Symposium Organizers
Francesca Maria Toma, Lawrence Berkeley National Laboratory
Akihiko Kudo, Tokyo University of Science
Roel Van de Krol, Helmholtz-Zentrum Berlin
Lianzhou Wang, University of Queensland
Symposium Support
ACS Energy Letters | ACS Publications
Bruker—Nano Surfaces Division
California Institute of Technology
Helmholtz-Zentrum Berlin (HZB)
Lawrence Berkeley National Laboratory– Joint Center for Artificial Photosynthesis
Wiley VCH Verlag GmbH &
Co. KGaA
ES7.1: CO2 Reduction
Session Chairs
Harry Atwater
Francesca Maria Toma
Roel Van de Krol
Monday PM, April 17, 2017
PCC North, 200 Level, Room 222 BC
9:30 AM - *ES7.1.01
Artificial Photosynthesis—The Selective CO2 Reduction Challenge
Harry Atwater 1
1 , California Institute of Technology, Pasadena, California, United States
Show AbstractThe design of highly efficient devices that generate fuels directly from sunlight, water, and carbon dioxide is both a formidable challenge and an opportunity that, if realized, could have a revolutionary impact on our energy system. In the past five years, considerable progress has been made in scientific discovery of key materials and mechanisms needed to realize artificial solar fuels generators, and advances modeling and design have generated a now quite widely embraced conceptual paradigm for a solar fuels generator. While we still lack sufficient knowledge to design solar-fuel generation systems with the ultimate efficiency, scalability, and sustainability to be economically viable, considerable advances have been made. I will describe work done in this area by the Joint Center for Artificial Photosynthesis and discuss JCAP's approach and recent advances in materials, measurements and device design to advance the understanding of selective carbon dioxide reduction.
10:00 AM - ES7.1.02
Optimizing C-C Coupling on Oxide Derived Copper Catalysts for Electrochemical CO2 Reduction
Yanwei Lum 1 2 , Binbin Yue 3 4 , Alexis Bell 1 5 , Joel Ager 1 2
1 Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States, 3 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 , Center for High Pressure Science and Technology Advanced Research, Shanghai China, 5 Department of Chemical & Biomolecular Engineering, University of California, Berkeley, Berkeley, California, United States
Show AbstractRecently, "oxide derived" copper catalysts have been extensively studied for electrochemical CO2 reduction. Their typical preparation process involves oxidizing metallic copper to form copper oxides which are then reduced back to their metallic state. Catalysts so prepared have been reported to have higher activity and better selectivity towards C2 and C3 (C2+) products compared to their original form. Herein, we systematically compare four different variations of these electrocatalysts (as reported by others in the literature) and show that changing the cation of the electrolyte can further enhance their selectivities and activities towards C2+ products. We utilize two electrolyte conditions, 0.1 M KHCO3 and 0.1 M CsHCO3, and find that the presence of the larger cation in the electrolyte yields increases in selectivity towards C2+ products from -0.7 V to -1.0 V vs RHE for all catalysts studied. On the best performing oxide-derived catalysts with Cs+, we observe up to ~70% selectivity towards C2+ products with only ~3% selectivity towards C1 products at -1.0 V vs RHE. At the same conditions with K+, only ~56% selectivity towards C2+ products was observed. Additionally, study of these different electrocatalysts under the same conditions, combined with in-depth characterization with techniques such as synchrotron x-ray diffraction and x-ray photoelectron spectroscopy, allows us to discern the key factors governing product selectivity. We find that morphology, grain size and surface roughness of the oxide derived layer are critical parameters which affect the catalytic performance. The grain size of an optimally performing catalyst should be small and the morphology has to be optimized so as to raise local pH and enhance selectivity to C2+ products but not result in CO2 mass transport limitations. Electrochemical transport models have also been developed to better instruct future morphological designs of such oxide derived catalysts.
10:15 AM - ES7.1.03
Strain Induced Changes in CO2 Electro-Reduction Pathway at Au-Pd Core-Shell Nanostructures
David Fermin 1 , Jo Humphrey 2 , Veronica Celorrio 1 , Elena Pastor 3 , Paola Quaino 4
1 , University of Bristol, Bristol United Kingdom, 2 , NPL, Teddington United Kingdom, 3 , Universidad de La Laguna, La Laguna Spain, 4 , Universidad Nacional del Litoral, Santa Fe de la Vera Cruz Argentina
Show AbstractCatalytic activity of metal overlayers on foreign support can be significantly altered from their individual components. These phenomena, commonly rationalised in terms of ligand and strain (also known as geometric) effects, originate from changes in the electronic structure of the metal overlayer, with the d-band being the most relevant for catalysis. In this contribution, we shall explore ligand and strain effects on the electrocatalytic reduction of CO2 at carbon-supported Au-Pd core-shell nanoparticles. Pd shells with average thicknesses of 1.3 to 10 nm grown on Au nanoparticles are characterised by effective strain values decreasing from 3.5 to less than 1%. The generation of products during the CO2 reduction were probed by off-line 1H NMR, differential electrochemical mass spectrometry (DEMS) and on-line electrochemical mass spectrometry (OLEMS). Experiments carried out at pH 4 in the presence of SO42- show an accute dependence of product selectivity with the applied potential and thickness of the Pd overlayer. In the case of 1.3 nm Pd shells, the faradaic efficiency towards CO2 reduction can reach values as high as 90%, with the CO being the key product generated. On the other hand, HCOO- , CH4 and C2H6 are detected in Pd shells with thickness of 5 and 10 nm. Electrochemical responses recorded duing CO2 reduction also show evidences of the formation of adsorbed CO intermediates with increasing Pd thickness. We further probe the interaction of CO with Pd nanoshells employing in-situ FTIR spectroscopy, concluding that overall affinity and orbital coupling of CO to Pd is weaker at strained shells. Finally, we use DFT to compute the contributions of strain and ligand effects on the d-band centre, concluding that the former has the most signfiicant contribution to the electronic structure of the Pd shells in this length scale, dictating the overall CO2 reduction pathway.
10:30 AM - ES7.1.04
Surface Science Insights into the Role of the Electrode Surface in Solar-Driven Pyridine-Catalyzed CO2 Reduction
Coleman Kronawitter 1 , Bruce Koel 2
1 Department of Chemical Engineering, University of California, Davis, Davis, California, United States, 2 Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractThe design of new technologies that shift U.S. power consumption away from fossil fuels toward sustainable alternatives must take into account the nation's large-scale need for stored chemical fuels. Photoelectrocatalytic CO2 reduction is one such technology, since it facilitates a process whereby solar energy is used to reduce CO2, a combustion product, into chemical fuels that are compatible with the existing U.S. energy infrastructure. We report here on experimental studies of well-defined surfaces to investigate the role of the electrode surface in pyridine-catalyzed CO2 reduction, which is reported to be associated with high yields for methanol and formic acid. Ambient pressure photoelectron spectroscopy (APPES) was used to spectroscopically identify in situ surface-bound species formed by the interaction of water on GaP(110), the most stable surface of GaP, at pressures up to 1 Torr. The data show that the interaction with water is characterized by the presence of a partially dissociated adlayer, with Ga-OH, P-H, and adsorbed molecular H2O species detected on the surface. This is consistent with previously published theoretical work that predicts the presence of this layer. In addition, we used isobaric APPES measurements at elevated pressures to probe the thermal stabilities of adsorbed species as well as the oxidation of surface Ga and P. We observe the surface hydride to be remarkably stable in the presence of water, which is notable given the critical role of hydride transfer to catalysts and CO2 during chemical fuel synthesis reactions in aqueous environments. It is hypothesized that the high observed stability of the hydride on GaP may contribute to its associated remarkable near-100% faradaic efficiency for methanol generation by solar-driven CO2 reduction. We have also obtained orbital-resolved information on the adsorption state of pyridine (C5H5N) on GaP(110) using scanning tunneling microscopy (STM). By examining the distribution of unoccupied molecular orbitals with high spatial and energetic resolution, we showed that scanning probe techniques can be used to positively identify the sites on pyridine susceptible to nucleophilic attack, consistent with frontier orbital theory. This technique can be used to explore the local reaction centers of adsorbed catalysts relevant to artificial photosynthesis. Our observations of the stable adsorption of both H and pyridine on this surface is notable, because it characterizes the proposed precursor state for the formation of adsorbed dihydropyridine, which could be a key hydride-shuttling catalyst for heterogeneous CO2 reduction.
10:45 AM - ES7.1.05
Investigations of Heterogeneous Processes for CO2 Reduction Involving Molecular Co-Catalysts
Coleman Kronawitter 1 , Bruce Koel 2
1 , University of California, Davis, Davis, California, United States, 2 , Princeton University, Princeton, New Jersey, United States
Show AbstractThe surface chemistry of molecular co-catalysts that enable the selective electrochemical reduction of CO2 to fuels is discussed. The presented experimental results focus on elucidating the role of the electrode surface in CO2 reduction reactions that are co-catalyzed by N-containing heteroaromatics. A combination of ex situ and operando vibrational spectroscopy measurements on model surfaces are used to supplement electrochemistry investigations. Specifically, we characterize the preferential adsorption sites, bonding interactions, and reactivities of N-containing heteroaromatics on platinum group metals and III-V semiconductors. Conclusions from experimental results on model systems are supported by calculations using density functional theory. This work assists in generating a molecular-level understanding of the heterogeneous processes important to the reaction mechanisms involved in the efficient (photo)electrocatalytic generation of carbon-containing fuels with high energy densities.
11:30 AM - *ES7.1.06
Solar CO2 Reduction Coupled with Water Oxidation—Semiconductor/Metal-Complex Hybrid System
Takeshi Morikawa 1 , Shunsuke Sato 1 , Takeo Arai 1 , Keita Sekizawa 1 , Tomiko Suzuki 1
1 , Toyota Central R&D Labs, Nagakute-shi Aichi-ken Japan
Show AbstractWe have proposed the novel concept of selective CO2 photoreduction using a hybrid photocatalyst comprising of semiconductor photosensitizer and metal-complex electrocatalyst [1-3]. The semiconductor/metal-complex system has recently been integrated to a monolithic tablet-shaped device of Ru-complex polymer/carbon/SiGe-jn/IrOx. By applying functions of selective reduction and oxidation reactions to the catalytic Ru-complex polymer/carbon and IrOx sites, respectively, the device operated in a single-compartment reactor in phosphate buffer at pH =6.4, and generated formate with a very high solar-to-chemical conversion efficiency of 4.6 %, which could make the future of solar CO2 reduction to useful chemicals more reliable [4].
In the hybrid system, transfer of conduction band electrons in a photoexcited state to metal-complex catalyst is the key factor, and p-type N-doped Ta2O5 [5] and p-type Fe2O3 [6,7] with an upward band-edge shift induced by N-doping facilitated the electron transfer resulting in the CO2 reduction reaction. We will discuss the unusual band shift of around 1 eV from a viewpoint of surface dipole moment [8], and correlated CO2 reduction reaction and hydrogen generation over the semiconductors with the upward shift [9,10]. New electrocatalysts applicable to the semiconductor/metal-complex system will also be presented.
[references]
[1] S. Sato, T. Morikawa, S. Saeki, T. Kajino, T. Motohiro, Angew. Chem. Int. Ed. 2010, 49, 5101-5105.
[2] T. M. Suzuki, H. Tanaka, T. Morikawa, M. Iwaki, S. Sato, S. Saeki, M. Inoue, T. Kajino, T. Motohiro, Chem. Commun. 2011, 47, 8673-8675.
[3] S. Sato, T. Arai, T. Morikawa, K. Uemura, T. M. Suzuki, H. Tanaka, T. Kajino, J. Am. Chem. Soc., 2011, 133, 15240-15243.
[4] T. Arai, S. Sato, T. Morikawa, Energy Environ. Sci., 2015, 8, 1998-2002.
[5] T. Morikawa, K. Saeki, T. M. Suzuki, T. Kajino, T. Motohiro, Appl. Phys. Lett. 2010, 96 142111.
[6] T. Morikawa, K. Kitazumi, N. Takahashi, T. Arai, T. Kajino, Appl. Phys. Lett. 2011, 98, 242108.
[7] T. Morikawa, T. Arai, T. Motohiro, Appl. Phys. Express, 2013, 6 041201.
[8] R. Jinnouchi, A. V. Akimov, S. Shirai, R. Asahi, O. V. Prezhdo, J. Phys. Chem. C 2015, 119, 26925.
[9] T. M. Suzuki, S. Saeki, K. Sekizawa, K. Kitazumi, N. Takahashi, T. Morikawa, Appl. Catal. B: Environ. 2017, 202 597.
[10] K. Sekizawa, T. Morikawa, submitted.
12:00 PM - ES7.1.07
High Efficiency Solar-Fuel Devices
Chengxiang (CX) Xiang 1
1 Joint Center for Artificial Photosynthesis, California Institute of Technology, Pasadena, California, United States
Show AbstractEfficient solar-fuel devices require synthetic assembly of light absorbers, electrocatalysts, membrane separators and electrolytes. Over the past years, we have developed a robust multi-physics, multi-dimensional, solar-fuel device model to guide the materials discovery, to define operational conditions and constraints and to optimize and explore viable device architectures for solar-driven water-splitting devices as well as solar-driven CO2 reduction devices. In particular, the modeling revealed operational constraints and strategies for system to perform sustainable and efficient solar-driven CO2 reduction at atmospheric conditions (400 ppm). The model also addressed ionic couplings between the catholyte and anolyte at different bulk pH values by the use of bipolar membranes.
Through the modeling and simulation guided device development, a range of stable and intrinsically safe solar-driven water-splitting devices with >10% conversion efficiency have been assembled and characterized by the JCAP prototyping team, including photocathode devices and photoanode devices, as well as devices that operated at alkaline conditions, at acidic conditions and at near-neutral pH conditions. Recently, we have also demonstrated a solar-driven CO2 reduction device with a record-high solar-to-fuel conversion efficiency of 10% under 1.0 sun illumination. The device operated in two electrolytes with different pHs to achieve the optimal performance and the lowest overpotentials for oxygen evolution reaction and CO2 reduction to a liquid product, formate with robust product separations.
12:15 PM - ES7.1.08
Highly Efficient Artificial Photosynthesis with Low-Cost Transition Metal Electrocatalysts in Aqueous Solution
Kun Jiang 1 , Samira Siahrostami 2 , Zhiyi Lu 2 , Chris Stokes 1 , Jens Norskov 2 , Yi Cui 2 , Haotian Wang 1
1 Rowland Institute, Harvard University, Cambridge, Massachusetts, United States, 2 , Stanford University, Stanford, California, United States
Show AbstractUtilizing solar energy to fix carbon dioxide (CO2) with water into chemical fuels and oxygen, a mimic process of photosynthesis in nature, is becoming increasingly important but still challenged by the low selectivity and activity especially in CO2 electrocatalytic reduction. Here we report a Ni-N doped graphitized carbon nanofiber as an aqueous CO2 reduction to carbon monoxide (CO) electrocatalyst, with a high Faradic efficiency of 93 % under a high current density of ~ 20 mA/cm2mg. Coupling with Li+ cycled CoO as a highly active oxygen evolution catalyst in neutral pH, and a commercialized solar cell, we demonstrate an artificial photosynthesis system with a solar to CO2 reduction efficiency of ~ 10 %.
12:30 PM - ES7.1.09
Fabrication of Copper Oxide Photocathode for CO2 Reduction
Guiji Liu 1 , Francesca Maria Toma 1
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractCO2 reduction using solar energy could not only help to 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 photo-electrode using electrochemical deposition, which is able to possess abundant active sites and high electrical conductivity, showing promise for substantially promoting the CO2 reduction.
12:45 PM - ES7.1.10
Photocatalytic Conversion of CO2 over Nanostructures into Solar Fuels
Yong Zhou 1 , Congping Wu 1 , Zhigang Zou 1
1 , Nanjing University, Nan Jing China
Show AbstractIn recent years, the increase of carbon dioxide (CO2) in the atmosphere has become a global environmental issue because of the serious problems, such as the “greenhouse effect”. The idea of mimicking the overall natural photosynthetic cycle of chemical conversion of CO2 into useful fuels has been consistently gaining attention for more than thirty years. Such artificial photosynthesis allows direct conversion of CO2 and water on photocatalysts into valuable hydrocarbon using sunlight at room temperature and ambient pressure to serve to reduce atmospheric CO2 concentrations while providing on a renewable carbon fixation and energy storage.
In this presentation, we will report the utilization of solar energy to highly efficient conversion of CO2 into renewable hydrocarbon fuel over structured nanomaterials. The geometric shape and exposure of specific crystal planes of the nanostructures as well as combination of graphene as a good electron collector and transporter are a requisite for the high level of photocatalytic reduction of CO2.
References
(1) Haijin Li, Yuying Gao, Yong Zhou, Fengtao Fan, Qiutong Han, Qinfeng Xu,Xiaoyong Wang, Min Xiao, Can Li, and Zhigang Zou, Nano Lett. 2016, 16, 5547.
(2) P. Li, Y. Zhou, Z. Zhao, Q. Xu, X. Wang, M. Xiao, and Z. Zou, J. Am. Chem. Soc. 2015, 137, 9547.
(3) H. Li, Y. Zhou, W. Tu, J. Ye, and Z. Zou, Adv. Funct. Mater. 2015, 25, 998.
(4) W. Tu, Y. Zhou, and Z. Zou, Adv. Mater. 2014, 26, 4607
ES7.2: Advanced Architectures for Solar Fuels
Session Chairs
Takeshi Morikawa
Bruce Parkinson
Monday PM, April 17, 2017
PCC North, 200 Level, Room 222 BC
2:30 PM - *ES7.2.01
Sunlight-Driven Hydrogen Formation by Membrane-Supported Photoelectrochemical Water Splitting
Nathan Lewis 1
1 , California Institute of Technology, Pasadena, California, United States
Show AbstractWe are developing an artificial photosynthetic system that utilizes sunlight and water as the inputs and produce hydrogen and oxygen as the outputs. We are taking a modular, parallel development approach in which three distinct primary components-the photoanode, the photocathode, and the product-separating but ion-conducting membrane-are fabricated and optimized separately before assembly into a complete water-splitting system. The design principles incorporate two separate, photosensitive semiconductor/liquid junctions that will collectively generate the 1.7-1.9 V at open circuit necessary to support both the oxidation of H2O (or OH-) and the reduction of H+ (or H2O). The photoanode and photocathode will consist of rod-like semiconductor components, with attached heterogeneous multi-electron transfer catalysts, which are needed to drive the oxidation or reduction reactions at low overpotentials. The high aspect-ratio semiconductor rod electrode architecture allows for the use of low cost, earth abundant materials without sacrificing energy conversion efficiency due to the orthogonalization of light absorption and charge-carrier collection. Additionally, the high surface-area design of the rod-based semiconductor array electrode inherently lowers the flux of charge carriers over the rod array surface relative to the projected geometric surface of the photoelectrode, thus lowering the photocurrent density at the solid/liquid junction and thereby relaxing the demands on the activity (and cost) of any electrocatalysts. A flexible composite polymer film will allow for electron and ion conduction between the photoanode and photocathode while simultaneously preventing mixing of the gaseous products. Separate polymeric materials will be used to make electrical contact between the anode and cathode, and also to provide structural support. Interspersed patches of an ion conducting polymer will maintain charge balance between the two half-cells.
3:00 PM - ES7.2.02
Nano-Photoelectrochemical Cell Arrays with Spatially Isolated Oxidation and Reduction Channels
Hang-Ah Park 1 , Siyuan Liu 1 , Youngseok Oh 1 , Paul Salvador 1 , Gregory Rohrer 1 , Mohammad Islam 1
1 , Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractPhotocatalysts and photoelectrochemical cells (PECs) have been broadly explored for diverse applications but suffer from poor efficiencies due to limited solar absorption, inadequate charge carrier separation, and redox half-reactions occurring in close proximity. We have developed massively parallel nano-PECs composed of an array of carbon nanotubes (CNTs) with photoanodic reactions occurring on TiO2 coated outer walls and photocathodic reactions occurring on Pt decorated inner walls of the CNTs, isolating reactions in separate flow channels. The PECs exhibit an enlarged solar absorption, efficient carrier separation, and a 1.8% photon-to-current conversion efficiency (a current density of 4.5 mA/cm2) under white-light irradiation. The design principles to control the nanoscale architecture in these cells can be readily adapted to myriads of photocatalysts for cost-effective solar utilization.
3:15 PM - ES7.2.03
Core-Shell Micro-Tube Array for Closing the Artificial Photosynthesis Cycle on a Nanometer Scale
Eran Edri 1 , Heinz Frei 1
1 , Lawrence Berkeley National Lab, Berkeley, California, United States
Show AbstractCrossover of intermediates and products of the water oxidation and CO2 reduction reactions are counter-productive to the making carbon-based fuels by light-driven processes. A separation of the reactions’ environment is required, but in a way that will minimize the potential loss and/or obviate the need for an extreme pH conditions. Designs that can compartmentalize the two reactions yet keep them electronically and ionically connected with a nanometer scale separation are needed.
Using an inorganic material with embedded conjugated organic molecules forms a molecularly tight material that facilitates proton conduction through the inorganic matrix, and energy specific electronic charge transport through the conjugated molecules. Casting this membrane externally on a tube made of a water oxidation catalyst and further decorating it outwardly with a CO2 reduction photo-catalyst, provides a construct that separate the reaction environments, yet connect the reaction sites electronically and protonically.
A fabrication path for making a centimeter scale array of such tubes will be presented, alongside our recent studies of the mechanism of charge transport through the conjugated molecules.
3:30 PM - ES7.2.04
Design of Photonic and Plasmonic Materials for Photocatalytic CO2 Reduction
Ashley Gaulding 1 , Ian Sharp 1 , Adam Schwartzberg 1 , Francesca Maria Toma 1
1 , Lawrence Berkeley National Lab, Berkeley, California, United States
Show AbstractThis work explores the effects of photonic structuring and localized surface plasmon resonance (LSPR) and on CO2 reduction reactions (CO2RR). We design photonic crystals where the photonic bandgap is tuned by changing the periodicity of the inverse opal cavities. The photonic bandgap of an inverse opal structure can be tuned by the material and pore size/periodicity. These photonic properties control how the incoming light is reflected and refracted within the material. We investigate a variety of photonic crystal materials, such as metal oxides, by infilling the voids of an opal template of polystyrene (PS) spheres to obtain an inverse opal photonic crystal by subsequently removing the PS spheres.
Similarly, the LSPR can be tuned in metal or degeneratively doped semiconductor nanoparticles by tuning the size, shape, and material composition. This increases absorption near the resonance peak. Additionally, hot carriers can be generated as the LSPR decays, providing the potential to access kinetic pathways normally difficult for desired products in CO2RR chemistry. Since we can tune the photonic bandgap simply by changing the size of the PS spheres in the template, we can investigate how matching the photonic bandgap with the plasmonic resonance of a metal particle affects CO2 reduction. We then study the effects of combining both of these physics phenomena on chemical transformation and selectivity in CO2 reduction. From this basis, one could potentially tune CO2RR processes to a desired hydrocarbon fuel product.
We use a combination of SEM, TEM, and XRD to analyze the morphological structure and crystallinity of our materials as well as material composition via EDX. Surface analysis of our structures is done using XPS. We characterize the films’ optical properties, monitoring shifts in the photonic bandgap and LSPR, via UV-Vis-NIR spectroscopy.
3:45 PM - ES7.2.05
Solid-State Architecture for a High-Current, Elevated-Temperature Photoelectrochemical Cell
Madhur Boloor 1 , Xiaofei Ye 1 , Liming Zhang 1 , Nick Melosh 1 , William C. Chueh 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractThe majority of photoelectrochemical (PEC) cells used for solar water splitting operate near room temperature with a liquid or polymeric electrolyte, and are limited by slow kinetics and thermodynamic losses at interfaces. We have recently shown that even a small temperature increase can substantially improve the minority carrier collection efficiency [1,2]. Additionally, using a detailed-balance approach, we show that a solid-state heterojunction device operating at 450°C could achieve solar-to-hydrogen efficiencies as high as 17% [3].
In this work, we develop a novel solid-state architecture for elevated temperature photoelectrochemistry using an oxide solid electrolyte. I will discuss the fabrication and characterization of the solid-state PEC cell composed of a BiVO4 light absorber and yttria-stabilized zirconia thin-film solid electrolyte. Our cell design enables us to achieve photocurrent densities above 100 mA/cm2. To the best of our knowledge, this is the highest current density observed in literature for a solid-state photoelectrochemical cell operating at an elevated temperature range of 300 to 400°C [4]. In addition to optimizing the oxide layers, the influence of current collector morphology and deposition technique on cell performance is also investigated. A UV diode and a concentrated solar simulator were used to understand the dependence of cell photocurrent and photovoltage on light intensity and wavelength.
[1] X. Ye, et al. J. Mater. Chem. A, 2015, 3, 10801-10810.
[2] L. Zhang, et al. Energy Environ. Sci., 2016, 9, 2044-2052.
[3] X. Ye, et al. Phys. Chem. Chem. Phys., 2013, 15, 15459-15469.
[4] G. Brunauer, et al. Adv. Funct. Mater., 2016, 26, 120–128.
ES7.3: Novel Concepts
Session Chairs
Nathan Lewis
Lianzhou Wang
Monday PM, April 17, 2017
PCC North, 200 Level, Room 222 BC
4:30 PM - ES7.3.01
Photoelectrochemical Tandem Configuration for Solar Water Splitting Exceeding 7% Using Photon Recycling
Jong Hyeok Park 1 , Jong Kyu Kim 2 , Pil Jin Yoo 3
1 , Yonsei University, Seoul Korea (the Republic of), 2 , Pohang University of Science and Technology (POSTECH), Pohang Korea (the Republic of), 3 , SKKU, Suwon Korea (the Republic of)
Show AbstractVarious tandem cell configurations have been reported for highly efficient and spontaneous hydrogen production from photoelectrochemical solar water splitting. However, there is a contradiction between the two main requirements of a front photoelectrode in a tandem cell configuration, namely, high transparency and high photocurrent density. Here we demonstrate a simple yet highly effective method to overcome this contradiction by incorporating a hybrid conductive distributed Bragg reflector on the back side of the transparent conducting substrate for the front photoelectrochemical electrode, which functions as both an optical filter as well as a conductive substrate for counter-electrode of the rear dye-sensitized solar cell. The hybrid conductive distributed Bragg reflectors were designed to be transparent to the long-wavelength part of the incident solar spectrum (λ>500 nm) for the rear solar cell, while reflecting back the short-wavelength photons (λ<500 nm) which can then be absorbed by the front photoelectrochemical electrode for enhanced photocurrent generation.
4:45 PM - *ES7.3.02
Photoelectrochemical Solar Energy Storage—Hydrogen Production vs Direct CO2 Reduction and Photoredox Flow Batteries
Bruce Parkinson 1
1 , University of Wyoming, Laramie, Wyoming, United States
Show AbstractThe time window for producing a photoelectrochemical solar energy system is closing. Unless new stable and efficient semiconductor materials with near optimum band gaps are discovered or conventional high mobility materials can be stabilized in electrolytes, present technology of photovoltaics coupled to electrolyzers will be more practical and cost effective. However producing hydrogen from water is still the preferred target rather than direct carbon dioxide reduction since carbon dioxide is much more easily reduced to useful fuels using gas phase reactors with renewable hydrogen. Many arguments to support this supposition will be given, In addition a new concept of a photodriven redox flow battery will be presented that circumvents many of the problems associated with multielectron/multiproton reactions associated with water splitting and has the additional advantage of allowing for thermal storage. A prototype system will be presented.
Symposium Organizers
Francesca Maria Toma, Lawrence Berkeley National Laboratory
Akihiko Kudo, Tokyo University of Science
Roel Van de Krol, Helmholtz-Zentrum Berlin
Lianzhou Wang, University of Queensland
Symposium Support
ACS Energy Letters | ACS Publications
Bruker—Nano Surfaces Division
California Institute of Technology
Helmholtz-Zentrum Berlin (HZB)
Lawrence Berkeley National Laboratory– Joint Center for Artificial Photosynthesis
Wiley VCH Verlag GmbH &
Co. KGaA
ES7.4: Advanced Characterization and Operando Studies
Session Chairs
Rainer Eichberger
Dunwei Wang
Tuesday AM, April 18, 2017
PCC North, 200 Level, Room 222 BC
11:30 AM - *ES7.4.01
In situ and Operando Characterization of CO2 Reduction Reaction Catalysts Using Soft and Hard X-Ray Spectroscopy
Marco Favaro 1 3 4 , Maryam Farmand 1 4 , Sean Fackler 1 4 , Walter Drisdell 1 4 , Ethan Crumlin 3 , Junko Yano 1 2
1 Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Lab, Berkeley, California, United States, 3 Advanced Light Sources, Lawrence Berkeley National Lab, Berkeley, California, United States, 4 Chemical Sciences Division, Lawrence Berkeley National Lab, Berkeley, California, United States, 2 Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Lab, Berkeley, California, United States
Show AbstractImproved catalysts for electroreduction of carbon dioxide are highly important for promoting the generation of carbon-based reduction products. To gain a fundamental understanding needed to tailor novel catalysts, in particular for the selectivity of the products, the information of the early steps of the electroreduction process on catalyst surfaces is important. We have optimized and utilized surface-sensitive soft and hard X-ray techniques, including grazing incident X-ray absorption spectroscopy, X-ray diffraction, and ambient pressure X-ray photoemission spectroscopy to investigate the interaction of metal catalytic surfaces with electrolytes and/or gases (CO2 and/or H2O) under in situ/operando conditions. We report ambient pressure X-ray photoelectron spectroscopy experiments to investigate the initial steps of adsorbed CO2 on a copper surface in the presence of water. We also studied the chemical nature of the catalyst surface using X-ray absorption spectroscopy with grazing incident geometry. We discuss some results based on recent experiments and we discuss the progress we have made in this endeavor.
12:00 PM - ES7.4.02
Corrosion Behavior of p-GaInP2 Thin Films for Photoelectrochemical Water Splitting Studied by Ambient Pressure X-Ray Photoelectron Spectroscopy
Monika Blum 1 , James Young 2 3 , Dagmar Kreikemeyer-Lorenzo 4 , Lothar Weinhardt 1 4 5 , Ethan Crumlin 6 , Todd Deutsch 2 , Clemens Heske 1 4 5
1 Chemistry and Biochemistry, University of Nevada, Las Vegas, Las Vegas, Nevada, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Materials Science and Engineering, University of Colorado Boulder, Boulder, Colorado, United States, 4 Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen Germany, 5 Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology, Karlsruhe Germany, 6 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractTo date, devices based on p-GaInP2 epitaxial thin films in a tandem configuration with p/n-GaAs represent the most promising material for photoelectrochemical (PEC) solar water splitting, achieving a 14.3% solar-to-hydrogen (STH) conversion efficiency [1]. Apart from its high efficiency, the material presents challenges for practical long-term PEC applications due to corrosion under operation. Recently, a nitrogen ion bombardment treatment of p-GaInP2, coupled with a Pt/Ru catalyst treatment, has been shown to prevent corrosion while retaining high efficiency. To further improve the material, it is essential to understand the corrosion behavior during PEC operation with and without treatment.
We have employed synchrotron-based ambient pressure (tender) x-ray photoelectron spectroscopy [2] to investigate the electronic and chemical structure of the p-GaInP2 films at the solid/liquid interface during operando measurements. The obtained information sheds light on the influence of electrolyte and applied potential on the material, including the corrosion behavior during PEC operation and the impact of sample surface treatments. Based on this data, the chemical surface composition can be retrieved, and insights into the photocathode/electrolyte interface can be gained.
References
[1] https://www.hydrogen.energy.gov/pdfs/review16/pd115_deutsch_2016_o.pdf
[2] S. Axnanda, Scientific Reports 5, 9788 (2015).
12:15 PM - ES7.4.03
X-Ray Characterization of Solar Fuels Catalysts under Operation
Walter Drisdell 1 , Jeremy Feaster 2 , Maryam Farmand 1 , John Lin 2 , Sean Fackler 1 , Alan Landers 2 , Jeffery Beeman 1 , Ryan Davis 3 , Thomas F. Jaramillo 2 , Junko Yano 1 , Apurva Mehta 3
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 , Stanford University, Stanford, California, United States, 3 , SLAC National Accelerator Laboratory, Menlo Park, California, United States
Show AbstractThe mission of the Joint Center for Artificial Photosynthesis is to develop a prototype device for the photoelectrichemical reduction of CO2 to fuels using sunlight. Current CO2 reduction catalysts, however, do not display enough efficiency or selectivity to be viable. To design improved catalysts, a fundamental understanding of the catalytic mechanisms for CO2 reduction is needed. We present a combined grazing incidence X-ray absorption spectroscopy (GI-XAS) and grazing incidence X-ray diffraction (GI-XRD) study of metallic and metallic alloy catalysts for CO2 reduction under operating conditions. These techniques allow probing of the top 2-3 nm of the catalyst surface, and reveal long-term evolution of catalyst surface structure and chemistry during operation. These findings demonstrate that the conditions at the catalyst surface deviate significantly from predictions, and demonstrate the power of this in situ characterization technique for determining mechanistic details of electrocatalysis.
12:30 PM - ES7.4.04
Soft X-Ray Spectroscopic Investigation of the CdS/Cu(In,Ga)S2 Interface in Thin Films for Photoelectrochemical Water Splitting
James Carter 1 , Bridget Elizan 1 , Monika Blum 1 , Kim Horsley 2 , Alex DeAngelis 2 , Wanli Yang 4 , Lothar Weinhardt 1 3 5 , Nicolas Gaillard 2 , Clemens Heske 1 3 5
1 , University of Nevada, Las Vegas, Las Vegas, Nevada, United States, 2 Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, Hawaii, United States, 4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen Germany, 5 Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology, Karlsruhe Germany
Show AbstractIn recent years, the efficiency of various solar devices has significantly increased on both the large and laboratory scale. To further drive the optimization of device efficiency, a detailed understanding of the chemical and electronic structure of the surfaces and interfaces is needed. Chalcopyrite-based compounds, such as Cu(In,Ga)(S,Se)2 (CIGSSe), are regarded as one of the most promising material systems for use as an absorber in highly efficient solar devices. In the context of photoelectrochemical (PEC) hydrogen generation, the band gap and band edges of these materials can be tailored and optimized to the solar spectrum and the water splitting potentials, respectively. This, in turn, makes them a promising candidate for utilization in PEC devices.
In this contribution, the chemical and electronic structure of Cu(In,Ga)S2 (CIGS) based thin films and their interfaces to CdS buffer layers are investigated by a number of different soft x-ray and electron spectroscopy techniques, both at UNLV and at Beamline 8.0.1 of the Advanced Light Source, Lawrence Berkeley National Lab. These techniques include x-ray photoelectron (XPS) and x-ray emission (XES) spectroscopy for determination of the surface and near-surface chemical composition, respectively, as well as complementary ultraviolet photoelectron (UPS) and inverse photoemission (IPES) spectroscopy for a derivation of the band gap. Combining XPS, UPS, and IPES, a full description of the interfacial band alignment can be obtained.
Our sample series, prepared at HNEI, consisted of CdS buffer layers with increasing thickness, deposited on a CIGS substrate by chemical bath deposition (CBD). The experimental results provide valuable information about the CdS/CIGS interface and will be discussed with respect to sample treatment, sulfurization, and buffer layer thickness.
12:45 PM - ES7.4.05
Characterization of BiVO4 Powders and Thin Films by Electron Microscopy
Hector Calderon 1 , Francesca Maria Toma 2 , Jason Cooper 2 , Ian Sharp 2 , Peter Ercius 3 , Oscar Cigarroa 1 , Juan Ernesto Neri 1
1 Dept. Fisica, Instituto Politecnico Nacional - ESFM, CDMX Mexico, 2 Chemical Sciences Division, LBNL-JCAP, Berkeley, California, United States, 3 Molecular Foundry-NCEM, LBNL , Berkeley, California, United States
Show AbstractBiVO4 is a candidate for solar light capture in a device transforming solar energy into a fuel. BiVO4 powders and thin films have been characterized by electron microscopy in order to determine the possible effect of selected elemental additions. The techniques in use include electron microscopy imaging (STEM and TEM) and EELS (electron energy loss spectroscopy). STEM is done by controlling the scanning rate in an effort to reduce electron dose and the corresponding beam damage. Imaging can be done to an atomic resolution level but the sampling dose is still of the order of 170 e-/Å2 s and both powder particles and thin films are quickly damaged after a single image. EELS in STEM produces clear differences between sample edge and interior volume in linear profiles as already shown in the literature for the pure compound. Nevertheless addition of alloying elements can greatly modify such patterns. Transmission electron microscopy is better suited to control the beam dose rate to levels of the order of 10 e-/Å2s that allow using exit wave reconstruction with focal series of at least 40 images. Atomic resolution and determination of the nature of columns from the corresponding contrast can be achieved with such a procedure. Measurements of lattice spacings and simulation of contrast allows characterization of both powder particles and thin films by determining their atomic arrangements as they were synthesized and without appreciable damage.
ES7.5: Understanding Interfaces
Session Chairs
Tuesday PM, April 18, 2017
PCC North, 200 Level, Room 222 BC
2:30 PM - *ES7.5.01
Charge Transfer Processes in Catalyzed Semiconductor Photoelectrodes
Shannon Boettcher 1 , Jingjing Qiu 1
1 , University of Oregon, Eugene, Oregon, United States
Show AbstractLight-absorbing semiconductor electrodes coated with electrocatalysts are key components of photoelectrochemical energy conversion and storage systems. Efforts to optimize these systems have been slowed by an inadequate understanding of the semiconductor-electrocatalyst interface. The semiconductor-electrocatalyst interface is important because it separates and collects photoexcited charge carriers from the semiconductor. The photovoltage generated across the interface drives “uphill” photochemical reactions, such as water splitting to form hydrogen fuel.
In this presentation I will describe efforts to understand the microscopic processes and materials parameters governing interfacial electron transfer between light-absorbing semiconductors, electrocatalysts, and solution, using simple theory and simulation1 coupled with new experimental “dual-working-electrode photoelectrochemistry” approaches.2 I will review our initial work on model catalyzed TiO2 single crystals,2 describe progress in understanding catalyzed α-Fe2O3 thin-film and n-Si photoelectrodes, and our latest development of electrochemical atomic force microscopy imaging techniques that allow for understanding interface effects at the nanoscale on a broad array of photoelectrodes.3
These results in sum provide new insight to guide the interface design of efficient water-splitting photoelectrodes, particularly in the case were the electrocatalyst layer is electrolyte permeable4 and forms so-called “adaptive” junctions where the effective interfacial barrier height to electron transfer depends on the charge state of the catalyst.5,6
(1) Mills, T. J.; Lin, F.; Boettcher, S. W. Theory and simulations of electrocatalyst-coated semiconductor electrodes for solar water splitting. Phys. Rev. Lett. 2014, 112, 148304.
(2) Lin, F.; Boettcher, S. W. Adaptive semiconductor/electrocatalyst junctions in water-splitting photoanodes. Nat. Mater. 2014, 13, 81-86.
(3) Nellist, M.; et.al. PeakForce Scanning Electrochemical Microscopy with Nanoelectrodes for High Resolution Electrochemical, Nanomechanical and Nanoelectrical Imaging In Preparation 2016.
(4) Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W. Cobalt-Iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts: The Role of Structure and Composition on Activity, Stability, and Mechanism. J. Am. Chem. Soc. 2015, 137, 3638-3648.
(5) Nellist, M. R.; Laskowski, F. A. L.; Lin, F.; Mills, T. J.; Boettcher, S. W. Semiconductor–Electrocatalyst Interfaces: Theory, Experiment, and Applications in Photoelectrochemical Water Splitting. Acc. Chem. Res. 2016, 49, 733-740.
(6) Lin, F.; Bachman, B. F.; Boettcher, S. W. Impact of Electrocatalyst Activity and Ion Permeability on Water-Splitting Photoanodes. J. Phys. Chem. Lett. 2015, 6, 2427-2433.
3:00 PM - *ES7.5.02
Probing Energetics and Kinetics at the Interface of the Photoelectrode and Water
Dunwei Wang 1 , Yumin He 1
1 , Boston College, Chestnut Hill, Massachusetts, United States
Show AbstractPhotocatalysis utilizes the energy delivered by light and enables chemical reactions that otherwise cannot take place. When used to power thermodynamically uphill reactions, photocatalysis offers a solution to large-scale solar energy storage. Despite over four decades of intense research, however, photocatalysis remains either too expensive or too inefficient or both. Poor understanding of the mechanisms behind the low performance is a key reason that limits the progress of this important field. To address this critical challenge, and to complement existing efforts focused on discovering new materials for photocatalysis, we present here a series of experiments designed to elucidate the working principles of photocatalysis. First, we carried out measurements of the system under quasi-equilibrium conditions, and the goal was to understand how the charge separation capability is influenced by the surface treatments of the photocatalyst. Next, we conducted intensity modulated photocurrent spectroscopy and established a quantitative correlation between surface treatment and charge transfer kinetics.[6] Afterward, we compared two co-catalyst systems, one heterogeneous and one homogeneous and found that they improve the performance of the photocatalyst by fundamentally different mechanisms.[7] Lastly, we employed synchrotron X-ray spectroscopy to provide a complete picture of surface energy evolution as a function of water splitting history.[8] The key value generated by this body of research lies in the fundamental understanding of the thermodynamics and kinetics of the photocatalyst/electrolyte interface, which is expected to serve as a foundation for future research on photocatalysis.
3:30 PM - ES7.5.03
Direct Observation of Photoelectrochemical Water Oxidation Intermediates on α-Fe2O3 Electrode Surfaces Employing Operando ATR–IR Spectroscopy
Omid Zandi 1 , Thomas Hamann 2
1 , University of Texas at Austin, Austin, Texas, United States, 2 , Michigan State University, East Lansing, Michigan, United States
Show AbstractA combination of proper band energy, electrochemical stability, and abundance places hematite (α-Fe2O3) among the most promising photoanode materials for photoelectrochemical (PEC) water splitting. It is well known that water oxidation efficiency on hematite is hindered by recombination of photogenerated holes with near-surface conduction band electrons. Previous studies have reported indirect evidence of oxidized surface states (holes) mediating the water oxidation reaction. In this work, the surface of hematite electrodes was probed using operando ATR-IR spectroscopy during PEC water splitting in order to gain insight on the chemical nature of surface holes. A potential and light-dependent IR absorption peak was reproducibly observed at 898 cm-1, which is correlated to surface trapped holes building up during current-potential measurements. This IR signature is assigned to a FeIV=O group, which is an intermediate in the PEC water oxidation reaction. Control experiments in contact with a hole scavenger and isotopically labeled water corroborate the assignment of this spectral feature to oxygen containing groups involved in the water oxidation reaction. These results establish the mechanism of PEC water oxidation on hematite by providing the first direct evidence of high-valent iron-oxo intermediates as the product of the first hole transfer reaction on the hematite surface.
3:45 PM - ES7.5.04
Amorphous Molybdenum Sulphide—Surface Water Dependent Properties, Humidity Sensing and Electrolyte Free Water Splitting
Torben Daeneke 1 , Kourosh Kalantar-zadeh 1
1 , RMIT University, Melbourne, New South Wales, Australia
Show AbstractAmorphous molybdenum sulphides are an emerging class of inorganic polymers which are predominantly utilized for their superior catalytic properties in the hydrogen evolution reaction. In this work we investigate interactions between water vapours and polymeric MoS4. We report that MoS4 is a highly hygroscopic semiconductor which can reversibly bind up to 7% of its weight as surface water. The reversibly bound moisture is determined to predominantly interact through van der Waals forces and hydrogen bonds with the inorganic matrix. The presence of surface water was found to have profound influence on the semiconductors properties, modulating the materials photoluminescence by over one order of magnitude and its conductivity in excess of 3 orders of magnitude. In-depth analysis of the moisture adsorption process implicate shared and bridging disulphide ligands as the binding site, rather than the apical and terminal sulphides or defect and molybdenum sites.
We utilized these newly discovered properties to design low energy dehumidifiers, highly sensitive conductometric moisture sensors and an ink based electrolyte less water splitting photocatalyst that relies entirely on the hygroscopic nature of MoS4 as the water source. While the efficiency of the hydrogen evolution process is lower than that observed when catalysis is conducted in liquid suspension, it is surprising that a process entirely reliant on the hygroscopic properties of MoS4 as a water source can lead to sustained hydrogen production.
Overall the findings are relevant to the field of photocatalytic hydrogen production, while also hinting at additional future applications of amorphous molybdenum sulphides.
ES7.6: Protection Layers
Session Chairs
Shannon Boettcher
Lianzhou Wang
Tuesday PM, April 18, 2017
PCC North, 200 Level, Room 222 BC
4:30 PM - ES7.6.01
Atomic Layer Deposited Transition Metal Oxide-Titania Alloys as Corrosion Resistant Schottky Contacts for Silicon Photoanodes
Olivia Hendricks 1 , Matthijs van den Berg 1 , Andrew Scheuermann 2 , Paul Hurley 3 , Christopher Chidsey 1 , Paul McIntyre 2
1 Chemistry, Stanford University, Stanford, California, United States, 2 Materials Science and Engineering, Stanford University, Stanford, California, United States, 3 , Tyndall National Institute, Cork Ireland
Show AbstractMIS structures are promising candidates for integrated solar driven water splitting. Traditionally, the metal 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. These 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 layers,2 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 atomic layer deposition (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 sufficiently high work function of 5.02 eV. Although TiO2-RuO2 alloys were ideal Schottky contacts to nSi, they lacked the long-term corrosion resistance required of a protection layer. RuO2 is known to be unstable at the conditions required for water oxidation. We also report on alloys of TiO2 and IrO2, which exhibit similar conductivity and photovoltage but with improved stability. Additionally, 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.
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 TiO 2 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 Junctionn: Atomic Layer Deposited TiO2 − RuO2 Alloy Schottky Contacts for Silicon Photoanodes. (2016). doi:10.1021/acsami.6b08558
4:45 PM - ES7.6.02
Controlling the “Leakiness” of TiO2 Protection Layers
Aafke Bronneberg 1 , Julius Plescher 1 , Christian Hoehn 1 , Sean Berglund 1 , Fatwa Abdi 1 , Roel Van de Krol 1
1 , Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin Germany
Show AbstractTitanium dioxide (TiO2) received renewed attention in the field of photoelectrochemical water splitting as corrosion protection layer for unstable, small-bandgap photoelectrodes.[1–4] In particular the fabrication of TiO2 with hole-conducting properties (also referred to as ‘leaky’ TiO2) proved key towards a stable PEC device.[4]
Although the interfacial charge transport properties and energetics of the Si/TiO2/catalyst/electrolyte system have been studied,[5–9] the origin of the hole-conducting properties of TiO2 is still not fully understood. A likely mechanism for the observed hole-conduction through the TiO2 protection layer is for hole transport to be mediated via mid-gap defect states in the TiO2 layer.[10] Evidence for the mid-gap states has been found in valence band spectra measured by UPS.[4, 5, 11] However, there are also reports of hole-conducting TiO2 layers that do not show the presence of mid-gap states.[7, 11]
We found that we can control the mid-gap state density by tuning the substrate temperature. We were able to link the amount of mid-gap states to the presence of Ti3+ species and found a clear correlation between the hole conductivity and the Ti3+ content. We will show that the presence of Ti3+ species is not due to the formation of Ti interstitials, O vacancies, and/or impurities (C, N), but most likely to an increased H content. The implications of these findings for the fabrication of TiO2 protection layers on photoelectrodes will be discussed.
[1] A. Paracchino et al., Nat. Mater. 2011, 10, 456–61, DOI: 10.1038/nmat3017
[2] B. Seger et al., RSC Adv. 2013, 3, 25902, DOI: 10.1039/c3ra45966g
[3] Y. W. Chen et al., Nat. Mater. 2011, 10, 539–544, DOI: 10.1038/nmat3047
[4] S. Hu et al., Science (80-. ). 2014, 344, 1005–9, DOI: 10.1126/science.1251428
[5] M. F. Lichterman et al., Energy Environ. Sci. 2015, 8, 2409–2416, DOI: 10.1039/C5EE01014D
[6] S. Hu et al., J. Phys. Chem. C 2016, 120, 3117–3129, DOI: 10.1021/acs.jpcc.5b09121
[7] B. Mei et al., J. Phys. Chem. C 2015, 119, 15019–15027, DOI: 10.1021/acs.jpcc.5b04407
[8] M. F. Lichterman et al., ECS Trans. 2015, 66, 97–103,
[9] M. H. Richter et al., ECS Trans. 2015, 66, 105–113, DOI: 10.1149/06606.0105ecst
[10] H. H. Pham et al., Phys. Chem. Chem. Phys. Phys. Chem. Chem. Phys 2015, 17, 541–550, DOI: 10.1039/c4cp04209c
[11] M. T. Mcdowell et al., ACS Appl. Mater. Interfaces 2015, 7, 15189–15199, DOI: 10.1021/acsami.5b00379
5:00 PM - ES7.6.03
Charge Transfer Characterization on Atomic Layer Deposited TiO2 Protective and Conductive Layers for Photoelectrochemical Solar Fuels
Carles Ros 1 , Teresa Andreu 1 , Juan Morante 1
1 , IREC, Catalonia Institute for Energy Research, Sant Adrià del Besòs 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.
5:15 PM - ES7.6.04
Enhanced Photoelectrochemical Efficiency and Stability Using A Conformal TiO2 Film on A Black Silicon Photoanode
Yanhao Yu 1 , Zheng Zhang 2 , Yue Zhang 2 , Xudong Wang 1
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 , University of Science and Technology Beijing, Beijing, Beijing, China
Show AbstractIntense charge recombination and weak electrochemical stability are main limitations that dragging the advance of black silicon (b-Si) in photoelectrochemical (PEC) solar-to-fuel production. Here we report a facile surface protecting strategy that can simultaneously promote the charge separation efficiency and improve the operational lifetime of b-Si PEC photoanode.[1] Through a low-temperature atomic layer deposition (ALD), a conformal amorphous TiO2 layer was uniformly deposited on the entire nanostructured surface of b-Si. Combined with an optimized Co(OH)2 thin film as the oxygen evolution catalyst, this b-Si/TiO2/Co(OH)2 photoelectrode was able to produce a saturated photocurrent density of 32.3 mA/cm2 at a low external potential of 0.43 V vs. SCE, noticeably exceeding planar Si and unprotected b-Si photoelectrodes. After systematically investigating their light absorption and charge separation characteristics, TiO2-induced photocurrent gain was revealed to be a result of the enhanced charge separation efficiency, which was caused by the effective passivation of defective sites on b-Si surface. Moreover, this ALD TiO2 layer can effectively elongate the functional endurance of b-Si from less than half an hour to four hours by decoupling the chemically unstable b-Si surface from the alkaline electrolyte. This research established a promising strategy for constructing efficient and stable b-Si electrochemical systems.
Reference
Yanhao Yu*, Z. Zhang*, (*equal contribution) Q. Liao, Z. Zhang, X. Yan, Y. Zhang, X. Wang. “Enhanced Photoelectrochemical Efficiency and Stability Using A Conformal TiO2 film on A Black Silicon Photoanode” Nature Energy, accepted in press.
5:30 PM - ES7.6.05
Passivating Silicon Photocathodes by Solution-Deposited Ni-Fe Layered Double Hydroxide for Efficient H2 Evolution in Alkaline Media
Jiheng Zhao 1 , Lili Cai 1 , Li Hong 1 , Xiaolin Zheng 1
1 , Stanford University, Stanford, California, United States
Show AbstractStabilizing silicon (Si) photoelectrodes for hydrogen evolution reaction (HER) is an important step towards cost-effective photoelectrochemical (PEC) water splitting devices, especially in alkaline solutions, since most efficient oxygen evolution reaction (OER) catalysts for photoanodes are only stable in alkaline solutions. To date, the reported most stable Si photocathode in alkaline media was protected by the atomic layer deposited (ALD) dense TiO2 layer with noble metal based catalysts on top. There is a need to further develop low cost deposition method and use earth abundant catalyst to protect Si photocathodes in alkaline solution. Herein, we report the first demonstration of using the hydrothermal method to deposit earth-abundant NiFe layered double hydroxide (LDH) to simultaneously passivate 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 13mA/cm2 at 0V vs RHE, durability of ~24hours in 1M KOH (pH ~13.5) electrolyte, and an onset potential ~0.3V vs RHE that is comparable to that of the reported p-n+ Si wafers. We believe that the combination of earth abundant and HER active NiFe LDH, and simple hydrothermal method represents a promising pathway towards making high performance, precious metal-free Si photocathodes for the PEC water splitting devices.
ES7.7: Poster Session
Session Chairs
Wednesday AM, April 19, 2017
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - ES7.7.01
Solar Fuel Production by Doping-Treated BiVO4
Won Jun Jo 1 , Karen Gleason 1 , Jae Sung Lee 2
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 , UNIST, Ulsan Korea (the Republic of)
Show AbstractHydrogen has been recognized as one of the most promising energy carriers for the future, because it can generate enormous energy by clean combustion chemistry without any greenhouse gas emissions. Water splitting under visible light irradiation is an ideal route to cost-effective, large-scale, and sustainable hydrogen production, but it is challenging, because it requires a rare photocatalyst that carries a combination of suitable band gap energy, appropriate band positions, and photochemical stability. To create this rare photocatalyst, we engineered the band edges of BiVO4 by simultaneously substituting In3+ for Bi3+ and Mo6+ for V5+ in the host lattice of monoclinic BiVO4, which induced partial phase transformation from pure monoclinic BiVO4 to a mixture of monoclinic BiVO4 and tetragonal BiVO4.[1]
The band-edge engineered BiVO4 has a slightly larger band-gap energy (Eg ~ 2.5 eV) than usual ‘yellow’ monoclinic BiVO4 (Eg ~ 2.4 eV), as supported by the unique color change to green, and higher (more negative) conduction band edge (− 0.1 VRHE at pH 7) than H+/H2 potential (0 VRHE at pH 7). Consequently, the band-edge engineered BiVO4 is able to split water into H2 and O2 under visible-light irradiation without using any sacrificial reagents (e.g. CH3OH or AgNO3). This outcome is the first example of a pure water-splitting photocatalyst responding to visble light without any noble-metal cocatalyst.[1]
The physical origin of the augmented photocatalytic behaviors of the band-edge engineered BiVO4 was illuminated through density functional theory (DFT) calculations as well as a variety of physical and electrochemical characterizations. Briefly, the In3+/Mo6+ dual dopant formation is more promoted within tetragonal BiVO4 rather than monoclinic BiVO4. This physicochemical tendency triggers the partial phase transition from pure monoclinic BiVO4 to a mixture of monoclinic BiVO4 and tetragonal BiVO4, which sequentially leads to unit-cell volume growth, compressive lattice-strain increase, conduction-band edge uplift, and band-gap widening. This In3+/Mo6+ doping-induced domino effect from phase transition to band edge engineering enables a previously unidentified mechanism to create a noble metal-free photocatalyst, accomplishing overall water splitting under visible-light irradiation. Therefore, the findings from this research possess great potential to realize a platform technology toward cost-effective, large-scale, and sustainable solar hydrogen harvest from water.[1]
Reference:
[1] Won Jun Jo et al, Phase Transition-Induced Band Edge Engineering of BiVO4 to Split Pure Water under Visible Light, Proceedings of the National Academy of Sciences, 2015, 112(45), 13774-13778.
9:00 PM - ES7.7.02
Cu2O Based Photostable Photoanodes by the Au Passivation for High-Efficiency Photoelectrochemical Applications
Hee Jun Kim 1 , Jae Won Lee 1 , Hak-Jong Choi 2 , Jeong Min Baik 1
1 , UNIST, Ulsan Korea (the Republic of), 2 , Korea University, Seoul Korea (the Republic of)
Show Abstractp-n heterostructure design based on Cu2O/ZnO film for enhanced photoelectrochemical and photoelectrocatalytic activities is reported, which mainly consists of four layers of Au film, TiO2 film, Cu2O film, and Au NPs. p-type Cu2O film and n-type ZnO film are produced by using electrodeposition and atomic layer deposition. The electrodeposition of Cu2O acquired high carrier concentration, resulting in large internal electric field between Cu2O and ZnO interface, which led to high charge separation efficiency of photogenerated charge carriers. The Au/Cu2O/ZnO/Au NPs film shows a remarkable enhancement in the photocurrent density in the UV-visible region, compared with bare ZnO film. The high performance is due to the optimized design of the metal-hetero junction structure, where the surface plasmon resonance efficiently absorb visible light and separate electron-hole pairs in the photoelectrode.
9:00 PM - ES7.7.03
Photoelectrochemical Hydrogen Production by Water Splitting over Dual-Functionally Modified Oxide—P-Type N-Doped Ta2O5 Photocathode Active under Visible Light Irradiation
Tomiko Suzuki 1 , Shu Saeki 1 , Keita Sekizawa 1 , Kousuke Kitazumi 1 , Naoko Takahashi 1 , Takeshi Morikawa 1
1 , Toyota Central R&D Labs Inc, Nagakute Japan
Show AbstractIt is widely accepted that photocatalytic hydrogen generation by water splitting and carbon dioxide recycling by reductive reaction are promising technologies as clean energy generation in the future. To this end, lots of semiconductor materials which can be activated under visible light irradiation have been developed to date. Recently, our group developed a N-doped Ta2O5 (N-Ta2O5) that absorbs visible light at wavelengths below 520 nm (Eg = 2.4 eV) by forming a new energy state in which the N 2p is above the O 2p in the bandgap [1, 2]. The N-doping also changed its photoresponse from anodic to cathodic due to formation of acceptor level by N. DFT calculation also suggested a highly negative position of the conduction band minimum (ECBM = -1.3 V vs NHE) was originated from surface dipole effect [3]. A highly selective visible light-induced reduction of CO2 to HCOOH was achieved by employing N-Ta2O5 linked with a Ru complex catalyst in acetonitrile containing triethanolamine as an electro donor [4, 5]. These characters as a semiconductor are completely different from those of conventional tantalum pentoxide (Ta2O5, Eg = 4.0 eV, = -0.4 V vs NHE) and tantalum oxynitride (TaON, Eg = 2.4 eV, = -0.3 V vs NHE) [6], which exhibit anodic photoresponses.
In this paper, we demonstrate H2 evolution over N-Ta2O5 by water splitting in aqueous solution [7]. Although the surface of N-Ta2O5 was found to be almost inert for hydrogen production by water splitting, photocathodic current over N-Ta2O5 films under visible light irradiation (λ ≥ 410 nm) was highly enhanced by surface modification with Pt, Rh or Au. Loading with Pt or Rh greatly enhanced the H2 evolution rate in an aqueous solution, increasing the rates by two orders of magnitude compared to that over unmodified N-Ta2O5 for both photoelectorodes and nanoscaled powders. We will discuss the difference in the cocatalyst appropriate for H2 evolution performance of between N-Ta2O5 and TaON from viewpoints of pH dependence, kind of electron donors and band alignment, which suggested electron transfer associated with these parameters might be the cause in the difference.
References
[1] T. Morikawa, et al., Appl. Phys. Lett., 96 (2010) 142111. [2] T. M. Suzuki, et al., J. Mater. Chem., 22 (2012) 24584. [3] R. Jinnouchi, et al., J. Phys. Chem. C., 119 (2015) 26925. [4] S. Sato, et al., Angew. Chem. Int. Ed., 49 (2010) 5101. [5] T. M. Suzuki, et al., Chem. Commun., 47 (2011) 8673. [6] K. Domen, et al., J. Phys. Chem. B., 107 (2003) 1798. [7] T. M. Suzuki, et al., Applied Catal. B., 202 (2017) 597.
9:00 PM - ES7.7.04
Enhanced Visible Light Photocatalytic Water Reduction of a g-C3N4/SrTa2O6 Heterojunction
Shiba Adhikari 1 2 , Abdou Lachgar 1 2
1 , Wake Forest University, Winston Salem, North Carolina, United States, 2 Wake Forest University, Centre for Energy Studies, Winston Salem, North Carolina, United States
Show AbstractA visible-light-active g–C3N4/SrTa2O6 (CN/STO) heterojunction was fabricated using melamine and the acid form (H2SrTa2O7.nH2O) of K2SrTa2O7.nH2O with Ruddlesden-Popper (RP) layered perovskite-type structure. Electron microscopy reveals that g-C3N4 nanofibres are anchored to the surface of SrTa2O6 resulting in an intimate contact between the two semiconductors. Diffuse reflectance UV-Vis spectroscopy demonstrates that the CN/STO heterojunction possesses intense absorption in the visible. The structural and optical properties endow CN/STO heterojunction with remarkably enhanced photocatalytic activity. The observed photocatalytic hydrogen generation of CN/STO heterojunction under visible light irradiation was found to be 9 times higher than that of pristine CN. This enhancement is attributed to well-aligned band positions of CN and STO and high separation and transfer of photogenerated electrons at the intimate heterojunction interface. A plausible mechanism for the observed enhanced photocatalytic activity of the heterojunction is proposed based on photoluminescence, time-resolved fluorescence emission decay, electrochemical impedance spectroscopy and band position calculations.
9:00 PM - ES7.7.05
Interface Engineering of Colloidal CdSe Thin Film Photocathodes for Solar-Driven Hydrogen Evolution
Hui Li 1
1 Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina, United States
Show AbstractColloidal semiconductor quantum dots-based photocathodes for solar-driven hydrogen evolution have attracted intensive attention due to their tunable size, nanostructure, crystalline orientation, and band-gap. Here, we report a thin film heterojunction photocathode composed of organic PEDOT:PSS, colloidal CdSe quantum dots, and atomic Pt, functionalizing as hole transport layer, semiconductor light absorber, and cocatalyst for hydrogen evolution, respectively. Through engineering of CdSe surface ligands by using tetrabutylammonium chloride for improving charge separation and the film conductivity, optimizing the thickness of PEDOT:PSS and atomic layer deposition Pt layer for enhancing holes and electrons collection, separately, thus the PEDOT:PSS/TBAC-CdSe/Pt heterogeneous photocathode produces a photocurrent of about 1.08 mA/cm2 (AM 1.5, 100 mW/cm2) at a potential of 0 V vs. RHE in neutral aqueous solution, which is nearly 12 times than that of the pristine CdSe photoelectrode. Furthermore, the composite photocathode shows an onset potential for water reduction as positive as 0.46 V vs. RHE and long-term stability for 24 h with negligible degradation. Electrochemical impedance spectroscopy and time-resolved photoluminescence results indicate that these enhancements are stem from the low bulk charges recombination, high interfacial transfer rate, and fast reaction kinetics. We believe that the interface engineering strategies could be applied to other colloidal semiconductors to construct highly efficient and stable heterogeneous photoelectrodes for solar fuel production.
9:00 PM - ES7.7.06
Unique Role of Metal Oxide 2D Nanosheet in Optimizing Catalyst Performance of Graphene for Oxygen Reduction Reaction
Xiaoyan Jin 1 , Seong-Ju Hwang 1
1 , Ewha Womans University, Seoul Korea (the Republic of)
Show AbstractHigh performance electrocatalysts for oxygen reduction reaction can be synthesized by the hybridization of exfoliated metal oxide nanosheets with N-doped graphene. The incorporation of metal oxide nanosheet into N-doped graphene matrix induces a marked increase of surface area via the weakening of strong inter-graphene interaction. Of prime importance is that the addition of metal oxide nanosheet leads to the creation of more pyridinic N sites and also to the decrease of the charge transfer resistance. The resulting metal oxide-N-doped graphene nanocomposites exhibit excellent electrocatalytic activity for oxygen reduction reaction with marked positive shift of on-set potential and improvement of stability. Such beneficial effects of the addition of metal oxide component are much more prominent for 2D nanosheet than for 0D nanoparticle, underscoring unique merit of 2D nanosheet morphology. The present study clearly demonstrates that exfoliated metal oxide 2D nanosheets can act as powerful building block for exploring novel efficient electrocatalysts.
9:00 PM - ES7.7.07
Direct Fabrication of BaNbO2N Crystals on Niobium Substrates for Visible-Light-Responsive Photoanode by Flux Coating under Ammonia Flow
Sayaka Suzuki 1 , Katsuya Teshima 1 2
1 Faculty of Engineering, Shinshu University, Nagano Japan, 2 Center for Energy and Environmental Science, Shinshu University, Nagano Japan
Show AbstractConverting the renewable solar energy into stockable and transplantable chemical fuels is considered to be a long-term solution to address global energy and environmental problems. Water splitting using photocatalysts have been researched because of possibility to supply clean hydrogen energy. Among photocatalytic materials, visible-light-responsive photocatalysts have been focused because they can utilize solar energy effectively compared to conventional photocatalyst which only work in ultraviolet region. BaNbO2N is one of the promising candidates of the perovskite-type oxynitride family for water splitting under visible light irradiation because it has narrow band gap of 1.7 eV and appropriate band-edge positions for oxidation and reduction of water. In this study, we report the fabrication of BaNbO2N crystal layers on niobium substrates using flux coating and their photoelectrochemical properties.
BaNbO2N crystal layers were fabricated using niobium foil, Ba(NO3)2 and some sodium chlorides (e.g., NaCl, KCl, and BaCl2) as the substrate, barium source and flux, respectively. Aqueous solutions of Ba(NO3)2 and flux were coated on metal substrates, and then the niobium substrates were heated at 700−950 °C in an ammonia flow.
Cube- and truncated cube like crystals were directly grown on the niobium substrate surface. From XRD pattern and EDS mapping images, the crystals were determined as BaNbO2N. In addition to BaNbO2N and Nb, the diffraction lines corresponding to Nb2N and NbN were observed. When the heating temperature was increased, the diffraction intensities for Nb2N and NbN were increased, but the photocurrent density at 1.23 VRHE was also increased. The details of crystal growth and photoelectrochemical properties will be presented in 2017 MRS Spring Meeting & Exhibit.
Acknowledgements: This research was partially supported by JSPS Grant-in-Aid for Scientific Research (A) 25249089.
Symposium Organizers
Francesca Maria Toma, Lawrence Berkeley National Laboratory
Akihiko Kudo, Tokyo University of Science
Roel Van de Krol, Helmholtz-Zentrum Berlin
Lianzhou Wang, University of Queensland
Symposium Support
ACS Energy Letters | ACS Publications
Bruker—Nano Surfaces Division
California Institute of Technology
Helmholtz-Zentrum Berlin (HZB)
Lawrence Berkeley National Laboratory– Joint Center for Artificial Photosynthesis
Wiley VCH Verlag GmbH &
Co. KGaA
ES7.8: Oxygen Evolution Catalysts
Session Chairs
Artur Braun
Francesca Maria Toma
Wednesday AM, April 19, 2017
PCC North, 200 Level, Room 222 BC
10:15 AM - ES7.8.02
Electronic Structure of Manganese Based Oxides under OER
Katarzyna Skorupska 3 2 , Travis Jones 3 , Sebastian Fiechter 1 , Marc Willinger 3 , Ramzi Farra 3 2 , Frederic Sulzmann 3 , Michael Havecker 3 2 , Juan-Jesus Velasco-Velez 3 2 , Cheng-Hao Chuang 4 , Philipp Kurz 5 , Axel Knop-Gericke 3 , Robert Schloegl 5 2
3 Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaf, Berlin Germany, 2 Department of Heterogeneous Reactions, Max-Planck-Institut für Chemische Energiekonversion, Mülheim a. d. Ruhr Germany, 1 Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin Germany, 4 Department of Physics, Tamkang University, New Taipei City Taiwan, 5 Institut für Anorganische und Analytische Chemie, Albert-Ludwigs-Universität Freiburg, Freiburg Germany
Show AbstractNature selected manganese as the chemical element capable of catalyzing oxygen evolution reaction (OER) [1]. The OER reaction is performed by the oxygen evolving complex (OEC) known also as water oxidation complex (WOC) with [Mn4Ox:Ca] core structure [2]. The inorganic equivalent of Mn-Ca clusters surrounded by amino acids is Ca-rich birnessite. These amorphous oxides are consisting of layers of edge sharing [MnO6]x octahedra with about 7Å spacing between them calcium ions are localized [3].
The properties of the Ca-birnessite powder will be compared with the screen-printed electrode layer on FTO support [4]. Changes taking place after the electrochemistry in OER conditions in 0.1M phosphate buffer pH7 will be discuss.
The studies were performed by means of X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy and transmission electron microscopy (TEM).
The quantitative and qualitative analysis of the obtained XPS spectra gave information on the chemical shift of the elements and their ratio, respectively. The oxidation state of the d-block metal was studied by Mn L-edge NEXAFS. The same method was used to probe the symmetry of the surrounding atoms in the first coordination sphere of the d0 metal at the Ca L-edge. The NEXAFS and XPS spectra of calcium were calculated using multiplet ligand-field theory (MLFT) and the Xclaim package[5]. The obtained results suggest a covalent interaction of the calcium ions with the oxygen ligands after the electrochemistry in phosphate buffer under OER conditions. This change was related to the change in oxidation state of manganese for the different samples studied by Mn L-edge. The calculated intensity ratios of Mn 2p to Ca 2p shows that the concentration of calcium in the birnessite catalyst decreases during the applied procedures. Deconvolution of the P 2p line for the sample after the electrochemical treatment shows remnants of phosphate buffer within the manganese based catalyst.
We will also report on the development of manganese oxide based electrodes for the oxygen evolution reaction designed for in-situ measurements. This is the optimal but also very challenging way to understand the role of catalytic centers under realistic reaction conditions. The first approaches and issues recording of Mn K-egde and Mn L-edge during the in-situ electrochemical deposition of manganese oxides on silicon nitride and graphene membranes followed by their characterization under OER conditions will be discuss.
References
1. F. A. Armstrong, Philos. Trans. R. Soc. Lond. B Biol. Sci. 363, 1263 (2008).
2. N. Cox, D. A. Pantazis, F. Neese, and W. Lubitz, Acc. Chem. Res. 46, 1588 (2013).
3. K. P. Lucht and J. L. Mendoza-Cortes, J. Phys. Chem. C 119, 22838 (2015).
4. S. Y. Lee, D. González-Flores, J. Ohms, T. Trost, H. Dau, I. Zaharieva, and P. Kurz, ChemSusChem 7, 3442 (2014).
5. J. Fernández-Rodríguez, B. Toby, and M. van Veenendaal, J. Electron Spectrosc. Relat. Phenom. 202, 81 (2015).
10:30 AM - ES7.8.03
Elucidating the Role of the Mn Oxidation State in Thin Film MnOx Oxygen Evolution Catalysts
Paul Plate 1 , Fatwa Abdi 1 , Christian Hoehn 1 , Peter Bogdanoff 1 , Roel Van de Krol 1 , Aafke Bronneberg 1
1 , Helmholtz-Zentrum Berlin, Berlin Germany
Show AbstractProtection of semiconductors against photocorrosion or photooxidation while maintaining high injection efficiencies is important to make photoelectrochemical water splitting a viable route for hydrogen production. Manganese oxide (MnOx) is a possible candidate as a protection layer; it has been reported to protect Si photoanodes from photooxidation while acting as a relatively efficient oxygen evolution catalyst [1,2]. In addition, it is non-toxic, earth abundant, and cheap. This dual protection-OER functionality, however, puts contradicting constraints on the MnOx layer. As a protection layer, the MnOx layer needs to be thicker than a few nanometers to avoid pinholes and successfully stabilize semiconductors. On the other hand, since catalysis is a surface process, a film with a high specific surface area (i.e., high surface-to-bulk ratio) is typically desired.
To investigate the optimal MnOx film thickness in terms of both the catalytic performance and protective function, we prepared thin MnOx films using atomic layer deposition (ALD). ALD has the advantage of depositing pinhole-free films with atomic layer precision, even on nanostructured surfaces. We find that a thin film of 2.5 nm effectively protects Si under oxidizing conditions, while maintaining its activity for the oxygen evolution reaction (OER). Currently, even thinner films are being prepared in order to find the lower limit in terms of stability. We also find that the OER current density depends on the thickness of the MnOx, consistent with earlier reports [1].
Another critical parameter that is known to influence the catalytic performance of MnOx is the Mn oxidation state [3]. With our in-line ALD/XPS system we measure the oxidation state of as-deposited and annealed MnOx without any air exposure. By tilting the sample stage with respect to the analyzer and thereby reducing the information depth, we can differentiate between bulk and surface oxidation states. As deposited samples consisting of a MnO/Mn2O3 mixed Phase. Air exposure leads to an increase of the Mn2O3 content, while the Mn(II)/Mn(II) ratio stays the same for bulk and surface. In Contrast vacuum annealed films are able to form at air a higher oxidized surface with respect to the bulk. We will discuss the effect of those oxidation states on the catalytic activity and present an outlook for semiconductor/catalyst electrode design for efficient solar water splitting.
[1] N. C. Strandwitz et al., J. Phys. Chem. C, 2013, 117, 4931-4936, DOI: 10.1021/jp311207x
[2] K. L. Pickrahn et al., Adv. Energy Mater., 2012, 2, 1269-1277, DOI: 10.1002/aenm.201200230
[3] A. Ramiréz et al., J. Phys. Chem. C, 2014, 118, 14073-14081, DOI: 0.1021/jp500939d
10:45 AM - ES7.8.04
Development of Solar Fuels Photoanodes through Combinatorial Integration of Ni-La-Co-Ce and Ni-Fe-Co-Ce Oxide Catalysts on BiVO4
Aniketa Shinde 1 , Dan Guevarra 1 , Santosh Suram 1 , Lan Zhou 1 , Guiji Liu 2 , Ian Sharp 2 , Francesca Maria Toma 2 , Joel Haber 1 , John Gregoire 1
1 , California Institute of Technology, Pasadena, California, United States, 2 , Lawrence Berkeley Laboratory, Berkeley, California, United States
Show AbstractSolar fuel generators entail a high degree of materials integration and efficient photoelectrocatalysis of the constituent reactions hinges upon the establishment of highly functional interfaces. Recent application of high throughput experimentation to interface discovery for solar fuels photoanodes has revealed several surprising and promising mixed-metal oxide coatings for BiVO4. Ni-La-Co-Ce and Ni-Fe-Co-Ce inkjet printed libraries were deposited on uniform, spin-coated BiVO4 thin films on FTO and scanning droplet cell measurements were done on these samples in pH 13 electrolyte. In situ optical spectroscopy measurements allowed a comparison between the metal oxide coatings and their extrinsic optical and electrocatalytic properties revealing that specific coatings lower the photoanode performance while select combinations of Ni, La, Fe, and/or Ce composition and total metal loading provided an increase in the maximum photoelectrochemical power generation for oxygen evolution. Initial high throughput discoveries were extended and validated through follow-up high throughput investigations and conventional photoelectrochemical measurements on larger area electrodes, as well as ab-initio calculations. The high throughput experimentation and informatics provides a powerful platform for both identifying the pertinent interfaces for further study and discovering high performance photoanodes for incorporation into efficient water splitting devices. Ce-rich coatings exhibited an enhancement in power conversion efficiency compared to bare BiVO4 and displayed high interface quality and maximum photoelectrochemical power conversion. Using sputter deposition of composition and thickness gradients on a uniform BiVO4 film, we systematically explored photoanodic performance as a function of the composition and loading of Fe-Ce oxide coatings. This combinatorial materials integration study not only enhances the performance of this new class of materials but also identifies Ce as a critical ingredient that merits detailed study.
ES7.9: III-V Semiconductors
Session Chairs
Roel Van de Krol
Lydia Wong
Wednesday PM, April 19, 2017
PCC North, 200 Level, Room 222 BC
11:30 AM - *ES7.9.01
Recent Advances in III-V Multijunction Semiconductor Photo-Electrochemical Water Splitting
Todd Deutsch 1 , James Young 1 , Myles Steiner 1 , Henning Doscher 2 1 , John Turner 1
1 , National Renewable Energy Laboratory, Lakewood, Colorado, United States, 2 , Philipps-Universität Marburg, Marburg Germany
Show AbstractIn order to economically generate renewable hydrogen fuel from solar energy using semiconductor-based devices, the U.S. Department of Energy Fuel Cells Technology Office has established technical targets of over 20% solar-to-hydrogen (STH) efficiency with several thousand hours of stability under operating conditions [1]. We have modeled attainable efficiencies of tandem absorbers that, for the first time, considered the absorption of sunlight by water [2]. We used this modeling to identify top and bottom semiconductor bandap combinations that should be targeted to achieve maximal STH efficiency.
We had to employ several key solid-state technological advances to achieve STH efficiencies exceeding 16%. The first improvement was to increase the device current via a non-lattice-matched 1.2 eV InGaAs grown using the inverted metamorphic multijunction technique developed by NREL’s III-V photovoltaics group. The second modification was to add a thin n-GaInP2 layer to p-GaInP2 to generate a "buried junction", which increased the photocurrent onset or Voc of the device by several hundred mV and enabled 14% STH efficiency. Finally, we increased the top junction photon conversion efficiency by adding an AlInP "window layer", which is commonly used in solid-state PV devices to reduce surface recombination. Through the use of a collimating tube, we measured our devices outdoors under direct solar illumination and verified over 16% STH conversion efficiency. I will also briefly introduce pitfalls of common experimental procedures that can influence the accuracy of measured STH efficiencies, which can be exaggerated for mulitjunction absorbers.
The largest loss in our current system is reflection at the semiconductor/electrolyte interface, so I will address the photon management strategies we use to achieve greater parity between measured efficiency and the theoretical limit. Capturing a significant portion of the ~25% of photons lost to reflection at this interface should allow the realization of devices that exceed 20% STH efficiency.
[1] http://energy.gov/sites/prod/files/2015/06/f23/fcto_myrdd_production.pdf
[2] H. Döscher et al., Energy Environ. Sci. 7, 2951 (2016).
12:00 PM - ES7.9.02
Solar-to-Hydrogen Efficiency—Shining Light on Photoelectrochemical Device Performance
James Young 1 , Henning Doscher 1 , John Geisz 1 , John Turner 1 , Todd Deutsch 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractDirect photoelectrochemical (PEC) hydrogen production aims to provide a clean and cost-effective solar fuel. Solar-to-hydrogen (STH) conversion efficiency is central to evaluating and comparing research results, and it largely establishes the prospect for successfully introducing commercial solar water-splitting systems. Present measurement practices do not follow well-defined standards, and common methods potentially impact research results and their implications. We demonstrate underestimated influence factors and experimental strategies for improved accuracy[1].
Our focus is tandem devices that have the prospect for greater STH efficiency[2], but increased complexity that requires more careful consideration of characterization practices. We perform measurements on an advanced version of the classical GaInP/GaAs design[3] while considering (i) calibration and adjustment of the illumination light-source; (ii) confirmation of the consistency of results by incident photon-to-current efficiency (IPCE), and (iii) definition and confinement of the active area of the device.
We initially measured 21.8% STH efficiency using a tungsten white-light source, a calibrated GaInP photovoltaic reference cell, and epoxy-encased photocathodes. In contrast, integrating experimental IPCE over the AM 1.5G solar irradiance showed that less than 10% STH conversion appeared conceivable. We then performed a set of on-sun measurements that gave 16.1% STH, before eliminating indirect light coupled to the sample by using a collimating tube and 13.8% STH efficiency thereafter. However, the value still vastly exceeded the current density expected according to the quantum efficiency measured via IPCE. Finally, suspecting that the illuminated area is poorly defined by epoxy, we use a compression cell for an epoxy-free area definition, resulting in 9.3% STH efficiency – a number also compatible with our IPCE results.
We propose applying the following standards for future PEC performance reporting: (i) traceable disclosure of the illumination-source configuration (lamp, filters, optics, PEC configuration) and/or its measured spectral distribution; (ii) thorough device-area definition (including confinement of the illumination area and avoidance of indirect light paths); (iii) complementary IPCE confirmation of the solar-generation potential; and (iv) proper consideration of faradaic efficiency.
[1] H. Döscher, J. L. Young, J. F. Geisz, J. A. Turner, and T. G. Deutsch, “Solar to hydrogen efficiency: Shining light on phoelectrochemical device performance,” Energy Environ. Sci. 2015.
[2] H. Döscher, J. F. Geisz, T. G. Deutsch, and J. A. Turner, “Sunlight absorption in water – efficiency and design implications for photoelectrochemical devices,” Energy Environ. Sci. 2014.
[3] O. Khaselev and J. A. Turner, “A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting,” Science. 1998.
12:15 PM - ES7.9.03
Photo-Induced Gain of Open-Circuit-Potential (OCP) in GaN Photoelectrodesfor Characterizing Defects and Photoelectrochemical Activity
Yuuki Imazeki 1 , Youhei Iwai 1 , Akihiro Nakamura 1 , Kayo Koike 1 , Kentaroh Watanabe 1 2 , Katsushi Fujii 3 , Masakazu Sugiyama 1 , Yoshiaki Nakano 1 2
1 Graduate School of Engineering, University of Tokyo, Bunkyo, Tokyo, Japan, 2 Global Solar Plus Initiative, University of Tokyo, Bunkyo, Tokyo, Japan, 3 , University of Kitakyusyu, Kitakyusyu, Fukuoka, Japan
Show AbstractDefects in a photoelectrode usually prevent its activity by decreasing carrier concentration and thus quasi-Fermi-level (qFL) splitting via recombination. It is, however, reported that some kind of defects near the reaction surface can enhance photocatalytic activity, such as the ones in TiO2 which enhances water oxidation [1]. Characterization of the defects in photoelectrode is, therefore, highly demanded for the realization of higher efficiency and better selectivity.
As a method of characterizing the defects in semiconductor photoelectrodes, we have proposed the measurement of open-circuit potential (OCP) as a function of irradiation intensity [2]. We measured the OCP of epitaxial n-type GaN photoelectrode, which corresponds to the qFL of electrons. OCP approached to the flat-band potential (FBP) under high irradiation intensity (> 10 mW/cm2) but the value at low intensity range was bound to a value which was dependent on the pretreatment of the GaN surface. The tendency was considered to reflect the pinning of qFL by the defect energy levels in the GaN electrode.
In order to confirm that the behavior of OCP versus light intensity gives us a clue to clarify the nature of defects in a semiconductor photoelectrode, we performed a systematic comparison of the OCP behavior for the GaN photoelectrodes to which surface defects were introduced intentionally with Ar fast atom beam (FAB) bombardment. The surface of epitaxial n-GaN photoelectrodes were treated with Ar FAB for 3min. The OCP of these photoelectrodes were then measured in 1M NaOH solution versus an Ag/AgCl reference electrode with a platinum wire as a counter electrode. Output of Xe lamp was irradiated to the electrode with its intensity varied using ND filters from 10-5 to 100 mW/cm2.
The OCP of non-treated n-GaN electrode stayed at -0.5 V v.s. Ag/AgCl under the light intensity of 10-5 to 10-4 mW/cm2. Above this intensity range, OCP increased drastically and exhibited an almost linear behavior with the logarithm of light intensity from 0.1 mW/cm2 until it reached -1.4 V with 100 mW/cm2. In reference to the flat-band potential of the n-GaN electrode (-1.5 V), it seems that the native defect states in n-GaN existed at 1.0 eV below the conduction band edge and these states pinned the qFL under the light intensity of 10-3 mW/cm2. For the electrode treated with Ar-FAB, OCP stayed at -0.2 V v.s. Ag/AgCl in the intensity range from 10-5 to 10-2 mW/cm2, and it never reached the flat-band potential. This observation suggests that additional defect levels were introduced by Ar-FAB at 1.3 eV below the conduction band edge and their density is so large that qFL was pinned up to a larger light intensity than the case of untreated n-GaN.
[1] M. Kong et al., JACS 2011, 133, 16414-16417
[2] Y. Iwai et al., IPS 2016, SF-P21.
12:30 PM - ES7.9.04
GaN-Based Nanopillars for Solar Water Splitting
Parvathala Reddy Narangari 1 , Siva Karuturi 1 , Joshua Butson 1 , Mykhaylo Lysevych 1 , Hoe Tan 1 , Chennupati Jagadish 1
1 , Australia National University, Canberra, Australian Capital Territory, Australia
Show AbstractResearch on renewable energy technologies has been intensified from the past decade due to the scarcity of fossil fuels and global warming caused by the by-products from the consumption of fossil fuels. Hydrogen generation from solar water splitting is one of the promising routes to secure sustainable, green, storable and portable form of energy. Gallium nitride-based alloys, such as InGaN, are attractive for solar water splitting due to their tunable band gap properties, ranging from far infrared (InN≈0.7 eV) to ultraviolet (GaN≈3.4 eV) based on alloy composition that can absorb the entire solar spectrum. Further, GaN exhibits good crystal quality and stability against chemical, photo corrosion and UV irradiation which are requisites for solar water splitting. In addition, nanostructures made from these materials offer several advantages such as enhanced sunlight absorption, increased surface area and improved carrier separation which are critical for photocatalytic applications.
In this work, we report on the fabrication of stable GaN-based nanopillar (NP) photoanodes for solar water splitting. Inductively coupled reactive ion etching (ICP-RIE) and random nanomask techniques were employed for the fabrication of n-doped GaN NPs. Structural and optical quality of GaN NPs is carried out by using SEM and photoluminescence studies respectively [1]. The solar water splitting performance of GaN photoanodes was measured using three electrode PEC setup in NaOH electrolyte. NP photoanodes exhibit superior PEC performance compared to their counterpart planar photoanodes[2]. Most importantly, doping concentration and NP dimensions play a critical role in controlling the PEC performance of GaN NP photoanodes. Electrochemical impedance and diffuse reflectance measurements were used to analyze the PEC performance of GaN NP photoanodes. Our ongoing work on NP photoanodes based on InGaN multi quantum wells (MQWs) for improved photocurrent conversions will also be presented.
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:
[1] N P Reddy et al., “Enhanced luminescence from GaN nanopillar arrays fabricated using top-down process”, Nanotechnology 27, 065304(2016)
[2] P R Narangari et al., “Improved Photoelectrochemical Performance of GaN Nanopillar Photoanodes”, under review
12:45 PM - ES7.9.05
Self-Oriented Sb2Se3 Nanoneedle Arrays on a Conductive Substrate for Photoelectrochemical Water Splitting Prepared by Simple Spin-Coating Method
Wooseok Yang 1 , Hyungsoo Lee 1 , Yunjung Oh 1 , Jooho Moon 1
1 , Yonsei University, Seoul Korea (the Republic of)
Show AbstractSynthesis of one-dimensional nanostructured chalcogenide compounds using nontoxic and abundant constituents provides an important pathway to the development of commercially feasible photoelectrochemical water splitting. In this study, vertically inclined Sb2Se3 nanoneedle arrays are successfully fabricated on a substrate via a facile spin-coating method without any complicated processes such as templating, seed formation, or use of a vapor phase. Growth mechanism of Sb2Se3 nanoneedle is investigated by observing structural evolution as a function of annealing temperature, spin-coating iterations, and capping agent concentration. Microstructural analyses indicate that a combination of the intrinsic anisotropy in Sb2Se3 and the specific chemical interaction yields this unique self-oriented nanostructure. Molecular chemistry in Sb2Se3 precursor solution formulation, particularly the role of capping agent, is elucidated by liquid Raman spectroscopy. Preferential growth of [001] from the initial single-crystalline Sb2Se3 occurs during the first spin-coated precursor film, but interfacial defects are generated upon subsequent spin-coating iterations, resulting in annual-ring-like growth of Sb2Se3 nanoneedles. After sequential surface modification with TiO2 and Pt, the resistance to charge transfer from the photoelectrode to the electrolyte decreases significantly, yielding a remarkable record-high photocurrent of 4.5 mA cm−2 at −0.2 VRHE.
ES7.10: Understanding and Improving Hematite
Session Chairs
Giulia Galli
Akihiko Kudo
Wednesday PM, April 19, 2017
PCC North, 200 Level, Room 222 BC
2:30 PM - *ES7.10.01
Intermediates in PEC Water Oxidation—How They Come and How They Go
Artur Braun 1
1 , Empa, Duebendorf Switzerland
Show AbstractAtmospheric oxygen that we live on is the result of a fundamental evolutionary step in photosynthesis which occurred on Earth 2-3 billion years ago [Biello 2009]. The water oxidation in natural photosystem II accounts for 50% of this oxygen. The man-made analog to this natural process is water oxidation in photoelectrochemical cells - one of the most complex processes in physical chemistry. I will showcase how modern x-ray spectroscopy methods have assisted in the understanding of the molecular pro-cesses which occur in natural [Bora 2013, Ralston 2000, Visser 2001] and in man-made "photosystems". A most recent example is the element and orbital specific identification of transient electron holes in iron oxide [Braun 2012], chemical surface intermediates [Bora 2011] and changes in the water molecules in the electrochemical double layer during photoelectrochemical water oxidation [Braun 2016]. I will also demonstrate how the electronic structure evolution is quantitatively paralleled by the electric transport properties of the electrodes during operation.
[Biello 2009] Biello D: The Origin of Oxygen in Earth's Atmosphere. In Scientific American 2009.
[Bora 2011] Bora DK et al. Journal of Physical Chemistry C 2011, 115:5619-5625.
[Bora 2013] Bora DK et al. JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA 2013, 190:93-105.
[Braun 2012] Braun A et al. Journal of Physical Chemistry C 2012, 116:16870-16875.
[Braun 2016] Braun A et al. Catalysis Today 2016, 260:72-81.
[Ralston 2000] Ralston CY et al.Journal of the American Chemical Society 2000, 122:10553-10560.
[Visser 2001] Visser H et al. Journal of the American Chemical Society 2001, 123:7031-7039.
3:00 PM - *ES7.10.02
Strategies to Improve the Performance of Semiconductor Photoelectrodes for Photoelectrochemical Water Splitting
Lydia Wong 1 , Prince Bassi 1 , Guru Dayal 1 , Yingfan Tay 1
1 School of Materials Science and Engineering, Nanyang Technological University, Singapore Singapore
Show AbstractAn ideal photoelectrochemical cell for overall water splitting system consists of photoactive semiconductors as the photoanode for oxygen generation and as the photocathode for Hydrogen generation. The ideal materials for these photoelectrodes need to possess certain qualities like band gap around 1.6 - 2 eV, good visible light absorption, long term aqueous and thermal stabilities apart from being cheap, abundant and non-toxic. These intrinsic properties often vary with the slightest change in stoichiometry of compounds or with the cationic/anionic substitution. This makes the research on photoelectrodes more complicated since there could be numerous compounds with different stoichiometry which increases immensely with ternary or quarternary compounds. To complicate matters, the photoelectrode performance is often limited by the poor charge extraction due to limited bulk transport (short diffusion length, low carrier concentration, low mobility, etc) and surface recombination.
In this presentation, the strategies adopted by our group to solve these challenges, particularly in the context of iron oxide based photoanode, will be presented. This includes nanostructuring, doping of transition metals, cation substitution, and surface passivation strategies. Unassisted water splitting using hematite photoanode stacked with a perovskite solid state solar cell is demonstrated. Finally, recent progress in novel materials for photoelectrodes developed in our group will also be presented.
ES7.11: Theory and Modeling
Session Chairs
Todd Deutsch
Roel Van de Krol
Wednesday PM, April 19, 2017
PCC North, 200 Level, Room 222 BC
4:30 PM - *ES7.11.01
Optimizing Solar Interfaces from First Principles—In Search for Descriptors
Giulia Galli 1 , Tuan Anh Pham 2 , Yuan Ping 3
1 , University of Chicago, Chicago, Illinois, United States, 2 , Lawrence Livermore National Laboratory, Livermore, California, United States, 3 , University of California, Santa Cruz, Santa Cruz, California, United States
Show AbstractWe discuss the results of first principles simulations [1] of interfaces present between photo-absorber, catalysts and water in photo-electrochemical cells. We focus on the identification of descriptors to be used to optimize photo-conversion properties.
[1] T.A.Pham, Y.Ping and G.Galli, Nature Materials 2016.
5:00 PM - ES7.11.02
Modelling the Electrochemical Interface—Applications to CO2 Reduction
Karen Chan 1 , Zachary Ulissi 1 , Jianping Xiao 1 , Michal Bajdich 1 , Leanne Chen 1 , Jens Norskov 1
1 , Stanford, SLAC, Stanford, California, United States
Show AbstractThe electroreduction of CO2 has the potential to store energy from intermittent renewable sources and to produce carbon-neutral fuels and chemicals [1, 2]. In recent years, theoretical studies of CO2 reduction have usually applied the computational hydrogen electrode model, which allows for the determination of the energies of reaction intermediates without explicitly treating the potential and the ions in solution [3]. In this talk, I will briefly review the application of this approach to CO2 reduction: the determination of lowest free energy pathways and correlation of theoretical activity volcanoes with experimental onset potentials [4, 5], and the computational screening of new catalysts [6]. I will then focus on recent developments in the explicit treatment of the electrochemical interface [7], required for an understanding of charge transfer barriers, kinetics, and selectivity. I will discuss the scaling of CO2 reduction barriers, field effects, C-C coupling [8], and the integration of these effects into microkinetic models.
[1] Whipple, D. T. & Kenis, P. J. a. Prospects of CO 2 Utilization via Direct Heteroge- neous Electrochemical Reduction. J. Phys. Chem. Lett. 1, 3451–3458 (Dec. 2010).
[2] Graves, C., Ebbesen, S. D., Mogensen, M. & Lackner, K. S. Sustainable hydrocarbon fuels by recycling CO 2 and H 2 O with renewable or nuclear energy. Renewable and Sustainable Energy Reviews 15, 1–23 (2011).
[3] Nørskov, J. K., Rossmeisl, J., Logadottir, A., Lindqvist, L., Kitchin, J. R., Bligaard, T. & Jónsson, H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 108, 17886–17892 (Nov. 2004).
[4] Shi, C., Hansen, H. a., Lausche, A. C. & Nørskov, J. K. Trends in electrochemical CO2 reduction activity for open and close-packed metal surfaces. Phys. Chem. Chem. Phys. 16, 4720–7 (Mar. 2014).
[5] Kuhl, K. P., Hatsukade, T., Cave, E. R., Abram, D. N., Kibsgaard, J. & Thomas, F. Electrocatalytic conversion of carbon dioxide to methane and methanol on transition metal surfaces (2014).
[6] Chan, K., Tsai, C., Hansen, H. A. & Nørskov, J. K. Molybdenum sulfides and selenides as possible electrocatalysts for CO2 reduction. ChemCatChem 6, 1899–1905 (2014).
[7] Chan, K. & Nørskov, J. K. Electrochemical Barriers Made Simple. J. Phys. Chem. Lett. 2663–2668 (2015).
[8] Montoya, J. H., Shi, C., Chan, K. & Nørskov, J. K. Theoretical Insights into a CO Dimerization Mechanism in CO2 Electroreduction. The journal of physical chemistry letters 6, 2032–2037 (2015).
5:15 PM - ES7.11.03
Understanding CO2 Reduction on Transition Metals
Karen Chan 1
1 , SLAC National Accelerator Laboratory, Menlo Park, California, United States
Show AbstractThe electrochemical reduction of CO2 has the potential to store energy from intermittent renewable sources and to produce carbon-neutral fuels and chemicals [1,2]. There are known electrode catalysts that can facilitate the process, but they are generally inefficient and the selectivity towards the desired products are often low[3]. Recent theoretical works have usually focused on the mechanism on copper, the only pure transition metal capable of reducing CO2 to alcohols and hydrocarbons at reasonable faradaic efficiencies [4-12]. In this talk, we examine the kinetics of CO reduction on transition and noble metal surfaces. We are primarily interested in CO2 reduction to more reduced products than CO, and so we focus on CO as the reactant. We use density functional theory calculations and an explicit solvent model of the electrochemical interface [13,14] to determine activation energies for the electrochemical steps. We show that the transition state energy for the CO hydrogenation process scales with the CO adsorption energy for metal surface catalysts. This scaling relation, combined with kinetic modeling, explains why rates do not vary much from one transition metal catalyst to the next, and why Cu is the best elemental metal catalyst for this process. On this basis, we propose two design criteria for efficient CO2 reduction catalysts.
1. Whipple, D. T. & Kenis, P. J. J. Phys. Chem. Lett/ 1,
3451-3458 (2010)
2.Graves, C. Ebbesen, S. D., Mogensen, M. & Lackner, K. S. Renewable and Sustainable Energy Reviews 15, 1–23 (2011).
3. Hori, Y. Modern Aspects of Electrochemistry, 89-189 (Springer New York, 2008).
4. Cheng, T., Xiao, H. & Goddard Iii, W. A. J. Phys. Chem. Lett. (2015).
5. Nie, X., Esopi, M. R., Janik, M. J. & Asthagiri, A. Angew. Chem. Int. Ed. 52, 2459--2462 (2013).
6. Nie, X., Luo, W., Janik, M. J. & Asthagiri, A. J. Catal. 312, 108--122 (2014).
7. Xiao, H., Cheng, T., Goddard, W. A. & Sundararaman, R. J. Am. Chem. Soc. 138, 483-486, (2016).
8. Calle-Vallejo, F. & Koper, M. T. M. Angew. Chem. Int. Ed. 52, 7282--7285 (2013).
9. Kortlever, R., Shen, J., Schouten, K. J. P., Calle-Vallejo, F. & Koper, M. T. M. J. Phys. Chem. Lett. (2015).
10. Hussain, J., Jonsson, H. & Skúlason, E. Faraday Discussions, (2016).
11. Hussain, J., Skúlason, E. & Jónsson, H. Procedia Computer Science 51, 1865-1871, (2015).
12. Goodpaster, J. D., Bell, A. T. & Head-Gordon, M.. J. Phys. Chem. Lett. 7, 1471-1477, (2016)
13.Chan, K. & Nørskov, J. K. J. Phys. Chem. Lett., 2663--2668 (2015).
14.Chan, K. & Nørskov, J. K. J. Phys. Chem. Lett., 1686-1690 (2016).
5:30 PM - *ES7.11.04
Catalysis on Nanoparticle Derived Aerogels
Alexander Eychmueller 1
1 , TU Dresden, Dresden Germany
Show AbstractGels and aerogels manufactured from a variety of nanoparticles available in colloidal solutions have recently proven to provide an opportunity to marry the nanoscale world with that of materials of macro dimensions which can be easily manipulated and processed, whilst maintaining most of the nanoscale properties [1]. The materials carry an enormous potential for applications, especially in catalysis. This is largely related to their extremely low density and high porosity providing access to the capacious inner surface of the interconnected nanoobjects they consist of. The aerogel materials may be further processed in order to achieve improvements in their properties relevant to applications in optical sensing and catalysis.
The commercialization of polymer electrolyte fuel cells (PEFC) is still hindered by the cathode electrocatalyst for the oxygen reduction reaction (ORR) not fulfilling the criteria of low cost, high performance, and high durability. We recently developed a facile strategy for the controllable synthesis of nanoparticle-based bimetallic PtxPdy aerogels with high surface area and large porosity, which act as highly active and stable catalysts for the ORR in PEFC cathodes. In addition to excellent durability the PtxPdy aerogels show superior electrocatalytic activity towards the ORR with the Pt80Pd20 aerogel exhibiting a five times mass activity enhancement compared to commercial Pt/C catalysts [2]. Further work is related to bi-metallic systems of PdNi structures (solid and hollow) and to Ni-Co-O oxide hollow nanosponges [3] as well as to electrocatalysis on pure Au aerogels [4].
References:
[1] a) J.L. Mohanan, I.U. Arachchige, and S.L. Brock: Science Vol. 307 (2005) p. 397 b) N. Gaponik, A.-K. Herrmann, and A. Eychmüller: J. Phys. Chem. Lett. Vol. 3 (2012) p. 8
[2] a) W. Liu, A.-K. Herrmann, D. Geiger, L. Borchardt, F. Simon, S. Kaskel, N. Gaponik, and A. Eychmüller: Angew. Chem. Int. Ed. Vol. 51 (2012) p. 5743 b) W. Liu, P. Rodriguez, L. Borchardt, A. Foelske, J. Yuan, A.-K. Herrmann, D. Geiger, Z. Zheng, S. Kaskel, N. Gaponik, R. Kötz, T.J. Schmidt and A. Eychmüller: Angew. Chem. Int. Ed. Vol. 52 (2013) p. 9849 c) W. Liu, A.-K. Herrmann, N.C. Bigall, P. Rodriguez, D. Wen, M. Oezaslan, T.J. Schmidt, N. Gaponik and A. Eychmüller: Acc. Chem. Res. 48 (2015) p. 154
[3] a) C. Zhu, D. Wen, S. Leubner, M. Oschatz, W. Liu, M. Holzschuh, F. Simon, S. Kaskel and A. Eychmüller: Chem. Comm. Vol. 51 (2015) p. 7851 b) B. Cai, D. Wen, W. Liu, A.-K. Herrmann, A. Benad and A. Eychmüller: Angew. Chem. Int. Ed. Vol. 54 (2015) p. 13101 d) C. Zhu, D. Du, A. Eychmüller and Y. Lin: Chem. Rev. Vol. 115 (2015) p. 8896
[4] a) D. Wen, W. Liu, D. Haubold, C. Zhu, M. Oschatz, M. Holzschuh, A. Wolf, F. Simon, S. Kaskel, A. Eychmüller: ACS Nano Vol. 10 (2016) p. 2559 b) D. Wen, W. Liu, A.-K. Herrmann, D. Haubold, M. Holzschuh, F. Simon, A. Eychmüller: Small Vol. 12 (2016) p. 2439
Symposium Organizers
Francesca Maria Toma, Lawrence Berkeley National Laboratory
Akihiko Kudo, Tokyo University of Science
Roel Van de Krol, Helmholtz-Zentrum Berlin
Lianzhou Wang, University of Queensland
Symposium Support
ACS Energy Letters | ACS Publications
Bruker—Nano Surfaces Division
California Institute of Technology
Helmholtz-Zentrum Berlin (HZB)
Lawrence Berkeley National Laboratory– Joint Center for Artificial Photosynthesis
Wiley VCH Verlag GmbH &
Co. KGaA
ES7.12: Towards Practical Systems
Session Chairs
Katherine Ayers
Miguel Modestino
Francesca Maria Toma
Roel Van de Krol
Thursday AM, April 20, 2017
PCC North, 200 Level, Room 222 BC
9:30 AM - *ES7.12.01
An Integrated Inorganic-Biological Hybrid System for a Complete Artificial Photosynthesis
Daniel Nocera 1
1 , Harvard University, Cambridge, Massachusetts, United States
Show AbstractNatural photosynthesis has evolved to combine the hydrogen, produced from solar-driven water splitting, with carbon dioxide to produce biomass. We have achieved an authentic and complete artificial photosynthesis. Using the tools of synthetic biology, a bio-engineered bacterium has been developed to convert carbon dioxide, along with the hydrogen as a result of the water splitting catalysis of the artificial leaf, into biomass and liquid fuels, thus closing an entire artificial photosynthetic cycle. This hybrid microbial | artificial leaf system scrubs 180 grams of CO2 from air, equivalent to 230,000 liters of air per 1 kWh of electricity. This hybrid device, called the bionic leaf, operates at unprecedented solar-to-biomass (10.7%) and solar-to-liquid fuels (6.2%) yields, greatly exceeding the 1% yield of natural photosynthesis.
10:00 AM - *ES7.12.02
Electrochemical Carbon Dioxide Reduction as an Alternative Source of Fuels and Chemicals
Sichao Ma 1 , Etosha Cave 1 , Kendra Kuhl 1 , George Leonard 1 , Nicholas Flanders 1
1 , Opus 12, Berkeley, California, United States
Show AbstractCost-effective industrial CO2 recycling using renewable energy sources (ECO2R) could form the basis of an artificial carbon cycle that replaces a wide range of products that are currently derived from fossil resources. ECO2R combines CO2, water, and electricity, and converts them into useful products using engineered metal catalysts. At full scale, this technology could eliminate our dependence on fossil resources by providing an alternative source of carbon-based compounds for fuels and commodity chemicals. However, commercial fuel and chemical production via ECO2R is challenging, because the current state of the technology is not cost-effective enough to compete with conventionally manufactured fuels and chemicals already on the market.
Opus 12 has also developed a prototype ECO2R reactor. The Opus 12’s prototype contains improved catalysts and a reactor design with high energy efficiency, high product selectivity, and high current densities. The key cost-drivers of ECO2R are long term stability, energy efficiency, product selectivity, and current density. We will present our technical progress regarding these key performance metrics. Based on these metrics, we will identify the technical barriers that need to be overcome to reduce the cost, including enhanced electrocatalysis [1-3] and achieving high current densities needed for a cost-competitive process because of limited carbon dioxide solubility. Using these metrics, we can estimate the current cost of ECO2R and present a roadmap of improvements needed to reduce further costs to compete with traditional chemicals and fuels derived from fossil resources.
[1] K.P. Kuhl, E.R. Cave, D.N. Abram, T.F. Jaramillo, New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces, Energy Env. Sci., 5 (2012) 7050-7059.
[2] K.P. Kuhl, T. Hatsukade, E.R. Cave, D.N. Abram, J. Kibsgaard, T.F. Jaramillo, Electrocatalytic Conversion of Carbon Dioxide to Methane and Methanol on Transition Metal Surfaces, Journal of the American Chemical Society, 136 (2014) 14107-14113.
[3] F.S. Roberts, K.P. Kuhl, A. Nilsson, High Selectivity for Ethylene from Carbon Dioxide Reduction over Copper Nanocube Electrocatalysts, Angewandte Chemie International Edition, 54 (2015) 5179-5182.
10:30 AM - ES7.12.03
A Monolithic and Scalable Device Based on Adapted Silicon HIT Photovoltaic Structures Enabling Bias-Free CO2 Conversion to Syngas
Felix Urbain 1 , Nina Carretero 1 , Teresa Andreu 1 , Cristobal Voz 2 , Guillermo Gerling 2 , Ramon Alcubilla 2 , Juan Morante 1
1 , IREC, Catalonia Institute for Energy Research, Sant Adrià de Besòs Spain, 2 Electronic Engineering Department, Universitat Politècnica de Catalunya, Barcelona Spain
Show AbstractWe report on a device for the direct photoelectrochemical conversion of CO2 to syngas (H2 + CO), which by design, is integrated, scalable to large areas, and compatible with state-of-the-art solar cell modules and electrocatalysts. While substantial advances in materials components (photoabsorbers and catalysts) and subassemblies that demonstrate CO2 reduction have been hitherto reported in the literature, the work has been focused primarily on making scientific progress rather than creation of efficient, durable, and scalable solar fuels device systems. This work aims to bridge that gap by giving a push also from the engineering site.
Within this contribution mainly three aspects will be addressed: (i) cathode material development, (ii) adaption and integration of silicon HIT solar cells as photoanodes, and (iii) reactor assembly and scalability. For (i) we investigate the deposition of single and compound nano-particle catalysts, such as Mo and Ag onto Ni-and Cu-foams, respectively by means of thermal- and electro-deposition. The performance of the as-produced (gas diffusion) cathodes is evaluated in terms of product selectivity, Faradaic efficiency, overpotentials, and stability. We demonstrate that the cathodes exhibit low overpotentials and stable H2:CO ratios between 1 and 3 with high Faradaic efficiencies over 90% using 1M KHCO3 as aqueous electrolyte.
The application of silicon HIT photovoltaic cells as photoanodes (ii) requires meeting challenges, such as increasing the photovoltage without impairing the photovoltaic efficiency; protection of the solar cell by robust coatings to increase the stability in aqueous electrolytes; and the decoration with catalysts ensuring an efficient oxygen evolution reaction (OER). The silicon HIT technology stands out because it is non-toxic, abundant, well understood and has recently acquired an industry leading level. We show that photovoltages up to 2.5 V with photocurrent densities up to 7.5 mA/cm2 can be reached by connecting four HIT cells in series. Furthermore, for the rear contact of the HIT cells we investigate different non-precious metallic (e.g. Ni and Ti) protection layers and explore the applicability of metallic foams (e.g. Ni and Cu) loaded with metallic particles as OER catalyst. We demonstrate OER overpotentials below 400 mV and the best photoanodes operate bias-free for over 100 hours at 7.5 mA/cm2 in 1M KOH (2-electrode configuration).
In the complete integrated reactor assembly (iii) we combine the optimized cathode and photoanode and analyze whether two-compartments (anolyte, catholyte) or one-electrolyte solution will be beneficial in terms of the device performance. Furthermore, based on the choice of the electrolyte, the membrane will be adjusted accordingly. We stepwise optimize the reactor (base unit), regarding clever packaging, efficient gas management, and electrolyte flow. The scalability is achieved by continuous repetition of the base unit (up to 50 cm2 active device area).
10:45 AM - ES7.12.04
Design, Analysis and Optimization of High Voltage Photovoltaic Electrolysis System for Solar Fuel Production from CO2
Gowri Sriramagiri 1 2 , Nuha Ahmed 1 2 , Wesley Luc 3 , Kevin Dobson 2 , Steven Hegedus 1 2 , Feng Jiao 3 , Robert Birkmire 2 4
1 Department of Electrical and Computer Engineering, University of Delaware, Newark, Delaware, United States, 2 , Institute of Energy Conversion, Newark, Delaware, United States, 3 Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware, United States, 4 Department of Physics & Astronomy, University of Delaware, Newark, Delaware, United States
Show AbstractGrowing interest in the use of CO2 as a feedstock for fuel generation has led to increased attention to efficient artificial carbon fixation. The challenges of integrated solar-to-carbon fuel converters, wherein the photovoltaic (PV) material is immersed in the electrolyte, are well-known: the need for unique PV cell designs; material incompatibility; corrosion; and optical losses. Here, a PV-electrolysis system is presented, where a well-established flow-cell electrolyzer is power-matched to a high-performance solar PV module. Two advantages of such a system design are: 1) use of standard PV cells external to the electrolyzer, which allows de-coupling the design, fabrication and operation of the PV system from that of the electrolyzer; and 2) enabling optimization of the PV configuration to maximize power coupling efficiency to the specific electrolyzer Tafel curve, with or without the use of electronic power-conditioning devices.
High-performance commercial, >20% efficiency, c-Si SunPower® C60 cells were used for the PV source. The high short-circuit current density, JSC > 40 mA/cm2, of these cells allows for high solar-to-fuel efficiencies, SFE, since it is directly proportional to the effective current density of operation, JOP. The process of the PV source design, including modeling of various configurations and calculating their SFE will be described and compared with experimental results. The electrolyzer used here is a sandwich-type flow-cell reactor, which comprises of a nanoporous silver (np-Ag) cathode and iridium nanoparticles sprayed onto a Nafion® membrane as the anode. This system is the first-reported deployment-ready solar-powered CO2 flow-cell electrolyzer.
The variables were the exposed area of the cells and number of cells in series, determining the current and voltage, respectively. The goal was to obtain the highest JOP, while providing the required operating voltage, VOP, to drive the electrolyzer near its peak Faradaic efficiency, FE, which is around 2.8 V for the one we used. We found from modeling different source configurations that the best SFE occurs when the operating I-V point, IOP-VOP, is closest to the maximum power point on the solar module I-V curve. The modules with smaller solar cell areas have smaller IOP and VOP, therefore low FE, yet their JOP is high. Thus there is a tradeoff between high JOP and low FE for small cell areas in a configuration and vice versa for higher areas. The SFE’s for several cell areas with 5 or 6 cells in series were determined, with best efficiency predicted for this setup being >7%, challenging current highest reported CO2 solar-electrolysis efficiencies [1]. Experimental results of 5-cell and 6-cell configurations, tested with optimal and suboptimal cell areas for comparison, will be reported.
References:
[1] Schreier, Marcel, et al. "Efficient photosynthesis of carbon monoxide from CO2 using perovskite photovoltaics" Nature communications 6 (2015)
11:30 AM - *ES7.12.05
Working Together to Enable Gigawatt Scale Renewable Hydrogen Production—Solar Fuels and Large Scale Electrolysis
Katherine Ayers 1 , Nemanja Danilovic 1 , Everett Anderson 1
1 , Proton OnSite, Wallingford, Connecticut, United States
Show AbstractRenewable sources of hydrogen are an essential component of grid decarbonization. Hydrogen is required in many industrial processes, including manufacture of ammonia and hydrocarbon refining, and is generated in quantities in excess of 50 million metric tons annually, mostly from natural gas. The ammonia production process currently consumes 2% of the world’s energy, largely due to the methane reforming step to make the hydrogen, which also contributes the majority of the greenhouse gas emissions (CO2). To make this process greener, hydrogen from non-fossil fuel sources is required. Renewable hydrogen is also needed for converting CO2 to liquid fuels and upgrading biomass/crude oil. At the same time, renewable sources of energy such as wind and solar have been growing in penetration on the electrical grid, and require energy storage solutions to level supply and demand. Hydrogen has been proposed as one means of storing the excess capacity. These concepts have been recently gaining momentum through H2@Scale, an initiative to look at the synergies between intermittent renewable energy sources, industrial applications, transportation, and other grid management considerations. Recently there has been significant publicized work in solar fuels (direct photoelectrochemical conversion of sunlight and water to hydrogen), as well as isolated work on components such as oxygen and hydrogen evolution catalysts. In parallel, commercial water splitting technology has advanced to larger and larger scale, and similar fuel cell technology can be leveraged to advance component and manufacturing development. In order to make the necessary multi-gigawatt scale impact in grid level integration and large scale renewable hydrogen supply, learnings from each field need to be leveraged. The product development cycle is long, and these near term solutions will be needed as building blocks and stepping stones for nascent technologies of today. This talk will focus on the evolution of membrane-based water electrolysis as a renewable hydrogen pathway, and implications for industrial and energy applications.
12:00 PM - ES7.12.06
User-on-Demand Power Supply System Operation Using the Concept of High Performance Solar to Hydrogen Conversion Device
Katsushi Fujii 1 2 3 , Shin Nakamura 2 , Yasuhiko Miwata 2 , Satoshi Wada 2 , Kensuke Nishioka 4 , Yoshiaki Nakano 3 , Masakazu Sugiyama 3
1 , University of Kitakyushu, Kitakyushu Japan, 2 , RIKEN, Wako Japan, 3 School of Engineering, The University of Tokyo, Tokyo Japan, 4 , University of Miyazaki, Miyazaki Japan
Show AbstractEnergy storage is important for the energy originate from nature energy source because the nature energy like sun light is fluctuated but the users want to use it on demand. Solar to hydrogen conversion is useful technology for the renewable energy storage. High performance conversion efficiency is expected when the solar cell and electrochemical cell is directly connected [1]. One of the key point to obtain the high efficiency for the direct connection of solar cell and electrochemical cell is the improvement of the maximum conversion efficiency from sun light to electricity of solar cell. The other is the operation point matching, that is, the operation voltage and current of electrochemical cell should be the same at the maximum power point of the solar cell. The maximum energy conversion efficiency was approached to be 24.4% with optimizing these key points [1].
Dominant and major nature to energy system of today in the world is, however, solar cell devices due to its cost performance and set up convenience. This means that the device has no storage system, thus, the generation power is fluctuated and affects power grid stability. However, energy efficiency points of view, the direct usage of the power generated by solar cell is the best way. As a result, it is better that the power is not only from the fuel cell using hydrogen produced by electrochemical water splitting but also from solar cell directly when the nature originate energy is used as on demand. That is, the good way to use of the power is from solar cell when the solar energy exists enough, and from fuel cell at cloudy and rainy days and at the night.
In order to evaluate such kind of user-on-demand power system operation, which energy is supplied from solar cell directly and fuel cell via hydrogen, a power supply system with crystalline silicon solar cell was assembled. Although the system is a kind of demonstration system and the maximum power supply is only about 50 W, the power supply is controlled by the user-on-demand based. The operation efficiency cannot be compared from the high performance system of 24.4% solar to hydrogen energy conversion, but the system is useful for the solar energy storage when it is required. The device voltage matching like the direct coupling of solar cell and electrochemical cell is difficult for this kind of system because the devices are the commercialized ones. Thus, the system requires DC/DC voltage conversion to operate the devices assembled in the system. Not only the device performance improvement but also the improvement of DC/DC conversion efficiency is important for the improvement of system efficiency.
[1] A. Nakamura et al., Appl. Phys. Express 8 (2015) 107101.
12:15 PM - ES7.12.07
A Durable and Efficient Solar Hydrogen Generator
Karl Walczak 1 , Gideon Segev 1 , David Larson 1 , Jeffery Beeman 1 , Chengxiang (CX) Xiang 2 , Frances Houle 1 , Ian Sharp 1
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 , CIT, Pasadena, California, United States
Show AbstractA safe and practical solar-driven water splitting system must be capable of efficient and stable operation under diurnal cycling with full separation of gaseous H2 and O2 products. Here, we presesnt a novel architecture that fulfills all of these requirements and provides a versatile approach for integrating catalysts and photovoltaic elements in a monolithic assembly, while also protecting chemically sensitive semiconductor components. The 1 cm2 proof-of-concept device uses a commercially available triple-junction photovoltaic cell with its illuminated photocathode protected by a composite coating comprising an organic epoxy with an embedded catalytic support. The device is compatible with operation in both 1 M H2SO4 and 1 M KOH, enabling flexibility in selection of semiconductor, electrolyte, membrane, and catalyst. This approach significantly relaxes material and process compatibility constraints associated with chemically and thermally sensitive materials combinations. Stable operation at a solar-to-hydrogen conversion efficiency of >10% is demonstrated under continuous operation, as well as under diurnal light cycling for at least four days, with simulated sunlight. The device was also validated with outdoor testing. Analysis of operational characteristics is enabled by comparison of a device model to experimental data.
12:30 PM - ES7.12.08
Developing Microfluidic Air-Based Solar-Hydrogen Generators
Miguel Modestino 2 , S. Mohammad H. Hashemi 1 , Christophe Moser 1 , Demetri Psaltis 1 , Matthias Neuenschwander 1 , Daniel Gregory 1
2 , New York University, Brooklyn, New York, United States, 1 , EPFL, Lausanne Switzerland
Show AbstractThe large scale deployment of solar energy technologies relies in our ability to develop practical energy storage solutions. Solar-hydrogen generators are an attractive solution to this challenge, as they couple the solar-absorption process with the production of energy-rich, storable and transportable hydrogen molecules. Several concepts of solar-hydrogen devices have been proposed in the past, but their implementation has been hindered by multiple technical and economic shortcomings. Simplifying the operation of these devices by employing water vapor feeds can aid in overcoming these hurdles. Specifically, operating electrolyzers with water-vapor can reduce the required water-splitting potential, eliminate catalysis limitations from bubble formation, and reduce the rate of degradation of the materials involved. Here, we present a microfluidic electrolysis device that can produce hydrogen directly from humid ambient air feeds. The device architecture consists of a set of microelectrodes that are covered by a thin Nafion film which serves as a medium for ion-conduction. Water diffuses from the air stream through the ionomer film and reacts at the anode to generate oxygen and protons. Protons then migrate towards the cathode and are reduced to produce hydrogen. We will describe how the device architecture, catalyst selection, and ionomer properties affect the balance between mass transport and kinetics, and ultimately determines the electrochemical performance of the system. Moreover, we will demonstrate how optimal device designs can lead to water electrolysis systems with current densities higher than 10 mA/cm2 at potentials below 1.8 V. Lastly, we will present early demonstrations of integrated solar water-splitting devices that operate directly under humid-air streams.
12:45 PM - ES7.12.09
Tandem Cell Approach for Artificial Photosynthesis
Gurudayal Gurudayal 1 , Lydia Wong 2 , Michael Graetzel 3 , Nripan Mathews 2 , Joel Ager 1
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 MSE, Nanyang Technological University, Singapore Singapore, 3 , EPFL, Lausanne Switzerland
Show AbstractA solar photoelectochemical system which could produce a chemical fuel would be a potential strategy towards global energy sustainability, complementing currently implemented solar photovoltaics. Solar-driven water splitting semiconducting photoelectrodes have received much recent attention. While unassisted solar water splitting driven by a single photoelectrode yields low overall conversion efficiencies due to the large overall voltage requirement, use of a tandem cell approach wherein the total photovoltage produced by complementary optical absorption across different semiconducting photoelectrodes allows greater flexibility in component selection and has achieved much higher laboratory efficiencies. The relatively low photovoltages generated by conventional photovoltaic materials (eg. Si, CIGS) has limited such systems to triple junction designs which increases complexity. Here, we utilize the relatively large open circuit voltages available from organic-inorganic halide perovskite (CH3NH3PbI3) solar cells to construct a tandem water splitting devices with manganese (Mn) doped Fe2O3 photoanodes. Mn doping in hematite helps to improve the charge carrier conductivity and was also found to be beneficial in charge transfer at electrode-electrolyte interface. In our initial work, we demonstrated a solar to hydrogen conversion efficiency of 2.4% [1].
The efficiency losses in this system have been analyzed. The total photopotential generated by our tandem system (1.87 V) exceeds both the thermodynamic and kinetic requirements (1.6 V), resulting in overall water splitting without the assistance of an electrical bias. However, the STH efficiency was limited by high onset potential and low photocurrent performance of hematite. Therefore, we worked to enhance the photocurrent of nanostructured hematite photoanode with SnOx overlayer deposition, which acts as a surface passivation layer and increases the photocurrent by 1.5 times compared to the Mn doped hematite. The onset potential was shifted cathodically by 250 mV via incorporation of a Co phosphate co-catalyst to improve the kinetics of water oxidation. When incorporated in series optically with a perovskite solar cell, a 3.6% solar to hydrogen conversion efficiency is achieved.
[1] Gurudayal et al., Nano Lett. 2015, 15, 3833–3839.
ES7.13: BiVO4 Photoelectrodes
Session Chairs
Akihiko Kudo
Lianzhou Wang
Thursday PM, April 20, 2017
PCC North, 200 Level, Room 222 BC
2:30 PM - *ES7.13.01
Elucidating Excited State Processes in Transition Metal Oxide Photoelectrodes
Ian Sharp 1 , Jason Cooper 1 , Lucas Hess 1
1 , Lawrence Berkeley National Lab, Berkeley, California, United States
Show AbstractDevelopment of practical photosystems for direct conversion of solar energy to chemical fuel requires semiconductor light absorbers that are simultaneously efficient, durable, and scalable. Thin film, transition metal oxide semiconductors are actively investigated to fill this critical role. However, the efficiencies of these systems typically fall far short of the thermodynamic limit implied by their bandgaps due to a mismatch between optical absorption depths and charge collection lengths. As such, establishing intrinsic and extrinsic limits to energy conversion efficiency for a given material requires improved understanding of light interactions with the semiconductor, as well as photocarrier trapping and recombination mechanisms. Among available tools, transient absorption spectroscopy is a widely used method for probing photocarrier relaxation processes and tracking competitions between undesired recombination and desired chemical reaction pathways. However, excited state spectra from inorganic semiconductors are typically characterized by broad and convoluted features. Such complexity has impeded progress in understanding the factors contributing to efficiency loss and, at times, resulted in inconsistent or ambiguous spectral assignments. In this work, we establish a quantitative description of the excited state absorption spectrum of a prototypical semiconductor photoanode, bismuth vanadate (BiVO4). Starting from a comprehensive portrait of electronic structure, obtained via X-ray spectroscopy, and the complex dielectric function, obtained via variable angle spectroscopic ellipsometry, we show that the complete spectral response can be described by considering physical models for bandgap renormalization, band filling, and free carrier absorption. Photo- and electro-reflectance effects are also considered and evaluated under steady state, as well as during the longer time scales that are relevant for chemical reaction. Based on these spectral assignments, improved understanding of photocarrier trapping, photocarrier recombination, and phonon interactions are established. The application of this physical model to other transition metal oxide photoelectrode systems provides general insights into the basic processes that affect solar energy conversion efficiencies.
3:00 PM - ES7.13.02
Time-Resolved Terahertz Study of Carrier Trapping and Polaron Formation in BiVO4
Rainer Eichberger 1 , Manuel Ziwritsch 1 , Soenke Mueller 1 , Hannes Hempel 2 , Fatwa Abdi 1 , Thomas Unold 2 , Dennis Friedrich 1 , Roel Van de Krol 1
1 Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin Germany, 2 Department Structure and Dynamics of Energy Materials, Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin Germany
Show AbstractMetal oxide semiconductors have received a great deal of attention within the past decade due to their many technological applications, particularly in the fields of photocatalysis and solar water splitting. However, they mostly remain far below the theoretical efficiency limit because of their poor carrier transport properties. Many metal oxides possess low conductivity due to the localization of the excess charge carriers that typically form polarons very rapidly after photoexcitation. We apply sub-ps THz spectroscopy on undoped and doped thin films of BiVO4 and a single crystal to get fundamental insight into the behavior of charge carriers and polarons in this material. We observe the temporal evolution of polaron formation leading to a buildup of a polaron population in parallel with carrier trapping dynamics in an early picosecond to nanosecond time window. A major advantage of THz spectroscopy is the possibility to measure frequency-dependent conductivity or mobility spectra, which gives a deeper insight into the nature of carrier transport and localization by transferring the signal into the Fourier space. We find direct time-resolved evidence of polaron formation in the mobility spectrum of BiVO4 via a resonance related to a prominent low-frequency vibrational mode that modulates polaron migration by phonon coupling. The polaron mobility and the vibrational frequency allow us to calculate the room temperature activation energy of ~ 90 meV for thermal polaron hopping. We affirm that THz spectroscopy that probes free and weakly bound charge carriers is an excellent technique for studying time-resolved carrier transport and localization effects. Our experimental findings will help to characterize and select future semiconductor candidates for more efficient use in photovoltaic and photocatalytic devices.
M. Ziwritsch, S. Müller, H. Hempel, T. Unold, F. F. Abdi, R. van de Krol, D. Friedrich, R. Eichberger, ACS Energy Lett., 1, 888 (2016)
F. F. Abdi, T. J. Savenije, M. May, B. Dam, R. van de Krol, J. Phys. Chem. Lett. 4, 2752 (2013)
J. Ravensbergen, F. F. Abdi, J. H. van Santen, R. N. Frese, B. Dam, R. van de Krol, J. T. M. Kennis, J. Phys. Chem. C, 118, 27793 (2014)
3:15 PM - ES7.13.03
Nanoscale Imaging of Charge Carrier Transport in Monoclinic Bismuth Vanadate Photoanodes via Atomic Force Microscopy
Johanna Eichhorn 1 , Jason Cooper 2 1 , Lucas Hess 1 , Dominik Ziegler 3 4 , David Larson 2 1 , Mary Gilles 1 , Ian Sharp 2 1 , Francesca Maria Toma 2 1
1 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 , Scuba Probe Technologies LLC, Alameda, California, United States, 4 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractPhotoelectrochemical water splitting is a promising route for efficient conversion of solar energy to chemical fuel. Among different photoelectrode materials, bismuth vanadate (BiVO4) is one of the most actively investigated oxide semiconductors due to its moderate bandgap, favorable conduction band position, and relatively long photocarrier lifetimes.[1] However, under relevant operating conditions, pristine BiVO4 thin films are subjected to degradation at the exposed surface facets. The degradation process in solution is accelerated by photoexcitation, which causes trapping of photogenerated holes at localized surface sites.[2] Therefore, developing approaches to stabilize these efficient semiconductor nanostructures requires a detailed understanding and control of charge separation, transport, and recombination mechanisms at their relevant length scales.
Here, we use photoconductive atomic force microscopy in combination with Kelvin probe force microscopy to correlate local surface morphology with generated photocurrent and contact potential difference (CPD) maps at individual grain facets in polycrystalline BiVO4 films. Furthermore, we employ scanning transmission X-ray microscopy to trace the changes in local chemical structure and composition in pristine and photodegraded BiVO4.
The photocurrent and CPD maps reveal the impact of different working conditions, such as bias voltage, excitation energy or excitation power, on the local charge carrier dynamics. Both for excitation above the bandgap (405 nm) and sub-bandgap illumination (532 nm), the photocurrent maps resolve the contributions from individual grains with nanometer spatial resolution. The photoelectrochemical performance of BiVO4 can be significantly enhanced by varying the oxygen vacancy defects and hydrogen impurities, through hydrogen annealing. Therefore, we compare the photocurrent generation of as-grown and hydrogen annealed BiVO4. This careful analysis allows us to identify locally the charge transfer and loss mechanisms in these materials which ultimately contribute to desired photocurrent generation or undesired photocorrosion.[3]
[1] J. K. Cooper et al., Chemistry of Materials, 2016, 28, 5761−5771
[2] F. M. Toma et al., Nature Communication, 2016, 7, 12012
[3] J. Eichhorn et al., in preparation
3:30 PM - ES7.13.04
Photoelectrochemical Water Oxidation of BiVO4 Photoanodes with 50 cm2 Active Area
Yimeng Ma 1 , Ibrahim Ahmet 1 , Ji-Wook Jang 1 , Fatwa Abdi 1 , Roel Van de Krol 1
1 Institute for Solar Fuels, Helmholtz-Zentrum Berlin, Berlin Germany
Show AbstractMetal oxide photoelectrodes are promising candidates for photoelectrochemical (PEC) and photocatalytic applications, as they are relatively stable, cheap, and scalable. Bismuth vanadate (BiVO4) is a particular promising photoanode material for water oxidation. Photocurrents exceeding 4 mA cm-2 at 1.23 VRHE have been reported by several groups,1 and the material shows a favorable onset potential of ~0.2 VRHE.2-3 However, these results were obtained from small-sized photoelectrodes (~1 cm2), in which limiting factors such as substrate conductivity, electrolyte pH gradient and mass transport were not considered.
We present PEC results of BiVO4 photoanodes with 50 cm2 active area fabricated using spray pyrolysis.4 By optimizing the spray process, we could deposit these large-area samples with little variation in the photocurrent density across the sample area. One of the main challenges to overcome during scale-up was the high FTO substrate resistance. We solved this by using nickel metal lines that were electrodeposited on the FTO in order to efficiently collect photo-excited electrons from BiVO4. Although partial oxidation of the nickel lines during BiVO4 deposition and post-deposition annealing could not be avoided, the conductivity of the lines remained sufficiently high. In addition, we have explored surface overlayers to improve the photocurrent onset potential and photocurrent density. Cobalt phosphate and several other materials were studied to evaluate their compatibility with a BiVO4-based photoanode, their PEC stability, and the water oxidation efficiency. Finally, we will present an overall solar-driven water splitting demonstrator based on a BiVO4 photoanode and a PV cell as bottom absorber on the scale of 50 cm2, and discuss the advantage of this strategy for solar fuel production.
References:
1. Pihosh, Y.; Turkevych, I.; Mawatari, K.; Uemura, J.; Kazoe, Y.; Kosar, S.; Makita, K.; Sugaya, T.; Matsui, T.; Fujita, D.; Tosa, M.; Kondo, M.; Kitamori, T., Photocatalytic generation of hydrogen by core-shell WO3/BiVO4 nanorods with ultimate water splitting efficiency. Scientific Reports 2015, 5, 11141.
2. Abdi, F. F.; Han, L.; Smets, A. H. M.; Zeman, M.; Dam, B.; van de Krol, R., Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode. Nat. Commun. 2013, 4, 2195.
3. Kim, T. W.; Choi, K. S., Nanoporous BiVO4 photoanodes with dual-layer oxygen evolution catalysts for solar water splitting. Science 2014, 343 (6174), 990-4.
4. Abdi, F. F.; Firet, N.; van de Krol, R., Efficient BiVO4 Thin Film Photoanodes Modified with Cobalt Phosphate Catalyst and W-doping. ChemCatChem 2013, 5 (2), 490-496.
3:45 PM - ES7.13.05
Pulsed Laser Deposition of Ternary Oxide Photoelectrodes—BiVO4 as a Model Material
Moritz Koelbach 1 , Karsten Harbauer 1 , Roel Van de Krol 1 , Klaus Ellmer 1
1 Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin Germany
Show AbstractOne of the main challenges in the field of solar water splitting is to find novel photoanode and photocathode materials that can be used as chemically stable wide-gap absorbers in tandem devices. Few—if any—of the simple binary oxides show the desired properties, and recent efforts in the field have therefore shifted towards the more complex multinary oxides. To study the fundamental properties and performance limitations of such novel photoelectrode materials, one needs to be able to deposit thin compact films of high electronic quality.
Pulsed laser deposition (PLD) is a versatile physical vapor deposition technique that meets these demands.[1] Up until now, this powerful tool has not been utilized often for the preparation of photoelectrodes for solar water splitting, partially due to the complexity of the PLD process. We have chosen n-type BiVO4, one of the most studied photoelectrode materials so far, as a model material for our PLD studies. BiVO4 has been successfully deposited using PLD by Rettie et al. and Murcia-López et al., but the fundamental PLD processes were not analyzed in detail.[2,3] Here, we report for the first time the parameters that influence the V-to-Bi ratio, film morphology, and photoactivity of the BiVO4 electrodes.
The stoichiometry of the deposited film is mainly influenced by the laser fluence and the oxygen background pressure during deposition. EDX and RBS measurements reveal that the V-to-Bi ratio increases with increasing laser fluence in vacuum. This can be attributed to a diffusion-limited preferential ablation of Bi. The addition of an oxygen background pressure results in a decreased V-to-Bi ratio. The reason for this is the faster thermalization (collisions with oxygen molecules) of the lighter element V, which was qualitatively verified with ion energy measurements and SRIM simulations. Combining these results allows control of the V-to-Bi ratio of the films. The structure, optical properties and photoactivity of the BiVO4 electrodes will be discussed as a function of the deposition conditions and post-deposition annealing parameters.
The results of this work provide important new insights into the PLD process of ternary photoelectrodes and helps to accelerate the preparation of new high-quality complex metal oxide photoelectrodes.
[1] H. U. Krebs et al., Adv. Solid State Phys. 2003, 43, 505–517
[2] A. J. E. Rettie et al., J. Phys. Chem. C 2014, 118, 26543–26550
[3] S. Murcia-López et al., ACS Appl. Mater. Interfaces 2016, 8, 4076–4085
ES7.14: Nitride and Oxynitride Absorbers
Session Chairs
Thursday PM, April 20, 2017
PCC North, 200 Level, Room 222 BC
4:30 PM - *ES7.14.01
Improvement of Water Oxidation Ability of Oxynitrides Aiming at Application to Photoanodes
Hideki Kato 1
1 IMRAM, Tohoku University, Sendai Japan
Show AbstractPhotocatalytic water spitting gathers much attention because of its potential for applications to solar energy conversion. Photocatalyst particles can also be used as photocathodes or photoanodes when suitable electrodes are fabricated from the photocatalyst particles. Many factors such as particle size, crystallinity, defect, band potential, and co-catalyst affect photocatalytic and/or photoelectrode properties. Among them, potentials of valence and conduction bands are regarded as quite important factors because they determine the reactivity of photogenerated electrons and holes, and the band gaps of photocatalyst materials. Here, I introduce the resent research on control of photocatalytic properties of oxynitride for water oxidation through tuning of band potentials of oxynitride photocatalysts.
The contribution of N 2p orbitals to the valence bands, which causes the formation of valence bands at higher (more negative) position than those in the general oxides, is essential for narrow band gaps of (oxy)nitrides. However, such high valence bands sometimes result in lack of the ability for water oxidation to O2; for examples, perovskite-type oxynitride LaTaON2 is not active for water oxidation in the absence of cocatalysts. Water oxidation is a fundamental step in photocatalytic and photoelectrochemical water splitting. Moreover, photoanodes in water splitting system should have ability for water oxidation. From this background, tuning of the valence band potential through control of the content of nitrogen in oxynitride has been examined with aim at achievement of O2 evolution over perovskite-type oxynitride photocatalysts absence of the cocatalyst modification. The solid solutions between LaTaON2 and NaTaO3 giving the general formula La1–xNaxTaO1+2xN2–2x have been successfully synthesized. The band gaps of the solid solutions became large as the fraction of oxide increased. The solid solutions with x = 0.2–0.75 exhibited activity for O2 evolution in addition to H2 evolution in the presence of sacrificial reagents under visible light irradiation although LaTaON2 (x = 0) and the sample of x = 0.1 did not show any activities for O2 evolution. The appearance of activity for O2 evolution is due to the increase in the driving force for water oxidation, which corresponds to potential deference between valence band maximum (VBM) and E(O2/H2O). Interestingly, another series of solid solutions between LaTaON2 and SrTiO3, La1–xSrxTa1–xTixO1+2xN2–2x (LSTTON), has larger driving force and exhibited remarkably higher activity for O2 evolution. LSTTON utilizes visible light with wavelength up to 580 nm. High activity for water oxidation suggests that LSTTON has potential to be applied as a visible light responsive photoanode.
5:00 PM - ES7.14.02
Fabrication of Ta3N5 Crystal Layers on Tantalum Substrate Using NaCl-Na2CO3 Flux Evaporation and Their Photoelectrochemical Properties
Minori Yanai 1 , Sayaka Suzuki 2 , Tetsuya Yamada 3 , Katsuya Teshima 2 3
1 Graduate School of Science and Technology, Shinshu University, Nagano Japan, 2 , Faculty of Engineering, Shinshu University, Nagano Japan, 3 , Center for Energy and Environmental Science, Shinshu University, Nagano Japan
Show AbstractPhotoelectorochemical (PEC) water splitting into hydrogen and oxygen has attracted great attention as a solution for environmental and energy problems. For PEC water splitting, 10 % or higher solar-to-fuel conversion efficiency is the ultimate goal. The photoanodes consist of n-type semiconductor and conductive substrate. The good connection between photocatalyst and the collective electrode would be required to obtain higher efficient photoanode. Ta3N5 is one of the most promising photocatalyst candidates for solar water splitting because of suitable band structure and band gap (2.1 eV). Ta3N5 photoanodes have been generally prepared by nitridation of precursor tantalum oxide, resulting in porous Ta3N5 due to the volume decrease. The porous Ta3N5 could lead to increase the number of photogenerated carrier recombination center and to degrade the contact to the substrate. In the previous study, Ta3N5 crystal layers were successfully fabricated using a flux coating method. The flux coating method is one of the crystal layer growth technique from high temperature solution. It expects to fabricate highly crystalline layers. In this study, we report a new approach to fabricating Ta3N5 crystal layers on tantalum substrate by supplying the flux via evaporation.
Tantalum foil was used as both the substrate and tantalum source, and the nitrogen part of Ta3N5 came from the ammonia gas used. NaCl and Na2CO3 powders were used as the fluxes. The mixed powders were put into a platinum cases, and then tantalum substrates were placed on the cases. The cases were heated at 700-900 °C under an ammonia gas flow.
The Ta3N5 crystal layers were directly fabricated on tantalum substrate using NaCl-Na2CO3 flux at 900 °C. Ta3N5 crystal layers had good connection with the substrates. The thicknesses of Ta3N5 crystal layers fabricated at 900 °C for 1 and 3 h were 560 and 680 nm, respectively. From surface FE-SEM images, cuboid Ta3N5 crystals were observed. XRD patterns of the products indicated that the main phase was Ta3N5, but there were diffraction lines corresponding to NaTaO3 and unknown phases. The maximum photocurrent density of Ta3N5 crystal layers modified with Co-Pi as a co-catalyst was 6.5 mA/cm2 at 1.23 V vs. RHE under AM 1.5 G simulated sunlight irradiation. By changing fabrication conditions such as holding time, holding temperature, and flux ratio, the crystal growth manner was also investigated.
Acknowledgements: This research was partially supported by JSPS Grant-in-Aid for Scientific Research (A) 25249089.
Symposium Organizers
Francesca Maria Toma, Lawrence Berkeley National Laboratory
Akihiko Kudo, Tokyo University of Science
Roel Van de Krol, Helmholtz-Zentrum Berlin
Lianzhou Wang, University of Queensland
Symposium Support
ACS Energy Letters | ACS Publications
Bruker—Nano Surfaces Division
California Institute of Technology
Helmholtz-Zentrum Berlin (HZB)
Lawrence Berkeley National Laboratory– Joint Center for Artificial Photosynthesis
Wiley VCH Verlag GmbH &
Co. KGaA
ES7.15: 2D Materials
Session Chairs
Gang Liu
Kazuhiro Takanabe
Friday AM, April 21, 2017
PCC North, 200 Level, Room 222 B
10:00 AM - *ES7.15.01
Direct Observation of Single-Atom Photocatalytic Reaction Centers for Hydrogen Production Using Two-Dimensional Oxide Nanosheet
Shintaro Ida 1
1 , Kyushu University, Fukuoka Japan
Show AbstractCo-catalysts play an important role in photocatalytic or photoelectrochemical water splitting for efficient decomposition of water into hydrogen, because photocatalysts without a co-catalyst generally exhibit poor activity. The co-catalyst is generally deposited as nanoparticles on the electrode surface or catalyst surface, and is believed to provide water reduction sites. However, the minimum size of a co-catalyst that can function as a reaction site and the detailed local environment of the catalytic centers of the photocatalytic reaction are not yet fully understood. Here, we show that single-atom sites in an Rh-doped two-dimensional titanium oxide crystal act as a co-catalyst for hydrogen production, and we describe observations of single-atom photocatalytic reaction centers. These results provide new insights for better understanding the role of the co-catalyst in the photocatalytic hydrogen production.
10:30 AM - ES7.15.03
Two-Dimensional g-C3N4/Ca2Nb2TaO10 for Efficient Visible Light Photocatalytic Hydrogen Evolution
Supphasin Thaweesak 1 , Miaoqiang Lyu 1 , Piangjai Peerakiatkhajohn 1 , Teera Butburee 1 , Bin Luo 1 , Hongjun Chen 1 , Lianzhou Wang 1
1 , Nanomaterials Centre, School of Chemical Engineering and AIBN, The University of Queensland, Brisbane, Queensland, Australia
Show AbstractScalable g-C3N4 nanosheet powder catalyst was prepared by pyrolysis of dicyandiamide and ammonium chloride followed by ultra-sonication and freeze-drying. Nanosheet composite that combines the g-C3N4 nanosheets and Ca2Nb2TaO10 nanosheets with various ratios were developed and applied as photocatalysts for solar hydrogen generation. Systematic studies reveal that the g-C3N4/Ca2Nb2TaO10 nanosheet composite with a mass ratio of 80:20 shows the best performance in photocatalytic H2 evolution under visible light irradiation, which is more than 2.8 times out-performing bare g-C3N4 bulk. The resulting nanosheets possess a high surface area of 34 m2/g, which provide abundance active sites for the photocatalytic activity. More importantly, the g-C3N4/Ca2Nb2TaO10 nanosheet composite shows efficient charge transfer kinetics at its interface, as evidenced by the photoluminescence measurement. The intimate interfacial connections and the synergistic effect between g-C3N4 nanosheet and Ca2Nb2TaO10 nanosheet with cascading electrons are efficiently suppressing charge recombination and improving the photocatalytic H2 evolution.
10:45 AM - ES7.15.04
Efficient Photoelectrochemical Hydrogen Production Using Wafer-Scale, Defect-Engineered Transition Metal Disulfide Thin Film Catalysts
Ki Chang Kwon 1 , Seokhoon Choi 1 , Ho Won Jang 1
1 , Seoul National University, Seoul Korea (the Republic of)
Show AbstractHydrogen appears as a next-generation clean energy source to replace fossil fuels. One of the most promising ways to produce hydrogen is photoelectrochemical (PEC) water splitting in which sunlight-illuminated photoelectrodes generate electrical current to transform water into useful hydrogen. However, the existing photoelectrodes such as Si with noble catalysts still suffer from low efficiency and poor stability. Moreover, the extremely high cost of the noble metal catalysts limits the wide use of water splitting photoelectrodes. Recently 2-dimensional transition metal disulfides (2D TMDs) have been attracted much attention as promising candidates to replace Pt, because 2D TMDs such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) have inherently large surface-to-volume ratios and possess high densities of catalytically active sites for hydrogen evolution reaction (HER).
We have investigated PEC activities of TMD thin films with sulfur vacancies or phosphorus dopants, and various alloyed TMD thin films which contain two different transition metals. We focused on characterizing micro- and nano-structures of the defect-engineered TMD thin films by using atomic resolution transmission electron microscopy (TEM). From the measurements, the enhanced PEC catalytic activities of defect-engineered TMD thin films compared to pristine TMD thin films are found and the crucial role of defect-like sulfur vacancies, phosphorus dopants, and substitutional or bond-breaking foreign transition metal dopants are unraveled for highly efficient TMD-based HER catalysts. Our results clearly show that the defect-engineered TMD thin film catalysts not only reduce the overpotential at electrolyte/solid interfaces but also stabilize the surface of solids for efficient water splitting using p-type semiconductor photocathodes such as Si and InP, leading to the state-of-the-art water splitting performance for novel-metal-free Si-based photocathodes. We believe that our approach provides a well-organized platform for developing efficient PEC water splitting system using earth-abundant and low-cost catalysts.
ES7.16: Chalcogenide Photoelectrodes
Session Chairs
Seong-Ju Hwang
Shintaro Ida
Friday PM, April 21, 2017
PCC North, 200 Level, Room 222 B
11:30 AM - *ES7.16.01
Surface-Modified Chalcogenide Thin Films as Efficient Photocathodes for Water Reduction
Shigeru Ikeda 1
1 , Konan University, Kobe Japan
Show AbstractSolar water splitting for production of hydrogen (H2) energy has attracted much attention as a feature environmentally friendly and cost-effective energy system. Utilization of inorganic semiconductor thin films in photoelectrochemical (PEC) water splitting is an ideal approach to yield pure H2 energy. Various semiconductors applied for solar cells have been studied for their applications in PEC H2 production because of their high absorption efficiency of sunlight.
The Cu2ZnSnS4 (CZTS) compound is a promising absorber material for the next-generation photovoltaic thin-film module. Due to its p-type semiconductive characteristic and optimum band gap for sunlight absorption (1.5 eV), the compound should also be promising as a photocathode for PEC water splitting.
One of the critical problems of PEC water splitting using a semiconductor photocathode is degradation and/or corrosion of the photocathode used, i.e., appreciable reduction of the photocurrent (H2 production) was usually observed during photoirradiation. Regarding CZTS-based PEC water splitting, the stability issue has been discussed in only one report, but the electrode used in that study had a poor photocurrent.
Recently, we found that the chemically deposited In2S3 buffer efficiently work as a good buffer layer for a CuInS2-based photocathode. Moreover, an In2S3/CdS hybrid buffer was applied for a Cu2ZnSnSe4-based photovoltaic (PV) system: it gave comparable PV performances compared to the conventional CdS buffer. These previous results and literature works motivated us to apply the In2S3 compound for the photocathode system. In this study, we employed a novel In2S3/CdS double layer for coverage of the CZTS film. By adding conventional Pt catalyst particles on the In2S3/CdS/CZTS hybrid film, the thus-obtained photocathode showed a significant enhancement of stability during photoirradiation for PEC water splitting. Application of the efficient photocathode for bias-free water splitting by using a BiVO4 counter photocathode was also demonstrated.
12:00 PM - ES7.16.02
Photoelectrochemical and Solid-State Properties of Wide Bandgap Copper Chalcopyrites for Renewable Hydrogen Generation
Nicolas Gaillard 1 , Kim Horsley 1 , Alex DeAngelis 1
1 , University of Hawaii, Honolulu, Hawaii, United States
Show AbstractPhotoelectrochemistry (PEC) is one of the most efficient methods to produce alternative fuels, although lab-scale systems efficiency, cost and durability are currently not at the level required to make this technology economically feasible. The chalcogenide material class, typically identified by its most popular alloy Cu(InGa)Se2, provides exceptionally good candidates to meet the requirements identified for cheap, sustainable solar fuels production. As we have previously reported, 1.7 eV CuGaSe2 offers high-saturated photocurrent densities (>15 mA/cm2) and high Faradaic efficiency (>85%). However, CuGaSe2 photocathodes suffer from non-ideal surface energetics, requiring additional photovoltaic (PV) cells to drive the water splitting process. The hybrid photo-electrode device, in which a PEC electrode is placed on top of a narrow bandgap solar cell, is by far the most efficient approach for unbiased solar water splitting. Unfortunately, CuGaSe2’s narrow bandgap makes this stacked integration with existing PV cells challenging.
In the present communication, we report on our efforts to synthesize 1.8-2.2 eV bandgap chalcopyrite materials for PEC water splitting. We first report on a method to convert narrow bandgap selenide-based chalcopyrites (e.g. CuInGaSe2) into wide bandgap absorbers. Using co-evaporated 1 μm-thick CuGaSe2 as a baseline system, we demonstrate that the substitution of selenium with sulfur can be accomplished through a simple annealing step. As a result, a dramatic change in optical properties was observed, with a bandgap increase from 1.6 eV (CuGaSe2) to 2.4 eV (CuGaS2). Then, by simply adjusting the indium content during the initial growth process, the bandgap of sulfurized copper chalcopyrite can be tuned to achieve bandgap values in the 1.8-2.0 eV range. X-ray diffraction data indicated successful bulk sulfurization by the shift of the prominent (112), (220), and (312) reflections to higher angles.
In a PEC configuration, a saturated photocurrent density of 5 mA/cm2 was achieved with 2.0 eV red CuInGaS2 photocathodes in 0.5M H2SO4 under AM1.5G simulated illumination. Also, Mott-Schottky analyses revealed an anodic shift of the flatband potential with increasing bandgap when compared to CuGaSe2. This suggests that the bandgap modification in sulfurized films primarily stems from a downward shift of the valence band, an ideal situation for p-type PEC systems. Subsequent solid-state measurements of CIGS2-based solar cells were performed to further characterize these new materials and understand their defect chemistry. Both quantum efficiency and current-vs-voltage characterizations revealed recombinations in wide bandgap chalcopyrites, especially those with high gallium content. New synthesis strategies will be presented to produce wide bandgap chalcopyrites compatible with the hybrid photo-electrode approach and capable of generating photocurrent densities over 10 mA/cm2.
12:15 PM - ES7.16.03
Cd-Doped CZTS Photocathode for Enhanced Photoelectrochemical Water Splitting
Ying Fan Tay 1 , Zhenghua Su 1 , Prince Bassi 1 , Wenjie Li 1 , Lydia Wong 1
1 , Nanyang Technological University, Singapore Singapore
Show AbstractCu2ZnSn(S,Se)4 (CZTSSe) has been widely studied as earth abundant photovoltaic material since the discovery of its photovoltaic effect in 1966 and the record efficiency is 12.7%. Its promising characteristics of a p-type nature, suitable band gap (~1.5eV) and efficient absorption coefficient make it also an attractive photocathode for photoelectrochemical water splitting. However, despite its promising characteristics, progress on CZTS as a photocathode is limited ever since its first application in 2010 where a photocurrent of 9mAcm-2 at 0VRHE was produced. Since then, the photocurrent has only improved slightly to 9.3mAcm-2 in 2015 by incorporating a dual buffer layer of CdS and In2S3 with CZTS. This photocurrent observed is still much lower than that observed in photovoltaic cells (~18mAcm-2), which highlights the importance of appropriate deposited overlayers to ensure efficent charge transfer from the photocathode to the electrolyte. However, due to the lack of systematic studies on different overlayers, the charge transfer kinetics and the requirements of overlayers are still not very well established. In our study, we have fabricated a solution processed Cu2Zn1−xCdxSnS4 photocathode for the first time and have conducted a systematic study of different overlayers and their impact on the charge transfer kinetics, stability, band alignment and onset potential of the photoelectrode through the use of Impedance Spectroscopy, X-ray photoelectron and Ultraviolet photoelectron Spectroscopy (XPS and UPS). We have also demonstrated that through Cd doping and depositing appropriate overlayers, we are able to achieve an enhanced photoelectrochemical performance for our photoelectrode.
12:30 PM - ES7.16.04
Feasible and Non-Expensive Photocathodes Based on Kesterites for Water Splitting
Carles Ros 1 , Sergio Giraldo 1 , Zhishan Luo 1 , Edgardo Saucedo 1 , Teresa Andreu 1 , Andreu Cabot 1 , Juan Morante 1
1 , IREC, Catalonia Institute for Energy Research, Sant Adrià del Besòs Spain
Show AbstractAn important approach towards an efficient and sustainable economy is storing solar energy into chemical fuels through photoelectrochemical (PEC) water splitting. Solar hydrogen, as a clean source instead of hydrocarbon steam reforming, can be used in CO2 hydrogenation for its conversion, among others, to methanol or synthetic natural gas as vector for chemical energy storage and contributing to the circular economy. Cheap and earth abundant materials, optimal band gap and electrolyte adaptability is mandatory for large scale industrialization and deployment of PEC technology.
Mathematical calculations have shown that dual absorber tandem photocathode/photoanode configurations, coupling a medium band gap photoelectrode (around 1.9 eV) with a small one (1.0 eV) could overcome the required efficiencies for large scale industrialization[1]. Chalcopyrite CI(G)S, and its earth abundant similar, kesterite CZTS/Se, offer a thin film low cost alternative to traditional silicon. Chalchogenide solar cells bandgap can be tuned from 1.0 to 1.5 eV with stoichiometric modification[2], with state of the art efficiencies ranging over 20 %. This versatility makes them very interesting to be implemented in PEC water splitting[3]. In this work, we present a high throughput CZTS/Se based photocathode for water splitting with MoS2 catalyst, protected from degradation by TiO2 for wide range of pH.
We demonstrate that titanium dioxide grown by ALD can be used as a transparent protective and conductive layer and MoS2 nanostructures as HER catalysts to obtain a kesterite CZTS/Se fully based on earth abundant materials photocathode with tunable band gap. Solar-to-Hydrogen half-cell stable efficiencies over 7% have been obtained with earth-abundant CZTS/Se, with room for photopotential improvement by deeply understanding band gap tunning. These photocathodes have been tested in electrolytes with pH ranging from 0.3 to 14, presenting no degradation over one hour stability in the harshest environments and high current throughput over 30 mA/cm2 both decorated with Pt and MoS2.
[1] S. Hu, C. Xiang, S. Haussener, A. D. Berger, and N. S. Lewis, “An analysis of the optimal band gaps of light absorbers in integrated tandem photoelectrochemical water-splitting systems,” Energy Environ. Sci., vol. 6, no. 10, p. 2984, Sep. 2013.
[2] S. Giraldo, M. Neuschitzer, T. Thersleff, S. López-Marino, Y. Sánchez, H. Xie, M. Colina, M. Placidi, P. Pistor, V. Izquierdo-Roca, K. Leifer, A. Pérez-Rodríguez, and E. Saucedo, “Large Efficiency Improvement in Cu2ZnSnSe4 Solar Cells by Introducing a Superficial Ge Nanolayer,” Adv. Energy Mater., vol. 5, no. 21, pp. 1–6, 2015.
[3] C. Ros, T. Andreu, S. Giraldo, Y. Sánchez, and J. R. Morante, “Conformal chalcopyrite based photocathode for solar refinery applications,” Sol. Energy Mater. Sol. Cells, pp. 1–5, 2016.
12:45 PM - ES7.16.05
Growth of P-Type CdSxSe1-x:Cu Absorber Layer for Overall Water Splitting in PEC Cells
Zi Ye 1 , Xudong Xiao 1
1 , The Chinese University of Hong Kong, Shatin Hong Kong
Show AbstractUnassisted overall water splitting via photoelectrochemical (PEC) light harvesting system as a promising approach has been developed for hydrogen fuel production. However, very few p-type semiconductor photocathodes can simultaneously satisfy all the harsh requirements for overall water splitting in PEC cells: visible light response, appropriate conduction/valence band edges and high stability. In this study, a series of p-type CdSxSe1-x:Cu light absorber which has a tunable bandgap for visible light response, suitable band edge positions for water splitting in basic solution was designed. The variable bandgap was achieved by controlling S/Se ratio in CdSxSe1-x:Cu absorber layer via a controllable co-evaporation process. By doping Cu, p-type conductivity was achieved. Then, the photo-corrosion which hindered the stability of traditional II-VI group n-type photoanode was minimized via a cathodic protection. A surface modification such as Pt nanoparticles was further applied to enhance both the photocurrent and stability. Ultimately, a stable p-type CdSxSe1-x:Cu absorber layer with visible light response in basic solution was prepared for overall water splitting in PEC Cells.
ES7.17: Particle-Based Systems
Session Chairs
Francesca Maria Toma
Roel Van de Krol
Friday PM, April 21, 2017
PCC North, 200 Level, Room 222 B
2:30 PM - *ES7.17.01
Quantitative Identification of Limitations in Semiconductor Particulate System for Photocatalytic Water Splitting
Kazuhiro Takanabe 1
1 , King Abdullah University of Science and Technology (KAUST), Thuwal Saudi Arabia
Show AbstractPowder photocatalytic system has potential for large scale application of solar fuel production. Understanding the photocatalytic reaction processes is challenging because of its complexity associated with various incidents happening at different spatial- and time-scale. In this study, crucial properties were first identified for designing an effective particulate system for water splitting. A simplified photocatalyst model was then used to solve simulations of semiconductor-metal devices. The electrocatalysts on the semiconductors surface are integral components providing anisotropic electric fields for charge separation in the semiconductor (and to enhance the kinetics of the redox reactions which were omitted from these simulations). In our simple model, the metal catalysts collect excited electrons to drive the hydrogen evolution reaction (ohmic junction); whereas the n-type semiconductor surface was considered as active for water oxidation under illumination and steady state water-splitting conditions (i.e. a Schottky type of contact was considered for the semiconductor-liquid junction interface). The established model enabled us to calculate the sensitivity of the quantum efficiency (QE) of the system to the various semiconductor properties; such as light absorption, carrier density, mobility and lifetime of the carriers, and to the dispersion of the metal particles, which create heterojunctions, considered as the driving force for charge separation. As a result, a pinch-off effect was prevalent underneath the hydrogen evolution site, suggesting an undesired energetic barrier for electron transport and transfer to the electrocatalyst. Using values reported in the literature, the simulation revealed that the QE was mostly governed by recombination of carriers in the bulk of the semiconductor particles. Presentation will also discuss the potential future target.
3:00 PM - *ES7.17.02
Solar-Driven Photocatalysts with Wide Spectrum Absorption and High Charge Separation Ability
Gang Liu 1
1 Shenyang National Laboratory for Materials Science, Institute of Metal Research, CAS, Shenyang China
Show Abstract
Converting solar energy into storable chemical energy with photocatalysts represents a promising way of utilizing solar energy. Photocatalysis can realize the splitting of water to release hydrogen and reduction of CO2 to produce solar fuels or chemicals including methane/methanol/CO. The search for photocatalysts with wide spectrum absorption and high charge separation ability is one central task in this area. Most stable photocatalysts usually suffer from large bandgap-controlled narrow light absorption range and also low charge separation ability. Developing the strategies to widen the absorption range and promote charge separation has been the active topics. In our study, we aim to realize the band-to-band redshift of the absorption edge by controlling the spatial distribution of electronic structure modifiers. These modifiers include dopant, vacancies and disordering. A series of red TiO2 and carbon nitride photocatalysts with wide spectrum absorption were developed by homogeneous modification with B/N dopants, anion vacancies and disordering. These photocatalysts give a promising activity in producing hydrogen from water splitting under visible light. Regarding the separation of photogenerated charge carriers, two studies will be introduced. 1) The electrons in metal oxides/sulfides/nitrides usually have a smaller effective mass than holes so that the population of electrons reaching photocatalyst surface is always larger than that of holes. While photocatalysis is dominantly controlled by holes involved half reaction. To address this intrinsic restriction, a strategy for reversing the population of electrons and holes reaching surface by modulating the band alignments of core and shell regions of single TiO2 particle was developed. 2) Control of spatial distribution of co-catalyst mediated by ferroelectric field in perovskite PbTiO3 was realized to promote the charge separation and thus improve photocatalytic activity.
3:30 PM - ES7.17.03
Modelling Photocatalytic, Particle-Based Water Splitting
Marc Schiffler 1 , Sophia Haussener 1
1 , EPFL, Lausanne Switzerland
Show AbstractPhotocatalytic reactions based on two sets of suspended semiconductor particles and a redox shuttle mediator are considered a safe and economic solution for solar water or CO2 splitting. However until now no reasonably-sized device demonstration has been made. The technological potential of this approach is unclear and it is not known how the choice of the components (semiconducting particles, electrolyte, mediator, etc.), the design of the system, and the operating conditions affect the performance of the system.
Here we present a transient zero-dimensional model which we developed in order to assess the feasibility and limits of the water splitting process and to provide basic guidance on the design choices. The transient model accounts for ideal solar absorption, ideal charge generation and transport, and the kinetic characteristics at the catalytic sites for two sets of catalyst-covered semiconductor particles. We solved for species transport (mediator species and protons) and conservation in the two reactor compartments, separated by a semipermeable Membrane.
The model allows for the evaluation of different photoabsorber and mediator materials, and for different concentrations and provides indication which combinations are favorable for two-step solar hydrogen production. The quasi tandem arrangement, where the two compartments are positioned on top of each other has shown improved performance compared to the side-by-side arrangement. The highest solar to hydrogen efficiencies could be achieved with a redox shuttle at a standard potential of 1 V vs. RHE in the quasi tandem arrangement.
The developed model provides a first basis for theoretical design guidelines, allows for the prediction of theoretical maximum efficiencies, and provides feasibility assessments of design and component combinations (particles and mediator) of particle-based photocatalytic water splitting devices.
ES7.18: Novel Oxide Absorbers
Session Chairs
Akihiko Kudo
Roel Van de Krol
Friday PM, April 21, 2017
PCC North, 200 Level, Room 222 B
4:15 PM - ES7.18.01
Discovery of Solar Fuels Photoanode Materials by Integrating High-Throughput Theory and Experiment
Santosh Suram 1 , Qimin Yan 2 3 , Jie Yu 1 4 6 , Lan Zhou 1 , Aniketa Shinde 1 , Paul Newhouse 1 , Wei Chen 4 , Guo Li 2 3 6 , Kristin Persson 4 5 , Jeffrey Neaton 2 3 7 , John Gregoire 1
1 Joint Center for Artificial Photosynthesis, California Inst of Technology, Pasadena, California, United States, 2 Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Department of Physics, University of California, Berkeley, California, United States, 4 Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 6 Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 5 Department of Materials Science and Engineering, University of California, Berkeley, California, United States, 7 , Kavli Energy NanoSciences Institute at Berkeley, Berkeley, California, United States
Show AbstractDiscovery of photoanode materials remains a primary challenge in the development of solar fuels generators. Since the initial demonstration of TiO2 as an overall water splitting photoelectrocatalyst, several metal oxide systems have been investigated especially for band gap energy in the desirable 1.2 to 2.8 eV range that strongly overlaps with the solar spectrum. However, forty years of experimental research has yielded just 12 metal-oxide photoanode compounds with the desirable band gap. Predictions of prior high-throughput theory approaches have yet to expand this list, partly due to limitations in prediction of electronic structure descriptors; establishing the need for tiered screening approaches that enable rapid yet reliable predictions. Additionally, experimental discoveries of semiconductor materials based on theory suggestions have shown that the composition with optimal performance is typically slightly off-stoichiometric from the predicted composition; necessitating the need for combinatorial high-throughput experimentation based follow up of theory predictions. We incorporate crystal structure characterization, high-throughput optical analysis and high-throughput photo-electrochemical measurements on continuous composition spread material libraries into a tiered screening pipeline that receives material suggestions from a tiered high-throughput theory pipeline. Wherein, the theory suggestions are a result of screening candidate materials for thermodynamic stability, band gap and valence band energy using appropriate theory for each screening criterion. Using this integrated tiered high-throughput theory and experiment pipeline we translate the hypothesis of the role of structural motifs in establishing an efficient photoanode into discovery of 12 new photoanodes, doubling the number of known metal-oxide photoanodes with desirable band gap.
4:30 PM - ES7.18.03
Fabrication of Fe2TiO5 Epitaxial Thin Films for Photoelectrochemical Applications
Motoki Osada 1 , Kazunori Nishio 2 3 , Yasuyuki Hikita 3 , Harold Hwang 2 3
1 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Geballe Laboratory for Advanced Materials, Department of Applied Physics, Stanford University, Stanford, California, United States, 3 Stanford Institute for Energy & Materials Sciences, SLAC National Accelerator Laboratory, Menlo Park, California, United States
Show AbstractDevelopment of good light absorbing semiconductors with high chemical stability in electrolytes is critical to achieving high efficiency solar-water splitting devices. Complex oxide semiconductors are potentially promising materials due to the high flexibility to manipulate their physical properties [1]. However, in contrast to the current best performing photoelectrodes using single crystalline epitaxial layers of conventional semiconductors [2], the study of complex oxides for photoelectrochemistry has been focused primarily on polycrystalline samples or nanostructured forms where control of crystal orientation, thickness, and defect types and densities are fundamentally limited. In order to assess the intrinsic potential of complex oxide photoelectrodes, study of the physical and electrochemical properties of oxide semiconductors in epitaxial thin film form is an effective approach, as demonstrated recently [3, 4, 5].
Here we present an epitaxial thin film growth study of Fe2TiO5 (Pseudobrookite), a prospective photoanode consisting of abundant elements Ti and Fe, with an ideal bandgap of ~2.18 eV, and high chemical stability [6]. Compared to the well-studied hematite (α-Fe2O3), incorporation of Ti is expected to add higher controllability over the conductivity of this semiconductor due to the accessibility to multiple valence states by both Fe and Ti, mitigating the very high electrical resistivity of 106Ωcm of α-Fe2O3 [7]. We used pulsed laser deposition to fabricate epitaxial Fe2TiO5 thin films with different orientations on single crystal substrates under controlled temperatures and partial oxygen pressures to construct a growth phase map. By carefully tuning the conditions, we successfully stabilized single-phase epitaxial thin films of Fe2TiO5 in (100) and (230) orientations as evidenced by X-ray diffraction and Raman spectroscopy. The room temperature resistivities of these films were on the order of 102Ωcm, much reduced from that of α-Fe2O3. These results provide new insight into Fe2TiO5 as a photoanode which we will discuss.
[1] A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253.
[2] O. Khaselev and J. A. Turner, Science, 1998, 280, 425.
[3] S. Kawasaki et al., J. Phys. Chem. C, 2014, 118, 20222.
[4] H. Mashiko et al., J. Phys. Chem. C, 2016, 120, 2747.
[5] L. C. Seitz et al., Science, 2016, 353, 1011.
[6] D. S. Ginley and M. A. Butler, J. Appl. Phys., 1977, 48, 2019.
[7] J. C. Launay and G. Horowitz, J. Cryst. Growth, 1982, 57, 118.
4:45 PM - ES7.18.04
Solar Water Oxidation on a Tri-Layered Bi2MoO6 / MoO3 Heterojunction Photoanode Prepared by an Anodization-Hydrothermal Intercalation Route
Shi Nee Lou 1 , Jason Scott 1 , Rose Amal 1 , Yun Hau Ng 1
1 , University of New South Wales, Sydney, New South Wales, Australia
Show AbstractTernary oxide, bismuth molybdate (Bi2MoO6), with an optical band gap of 2.7 eV, is an emerging semiconductor in the field of photocatalysis and photo-electrochemical (PEC) water splitting. Most synthesis methods yield Bi2MoO6 as powders where transforming these powders into thin films (electrodes) for PEC applications remain a significant challenge owing to the generally weaker interaction between the powder and electrode substrate. Anodization is an effective method to grow nanostructured metal oxide thin films with a high surface-area-to-volume ratio on their starting metal foils. However anodization typically produces only simple oxides. One approach to produce more complex metal oxides is via hydrothermal treatment where, for instance, Bi2MoO6 particles can be synthesised from bismuth nitrate and molybdate precursors. This motivated us to investigate the direct synthesis of a ternary oxide Bi2MoO6 thin film by anodizing Mo foil followed by hydrothermal processing in a bismuth salt solution. Herein, we demonstrate the feasibility of converting a simple as-anodized MoO3 thin film into a ternary Bi2MoO6 electrode by intercalating Bi3+ cations into the interlayer gaps of MoO3 using a hydrothermal process1. The unique approach removes the need for preformed particles to prepare the film, in turn providing a robust and stable anode for PEC water splitting.
Bi2MoO6 / MoO3 thin films produced using this technique possess a tri-layered structure comprising a thin layer of pure-phase MoO3 sandwiched between an upper layer of Bi2MoO6 and the original Mo metal foil. The ensuing Bi2MoO6 / MoO3 ternary electrode exhibits an enhanced PEC performance, providing a 79% enhancement in anodic photocurrent density compared to the unmodified MoO3 thin film under a positive bias of 0.4 V vs. Ag/AgCl. The better performance was attributed to: (i) the narrow optical band gap of Bi2MoO6, which extended the absorption of light by the film into the visible range and; (ii) the well-aligned band structure of MoO3 and Bi2MoO6. The Bi2MoO6 / MoO3 thin film electrode was subsequently utilised as a photoanode for PEC water splitting. The Bi2MoO6 / MoO3 thin film electrode provided high Faradaic photocurrent-to-O2 conversion efficiency (~93%) for PEC water splitting under UV illumination and, importantly, exhibited excellent photostability as a consequence of the unique synthesis method.
Reference:
1. Lou, S. N.; Scott, J.; Iwase, A.; Amal, R.; Ng, Y. H., Photoelectrochemical water oxidation using a Bi2MoO6/MoO3 heterojunction photoanode synthesised by hydrothermal treatment of an anodised MoO3 thin film. Journal of Materials Chemistry A 2016, 4 (18), 6964-6971.
5:00 PM - ES7.18.05
A Novel Perovskite Oxides for Visible Light Photocatalytic Hydrogen Production
Zaicheng Sun 1
1 , Beijing University of Technology, Beijing China
Show AbstractPerovskite oxides(ABO3) are one of the most important families of materials exhibiting properties suitable for numerous technological applications. For example, SrTiO3 and others have shown excellent photocatalytic properties in UV region. The A site is occupied by the larger cation, while the B site is occupied by the smaller cation in ABO3 crystal structure. Generally, B site cations are strongly bonded with the oxygen, while A site cations have relatively weaker interactions with oxygen. Perovskites may exhibit different electronic and optical properties depending on the ionic radii and electronegativity of the B site cations. According to the band gap-engineering strategy explored in a previous theoretical studies, it has been demonstrated that BO6 tilting distortions from the ideal high-symmetry cubic ABO3 structure have a strong impact on the band gaps of perovskites.Herein, we firstly synthesized a new type of perovskite Ba2FeNbO6 (BFNO) with mixed Fe and Nb on the B site, which exhibit strong visible light absorption up to 550 nm (Eg=2.29 eV). It indicates that perovskite oxides with narrow band gap could be obtained via mixed metal elements on the B site. This further confirm the Rappe’s theoretical prediction that the BO6 octahedral distortions induced by the B-cation off-center displacements have a strong effect on the band gap and propose a strategy to reduce the band gaps of perovskites. However, it exhibits low photocatalytic performance due to poor charge separation efficiency. In order to promote the photocatalytic performance, the solid solutions of SrTiO3-Ba2FeNbO6 (ST-BFNO) nanocrystals were prepared via molten salt route. ST-BFNO exhibits stable photocatalytic H2 production capability.