that this CeO2 – xSnO2 / Ce2Sn2O7 pyrochlore cycle based on stoichiometric chemistry may open up a new avenue for rationally optimizing the redox materials for thermochemical cycling, thus holding great promise for identifying promising candidate for solar fuels production.
4:45 PM - ES02.03.10
Low-Temperature Thermochemical Water-Splitting Using Poly-Cation Oxides
Shang Zhai 1 , Jimmy Rojas 1 , Hyungyu Jin 2 , Nadia Ahlborg 1 , William C. Chueh 1 , Arun Majumdar 1
1 , Stanford University, Stanford, California, United States, 2 , Pohang University of Science and Technology (POSTECH), Seoul Korea (the Republic of)
Show AbstractThermochemical water splitting (TWS) has been long pursued for H2 production, but the need for high operating temperature (>1500oC) makes it incompatible with the chemical industry infrastructure. We designed and synthesized a new class of metal oxides and experimentally demonstrated O2 evolution at 1100oC followed by H2 evolution per gram of oxide at 600oC - even with 100 ppm background H2. A thermodynamic analysis helped identify the enthalpy and entropy ranges for TWS at temperatures ~1000oC or below. The experiments also showed diffusion limited kinetics for both O2 and H2 evolutions, indicating a potentially better performance with micro/nano-structure. The poly-cation design may have improved both phase transition and nonstoichiometric effects during TWS cycles, making it thermodynamically outperform state-of-the-art TWS oxides, especially at low temperature. Thermodynamic, chemical and structural characterization was conducted to investigate the detailed mechanism of this new class of materials. The large design space of such oxides offers the promising prospects of TWS at temperatures <1000oC.
ES02.04: Poster Session I
Session Chairs
Thomas Fischer
Simelys Hernandez
Avner Rothschild
Tuesday AM, November 28, 2017
Hynes, Level 1, Hall B
8:00 PM - ES02.04.01
Graphene Quantum Dots Immobilized Mesoporous N-TiO2 Thin Films for Efficient Photocatalytic Water Oxidation
Namal Wanninayake 1 , Syed Islam 1 , Allen Reed 1 , Joseph Strzalka 2 , Stephen Rankin 1 , Doo Young Kim 1
1 , University of Kentucky, Lexington, Kentucky, United States, 2 , Argonne National Laboratory, Lemont, Illinois, United States
Show AbstractRecently we reported that the plasma-induced N2/Ar treatment is an efficient method to enhance the visible-light absorption and the photocatalytic activities of mesoporous TiO2. A significant band gap reduction (from 3.5 eV to 2.9 eV) was achieved upon the N2/Ar plasma treatment of TiO2, and the photocatalytic water oxidation showed 70 times enhancement with nitrogen-doped TiO2 (N-TiO2) under solar simulator (AM1.5G). This work is an effort to enhance the visible-light-driven photo-activity of N-TiO2 further by the sensitization with graphene quantum dots (GQDs). GQDs are new members of carbon material family whose physicochemical properties are unique. In this study, GQDs were synthesized by the chemical oxidation of carbon nano-onions. The synthesized GQDs have a lateral dimension of 5 nm with the thickness of a few nm. Furthermore, the disk-shaped sp2-carbon network of GQD consists of many chemical functional groups such as hydroxyl, epoxy, and carboxylic groups. These functional groups can play a significant role by tuning the surface property of the N-TiO2. In this study, the cubic ordered mesoporous TiO2 films were prepared using a surfactant templated sol-gel method. Then, TiO2 films were treated with N2/Ar plasma for the incorporation of substitutional N atoms. The direct immobilization of GQDs on to N-TiO2 was accomplished by hydrothermally reacting GQDs and N-TiO2. The successful immobilization of GQDs onto N-TiO2 was probed by scanning electron microscopy, zeta potential, and contact angle measurements. Furthermore, the pore orientation of TiO2 before and after the plasma treatment was confirmed by the Grazing-Incidence Small-Angle X-ray Scattering technique. Chronoamperometry was employed to determine their photocatalytic performance. Kinetic photocurrent decay analysis revealed that GQDs reduce the charge recombination. GQD-immobilized N-TiO2 showed about 2X photocurrent enhancement compared to unmodified N-TiO2, indicating the significant role of GQDs. We conclude this enhancement originates from the synergistic effect of N-TiO2 and GQDs, including enhanced visible light absorption, proper band alignment, efficient charge separation at the GQD/N-TiO2 interface and the surface catalytic sites induced by GQDs.
8:00 PM - ES02.04.02
Ultrathin Nanotubes for Harnessing Solar Energy—Solar Fuel and Electricity
Menna Said 1 , Nageh Allam 1 2
1 , The American University in Cairo, Cairo Egypt, 2 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractHollow TiO2 nano-architectures show much promise for the design of highly active nanostructured catalysts due to their low density, high strength, high active surface area, and improved light harvesting characteristics. For example, TiO2 hollow nanoparticles have shown exceptional performance upon their use as photoanode materials in DSSCs mainly because of their large surface area, enabling the adsorption of an enormous amount of dye. However, for applications such as water splitting, the small grain boundaries of the nanoparticles can be problematic as they act as recombination centers of charge carriers, resulting in a short lifetime of the electrons. On the other hand, the nanotubular architecture with its one dimensional structure and ordered morphology offers the advantage of directed electron transport and electron/hole pair separation. However, nanotubes have a lower surface area compared to nanoparticles. To have both advantages, a high surface area and directional charge transfer, the nanotubular architectures need to have lengths that are close to those of the nanoparticles. Herein, we report a facile and cost-effective synthesis method, galvanostatic anodization, of Sub-100 nm TiO2 tubular architectures. We also report on the control of the growth of the photoactive {001} facet. The fabricated nanotubes are partially crystalline with high photoactivity towards water splitting and solar to-electric conversion. Mott–Schottky, transient photocurrent and incident photon-to-current efficiency (IPCE) analyses indicate a faster electron transfer at the nanotube/electrolyte interface. The sub-100 nm tubes showed a maximum conversion efficiency of 9.3% upon their use in dye-sensitized solar cell devices.
8:00 PM - ES02.04.03
Titanium Nitride Interfacial Layer—A Key Route for Efficient and Stable Thin-Film NiP2 Hydrogen Evolution Catalysts on Silicon Photocathodes
Shinjae Hwang 1 , Spencer Porter 1 , Anders Laursen 1 , Hongbin Yang 1 , Mengjun Li 1 , Slava Manichev 1 , Karin Calvinho 1 , Voshadhi Amarasinghe 1 , Martha Greenblatt 1 , Eric Garfunkel 1 , G Dismukes 1
1 , Rutgers, The State University of New Jersey, Piscataway, New Jersey, United States
Show AbstractThe development of highly efficient photoelectrochemical cells (PECs) that can split water into H2 and O2 is a grand challenge in the field of renewable energy. Replacing costly noble metal electrodes and establishing stable devices are major challenges. Low-cost, efficient electrocatalysts using transition metal phosphides (TMP) for hydrogen evolution have been developed, but few reports have demonstrated their successful integration with a photoabsorber owing to unstable junction formation. To prepare a monolithic junction, a catalyst/diffusion barrier/photoabsorber stack consisting of NiP2, TiN, and n+p-Si made up the photocathode reported herein. Crystalline TiN creates an electron conducting, transparent interface that prevents atomic diffusion during high temperature fabrication and maintains clean interfaces. A 6.8 nm film of crystalline NiP2 on TiN (4 nm)/n+p-Si retained 97% of photovoltage, produces similar Jsc compared to bare Si, and achieves a turnover frequency of 1.04 H2 s-1 at -100 mV applied electrical potential. When used as photocathode it requires only -150 mV overpotential above the benchmark Pt/TiN/n+p-Si to achieve a current density of -10 mA/cm2. This photocathode also maintains a stable hydrogen evolution photocurrent (±10%) without failure for at least 125 hours in acid (0.5 M H2SO4), the duration of testing. Supported by a joint NSF-CBET/DOE-EERE-FCTO grant.
8:00 PM - ES02.04.04
Photocatalytic Conversion of Carbon Dioxide by Water as an Electron Donor over Rh3+ -Substituted Gallium Oxide Photocatalyst
Soichi Kikkawa 2 , Kentaro Teramura 2 1 , Hiroyuki Asakura 2 1 , Saburo Hosokawa 2 1 , Tsunehiro Tanaka 2 1
2 , Graduate School of Engineering, Kyoto University, Kyoto-shi Japan, 1 , ESICB, Kyoto University, Kyoto-shi Japan
Show AbstractThe photocatalytic conversion of CO2 is one of the best ways for the conversion of CO2 toward chemical resources such as CO, HCOOH, HCOH, CH3OH, CH4 at ambient temperature and ordinary pressure. We have reported that Ag-loaded Ga2O3 (hereinafter, referred to as Ag/Ga2O3) shows the high conversion of CO2 toward CO for the photocatalytic conversion of CO2 by H2O as an electron donor. Unfortunately, Ag/Ga2O3 functioned only at less than 300 nm wavelength of light. In this study, we synthesized Ga2O3-based photocatalysts in which Rh ions were substituted for the Ga site (Ga2O3:Rh) for the photocatalytic conversion of CO2 driven at more longer wavelength of photoirradiation.
Ga2O3:Rh were prepared by a typical preparation method using a Ga(NO3)3 precursor. An aqueous solution of NH3 was dropped into a mixed solution of Ga(NO3)3 and RhCl3. Yellow powder of Ga2O3:Rh was obtained by calcination of the precipitate at 1173 K for 6h. Total of 1 wt% of Ag and Cr co-catalyst were loaded by an impregnation method. The photocatalytic conversion of CO2 by H2O was carried out using an inner-irradiation type reactor. 0.5 g of photocatalyst was dispersed in an aqueous solution of NaHCO3 (0.1 M, 1.0 L). The suspension was irradiated under a 400 W high-pressure mercury lamp though a Pyrex® glass filter equipped with a cooling water system. CO2 gas (99.999 %) was bubbled into the solution at a flow rate of 30 mL min−1. The photocatalytic reaction under a Xe lamp with a cut-off filter was carried out in a closed circulation system connected to a vacuum line.
Considerable amount of CO (ca. 6 µmol h-1) was obtained over Ga2O3:Rh photocatalysts in the photocatalytic conversion of CO2. O2 as oxidation product of H2O was observed at stoichiometric ratio to the total amount of CO and H2 as reduction products, continuingly. UV-vis DRS of Ga2O3:Rh showed that the absorption edge shifted to 370 nm and the new broad band was observed between 400 and 500 nm. XRD patterns and Rh-K edge and Ga-K edge XANES spectra of Ga2O3:Rh suggested that the Rh species were considered to be trivalent and substituted into the octahedral Ga site of β-Ga2O3. DFT calculations of Ga2O3:Rh revealed that the new energy level was formed by the Rh3+ doping. We monitored the formation rates of H2 using MeOH as a sacrificial reagent under photoirradiation in the different wavelength regions. The evolution of H2 was observed when cut-off filters (from UV-29 to UV-35) were employed. In addition, it depended on the absorption of Ga2O3:Rh in UV-vis DRS, indicating that Ga2O3:Rh functions as a photocatalyst using the absorption which absorption edge shifted to 370 nm by Rh3+ doping.
8:00 PM - ES02.04.05
Enhanced Electrochemical CO2 Reduction by Adsorbed CN and Cl on Au Electrode
Minhyung Cho 1 , Jun Tae Song 1 , Seoin Baek 1 , Youngkook Kwon 2 , Yousung Jung 1 , Jihun Oh 1
1 , KAIST, Daejeon Korea (the Republic of), 2 , KRICT, Daejeon Korea (the Republic of)
Show AbstractThrough electrochemical CO2 reduction, CO2 can be converted to value-added products like CO. These electrochemical CO2 reduction studies mostly have been carried out under aqueous circumstances, where hydrogen evolution reaction (HER) always competes with CO2 reduction reaction (CO2RR) because of its redox potential. Therefore, many researchers have been studied to suppress HER and increase CO2RR selectivity through modifying metal surface using the additives such as anchoring agents, anions, etc. However, there are only a few studies about modifying Au surface with additives.
Here, we present the role of adsorbed Cl and CN on Au electrodes for electrochemical CO2RR. At first, we show the ability of adsorbed Cl and CN on Au for the high CO selectivity toward CO2RR by density functional theory (DFT) calculation using Vienna Ab initio Simulation Package (VASP). Then, we prepared Cl and CN adsorbed Au electrodes by simple electrodepostion of Au in aqueous solutions with various Au salts (i.e. KAuCl4, KAu(CN)2). We confirm that the CO2RR of electroplated Au shows higher CO selectivity than evaporated Au films. The adsorbed Cl and CN on Au electrodes were observed with XPS and in electrochemical CO2RR experiments, CN and Cl adsorbed Au electrodes show around 80% CO selectivity at –0.39 V while evaporated Au shows below 20%. The more detail experimental data like jCO, Tafel plot and electrode stability will be presented.
8:00 PM - ES02.04.06
Ordered Cu Mesostructures for Selective C2 Product Formation During Electrochemical CO2 Reduction
Hakhyeon Song 1 , Youngkook Kwon 2 , Jihun Oh 1
1 , Korea Advanced Institute of Science and Technology, Daejeon Korea (the Republic of), 2 , Carbon Resources Institute, Korea Research Institute of Chemical Technology, Daejeon Korea (the Republic of)
Show AbstractElectrochemical CO2 reduction reaction (CO2RR) is promising to convert intermittent renewable energy into value-added fuels and chemical feedstocks. Cu-based electrocatalysts are attracting much attentions due to their unique ability to produce various hydrocarbons from electrochemical CO2RR. In particular, Cu mesostructures such as meso-pores, nanowires and nano-foam structures have been shown to produce C2 products with enhanced selectivity at low overpotentials because of high local pH in mesostructures [1-3].
Herein, we demonstrate Cu inverse opal (IO) mesostructures for highly enhanced C2 products formation from electrochemical CO2RR. Cu IOs are three-dimensionally ordered Cu networks with controlled thicknesses of hexagonally closed packed spherical void layers. In order to form Cu IOs, firstly, we electrophoretically deposited polystyrene (PS) beads with 600 nm in diameter on a Au (200 nm)/Ti (20 nm)/Si substrate and then electro-deposited Cu in the PS template with acidic Cu sulfate solutions. By controlling deposition time, the thickness layers of Cu IO are adjusted as 3, 6 and 12 layers, and then Cu IOs with controlled thickness was formed after removal of the PS template in toluene. Electrochemical CO2RR of our Cu IOs were conducted in 0.1 M KHCO3 and the reaction products were measured using gas and liquid chromatography. The electrochemical product analysis reveals that our Cu IOs suppress H2 evolution reaction and significantly enhance CO2RR catalytic activity with reduced overpotential. For example, our Cu IO with 6 layers shows ~30 and ~10% Faradaic efficiencies (FEs) of C2H4 and C2H5OH, respectively, with a strong suppression of methane (~3%FE) at -0.8 V (RHE). The superior CO2RR activity of our Cu IOs is believed to originate from the increased surface area and local pH variation in the mesostructures. Detailed analysis of the thickness layers-dependent CO2RR activity of Cu IO will be discussed.
1. Ma, M., K. Djanashvili, and W.A. Smith, Controllable Hydrocarbon Formation from the Electrochemical Reduction of CO2 over Cu Nanowire Arrays. Angewandte Chemie, 2016. 128(23): p. 6792-6796.
2. Yang, K.D., et al., Morphology Directed Selective Production of Ethylene or Ethane from CO2 on a Cu Mesopore Electrode. Angewandte Chemie International Edition, 2017. 56(3): p. 796-800.
3. Dutta, A., et al., Morphology Matters: Tuning the Product Distribution of CO2 Electroreduction on Oxide-Derived Cu Foam Catalysts. ACS Catalysis, 2016. 6(6): p. 3804-3814.
8:00 PM - ES02.04.07
Enhanced Photoelectrochemical Performance of Hydrogenated Fe2O3/TiO2 Heterostructure
Nisha Kodan 1 , Aadesh Singh 1 , Bodh Mehta 1
1 Department of Physics, IIT Delhi, New Delhi, Delhi, India
Show AbstractNanostructured metal oxide based photoelectrochemical (PEC) cells are under worldwide attention as the method to generate clean energy i.e. solar hydrogen. The efficient separation of photogenerated electron−hole pairs and stability against corrosion are critical preconditions for a photoelectrode to achieve a high photoelectrochemical performance. In this work it is shown how both criteria can be met by employing a heterostructure of iron oxide (Fe2O3) and titanium dioxide (TiO2) as the photocatalyst. The stoichiometric thin films of nanostructured Fe2O3 were grown by rf sputtering under Argon atmosphere at a fixed rf power of 80W and at fixed substrate temperature. The obtained thin films (Fe2O3) were further annealed in hydrogen atmosphere at 300° for 10 hours to obtain hydrogen treated (H:Fe2O3) thin films. To obtain heterostructure Fe2O3 and H:Fe2O3 thin films are coated with thin overlayer of TiO2 using sol-gel spin coating technique. Prepared Fe2O3, Fe2O3/TiO2 and H:Fe2O3/TiO2 thin films were characterized by glancing incidence x-ray diffraction, atomic force microscopy, scanning electron microscopy, x-ray photoelectron spectroscopy and optical spectroscopy to understand the structural, morphological, optical and photoelectrochemical properties. Fe2O3 and TiO2 thin films grown with preferred photocatalytic hematite and anatase structure with no change in phase on hydrogen treatment has been confirmed by XRD and Raman analysis. Measurements of current versus applied voltage were performed under dark and illumination in order to assess the performance of Fe2O3, Fe2O3/TiO2 and H:Fe2O3/TiO2 thin films as photoanodes for hydrogen generation in PEC cell. Annealing in hydrogen ambient improved photoelectrochemical properties over the visible range of the solar spectrum as compared to pure Fe2O3/TiO2 samples. We observed approximate three times higher photocurrent density response in H:Fe2O3/TiO2 samples (2.2 mA/cm2) as compared to Fe2O3/TiO2 (0.8 mA/cm2) samples.
8:00 PM - ES02.04.08
Enhanced Solar Light-Active Photocatalysis Based on Upconversion Nanocrystal Embedded Mesoporous Carbon-TiO2 Hybrid Films
Hannah Kwon 1 , Kyungwha Chung 1 , Yu Jin Jang 1 , Jiseok Lee 2 , Dong Ha Kim 1
1 , Ewha Womans University, Seoul Korea (the Republic of), 2 , Ulsan National Institute of Science and Technology, Ulsan Korea (the Republic of)
Show AbstractEnvironmental pollution has been recognized as a serious problem in global society. Researchers have suggested diverse potential pathways to solve the problem. The use of photocatalysis has been actively investigated and employed to resolve the issue of environmental remediation. Titanium dioxide (TiO2) has been mostly used in the field of photocatalysis and solar energy conversion due to its good stability, nontoxicity, and high activity. Mesoporous structures have been actively studied due to their large accessible surface area and excellent connectivity, in which both effective charge carrier transfer and efficient mass flow of the reactants are attained. Standard available amphiphilic triblock copolymers, typically Pluronic P123, have been commonly used as the structure-directing agents for the construction of highly organized mesoporous TiO2 (mTiO2). However, the large bandgap (~3.2 eV) limits its sunlight absorption to UV region of the solar spectrum, which only possesses 5% of total solar energy. Hence, a number of methods were introduced to increase absorption of longer wavelengths sunlight ranging from visible to near-infrared region. One advanced strategy to enhance such panchromatic photocatalytic activity is to utilize upconversion nanocrystals (UCNs). Pair of some lanthanides elements can induce absorption of long-wavelength radiation in the IR or NIR region and emit shorter wavelength, higher energy radiation from UV to IR by a process called upconversion. Herein, we report a concept for the fabrication of Gd3+-doped β-NaYF4:Yb3+,Tm3+ (UCNs) embedded carbon TiO2 hybrid film with mesoporous textures (mC-TiO2) on the surface by direct carbonization of block copolymer templates in order to maximize the photocatalytic performance under one-sun illumination. Photocatalytic activities for the mTiO2, mC-TiO2 and mC-TiO2/UCN hybrid films were validated and compared in terms of the degradation of NB (Nitrobenzene) under UV, visible and NIR light irradiation, respectively. 77 and 65% of NB molecules were decomposed in 3 h in the presence of the as-prepared mC-TiO2 and mC-TiO2/UCN, respectively, demonstrating markedly enhanced performance compared with mTiO2 which decomposed 49% of NB. Furthermore, we explored the photocatalytic activity of the samples under NIR (λ = 980 nm laser) light illumination for 3 h. The results indicate that mTiO2 and mC-TiO2 did not show discernable photocatalytic activity under NIR light, i.e., 39 and 48% NB decomposition, respectively. In contrast, 63% of NB was degraded by mC-TiO2/UCN, evidencing a remarkably higher photocatalytic activity than mTiO2 and mC-TiO2. Conclusively the mC-TiO2/UCN hybrid fim showed the highest photocatalytic activity, i.e., 86% degradation of NB within 5 h. Thus, a simple but viable strategy to maximize the photocatalytic performance was suggested via the synergetic effect from the constituent elements, in which effective energy transfer was mediated from UCN to mC-TiO2.
8:00 PM - ES02.04.09
Photoelectrochemical Properties of P-Type Bilayer CuBi2O4|CuO Thin Films by a Simple Solution Process
Byeong-Uk Choi 1 , Sangwoo Ryu 1 , Jihun Oh 1
1 Graduate School of EEWS, Korea Advanced Institute of Science and Technology, Yuseong-gu, Daejeon, Korea (the Republic of)
Show AbstractMetal oxide semiconductors are drawing much attention for low cost photoelectrochemical (PEC) water splitting cells because of material abundance and facile synthetic methods [1]. CuBi2O4 is a p-type semiconductor with band gap of 1.5-1.8 eV and is promising for a top layer in double junction tandem PEC devices [2]. However, CuBi2O4 (CBO) is suffering from low hydrogen evolution reaction (HER) photocurrent density from poor charge transport in CuBi2O4 and charge injection kinetics at the CBO/electrolyte interface.
Here, we demonstrate CuBi2O4 and CuO bilayer thin films prepared by simple solution process for improved PEC HER. A CuO layer is introduced to enhance light absorption for its smaller band gap (~ 1.2 eV). Films are prepared by drop-casting a solution containing CuBi2O4 and CuO precursors dissolved in ethanol/acetic acid mixtures on a fluorine-doped tin oxide (FTO) coated glasses, followed by annealing at 550oC. Bilayered configurations of FTO|CuO|CuBi2O4 or FTO|CuBi2O4|CuO were fabricated to compare PEC oxygen reduction reaction (ORR) performance of the bilayer films. The PEC ORR performance was higher when CuO layer was on top of CuBi2O4, compared to CuBi2O4 on CuO, doubling the current density from -0.44 mA/cm2 to -0.82 mA/cm2 at +0.4 V vs RHE under AM 1.5G illumination. We suggest that conduction band edge alignment of CuO and CuBi2O4 improves charge separation between the FTO|CuBi2O4|CuO and electrolyte interface. Detailed investigation of the band structure of CuBi2O4 and CuO bilayer and PEC HER performance will be provided in the presentation.
[1] D. Kang et al., Chem. Rev. 2015, 115, 12839−12887
[2] S. Berglund et al., Chemistry of Materials, 2016, 28, 4231−4242
8:00 PM - ES02.04.10
Enhancing CO Selectivity of Electrochemical CO2 Reduction by Three Dimensional Au Nanoarchitecture Electrocatalysts
Jongmin Kim 1 , Minhyung Cho 1 , Jihun Oh 1 , Yeon Sik Jung 1
1 , Korea Advanced Institute of Science and Technology (KAIST), Daejeon Korea (the Republic of)
Show AbstractElectrochemical reduction of CO2 into a chemical feedstock has attracted much attention due to increasing demands for renewable energy sources. However, electrochemical CO2 reduction systems generally suffer from poor energy conversion efficiency and low product selectivity due to limitations of catalytic performances. In this study, we demonstrate a highly selective and efficient 3-dimensional Au nanoarchitecture electrocatalysts fabricated through solvent-assisted nanotransfer printing process (s-nTP). Based on the engineering of grain boundary density and crystalline plane of Au nanowires constituting the Au catalyst nanoarchitectures, high faradaic efficiency and current density were achieved. Furthermore, diffusional gradients induced by systematically engineered 3-dimensional nanoarchitectures further suppress hydrogen evolution reaction while preserving the high rates of CO2 reduction to CO, and thus a high CO faradaic efficiency (94%) at a low overpotential (390 mV) with a current density of 1.32 mA/cm2 was chieved. We believe that this new strategy will provide a new way to fabricate highly efficient electrodes for CO2 reduction system with outstanding scalability and controllability.
8:00 PM - ES02.04.11
Protection and Photovoltage Improvement of N-Type Semiconductor Photoanode for Water Oxidation by Atomic-Layer Deposition of Cobalt Oxide
Xinghao Zhou 1 , Rui Liu 1 , Ke Sun 1 , Kimberly Papadantonakis 1 , Bruce Brunschwig 1 , Nathan Lewis 1
1 , California Institute of Technology, Pasadena, California, United States
Show AbstractWater oxidation is a key process for sustainable hydrogen evolution reaction, as well as CO2 reduction process. Single Crystalline n-Si(100) photoanode coated with a thin ~50 nm film of cobalt oxide fabricated using atomic-layer deposition (ALD), exhibited 570 mV photovoltage, 11% equivalent photovoltaic efficiency, and operated continuously for over 100 days ( ~2500 h) in 1.0 M KOH(aq) under simulated solar illumination. The ALD CoOx thin film could also stabilize other n-type semiconductors in strong alkaline electrolytes, as well as improve the photovoltages under simulated solar illumination. The multi-functional ALD CoOx thin film: (i) formed a heterojunction with the n-type semiconductor that provided an improved photovoltage under 1 Sun of simulated solar illumination; (ii) stabilized photoanodes that are otherwise unstable when operated in aqueous alkaline electrolytes; (iii) catalyzed the oxidation of water, thereby reducing the kinetic overpotential required for the oxygen evolution reaction (OER) and increasing solar conversion efficiency. This method provides a simple, effective method for enabling the use of n-type single crystalline silicon and other semiconductors as efficient and durable photoanodes in fully intergrated solar fuel generators.
8:00 PM - ES02.04.12
Mesoporous Quaternary Semiconductor Oxides for Improved Photocatalytic Hydrogen Production
Tobias Weller 1 , Morten Weiss 1 , Lukas Specht 1 , Roland Marschall 1
1 , Justus-Liebig-Univ Giessen, Giessen Germany
Show AbstractDetailed and reliable studies on the optimum pore morphology of mesoporous photocatalysts for photocatalytic water splitting are surprisingly not available. Mesoporous materials often exhibit pore walls of only several nanometers, which can be favorable for photocatalysis due to very short diffusion pathways for charge carriers.
We chose the complex semiconductor photocatalyst CsTaWO6 as model system for studying the optimum mesoporous morphology of semiconductors for hydrogen production. CsTaWO6 is known to generate hydrogen from alcoholic solutions without any co-catalyst,[1] is known to be amenable for homogeneous anion doping from the gas phase,[1,2,3] and exhibits only one crystal structure (cubic defect-pyrochlore structure) without any temperature-induced phase transitions known. The latter is crucial to avoid any influence on photocatalytic activity by the formation of multicomponent heterojunctions.[4]
We have prepared highly crystalline mesoporous CsTaWO6 via evaporation-induced self-assembly (EISA) process using the commercial block-copolymer P-123,[5] and via hard-templating in mesoporous silica KIT-6.[6]
Different additives during soft-templating were used to vary only the pore morphologies, resulting in BET surface areas measured by N2 physisorption up to 78 m2 g-1 and pore sizes ranging from 5 to 10 nm, depending on the additives. Although pore ordering is not fully established, we find photocatalytic hydrogen evolution activities being independent on surface area, but rather depending on pore size distribution, indicating that transport of electrolyte and gaseous products in the pores are rate determining factors in photocatalytic hydrogen production.
Hard-templated phase pure samples of mesoporous CsTaWO6 with surface area up to 115 m2 g-1 confirm the hydrogen evolution dependence on pore size rather than surface area.
Moreover, we have identified an optimum crystallite size for CsTaWO6 to maximize hydrogen production, by investigating size-defined single-crystalline nanoparticles of CsTaWO6 prepared via hydrothermal synthesis.[7] 13 nm were identified to be the ideal crystallite size, as an optimum ratio of surface recombination and charge carrier diffusion pathway.
Currently, we are developing periodically ordered mesoporous CsTaWO6 via soft-templating for further investigations.
[1] A. Mukherji, R. Marschall, A. Tanksale, C. Sun, S. C. Smith, L. Wang, G. Q. (Max) Lu Adv. Funct. Mater., 2011, 21, 126.
[2] R. Marschall, A. Mukherji, A. Tanksale, C. Sun, S. C. Smith, L. Wang, G. Q. (Max) Lu, J. Mater. Chem., 2011, 21, 8871.
[3] R. Marschall, L. Wang, Catal. Today, 2014, 225, 111.
[4] R. Marschall, Adv. Funct. Mater., 2014, 24, 2421.
[5] T. Weller, J. Sann, R. Marschall, Adv. Energy Mater., 2016, 6, 1600208 (1-9).
[6] M. Weiss, S. Waitz, R. Ellinghaus, T. Weller, R. Marschall, RSC Adv., 2016, 6, 79037.
[7] T. Weller, L. Specht, R. Marschall, Nano Energy, 2017, 31, 551.
8:00 PM - ES02.04.13
A New Synthesis Approach for Carbon Nitrides—Poly (Triazine Imide) and Its Photocatalytic Properties
Leonard Heymann 1 , Christian Klinke 1
1 Department of Chemistry, University of Hamburg, Hamburg Germany
Show AbstractAfter being first synthesized in the 1830s by Justus von Liebig carbon nitride materials have not been paid particular attentions by the scientific community. This changed in the 1990s when a carbon nitride phase, so called β-C3N4, was proposed with a hardness comparable to diamond. This interest even increased when it was shown in the early 2000s that melon, often referred as g-C3N4, has photocatalytic properties, e.g. water splitting and reduction of carbon dioxide. Beside melon other carbon nitride materials have been described in the last years, e.g. poly (triazine imide) and poly (heptazine imide) with intercalated ions.
We present a new synthesis method for a carbon nitride, which we identify as poly (triazine imide) PTI. In contrast to the common thermal induced poly condensation reactions this synthesis method relies on polymerization of melamine by radical species. These species were formed during electrolysis of water, e.g. hydroxyl radicals.
The as-obtained brownish product shows interesting properties compared to melon. It has a significant solubility in water at high pH values, which we ascribe to deprotonation. Therefore, it was possible to conduct liquid state NMR spectroscopy to characterize the product. In combination with powder XRD, FTIR spectroscopy and XPS it was possible to identify the product as poly (triazine imide).
The as-obtained PTI has photocatalytic properties, which were investigated by degradation of a cationic dye (methylene blue) and an anionic dye (naphthol yellow S) at different pH values. It was shown that the adsorption of methylene blue is increased at higher pH values as well as its degradation rate. For naphthol yellow S the contrary effect is observed. These properties can be used for selective photocatalysis. Additionally, the influence of different active species in the degradation process was investigated.
PTI is a promising carbon nitride material with photocatalytic properties. The presented synthesis method enables new approaches to improve the photocatalytic properties. This improvement may be due to molecular doping, which could shift the absorption edge or even increase the quantum efficiency. PTI and its synthesis method have a lot of factors, which could lead to an increase of the water splitting abilities of carbon nitride materials.
8:00 PM - ES02.04.14
Enabling Unassisted Solar-Powered High Voltage Redox Battery by Ta3N5 and GaN/Si
Qingmei Cheng 1 , Weiqiang Fan 1 , Yumin He 1 , Peiyan Ma 1 , Srinivas Vanka 2 , Shizhao Fan 2 , Zetian Mi 2 , Dunwei Wang 1
1 , Boston College, Chestnut Hill, Massachusetts, United States, 2 , McGill University, Montreal, Quebec, Canada
Show AbstractThe utilization of intermittent solar energy is craving for efficient and scalable energy storage technologies for continuous supplies. Solar rechargeable redox flow battery, based on the combination of a photoelectrochemical cell and a redox flow battery, provides such an innovative and promising approach for solar energy utilization through the direct photoelectrochemical charging of the redox couples in a redox flow battery. However, the development of this technology has been hindered by low open-circuit potential (typically <0.8 V), largely due to the insufficient photovoltages inherent to photoelectrode choices. To solve this critical challenge, here we demonstrated that an overall photovoltage larger than 1.4 V could be realized by a dual-photoelectrode system, namely a Ta3N5 photoanode and a GaN nanowire/Si photocathode. Additionally, the photoelectrode system featured high current densities, negligible side-reactions, and good stabilities. As a result, a 1.2 V alkaline anthraquinone/ferrocyanide redox battery was directly charged by solar light bias-free with a high solar-to-chemical conversion efficiency of 3.0%. Moreover, the photocharged battery could be discharged like a normal redox battery with a high output voltage and appreciable efficiency. The demonstration opens up new doors toward practical solar-powered redox flow batteries by providing a desirable photoelectrode platform.
8:00 PM - ES02.04.15
Room-Temperature Synthesis of Nitride Nanocatalysts for Electrochemical Water Splitting
Jin Soo Kang 1 2 , Heejong Shin 1 2 , Yoon Jun Son 1 2 , Myeong Jae Lee 1 2 3 , Subin Park 1 2 , Yung-Eun Sung 1 2
1 School of Chemical and Biological Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Center for Nanoparticle Research, Institute for Basic Science, Seoul Korea (the Republic of), 3 School of Advanced Materials Engineering, Kookmin University, Seoul Korea (the Republic of)
Show AbstractOwing to the forthcoming depletion of fossil fuels and considerations on global warming, development of sustainable energy sources is regarded as an issue of paramount consideration. Recent advances in fuel cell technologies are raising the expectations for practical utilization of H2 as energy sources, and eco-friendly H2 production is thereby an important issue. For this reason, intensive investigations have been performed lately on the design and synthesis of electrocatalysts for water splitting.
In hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), Pt and IrO2 respectively stand as state-of-the-art catalysts with excellent activity and reliability. In order to replace these costly materials, various transition metal compounds with favorable electronic and geometric structures have been introduced, and they manifested decent performances in water splitting. However, complicated nano-architecturing processes and harsh synthesizing conditions are impeding the practical utilizations of the compound catalysts. In this presentation, we propose a new strategy for the preparation of transition metal compound catalysts. As model catalysts, nitride materials were prepared in an inert condition without any elevation of temperatures. Complete nitridation was confirmed by various physicochemical characterizations including the synchrotron X-ray analyses, and the effectiveness of our technique was verified. In electrochemical water splitting, nitride catalysts exhibited high activities that were even comparable to noble metal-based counterparts. Moreover, long-term stabilities were confirmed by continuous potential sweeps in wide potential ranges.
8:00 PM - ES02.04.16
Nanostructured Mo-TiO2 Photoanodes for Solar Fuels
Miriam Regue 1 2 , Katherine Armstrong 4 , Emma Richards 4 , Andrew Johnson 1 3 , Salvador Eslava 1 2
1 Chemistry, Centre for Sustainable Chemical Technologies, University of Bath, Bath United Kingdom, 2 Chemical Engineering, University of Bath, Bath United Kingdom, 4 , School of Chemistry, Cardiff University, Cardiff United Kingdom, 3 Chemistry, University of Bath, Bath United Kingdom
Show AbstractSociety is facing a global energy crisis. By 2050, it is expected that world energy demand will double and both oil and natural gas resources will not be able to cope with such demand, making it impossible to live in a fossil fuels based society.1 In this energy consuming culture, the need to diversify our energy portfolio is paramount. Semiconductor photocatalysts, such as metal oxides, could play an important role owing to their capability to exploit solar energy to produce solar fuels, such as hydrogen. Titanium dioxide (TiO2) is the most studied semiconductor, however its (often) poor surface area, large band-gap and fast recombination of electron and holes limit its practical application.2 The doping of TiO2 with external metals such as Mo, Ta, Ni can minimise the fast charge recombination and enhance the photocatalytic performance.3 Here we present a new strategy for the formation of nanostructured Mo doped TiO2 (Mo:TiO2) photoanodes using a single source precursor. A heterometallic titanium-molybdenum oxo alkoxy cage is successfully used as a single source precursor for the formation of a Mo:TiO2 photoanode film using spray pyrolysis.
The resulting Mo:TiO2 photoanodes have been characterised by TEM, SEM, UV-vis spectroscopy, XRD, XPS, Raman spectroscopy, linear sweep voltammetry and electrochemical impedance spectroscopy to relate their physical properties to their photocatalytic performance as photoanodes in photoelectrochemical cells. It was found that calcination temperature is a crucial variable affecting their final performance due to its effects on the crystallinity and morphology of the resulting nano-materials. Mo:TiO2 photoanodes prepared at 700°C present better photo stability and a twofold increase in photocurrent performance (0.20 mA cm-2) in comparison to pure TiO2 photoanodes (0.10 mA cm-2) under AM 1.5G illumination calibrated to one sun. The superior performance of Mo:TiO2 over pure TiO2 is attributed to a higher surface area, Mo content, band-gap reduction and lower charge transfer resistance.
1 S. Styring, Faraday Discuss., 2012, 155, 357–376.
2 X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891–2959.
3 M. Ni, M. K. H. Leung, D. Y. C. Leung and K. Sumathy, Renew. Sustain. Energy Rev., 2007, 11, 401–425.
8:00 PM - ES02.04.17
Haematite Nanorods Activated by High Annealing Temperature for Sunlight-Driven Water Oxidation Reaction
Flavio De Souza 1 2 , Waldemir Carvalho 1 , Dereck Muche 2 , Ricardo Castro 2
1 , University Federal-ABC, Santo Andre-SP Brazil, 2 Materials Science & Engineering, University of California, Davis, Davis, California, United States
Show AbstractThe performance of nanostructures on photoelectrochemical water-splitting devices is fundamentally governed by the capability to promote efficient photogenerated charge separation reducing the recombination rate. Here we show that chlorine species originated from commonly used iron precursors annihilate haemetite nanorods photocurrent by providing recombination pathways. Although haemetite nanorod films could be obtained by thermal decomposition of iron oxyhydroxide phase (β-FeOOH), indistinguishable photocurrent responses under dark and sunlight irradiation conditions were observed until the nanorods were annealed (activated) at 750 °C. The annealing led to observable chlorine species elimination and allowed photocurrent responses of 1.3 mA.cm-2 at 1.23 V vs RHE, which is comparable to the best results found in literature, suggesting that residual chlorine species from the synthesis can act as electron trap and recombination sites for photogenerated holes.
8:00 PM - ES02.04.18
LaFeO3 Perovskite Films for Photoelectrochemical Water Splitting
Emma Freeman 1 2 , David Fermin 2 , Veronica Celorrio 2 , Salvador Eslava 1
1 Chemical Engineering, University of Bath, Bath United Kingdom, 2 Chemistry, University of Bristol, Bristol United Kingdom
Show AbstractRenewable and sustainable alternatives to fossil fuels are needed to limit the impact of global warming. Using hydrogen within fuel cells in which the H2 is attained from photoelectrochemical water splitting is one such alternative. Titania is one of the most common photocatalysts for water splitting but is limited due its wide band gap and hence inactivity in the visible light region, therefore viable substitutes are required. Perovskite oxides such as lanthanum ferrite (LaFeO3) have shown to be active for water splitting, and benefit from high structural flexibility and smaller band gaps, however have relatively low surface area. Surface modification of LaFeO3 on a nanostructure level, can be conducted to increase surface area and enhance overall catalytic efficiency. Here we present this can be achieved through calcination of catalyst precursors with graphene oxide (GO). GO has a unique nanostructure consisting of a 2D honeycomb network of high surface area, and on calcination with metal nitrate precursors during a sol-gel synthesis, pure LaFeO3 is formed in which GO’s nanostructure is replicated. These new templated perovskite nanoflakes display higher BET surface area and achieve photocurrent densities of more than quadruple that attained from pristine LaFeO3. This relationship was also observed for an ionic liquid based synthesis, where on templating with GO more than doubles photocatalytic activity. With addition to templating, doping with co-catalysts such as copper can be used to further enhance photocatalytic activity, by providing an additional effective reductive site for hydrogen evolution. On doping with copper the photoactivity increases by a factor of 9, additionally using doping in conjunction with GO templating further increases activity by a factor of 4. Overall, it has been demonstrated that the use of graphene oxide as a sacrificial support and the use of co-catalysts such as copper, can yield vast improvements in photocatalytic activity for water splitting.
S. Eslava, A. Reynal, V. G. Rocha, S. Barg and E. Saiz, J. Mater. Chem. A, 2016, 4, 7200–7206.
V. Celorrio, K. Bradley, O. J. Weber, S. R. Hall, and D. J. Fermin, ChemElectroChem, 2014, 1, 1667-1671
8:00 PM - ES02.04.19
Solar-Driven Water Splitting Device of Anodized Ni-Fe Catalytic Substrate for Oxygen Evolution
Young Jin Song 1 , Wan Jae Dong 1 , Sungjoo Kim 1 , Gwan Ho Jung 2 , Kisoo Kim 2 , Jong-Lam Lee 1
1 , Pohang University of Science and Technology, Pohang Korea (the Republic of), 2 , POSCO, POHANG Korea (the Republic of)
Show AbstractPhoto-assisted water splitting is one of promising technologies to produce hydrogn fuels. Efficient oxygen evolution reaction (OER) catalysts are required for water spliting, since total water splitting reaction is severely constrained by sluggish kinetics of OER. Thus, main challenge for water splitting is lowering OER overpotential and integrating OER catalysts with solar cells. Recently, most OER catalysts are powders coated on glassy carbon or metallic foams with polymeric binders in literature. However, there are several problems like slow charge transfer, poor stability and impossibility to integrate with solar cells. Such problems were solved by anodizing Ni-Fe foil. Also, electrodeposited Ni-Fe foil has a film structure with flatness, so it could be used as the catalytic substrates of solar cells.
Here, we fabricated Ni-Fe oxyhydroxide film by anodizing Ni-Fe film, which exbilits low OER overpotential of 0.251 V and excellent stability for 36 h in 1 M KOH solution. We also utilized the catalyst as a substrate of an amorphous silicon (a-Si:H) solar cell to demonstrate a monolithic photo-assisted water splitting device with a structure of Ni-Fe foil / Ag (120 nm) / Al-doped ZnO (60 nm) / p-i-n a-Si:H (250 nm) / ITO (60 nm). When the device was illuminated in the electrolyte, photocurrent lowered OER overpotential by 0.8 V. It was the first monolithic device which can be easily adapted in the industry to fabricate highly efficient solar-driven water splitting devices.
8:00 PM - ES02.04.20
Hematite Photoanodes Surface Passivation with TiO2 Layers for Efficient Solar Water Splitting
Andre Luiz Freitas 1 , Waldemir Carvalho 1 , Turkka Salminen 2 , Harri Ali-Löytty 2 , Kimmo Lahtonen 2 , Mika Valden 2 , Tapio Niemi 2 , Flavio De Souza 1
1 , UFABC, Santo Andre Brazil, 2 , Tampere University of Technology, Tampere Finland
Show AbstractThe surface modification of hematite (α-Fe2O3) photoelectrodes for solar water splitting with overlayers has been considered a promising alternative to overcome its lower oxidation kinetics, which has been regarded as one of the major drawbacks on energy conversion efficiency. This strategy seeks to improve the charge-separation process through the semiconductor–liquid interface. Here we describe the ALD deposition of a TiOx thin layer (2 or 5 nm) in different stages of α-Fe2O3 nanorods production (on FTO, FeOOH and α-Fe2O3). The pure and TiOx-hematite photoanodes have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Uv-vis spectroscopy. Photoelectrochemical measurements showed a consistent difference when performed at front or back-side illumination, where all TiOx modified electrodes exhibited superior photocurrent results at front-side illumination. This higher photocurrent response can be related to the surface state trap removal, which favor the photogenerated hole diffusion through the semiconductor-liquid interface reducing it higher recombination loss. Moreover, it was observed a water oxidation onset potential shift to the cathodic potential on TiOx modified hematite photoanodes, which are strictly related to the slow water oxidation kinetics. The impedance spectroscopy measurements confirm that the TiOx overlayer leads a significant improvement on charge transport through the solid-liquid interface by passivating the surface states, but the bulk recombination probably remain significant that limits a higher efficiency. This finding showed that the addition of Ti over the hematite surface can be useful strategy to boost its performance, as photoanode, in a photoelectrochemical device. This work was supported by FAPESP grant 2014/50516-6 and Academy of Finland grant 2014/284652
8:00 PM - ES02.04.21
Visible-Light Active, Dopant-Free Anatase Nanocrystals for Efficient Solar Fuel Generation—The Role of Defect States
Nageh Allam 1 , Aiat Hegazy 2
1 , American University in Cairo, New Cairo Egypt, 2 , National Research Center, Cairo Egypt
Show AbstractSolar-driven water splitting promises a step towards large-scale solar energy storage. However, the bottleneck is always the poor performance of the used photoanodes, particularly in their capability to harvest the visible light. Herein, we report the design of visible-light active anatase titanium dioxide nanocrystals (sub 10 nm in size and with a surface area of ∼99 m2/g) using the sol-gel method followed by hydrothermal treatment with H2O2 at relatively low temperature (180 °C). The fabricated nanocrystals demonstrate a band gap of 2.85 eV, with an increased amount of surface defects that overcome the negative effects of bulk defects as revealed by the positron annihilation measurements. These prepared nanocrystals have notable enhancement in solar light harvesting and water splitting efficiency compared to the commercial Degussa P25 counterpart. The photoactivity, structural and electrochemical behavior of the synthesized nanocrystals were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), photocurrent measurements, photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), positron annihilation, and Doppler broadening analysis. The positron annihilation and Doppler broadening results provide new vision into the major role of defects in visible-light photocatalytic activity. Upon their use as photoanodes to split water, the H2O2-treated nanocrystals showed unprecedented performance, resulting in photocurrents of 3.8 mA/cm2. Our study introduced a new route to tune the bandgap and the functionality of semiconducting photocatalysts by tuning their size and surface-to-bulk defects.
8:00 PM - ES02.04.22
Three-Dimensional p-n TiO2/CuO Nanofiber Films for High Performance Solar-Driven Water Splitting
Menna Hasan 1 , Nageh Allam 1
1 , American University in Cairo, New Cairo Egypt
Show AbstractUsing hydrogen as a clean fuel has many advantages over hydrocarbon fuels. Herein, we introduce a novel electrode material made of p-n junction of TiO2/CuO electrospun nanofibers for solar hydrogen generation. The photocatalytic activity of electrospun Ti-Cu oxide nanofibers was compared to that of pure TiO2 nanofibers. The effect of different annealing atmospheres on the crystal structure and the photocatalytic activity was also studied. The Ti-Cu oxide nanofibers possess distinct characteristics including, high surface area, shifting the absorption edge of TiO2 into the visible region of the light spectrum, besides forming a p–n junction, which is very effective in separating the photogenerated charge carriers and hence enhancing the photocatalytic activity. For example, annealing in oxygen resulted in the formation of rutile structure while annealing in air resulted in the formation of anatase structure. The anatase structure showed almost 4 times higher current upon the use of the materials to split water photoelectrochemically. However, upon incorporating CuO with TiO2, the photocurrent of the rutile structure increased by almost five times, but still lower than that of the anatase counterpart. Adding CuO to the anatase structure also enhanced the photocatalytic activity of the anatase structure, showing almost double the photocatalytic activity of pure anatase nanofibers. Time-transient measurements showed the exceptional stability of the fabricated materials. The Mott-Schottky measurements showed that the combination of CuO resulted in smaller band gap with higher density of states. This study indicates that incorporating other semiconductors with TiO2 can help overcome the limitations of TiO2 and enhance the charge transfer process.
8:00 PM - ES02.04.24
Enhanced Photoelectrochemical Performance of Graphene Wrapped Copper Oxide Nanowires
Subish John 1 , Somnath Roy 1
1 , Indian Institute of Technology Madras, Chennai India
Show AbstractHydrogen generation by photoelectrochemical (PEC) water splitting is considered a renewable and sustainable pathway to obtain clean energy. In PEC, a semiconducting material harness the solar energy and converts it into chemical energy. However, the major limiting factors in PEC process are light absorption, charge separation and surface chemical reactions. Although metal oxides such as TiO2, ZnO, Fe2O3 etc. have been extensively investigated, the inherent n-type conductivity makes these materials suitable as photoanode. On the other hand, materials with p-type conductivity such as CuO work as photocathode and also a hydrogen evolving electrode, thereby minimising the losses. CuO, with a band gap of 1.5 eV is a suitable visible light absorbing material. Also, moderate electrical conductivity, abundance and low cost makes it a promising material for water splitting. On the other hand, 2D graphene and its derivatives such as graphene oxide (GO) and reduced graphene oxide (rGO) are being extensively investigated due to exciting properties such as very high surface area, good transparency, flexibility for functionalization and good electron conductivity. In particular, rGO behaves like a semiconductor which, in contact with a photo-catalyst, allows separation of photo-generated electrons from the corresponding holes.
In this work, we report the fabrication of rGO functionalized CuO nanowires and their application in PEC water splitting. CuO nanowires were fabricated by electrochemical anodization of Cu foil followed by annealing. Analysis of the X-ray diffraction patterns shows the formation of CuO phase. Further, rGO/CuO composite was obtained through electrophoretic deposition using GO solution. The resultant “rGO wrapped CuO nanowires” were characterised by scanning electron microscopy (SEM), Raman spectroscopy, diffused reflectance spectroscopy (DRS) and photoelectrochemical measurements. PEC measurement of rGO/CuO sample shows enhanced photocurrent compared to that of bare CuO samples (without rGO wrapping). Impedance spectroscopy measurements confirm enhance charge separation at the CuO-rGO/electrolyte interface, which indicate higher electron injection into the electrolyte driven by the rGO layers on CuO. A complete analysis of these results and data will be presented.
8:00 PM - ES02.04.25
Nanostructuring Metal Oxide Photocatalysts and Photoelectrodes for Solar Water Splitting
Salvador Eslava 1
1 , University of Bath, Bath United Kingdom
Show AbstractFacile, effective and greener approaches for the synthesis of nanostructured materials are key in the development of photocatalysts and photoelectrodes for artificial photosynthesis. Here I will present the recent developments we have achieved in the preparation of nanostructured photocatalysts and photoelectrodes of TiO2, WO3, LaFeO3, and Fe2O3. We put emphasis not only on tuning their final morphology to maximize their surface area but also on finding greener approaches that will be more sustainable and easier to commercialize. For example, exploiting the amphiphilic properties of graphene oxide and its oxygen functional groups, we have successfully imparted two-dimensionality features to TiO2 and LaFeO3 photocatalysts and photoelectrodes, boosting their final performance [1]. We have also successfully found greener approaches using naturally occurring acids to anodize tungsten foil and prepare WO3 photoanodes, avoiding the frequent but dangerous use of HF acid [2]. Finally, we have also found a greener and a more sustainable deep eutectic solvent based on food additives and fertilisers for the microwave synthesis of hematite Fe2O3 nanoparticles that can be doctor bladed for successful hematite Fe2O3 photoanodes [3]. In a nutshell, this presentation will cover the recent advances in my group in tailoring the nanostructure of photocatalysts and photoelectrodes for solar water splitting, together with the characterization that relates their properties to their activity.
[1] Eslava, S., Reynal, A., Rocha, V. G., Barg, S. and Saiz, E., “Using graphene oxide as a sacrificial support of polyoxotitanium clusters to replicate its two-dimensionality on pure titania photocatalysts”. Journal of Materials Chemistry A 2016, 4 (19), pp. 7200-7206.
[2] Zhang, J., Salles, I., Pering, S., Cameron, P.J., Mattia, D., Eslava, S. “Nanostructured WO3 photoanodes for efficient water splitting via anodisation in citric acid, RSC Advances 2017, accepted with minor revision
[3] Hammond, O.S., Eslava, S., Smith, A. J. Edler, K. “Microwave-Assisted Deep Eutectic-Solvothermal Preparation of Iron Oxide Nanoparticles for photoelectrochemical solar water splitting”, J. Mater. Chem. A 2017, in press.
8:00 PM - ES02.04.26
Water Oxidation via Silicon Photoelectrodes Protected by Thin Boron-Doped Diamond Films
Petr Ashcheulov 1 , Andrew Taylor 1 , Mariana Klementová 1 , Jaromir Kopeček 1 , Pavel Hubik 1 , V. Mortet 1 , Florian Le Formal 2 , Kevin Sivula 2
1 , Institute of Physics of the Czech Academy of Sciences, Prague Czechia, 2 Laboratory for Molecular Engineering of Optoelectronic Nanomaterials, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne Switzerland
Show Abstract
Silicon exhibits significant advantage over wide band gap semiconductors traditionally practiced in photoelectrochemical (PEC) cells because it absorbs a substantial fraction of the solar spectrum. Silicon photoelectrodes with np(Si) buried-junction structure (BJ-PEC) are capable to achieve high solar-to-fuel conversion efficiencies since such approach allows for the control of electric field build-up independently on water splitting catalysis [1]. Yet, realization of BJ-PEC devices based on Si must account for its inherent corrosion at the highly oxidative potentials required for water oxidation.
Synthetic diamond is known for its superior chemical, optical and electrical properties. Diamond in the form of thin polycrystalline film grown on silicon via chemical vapor deposition (CVD) can be optimized for high metal-like conductivity when doped with boron, such that it holds a potential to substitute metal-oxide based (ITO, FTO) anodes used in the BJ-PEC cells [1-3]. Outstanding corrosion resistance, good electrical conductivity and favorable alignment of valence bands between Si and boron-doped diamond predestine the potential of diamond films for the application in BJ-PEC devices based on Si as protective and electrically conductive coating.
Buried-junction np-Si photoelectrodes were coated by thin boron-doped nanocrystalline diamond (B-NCD) layers [4,5]. CVD growth of B-NCD were optimized towards high optical transmission and high electrical conductivity of layers. The effect of B-NCD film deposition process on the intrinsic properties of the buried-junction Si photoelectrodes has been investigated. B-NCD anodes were functionalized with oxygen-evolving catalysts to assist water oxidation. Electrochemical properties of B-NCD coated BJ-PEC Si photoelectrodes have been characterized by cyclic voltammetry.
References
1 Cox, C.; Lee, J.Z.; Nocera, D.G.; Buonassisi, T., Proc. Nat. Acad. Sci., 2014, 111 (39), 14057–14061
2 P. Ashcheulov, A. Taylor, J. More-Chevalier, A. Kovalenko, Z. Remeš, J. Drahokoupil, P. Hubík, L. Fekete, L. Klimša, J. Kopeček, J. Remiášová, M. Kohout, O. Frank, L. Kavan, V. Mortet, Carbon 119 (2017) 179 - 189.
3 Kovalenko, A.; Ashcheulov, P.; Guerrero, G.; Heinrichová, P.; Fekete, L.; Vala, M.; Weiter, M.; Kratochvílová, I.; Garcia-Belmonte, G., Sol. Energ. Mat. Sol. C., 2015, 134, 73–79.
4 P. Ashcheulov, M. Kusko, F. Fendrych, A. Poruba, A. Taylor , A. Jäger, L. Fekete, I. Kraus, I. Kratochvílová, Phys. Status Solidi A 12 (2014) 1 - 6.
5 Taylor, A.; Fekete, L.; Hubík, P.; Jager, A.; Janíček, P.; Mortet, V.; Mistrík, J.; Vacík, J., Diam. Relat. Mater., 2014, 47, 27–34.
Symposium Organizers
Thomas Fischer, University of Cologne
Fabio Di Fonzo, Istituto Italiano di Tecnologia
Rita Toth, Swiss Federal Laboratories for Materials Science and Technology (EMPA)
Mmantsae Diale, University of Pretoria
Symposium Support
Kenosistec
Nature Catalysis | Springer Nature
Sustainable Energy &
Fuels | The Royal Society of Chemistry
ES02.05: CO2 Reduction I
Session Chairs
Tuesday AM, November 28, 2017
Hynes, Level 3, Room 306
8:30 AM - ES02.05.01
Highly Concentrated CO Evolution for Photocatalytic Conversion of CO2 by H2O as an Electron Donor
Kentaro Teramura 1
1 , Kyoto University, Kyoto Japan
Show AbstractThe reduction in human induced emissions of CO2 from automobiles, factories, power station etc., over the next 15 years is currently one of the most important issues facing the planet. We should therefore attempt to develop industrial processes using CO2 as a feedstock in order to build a sustainable society in the near future. Linear CO2 molecules adsorbed on the surface of the solid bases are converted into unique structures, such as bicarbonate and carbonate species possessing lattice oxygen atoms. We believe that the process involves the capture and distortion of CO2 upon adsorption on a solid base through activation by photoirradiation. Unstable CO2 species adsorbed onto the surface can then be reduced by electrons with protons derived from H2O (CO2 + 2e－ + 2H+ → CO + H2O). These days, we succeeded in designing highly selective photocatalytic conversion of CO2 by H2O as the electron donor, by the simultaneous use of an inhibitor of the production of H2 and a material for CO2 capture and storage, such as ZnGa2O4/Ga2O3, La2Ti2O7, SrO/Ta2O5, ZnGa2O4, ZnTa2O6, and Sr2KTa5O15 with the modification of Ag cocatalyst. An isotope experiment using 13CO2 and mass spectrometry clarified that the carbon source of the evolved CO is not the residual carbon species on the photocatalyst surface, but the CO2 introduced in the gas phase. In addition, stoichiometric amounts of O2 evolved were generated together with CO.
8:45 AM - ES02.05.02
Developing Earth-Abundant Transition Metal Catalysts for Highly Efficient CO2 Conversion to Fuels
Haotian Wang 1
1 , Harvard University, Cambridge, Massachusetts, United States
Show AbstractElectrochemical CO2 reduction to chemcials or fuels is becoming increasingly important but still challenged by low selectivity, low activity, and strong competition with hydrogen evolution reaction in aqueous solutions. Here we present a few examples of earth-abundant transition metal catalyst, with proper electronic structure tuning, for highly selective CO2 conversion to CO, as well as higher value C2+ products such as ethylene, ethanol, and n-proponal. Combined with transition metal oxide oxygen evolution catalyst and a commercialized solar cell, we demonstrate a ~ 10 % artificial photosynthesis efficiency, higher than the photosynthesis in nature.
9:00 AM - *ES02.05.03
Solar Refineries Based on Highly Selective Photo-Driven CO2 Reduction Using Nanosized Earth-Abundant Catalysts
Juan Morante 1 2 , Felix Urbain 1 , Carles Ros 1 , Nina Carretero 1 , Teresa Andreu 1 2 , Maria-Dolores Hernández-Alonso 3 , German Penelas-Perez 3
1 , IREC, Sant Adria del Besos Spain, 2 , University of Barcelona, Barcelona, Barcelona, Spain, 3 , Repsol Technology Center, Madrid, Madrid, Spain
Show AbstractTo develop solar refineries, the conversion of carbon dioxide (CO2) into value-added chemicals and fuels, preferably using renewable energy and earth-abundant materials, becomes a key priority. In this contribution, we report on the development of an integrated upscalable photoelectrochemical device for the solar-driven conversion of CO2 to synthesis gas (syngas) or alternatively to formic acid. The filter-press type cell device consists of a cathode made of a highly conductive scaffold which is coated with an appropriate low-cost nanosized transition metal as catalyst to perform the CO2 reduction (CO2R) to the target product. This electrode is combined with a highly efficient photoanode, such as silicon based structures duly protected and able to exhibit photocurrent density higher than 40mA/cm2, or other photovoltaic semiconductor-based equivalent structures. Likewise, these ones are also decorated with 3D based nanostructures as catalyst to facilitate the oxygen evolution reaction (OER). Catholyte and anolyte compartments are separated by a bipolar membrane (BPM) allowing operating the complete CO2R system at two different pH values if it is required. Under photoelectrolysis conditions, the photovoltage of the photoanode was tuned between 0.6 V and 2.4 V by connecting up to four single junction cells in series and thus, reducing the overall cell voltage by increasing solar energy utilization up to achieving free bias conditions required for pure artificial photosynthesis under 100 mW/cm2 illumination of the complete device. With this concept we demonstrate industrial feasible solar-to-fuel (STF) conversion efficiency values. Influence of the catalyst nanoparticle size on the performance, faradaic efficiency and final productivity will also be discussed considering overpotencials, Taffel plots and the overall characteristics of the selected catalyst according to the target CO2R product.
9:30 AM - ES02.05.04
Pulse Plating of Copper Nanostructures onto Gas Diffusion Layers for the Electroreduction of Carbon Dioxide to Hydrocarbons
Sujat Sen 1 , Brian Skinn 2 , Tim Hall 2 , E.J. Taylor 2 , Fikile Brushett 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , Faraday Technology, Englewood , Ohio, United States
Show AbstractUtilizing carbon dioxide (CO2) as a chemical feedstock has been identified as a means of reducing greenhouse gas emissions and moving towards a carbon neutral energy cycle [1]. A promising approach for CO2 conversion is electrocatalytic reduction to selectively generate hydrocarbons such as ethylene and propylene that are the primary building blocks of the petrochemical industry [1]. The production of these precursors by conventional means from petroleum feedstocks is energy intensive, requiring high temperatures and pressures. Hence, electrochemical methods, which can operate at conditions much closer to ambient, represent a potentially less costly and more sustainable alternative.
Prior reports have demonstrated electroreduction of CO2 to hydrocarbons on copper (Cu)-loaded gas diffusion layers (GDLs) to obtain ethylene, with the best performance to date at a potential of -0.8 V RHE, yielding a current density of 200 mA/cm2, and a current efficiency of 46% [2]. These studies have typically used Cu nanoparticles, mixed with an ionomer and spray coated onto a microporous carbon layer (MPL) supported by a carbon fiber substrate (CFS), to form a gas diffusion electrode (GDE). This approach limits the electroreduction process due to 1) low catalyst specific surface area due to the relatively large Cu particle size (40-100 nm), and 2) poor utilization of a fraction of the Cu catalyst particles, which are surrounded by ionomer and thus not in electrical contact with the MPL. Previous work directed towards platinum (Pt) catalyst utilization in polymer electrolyte fuel cell GDEs demonstrated a novel “electrocatalyzation” approach to obtain highly dispersed ~5 nm Pt catalyst particles using pulse-reverse electrodeposition [3]. Additionally, since the Pt was electrodeposited through an ionomer pre-coated on the MPL surface, the catalyst was inherently in electronic and ionic contact within the GDE and catalyst utilization was enhanced. Such an electrocatalyzation approach has also been successfully demonstrated for tin-based GDEs for the conversion of CO2 to formate [4].
Herein we investigate the electrocatalytic performance of novel Cu-coated GDEs prepared by direct electrodeposition onto GDL substrates using pulse-reverse waveforms. We demonstrate the potential for significant enhancements in catalytic activity due to 1) increased control of particle size, nucleation site density, and surface texture, as well as 2) improved Cu catalyst utilization via enhanced electronic and ionic contact with the MPL. Electrolysis experiments were conducted in a lab-scale reactor with gas-phase CO2 delivered across a GDE to determine the catalyst activity and stability as a function of deposition parameters and cell operating conditions.
References
[1] G. Centi et al., ChemSusChem 4(9), 1265 (2011)
[2] S. Ma et al., J. Power. Sources, 301, 219-228 (2016)
[3] E. J. Taylor et al., J. Electrochem. Soc., 139(5), L45 (1992)
[4] S. Sen et al., MRS Adv., 2(8), 451-458 (2017)
9:45 AM - ES02.05.05
Pd@HyWO3-x Nanowires Efficiently Catalyze the CO2 Heterogeneous Reduction Reaction with a Pronounced Light Effect
Young Feng Li 1 , Navid Soheilnia 1 , Eva Pütz 1 , Mark Greiner 2 , Ulrich Ulmer 1 , Thomas Wood 1 , Geoffrey Ozin 1
1 , University of Toronto, Toronto, Ontario, Canada, 2 , Max Planck Institute for Chemical Energy Conversion, Mülheim Germany
Show AbstractThe design of photo-catalysts to reduce CO2 is a necessary step for the effective utilization of CO2. WO3 is commonly regarded as a promising photoanode material for catalyzing oxidation reactions due to its relatively low valence and conduction band positions.1 It is less studied as a catalyst for photoreduction reactions but more recently, defected WO3 has gained interest for driving reduction reactions due to the reducing ability of its oxygen vacancies2 and high capacity for incorporating H in its lattice in the form of a hydrogen tungsten bronze with W(V) centers and OH groups.
Herein, we report the use of Pd and Ni decorated WO3 nanowires as photo-catalysts to reduce CO2 with H2. TGA, UV-Vis, and XPS were used to investigate the formation of HyWO3-x via the Pd promoted H2 spillover effect. The high population of OH groups is proposed to facilitate the capture of CO2 whereas the oxygen vacancies along with illumination are proposed to facilitate the activation of CO2. CO production was observed to significantly increase upon illumination, reaching 3.0 mmol gcat-1 hr-1 at 250°C. The light-assistance was also manifested as a drastic change in the reaction orders of H2 and CO2, shifting from a more CO2 dependent process in the dark to a more H2 dependent process under illumination, interpreted as the light-enhancement of CO2 dependent steps. The results of this study provide valuable insight into the design of photocatalysts for CO2 reduction, highlighting the importance of the surface chemistry occurring on metal decorated redox active metal oxides.
1. T. Zhu et al., ChemSusChem, vol. 7, pp. 2974-2997, 2014
2. G. Xi et al., Angew. Chem., Int. Ed. vol. 51, pp. 2395-2399, 2012
10:30 AM - ES02.05.06
Photocatalytic CO2 Reduction in Nanoporous Gold Photoelectrodes
Alex Welch 1 , Joseph Duchene 1 , Giulia Tagliabue 1 , Artur Davoyan 1 , Wen-Hui Cheng 1 , Harry Atwater 1
1 , California Institute of Technology, Pasadena, California, United States
Show AbstractPhotocatalysts present a route to generate clean and renewable energy by converting sunlight into chemical energy. Particularly, plasmonic photocatalysts show promise for a variety of chemical reactions, but the fundamental processes are not yet fully understood. It remains unclear as to whether it is a plasmon resonant energy transfer or hot carrier injection that drives the chemical reaction. Here we report a nanoporous gold (np-Au) photoelectrode that exhibits light-dependent selectivity for CO2 reduction to CO in aqueous electrolytes. The np-Au structure is fabricated via electron beam co-deposition of a gold/silver (Au/Ag) alloy of tunable elemental composition (10/90 to 30/70) and variable thickness (0.1 μm to 1 μm), followed by a chemical etch of silver with nitric acid to yield a monolithic, np-Au structure. Depending on the etching temperature,we can alter the feature size from ca. 10nm to 25nm. The np-Au films exhibit 5-15x larger electrochemical surface area than a comparable planar Au film while also possessing near-unity absorption throughout a broad portion of the visible spectrum from 400nm to 600 nm). Our photoelectrochemical analysis indicates that the np-Au electrode exhibits a significant change in selectivity for the production of CO vs. H2 when operating under illumination as compared to dark conditions. We have experiments underway to understand the selectivity of the process as a function of illumination wavelength and incident light power. This np-Au scaffold can also be used as a platform to explore other metal electrocatalysts, where we hope to tune selectivity for desired products through careful alteration of the alloy composition.
10:45 AM - ES02.05.07
Reversible Surface Reconstruction of Copper Electrodes Spectroscopically Observed During Carbon Dioxide Reduction
Matthias Waegele 1
1 , Boston College, Chestnut Hill, Massachusetts, United States
Show AbstractAs copper is the only metal capable of electrochemically reducing carbon dioxide to hydrocarbons at significant reaction rates, a better understanding of the key factors that determine the reactivity and selectivity of this prototype catalyst is essential for the development of industrially viable catalysts for carbon dioxide reduction. Recent studies have highlighted that the nanoscale surface structure plays a critical role in determining the catalytic properties of this electrocatalyst. Although previous work has explored the morphology of copper electrodes before and after electrolysis, the surface structure of the electrode under carbon dioxide reduction has not been probed to date. Here we present the first observation of reversible reconstruction of copper electrodes induced by surface-adsorbed carbon monoxide, which is intermediately formed during carbon dioxide reduction. Employing surface-sensitive infrared and Raman spectroscopies, the formation of nanoscale features on the surface is demonstrated by the appearance of a new CO stretch band in infrared spectra and a marked enhancement of the surface-enhanced Raman effect. The newly formed features are composed of under-coordinated copper atoms with a higher binding affinity to the CO intermediate compared to sites available before reconstruction. Due to their reversible nature, these structural changes have escaped previous ex situ studies. The significant difference in CO binding energies implied by our data is expected to profoundly affect the catalytic properties of the electrode and will need to be taken into account in future investigations of electrocatalytic copper.
11:00 AM - ES02.05.08
Tunable and Bifunctional Micropatterned Catalysts for Control of Oxygenate Selectivity In Electrochemical CO2 Reduction
Yanwei Lum 1 2 3 , Joel Ager 1 2 3
1 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 2 , Joint Center for Artificial Photosynthesis, Berkeley, California, United States, 3 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractElectrochemical reduction of CO2 in aqueous media is a strategy for sustainable production of fuels and commodity chemicals. Cu is the only catalyst which converts CO2 to significant quantities of hydrocarbons and oxygenates. Here we demonstrate that oxygenate products can be favored over hydrocarbons by positioning a local source of CO generated by a CO producing catalyst in close proximity to a Cu catalyst. This is demonstrated in a bimetallic system comprising of Cu dots/lines patterned directly onto a Ag substrate, allowing for the ratio of Cu to the CO generating metal to be precisely controlled. Controlling the relative areas of Ag and Cu allows for tuning of the oxygenate to ethylene ratio from 0.59 to 2.39 and an increase in oxygenate faradaic efficiency from 21.4% to 41.4%, while maintaining the selectivity to C2/C3 products in the 50-65% range. By utilizing crossover of CO to Cu, a sequential catalysis pathway is created which yields insights into the control of oxygenate selectivity in aqueous CO2 reduction.
11:15 AM - *ES02.05.09
Current Status and Future Implementation of Syngas Production from Electrochemical Reduction of CO2
Simelys Hernandez 1 2 , M. Amin Farkhondehfal 1 , Guido Saracco 2 1 , Nunzio Russo 1
1 , Politecnico di Torino, DISAT, Torino Italy, 2 CSFT, Istituto Italiano di Tecnologia, Turin Italy
Show AbstractThe CO2 that comes from the use of fossil fuels accounts for about 65 % of the global greenhouse gas emission, and it plays a critical role in global climate changes. Among the different strategies that have been considered to address the storage and reutilization of CO2, the transformation of CO2 into chemicals or fuels with a high added-value is considered a winning approach. This transformation is able to reduce the carbon emission and induce a “fuel switching” that exploits renewable energy sources (e.g. sunlight). Among all the proposed methods, the electrocatalytic reduction of CO2 is considered an interesting technology for the storage and reutilization of CO2 from both economic and environmental points of view. It can be used to transform CO2 into CO, formic acid, alcohols or higher molecular weight hydrocarbons, such as oxalic acid. However, the main challenge for the establishment of this technology, at an industrial level, is to find suitable electro-catalysts as well as optimized process conditions for the selective production of a single compound with a high conversion efficiency. Since the electrochemical reduction of CO2 is generally performed in aqueous media, the hydrogen evolution reaction (HER) from the reduction of water or protons (H+) is in inevitable rivalry with the CO2 conversion. Hence, the intrinsic nature of the electrolysis process could be exploited, in a competitive approach, by combining CO2 reduction and HER for the production of syngas. The great advantage of producing syngas, instead of another direct CO2 reduction product, is the fact that there are several possible options for further development of engineered products in relation to the H2/CO ratio of the mixture, to generate ammonia or more reduced products, like alcohols and hydrocarbons (via Fischer-Tropsch catalysis). In such context, the aim of the here presented review1 has been to gather and critically analyze the main efforts that have been made and results that have been achieved concerning the electrochemical reduction of CO2 for the production of CO. The different methods, catalysts and reactor systems that have been used for this purpose have been outlined. It is worth noticing that, although remarkable activities have been undertaken and scientists have achieved high efficiency and selectivity with acceptable kinetics, there are still some serious obstacles to overcome before this process can become viable. In fact, the most efficient catalysts for the reduction of CO2 to CO are based on noble metals; long-term tests and a complete understanding of the deactivation mechanisms have still not been investigated for the most promising catalysts and proposed systems so far only produce CO in μmoles per minutes that is quite far from an industrial productivity levels. Thus, challenges and prospective trends towards a practical application of this technology will be emphasized.
1 Green Chemistry, 2017, 19, 2326 - 2346
11:45 AM - ES02.05.10
Copper-Based P-Type Ternary Metal Oxides as Photocathodes for Solar Fuel Generation
Jiangtian Li 1 , Deryn Chu 1
1 , U.S. Army Research Laboratory, Adelphi, Maryland, United States
Show AbstractSolar driven fuel generation by water splitting and CO2 reduction is an attractive strategy to supply the sustainable energy. The nature of p-type semiconductors facilitate the photocathodic reduction chemistry. Copper based ternary metal oxides represent a promising class of photocathode candidates for solar energy applications. The barriers facing the copper ternary oxides however are the stability and efficiency in aqueous solution. In this presentation, we will talk about our recent experimental and theoretical efforts to engineer this kind of photocathode materials in view of energy band structures by controlling the surface states, energetic level alignment as well as surface catalysts with the aim to improve their capability for anti-photocorrosion and solar conversion efficiency. The resultant nanostructures could be employed as photocathode in both half and whole photoelectrochemical cell for solar fuel generation.
ES02.06: CO2 Reduction II
Session Chairs
Tuesday PM, November 28, 2017
Hynes, Level 3, Room 306
1:30 PM - *ES02.06.01
Integrated Photoelectrodes for CO2 Reduction
Francesca Maria Toma 1 , Guiji Liu 1 , Ashley Gaulding 1
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractCO2 reduction using solar energy could not only help reduce CO2 emission, but also holds promise to meet the increasing demand for global energy. Over the past three decades, researchers have evaluated lots of materials for CO2 reduction in aqueous solutions. Among those, Cu based materials have been the focus of most CO2 reduction studies due to its capability of producing hydrocarbon products. Herein, we aim to construct a copper oxide based photoelectrodes, as they possess abundant active sites to substantially promoting CO2 reduction. Approaches to stabilization of Cu2O through fabrication of p-n junctions will be presented.
2:00 PM - ES02.06.02
Photothermal Catalyst Engineering—Hydrogenation of Gaseous CO2 with High Activity and Tailored Selectivity
Jia Jia 1 , Nazir Kherani 1 , Doug Perovic 1 , Geoffrey Ozin 1
1 , University of Toronto, Toronto, Ontario, Canada
Show AbstractWe have designed and implemented a library of heteronanostructured catalysts, denoted Pd@Nb2O5, comprised of size-controlled Pd nanocrystals interfaced with Nb2O5 nanorods. We demonstrate that the catalytic activity and selectivity of CO2 reduction to CO and CH4 products can be systematically tailored by varying the size of the Pd nanocrystals supported on the Nb2O5 nanorods. Using large Pd nanocrystals, we achieve CO and CH4 production rates as high as 0.75 mol h-1 gPd-1 and 0.11 mol h-1 gPd-1, respectively. By contrast, using small Pd nanocrystals, a CO production rate surpassing 18.8 mol h-1 gPd-1 is observed with 99.5% CO selectivity. These performance metrics establish a new milestone in the champion league of catalytic nanomaterials that can enable solar-powered gas-phase heterogeneous CO2 reduction. The remarkable control over the catalytic performance of Pd@Nb2O5 is demonstrated to stem from a combination of photothermal, electronic and size effects, which is rationally tunable through nanochemistry.
References:
(a) J. Jia, C. Qian, Y. Dong, Y. F` Li, H. Wang, M. Ghoussoub,K. T. Butlerb, A. Walsh, G. A. Ozin. “Heterogeneous Catalytic Hydrogenation of CO2 by Metal Oxides: Defect Engineering - Perfecting Imperfection” Chem. Soc. Rev. 2017.
(b) J. Jia, P. G. O'Brien, L. He, Q. Qiao, T. Fei, L. M. Reyes, T. E. Burrow, Y. Dong, K. Liao, M. Varela, S. J. Pennycook, M. Hmadeh, A. S. Helmy, N. P. Kherani, D. D. Perovic, G. A. Ozin, “Ambient Temperature Visible and Near Infrared PhotothermalCatalysed Hydrogenation of Gaseous Carbon Dioxide over Nanostructured Pd@Nb2O5” Adv. Sci. 2016
(c) J. Jia, H. Wang, Z. Lu, P. G. O'Brien, M. Ghoussoub, P. Duchesne, Z. Zheng, P. Li, Q. Qiao, L. Wang, A. Gu, A. A. Jelle, Y. Dong, Q. Wang, K. K. Ghuman, T. Wood, C. Qian, Y. Shao, M. Ye, Y. Zhu, Z. H. Lu, P. Zhang, A. S. Helmy, C. V. Singh, N. P. Kherani, D. D. Perovic, and G. A. Ozin. “Photothermal Catalyst Engineering: Hydrogenation of Gaseous CO2 with High Activity and Tailored Selectivity” Adv. Sci. Under review.
2:15 PM - ES02.06.03
High Pressure Thermal Evaporation-Based Nanoporous Metal Catalysts for CO2 Reduction
Sangwoo Ryu 1 , DongYeon Kim 2 , Youngkook Kwon 2 , Nam Woon Kim 1 , Jihun Oh 1
1 Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon Korea (the Republic of), 2 Carbon Resources Institute, Korea Research Institute of Chemical Technology (KRICT), Daejeon Korea (the Republic of)
Show AbstractFormation of highly porous structure of metal catalysts for the electrochemical conversion of CO2 has been investigated extensively to raise the selectivity of the products and to lower electrochemical overpotential. Various methods to form nanoporous structures such as controlling electrodeposition parameters, dealloying, and applying metal-organic frameworks, etc., have been explored to enhance the performance and to elucidate the reaction mechanism of CO2 reduction reaction (CO2RR). Here we introduce a new route to form high purity nanoporous metal catalysts based on high pressure thermal evaporation, also known as black metal, and its electrochemical application to CO2RR. Regardless of metal catalysts, fractal-like highly porous structures with the porosity over 99% formed at a few Torr of Ar lower the overpotential of CO2RR compared to dense thin films. Transition to porous columnar structures caused by decreasing working pressure results in significantly different characteristics of CO2RR. In particular, for Cu, the only metal catalyst that can convert CO2 to hydrocarbons, high selectivity of C2H4 is obtained with no detectable CH4 and quite limited CO when the nanoporous structure formed at high pressure is used. We discuss the origin of this tunable selectivity, and moreover, possible engineering through surface oxidation for more enhanced activity and selectivity of CO2RR.
2:30 PM - ES02.06.04
Engineering Electronic Structure of Metal Nanocrystals for CO2 Reduction
Min Liu 1 2 , Edward (Ted) Sargent 2
1 , Central South University, Changsha, ON, China, 2 , University of Toronto, Toronto, Ontario, Canada
Show AbstractElectrochemical reduction of carbon dioxide (CO2) to hydrocarbons is an ideal strategy to store off-grid renewable-electricity and to alleviate atmospheric greenhouse effects. A major impediment to improving CO2 reduction reaction (CO2RR) technologies are the limitations on catalyst performance, where most traditional electrode materials have low activity and low selectivity. We took the view that modification of catalyst electronic structure should in principle improve the electrochemical reactions. Here we report a new approach, one that seeks to produce vacancy-rich metallic nanocrystals, and find that the new design strategy leads to dramatic enhancement for CO2RR. We synthesized vacancy-rich metallic nanocrystals for CO2 reduction reaction (CO2RR) by using colloidal quantum dots (QDs) through a facile cathodic reduction process. Through tuning the amount of vacancies, we implemented the desired electronic structure that lead to a record-efficient CO2RR catalyst. The resultant vacancy-rich metallic nanocrystals exhibits the highest current density for formate of ~32 mA/cm2 at the low potential of −0.2 V, with no signs of degradation after ~80 hours of operation. This performance surpasses previously reported metal or metal oxide electrodes evaluated under comparable conditions. X-ray absorption spectroscopy (XAS) and computational studies, taken together, reveal a synergistic interplay of vacancy in producing a favorable local coordination environment and electronic structure that enhance the CO2 energetics favorable for CO2RR.
2:45 PM - ES02.06.05
Kinetics and Material Stabilities for Membrane-Supported H2O/CO2 Splitting
Xiao-Yu Wu 1 , Ahmed Ghoniem 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractIn this presentation, we compare the impacts of different oxygen sources (i.e., water and carbon dioxide) in membrane-supported H2O/CO2 splitting. Mixed ionic-electronic conductive perovskite La0.9Ca0.1FeO3-δ (LCF-91) is examined in our laboratory [1] [2] [3]. At intermediate temperatures (850 – 990oC), hydrogen or syngas are produced from H2O/CO2 splitting on the feed side of the oxygen permeable membrane. Oxygen diffuses through the membrane driven by the chemical potential across, which is generated by fuel oxidation on the sweep side. In this process, heat from solar or other renewable sources is stored in the form of chemical energy, and the abundant and low-cost H2O/CO2 sources are upgraded into value-added chemicals.
Chemical kinetics and material stabilities are compared between H2O and CO2 splitting on the LCF-91 membrane. First, kinetics modeling shows that the activation energy for the H2O splitting reaction is 7.88 kJ/mol, which is two orders of magnitude lower than that of CO2 splitting (i.e., 364 kJ/mol). The splitting rate is proportional to the oxygen flux across, and membrane-supported H2O splitting leads to oxygen fluxes about 0.5 μmol/cm2-s higher than CO2 splitting. Besides, kinetics modeling also reveals that the limiting step is always the fuel oxidation in water splitting, while it changes from CO formation reactions on the feed-side to fuel oxidation reactions on the sweep-side as the temperature is raised in CO2 splitting.
Second, both H2O and CO2 splitting modify the surface morphology and lead to impurities on the feed side of the LCF-91 membrane. SEM and EDS analysis show that CO2 splitting creates Ca- and Fe-enriched clusters on the surface, while H2O splitting corrodes the surface such that hardly any original grains can be identified. Additionally, XRD shows that impurities such as carbonates and brownmillerite (Ca2Fe2O5) exist on the CO2 splitting surface, while the only observable impurity on the H2O splitting surface by XRD is the silicate species from the reactor or materials. We also examine the H2O splitting surface with Auger electron spectroscopy, and find that the LCF-91 perovskite decomposes into Fe-enriched and La-enriched particles on the surface.
Reference:
[1] Dimitrakopoulos, G., and Ghoniem, A. F., 2017, "Developing a multistep surface reaction mechanism to model the impact of H2 and CO on the performance and defect chemistry of mixed-conductors," J. Membr. Sci., 529, pp. 114-132.
[2] Wu, X. Y., Ghoniem, A. F., and Uddi, M., 2016, "Enhancing co-production of H2 and syngas via water splitting and POM on surface-modified oxygen permeable membranes," AlChE J., 62(12), pp. 4427-4435.
[3] Wu, X. Y., and Ghoniem, A. F., 2017, "Reaction kinetics of CO2 thermochemical reduction on perovskite LCF membranes " in preparation.
3:30 PM - *ES02.06.06
Renewable Fuels and Feedstocks via CO2 Electroreduction to C2 and Above
Edward (Ted) Sargent 1
1 Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario, Canada
Show AbstractThe increased availability of cost-effective renewable electricity from solar and wind sources demands cost-effective, long-term (e.g. seasonal), energy storage. Chemical routes based on CO2 reduction to carbon-based fuels and feedstocks hold the advantages of high energy density and availability of an existing carbon-based distribution and utilization infrastructure. We summarize recent advances in CO2RR, including both at the level of catalyst and systems. In catalysts, we highlight increased understanding of guidelines to achieve products with high specificity. In systems, we present advances in utilizing flow cells that implement high partial current densities for CO2RR and also aid in selectivity.
4:00 PM - ES02.06.07
Perovskite La0.6(Sr,Ca)0.4CrxMn1-xO3 Solid Solutions for Solar-Driven Thermochemical CO2 Splitting
Alfonso Carrillo 1 2 , Thierry Moser 2 , Alexander H. Bork 1 2 , Jennifer Rupp 1 2
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , ETH Zürich, Zurich Switzerland
Show AbstractSolar-to-fuel technology is a promising approach to transform an abundant renewable source into fuels, while reducing CO2 emissions.1 In particular, 2-step solar-thermochemical cycles operate on redox processes of oxides whereby H2O and CO2 are transformed into H2 and CO. A promising material used in this process is CeO2, however, it exhibits a high reduction temperature, 1500 °C, which implies technical challenges for reactor design and heat re-radiation losses. Traditionally redox schemes operate in temperature-swing regimes. Namely, oxidation is performed at 500 °C lower than reduction, implying additional energy penalties for the process. Alternatively, thermochemical cycles could be operated under isothermal conditions benefiting from faster kinetics and long-term materials stability.1 Thus, isothermal conditions could be especially beneficial for promising perovskite compositions which can operate at significantly decreased temperature, but which exhibited slower kinetics than CeO2.2,3,4,5
Thermodynamic calculations performed to La0.6Sr0.4CrxMn1-xO3 solid solutions showed that these perovskites would be able to split CO2 in both non-isothermal and isothermal operation. Based on that, La0.6Sr0.4CrxMn1-xO3 perovskite powders were evaluated in that context, covering a full compositional range of chromium to manganese doping for the transition metal ion in the perovskite. Although O2 release decreased with increasing addition of Cr, La0.6Sr0.4Cr0.85Mn0.15O3 exhibited better performance than La0.6Sr0.4MnO3 under isothermal redox cycling operation, namely 350 µmol/g and 250 µmol/g respectively, when the reaction was carry out at 1400 °C and pCO2=0.5 atm.
Furthermore, Ca-doping on the A-site was study as an efficient alternative to manipulate structurally the lattice and ultimately increase CO yields. It was found that structural modifications triggered by Ca doping enhanced for La0.6Ca0.4CrxMn1-xO3 the O2 release during the thermal reduction process, which has direct implications on the amount of fuel produced during the CO2 splitting step. For all the solid solutions compositions, Ca-doping enhanced the amount of CO produced regardless the reaction configuration evaluated when compared with La0.6Sr0.4CrxMn1-xO3, e.g., La0.6Ca0.4Cr0.5Mn0.5O3 produced 49% higher CO yield than La0.6Sr0.4Cr0.5Mn0.5O3 under isothermal operation.
In conclusion, La0.6Sr0.4Cr0.85Mn0.15O3 and especially La0.6Ca0.4CrxMn1-xO3 series show enhanced fuel production and may be interesting alternatives when operated isothermally for solar-thermochemical fuel reactors.
1. Muhich, C. et al., A. W. Wiley Int. Reviews: En. and Environ., 5(3), 261–287. (2016).
2. Kubicek, M., Bork, A. H.& Rupp, J. L. M. J. Mater. Chem. A, (2017).
3. McDaniel, et al. Energy & Environmental Science, 6(8), 2424. (2013).
4. Bork, A. H., Kubicek, M., Struzik, M. & Rupp, J. L. M. J. Mater. Chem. A3, 15546–15557 (2015).
5. Bork, A. H., Povoden-Karadeniz, E. & Rupp, J. L. M. Adv. Energy Mater.7, 1601086 (2017).
4:15 PM - ES02.06.08
Enhancement of Water Splitting Performance in Pulsed Laser Deposited BiVO4-WO3 Thin-Film Photoanode in Cooperation with Cobalt Phosphate
Sang Yun Jeong 1 , Taemin Kim 2 , Do Hyun Kim 3 , Hye-Min Shin 1 , Jaesun Song 1 , Myung-Han Yoon 1 , Chung Wung Bark 3 , Ho Won Jang 2 , Sanghan Lee 1
1 , Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of), 2 , Seoul National University, Seoul Korea (the Republic of), 3 , Gachon University, Seongnam Korea (the Republic of)
Show AbstractEnergy harvest from solar light has been attracted attention since the Sun gives unlimited energy sources. Photoelectrochemical (PEC) cell is emerging techniques for the utilization of the solar radiation and has unique features that the light energy can be converted to chemical fuels. Among various photoanode materials in PEC cell, bismuth vanadate (BiVO4) is intensively investigated because of its low bandgap energy of 2.4 eV for visible light hydrogen gas generation. Recently, we reported on excellent water splitting performance of high quality BiVO4 thin film grown by pulsed laser deposition (PLD)[1]. The improved performance was attributed to the increased surface area of the film as a result of the Volmer-Weber grain growth during the PLD process. Further enhancement of device performance can be obtained by formation of heterojunction with other materials such as tungsten oxide (WO3) which has ability to suppress the carrier recombination in BiVO4 layer during operation. In this research, exploiting the capability of in-situ multilayer heterostructure growth using PLD, we have fabricated high quality BiVO4-WO3 heterostructure polycrystalline thin films. The grown BiVO4 have photocatalytically active phase of scheelite-monoclinic structure and show visible light absorption which were confirmed by X-ray diffraction (XRD) and UV-vis spectroscopy, respectively. The performance of the BiVO4–WO3 heterostructure grown on glass substrate covered with F-doped tin oxide (FTO) exceeds 3.0 mA/cm2 at 1.23 V versus the potential of the reversible hydrogen electrode (VRHE) under AM 1.5G illumination. Furthermore, based on the excellent performance of the BiVO4-WO3 heterostructure, a cobalt phosphate as oxygen evolution co-catalyst was deposited on the surface of the film so that the water splitting was realized using neutral water.
References
[1] Jeong, S. Y., Choi, K. S., Shin, H.-M., Kim, T. L., Song, J., Yoon, S., Jang, H. W., Yoon, M.-H., Jeon, C., Lee, J., Lee, S. Enhanced Photocatalytic Performance Depending on Morphology of Bismuth Vanadate Thin Films Synthesized by Pulsed Laser Deposition. ACS Appl. Mater. Interfaces 2017, 9, 505-512.
4:30 PM - ES02.06.09
Manganese-Cobalt Oxide Nanoparticle Growth and Electrocatalytic Performance for Oxygen-Evolution-Reaction
Manuel Gliech 1 , Malte Klingenhof 1 , Arno Bergmann 1 , Martin Gocyla 2 , Marc Heggen 2 , Rafal Dunin-Borkowski 2 , Peter Strasser 1
1 , TU Berlin, Berlin Germany, 2 Ernst-Ruska Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, Jülich Germany
Show AbstractThe storage of electric energy has become one of the main challenges in infrastructure development. Electrocatalytic water splitting is a promising of chemical energy transformation wherein the anodic oxygen evolution reaction (OER) is the efficiency-limiting factor. Scalable, cost-efficient and robust synthetic routes toward well-defined solid state structures are major objectives in this field. Manganese based oxides have attracted particular attention as non-noble metal catalysts for the OER. Due to their high abundance, low price and ecological harmlessness, those materials are promising candidates for a commercial application. Therefore insights into the relations between synthesis conditions, composition, structure and activity as well as stability are of fundamental interest.
Here we present a study on the particle growth of anisotropically shaped manganese-cobalt oxide nanoparticles. These mixed oxides with elemental ratios ranging from pure manganese oxide to pure cobalt oxide were characterized with respect to their crystal structure, morphology and their elemental distribution throughout the particles using X-ray diffraction, transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy techniques.
Furthermore we tested the electrocatalytic performance towards OER in alkaline conditions of the mixed oxides and compare it to the performance of physical mixtures of the single metal oxides with equivalent Mn/Co ratios. We observed a significant impact of the cobalt fraction on the morphology, while the general crystal structure and phase remain unchanged. We also found a non-linear relation between composition and activity for the mixed oxides. Our results will help to gain the fundamental understanding of the formation of mixed metal oxide nanostructures and their electrocatalytic activity.
4:45 PM - ES02.06.10
Fluorinated BiVO4—Enhancement of Photoelectrochemical Performance for Water Oxidation by Fluorine Incorporation
Martin Rohloff 1 2 , Björn Anke 1 , Martin Lerch 1 , Anna Fischer 2
1 Institut für Chemie, Technische Universität Berlin, Berlin, Berlin, Germany, 2 Institut für anorganische und analytische Chemie, Albert-Ludwigs-Universität Freiburg, Freiburg, Baden-Würtemberg, Germany
Show AbstractThe n-type semiconductor bismuth vanadate (BiVO4) has recently gained a lot of attention as photoanode material for visible-light induced water oxidation. Its absorption in the visible domain (band gap energy of 2.4 eV), its suitable band edge positions compared to the OER half reaction, its stability against photo-corrosion as well as its low cost, make BiVO4 one of the most interesting ternary oxide materials for light-induced oxygen evolution from water. One major drawback for BiVO4 is however its poor bulk electronic conductivity; problem, which can be overcome by doping as well as by improved structural design.
In contrast to well-known cation doping, we present a new approach of anion substitution in BiVO4 photoanodes. A new solid-vapor reaction method for the fluorination of BiVO4 powder at ambient pressure and inert gas atmosphere was developed to reproducibly substitute oxygen by fluorine in the monoclinic scheelite lattice. Obtained powder samples were characterized extensively by means of chemical and structural analysis as well as by electron microscopy. Using electrophoretic deposition and taking advantage of the low-temperature sintering of BiVO4, the as-synthesized F-modified powders were processed to photoanodes, allowing for their (photo)electrochemical characterization in terms of visible-light induced water oxidation. Photocurrent transient analysis revealed that surface hole recombination is drastically reduced by F-incorporation in the lattice. Additionally, Mott-Schottky-type electrochemical impedance spectroscopy revealed the F:BiVO4 material to profit from a higher amount of free charge carriers as well as from a cathodically shifted flat band potential (when compared to unmodified BiVO4). As a result, largely increased photocurrents were obtained for the fluorine-doped BiVO4 anode.
Our results demonstrate anion doping to be a viable tool to optimize photoanode properties with respect to photoelectrochemical water oxidation.
This work was funded by the DFG SPP1613 program.
Symposium Organizers
Thomas Fischer, University of Cologne
Fabio Di Fonzo, Istituto Italiano di Tecnologia
Rita Toth, Swiss Federal Laboratories for Materials Science and Technology (EMPA)
Mmantsae Diale, University of Pretoria
Symposium Support
Kenosistec
Nature Catalysis | Springer Nature
Sustainable Energy &
Fuels | The Royal Society of Chemistry
ES02.07: Long-Term Stabilization of Solar Fuel Systems
Session Chairs
Wednesday AM, November 29, 2017
Hynes, Level 3, Room 306
8:30 AM - ES02.07.01
Design Rules of an Amorphous Silicon Carbide (a-SiC:H) Thin-Film Photocathode for Solar Water Splitting
Paula Perez Rodriguez 1 , Ibadillah Digdaya 1 , Andrea Mangel Raventos 1 , Miroslav Zeman 1 , Wilson Smith 1 , Arno Smets 1
1 , Delft University of Technology, Delft Netherlands
Show AbstractEven though solar energy is assumed to play a crucial role in the future energy scenario, energy storage is expected to be one of the main bottlenecks in the implementation of solar energy by tackling the daily and seasonal fluctuations. An effective way to tackle the seasonal fluctuations in particular is the production of solar fuels by water splitting. Direct hydrogen production using solar water splitting has attracted considerable attention in the last years. In particular, photoelectrochemical (PEC) devices, are a form of solar water splitting that provides a simple and elegant solution to hydrogen production with solar energy.
In this study, the design rules of an amorphous silicon carbide (a-SiC:H) are determined. a-SiC:H is selected as a suitable material for photoelectrochemical (PEC) water splitting due to its relatively low bandgap and relatively high stability. However, the traditional device structures used in the field are insufficient to ensure maximum charge carrier collection. Thus, a back surface field has been introduced on a p/i structured photocathode, such as an n-doped nanocrystaline silicon oxide (nc-SiOx:H) layer. When this layer is implemented, the photovoltage and photocurrent of the device can be improved, reaching values higher than 0.8 V and 10 mA/cm2, respectively. This results from enhancing the internal electric field of the photocathode and increasing the charge carrier selectivity at the interfaces. Moreover, it can also be beneficial if a protective layer of n- TiO2 is used. The (n)nc-SiOx:H can then act as an intermediate layer between the intrinsic a-SiC:H and the protective TiO2 layer, reducing the recombination at this surface, and thus further improving the overall voltage and current performance. Regarding the thickness optimization of these layers, it was found that a minimum thickness of 20 nm for the (n)nc-SiOx:H deposited by PECVD layer was necessary to avoid pinholes and tunnelling, and to create the necessary electric field for charge separation. Finally, the optical parameters should also be optimized to increase light absorption. The photocurrent was further increased by tuning the absorber layer, arriving at a thickness of 150 nm, after which the current saturates to 10 mA cm-1 at 0 V vs. RHE.
In summary, this work determines the main design rules of an a-SiC:H photocathode. The main parameters to consider are the creation of a back surface field that can effectively separate and collect the maximum amount of charge carriers, and the light absorption in the intrinsic layer of the device. It is suggested that the improvement of the back surface field of the photocathode is able to increase the photocurrent by improving the electric field inside the material and the selectivity of charge carriers at the interfaces. This can be directly translated to a higher hydrogen production. In addition, choosing the correct layer combination can reduce the photocathode corrosion and increase its lifetime.
8:45 AM - ES02.07.02
Photo-Induced Interface to Enhance the Performance of Tantalum Nitride for Solar Water Oxidation
Yumin He 1 , Peiyan Ma 2 , Shasha Zhu 1 , Mengdi Liu 1 , Qi Dong 1 , Jeremy Espano 1 , Xiahui Yao 1 , Dunwei Wang 1
1 , Boston College, Chestnut Hill, Massachusetts, United States, 2 , Wuhan University of Technology, Wuhan China
Show AbstractSemiconductor/electrolyte interface plays important roles in defining the performance of photocatalysts but is poorly studied. Here we report a surprising observation that unique reactions at this interface under photoelectrochemical (PEC) conditions lead to unusual performance enhancement of Ta3N5 for solar water oxidation reactions. We chose Ta3N5 as the prototypical material platform for the present study for two important reasons. First, the physical properties of Ta3N5 render it an appealing material choice for solar water splitting applications, including a 2.1 eV direct bandgap and suitably positioned band edge positions that straddle water reduction/oxidation potentials. Indeed, near theoretical limit photocurrents have been measured on Ta3N5 under photoelectrochemical (PEC) conditions. Second, poor stability is a critical issue that limits the prospect of Ta3N5 as a practical photoelectrode material for solar water splitting. The large gap between the promises and the measured performance makes it significant to stabilize Ta3N5 under PEC water oxidation conditions. In our study, when the Ta3N5 was decorated with Co(OH)2 nanosheet, it is found that only under illumination conditions does a favorable interface form, leading to a continuous improvement of the performance of Ta3N5. The effect is in stark contrast to bare Ta3N5, which would degrade rapidly under similar conditions. Experimental characterization confirmed that the effect is due to the unique chemical interactions between Ta3N5 and Co(OH)2 in the presence of light, which leads to improved surface energetics and kinetics on the photoelectrode/water interface. Our approach represented a new direction toward stabilizing the Ta3N5 for solar fuel application.
9:00 AM - ES02.07.03
Photocorrosion-Limited Maximum Efficiency of Solar Photoelectrochemical Water Splitting
Jun-Wei Luo 1 , Ling-ju Guo 2 , Su-Huai Wei 3 , Shu-Shen Li 1
1 , Institute of Semiconductors, Chinese Academy of Sciences, Beijing China, 2 Chinese Academy of Sciences (CAS) Key Laboratory of Nanosystem and Hierarchy Fabrication, National Center for Nanoscience and Technology, Beijing China, 3 , Beijing Computational Science Research Center, Beijing China
Show AbstractPhotoelectrochemical (PEC) water splitting to generate hydrogen is one of the most studied methods for converting solar energy into clean fuel because of its simplicity and potentially low cost. Based on the solar spectrum, the theoretical solar-to-hydrogen (STH) conversion efficiency limit is 30% for a single-bandgap semiconductor. However, despite over 40 years of intensive research since the first demonstration of PEC water splitting by Fujishima and Honda in 1972, PEC water splitting remains in its early stages with stable efficiencies far less than 10%, a benchmark for commercial applications. Currently, massive efforts have been made to identify the best semiconductor photocatalysts with both a high STH conversion efficiency and long-term resistance to photocorrosion for solar PEC water splitting. Here, we examined the photocorrosion stabilities of 202 semiconductors under PEC water-splitting conditions; these semiconductors are known to either catalyze overall water splitting or reduce or oxidize water in the presence of an external sacrificial redox agent. We reveal that the desired photocorrosion stability sets a limit of 2.48 eV (relative to the normal hydrogen electrode (NHE)) for the highest possible potential of the valence band (VB) edge of a photocorrosion-resistant semiconducting photocatalyst. This limit is also supported by a derived correlation between the Gibbs free energy and the VB potential of a semiconductor. The disparity between the stability-limited VB potential at 2.48 V and the oxygen evolution reaction (OER) potential at 1.23 V vs NHE reduces the maximum STH conversion efficiency to approximately 8% for practical single-bandgap PEC water splitting. Based on this understanding, we suggest that the most promising strategy to overcome this 8% efficiency limit is to decouple the requirements of efficient light harvesting and chemical stability by protecting the active semiconductor photocatalyst surface with a photocorrosion-resistant oxide coating layer.
9:15 AM - ES02.07.04
Atomic Layer Deposited Transition Metal Oxide-Titania Alloys—Corrosion Resistant Schottky Contacts and Catalysts for Silicon Photoanodes
Olivia Hendricks 1 , Andrew Meng 1 , Andrew Scheuermann 1 , Paul Hurley 2 , Christopher Chidsey 1 , Paul McIntyre 1
1 , Stanford University, Stanford, California, United States, 2 , Tyndall National Institute, Cork Ireland
Show AbstractMetal-insulator-semiconductor (MIS) structures are promising candidates for integrated solar-driven water splitting devices. Typically, the "metal" layer serves a dual purpose, catalyzing water oxidation while also setting the built-in field for extracting photogenerated carriers from the semiconductor. Recently, incorporation of additional protection layers into the MIS junction has made it possible to use semiconductor materials, such as silicon, that would normally be unstable under the conditions required for water oxidation. The protection layers, however, have often compromised the photovoltage obtained from these devices. Although the theoretical maximum photovoltage for silicon photovoltaics is 700-800 mV,1 maximum photovoltages of ~400 mV have been reported for nSi/TiO2/metal photoanodes protected by highly conductive TiO2 layers2 with some devices achieving only 200-250 mV.3
Our goal is to develop a more ideal protection layer using transition metal oxide-titania alloys synthesized by ALD. An ideal protection layer should be corrosion resistant, highly conductive, and enable a high photovoltage. For n-type silicon Schottky photoanodes, this means that the protection layer must also possess a high work function. We previously demonstrated that TiO2-RuO2 alloys composed of 13-46% Ru were highly conductive and consistently achieved photovoltages >525 mV without any post-processing anneals.4 The built-in field was set by the TiO2-RuO2 alloy, which possessed a high work function of 5.02 eV. Although TiO2-RuO2 alloys are effective Schottky contacts to nSi, they lack the long-term corrosion resistance required of a protection layer. RuO2 is known to be unstable under the conditions required for water oxidation. Here we report on ALD alloys of TiO2 and IrO2, which exhibit both high electrical conductivity and photovoltage > 600 mV on nSi, and which exhibit improved stability during water oxidation. Additionally, these TiO2-IrO2 alloys are capable of catalyzing oxygen evolution. Thus, TiO2-IrO2 alloys have the potential to be an “all-in-one” catalyst, Schottky contact, and corrosion resistant protection layer for high quality semiconductor photoanodes.
References:
1. Green, M. A. Limits on the Open-circuit Voltage and Efficiency of Silicon Solar Cells Imposed by Intrinsic Auger Processes. IEEE Trans. Electron Devices ED-31, 671–678 (1984).
2. Hu, S. et al. Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation. Science. 344,1005–1009 (2014).
3. McDowell, M. T. et al. The Influence of Structure and Processing on the Behavior of TiO2 Protective Layers for Stabilization of n-Si/TiO2/Ni Photoanodes for Water Oxidation. ACS Appl. Mater. Interfaces. 7,15189–15199 (2015).
4. Hendricks, O. L. et al. Isolating the Photovoltaic Junction : Atomic Layer Deposited TiO2 − RuO2 Alloy Schottky Contacts for Silicon Photoanodes. (2016). doi:10.1021/acsami.6b08558
9:30 AM - *ES02.07.05
How to Build High-Efficiency, Durable and Low-Cost Photoelectrodes for Solar Water Splitting
Dunwei Wang 1
1 , Boston College, Chestnut Hill, Massachusetts, United States
Show AbstractPhotocatalysts have the potential to directly harvest solar energy and store it in chemical bonds but are often too inefficient or expensive or both. An important reason for the issues is the complex properties expected from a single material, including good optoelectronic performance, chemically active toward targeted reactions while resistant against corrosion. On top of these considerations, photocatalysts are also expected to be composed of earth abundant elements. Increasingly, it is recognized that these property expectations may be met simultaneously only on engineered complex materials. For instance, approaches have been proposed and shown highly promising by combining efficient light absorbers with active co-catalysts. Passivation is also popularly studied to enable the utilization of efficient but unstable materials. How these different material components influence each other is therefore important but remains poorly understood. In this presentation, we share our latest efforts aimed at correcting this critical deficiency. We show that surface modifications to semiconducting photocatalysts play important roles in defining the overall performance of the system, but through fundamentally different mechanisms. The results will be presented on three different material platforms, iron oxide, bismuth vanadate, and Si, for water oxidation and reduction, respectively. These results set the stage for future efforts of building photocatalysts that are both efficient and low cost.
10:30 AM - *ES02.07.06
Semiconductor Quantum Dot Solar Cells Developed by Optimising Interfacial Charge Transfer Dynamics
Yasuhiro Tachibana 1 2
1 , RMIT University, Bundoora, Victoria, Australia, 2 , Osaka University, Osaka Japan
Show AbstractSemiconductor quantum dot (QD) is one of the most attractive nanomaterials for solar energy conversion devices. With their relatively large extinction coefficients and a wide light absorption range over visible wavelengths, QDs can be effective light absorbers. However, despite these attractive properties, when they are employed in solar cells, their function, particularly exciton states, charge separation and recombination dynamics has not been well understood. For example, the excited electron and hole can readily be trapped by the surface states, losing initial excited energy, however their relations to the solar cell function is still not clear. In this presentation, we will discuss underlying key parameters controlling interfacial charge separation and recombination dynamics [1], and relationship of the nanostructures with the interfacial electron transfer reactions [2].
Several types of QDs with a narrow size distribution are synthesized to control the potential energy levels of the conduction and valence bands [3,4]. We analysed influence of QD surface states on the interfacial charge transfer dynamics [2], and their interfacial structure is modified to control the dynamics.
This work was financially supported by the JST PRESTO program (Photoenergy Conversion Systems and Materials for the Next Generation Solar Cells), the Collaborative Research Program of Institute for Chemical Research, Kyoto University (2017-75) and by JSPS KAKENHI Grant Number JP16K05885, Japan. The author also acknowledges the Australian Research Council (ARC) LIEF grant (LE170100235) and the Office for Industry-University Co-Creation, Osaka University, for the financial supports.
References
Y. Tachibana, et al. ACS Appl. Mater. Interfaces, 8(22) 13957-13965 (2016).
Y. Tachibana, et al. J. Phys. Chem. C, 119(35) 20357-20362 (2015).
Y. Tachibana, et al. Phys. Chem. Chem. Phys., 17(4), 2850 - 2858 (2015).
Y. Tachibana, et al. J. Mater. Chem. C, 5, 2182 - 2187 (2017).
11:00 AM - ES02.07.07
Stabilizing Silicon Photocathodes by Solution-Deposited Ni-Fe Layered Double Hydroxide for Efficient H2 Evolution in Alkaline Media
Jiheng Zhao 1 , Lili Cai 1 , Hong Li 1 , Xiaolin Zheng 1
1 , Stanford University, Stanford, California, United States
Show AbstractAn important pathway towards cost-effective photoelectrochemical (PEC) solar water-splitting devices is to stabilize and catalyze silicon (Si) photocathodes for hydrogen evolution reaction (HER). This is the particular case for alkaline solutions, since most efficient oxygen evolution reaction (OER) catalysts for photoanodes are only stable in such solutions. To date, the most stable Si photocathode in alkaline media is protected by the atomic layer deposited (ALD) dense TiO2 layer and catalyzed by noble metal-based catalysts on top. However, the ALD process is difficult to scale up and the noble metals are expensive. Thus, there is a need to develop scalable deposition methods and use earth abundant catalysts to protect and catalyze Si photocathodes in alkaline solutions. Herein, we report the first demonstration of using a scalable hydrothermal method to deposit earth-abundant NiFe-layered double hydroxide (LDH) to simultaneously protect and catalyze Si photocathodes in alkaline solutions. The NiFe LDH passivated/catalyzed p-type Si photocathode shows excellent HER performance with a current density of 7 mA/cm2 at 0 V vs RHE, an onset potential of ~0.3 V vs RHE that is comparable to that of the reported p-n+ Si wafers, and durability during 24 hours of testing at 10 mA/cm2 in 1M KOH (pH ~14) electrolyte.
11:15 AM - ES02.07.08
A Case Study of a Record-Breaking Long-Life Silicon-Based Photocathode
Dowon Bae 1 , Brian Seger 1 , Thomas Pedersen 2 , Ole Hansen 2 , Peter Vesborg 1 , Ib Chorkendorff 1
1 Physics, Technical University of Denmark, Kgs. Lyngby Denmark, 2 Micro- and Nanotechnology, Technical University of Denmark, Kgs. Lyngby Denmark
Show AbstractPhotoelectrochemical (PEC) water splitting is a promising approach to provide clean and storable fuel (i.e. hydrogen). However, major challenges still have to be overcome before commercialization can be achieved. One of the largest barriers to overcome is to obtain a stable PEC reaction in either strongly basic or acidic electrolytes without degradation of the semiconductor photoelectrodes.1 In this work, we discuss fundamental aspects of protection strategies for achieving stable solid/liquid interfaces. Besides, we also cover protection layer approaches and their stabilities for a wide variety of experimental photoelectrodes for water splitting.
Then as a case study, transparent TiO2 protected crystalline Si-based photoelectrodes for hydrogen evolution reaction (HER) are discussed in-depth. The detailed working principles based on the band alignment are outlined to understand how the carriers can be injected and transferred to solid/liquid interfaces in PEC systems. Notably, we demonstrate an extremely stable HER at pH 0 for more than 80 days under the red light (λ ≥ 635 nm) using a TiO2 protected MOS (metal-oxide-semiconductor) based c-Si photocathode.2 So far, this is the longest stability reported in photocatalytic water reduction. Importantly, the long-term stability experiment contains day/night cyclic test that can evaluate the intrinsic stability of the protection layer.
Detailed analysis using SEM (scanning electron microscopy), XPS (X-ray photoelectron spectroscopy) and ICP-MS (inductively coupled plasma mass spectrometry) revealed that the degradation in photocurrent mainly results from carbon contamination, which decreases reactivity by catalyst contamination and blocks the light absorption.
1 D. Bae, B. Seger, P. C. K. Vesborg, O. Hansen and I. Chorkendorff, Chem. Soc. Rev., 2017.
2 D. Bae, T. Pedersen, B. Seger, B. Iandolo, O. Hansen, P. C. K. Vesborg and I. Chorkendorff, Catal. Today, 2016.
11:30 AM - *ES02.07.09
Self-Assembled Gradient of 3D/ 2D Hybrid Perovskite to Boost Efficiency and Durability of Solar Cells
Mohammad Nazeeruddin 1 , Kyung Taek Cho 1 , Giulia Grancini 1
1 , Ecole Polytechnique Federale Lausanne, Sion Switzerland
Show AbstractOrganic-inorganic lead halide perovskites have shown impressive power conversion efficiency (PCE) in a range of solar cell architectures.1 However, the poor photo- and thermal stability and hysteresis issues are preventing for their practical applications including water splitting into hydrogen and oxygen. Interface engineering of perovskite solar cell is crucial for stability and high efficiency.2 Here we demonstrate an innovative approach of realizing high efficient and stable PSCs by utilizing 2D perovskite interlayer. We show insertion of molecularly engineered two-dimensional (2D) perovskite layer on top and the bottom of the 3D perovskite lead to enhanced stability and decreased hysteresis without affecting the efficiency. The devices with interface engineering of inserted 2D perovskite on top of three-dimensional (3D) bulk perovskite film yielded 20.7% power conversion efficiency, which after illumination under one sun for 500 hours at 50°C retained 96% of its initial value.
References
(1). www.nrel.gov/ncpv/images/efficiency_chart.jpg.
(2). Grancini, G.; Roldan-Carmona, C.; Zimmermann, I.; et al.NATURE COMMUNICATIONS , 8, 15684 (2017).
ES02.08: Bioinspired Solar Fuels
Session Chairs
Wednesday PM, November 29, 2017
Hynes, Level 3, Room 306
1:30 PM - *ES02.08.01
Nano-Bio Hybrids Based on Natural and Artificial Proton Pump for Photocatalytic Hydrogen Production
Elena Rozhkova 1 , Peng Wang 1 2
1 , Argonne National Laboratory, Lemont, Illinois, United States, 2 State Key Laboratory of Crystal Materials, Shandong University, Jinan China
Show AbstractNature, besides inspiring scientists and engineers conceptually, also provides biomolecules, complexes and machineries which owing their inherent functionality can be employed as backbones for engineering of advanced bio-inspired hybrid photocatalysts. We have been developing photons energy transformation architectures using natural sunlight-driven chlorophyll-independent machinery of retinol-centered proton pump from Halobacteria. In natural systems proton pumps are capable of carrying ions against an electrochemical potential — up to 250 millivolts, which is utilised to fuel ATP synthesis. This biological mechanism is driven by photo-izomerization of the retinol cofactor and does not include charge separation or water splitting. We demonstrated that isolated proton pumps can be assembled with semiconductor nanoparticles in engineered efficient photocatalytic water splitting systems capable of hydrogen production at ambient conditions under visible light. Further, modern "synthetic life" technology of cell-free membrane protein synthesis was engadged to construct entirely man-made nano-bio catalytic architecture. Robustness and flexibility of this approach allow for further systemic manipulation at the nanoparticle−bio interface toward directed evolution of energy transformation materials and artificial systems.
2:00 PM - ES02.08.02
Photoactive Blend Morphology Engineering through Tunable Aggregation in All Polymer Solar Cells
Gang Wang 1 , Antonio Facchetti 1 , Tobin Marks 1
1 Department of Chemistry, The Materials Research Center, and The Argonne-Northwestern Solar Energy Research Center, Northwestern University, Evanston, Illinois, United States
Show AbstractPolymer aggregation plays a critical role in determining the miscibility of donor and acceptor polymeric semiconductors as well as the performance of all polymer solar cells (APSCs). However, several aspects governing how polymer degree of texturing and aggregation affects photoactive blend film microstructure remain to be clarified. Here, we study the effect of aggregation by systematically varying the number-average molecular weight (Mn) of both the amorphous donor polymer PBDTT-FTTE and the semi-crystalline acceptor polymer P(NDI2OD-T2). The resulting APSC active layer morphologies and device performances are evaluated with transmission electron microscopy (TEM), grazing incidence wide-angle X-ray scattering (GIWAXS), and space charge limited current (SCLC) measurements. Additionally, coarse-grained modeling simulations were performed to gain deeper insights into the effect of polymer aggregation on the blend morphology. Importantly, the computed average distance between the donor and the acceptor polymers correlates with the photovoltaic parameters such as short-circuit current density and is a useful metric for understanding the blend film mixing. Our results demonstrate that when using polymers with different texturing abilities the effects on APSC performance and blend morphology are different than when using crystalline polymers. This work provides a guideline for the optimization of APSCs through variation of polymer templating capacity and bulk-heterojunction morphology.
2:15 PM - ES02.08.03
Self-Assembling High Mechanical Strength Liposomes for Integrated Photocatalytic Hydrogen Production
Jia Tian 1
1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractIntegrating all the necessary molecular components into a durable artificial system for photocatalysis reactions is a long-term goal in self-assembly chemistry. Liposomes as nature-made scaffolds play a critical role in numerous photosynthetic organelles, such as plant chloroplasts, which integrate light absorption, charge transport and catalysis to create chemical bonds using light energy in one body. However, the development of real industrialized applications were extremely restricted by their complicated molecular structures and flimsy mechanical properties. Herein, we synthesized a series of novel amphiphiles that exhibits extraordinarily high tensile strength. Based on this liposome platform, a noble metal free integrated photocatalytic system was constructed in water by using cationic porphyrin as hydrophilic heads and anionic cobalt complexes as catalysts. Photocatalytic production of hydrogen is observed in the three-dimensional environment of the scaffold and the material is easily embedded in the membrane or lumen of the liposome.
3:30 PM - ES02.08.04
Bacteriorhodopsin/Semiconductor Nanotube Arrays Hybrid System for Enhanced Photoelectrochemical Water Splitting
Nageh Allam 1
1 , American University in Cairo, New Cairo Egypt
Show Abstract
In recent years, considerable efforts have been made to improve the performance of photoactive nanostructured materials for water splitting applications. Herein, we report on the assembly and use of a bacteriorhodopsin (bR)/TiO2nanotube array hybrid electrode system. Photoanode materials composed of ∼ 7μm long self-ordered and vertically oriented nanotube array of titanium dioxide films were fabricated via the anodization of Ti foil in formamideelectrolytes containing NH4F at room temperature followed by sensitization of the electrodes with bR. The stability of bR on the TiO2 surface was found to depend on the pretreatment process of the TiO2 films. Our results demonstrate the opportunity to fabricate fairly stable bR/TiO2 hybrid electrodes that can be used as photoanodes for photoelectrochemical water splitting. Under AM 1.5 illumination (100 mW/cm2), the hybrid electrodes achieved a photocurrent density of 0.65 mA/cm2 which is a ∼ 50% increase over that measured for pure TiO2 nanotubes (0.43 mA/cm2) fabricated and tested under the same conditions. In the presence of a redox electrolyte, the photocurrent increased to 0.87 mA/cm2. To the best of our knowledge, this is the first report on the use of bR/TiO2 hybrid electrodes in photoelectrochemical water oxidation cells. We believe the proton pumping property of bR can be used in a variety of applications, especially those related to third generation photovoltaic cells.
3:45 PM - ES02.08.05
Photoelectrode Architectures for Hybrid Organic/Inorganic Photoelectrochemical Water Splitting
Francesco Fumagalli 1 , Sebastiano Bellani 1 , Hansel Comas Rojas 1 , Alessandro Mezzetti 1 , Guglielmo Lanzani 1 , Maria Rosa Antognazza 1 , Fabio Di Fonzo 1
1 , IIT, Milano Italy
Show AbstractHydrogen production through renewable sources, rather than fossil fuels, represents the missing element towards a carbon neutral energetic cycle. One of the most promising approaches is the direct conversion of solar energy into chemical fuels at a low cost semiconductor/water junction. Despite the theoretical simplicity of such a device, different limitations on suitable semiconductor materials’ characteristics have hindered its development. In the last years, the capability of semiconductive polymers/fullerene-based acceptors compounds to steadily drive photo-generated electrons towards an electrocatalyst in a water environment was demonstrated.
We present a study of different architectures of hybrid organic-inorganic H2 evolving photocathodes based on semiconducting polymeric absorbers. The relevance of this study can be summarized in few key points: (i) high performances with photocurrents up to 8mA/cm2 at 0VRHE; (ii) optimal process stability with 100% faradaic efficiency along electrode’s lifetime; (iii) excellent energetics with onset potential as high as +0.7VRHE; (iv) promising operational activity of more than 10 hours and (vi) by-design compatibility for implementation in a tandem architecture. Collectively, this set of features establishes hybrid architectures employing organic semiconductors and organic photoelectrochemical systems as promising candidates for efficient solar fuel production.
Suitable materials were first investigated. Different PVD or solution processed inorganic interfacial layers (MoO3, WO3, CuI, TiO2/Pt) and their influence on performances have been assessed, enlightening the working principles and limiting factors of actual implementations. We show the photocatalityic activity and long-term stability of a catalysed bulk heterojunction and the effect of selective contacts on performances is investigated separately. Introduction of an electron selective layer increases the photocurrent response while hole blocking layers shift the onset potential towards positive voltages allowing operation with a tandem photoanode and/or a PV cell.
Secondly the influence of system nanostructuration was assessed, the development of multi-layer systems based on structured absorbers in a host/guest architecture allowed us to orthogonalize light absorption and photogenerated carrier collection.
Finally, seeking the realization of an efficient and cost-competitive photocathode we combined PVD and solution-processed techniques realizing a 2”x2” photoelectrode in order to show the potential of cheap, large-scale production of organic photoelectrochemical systems. Such a system exhibits 0.8mAcm-2 at 0VRHE, an onset potential of +0.7VRHE and stability of over 1hr.
This work opens the way to the exploitation in photoelectrochemistry of organic semiconductors developed for OPVs and to the realization of a new generation of water splitting devices for renewable and low cost direct conversion of sunlight into H2.
4:00 PM - ES02.08.06
High-Performance Organic Photocathodes Stabilized by Low-Temperature Atomic Layer Deposition of Titania
Ludmilla Steier 1 , Matthew Mayer 2 , Hansel Comas 4 , Sebastiano Bellani 5 , Laia Forcada 1 , Maria Rosa Antognazza 5 , James Durrant 1 , Michael Graetzel 3
1 , Imperial College London, London United Kingdom, 2 , Helmholtz-Zentrum Berlin, Berlin Germany, 4 , Instituto Superior de Tecnologías y Ciencias Aplicadas, Havana Cuba, 5 , Istituto Italiano di Tecnologia, Milano Italy, 3 , École Polytechnique Fédérale de Lausanne, Lausanne Switzerland
Show AbstractFor the past decades, research on novel photocathodes for photoelectrochemical (PEC) water splitting has focused predominantly on inorganic photoabsorber materials. In the past five years however, organic photoabsorbers entered the PEC scene and, since, have shown remarkable photocatalytic performances.1-3 Organic semiconductors offer much higher flexibility in their synthesis allowing for tunable HOMO/LUMO positions and absorption characteristics. Furthermore, they offer low-cost roll-to-roll processing and hence are very attractive for upscale of PEC devices. Organic photocathodes based on a buried junction approach could reach photovoltages close to the open circuit potential of the equivalent photovoltaic and relatively high photocurrent densities, showing the great potential of these electrodes.1-3
As a drawback, these organic photocathodes have suffered severe stability issues in the relatively harsh PEC water splitting environment. Stability is a key necessity for any PEC operating device. As a solution to this stability issue, we developed a low-temperature atomic layer deposition (ALD) process of TiO2 suitable for various organic blend materials. The importance of a uniform and pinhole-free titania film on the blend material will be discussed in terms of i) electron extraction and ii) protection against corrosion of the underlying materials. I will focus in detail on two high-performance photocathode designs and will demonstrate benchmark operating stabilities.4 Our work on low-temperature ALD titania is a cornerstone of new research on efficient and stable organic PEC photocathode designs.
1. Fumagalli, F.; Bellani, S.; Schreier, M.; Leonardi, S.; Rojas, H. C.; Ghadirzadeh, A.; Tullii, G.; Savoini, A.; Marra, G.; Meda, L., et al. Hybrid Organic-Inorganic H-2-Evolving Photocathodes: Understanding the Route Towards High Performance Organic Photoelectrochemical Water Splitting. Journal of Materials Chemistry A 2016, 4, 2178-2187.
2. Guerrero, A.; Haro, M.; Bellani, S.; Antognazza, M. R.; Meda, L.; Gimenez, S.; Bisquert, J. Organic Photoelectrochemical Cells with Quantitative Photocarrier Conversion. Energy & Environmental Science 2014, 7, 3666-3673.
3. Rojas, H. C.; Bellani, S.; Fumagalli, F.; Tullii, G.; Leonardi, S.; Mayer, M. T.; Schreier, M.; Gratzel, M.; Lanzani, G.; Di Fonzo, F., et al. Polymer-Based Photocathodes with a Solution-Processable Cuprous Iodide Anode Layer and a Polyethyleneimine Protective Coating. Energy & Environmental Science 2016, 9, 3710-3723.
4. Steier, L.; Bellani, S.; Comas-Rojas, H.; Pan, L.; Laitinen, M.; Sajavaara, Di Fonzo, F.; T.; Grätzel, M.; Antognazza, M. R.; Mayer, M. T. Stabilizing organic photocathodes by low-temperature atomic layer deposition of TiO2. Sustainable Energy Fuels, 2017, DOI: 10.1039/C7SE00421D
4:15 PM - ES02.08.07
Microbial-Electrocatalytic Reduction of Carbon Dioxide to Formate and Hydrogen
Liviu Dumitru 1 , Stefanie Schlager 1 , Hathaichanok Seelajaroen 1 , Marianne Haberbauer 2 , Christine Hemmelmair 2 , Tanja Benninger 3 , Achim Hassel 3 , Niyazi Serdar Sariciftci 1
1 Linz Institute of Organic Solar Cells, Johannes Kepler Universität Linz, Linz Austria, 2 , The Austrian Centre of Industrial Biotechnology (acib GmbH), Linz Austria, 3 Institute for Chemical Technology of Inorganic Materials (ICTAS), Johannes Kepler Universität Linz, Linz Austria
Show AbstractThe fast increase of carbon dioxide (CO2) emissions all around the globe has become a major concern for the society and scientists from different research areas, who are struggling to find new and efficient ways for dealing with this issue. Various strategies have been tested and implemented, such as capture, storage and utilization of CO2 [1]. But converting CO2 into useful organic compounds and fuels is regarded economically more appealing and a long term, sustainable solution. Hydrogenation of CO2 to formic acid has been extensively investigated and achieved by using transition-metal complexes either in organic solvents or water along with amines, (bi)carbonates and hydroxides as additives[2][3]. The main drawback of this approach is that it requires high temperature and pressure to complete the reaction[4]. However, carbon dioxide can also be reduced at ambient temperature and pressure using photochemical[5], enzymatic[5] or (bio)-electrochemical techniques[6][7], to produce formate, methanol, or methane.
Microbial electrosynthesis cells (MECs) offer a great potential for sustainable, highly efficient, and selective CO2 reduction[7]. In this study we report on carbon dioxide reduction to formate and hydrogen (as by-product), in a MEC. The microorganisms (Methylobacterium extorquens), which serve as a biocatalyst, were grown on a carbon-felt cathode. Formate and hydrogen were generated by direct electron injection, without the need of additional mediators[8] or cofactors, when CO2 was bubbled into the cathode's compartment. The long-term performance of the cell was monitored (for more than one year), while the working parameters were constantly optimized. A promising approach towards green and efficient CO2 reduction to valuable products is proposed.
[1] A.J. Hunt, E.H.K. Sin, R. Marriott, J.H. Clark, ChemSusChem., 3: 306–322 , 2010.
[2] W. Wang, S. Wang, X. Ma, J. Gong, Chem. Soc. Rev., 40: 3703 , 2011.
[3] R. Tanaka, M. Yamashita, K. Nozaki, J. Am. Chem. Soc., 131: 14168–14169 , 2009.
[4] S. Wang, G.Q. (Max) Lu, G.J. Millar, Energy & Fuels., 10: 896–904 , 1996.
[5] M. Aresta, A. Dibenedetto, T. Baran, A. Angelini, P. Labuz, W. Macyk, Beilstein J. Org. Chem., 10: 2556–2565 , 2014.
[6] S. Schlager, L.M. Dumitru, M. Haberbauer, A. Fuchsbauer, H. Neugebauer, D. Hiemetsberger, A. Wagner, E. Portenkirchen, N.S. Sariciftci ChemSusChem., 9: 631–635 , 2016.
[7] S. Schlager, M. Haberbauer, A. Fuchsbauer, C. Hemmelmair, L.M. Dumitru, G. Hinterberger, H. Neugebauer, N.S. Sariciftci ChemSusChem., 10: 226–233 , 2017.
[8] H. Hwang, Y.J. Yeon, S. Lee, H. Choe, M.G. Jang, D.H. Cho, S. Park, Y. H. Kim Bioresour. Technol., 185: 35–39 , 2015.
4:30 PM - ES02.08.08
Mo-Doped BiVO4 Thin Films—Improved Photoelectrochemical Performance Regarding Water Oxidation Achieved by Tailored Structure and Morphology
Martin Rohloff 1 2 , Björn Anke 1 , Martin Lerch 1 , Anna Fischer 2
1 , Technische Universität Berlin, Berlin Germany, 2 Institut für Anorganische und Analytische Chemie, Albert-Ludwigs-Universität, Freiburg, Baden-Würtemberg, Germany
Show AbstractThe n-type semiconductor bismuth vanadate (BiVO4) has recently gained a lot of attention as photoanode material for light induced water oxidation. Its absorption in the visible domain (band gap energy of 2.4 eV), its suitable band edge positions compared to the OER half reaction, its stability against photo-corrosion as well as its low cost makes BiVO4 one of the most interesting ternary oxide materials for light-induced oxygen evolution from water. One major drawback for BiVO4 is its poor bulk electronic conductivity, which however can be overcome by doping as well as by improved structural design.
In here we present a new, one-step synthesis method allowing the direct deposition of BiVO4 and Mo-doped BiVO4 thin film photoanodes. Starting from suitable Bi, V and Mo precursor sols, BiVO4 and Mo-doped BiVO4 thin films can be fabricated by simple dip coating. Structural (GI-XRD, EBSD, SAED) and morphological characterization (SEM, TEM) reveal that the thin films crystallize in a favorable monoclinic scheelite structure in micrometer large, two-dimensional, single-crystalline porous domains. Mott-Schottky and SEM analysis reveals that the amount of Mo within the anode material influences the amount of free charge carriers and the size and structure of the single-crystalline domains, respectively. Photocurrent transient analysis reveals drastically improved photoanode kinetics, i.e. reduced surface hole recombination. Consequently, the photoelectrochemical performance regarding water oxidation of the here presented Mo-doped BiVO4 photoanodes turns out to be among the highest reported so far.
Our results reveal a direct connection between molybdenum content, film structure/morphology and photoelectrochemical performance for Mo-doped BiVO4 thin films and highlight the importance of rational thin film design for high-performing photoanodes.
This work has been funded by the DFG SPP1613 project.
4:45 PM - ES02.08.09
Colloidal Nanocrystals to Advance Studies in CO2 Conversion
Raffaella Buonsanti 1
1 Department of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, Sion Switzerland
Show AbstractStorage of intermittent renewable energy in chemical bonds is an important research area to build a more sustainable society. Artificial photosynthesis mimics the natural photosynthesis by converting sunlight, water and carbon dioxide into value-added chemicals (such as fuels for transportation like hydrogen or hydrocarbons but also useful chemicals like ethylene, which is a feedstock for petrolchemicals). Many scientific challenges still exist for the technological implementation of water splitting and CO2 photo/electrochemical devices. Our goal is to address some of these challenges by tailor-making material platforms based on atomically-defined colloidal nanocrystals (NCs).
In this talk, I will focus on NCs for the conversion of CO2 into value-added chemicals. We have recently showed that the material tunability afforded by colloidal chemistry allows to build unambiguous structure/properties between Cu NCs of different sizes (8nm to 60nm) and shapes (cubes and spheres) and their behaviour as electrocatalysts for CO2 reduction. An unexpected selectivity trend was found, with 44nm nanocubes showing an impressive CO2 conversion efficiency to ethylene, which is one of the highly desirable products (Figure 1). [2] I will discuss our recent stability studies by HR-TEM and theoretical calculations, which are elucidating the mechanims behind such a behaviour. [3] Furthermore, our results with more complex hybrid systems comprising Cu NCs as building blocks (metastable Cu-based alloys and NC/metal organic hybrids [4]) will be briefly discussed as examples of multifunctional platforms to tune selectivity in the CO2 conversion reaction. The synthetic challenges of such materials and our strategies to access them in a programmable manner will be highlighted.
1) C. Gadiyar et al., J. Phys. D: Appl. Phys., 2017, 50, 074006.
2) A. Loiudice, Angew. Chem. Int. Ed., 2016, 55, 5789.
3) J. Huang et al., submitted
4) I. Luz, Chem. Mater., 2016, 28, 3839.
ES02.09: Poster Session II
Session Chairs
Vincent Artero
Fabio Di Fonzo
Bruce Koel
Thursday AM, November 30, 2017
Hynes, Level 1, Hall B
8:00 PM - ES02.09.02
Silver Nanoparticles-Decorated Titanium Oxynitride Nanotube Arrays for Enhanced Solar Fuel Generation
Nageh Allam 1 , Khalid Soliman 2 , Ahmed Khalifa 1 , Siham AlQaradawi 3
1 Energy Materials Laboratory (EML), School of Sciences and Engineering, The American University in Cairo, New Cairo Egypt, 2 Physical Chemistry Department, National Research Centre, Cairo Egypt, 3 , Department of Chemistry and Earth Sciences, Qatar University, Doha Qatar
Show AbstractWe demonstrate, for the first time, the synthesis of highly ordered titanium oxynitride nanotube arrays sensitized with Ag nanoparticles (Ag/TiON) as an attractive class of materials for visible-light-driven water splitting. The nanostructure topology of TiO2, TiON and Ag/TiON was investigated using FESEM and TEM. The X-ray photoelectron spectroscopy (XPS) and the energy dispersive X-ray spectroscopy (EDS) analyses confirm the formation of the oxynitride structure. Upon their use to split water photoelectrochemically under AM 1.5 G illumination (100 mW/cm2, 0.1 M KOH), the titanium oxynitride nanotube array films showed significant increase in the photocurrent (6 mA/cm2) compared to the TiO2 nanotubes counterpart (0.15 mA/cm2). Moreover, decorating the TiON nanotubes with Ag nanoparticles (13 ±2 nm in size) resulted in exceptionally high photocurrent reaching 14 mA/cm2 at 1.0 VSCE. This enhancement in the photocurrent is related to the synergistic effects of Ag decoration, nitrogen doping, and the unique structural properties of the fabricated nanotube arrays
8:00 PM - ES02.09.03
Sn-Addition over Haematite Photoanode Enhances the Light Induced Water Oxidation Activity
Andre Luiz Freitas 1 , Flavio De Souza 1
1 , UFABC, Santo Andre Brazil
Show AbstractThis work describes a study of haematite photoanodes processed by a chemical route using a microwave synthesis reactor assisting hydrothermal condition. Since the synthesis product is in the hydrated phase an additional thermal treatment at 750°C for 30 min is needed to obtain the phase of interest. This additional treatment was conducted in two different atmospheres (Air and N2 flux). A systematic investigation was also conducted to obtain the optimum surface modification with a Sn4+ layer over haematite. XRD patterns confirmed the haematite as obtained phase with crystals preferentially oriented toward to (110) plane. Top-view scanning electron microscopy images exhibit photoanodes composed by nanorods with length estimated around 100 nm by cross-sectional images analysis. As result, the photoanodes Sn-modified followed by thermal treatment seems to induce the dopant segregation and reducing the energy for charge separation (lowest resistance to charge transfer) achieving a photocurrent response from the order of 1.10 mA.cm-2. While the N2 atmosphere may act reducing the bulk recombination (solid-solid interface such as, haematite-haematite interfaces or haematite-substrate interface) also helping to increase the electron collection through the substrate. A synergetic effect by combining these two strategies indicates that the oxygen-deficient atmosphere prevents the formation of a resistive layer of SnO2. Finally, further investigations are being conducted to obtain a high efficient haematite photoanode active for water oxidation.
Acknowledgements:
We gratefully acknowledge financial support from the Brazilian agencies of FAPESP (Grants 2011/19924-2, 2013/07296-2 and 2014/11736-0).
8:00 PM - ES02.09.04
One-Step Hydrothermal Deposition of Ni:FeOOH onto Photoanodes for Enhanced Water Oxidation
Lili Cai 1 , Jiheng Zhao 1 , Hong Li 1 , Joonsuk Park 1 , In Sun Cho 1 , Hyun Soo Han 1 , Xiaolin Zheng 1
1 , Stanford University, Stanford, California, United States
Show AbstractThe realization of efficient photoelectrochemical (PEC) water splitting requires effective integration of earth-abundant active oxygen evolution catalysts (OECs) with diverse photoanodes. Although many good OECs have been investigated on conductive substrates under dark conditions, further studies are needed to evaluate their performance when integrated with photoanodes under illumination. Such studies will be facilitated by developing effective coating methods of OECs onto diverse photoanodes. Here, we report a one-step hydrothermal process that conformally coats various photoanodes with ultrathin Ni-doped FeOOH (Ni:FeOOH) OECs. The coated Ni:FeOOH, due to its unique open tunnel structure, tunable Ni doping concentration, and high coating/interface quality, lowers the onset potential of all of the photoanodes investigated, including WO3/BiVO4, WO3, α-Fe2O3, TiO2 nanowires, BiVO4 films, and Si wafers. We believe that this simple and yet effective hydrothermal method is a useful addition to the existing deposition techniques for coupling OECs with photoanodes and will greatly facilitate the scale-up of efficient PEC devices.
8:00 PM - ES02.09.05
Photoelectrochemical Studies of GaN Based Nanopillars Fabricated Using Top-Down Approach
Parvathala Reddy Narangari 1 , Siva Karuturi 1 , Joshua Butson 1 , Rowena Yew 1 , Mykhaylo Lysevych 1 , Hoe Tan 1 , Chennupati Jagadish 1
1 , Australia National University, Canberra, Australian Capital Territory, Australia
Show AbstractThe research interest in photoelectrochemical (PEC) hydrogen generation is ever growing owing to its ability to produce clean and portable form of energy. However, the lack of narrow energy band gap materials with high photo-corrosion resistance is the main barrier for this technology towards commercialization. The band gap tunability with varying In content and high chemical stability makes InxGa1-xN a superior candidate for photoelectrodes. In addition, nanostructures reduce the carrier diffusion length and increase its absorption and semiconductor/electrolyte interface area which results in enhanced PEC performance of photoelectrodes.
In this report, we present GaN based nanopillar (NP) photoanodes fabricated by using top-down approach and their PEC performance. GaN based NPs were fabricated by inductively coupled plasma-reactive ion etching (ICP-RIE) of randomly mask patterned GaN wafers, grown in metal organic chemical vapour deposition (MOCVD). Scanning electron microscope and micro-photoluminescence were employed to study the structural and optical quality of the NPs respectively. PEC studies of the photoanodes were carried out in 1M NaOH electrolyte under one sun illumination using a three-electrode configuration PEC set up. From our results, the NP photoanodes exhibit superior PEC performance compared to their counterpart planar photoanodes. Carrier concentration, NP dimensions and In content in InxGa1-xN influenced the onset potential and over-potentials of the photoanodes. Moreover, In content only affects the photocurrent density of the NP photoanodes and not that of the planar photoanodes. Photoluminescence, diffuse reflectance and electrochemical impedance measurements are used to further understand these results.
Acknowledgment
We acknowledge the Australian Research Council for financial support and the Australian National Fabrication Facility for providing access to the growth and fabrication facilities
References
Parvathala Reddy Narangari, Siva Krishna Karuturi, Mykhaylo Lysevych, Hark Hoe Tan, and Chennupati Jagadish, “Improved Photoelectrochemical Performance of GaN Nanopillar Photoanodes” Nanotechnology 28, 154001(2017)
N. Parvathala Reddy, Shagufta Naureen, Sudha Mokkapati, Kaushal Vora, Naeem Shahid, Fouad Karouta, Hark Hoe Tan, Chennupati Jagadish, “Enhanced luminescence from GaN nanopillar arrays fabricated using a top-down process”, Nanotechnology 27, 065304 (2016)
8:00 PM - ES02.09.06
Li Electrochemical Tuning of Metal Oxide for Highly Selective CO2 Reduction
Kun Jiang 1 , Haotian Wang 1
1 , Harvard University, Cambridge, Massachusetts, United States
Show AbstractEngineering active grain boundaries (GBs) in oxide-derived (OD) electrocatalysts is critical to improve the selectivity in CO2 reduction reaction (CO2RR), which is becoming an increasingly important pathway for renewable energy storage and usage. Different from traditional in situ electrochemical reduction under CO2RR conditions, where some metal oxides are converted into active metallic phases but with decreased GB densities, here we introduce the Li electrochemical tuning (LiET) method to controllably reduce the oxide precursors into interconnected ultrasmall metal nanoparticles with enriched GBs. By using ZnO as a case study, we demonstrate that the LiET-Zn with freshly exposed GBs exhibits a CO2-to-CO partial current of ∼23 mA cm−2 at an overpotential of −948 mV, representing a 5-fold improvement from the OD-Zn with GBs eliminated during the in situ electro-reduction process. A maximal CO Faradaic efficiency of ∼91.1% is obtained by LiET-Zn on glassy carbon substrate. The CO2-to-CO mechanism and interfacial chemistry are further probed at the molecular level by advanced in situ spectroelectrochemical technique, where the reaction intermediate of carboxyl species adsorbed on LiET-Zn surface is revealed.
8:00 PM - ES02.09.07
Enhancing the Solar Energy Conversion Efficiency of Bi2S3 Thin Films by Tailored Annealing
Zhehao Zhu 1 , Satish Iyemperumal 1 , Kateryna Kushnir 1 , N. Aaron Deskins 1 , Lyubov Titova 1 , Ronald Grimm 1 , Drew Brodeur 1 , Pratap Rao 1
1 , Worcester Polytechnic Institute, Worcester, Massachusetts, United States
Show AbstractNon-toxic metal sulfides with moderate band gaps are desired for efficient generation of electricity or fuels from sunlight via photovoltaic or photoelectrochemical energy conversion. Bi2S3 is a non-toxic n-type semiconductor, which has been commonly synthesized in the form of quantum dots or nanocrystalline films by the successive ion layer adsorption and reaction (SILAR) method. Despite a favorable optical band gap of 1.4 eV, such films have not achieved high solar energy conversion efficiencies to date. We hypothesize that this is in part due to the presence of sulfur vacancies that, according to our density functional theory calculations, form a deep trap state in the band gap of Bi2S3, which can act as a strong recombination channel for photoexcited charges. Here, we report a microcrystalline Bi2S3 thin-film photoanode synthesized by high-temperature annealing of solution-deposited nanocrystalline Bi2S3 in a sulfur vapor environment, which simultaneously increases the grain size and phase purity of Bi2S3, fills in sulfur vacancies, and improves optical absorption. Time-resolved terahertz spectroscopy (TRTS) reveals that sulfur annealing increases the photoexcited carrier lifetime from sub-picosecond to ~30 picosecond, while the internal quantum efficiency of a photoelectrochemical device is increased 4-fold from ~10% to ~40%. In addition, TRTS reveals that the intra-grain carrier mobility in the sulfur-annealed films is ~165 cm2/Vs and the long-range mobility is ~111 cm2/Vs at short times, indicating that carries are able to hop across grain boundaries. These results indicate that annealing in sulfur vapor can produce high solar energy conversion efficiencies in Bi2S3 by achieving carrier diffusion lengths of several hundred nanometers, which is similar to the light absorption depth.
8:00 PM - ES02.09.08
Z-Scheme-Type Photocatalyst Panels Covered with Amorphous Oxide Layer for Efficient Water Splitting under Ambient Pressure
Sayuri Okunaka 1 2 , Hiromasa Tokudome 1 2 , Qian Wang 3 , Takashi Hisatomi 3 , Qingxin Jia 3 , Kazunari Domen 3
1 Research Institute, TOTO LTD., Chigasaki-city, Kanagawa, Japan, 2 , Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem), Tokyo Japan, 3 Chemical System Engineering, The University of Tokyo, Bunkyo-ku, Tokyo, Japan
Show AbstractPhotocatalytic water splitting has attracted considerable attention due to the potential application for clean hydrogen production from water by utilizing abundant solar light. Photocatalyst panels, in which films composed of photocatalyst particles are fixed on an inexpensive substrate, require neither energy nor troublesome process, and thereby have a potential to be employed as a cost-effective system on a large scale.1)
Our group recently prepared the composite-type photocatalyst panels consisting of a mixture of visible-light responsive photocatalysts (Rh-doped SrTiO3 and BiVO4) and gold (Au) nanocolloids, by simple screen-printing on glass substrates.2) We found that the resulting sheets can split water into H2 and O2 via the Z-scheme type mechanism with a relatively high efficiency. However, the photocatalytic water splitting activity of the sheets decreased with increasing gas pressure in the system because of the undesirable backward reactions (e.g. water formation from the evolved H2 and O2, photoreduction of O2).
In this work, we employed the amorphous oxide layer, which is inactive for backward reactions, on the photocatalyst panels to achieve efficient water splitting.
References:
1) A. Xiong, G. Ma, K. Maeda, T. Takata, T. Hisatomi, T. Setoyama, J. Kubota, K. Domen, Catal Sci Technol., 2014, 4, 325–328.
2) Q. Wang, T. Hisatomi, Q. Jia, H. Tokudome, M. Zhong, C. Wang, Z. Pan, T. Takata, M. Nakabayashi, N. Shibata, Y. Li, I-D. Sharp, A. Kudo, T. Yamada, K. Domen, Nat. Mater., 2016, 15, 611–615.
8:00 PM - ES02.09.10
Investigating Organic Photoabsorbers for Hybrid Organic/Inorganic Photoelectrochemical Water Splitting Devices
Antonio Alfano 1 2 , Alessandro Mezzetti 2 3 , Francesco Fumagalli 2 , Fabio Di Fonzo 2
1 Chemistry, Material and Chemical Engineering "Giulio Natta", Politecnico di Milano, Milan Italy, 2 , Istituto Italiano di Tecnologia - Center for Nano Science and Technology, Milan, MI, Italy, 3 Energy, Politecnico di Milano, Milan, MI, Italy
Show AbstractThe field of hybrid organic photoelectrochemical (HOPEC) water splitting is rapidly emerging as a clean technology employing organic semiconductors for the production of molecular hydrogen as a renewable chemical fuel alternative to the intensively exploited hydrocarbons. Great effort is currently dedicated to the realization and optimization of hybrid photoelectrodes to efficiently harvest the solar radiation. While good results have been achieved by properly tuning the materials used as selective contacts1, few work has been done on testing and developing organic semiconductors heterojunctions – besides P3HT:PC61BM blends – that can efficiently perform in the framework of a PEC cell.
Past projects from our research group allowed to develop good performing architectures that enhanced the properties of the state-of-the-art bulk heterojunction P3HT-PC61BM, either improving its photocurrent performances2 or extending its operational lifetime to the range of several hours3. Using these results as the starting point, we moved towards investigating other polymeric photo-absorbers using as guidelines the results obtained in the field of organic photovoltaic (OPV).
In our contribution we present the most relevant outcomes of our research with emphasis on the most promising materials for HOPEC applications. Various categories of organic materials are to be presented, on which a preliminary characterization has been performed. Main relevance is given to the new class of donor-acceptor co-polymers, which have shown great potential in the field of OPV. Promising results are provided for recently developed materials, such as PCDTBT, PTB7 and PCE10. Moreover, non-fullerene acceptors like IDTBR and IDFBR are tested in combination with the aforementioned polymers, since the peculiar molecular structure of this class of materials is found to be responsible of a sharp increase in open circuit voltage4-5. With the aim of extending the current library of suitable organic semiconductors for HOPEC applications, a careful electrochemical characterization is performed on each organic material, testing their photoactivity in a complete and optimized photocathodic architecture. Furthermore, electrochemical impedance spectroscopy (EIS) measurements are performed to evaluate the dynamics of photogenerated charge carriers and how they are affected by the presence of the electrolytic environment with respect of the intensively studied behaviour in air.
(1) Fumagalli, F. et al. J. Mater. Chem. A 2016, 4, 2178–2187.
(2) Rojas, H.et al. Energy Environ. Sci. 2016, 9, 3710-3723 .
(3) Mezzetti, A. et al. Faraday Discuss. 2017, 198, 433–448.
(4) Baran, D. et al. Energy Environ. Sci. 2016, 9, 3783–3793.
(5) Baran, D et al. Nature Mat. 2017, 16, 363-369.
8:00 PM - ES02.09.11
Dynamic Semiconductor Junctions Based Photoelectrochemical Cells
Jinyoung Jung 1 , Jinyonug Yu 1 , Jung-Ho Lee 1
1 , Hanyang University, Ansan Korea (the Republic of)
Show AbstractThe built-in potential at the semiconductor junctions is a thermodynamic driving force for separating charge carriers of electrons and holes, which is fundamental requisite in photoelectrochemical (PEC) water splitting devices. While, in general, a variety of solid-state junctions are developed the built-in potentials fixed during PEC operation (noted as static junctions), recently, dynamic change of built-in potentials have been realized by adopting a new type of semiconductor junctions (so-called dynamic junctions). Here, a fundamental question we can have is on whether the conventional PEC operation principle well-established in the static junction can be adopted in the dynamic junction. To verify this, we systematically investigated and compared the PEC performances for the static junction and the dynamic junction. The dynamic junction is found to reveal distinct result that a maximum thermodynamic potential generated in PEC device (i.e., open circuit potential, Voc) could exceed a photovoltage value regarding the photovoltaic losses evolved in charge generation, separation, and recombination. These finding offer insight into a new way in maximizing the Voc of PEC devices.
8:00 PM - ES02.09.12
Gold Nanoparticle Decorating Hematite Nanorod Enhanced Sunlight-Driven Water Splitting Activity
Aryane Tofanello 2 , Andre Luiz Freitas 2 , Turkka Salminen 1 , Harri Ali-Löytty 1 , Kimmo Lahtonen 1 , Waldemir Carvalho 2 , Mika Valden 1 , Tapio Niemi 1 , Flavio De Souza 2
2 , Federal University of ABC , Santo André Brazil, 1 , Tampere University of Technology, Tampere Finland
Show AbstractN-type hematite semiconductor is the most recognized metal transition oxide material, as a photoanode, for photoelectrochemical (PEC) cell, but it commercial application is limited to the sluggish water oxidation kinetics, poor photogenerated charge separation, etc. It’s well known that the metal nanoparticles [such as gold (AuNPs)] under visible light irradiation exhibit a localized surface plasmon resonance (LSPR), that combined with metal transition oxide might enhance the photon penetration through the oxide structure leading higher PEC device efficiency. Herein we investigated the AuNP modified hematite vertical nanorod with different length (film thickness) designed using a chemical route under hydrothermal condition at low temperature. The hematite nanorod surface were decorated by electrophoretic deposition of AuNPs with nominal diameters of 20 nm or 50 nm. The highest photocurrent performance at 1.23 VRHE was observed in a thinner hematite nanorod and AuNPs film. The role of AuNPs decorating hematite nanorod were investigated by X-ray diffraction technique showing an expected single X-ray pattern related to the hematite phase, while the SEM and TEM images revealed the presence of AuNPs on the hematite surface. Electrochemical impedance spectroscopy (EIS) showed that the thinner AuNPs deposited over the nanorod surface decrease the resistance for charge transport increasing the electronic conductivity and the material performance under simulated PEC application. This work was supported by FAPESP grant 2014/50516-6 and Academy of Finland grant 2014/284652.
8:00 PM - ES02.09.13
Synthesis of Tungsten Disulfide Nano-Porous Film and the Effect of Its Orientation and Bonding on Solar Energy Conversion Behavior
Tao Yan 1 , Pratap Rao 1 , Morgan Stefik 2 , Kayla Lantz 2 , N. Aaron Deskins 1
1 , Worcester Polytechnic Institute, Worcester, Massachusetts, United States, 2 Chemistry & Biochemistry, University of South Carolina, Columbia, California, United States
Show AbstractA typical semiconductor in the transition metal dichalcogenide (TMD) family, tungstendisulfide (WS2), has drawn lots of interest for solar energy conversion because of its unique optical and electrical properties. However, because of the layered structure of WS2, the orientation of WS2 crystals and the bonding between WS2 and other materials has a large influence on its performance. Our incident photon-to-current conversion efficiency (IPCE) and photoelectrochemical (PEC) measurements on single crystal WS2 with and without a protective TiO2 coating demonstrate that edge-on WS2 crystal orientation results in higher charge transport efficiency than does parallel orientation. Therefore, we have successfully synthesized IF-WS2 (inorganic fullerene-WS2) thin films by sulfurization of WO3 nano-porous thin films, preserving the original morphology. We anticipate that such films can take advantage of the superior edge-on charge transport to achieve efficient solar energy conversion from WS2.
8:00 PM - ES02.09.15
Highly Efficient Ambient Temperature CO2 Photomethanation Catalysed by Nanostructured RuO2 on a Silicon Supports
Abdinoor Jelle 1 , Paul O'Brien 2 , Mohamad Hmadeh 3 , Kulbir Ghuman 1 , Chandra Singh 1 , Doug Perovic 1 , Geoffrey Ozin 1
1 , University of Toronto, Toronto, Ontario, Canada, 2 , York Univeristy, Toronto, Ontario, Canada, 3 , American University of Beirut, Beirut Lebanon
Show AbstractSunlight-driven conversion of greenhouse gas CO2 to value-added chemicals is of great technological importance and has the potential to ultimately provide a more sustainable alternative to fossil fuels. Of particular interest is gas-phase photomethanation of CO2 using nanostructured Sabatier reaction catalysts. Here, we show that the Sabatier reaction can be activated using high intensity solar simulated light over nanostructured RuO2 nanocrystals grown on one-dimensional black silicon nanowire support. Additionally, we report the first example of photomethanation of CO2 over highly dispersed nanostructured RuO2 catalysts on three-dimensional silicon photonic crystal supports.
Photomethanation rates as high as 8 mmolgcat-1hr-1 and 4.4 mmolgcat-1hr-1 at ambient temperatures under high intensity solar simulated irradiations have been achieved using one-dimensional silicon nanowires support and three three-dimensional silicon photonic crystal support respectively. These rates are more than an order of magnitude greater than the thermal reaction for the same catalyst in the dark. The high absorption strengths, low reflective losses and unique light harvesting properties of the silicon-based supports across the entire solar spectral wavelength range coupled with the large surface area provided by the RuO2 nanoparticles are proposed to be responsible for the high methanation rates of the photocatalyst. Furthermore, DFT calculations were performed in order to understand the interaction of the CO2 and the H2 with the photocatalysts and a CO2 photomethanation mechanism was elucidated. Continued improvement of light-powered Sabatier reaction catalysts could enable the development of solar refineries for converting gas-phase CO2 to value-added chemicals and fuels.
8:00 PM - ES02.09.16
A Novel Approach to a Manganese Nitride for High-Performance Electrocatalytic Water Oxidation
Carsten Walter 1 , Prashanth Menezes 1 , Matthias Driess 1
1 , Technical University of Berlin, Berlin Germany
Show AbstractRising demand for energy and decreasing resources of fossil fuels will affect both, economy and ecology. Although, several materials are known to be active for hydrogen evolution reaction (HER), the bottleneck in overall water splitting is the oxygen evolution reaction (OER), and is challenging due to the transfer of four electrons and four protons with sluggish kinetics. In nature, plants convert photon energy into chemical energy within the oxygen evolving center (OEC) of the photosystem II (PSII) which contains manganese and calcium as the metal centers to form a cubane-like Mn4CaO5 cluster. Motivated by this, numerous bioinspired heterogeneous manganese based catalysts such as oxides, phosphates and even mixed manganese oxides have been extensively explored in the last few years, however, the attained OER activities still need to be improved in order to have their use in practical applications.
Here we present, the synthesis of a manganese nitride (Mn3N2) starting from a molecular precursor using a simple nitridation and further investigate it for water oxidation catalysis for the first time. The “bottom up” approach with molecular precursors to synthesize materials often has numerous advantages over the conventional synthetic techniques that includes relatively low-temperature synthesis, rational access and shape selectivity of materials on the nanoscale, tunable electronic properties, well-defined distribution, control of particle size as well as better homogeneity in the product. The as-synthesized Mn3N2 was well characterized by various spectroscopic, microscopic and analytical methods. The Mn3N2 catalyst was electrophoretically deposited on fluorine doped tin oxide (FTO) and nickel foam (NF) working electrodes and tested for OER using a three electrode set-up. Remarkably, the Mn3N2 exhibited superior catalytic activity with merely an overpotential of 390 mV on FTO and 270 mV on NF at a current density of 10 mAcm-2. The attained activity here was clearly higher than recently reported manganese based catalysts as well as comparable to precious metal oxides. In addition to the electrocatalytic activity, we successfully uncover surface-structure, structure-activity relation as well as the nature of active species involved in catalysis of Mn3N2 and will be presented.
8:00 PM - ES02.09.17
Nanostructured Ga0.51In0.49P Photocathodes for Broadband Absorption Enhancement in Solar Water Splitting
Haneol Lim 1 , James Young 3 , Dongseok Kang 1 , Todd Deutsch 3 , Jongseung Yoon 1 2
1 Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Department of Electrical Engineering, University of Southern California, Los Angeles, California, United States
Show AbstractPhotoelectrochemical (PEC) solar water splitting has attracted growing attention due to its potential to economically produce hydrogen using solar energy. III-V compound semiconductors represent excellent materials candidate for photoelectrodes in solar water splitting due to their superior photophysical properties for generating and transporting photogenerated charge carriers. However, optical losses at the semiconductor/electrolyte interface due to Fresnel reflection (~25%) limit the absorption of light and thus efficiency in solar-driven PEC water splitting. Here we present an approach that can address this challenge by introducing a random nanostructure on the electrode surface for broadband antireflection and absorption enhancement. Silver nanoparticles were electrolessly deposited on the surface of Ga0.51In0.49P photocathodes with and without buried pn-junction, which served as an etch mask in subsequent inductively coupled plasma reactive ion etching (ICP-RIE). The resulting randomized nanostructure with a graded refractive index exhibited excellent antireflection properties (less than 1%) over all wavelengths in the visible region. Detailed morphological, optical, and photoelectrochemical studies of nanostructured Ga0.51In0.49P photocathodes together with numerical optical modeling elucidated underlying physics and optimal design rules in the reported system.
8:00 PM - ES02.09.19
Probing Facet-Dependent Energetics and Kinetics of Photoanode towards Solar Water Oxidation—Case Study on Hematite
Wei Li 1 , Yumin He 1 , Qi Dong 1 , Da He 1 , Yanyan Zhao 1 , Dunwei Wang 1
1 , Boston College, Chestnut Hill, Massachusetts, United States
Show AbstractSolar energy is considered as one of the most ideal renewable energies, which can meet the rising energy demand of human beings. Given its intermittent nature, it is desired to convert solar energy in chemical bonds for ease of storage, transportation and utilization. Photoelectrochemical (PEC) water splitting promises a direct route for solar-to-chemical energy conversion. Despite decades of intense research, the efficiency remains low. In addition with discovering new material candidates, significant efforts have been devoted to understanding the mechanisms behind the low performance. It is noted that charge separation capability within space charge region and surface charge transfer kinetics greatly influence PEC performance. However, the link between surface properties and PEC performance is still missing due to the complexity of surface heterogeneity. Specifically, the uncertainty of exposed facets complicates the investigations, especially for nanostructured materials.
Here, we apply hematite as a platform, systematically compare (001) and (012) facet exposed photoanode on particle level, featuring disc and cube morphology, respectively. We are aiming to provide insights about how different exposed facet contributes to surface energetics & kinetics, and its dependency on PEC activity towards water splitting. The combination of intensity modulated photocurrent spectroscopy (IMPS), photoelectrochemical impedance spectroscopy (PEIS) and open circuit potential (OCP) measurement have been adopted to probe the surface kinetics & energetics accordingly. From kinetics perspective, although (012) cubes exhibit better charge transfer rate constant, on the other hand, these more active sites also act as surface hole/electron recombination centers, which is consistent with the previous report on TiO2. From energetics perspective, undesired Fermi level pining is induced on (012) cubes, resulting in smaller band bending. Additionally, (001) discs has more cathodic flat band potential promising larger photovoltage. These revealed information could help us understand different current-voltage characteristics caused by exposed facet, more importantly, establish correlation between surface heterogeneity and PEC activity towards water splitting.
8:00 PM - ES02.09.20
An Electrochemical Cycling Strategy towards Sustainable Ammonia Synthesis Using N2 and H2O at Atmospheric Pressure
Joshua McEnaney 1 , Aayush Singh 1 , John Schwalbe 1 , Jakob Kibsgaard 1 , John Lin 1 , Matteo Cargnello 1 , Thomas F. Jaramillo 1 , Jens Norskov 1
1 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractAmmonia production for fertilizer is imperitive for sustaining the massive and growing human population, and is also reasonably attractive as an alternative fuel or hydrogen storage chemical. However, the primary method of synthetic ammonia production, the Haber Bosch process, is resource demanding and unsustainable. The massive scale of ammonia production of consumes over 1% of the entire global energy supply and releases ~450 million metric tons of CO2 into the atmosphere each year due to its dependence on fossil fuels for H2 production. In this work, we demonstrate a generalized alternative pathway toward sustainable ammonia synthesis via an electrochemical cycling strategy. This electrification strategy is amenable to be powered by renewable energy and to be locally implemented where the ammonia fertilizer needed by circumventing the high pressure and H2 gas requirements of the Haber Bosch process and instead using electricity and H2O. The process can be summarized by three distinct steps of surface preparation, nitrogen activation, and ammonia synthesis which can be combined and cycled for continuous ammonia production. The feasibility and general applicability of this cycle were evaluated with theoretical analyses of potential dependent material thermodynamics and relevant diffusion energy barriers. The process steps are characterized with specific electrochemical and materials chemistry techniques. Ammonia is quantified by UV-Visible colorimetric analysis and confirmed to be from N2 via isotopic labeling in Fourier transform infrared radiation studies. We experimentally demonstrate an initial current efficiency of 88.5% toward ammonia production via this process and we begin to evaluate the potential of this strategy for viability. Our efforts continue to expand this strategy to novel materials systems and processes.
8:00 PM - ES02.09.21
Cu-Delafossite Double-Shelled 2D Opal Photocathodes for Enhanced Photoelectrochemical Response
Yunjung Oh 1 , Wooseok Yang 1 , Jeiwan Tan 1 , Hyungsoo Lee 1 , Jaemin Park 1 , Jooho Moon 1
1 , Yonsei Univ, Seoul Korea (the Republic of)
Show AbstractPhotoelectrochemical (PEC) water splitting that converts the solar light into hydrogen carrier has received significant attention as a promising strategy to address the energy crisis and environmental pollution issues. In the past few years, enormous effort has been devoted to novel nanostructured photoelectrodes in order to boost the photoresponse capabilities. Both opal and inverse-opal nanostructured photonic crystals have drawn noticeable interests for PEC applications due to their light-harvesting abilities as slow light. Recently, Oh et al. reported 2 dimensional (2D) opal photocathode composed of a dry-state rubbing-induced monolayer of CuFeO2 shelled microspheres (ACS Appl. Mater. Interfaces 2017, 9, 14078). Importantly, it was firstly proposed as a new strategy that the opal photonic crystal is applied to PEC devices, showing comparable photocurrents to thin film counterpart. However, 2D opal CuFeO2 photocathode struggled between the effective slow light size of microspheres and short diffusion length of photogenerated carriers, resulting in low incident-photon-to-electron conversion efficiency. Therefore, the new approach is needed to further improve the photocurrent density by enhancing the charge transport and separation ability. Generally, constructing double-shelled heterojunction of dissimilar semiconductors have been usually regarded as an effective method to promote charge separation and prolong the lifetime of photogenerated carriers for inverse-opal PEC devices (e.g., TiO2-CdS, WO3-BiVO4, etc.). However, synthesizing monodispersed microspheres of double-shelled photoactive into opal structure poses a great challenge. Herein, we present a new synthetic pathway to fabricate 2D opal composed of assembled porous silica microspheres decorated with inner CuFeO2/outer CuMO2 (M = Ga or Al). The well-engineered Cu-delafossite double-shelled photocathode exhibits an increase in the photocurrent than CuFeO2 single-shelled photocathode. More precisely, surface photovoltage spectra and electrochemical analysis provide us to understand the factors affecting photocurrent enhancement in which the formation of a CuFeO2-CuMO2 junction assists electron–hole separation. This work sheds light on the importance of the heterojunction interfaces for achieving optimal charge separation in opal architecture.
8:00 PM - ES02.09.22
Large-Scale Fabrication of Z-Scheme-Type Photocatalyst Panels Using a Nanoparticulate ITO Electron Mediator for Solar Water Splitting
Hiromasa Tokudome 1 2 , Sayuri Okunaka 1 2 , Hiroyuki Kameshige 1 2 , Toshio Nakamura 1 2 , Qian Wang 3 , Takashi Hisatomi 3 , Qingxin Jia 3 , Kazunari Domen 3
1 Research Institute, TOTO Ltd., Chigasaki Japan, 2 , Japan Technological Research Association of Artificial Photosynthetic Chemical Process (ARPChem), Tokyo Japan, 3 Chemical System Engineering, The University of Tokyo, Bunkyo-ku, Tokyo, Japan
Show AbstractPhotocatalytic water splitting utilizing solar energy has been attracted much attention as a promising technology for renewable hydrogen production. For the practical application, it is necessary to develop a scalable, versatile and cost-effective system along with highly efficient visible light responsive photocatalysts capable of effectively utilizing sunlight. The authors’ group reported that photocatalyst panels, wherein porous GaN:ZnO film mixed with SiO2 particles was immobilized on a glass substrate, exhibited comparable activity to that of a conventional powder suspension1). We also recently developed a new photocatalyst panel consisting of a hydrogen evolution photocatalyst, an oxygen evolution photocatalyst and a gold nanocolloid as an electron mediator for Z-scheme water splitting2). This panel showed relatively high activity with a solar-to-hydrogen energy conversion efficiency of 0.1%. In this study, we prepared a large-scale Z-scheme-type photocatalyst panel using a low-cost indium-tin-oxide (ITO) nanoparticle as a transparent mediator and examined the water splitting activity.
Z-scheme-type photocatalyst panels were fabricated by a scalable screen-printing method using a viscous paste including two kinds of visible-light-responsive photocatalyst particles (La, Rh-codoped SrTiO3 and Mo-doped BiVO4 for hydrogen and oxygen evolution, respectively) and ITO nanocolloids. The panels produced hydrogen and oxygen from pure water in the stoichiometric ratio under visible light irradiation. The water splitting activity was greatly enhanced with the increase in the amount of ITO colloids, indicating that ITO nanoparticles served as an electron mediator between two photocatalysts. The size of the Z-scheme-type panel can be expanded to 30×30 cm2.
1) A. Xiong, G. Ma, K. Maeda, T. Takata, T. Hisatomi, T. Setoyama, J. Kubota, K. Domen, Catal. Sci. Technol., 2014, 4, 325–328.
2) Q. Wang, T. Hisatomi, Q. Jia, H. Tokudome, M. Zhong, C. Wang, Z. Pan, T. Takata, M. Nakabayashi, N. Shibata, Y. Li, I-D. Sharp, A. Kudo, T. Yamada, K. Domen, Nat. Mater., 2016, 15, 611–615.
8:00 PM - ES02.09.23
Overcoming the Limitations of Bismuth Vanadate Photoanodes through Sulfur Incorporation
Marlene Lamers 1 , Wenjie Li 2 , Marco Favaro 1 , David Starr 1 , Roel Van de Krol 1 , Lydia Wong 2 , Fatwa Abdi 1
1 Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin Germany, 2 School of Materials Science and Engineering, Nanyang Technological University, Singapore Singapore
Show AbstractMetal oxides are attractive photoelectrode materials for photoelectrochemical (PEC) water splitting, due to their stability in aqueous solutions and low cost. Among them, bismuth vanadate (BiVO4) is particularly interesting, since it is currently the highest performing metal oxide photoanode [1,2]. However, with reported photocurrents already within 90% of the theoretical maximum [1], the solar-to-hydrogen conversion efficiency of BiVO4 is limited by its relatively wide bandgap of 2.4 eV. It is also still subjected to its poor carrier transport properties; high photocurrents (> 5.0 mA/cm2) are only achieved with nanostructuring, which poses additional complexities (e.g., light scattering) for the design of a PEC tandem device for water splitting. In this work, we demonstrate that the limiting factors of BiVO4 can be alleviated by the incorporation of sulfur (S). By incorporating different sulfur concentrations, we observed a systematic decrease in the bandgap of BiVO4, up to 300 meV. This result is further supported by performing hard X-ray photoelectron spectroscopy (HAXPES), which shows that the valence band of the sulfur incorporated BiVO4 is shifted to smaller binding energies (i.e., closer to the Fermi level). Simultaneously, time resolved microwave conductivity (TRMC) measurements reveal an improvement of charge carrier transport by the incorporation of sulfur; the mobility increases from 0.06 cm2V-1s-1 for the unmodified BiVO4 to 0.13 cm2V-1s-1 for the sulfur incorporated BiVO4. A complete phase map was constructed as a function of the sulfur concentration, from pure monoclinic BiVO4 to fully sulfurized BiVO4, by performing X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF). Based on these results, the interplay between the concentration of sulfur and the structural, optical, and electronic properties in BiVO4 as well as the PEC performance were systematically elucidated.
[1] Pihosh et al., Sci. Rep. 5:11141 (2015)
[2] Abdi et al., Nat. Commun. 4:2195 (2013)
8:00 PM - ES02.09.24
Nanostructured P-Si with TiO2 Passivation Layer and Metal Catalyst for Stable and Efficient Water Splitting
Changyeon Kim 1 , Ho Won Jang 1
1 Materials Science and Engineering, Seoul National University, Seoul, Seoul, Korea (the Republic of)
Show AbstractGlobal warming, caused by greenhouse gases, has been emerging as one of the biggest environmental issue. The raising temperature could be very harmful to us and our earth. The greenhouse gases, such as carbon dioxide, methane, nitrous oxide, is emitted through the burn of fossil fuels. Therefore, new eco-friendly energy source, can be replaceable the fossil fuels, and production of that are needed to be developed.
The solar water splitting has been attractive to promising approach. Generating hydrogen from water by solar energy allows for an unlimited amount of hydrogen to be stored and used as green fuels. For efficient production of hydrogen, there are several key factors to be needed; 1 proper energy band gap to absorb sunlight, 2 larger light absorption, 3 fater charge separation to prevent recombination of electron and hole pairs, 4 stability.
p-Si photocathode has narrow band gap (1.12 eV) to absorb the wide range of the solar spectrum. So it could generate many electron-hole pairs and so larger photocurrent could be obtained. And nanostructured p-Si play a role to scatter the solar light, so enhance the absorption of solar energy. However, the silicon photocathode is unstable at electrolyte and suffers from fast photocorrosion due to the position of thermodynamic redox potentials, and the solar to hydrogen conversion efficiency of bare silicon photocathode is largely suppressed. To solve this problem, desigining a surface passivation layer that can protect against chemical and photo-induced corrosion has beed studied these days. Also, some metal catalysts have been reported to be enable to enhance the efficiency of solar to hydrogen efficiency.
In this study, we are going to fabricate nanostructured p-Si with TiO2 passivation layer and deposit metal catalysts onto that. TiO2 layer, also, forms type 2 heterojunction with the p-Si and produces a band offset, which can improve the efficiency of separation of electron-hole pairs while preventing recombination. Finally TiO2 passivation layer would improve the statibility by preventing photocorrosion of p-Si. And metal catalysts would enhance the efficiency.
8:00 PM - ES02.09.25
Formation of Nanoporous Structure of Metal Catalysts by High Pressure Thermal Evaporation and Electrochemical Applications
Sangwoo Ryu 1 , Minhyung Cho 1 , Jaehoon Kim 1 , Kwang Min Baek 3 , Hyungcheoul Shim 2 , Jihun Oh 1
1 Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon Korea (the Republic of), 3 Department of Materials Science and Engineering, Korea Institute of Science and Technology (KAIST), Daejeon Korea (the Republic of), 2 Department of Applied Nano-mechanics, Korea Institute of Machinery & Materials (KIMM), Daejeon Korea (the Republic of)
Show AbstractFabrication of highly porous structures with functionality is one of unending demands through conventional catalysis to recently emerging energy conversion and storage. Here we introduce a physical vapor deposition-based technique for the formation of high purity nanoporous structure of various metal catalysts. Typical evaporation process under high vacuum results in dense thin films of sources. However, simply by evaporating metal sources at relatively high pressure of a few Torr of inert gas, fractal-like nanoporous structures of metals can be achieved with porosity over 99%. It utilizes homogenous nucleation of evaporated atoms due to multiple collisions during the traverse in the vacuum chamber and diffusion-limited aggregation of nanoclusters on the substrate. The morphology and porosity can be controlled by adjusting process parameters such as back ground gas pressure, substrate temperature, evaporation rate, etc. We exhibit the evolution of nanoporous structures for various metal catalysts with different melting temperature as these parameters change. Moreover, we demonstrate how these nanoporous structures can be applied to electrochemical reactions such as battery and CO2 reduction.
8:00 PM - ES02.09.26
Fabrication of Micropatterned, Flexible, Transparent or Reflective Thin-Film Photoelectrodes via a Template Stripping Method for Enhanced Light Absorption
Silvan Suter 1 , Diana Moreno Garcia 1 , Sophia Haussener 1
1 , Laboratory of Renewable Energy Science and Engineering, Lausanne, Vaud, Switzerland
Show AbstractThin film metal oxide semiconductors have shown to overcome their major drawbacks of low bulk hole mobility by reducing the material thickness to a few nanometers. However, much thicker films are needed to achieve sufficient light absorption. Efforts have been invested in the structuring of the photoelectrodes to improve their light absorption, but the morphologies are either expensive to fabricate or their shapes strongly depend on the deposition method with limited control for optimization.
We developed a platform based on a template stripping method to fabricate enhanced light-absorbing patterned thin film photoelectrodes with a precise control on the microstructure at low costs. We utilized mature silicon patterning technologies to fabricate a negative silicon template for our photoelectrode design. After applying a sacrificial layer, a multilayered thin film photoelectrode is deposited in the inverse order, starting from the photoactive semiconductor, adding a conductive backing layer and finishing with a polyimide support layer. Anodic dissolution of the sacrificial layer in salt water strips the photoelectrode from the template, which can be reused multiple times.
We applied this method to fabricate patterned, thin film hematite (α-Fe2O3) photoelectrodes with i) transparent and ii) reflective backing layers. As templates, various wedge and pyramid pattern were fabricated by anisotropic KOH etching of silicon after masking the wafer by photolithography. An aluminum sacrificial layer was followed by a 20 nm thin atomic layer deposited hematite film. Indium tin oxide was used as a transparent conductive layer and a silver-gold alloy was utilized to create a reflective conductive layer.
The proposed fabrication process proved to have several advantages: The very precise, controllable microstructures from the silicon template can be transferred to the hematite photoelectrodes and the semiconductor layer benefits from the inverse approach by exhibiting a smooth surface. In addition, the hematite layer can be annealed at high temperatures before the backing layers are applied, avoiding thermal etching of the reflective metal layer and decomposition of the support substrate. And last, the fabricated photoelectrodes are flexible due to the non-rigid nature of the polyimide film.
Photoelectrochemical experiments showed up to 20% higher photocurrents for the best patterned electrode compared to a flat electrode. Given that those patterns of the electrode were not yet optimized, it shows the potential of the proposed approach. Furthermore, exact calculations of the light absorption solving the Maxwell equations will follow to determine the optimal micropattern with the corresponding semiconductor thickness. Our platform will allow us to transfer the calculated optimal configuration to a real photoelectrode.
8:00 PM - ES02.09.27
Hierarchical Titanium Nitride Scaffold for Silver- and Gold-Based Catalysts for CO2 Electrochemical Reduction
Giorgio Giuffredi 1 2 , Andrea Perego 1 2 , Simelys Hernandez 3 4 , M. Amin Farkhondehfal 3 , Guido Saracco 3 4 , Fabio Di Fonzo 1
1 , Center for Nano Science and Technology - Istituto Italiano di Tecnologia (IIT@Polimi), Milano Italy, 2 Department of Energy, Politecnico di Milano, Milano Italy, 3 Department of Applied Science and Technology (DISAT), Politecnico di Torino, Torino Italy, 4 , Center for Sustainable Future Technologies (CSFT@POLITO), Torino Italy
Show AbstractCarbon dioxide coming from the combustion of fossil fuels accounts for 65% of the global greenhouse gases emissions, playing a critical role in the current climate changes. In order to limit and reduce global CO2 emissions, several strategies for carbon dioxide capture, sequestration and reutilization have been considered.
Among these strategies, transforming CO2 into fuels with higher added value represents a smart approach, because it reduces CO2 emissions and converts renewable energies-generated electricity into the chemical energy of a fuel. In particular, the electrochemical reduction of CO2 represents an interesting technology to convert carbon dioxide to CO – and more complex hydrocarbons – in a cost-effective and environmental-friendly way. However, much research is still needed to find suitable catalysts and optimized working condition in order to produce a single compound from CO2 with a satisfying conversion efficiency and with limited overpotential applied.
In this contribution, we report about a Titanium Nitride (TiN) hierarchical nanostructure, employed as catalyst support for Ag e Au-based catalysts deposited via chemical or electrochemical methods. The mesoporous, quasi 1D, tree-like TiN structure is grown by Pulsed Laser Deposition (PLD), a process where morphology can be fine-tuned by controlling the gas dynamics during the deposition. With this technique, we are able to obtain a mesoporous scaffold with high surface area, up to 200 m2g-1.
The scaffold is then decorated with Au and Ag nanostructured catalysts, which catalyze CO2 reduction to CO with limited driving force and show high activity. By tuning the process parameters, different catalyst morphologies are obtained, from nanoparticles with 50-100 nm diameter to dendrites with high aspect ratio and high surface area.
The material is then characterized physically and electrochemically, showing good current density with a reduced overpotential for CO2 reduction. The influence of both scaffold and catalyst morphology on the electrochemical performances and activity is investigated, and a quantitative analysis of the reaction products is carried out.
This contribution show the potential of a combined PLD/chemical fabrication technique for a scaffold/catalyst material with tunable morphology and electrochemical properties, that displays a good performance for CO2 electrochemical reduction.
8:00 PM - ES02.09.28
Black Composite Nanotube Arrays for Enhanced Solar Fuel Generation
Mariam Elgamal 1 , Ahmed Khalifa 1 2 , Nageh Allam 1
1 , American University in Cairo, Cairo Egypt, 2 Material Science, Northeastern University, Boston, Massachusetts, United States
Show AbstractThe consumption of the world’s fossil fuel and their hazardous pollution impact on the atmosphere makes photoelectrochemical water splitting a clean and renewable means to generate hydrogen fuel. Quaternary oxide materials are promising photoanodes because they offer improved control in tuning their optical and electronic properties over simpler metal oxides. Through alloying, the electronic structure of TiO2 can be altered by introducing new states in the band gap or through the narrowing of the band gap itself. Molybdenum oxide and Iron oxide are of interest in photocatalysis because of their earth abundance and narrow band gap. We report on the novel fabrication of self-ordered, vertically oriented Ti-Mo-Fe (TMF) oxide nanotubes via anodization in aqueous, formamide, and ethylene glycol-based electrolytes containing NH4F at room temperature. Anodized samples were annealed at 500 °C in oxygen and hydrogen atmosphere. The samples were characterized by SEM, XRD and XPS techniques. The nanostructure topology was found to be dependent on electrolyte, anodization time and applied voltage. Transient current measurements under AM 1.5 illumination (100 mW cm-2) showed a 6-fold enhancement in current for the hydrogen annealed TMF over oxygen annealing. Dramatic shift in the flat band position was concluded from Mott−Schottky analysis. Donor density calculations revealed inter-band gap states in H2 annealed samples indicating the abundance of lower cation oxide states. The prepared catalyst shows a tremendous application in water splitting and hydrogen fuel generation.
8:00 PM - ES02.09.29
Engineering Interfaces for Enhanced Carrier Extraction, Stability, and Catalysis in Copper Chalcopyrite Photoelectrochemical Arrays
David Palm 1 , Thomas Hellstern 1 , Alex DeAngelis 2 , Nicolas Gaillard 2 , Thomas F. Jaramillo 1
1 Chemical Engineering, Stanford University, Stanford, California, United States, 2 Hawaii Natural Energy Institute, University of Hawaii, Honolulu, Hawaii, United States
Show AbstractIn collaboration with Dr. Nicolas Gaillard’s team at HNEI, thin film light absorbers from the copper chalcopyrite [CuInGa(Se,S)2] class of materials have been developed as highly photo-active electrocatalytic arrays. Previously, these materials have shown promise for photovoltaic applications given their utility as potentially scalable and bandgap-tunable thin-film light absorbers. As inherently p-type semiconductors, these films can be adapted as photocathodes for carrying out the hydrogen evolution reaction. In the Jaramillo group, our focus has been on understanding the complex interfaces between the light-absorber, carrier-extraction layer, protective overlayer, catalyst, and electrolyte in the photoelectrochemical device.
Poor energetic alignment between the Fermi level of these chalcopyrite materials and the reduction potential for producing hydrogen precludes optimal energy storage from a device comprised solely of a semiconductor-electrolyte junction. With this in mind, we have incorporated a thin film n-type semiconductor layer for the formation of a built-in electric field, in order to maximize the extraction of minority carriers. We have demonstrated promising results using cadmium sulfide (CdS) as such, with the measured photovoltage for these electrodes (> 0.75 V) rivaling that of all other Cu chalcopyrite photoelectrodes, to the best of our knowledge.1,2,3 Utilizing thin-film TiO2 and MoS2 as protective overlayers, and both MoS2 and Pt layers as catalysts, we have engineered these photoelectrode arrays to achieved sustained hydrogen evolution over the course of hours of continuous illumination in acidic electrolyte. Additionally, pairing this device in tandem configuration with a GaAs photovoltaic driver has allowed for the demonstration of sustained light-driven water splitting under simulated one-Sun illumination.
References
1 Moriya, Makoto, et al. Journal of the American Chemical Society, 2013, 135.10, 3733-3735.
2 Kumagai, Hiromu, et al. The Journal of Physical Chemistry C, 2014, 118.30, 16386-16392.
3 Mali, Mukund G., et al. ACS Applied Materials & Interfaces, 2015, 7.38, 21619-21625.
8:00 PM - ES02.09.30
Core-Shell Au@MxOy Nanoparticles for Enhanced Oxygen Evolution Catalysis
Alaina Strickler 1 , Maria Escudero-Escribano 2 , Thomas F. Jaramillo 1 3
1 Chemical Engineering, Stanford University, Stanford, California, United States, 2 Nano-Science Center, Department of Chemistry, University of Copenhagen, Copenhagen Denmark, 3 SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, California, United States
Show AbstractOxygen evolution electrocatalysis is important for many renewable energy technologies, including sustainable fuel production and energy storage via (photo)electrochemical water splitting, CO2 reduction, and metal-air batteries. Currently, efficiencies of many of these technologies are limited by the slow kinetics of the oxygen evolution reaction (OER), necessitating the development of high performance OER electrocatalysts. Recently, 3d-transition metal-oxide films have demonstrated significant OER activity enhancement when deposited on Au supports; however, observed enhancement is restricted to thin films with limited surface area and transferability to membrane electrode assembly systems. In this work, we translate the beneficial impact of Au supports to high surface area, device-ready, core-shell nanoparticles, where a Au-core is surrounded by an OER-active transition metal oxide-shell (Au@MxOy where M=Ni, Co, Fe, and CoFe). This presentation details the synthesis, material characterization, and electrochemical performance of Au@MxOy nanoparticles. Through a systematic investigation, we demonstrate universal activity enhancement of Au@MxOy nanoparticles over corresponding Au-free oxide nanoparticles. The best performing nanoparticle system, Au@CoFeOx, demonstrates both high activity and stability with an overpotential of 328 ± 3 mV over two hours at 10 mA cm-2, illustrating that coupling Au and mixed metal-oxide effects in a core-shell nanoparticle morphology is a promising route to high performance, device-ready OER catalysts.
8:00 PM - ES02.09.31
Chemical Vapor Deposited Graphene for Photocorrosion Inhibition in Electrodeposited Cu2O Photocathodes
Chandan Das 1 , K. R. Balasubramaniam 1
1 , Indian Institute of Technology Bombay, Mumbai India
Show AbstractHydrogen being a clean energy carrier can be a promising approach and long-term solution to store solar energy via photoelectrochemical (PEC) water splitting. One of the key problems associated with this process has been identified to be the lack of inherently stable absorber material in PEC condition. Also, non-toxic, earth-abundant materials possessing requisite properties are essential for device fabrication from an economic perspective. Here, a systematic investigation has been carried out using electrodeposited Cu2O as a light absorber, coated with chemical vapor deposited (CVD) graphene as a protective layer. CVD graphene being a carbon-based transparent conductor is highly robust in acid or basic medium also possesses a work function below Cu2O resulting in facile electron transfer. Graphene as a protective layer significantly enhances the life of Cu2O photocathode, retaining its own structural properties during PEC experiment. However, the formation of a few cracks in such a thin layer (less than 1 nm) while transferring on rough electrodeposited Cu2O cannot screen the underneath absorber layer completely. This aberration in graphene requires another suitable material on top for blocking the microcracks. Although the use of TiO2 on top offers complete protection, TiO2 itself loses its properties due to the amorphous nature followed by reductive transformation from Ti4+ to Ti3+. In this study, an alternative way is proposed for the complete protection of Cu2O photocathodes along with enhancing the photocurrent by modifying the Cu2O electrodeposition process.
Symposium Organizers
Thomas Fischer, University of Cologne
Fabio Di Fonzo, Istituto Italiano di Tecnologia
Rita Toth, Swiss Federal Laboratories for Materials Science and Technology (EMPA)
Mmantsae Diale, University of Pretoria
Symposium Support
Kenosistec
Nature Catalysis | Springer Nature
Sustainable Energy &
Fuels | The Royal Society of Chemistry
ES02.10: Materials Characterization and Modeling I
Session Chairs
Thursday AM, November 30, 2017
Hynes, Level 3, Room 306
8:30 AM - ES02.10.01
In Operando XAS Study of MnOx Oxygen Evolution Catalysts and MnOx Modified BiVO4 Photoanodes
Lifei Xi 1 , Christoph Schwanke 1 , Fatwa Abdi 1 , Sebastian Fiechter 1 , Klaus Ellmer 1 , Roel Van de Krol 1 , Kathrin Lange 1 2
1 , Helmholtz-Zentrum Berlin, Berlin Germany, 2 , Universität Bielefeld, Bielefeld Germany
Show AbstractPhotoelectrochemical (PEC) water splitting is a potentially scalable method to store solar energy in the form of renewable hydrogen fuels. BiVO4 has recently emerged as one of the most promising photoanode materials: favorable optical band gap (~2.4 eV), a favorable conduction and valence band edge position and relatively stability. Loading co-catalysts on BiVO4 photoanodes was previously found to be an effective way to improve the photocurrents and reduce the onset potentials. Manganese oxides (MnOx) have been widely recognized as promising catalysts for water oxidation.The information of the percentage of different Mn oxides under real conditions as well as metal oxidation state is impossible to be obtained by X-ray PDF, electrochemistry, XPS or UV-Vis spectroscopy. In this study, we in situ and operando studied MnOx oxygen evolution catalysts (OECs) using XAS in transmission mode and in situ EXAFS in fluorescence mode. XA spectra were obtained for freshly prepared and activated MnPi films under different potentials and after different potential durations. Via linear combination fitting the contribution of different Mn species was revealed from the L-edge spectra. The XAS results show that the freshly prepared film at OCP contains a dominant contribution of MnO2 (~75 %) and a contribution from a birnessite-like material (~25%). No or only neglectable percentage of MnO, Mn3O4 or Mn2O3-like Mn species were found in the freshly prepared sample. After 51 min of in situ activation, the birnessite-contribution increased to 75 % in the spectrum. We correlate these changes to the material conversion into an efficient OER catalyst. EXAFS results are found to be consistent with our L-edge spectra fitting results.
Although we studied MnOx as electrocatalyst using soft XAS in situ condition, it may not necessarily be the same when deposited on a semiconductor. We deposited MnOx on BiVO4 photoanode and found that the PEC performance of BiVO4 photoanodes can be improved after deposition of a MnOx catalyst layer. We further investigated the electronic structure of the layer measuring Mn L-edge XAS spectra using in situ and operando soft XAS by varying the applied potentials and illumination conditions. Using the linear combination method, information on different Mn species and Mn oxidation states could be obtained. We found that charge transfer at the MnOx / electrolyte interface is affected by band bending which relates to the applied and built-in potential. With increasing potential the electronic properties of the MnOx layer and its structure are changing, leading to a birnessite-type layer structure associated with an electron transfer from the MnPi film to the BiVO4 photoanode. The present work should benefit the potential applications of other OECs on photoanodes. The present work should benefit the potential applications of other OECs and OECs modified photoanodes.
8:45 AM - ES02.10.02
Mechanistic Investigation of Electrocatalytic Water Oxidation by Uniform 10 nm Sized MnO Nanoparticles
Hongmin Seo 1 , Kyoungsuk Jin 1 , Toru Hayashi 2 , Ryuhei Nakamura 2 , Ki Tae Nam 1
1 Department of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 , RIKEN Center for Sustainable Resource Science, Saitama Japan
Show AbstractWater splitting is regarded as an attractive process towards environmentally sustainable energy schemes because hydrogen energy generated from electrolysis is a promising alternative resource to fossil fuels without undesirable by-products. The slow kinetics of oxygen evolving reaction (OER), an anodic half reaction, has been a major bottleneck for the overall efficiency of the water splitting reaction. To progress OER, four electrons and four protons must be extracted from two water molecules and high activation energy is required for O-O bond formation, rate determining step for OER.
In nature, the water oxidation complex (WOC) of photosystem II efficiently catalyzes the OER under neutral conditions. The WOC consists of a cubical Mn4CaO5 cluster and its unique organic ligand environment. It mediates the OER with a markedly higher turnover frequency (TOF) number (~25,000 mmolO2 mol-1Mn s-1) and extremely lower overpotential (~160 mV) than have been achieved to date by synthetic catalysts. Inspired by WOC, Mn-based OER catalysts have been widely investigated for the further application of electrochemical water splitting.
Recently, we have been developed sub-10-nm-sized monodispersed partially oxidized manganese oxide nanoparticles (MnO NPs) which exhibit superior catalytic activity and unique electrokinetics under neutral condition, compared to their bulk counterparts.1 In the present work, to reveal the origin of the exceptional catalytic activity, we investigated the electrochemical water oxidizing mechanism mediated by the MnO NPs using integrated in-situ spectroscopic and electrokinetic analyses. We successfully demonstrated that, in contrast to previously reported manganese (Mn)-based catalysts, Mn(III) species are stably generated on the surface of MnO nanoparticles via a proton-coupled electron transfer pathway. Furthermore, we confirmed as to MnO NPs that the one-electron oxidation step from Mn(II) to Mn(III) is no longer the rate determining step for OER and that Mn(IV)=O species are generated as reaction intermediates during catalysis. Specifically, the sequential oxidation of Mn and generation of Mn(IV)=O species were directly monitored by various spectroscopic analyses, including EPR, in-situ XANES, UV-Vis, and Raman spectroscopy, by virtue of high surface-area-to-volume ratio of the MnO NPs. We believe that the unique water-oxidizing mechanism of the MnO NPs is attributed to the high catalytic activity of this material for the OER at neutral pH.
9:00 AM - *ES02.10.03
Structure-Activity Correlations for Cobalt-Based Oxides and (Oxy)Hydroxides as Oxygen Evolution Reaction (OER) Catalysts
Zhu Chen 1 , Bruce Koel 1
1 , Princeton University, Princeton, New Jersey, United States
Show AbstractTransition metal oxides (TMOs) are promising catalysts for the oxygen evolution reaction (OER) with the potential to replace precious metal-based catalysts (e.g. IrOx and RuOx). While significant improvements to the OER activity of TMOs have been made by tailoring the morphology and crystal structure of the catalysts, incorporating dopants, as well as using conductive supports, clear structure-activity correlations remain elusive due to the complex composition and structure of TMO catalysts. In this contribution, we report on utilizing a range of spectroscopic techniques for characterization to investigate structure-activity correlations for cobalt-based oxides and (oxy)hydroxides—an important class of OER catalysts. Through a series of systematic investigations, we have determined the (111) facet of Co3O4 to be by highly active by comparing the OER activity of morphology-controlled nanoparticles as well as the promoting effect of Ni incorporation in CoOOH toward the OER activity. The beneficial role of Ni was found to reduce charge transfer resistance and stabilize surface hydroxyl groups when incorporated into CoOOH based on impedance analysis and ambient pressure photoemission studies. Further investigation of the Ni-modified CoOxHy catalysts using operando Raman spectroscopy reveals transformation of a spinel Co3O4-like structure into a more active (oxy)hydroxide structure that accompanies improved OER activity with Ni incorporation.
9:30 AM - ES02.10.04
Real-Time Product Detection for the Electrochemical Reduction of CO2 via Selected-Ion Flow-Tube Mass Spectrometry
Peter Lobaccaro 1 2 , Lily Mandal 1 2 , Jens Martin 1 2 , Joel Ager 2 3 4 , T. Venky Venkatesan 1 2
1 , National University of Singapore, Singapore Singapore, 2 , Singapore Berkeley Research Initiative for Sustainable Energy, Singapore Singapore, 3 , Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States
Show AbstractThe analytical equipment utilized to detect the products of electrochemical CO2 reduction (CO2R) have remained largely unchanged since the seminal work performed by Hori in the 1980s.1,2 Typically online gas chromatography is used to periodically sample gaseous products and liquid products are sampled once at the end of an experiment using HPLC or NMR. However, real-time quantification of the gaseous and liquid products would be highly desirable. Particularly for the dynamics of liquid product formation, very little is known at this time due to a lack of suitable analytical equipment.
Some techniques have already been developed to address this problem. Notably, the differential electrochemical mass spectrometry (DEMS) system developed by Bell and co-workers3 and the online electrochemical mass spectrometer (OLEMS) system developed by Koper and co-workers.4 While these systems show promise, they have insufficient sensitivity or are unable to decouple, and thus detect, all of the products of CO2R, especially the liquid products. This deficiency can be generally ascribed to the use of electron ionization mass spectrometry (EI-MS).
Here, we present for the first time the use of selected-ion flow-tube mass spectrometry (SIFT-MS) to detect the liquid and gaseous products of CO2R in real-time. SIFT-MS utilizes “soft” chemical ionization, which creates simple 1 or 2 component fragmentation patterns per analyte, as opposed to the complex patterns produced in EI-MS, which enables the analysis of complex mixtures of similar compounds like that present in CO2R (up to 20 reported gaseous and liquid products). We further show that this analytical technique enables quantifiable detection, as confirmed by more traditional analytical techniques run in parallel. In this presentation some test cases for the applications of SIFT-MS are shown. The rapid screening of catalyst materials is demonstrated by greatly reducing the amount of time required to analyze the voltage dependent product distribution per catalyst. Furthermore we demonstrate how insight into the mechanism of CO2R can be provided by elucidating the time evolution of liquid products.
1. Y. Hori, A. Murata, R. Takahashi and S. Suzuki, J. Am. Chem. Soc., 1987, 109, 5022–5023.
2. Y. Hori, K. Kikuchi and S. Suzuki, Chem. Lett., 1985, 1695–1698.
3. E. L. Clark, M. R. Singh, Y. Kwon and A. T. Bell, Anal. Chem., 2015, 87, 8013–8020.
4. A. H. Wonders, T. H. M. Housmans, V. Rosca and M. T. M. Koper, J. Appl. Electrochem., 2006, 36, 1215–1221.
9:45 AM - ES02.10.05
Assembly Mode of Perylene Based Nanosheets Revealed by Wide Angle X-Ray Scattering
Boris Harutyunyan 1 , Adam Dannenhoffer 1 , Sumit Kelwaramani 1 , Taner Aytun 1 , Daniel Fairfield 1 , Samuel Stupp 1 , Michael Bedzyk 1
1 , Northwestern University, Evanston, Illinois, United States
Show AbstractMolecular packing in light harvesting 2D assemblies of photocatalytic materials is a critical factor for solar-to-fuel conversion efficiency. However, structure-function correlations have yet to be fully established. This is partly due to the difficulties in extracting the molecular arrangements from the complex 3D powder averaged diffraction patterns of 2D lattices, obtained via in-situ wide-angle X-ray scattering. We develop a scattering theory formalism and couple it with a simple geometrical model for the molecular shape of various perylene monoimide (PMI) based assemblies. This generally applicable method fully reproduces the measured diffraction pattern including the asymmetric line-shapes for the Bragg reflections and yields the molecular packing arrangement within a 2D crystal structure with a high degree of detail. We find an approximate herringbone and edge-centered herringbone structures for the PMI fused aromatic rings and ordering of the carboxypentyl chains above and below the nanosheets. Packing modes show strong dependence upon the character of substituent chains. This structural difference is correlated to our measurements of the catalytic performance of nanosheets. The theory further reveals the structural modification occurring at the observed phase transition in propyl-PMI based nanosheets whose high temperature phase exhibits enhanced catalytic activity as compared to its lower temperature phase.
10:30 AM - ES02.10.06
Investigating the Role of Copper Oxide in Electrochemical CO2 Reduction in Real Time
Lily Mandal 1 , Mallikarjuna Motapothula 1 , Dan Ren 1 , Peter Lobaccaro 1 , Yang Ke 2 3 , Matthew Sherburne 4 , Victor Batista 3 , Yeo Boon Siang 1 , Joel Ager 4 , Jens Martin 1 , T. Venky Venkatesan 1
1 , National University of Singapore, Singapore Singapore, 2 Department of Chemistry, Yale University, New Haven, Illinois, United States, 3 Yale Energy Sciences Institute and Department of Chemistry, Yale University, New Haven, Connecticut, United States, 4 Department of Materials Science and Engineering, University of California, San Francisco, California, United States
Show AbstractOur work is focused on mechanistic aspects of electrochemical CO2 reduction (CO2R) on copper oxide that are critical to selectivity. Compared to polycrystalline copper, copper oxide surfaces might offer higher selectivity and lower overpotentials for CO2R.. However, it remains unclear whether the oxide itself is a catalyst, a promotor of catalytic copper nanostructures, or a source of subsurface oxygen sites that enhance the catalytic performance.
Ex situ measurements are not reliable since copper catalysts are easily oxidized, within seconds when kept in the CO2R setup under open circuit conditions (no current flowing). Thus, we focus on characterization based on state-of-the-art in-operando techniques. We introduce time selected ion flow tube mass spectrometry (SIFT-MS) for product characterization in real time. To the best of our knowledge, this paper reports the first implementation of SIFT-MS as applied to the study of electrochemical CO2R, enabling detection and quantification of all gaseous products, except hydrogen and carbon monoxide in a time scale of 0.1–10 sec.
SIFT-MS, measurements and characterization of three different Cu2O precursors, in concert with chronopotentiometry and in-situ Raman spectroscopyshow that surface Cu-oxide is electrochemically inactive for CO2 reduction. Our experimental findings, supported by DFT computational modeling, show that CO2R products are not formed so long as Cu2O is present at the surface, since Cu2O reduction is more favorable than CO2R.
10:45 AM - ES02.10.07
In Situ Spectroelectrochemical Approaches to Study Molecular Interfaces on Conductive Oxides—Immobilization of Electrocatalysts Using Diazonium Chemistry
Tomos Harris 1 2 , Anna Fischer 2 , Ingo Zebger 1 , Matthias Schwalbe 3 , Inez Weidinger 4 , Robert Götz 2 4 , Pierre Wrozlek 3 , Peter Hildebrandt 1
1 Institut für Chemie, Technische Universität Berlin, Berlin Germany, 2 Institut für Anorganische und Analytische Chemie, University Freiburg, Freiburg Germany, 3 Institut für Chemie, Humboldt Universität zu Berlin, Berlin Germany, 4 Fachrichtung Chemie und Lebensmittelchemie, Technische Universität Dresden, Dresden Germany
Show AbstractStorage of (renewable) energy in the form of chemical energy (e.g. hydrogen) for later consumption using (photo)electrochemical cells requires the development of stable and efficient catalysts. Molecular (e.g. biomimetic) catalysts can exhibit exceptionally high turnover frequencies (TOFs), low over potentials and high selectivies. Their limited stabilities especially when driven by electrodes can be overcome by immobilization on conductive metal oxides.
Current strategies for immobilising electrocatalysts on oxide surfaces use anchoring groups such as phosphonic acids, carboxylic acids, silanes and their derivatives. While each approach has its own merit e.g. high stability or good charge transfer properties, one usually comes at the expense of the other. Development of new approaches is hence required to build chemically and electrochemically robust interfaces which allow efficient and fast charge electron transfer rates. In this context, development of in-situ spectroscopic techniques providing real-time monitoring of interface formation, catalyst immobilisation and system evolution under reaction conditions will allow for a rational design of immobilization strategies, far beyond simple trial and error approaches.
Of particular interest is an in-situ IR spectroelectroscopic approach that we have developed for nanostructured oxide electrodes, providing unprecedented information on interface formation, stability and structure that is not accessible with conventional characterization approaches, such as UV-vis or purely electrochemical methods. In-situ IR spectroscopy is a powerful tool for researchers working on oxide-based electrochemical devices, such as electrolyzers, sensors, or dye-sensitized solar cells and photocatalytic cells i.e. anywhere inorganic-molecular interfaces play an important role in a device’s performance.1,2
In this work, diazonium electrochemistry was successfully used for the first time to covalently attach molecular catalysts onto a porous transparent conducting metal oxide surface, in particular precious metal-free ORR and water oxidation catalysts. In-situ ATR-IR was used to demonstrate the high electrochemical and chemical stability of the deposited interface, while in-situ Resonance Raman spectroscopy was used to determine the co-ordination of the immobilized species and to demonstrate their excellent electrochemical accessibility. In addition, in-situ UV-Vis spectroscopy was used to probe the immobilized species redox behaviour and X-ray Photoelectron spectroscopy was used to determine the interfacial structure. This culminated in functioning precious-metal free electrocatalytic devices.1
1Harris, T.G.A.A. et al. Diazonium salts for attaching molecular catalysts to metal oxide surfaces. In preparation.
2Harris, T.G.A.A. et al. In-situ spectroelectrochemical studies into formation and stability of robust diazonium-derived interfaces on gold electrodes for the immobilization of an oxygen-tolerant hydrogenase. Submitted.
11:00 AM - *ES02.10.08
Synchrotron Radiation Spectromicroscopy for the Characterization of Materials
David Shuh 1
1 , Lawrence Berkeley National Lab, Berkeley, California, United States
Show AbstractStrategies for improving materials systems for solar fuels increasingly rely on high performance and spatially-resolved characterization methods that are amenable to in-situ and in-operando operation. To accelerate the discovery and development of new materials systems, it is critical to integrate characterization efforts into the overall comprehensive approach to development. This contemporary endeavor depends on the development, use, and access to state-of-the-art tools for materials characterization at all stages of development. A class of characterization techniques that are essential to the development of new materials systems important to the overall energy sciences, including solar fuel systems, are those provided by synchrotron radiation-based approaches.
In the soft x-ray regime, x-ray absorption spectroscopy, x-ray emission spectroscopy, x-ray photoelectron spectroscopy, and resonant inelastic x-ray scattering, are the core approaches currently being utilized. One of the most impactful of these methods is soft x-ray spectromicroscopy and ptychography with the scanning transmission x-ray microscope (STXM) at the Molecular Environmental Science (MES) Beamline 11.0.2 of the Advanced Light Source (ALS). The MES STXM is regularly capable of imaging with a spatial resolution of 20 nm and can directly probe the light element K-edges below 2 keV via x-ray absorption. The STXM can be employed on a diverse range of samples including particulates, highly air-sensitive materials, and can be operated in an in-situ/in-operando mode for many classes of experiments. The use of STXM ptychography improves the spatial resolution to the true nanoscale approaching 3 nm. Representative results from several particulate-based systems will be presented highlighting the utility of the STXM for a broad range of materials research. A critical discussion will follow on future developmental opportunities for STXM, as well as other synchrotron radiation x-ray methods including those in the tender and hard x-ray regimes.
11:30 AM - ES02.10.09
Towards Computational Screening of Solar Thermal Fuels across the Complete Fuel Cycle
Khoa Le 1 3 , Rebecca Szabo 1 3 , Tim Kowalczyk 1 2 3
1 Department of Chemistry, Western Washington University, Bellingham, Washington, United States, 3 Institute for Energy Studies, Western Washington University, Bellingham, Washington, United States, 2 Advanced Materials Science and Engineering Center, Western Washington University, Bellingham, Washington, United States
Show AbstractThe prospect of storing solar energy through photoisomerization of organic materials is impeded by low energy density relative to batteries and combustion fuels. Nevertheless, materials design principles for these so-called photoswitching solar thermal fuels (STFs) have enabled the preparation of STF morphologies that can significantly enhance the energy density of the material. Here we employ electronic structure-based simulations to develop design principles for STFs which include properties of the electronically excited state on which the photoisomerization occurs. By including excited-state information, we can more completely characterize the performance of STF materials. This study examines four key properties of STFs derived from azobenzene and norbornadiene scaffolds using ground- and excited-state density functional tight binding (DFTB) methods: the absorption spectrum, photoisomerization quantum yield, reverse isomerization activation barrier, and isomerization enthalpy. The DFTB predictions are benchmarked against density functional theory and experimental characterization where available. Computational screening of STF candidate libraries enables a ranking of candidate materials with respect to each of the four key STF properties and provides a platform for elucidating a more complete set of design principles.
11:45 AM - ES02.10.10
Dynamic Microfluidic Interfaces in Light-Fired Catalysis for Solar Fuels
L. Criante 1
1 , Istituto Italiano di Tecnologia, Milan Italy
Show AbstractArtificial photosynthesis, a process which captures sunlight to store its energy in the chemical bonds of molecular hydrogen and oxygen, has been demonstrated using photo-electrochemical cells which combine light absorbing molecules for photo-induced redox processes, water oxidation catalysts, hydrogen evolving catalysts, anode/cathode for an external circuit pathway for electrons, and a membrane to physically separate the products.
Although the efficiency of these systems has steadily increased in recent years, in most of the processes shown to date, the long-term devices performance and the materials stability remains problematic yet in search of a solution by undermining the real sustainability of the process.
Here we want to explore an alternative approach in light-fired catalysis and preparation of solar fuels by taking advantage of heterogeneous phases in microfluidic systems to form dynamic photoactive interfaces to create a new generation of sustainable artificial photosynthetic devices. Mimicking the capillaries of a natural leaf, dynamic microfluidic interfaces enable a programmed reversible organization of functional pigments into photosynthetic architectures in which vectorial photoinduced energy- and electron-transfer processes yield long-lived charged species in different liquid phases
This is achieved by engineering tangential microfluidic channels with appropriate geometries in which laminar flows do not mix. This results in short diffusion distances, low mass transport limit, and product removal favoring forward reactions. Exploiting the different solvophobic properties, selected Vis-adsorbing chromophores are synthesized to undergo self-assembly at the two interfaces, forming photosynthetic architectures in which antenna (Ant) molecules will fuel donor (D) and acceptor (A) electron mediators via non-covalent recognition sites. Sensitized D* and A* will react independently with the chosen heterogeneous catalysts for water splitting. Successive split of the flows causes the disassembly of the interfacial organization, restoring all the photoactive species and preserving them from degradation reactions, thereby enabling their clean recycling. In this way a microfluidic approach to this artificial photosynthesis is beneficial since it uses less reagents, enables precise flow conditions, shorter reaction and times, in a reusable platform.
ES02.11: Materials Characterization and Modeling II
Session Chairs
Thursday PM, November 30, 2017
Hynes, Level 3, Room 306
1:30 PM - ES02.11.01
Characterizing the Structural Overpotentials Induced by Hydrogen Evolution on Spatially-Distributed Electrocatalyst-Semiconductor Interfaces
Zebulon Schichtl 1 , Robert Coridan 1
1 Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractThe spatial distribution of an electrocatalyst layer at a semiconductor-liquid junction (SLJ) can have mixed effects on the photoelectrochemical energy conversion efficiency of the junction. A layer of discrete electrocatalytic metal particles can preserve the photovoltage-producing energetics of the SLJ and reduce parasitic light reflection or absorption versus a continuous metal layer. However, the electrochemically active area on a discrete surface is smaller than on a continuous surface, and the additional overpotential required to achieve the same area-normalized current density reduces the efficiency of the electrode. For gas-evolving reactions like splitting water to H2 and O2, the gas-solid-electrolyte three-phase boundary formed by a growing bubble can block discrete catalytic areas, further reducing catalytic activity and adding an effective ‘structural overpotential’. Here, we investigate the relationship between the organization of electrocatalysts on a semiconductor interface and the energy conversion efficiency of that interface for the gas-evolving hydrogen evolution reaction in aqueous systems. We describe an empirical model for bubble evolution, coalescence, and detachment on electrocatalyst-semiconductor interfaces based on high-speed x-ray imaging experiments. With this model, we are able to estimate the collective effects of the surface structure on the rate of gas evolution, and elucidate strategies for engineering interfaces to maximize the efficiency of the interface through control of the three-phase boundary. Finally, we outline the consequences and benefits the organization of electrocatalysts can have on photoelectrochemical device design.
1:45 PM - ES02.11.02
Linker-Controlled Polymeric Photocatalyst for Highly Efficient Hydrogen Evolution from Water
Yiou Wang 1 , Mustafa Bayazit 1 , Savio Moniz 1 , Natalia Martsinovich 2 , Junwang Tang 1
1 , University College London, London United Kingdom, 2 , The University of Sheffield, Sheffield United Kingdom
Show AbstractPolymeric photocatalysts have been identified as promising materials for H2 production from water due to their comparative low cost and facile modification of the electronic structure. However, the majority only respond to a limited wavelength region (<460 nm) and exhibit fast charge recombination. Our density-functional theory (DFT) calculations have identified an oxygen-doped polymeric carbon nitride structure with heptazine chains linked both by oxygen atoms and by nitrogen species, which results in a reduced band gap and efficient charge separation. A novel synthetic method has then been developed to control both surface hydrophilicity and more importantly, the linker species in a polymer, which highly influences the band gap and charge separation. As such, the synthesized polymer can be excited from UV via visible to even near-IR (800 nm) wavelengths, resulting in a 25 times higher H2 evolution rate (HER) than the previous benchmark polymeric g-C3N4 (λ>420 nm), with an apparent quantum yield (AQY) of 10.3% at 420 nm and 2.1% at 500 nm, measured under ambient conditions, which is closer to the real environment (instead of vacuum conditions). This oxygen and nitrogen co-linked heptazine (ONLH) polymer is potentially applicable in tandem systems for solar energy conversion as photoelectrodes and can also be used in environmental purification. The strategy used here thus paves a new avenue to dramatically tune both the light absorption and charge separation to increase the activity of polymeric photocatalysts via careful control of the polymerization process.
Reference:
1. Y. Wang, M. K. Bayazit, S. Moniz, Q. Ruan, C. C. Lau, N. Martsinovich and J. Tang, Energy Environ. Sci., 2017, DOI: 10.1039/C7EE01109A.
2. R. Godin, Y. Wang, M. A. Zwijnenburg, J. Tang and J. R. Durrant, J. Am. Chem. Soc., 2017, 139, 5216-5224.
3. Q. Ruan, W. Luo, J. Xie, Y. Wang, X. Liu, Z. Bai, C. J. Carmalt and J. Tang, Angew. Chem. Int. Edit., 2017, DOI: 10.1002/anie.201703372
2:00 PM - *ES02.11.03
Understanding and Optimization of Transition Metal Oxides as Anode Materials for Water Splitting—Role of Crystallographic Orientation and Doping
Rossitza Pentcheva 1
1 Department of Physics and Center for Nanointegration (CENIDE), University of Duisburg-Essen, Duisburg Germany
Show AbstractTransition metal oxides emerge as promising materials for application in energy conversion and catalysis due to the broad variability of their properties related to the interplay of lattice, charge, orbital and spin degrees of freedom. Based on density functional theory calculations (DFT) taking into account static electronic correlations we explore trends in the catalytic activity of transition metal oxides with spinel and corundum structure as a function of crystallographic orientation and doping. In particular, we investigate the role of Ni and Sn substitution to improve the performance of CoFe2O4 [1] and Fe2O3, respectively. Analysis of the electronic properties, i.e. variation of oxidation state and orbital polarization provides mechanistic understanding of the energetic trends.
Work in collaboration with Hamidreza Hajiyani. Support by the DFG within priority program SPP1613, project PE883/9-2 and the computational time at the Leibnitz Rechenzentrum (grant pr87ro) and at magnitUDE is gratefully acknowledged.
[1] K. Chakrapani, G. Bendt, H. Hajiyani, I. Schwarzrock, T. Lunkenbein, S. Salamon, J. Landers, H. Wende, R. Schlögl, R. Pentcheva, M. Behrens, S. Schulz, Chem Cat. Chem. (2017), DOI: 10.1002/cctc.201700376
2:30 PM - ES02.11.04
Understanding Activity Trends in Electrochemical Water Oxidation to Form Hydrogen Peroxide
Xinjian Shi 2 , Xiaolin Zheng 1
2 , Stanford University, Stanford, California, United States, 1 , Stanford University, Stanford, California, United States
Show AbstractElectrochemical production of hydrogen peroxide (H2O2) from water oxidation could provide a very attractive route to locally produce a chemically valuable product from an abundant resource. Herein using density functional theory (DFT) calculations, we predict trends in activity for water oxidation towards H2O2 evolution on four different metal oxides i.e., WO3, SnO2, TiO2 and BiVO4. The DFT predicted trend for H2O2 evolution was further confirmed by our experimentally measurements. Moreover, we identified that BiVO4 has the best H2O2 generation amount of those oxides and can achieve faraday efficiency about 98% for H2O2 production.
2:45 PM - ES02.11.05
Overall Water Splitting Mechanism on CoO Nanoparticles
Kyoung-Won Park 1 , Alexie Kolpak 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractIt has been experimentally observed that the size reduction of CoO particles to nanometer range induces photocatalytic activity so that water molecules can be dissociated into stoichiometric H2 and O2 at room temperature without externally applied potential or co-catalyst. The photocatalytic activity of the CoO nanocrystals was suggested to be a result of an upward shift of the band edges such that the band gap covers both the water reduction and oxidation potential levels, enabling overall water splitting to occur. Nevertheless, the fundamental reason behind the band edge shift has been unknown. In this study, we identify the thermodynamically stable morphology of CoO nanocrystals in a water environment by using first-principles density functional theory (DFT) calculations. By performing explicit solvent calculations, we find the band edge positions of the different facets to be in good agreement with experimentally observed values. We discuss why CoO surfaces have different band edge positions in water based on the degree of band bending at the interface of each CoO slab/water. In addition, we demonstrate a charge separation mechanism of photogenerated carriers and elucidate the H2 and O2 evolution reaction mechanisms on the relevant surface facets of CoO nanocrystals by comparing the energy states of the photogenerated carriers and the transition states of the reactions. This work provides a general understanding of how the overall water splitting phenomena can occur in monolithic nanoparticle photocatalysts without a co-catalyst or an external bias.
3:30 PM - *ES02.11.06
Photoreaction of Ethanol over Model Surfaces—Anatase TiO2(101) and Rutile TiO2(110) Single Crystals—A Combined Scanning Tunneling Microscopy (STM) and Online Mass Spectrometry
K Katsiev 1 , George Harrison 2 , Geoff Thornton 2 , Hicham Idriss 1 2
1 Catalysis, SABIC, KAUST Saudi Arabia, 2 Chemistry, University College London, London United Kingdom
Show AbstractWithin the context of surface reactions of renewable for energy applications the reactions of ethanol have been investigated over anatase TiO2(101) and rutile TiO2 (110) single crystals by Scanning Tunneling Microscopy (STM) and on-line mass spectrometry. The objective is to study adsorbate species in the dark and post UV illumination, in the absence and presence of O2 or Au nanoclusters, in order to extract reaction parameters under photo-excitation. On anatase (101) TiO2 single crystal the reaction rate for the photo-oxidation of ethanol to acetaldehyde is found to be strongly dependent on O2 partial pressures and surface coverage with an order of the reaction for O2 close to 0.15. Carbon-carbon bond dissociation leading to CH3 radicals in the gas phase was found to be a minor pathway, which is contrary to the case of TiO2 rutile (110) single crystal, as previously reported by our group and others. Our STM images distinguished two types of surface adsorbates upon ethanol exposure that can be attributed to molecular and dissociative modes. Upon UV exposure at (and above) 3×10-8 mbar O2, a third species is identified as a reaction end-product, attributed to acetate/formate species, in line with XPS C1s measurements. The room temperature photo-oxidation of ethanol has also been investigated over a rutile TiO2(110) single crystal by STM and on-line mass spectrometry, in the presence of O2 to determine adsorbate species remaining post UV exposure and emitted gas phase products. In addition to acetaldehyde, methyl radical was detected in the mass spectrometry resulting from the photo-fragmentation of an acetaldehyde-O complex. Formic acid species bound to five-fold coordinated titanium cations, Ti5c, in a bi-dentate mode were identified by STM on the surface after the reaction. The effect of O2 partial pressure on the reaction selectivity demonstrated a dominance of the photo-fragmentation process with increasing pressure unlike the case of anatase TiO2(101) single crystal. In addition, the effect of Au clusters on the reaction of the TiO2(110) has been investigated for hydrogen production under photo-irradiation in UHV conditions. This is the first time hydrogen is produced catalytically over any model semiconductor in ultrahigh vacuum conditions. It was found that particle size in the range 0.4 to 0.8 nm do not change the reaction rate yet their particle density does. Detailed reaction mechanism for both reduction and oxidation reactions are addressed based on structural and kinetic information from the UHV systems and compared to those of powder forms in liquid-solid photocatalytic reactions conditions.
4:00 PM - ES02.11.07
Charge Transfer Characterization on Atomic Layer Deposited TiO2 Protective and Conductive Layers for Photoelectrochemical Solar Fuels
Carles Ros 1 , Teresa Andreu 1 , Maria-Dolores Hernández-Alonso 2 , German Penelas-Perez 2 , Jordi Arbiol 3 , Juan Morante 1
1 , IREC, Catalonia Institute for Energy Research, Sant Adria del Besos Spain, 2 , Repsol Technology Center, Mostoles, Madrid, Spain, 3 , Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology (BIST), Campus UAB, Bellaterra, Barcelona, Spain
Show AbstractTo enable silicon to be used as photocathode for hydrogen evolution, protective and conductive coatings are needed stable from acidic to basic electrolytes while transparent to light and with proper band alignment to be deposited on silicon. One of the best candidates is Titanium Dioxide, as it is known to be stable in wide range of pH.
In this contribution, we analyze the influence of ALD growth temperature on the charge transfer across protective TiO2 coatings for front illuminated silicon photocathodes. As temperature is increased, layer resistivity is reduced. A minimum growth temperature is required for charge transfer, directly related to layer crystallization between 100 and 200 oC. From conductive AFM images, we have proven that the conduction path is the crystalline structure of TiO2; and that amorphous layers and grain boundaries are highly resistive. Conduction across the protective layer can be increased by using higher deposition temperatures with more stable TiO2 phases and reducing defects and charge traps, obtaining higher fill factors up to 0.73 and 9 % half-cell Solar-to-Hydrogen conversion efficiencies.
A thin titanium layer of 5 nm, used to protect silicon from oxidation, has an important role also in enhancing the TiO2 nucleation and crystallization although reducing light transmission. Au/TiO2/Ti/Si structures have been prepared and studied for determining the I(V) curves revealing the conduction through the passivation layer. These curves show a hysteresis behavior correlated with the oxygen vacancies movement. Also, crystallized TiO2 is demonstrated to be mandatory for long term stability, and over 300h continuous operation is proven.
[1] C. Ros, T. Andreu, M. D. Hernández-Alonso, G. Penelas-Pérez, J. Arbiol, and J. R. Morante, “Charge transfer characterization of ALD-grown TiO2 protective layers in silicon photocathodes,” ACS Appl. Mater. Interfaces, 2017.
4:15 PM - ES02.11.08
Quantitative Structural and Transport Analysis of Morphologically-Complex Photoelectrodes
Silvan Suter 1 , Yannick Gaudy 1 , Sophia Haussener 1
1 , Laboratory of Renewable Energy Science and Engineering, EPFL, Lausanne, Vaud, Switzerland
Show AbstractThe morphology of semiconductor photoelectrodes significantly affects the performance of photoelectrochemical devices. Complex anisotropic morphologies are important to overcome performance limiting bulk transport properties of semiconductor materials, but are often also an unintended outcome of the fabrication process. A better understanding of morphology-induced transport limitations of photoelectrodes is needed.
We used a coupled experimental-numerical approach to quantitatively characterize morphologically-complex photoelectrodes (nano- to micrometer thick semiconductor-films composed of mesoscopic structural units with nano-scale structural details). We utilized a 3D-microscopy method, FIB-SEM tomography [1] with a high resolution of 4x4x4 nm3, to obtain a grey value array representing the photoelectrode morphology. The digital structure was segmented based on trainable machine-learning algorithms to subsequently quantify performance-related morphological parameters.
We applied this method to two distinct photoelectrodes, different in structure, composition, and scale: i) a particle-based lanthanum titanium oxynitride electrode with a film thickness of a few micrometers [2], and ii) a ‘cauliflower-like’ structured hematite electrode with a film thickness of a few hundred nanometers [3]. The digitalized morphology of each of the two films was used to quantify specific surface, mean feature dimensions, and film homogeneity. Further, the structural characteristics in the meso and nano-scale, including the shape and orientation of these structural details, were quantified.
The digitalized photoelectrode morphologies were then used in direct pore-level simulations to understand transport within and around the semiconductor. Charge carrier generation rates in the semiconductor phase were calculated by an electromagnetic wave propagation simulation based on spatially resolved material density profiles. The generation rates were mapped onto the semiconductor-electrolyte interface and limitations in the diffusive ion transport in the electrolyte were investigated with a finite volume solver.
The FIB-SEM tomography, with its high, nanometer-scale resolution, reveals precise structural information of semiconductor films at the submicrometer scale. The methodology proofs to be applicable to various photoelectrodes and provides a unique insight into their morphologies. The analysis of the 3D-data obtained allows for the qualitative and quantitative assessment of performance-related morphological parameters, and can characterize and identify limiting transport phenomena in the structure in order to guide the morphology and fabrication of optimized photoelectrodes.
References
[1] M. Cantoni, L. Holzer, MRS Bull. 2014, 39, 354.
[2] A. E. Maegli, S. Pokrant, T. Hisatomi, M. Trottmann, K. Domen, A. Weidenkaff, J. Phys. Chem. C 2013, 118, 16344.
[3] S. D. Tilley, M. Cornuz, K. Sivula, M. Grätzel, Angew. Chemie 2010, 122, 6549.
4:30 PM - ES02.11.09
Quantification of the Loss Mechanisms in Emerging Water Splitting Photoanodes through Empirical Extraction of the Spatial Charge Collection Efficiency
Gideon Segev 1 , Chang-Ming Jiang 1 , Hen Dotan 2 , Jason Cooper 1 , Jeffery Beeman 1 , Daniel Grave 2 , Ian Sharp 1 , Avner Rothschild 2
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Materials Science and Engineering, Technion–Israel Institute of Technology, Haifa Israel
Show AbstractThe operando quantification of surface and bulk losses is key to developing strategies for optimizing photoelectrodes and realizing high efficiency photoelectrochemical solar energy conversion systems. This is particularly true for emerging thin film semiconductors, in which photocarrier diffusion lengths, surface and bulk recombination processes, and charge separation and extraction limitations are poorly understood. Insights into mechanisms of efficiency loss can guide strategies for nanostructuring photoelectrodes, interface engineering, and catalyst incorporation. However, few experimental methods are available for direct characterization of dominant loss processes under photoelectrochemical operating conditions. In this contribution, we quantify the spatial charge collection efficiency in γ-Cu3V2O8 photoanodes, which is defined as the fraction of charge carriers photogenerated at every point in the bulk of the photoelectrode that contribute to the measured current, by combining optical modeling and incident photons to current efficiency (IPCE) measurements. By comparing the spatial collection efficiency profiles at different operating potentials, we show that increasing the applied potential primarily acts to reduce surface recombination rather than to increase the thickness of the space charge region under the semiconductor/electrolyte interface. Comparing the spatial charge collection efficiency in the presence and absence of a sacrificial reagent allows surface losses from electronically active defect states to be distinguished from performance bottlenecks arising from slow reaction kinetics. By considering only the surface losses, the performance limit for ideal nanostructured γ-Cu3V2O8 photoanodes can be derived. Combining all these insights promotes a complete understanding of the photoanode performance and its potential as a water splitting photoanode. Thus, application of the spatial collection efficiency extraction method to other new materials can greatly accelerate the search for new candidate materials for solar water splitting devices.
4:45 PM - ES02.11.10
Computational Study of Cu Surfaces for CO2/CO Reduction Catalysts
Liang Cao 1 , Tim Mueller 1
1 Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractCu and Cu-based alloy catalysts are one of the most successful electrode catalysts for the electrochemical CO2/CO reduction, as a promising means for artificial carbon recycling and solar-fuel energy conversion. Recently, the Cu nanowires synthesized by reducing pre-grown CuO nanowires were reported to have high activities and selectivities towards CO2/CO reduction at low overpotentials (more positive than –0.5 V versus RHE). Experimentally, the optimized Cu nanowires reduced at relatively low temperatures (e.g. 150 °C) appear to have a substantially larger fraction of (110) facet on the surface than those reduced at high temperatures (e.g. 300 °C). To investigate this structure-activity relationship at the atomic level, we have built free energy diagrams for CO2 reduction to CO(g) and further CO(g) reduction to C2 species on Cu surfaces. Using a thermodynamic free energy diagram, among the five prevalent Cu facets: (211), (110), (110)-rec, (100), and (111), the Cu(110) surface has the least negative calculated onset potentials for both CO2 and CO reduction, with values close to the experimentally observed onset potentials. Thus, our experimental characterization and computational simulations indicate that the metastable Cu(110) surface, or metastable surface with similar structural features, may be responsible for the high activities and selectivities at low overpotentials.
Symposium Organizers
Thomas Fischer, University of Cologne
Fabio Di Fonzo, Istituto Italiano di Tecnologia
Rita Toth, Swiss Federal Laboratories for Materials Science and Technology (EMPA)
Mmantsae Diale, University of Pretoria
Symposium Support
Kenosistec
Nature Catalysis | Springer Nature
Sustainable Energy &
Fuels | The Royal Society of Chemistry
ES02.12: Heterostructures I
Session Chairs
Friday AM, December 01, 2017
Hynes, Level 3, Room 306
8:30 AM - ES02.12.01
Novel Strategy of Optically and Electrochemically Decoupled Photoelectrode Structure for Highly Efficient Solar-Driven Water Splitting
Seungtaeg Oh 1 , Hakhyeon Song 1 , Jihun Oh 1
1 , Korea Advanced Institute of Science and Technology (KAIST), Daejeon Korea (the Republic of)
Show AbstractPhotoelectrochemical (PEC) water splitting cells have attracted great attention to convert solar energy to a clean and storable fuels [1]. Since most of semiconductors have the poor catalytic properties for water reduction or oxidation, PEC cells are usually used with metal cocatalysts to reduce the kinetic overpotential for highly efficient PEC water splitting [2] However, metal particles block the sunlight and the efficiency of photoelectrodes is limited by the competition between light absorption and electrocatalysis at a restricted cocatalyst area on a conventional photoelectrode [3].
Here, we present a new photoelectrode architecture to decouple optical absorption and electrocatalysis using locally defined nanostructured cocatalysts for efficient PEC water splitting reaction. Nanostructured cocatalyst on photoelectrodes can reduce the overpotential by increasing electrochemically active sites. In addition, when formed locally on a photoelectrode, it can allow light absorption of the photoelectrode. Therefore, the locally defined nanostructured cocatalysts can achieve higher efficiency than the conventional particle-like cocatalysts from the increased surface area. As a model system, we fabricated a Si photoanode with micro-patterned Ni inverse opal (IO) for PEC water oxidation in alkaline solution and also performed a systematic study to find an ideal structure by controlling independently surface area and/or footprint of Ni IO cocatalyst. In this work, it is shown that the overpotential of Si photoanodes with Ni IOs decreased continually with increasing height of Ni IO when it is lower than 7 layers. In addition, PEC OER performance of Si photoanodes with micro-patterned Ni IOs increased continually with decreasing footprint of Ni IO cocatalyst. As a result, an optimized Si photoanode with Ni IOs exhibits 120 mV less overpotential for water oxidation than a Si photoanode with planar Ni micropatches with same geometrical footprint in 1 M KOH under simulated 1 sun illumination. Finally, we locally formed NiFe IOs on Si photoanodes and our Si photoanode with micro-patterned NiFe IOs produces 31.2 mA/cm2 photocurrent density at water oxidation potential (1.23 V .vs RHE) which is among highest performance in Si based photoanode for water oxidation and it operates continually for 11 hours at the potential producing saturating photocurrent density without any damage and degradation.
Reference
1 Lewis, N. S. & Nocera, D. G. Powering the planet: chemical challenges in solar energy utilization. Proc Natl Acad Sci U S A 103, 15729-15735, (2006).
2 Walter, M. G. et al. Solar water splitting cells. Chemical reviews 110, 6446-6473 (2010).
3 Oh, S. & Oh, J. High Performance and Stability of Micropatterned Oxide-Passivated Photoanodes with Local Catalysts for Photoelectrochemical Water Splitting. J. Phys. Chem. C, 120, 133-141 (2015).
8:45 AM - ES02.12.02
Investigation of Sb2Se3 Nanostructures as a Photocathode for Photoelectrochemical Water Splitting
Wooseok Yang 1 , Yunjung Oh 1 , Jeiwan Tan 1 , Hyungsoo Lee 1 , Jaemin Park 1 , Jooho Moon 1
1 , Yonsei University, Seoul Korea (the Republic of)
Show AbstractSb2Se3, which is non-toxic and abundant binary semiconductor, has emerged as a promising light absorbing material for solar energy conversion devices. Recently, our group reported Sb2Se3 nanostructure obtained by facile spin-coating method and its application to photoelectrochemical (PEC) water splitting device (J. Mater. Chem. A, 2017, 5, 2180). However, our previous work has focused on the synthetic method of Sb2Se3 nanostructure without demonstrating detailed PEC properties of Sb2Se3 nanostructures. Although the optical and electrical properties of Sb2Se3 thin films have been already reported, in order to demonstrate the feasibility of Sb2Se3-based PEC water splitting, better understanding of Sb2Se3 itself toward the perspective of photoelectrochemistry is greatly necessary. In addition, the performance of one dimensional nanostructured Sb2Se3 array has not yet to be investigated in detail. In this talk, we will present in-depth characterizations of Sb2Se3 nanostructures including PEC properties (pH dependent performance and stability), accurate band position, electrical properties and role of surface modification. Ultraviolet photoelectron spectroscopy and UV-vis spectroscopy reveal that our Sb2Se3 nanostructures have desirable conduction band edge for photocathode materials, i.e., more negative than the water reduction potential (0 V vs reversible hydrogen electrode, RHE). The performance of the photocathode is also found to be highly dependent on both pH value and surface modification. With optimization of surface modification, our Sb2Se3 nanostructure based photocathode exhibits remarkable performance, reaching a 12 mA cm-2 at 0 V vs RHE under AM 1.5 G illumination. Our findings clearly demonstrate the possibility of our Sb2Se3 nanostructures as promising photocathode materials and novel approach for further improvement will be discussed.
9:00 AM - *ES02.12.03
Design, Performance and Stability of Low-Cost Materials for Photocatalytic Solar Water Splitting
Lionel Vayssieres 1
1 , Xi'an Jiaotong University, Xi'an China
Show AbstractLatest advances in design strategies and atomic-scale fundamental understanding of the performance and stability of advanced materials for water oxidation and overall water splitting will be presented. The detailed effects of dimensionality, confinement, surface chemistry, orbital character and symmetry, interfacial electronic structure, polarization, electrical properties and surface termination will be demonstrated on low-cost and earth-abundant semiconductors for efficient and large scale hydrogen generation from sunlight and (sea)water.
9:30 AM - ES02.12.04
Optimizing Structuring Geometries for Water Splitting Photoelectrodes
Herman Kriegel 1 , Mauricio Schieda 1 , Iris Herrmann-Geppert 1 3 , Dmitriy Voronov 2 , Deirdre Olynick 2 , Thomas Klassen 1 3
1 , Helmholtz-Zentrum Geesthacht, Geesthacht Germany, 3 , Helmut Schmidt University, Hamburg Germany, 2 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show Abstract
Semiconductor photoelectrodes based on structured conducting substrates have been increasingly studied lately, as they enable higher performance by spatially dissociating the processes of light absorption and transport of photogenerated carriers.
In this work, we identify the optimal surface structuring geometries for water splitting, using a set of experimental model electrodes, based on lithography-etched substrates coated with titanium dioxide by atomic layer deposition as a model absorber.
FDTD simulations are in excellent agreement with the broadband reflectance reduction of these model structured photoelectrodes. Additionally, the surface structuring cross-section profile has a significant effect on the photocurrent as a function of the etch depth.
For the case of sub-micrometer periods, line and pillar patterns show optimal performance near 1µ etch-depth, and a rapid decrease in performance at higher etch-depths. In this period range, photocurrents are more than 20% higher for lines than for pillars, at similar coating thickness and etch depths.
9:45 AM - ES02.12.05
PEC-GC-Coupled In Situ Analysis of 3D-Nanowire Networks for Photocatalytic Water Splitting
Titus Lindenberg 1 , Kasper Wenderich 1 , Wouter Maijenburg 1
1 , Martin Luther University Halle-Wittenberg, Halle (Saale) Germany
Show AbstractOxide semiconductors from abundant elements are of special interest for photocatalytic water splitting in order to keep the final assembly cheaper and therefore more easily scalable. Unfortunately, many of these materials suffer from the drawback of having a small charge carrier diffusion length while having a much higher light penetration depth (e.g. ~ 50 nm vs. ~3 μm for CuBi2O4, respectively) [1]. Therefore, if one wants to use thin film electrodes of these materials, one needs to find a trade-off between these two features.
This challenge can be overcome by using quasi-1D-nanostructured electrodes, e.g. nanowires. As these nanowires possess two different length scales, more incident solar light can be absorbed along the nanowire length, while the nanowire diameter provides a short pathway for the generated charge carriers to the reaction sites at the nanowire/electrolyte interface.
Keeping scalability in mind, we chose electrodeposition as a cheap and simple synthesis method, as it can be performed at atmospheric pressure and moderate temperatures. By using alumina membranes with interconnected pores [2] as a template for electrodeposition of CuBi2O4 nanowire networks, we managed to simultaneously improve the light absorption and charge carrier transport. Furthermore, light scattering within the nanowire network reduced the reflectance on the electrolyte/electrode interface, which is another drawback for thin film CuBi2O4 electrodes [1]. CuBi2O4 is an interesting p-type semiconductor with a bandgap Eg of ~1.8 eV [3], with suitable band positions for the reduction of protons to hydrogen.
We combined measurements in a photoelectrochemical cell (PEC) with gas chromatography in order to obtain in-situ information on the amount and ratio of the evolving H2 and O2 gases which can be directly correlated to the processes occurring in the PEC. This enables us to determine the faradaic efficiency and to identify side reactions.
Due to the relatively high bandgap of CuBi2O4, the application as top absorber in a dual absorber device might be a promising option [1]. Future research will therefore focus on the deposition of core-shell nanowires in which the CuBi2O4 is combined with a semiconductor with smaller bandgap to improve light-to-hydrogen efficiencies.
References
[1] S.P. Berglund et al., Chemistry of Materials, vol. 28, no. 12, pp. 4231–4242, Jun. 2016.
[2] J. Martín et al., Nature Communications, vol. 5, p. 5130, Oct. 2014.
[3] K. Sivula and R. van de Krol, Nature Reviews Materials, vol. 1, p. 15010, Jan. 2016.
10:30 AM - *ES02.12.06
Complex Composition and Shape Thin- and Ultra-Thin Films and Porous Materials through Solution Chemistry
Gunnar Westin 1
1 , Uppsala Univ, Uppsala Sweden
Show AbstractRobust low cost synthesis routes achieving complex nano-materials are required for practical application in many areas of sustainable energy conversion. Solution based processing routes are probably the best suited for preparation of these typically multi-phase, multi-elemental nano-materials in one or few steps. Here we will describe solution based synthesis routes to metals, complex oxides and, metal-in-oxide nano-composites in the forms of thin- and ultra-thin films and porous structures. In addition some examples of up-scaling will be presented. Systems that will be discussed are; (i) nano-phase sponges of metals for use as catalyst supports and electrodes; metal-in-oxide nano-composites for hydrogen activation in photo-catalysts, fuel-cells and FT or dry reforming reactions and; oxide semi-conductors for use in solar cells and up-conversion devices. (ii) Thin- and ultra-thin (1-5 nm) oxide, metal and metal-in-ceramic nano-composite films on flat and porous structures. The focus will be on the influence of the alkoxide or salt based precursors and processing parameters on the target material structure and quality, although some properties of the materials are also given. An array of experimental techniques were used for elucidation of the reaction steps along the solution and thermal processing and a detailed description of the target materials including; XRD, IR and Raman spectroscopy, TEM, SEM, TGA, DTA/DSC and XPS.
11:00 AM - ES02.12.07
Cu2O Nanowire Arrays and Networks Electrodeposited in Etched Ion-Track Membranes as Photocathodes for Solar Hydrogen Production
Florent Yang 1 , Jan Kugelstadt 1 , Dimitri Korjakin 1 , Mercedes Carrillo Solano 1 , Wouter Maijenburg 1 , Christina Trautmann 1 2 , Maria Eugenia Toimil-Molares 1
1 , GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt Germany, 2 , Technische Universität Darmstadt, Darmstadt Germany
Show AbstractResearch on the synthesis and characterization of semiconductor nanowires is continuously increasing especially for applications in the field of energy conversion, such as for instance solar cells, photocatalysis, and photoelectrochemical water splitting for hydrogen production [1]. One attractive approach to improve the solar to hydrogen (STH) conversion efficiency is the use of nanowire structures since, in comparison to films, they offer larger surface areas and permit to dramatically reduce the ratio of the minority charge carrier diffusion length over the light absorption depth [2]. Among the various materials studied as photocathodes for hydrogen production via water splitting, Cu2O is a promising candidate. An STH conversion efficiency of ~18% has been predicted for this p-type semiconductor with a band gap of 2 eV [3]. Moreover, Cu2O has favorable band energy positions for water splitting and is also earth-abundant, scalable, non-toxic, and compatible with low-cost fabrication processes. Its chemical stability in aqueous solution can be improved by adding protection layers.
Among the various methods available to synthesize Cu2O nanowires, we apply ion-track nanotechnology [4]. The fabrication and characterization of parallel and highly textured Cu2O nanowire arrays, as well as networks obtained by electrodeposition in etched ion-track membranes, will be presented. Nanowire diameter, length, number density, and crystallinity are adjusted in a controlled and systematic manner during the synthesis. After removal of the polymer membrane in an organic solvent, the freestanding nanostructures are coated by an additional electrodeposited Cu2O layer to avoid direct contact between the electrolyte and the metallic support layer. For this additional step, several strategic approaches will be presented and discussed by comparing their photoelectrochemical (PEC) performance. The formation of a p-n junction by a conformal TiO2 overlayer thin-film is applied by atomic layer deposition to enable efficient charge separation and collection but also serves as a protection layer against corrosion. Finally, the electrodeposition of Pt or RuOx catalyst is also performed to improve the chemical stability and to reduce the kinetic barrier for the water reduction reaction by shifting the onset potential to higher anodic potential. PEC measurements on these nanowire-based electrodes will be presented, and the influence of their geometrical characteristics on their photoelectrochemical performance will be discussed.
References
[1] A. I. Hochbaum, P. Yang, Chem. Rev. 2010, 110, 527-546.
[2] R. van de Krol, M. Grätzel, Photoelectrochemical Hydrogen Production, Springer, 2012.
[3] A. Paracchino, V. Laporte, K. Sivula, M. Grätzel, E. Thimsen, Nat. Mater. 2011, 10, 456-461.
[4] M. E. Toimil-Molares, Beilstein J. Nanotechnol. 2012, 3, 860-883.
11:15 AM - ES02.12.08
Graphene Related 2D Crystals and Hybrid Systems for High-Efficiency, Solution-Processed, Large-Area, Flexible, Stable Electrocatalysts and Photocathodes for Hydrogen Evolution Reaction
Sebastiano Bellani 1 , Francesco Bonaccorso 1 , Leyla Najafi 1 , Mirko Prato 2 , Antonio Del Rio 1 , Alberto Ansaldo 1 , Iwaan Moreels 1 3 , Beatriz Garcia 1 3 , Reinier Oropesa-Nuñez 1
1 Graphene Labs, Istituto Italiano di Tecnologia, Genova Italy, 2 Materials Characterization Facility, Istituto Italiano di Tecnologia, Genova Italy, 3 Nanochemistry, Istituto Italiano di Tecnologia, Genova Italy
Show AbstractMolecular hydrogen (H2) produced from electrochemical water splitting has attracted growing attention due to its highest energy density and environmental friendliness. The best-known effective H2 evolution reaction (HER)-electrocatalysts are platinum-group elements but their high cost and scarcity hinder massive commercial applications. Therefore, HER-electrocatalysts based on earth-abundant and electrochemically stable materials are being pursued for viable and sustainable hydrogen production perspectives. Recently, two dimensional-transition metal dichalcogenides (2D-TMDs) have been reported as high-performance HER-electrocatalyst in terms of electrocatalytic activity and stability.1,2 Here we design solution-processed hybrid heterostructures between graphene flakes or single-walled carbon nanotubes (SWCNTs) and 2D-TMDs (MoS2 and MoSe2),3,4 achieving remarkable overpotential at 10 mA cm-2-cathodic current density (n10) in both acid and alkaline electrolytes. Notably, we produce graphene and 2D-TMDs from their parent bulk crystals in suitable liquids to yield dispersions by liquid phase exfoliation (LPE).5,6 This permits to formulate functional inks, which can be processed by large-scale, cost-effective solution processed techniques reaching high-electrocatalytic performance compatibly with high-throughput industrial implementation.2 Taking advantage of our on-demand properties-tuned 2D-material inks, we also exploit graphene and 2D-TMDs as charge transport layer (HTL) for recently emerged hybrid H2-evolving organic photocathodes,7,8 boost their efficiency and durability.9,10 Our 2D-material-based interface engineering permits to achieve record high performances concerning all-solution-processed photocathodes. In fact, our devices show photocurrent at 0 V vs. RHE (J0V vs RHE) of -6.01 mA cm-2, onset potential (Vo) of 0.6 V vs. RHE, ratiometric power-saved efficiency (φsaved) of 1.11% and operational activity of 20 hours. Moreover, our photocathodes are demonstrated to be effective in different pH environment ranging from acid to basic, showing J0V vs RHE exceeding -1 mA cm-2. This is pivotal for their exploitation in tandem configurations, where photoanodes operate only in restricted electrochemical conditions. Furthermore, we demonstrate the up-scaling feasibility of our approach by fabricating a large-area (9 cm2) flexible (onto ITO-PET substrate) photocathodes, with remarkable J0V vs RHE of -2.8 mA cm-2, Vo of 0.45 V vs. RHE and φsaved of 0.31%.
1. M. Pumera et al., J. Mater. Chem. A, 2014, 2, 8981.
2. F. Bonaccorso et al., Science, 2015, 347, 1246501.
3. L. Najafi et al., submitted.
4. L. Najafi et al., submitted.
5. F. Bonaccorso et al., Adv. Mater., 2016, 28, 6136.
6. F. Bonaccorso et al., Mater. Today, 2012, 15, 564.
7. F. Fumagalli et al., J. Mater. Chem. A, 2016, 4, 2178.
8. H. C. Rojas et al., Energy Environ. Sci., 2016, 9, 3710.
9. S. Bellani et al., J. Mater. Chem. A, 2017, 5, 4384
10. S. Bellani et al., submitted.
11:30 AM - ES02.12.10
Nanophotonic Design for Achieving Maximum Short Circuit Current via Effectively Transparent Catalyst on High Efficiency Silicon Micro Antenna Photocathodes for Hydrogen Evolution
Sisir Yalamanchili 1 , Paul Kempler 1 , Nathan Lewis 1 , Harry Atwater 1
1 , California Institute of Technology, Pasadena, California, United States
Show AbstractWe demonstrate a nanophotonic architecture to achieve extremely high absorption in Silicon resulting in high efficiency photoelectrodes. The architecture utilizes relatively high loadings of catalyst with minimal compromise on light absorption to achieve ~40 mA/cm2 short circuit current.
We report ordered, high aspect ratio, tapered Si microwire arrays that exhibit an extremely-low angular (0o to 50o) and spectrally averaged reflectivity of <1% of the incident 400 nm - 1100 nm illumination. After isolating the microwires from their substrate with a polymer infill and peel off process, the arrays absorb 89.1% of angular averaged incident illumination (0o to 50o) in the equivalent volume of a 20 micron thick Si planar slab. The absorption is slightly below the 4n2 classical light trapping limit of a 20 micron thick Si slab for most of the solar spectrum, and exceeded the limit at wavelengths near the Si band gap (1050 nm – 1100 nm), reaching 99.5% of the classical light trapping limit between 400 nm - 1100 nm. The remarkable optical characteristics – minimal reflection and high absorption of the tapered Si microwire arrays is due to efficient coupling of incident light to the waveguide modes in Si microcone arrays that act like an antenna. The efficient coupling is possible due to the sharp tip with a radius as small as 25nm, where the incident light couples to waveguide modes and the 75um long length enables the propagation of light especially of long wavelengths to maximize the probability of absorption.
We then selectively deposit Hydrogen evolution catalysts such as- Pt, and earth abundant catalyst CoP specifically at the tip of the Si arrays. We demonstrate that even under relatively high catalyst loadings a short circuit current loss of just ~1.5 mA/cm2 is observed compared to bare electrodes with no catalyst loading reaching to nearly 37 mA/cm2. The thickness of the catalyst is optimized such that there is minimum loss in the coupling of incident light to the tip of the microwires and minimum overpotential for hydrogen evolution reaction. The Platinum catalyst is deposited via sputtering over wax masked arrays to ensure the deposition to be specifically at the tip of the conical arrays. Cobalt Phosphide is photoelectrochemically deposited using a blue light emitting diode to concentrate the deposition at the tapered microwire tips. The measurements were done in 0.5M Sulfuric acid solution with an Ag/AgCl2 reference electrode and a carbon counter, under illumination from a Xenon arc lamp. This architecture results in <5% short circuit current loss, and therefore the catalyst in spite of high loading is effectively transparent.
Our calculations show that given the measured lifetime of photo generated carriers of 0.75us in the microwire arrays embedded in PDMS peeled off from their substrates, the implied open circuit voltage is >0.6V. Higher open circuit voltages close to 0.7V could be achieved in principle with improved surface passivation of sidewalls of these arrays.
ES02.13: Heterostructures II
Session Chairs
Friday PM, December 01, 2017
Hynes, Level 3, Room 306
1:30 PM - ES02.13.01
Quasi-1D Hyperbranched Nanostructures for Low-Voltage Photoelectrochemical Water-Splitting
Alessandro Mezzetti 1 2 , Alessandra Tacca 3 , Laura Meda 3 , Fabio Di Fonzo 1
1 , Istituto Italiano di Tecnologia , Milano Italy, 2 , Politecnico di Milano, Milano Italy, 3 , Istituto ENI Donegani, Novara Italy
Show AbstractPhotoelectrochemical water-splitting (PEC WS) is an expanding field of research for the renewable production of hydrogen that exploits semiconducting photoelectrodes to convert solar energy into chemical energy to cleave water molecules. In the last years, a great deal of effort has been invested to improve the maximum photocurrent obtained by the photoelectrodes, as it represents the amount of hydrogen/oxygen evolved by the system. This practice is mainly ascribed to the experimental ease of separately testing photocathodes and photoanodes, in a half-cell configuration. However, a full PEC system operates at the matching point between the cathode and anode photocurrent characteristics and therefore benefits not only from a high yield, but also from an early onset potential.
In this contribution, we present two different works where pulsed laser deposition (PLD) is employed to fabricate quasi-1D hyperbranched nanostructures of transition metal oxides (TiO2 and WO3). The deposited films resemble a forest of high aspect-ratio, tree-like structures made of crystalline nanoparticles and represent an innovative compromise between nanorods/nanotubes and conventional isotropic nanoparticle films. The hyperbranched nanostructures are used in PEC photoanode architectures, achieving good photocurrent densities at very low applied biases. This peculiar feature is attributed to the strong directional growth and low defectivity of the nano-trees, efficiently extracting and transporting the photogenerated charges. To better understand the phenomenon behind these performances, a thorough investigating is performed with different physical and electrochemical techniques.
Optimized photoanode heterostructures comprising quasi-1D hyperbranched TiO2 nano-trees as the supporting scaffold and a thin CdS layer as the absorber display an onset potential of -0.43 VRHE and a saturation photocurrent density of 6.6 mA/cm2 at 0.35 VRHE, with an internal conversion efficiency above 80%. These figures of merit currently represent the state-of-the-art for TiO2/CdS heterostructures for water-spitting applications. Quasi-1D hyperbranched WO3 nano-trees are used in single material photoanodes, exhibiting an onset potential as low as 0.4 VRHE and saturation photocurrent densities of 1.85 mA/cm2 at 0.8 VRHE. Despite the saturation photocurrent is below the state-of-the-art value, in the low voltage range the optimized hyperbranched devices outperform all the other competitors.
These results show that optimized photoelectrodes with low onset potential can be obtained with hyperbranched nanostructured films employed as the supporting scaffold for charge transport. The flexibility of the PLD technique opens up for wide range of different materials that can be employed in future architectures, for photoanodes and for photocathodes applications alike.
1:45 PM - ES02.13.02
Photoanode with Enhanced Performance Achieved by Coating BiVO4 onto ZnO-Templated Sb-Doped SnO2 Nanotube Scaffold
Lite Zhou 1 , Yang Yang 1 , Jing Zhang 1 , Pratap Rao 1
1 , Worcester Polytechnic Institute, Worcester, Massachusetts, United States
Show AbstractPhotoelectrochemical (PEC) water splitting is one of the most promising ways to convert solar energy into hydrogen. BiVO4 has recently been proven to be the top performing metal oxide photoanode in the world. The performance of BiVO4 photoanodes, especially under front-side illumination, is limited by the modest charge transport properties of BiVO4. Core/shell nanostructures consisting of BiVO4 coated onto a conductive scaffold are a promising route to improving the performance of BiVO4-based photoanodes. Here, we investigate photoanodes composed of thin and uniform layers of BiVO4 particles coated onto Sb-doped SnO2 (Sb:SnO2) nanotube arrays that were synthesized using a sacrificial ZnO template with controllable length and packing density. We demonstrate a new record for the product of light absorption and charge separation efficiencies (ηabs × ηsep) of ∼57.3 and 58.5% under front- and back-side illumination, respectively, at 0.6 VRHE. Moreover, both of these high ηabs × ηsep efficiencies are achieved without any extra treatment or intentional doping in BiVO4. These results indicate that integration of Sb:SnO2 nanotube cores with other successful strategies such as doping and hydrogen treatment can increase the performance of BiVO4 and related semiconductors closer to their theoretical potential.
2:00 PM - ES02.13.03
Understanding Energy Conversion and Loss Mechanisms in Ternary Metal Oxide Photoelectrodes—The Case of Copper Vanadate
Chang-Ming Jiang 1 , Jason Cooper 1 , Ian Sharp 1
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractThe events governing overall photon-to-current conversion efficiency in semiconductor photoelectrodes include light absorption, photocarrier separation, bulk transport, and charge transfer across interfaces. However, these processes occur over a vast range of time scales and are influenced by a variety of factors that can be difficult to disentangle in a single compound. Copper vanadate (CVO) – an emerging photoanode material system that supports a variety of room temperature stable phases depending on the Cu:V stoichiometry – represents a unique platform for studying the role of composition, phase, and surface defects on PEC characteristics. In this study, thin films of four CVO phases: Cu5V2O10, Cu11V6O26, γ-Cu3V2O8, and β-Cu2V2O7, were prepared by reactive co-sputtering. With the presence of sacrificial hole acceptor, it is found that the Cu-rich phases exhibit more negative onset potentials and higher photocurrent densities, though significant deviations from ideality are observed for all compositions. To gain insight into the factors affecting efficiency loss, we use a suite of complimentary experimental tools (including spectroscopic ellipsometry, transient absorption, and transient photoccurent) to determine trends in basic optical properties, photocarrier dynamics, and surface state-induced charge trapping. Systematic trends with Cu:V ratio reveal competing properties, in which increasing Cu content provides stronger light absorption but higher concentrations of electronically active defect states. While the nature of these highly correlated compounds as charge transfer insulators suggests that electron transport should limit photocurrent generation, the opposite is observed. Analysis of the influence of composition on the basic processes associated with energy conversion provides insight into performance limitations in emerging ternary transition metal oxides and offers strategies for improving their function for PEC applications.
2:15 PM - ES02.13.04
Model Mass-Selected NiFe(OH)x Catalysts for the Oxidation of Water
Claudie Roy 1 , Béla Sebök 1 , Elisabetta M. Fiordaliso 2 , Søren Bertelsen Scott 1 , Jakob Ejler Sørensen 1 , Anders Bodin 1 , Daniel Trimarco 1 , Jakob Kibsgaard 1 , Christian Danvad Damsgaard 2 , Ifan Stephens 1 , Ib Chorkendorff 1
1 Surface Physics and Catalysis, Technical University of Denmark, Kgs Lyngby Denmark, 2 Center for Electron Nanoscopy, Technical University of Denmark, Kgs. Lyngby Denmark
Show AbstractHydrogen is one of the world's most important chemicals with a global production rate of approximately 50 billion kg per year. Synthesis of hydrogen using electrochemistry is very attractive as it can be produced from water in a sustainable manner. However, cost-competitive electrochemical hydrogen production is hindered by the sluggish kinetics of the oxygen evolution reaction. NiFe oxy-hydroxides are the current state-of-the-art catalysts for oxygen evolution in alkaline solution [1-3]. The origin of their activity is currently under intense debate. For example, there is uncertainty whether the bulk or the surface is active. In order to improve the catalysis of this reaction, it is important to gain further understanding of the factors controlling the intrinsic activity. To this end , in our current study, we investigated well-characterized model systems of NiFe nanoparticles. We deposited mass-selected NiFe nanoparticles with different sizes on polycrystalline gold substrates to create well-defined model-systems. Our laboratory previously used this approach to elucidate oxygen reduction [4] and hydrogen evolution [5].
A wide range of techniques was used to characterize our model system: X-ray Photoelectron Spectroscopy, Ion Scattering Spectroscopy, X-ray diffraction, Scanning Electron Microscopy and Transmission Electron Microscopy. We tested the catalysts for oxygen evolution in 1 M KOH using a rotating ring disk electrode set-up. The activity as function of particle size and interparticle distances was evaluated, as well as measuring the stability against corrosion. The particles exhibit turnover frequencies that are amongst the highest ever reported for oxygen evolution in alkaline media. Finally, we used 18O isotope labeling together with a novel in-situ electrochemical mass spectrometry technique [6] to determine whether lattice oxygen participates in the reaction.
[1] M. K. Debe, S. M. Hendricks, G. D. Vernstrom, M. Meyers, M. Brostrom, M. Stephens, Q. Chan, J. Willey, M. Hamden, C. K. Mittelsteadt, C. B. Capuano, K. E. Ayers and E. B. Anderson, J. Electrochem. Soc., 159, K165 (2012).
[2] Dionigi, F. and Strasser, P., Adv. Energy Mater. (2016) 1600621.
[3] Bryan M. Hunter, James D. Blakemore, Mark Deimund, Harry B. Gray, Jay R. Winkler, and Astrid M. Müller, J. Am. Chem. Soc. 136 (2014) 13118.
[4] Hernandez-Fernandez, P., Masini, F., McCarthy, D. N., Strebel, C. E., Friebel, D., Deiana, D., Malacrida, P., Nierhoff,
A., Bodin, A., Wise, A. W., Nielsen, J. H., Hansen., T. W., Nilsson. Stephens, I. E. L., Chorkendorff, I. Nat. Chem, 6 (2014)
32.
[5] Kemppainen, Erno; Bodin, Anders; Sebök, Béla; Pedersen, Thomas; Seger, Brian; Mei, Bastian Timo; Bae, Dowon; Vesborg, Peter Christian Kjærgaard; Halme, J.; Hansen, Ole; Lund, P. D.; Chorkendorff, Ib., Energy Environ. Sci. 8 (2015) 2991.
[6] Trimarco, Daniel; Pedersen, Thomas; Hansen, Ole; Chorkendorff, Ib; Vesborg, Peter C.K., Rev. Sci. Instrum. 8 (2015) 075006.
2:45 PM - ES02.13.06
Carrier Transfer Mechanism of NiO Loaded N-Type GaN Photoanodic Reaction for Water Oxidation from Comparison between Band Model and Optical Measurements
Katsushi Fujii 1 2 , Kayo Koike 2 , Takenari Goto 3 , Shin Nakamura 3
1 , University of Kitakyushu, Kitakyushu Japan, 2 Advanced Photonics Technology Development Group, RIKEN, Wako, Saitama, Japan, 3 Nakamura Lab., Innovation Center, RIKEN, Wako, saitama, Japan
Show AbstractThe electrochemical catalytic effects of NiO have been investigated from its chemical reaction point of view. It is said that the impurity of Fe in NiO has an important role for the electrochemical water oxidation. In contrast, the details of NiO on semiconducting photoanode is still obscured. NiO is reported to be a p-type semiconductor due to its nonstoichiometric property of the Ni vacancy existence with the band gap of 3.7 eV [1].
The NiO island-like loading on n-type GaN improves its photoanodic current and prevents its photoanodic corrosion, although the NiO covered area is a part of GaN surface. Especially, the suppression of photoanodic corrosion, which has the contact area between GaN and electrolyte, is worth to analyze to prevent the photoanodic corrosion. The carrier transfer mechanism from GaN to NiO is discussed in this report using with the band lineup model and optical measurements.
The reported valence band edge of NiO is close to water oxidation redox energy from the calculation with its band gap and electron affinity of 1.4 eV [2]. The hole transportation is expected from the valence band edge of GaN to the valence band edge of NiO from this evaluation. The real NiO electrocatalyst on GaN is, however, probably the mixture of NiO and NiOOH due to its hydroxide form of precursor and due to its annealing temperature of around 473 K [1]. Thus, the band structure of real NiO electrocatalyst is expected to be different from the ideal one.
The photo absorption peak at around 3.9 eV was clearly observed from the NiO layer with higher annealed temperature (different precursor with 823 K annealing) on sapphire substrate but it was not observed from the NiO electrocatalyst originated from hydroxide. The peak was reported to be a d-d transition absorption [3]. A broad absorption peak at around 3.25 eV, which was probably amorphous-like structure, was observed from both of the samples. The other observations were supported this result. That is, although the absorption peak observation is difficult from the sample of NiO on GaN on sapphire substrate due to its interference phenomenon of GaN layer, the absorption from around 3.40 eV was observed. The light absorbed area from the optical absorption measurements and band edge emission of GaN at around 3.45 eV by photoluminescence (PL) were observed but the light absorption of YL at around 2.2 eV by PL was not clear from the mapping observation.
In summary, the valence band edge of NiO is probably located near the water oxidation redox energy, and the hole from the valence band edge of GaN is expected to transfer to the NiO valence band edge, which is related to the water oxidation.
[1] B. Sasi et al., Vacuum 68 (2003) 149.
[2] T. M. Ramond et al., J. Molecular Spectroscopy 216 (2002) 1.
[3] V. I. Sokolov et al., Phys. Rev. B 86 (2012) 115128.
3:30 PM - ES02.13.07
Hierarchical Amorphous Molybdenum Sulfide Nanostructures for Hydrogen Evolution
Giorgio Giuffredi 1 2 , Andrea Perego 1 2 , Piero Mazzolini 1 , Fabio Di Fonzo 1
1 Center for Nano Science and Technology, Istituto Italiano di Tecnologia (IIT@Polimi), Milano Italy, 2 Department of Energy, Politecnico di Milano, Milano Italy
Show AbstractHydrogen, in the future energy scenario, is the most promising energy vector for storing and generating electricity. Electro- and photoelectrocatalytic, renewable energies-driven water splitting can be an appealing, CO2-free technology to produce hydrogen, but it needs to overcome several challenges regarding the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).
For the HER, the most employed catalyst is Pt, which exhibits high activity but is expensive and scarce. Efforts have been thus devoted to find an alternative catalyst that is abundant, non-expensive and yields still comparable activity.
In particular, molybdenum sulfide (MoSx) materials – in crystalline (c-MoS2), amorphous (a-MoSx) or cluster form – have gained significant attention thanks to the remarkable catalytic activity and chemical stability in acidic media.
Notwithstanding their impressive performance, the catalytic HER mechanism for c-MoS2 is still unclear [1] and even less is known regarding the mechanism and the role of morphology for a-MoSx, despite its activity being higher than c-MoS2[1],[2],[3] . Classic synthesis methods for MoS-based catalysts also employ energy-intensive processes like high temperature sulfidization.
In this contribution, we propose a nanostructured, mesoporous amorphous molybdenum sulfide catalyst grown by Pulsed Laser Deposition (PLD). With this method, the catalyst can be deposited straightforwardly in a room temperature, energy-efficient process. By controlling the gas dynamics during the process, it is also possible to tune the morphology of the material to maximize the catalyst surface area.
Both a physical and electrochemical characterization of the a-MoSx catalyst are performed, and the variation of chemical activity with morphology and composition is explored.
Our experiments in a 0.5 M H2SO4 electrolyte show that the PLD-grown a-MoSx achieves high current density, up to 500-600 mA/mg at -0.21 V vs RHE, with a good onset potential of -0.1 V vs RHE. Despite having lower activity than state-of-the-art Pt, the PLD-grown catalyst has an increased active area that, in the same electrode volume, can compensate for the minor performance.
The relationship between catalyst composition variation and kinetic properties is tackled, showing a Tafel slope of 50 mV/dec. Despite being higher than the Tafel slope for Pt (30 mV/dec), this value is comparable to other c-MoS2 based catalysts found in the literature.
Finally, electrochemical performance of the catalyst under 1.5 AM illumination is also investigated.
These results show the potential of a PVD-based, energy-efficient technique for the fabrication of a MoS-based HER catalyst with tunable morphological and electrochemical properties, that could reach the performance of the actual state-of-the-art Pt catalyst.
[1] Tran, P.D. et al, Nature Materials 2016 (15)
[2] Merki, D. et al, Chem. Sci., 2011, 2, 1262
[3] Benck, J.D. et al, ACS Catal. 2012, 2
3:45 PM - ES02.13.09
Alkali-Metal Manganese Borophosphates as a Novel Class of Materials for Water Oxidation
Prashanth Menezes 1 , Carsten Walter 1 , Matthias Driess 1
1 , Technical University of Berlin, Berlin Germany
Show AbstractThe development of highly efficient and remarkably stable water oxidation catalysts is required for the realization of practically viable water-splitting systems and therefore, transition metal (TM) catalysts with low-cost, earth-abundant and environmentally benign have attracted much attention in the last few years. Interestingly, inspired by the natural presence of the highly oxygen evolution reaction (OER) active CaMn4O5 cluster in the oxygen evolving complex (OEC) of photosystem II, numerous structurally diverse crystalline as well as amorphous manganese based catalysts have been investigated for water oxidation. However, most of the catalysts explored are mainly oxides or phosphates. In search of efficient catalysts for water oxidation, we explore an entirely new class of materials “borophosphates” which are intermediate compounds of systems MxOy–B2O3–P2O5–(H2O) (M = main group or TM).
Alkali-metal manganese (II) borophosphates were synthesized by a mild hydrothermal synthesis and were well characterized by various state-of-the-art techniques. The catalysts were deposited electrophoretically on fluorinated tin oxide (FTO) and nickel foam (NF) electrode substrates. When applied as oxygen electrodes, the catalysts displayed tremendous activity yielding very low overpotentials in both alkaline as well as in neutral media. The overpotential acquired here are superior to most of the manganese based oxide catalysts and very close to the noble metal derived catalysts. From the spectroscopic and microscopic study, it was uncovered that in alkaline conditions, the catalysts formed hydroxide-oxyhydroxide amorphous shell on the surface that serves as the active species for water oxidation. Strikingly, no such shell formation was observed in neutral media, suggesting stabilization of the structure as well as in the manganese oxidation state. This and other new exciting findings will be discussed.