Samuel Mao, University of California, Berkeley
Lionel Vayssieres, Lawrence Berkeley National Laboratory
Heli Wang, National Renewable Energy Laboratory
Dunwei Wang, Boston College
Monday PM, April 06, 2015
Moscone West, Level 3, Room 3011
2:30 AM - *J2.01
Solution-Processed Photocathodes for Solar Water Splitting Tandem Cells
Kevin Sivula 1
1Eacute;cole Polytechnique Feacute;deacute;rale de Lausanne (EPFL) Lausanne SwitzerlandShow Abstract
A practical solar water splitting device that can produce H2 at a cost less than PV + electrolysis is difficult to envision without it possessing a simple construction that employs widely available materials and inexpensive processing techniques. A tandem cell consisting of 2 solution-processed photoelectrodes (photoanode/photcathode) can reasonably reach this goal. While n-type oxide semconductors have been established as viable photoanode materials, cheap and stable p-type photocathodes are less developed. In this presentation our gorups&’ progress in the development of solution-processed stable photocathodes is presented and their application toward overall photoelectrochemical water splitting tandem cells is demonstrated. Three classes of promising materials are discussed: copper-based oxides in the delafossite phase, ternary and quaternary chalcogenides, and 2D transition metal chalcogenides. In each case the challenges to maximize photogenerated charge collection in thin films prepared by solution-based deposition approaches are highlighted and the use of bottom-up nanostructuring to gain insight into routes for improvement and overcome these challenges are established. The optimization of these materials for solar water reduction using overlayers and catalysts, and the stability under reasonable operation conditions are addressed. Routes to improve the unassisted and overall solar water splitting performance in tandem cells with commonly used oxide photoanodes are finally outlined.
3:00 AM - *J2.02
High-Efficiency Tandem Absorbers for Economical Solar Hydrogen Production
Todd Deutsch 1 James Luke Young 2 1 Henning Doescher 1 3 Heli Wang 1 John A. Turner 1
1National Renewable Energy Laboratory Golden United States2University of Colorado Boulder United States3Technische Universitauml;t Ilmenau Ilmenau GermanyShow Abstract
This talk will present an overview of our research strategy to develop a semiconductor-based device capable of over 20% solar-to-hydrogen efficiency with several thousand hours of stability under operating conditions. Our goal is to improve solar-to-hydrogen (STH) efficiency from 12.4% to over 20% by developing novel tandem semiconductor materials and configurations. Our approach focuses on classes of materials that have either demonstrated exceptionally high efficiency or theoretically can produce highly efficient materials. The four tandem material sets in our research portfolio are III-Vs on GaAs, InGaN on Si, III-V-N on Si, and dual photoelectrode pairings. We are evaluating catalytic nitride, oxide, and sulfide-based semiconductor surface modifications to extend durability from a few hundred hours to nearly 1000 hours, with a stretch target of 3500 hours. Cost reductions in the synthesis of high-efficiency III-V photoelectrochemical devices could be realized from emerging epitaxial synthesis technologies such as spalling, epitaxial lift-off, or close-space vapor transport to yield economical hydrogen production. Our group addresses synthesis cost, in a limited capacity, by investigating novel material configurations.
Ultimately, we plan to demonstrate a prototype photoreactor that produces 3 L of standard hydrogen within an 8-hour period under moderate solar concentration (10x), which can be accomplished using only 6 cm2 of a 20% STH material.
3:30 AM - J2.03
Photoelectrochemical Conversion on Prospective 2-D Layered Chalcogenide Photoelectrodes for Solar Water-Splitting: A Spatially Resolved Study of the Role of the Surface Motifs
Jimmy John 1 Jesus Velazquez 1 Daniel Esposito 3 2 Adam Pieterick 1 Ragip Pala 1 Rebecca Saive 1 Shane Adam Ardo 4 Bruce Brunschwig 1 Nathan S. Lewis 1
1California Institute of Technology Pasadena United States2National Institute of Standards and Technology Gaithersburg United States3Columbia University New York United States4University of California - Irvine Irvine United StatesShow Abstract
Layered transition metal dichalcogenides of the general formula, MX2 (where M= Mo, W and X=S, Se), are of significant interest as photoelectrode materials in solar water-splitting due to their favorable bandgap, high absorption coefficients and good photoelectrochemical stability. However, the surfaces of these materials are highly heterogeneous exhibiting macroscopic terraces and step edges; along with other micro/nano-scale defects. It has been suggested previously that the collection of photogenerated carriers at the semiconductor/electrolyte interface occurs efficiently on smooth flat terraces; whereas the step edges and other defects act as carrier recombination sites. The efficiency of the overall photoconversion process in such materials, hence, would be crucially determined by the interfacial charge transfer occurring at these surface motifs and their surface densities. Thus, understanding the charge transfer process at the microscopic level on the surface of these materials is foundational to their application in solar water-splitting.
To this end, we have carried out spatially resolved local photo-current measurements on layered chalcogenide photoelectrodes using the technique of laser beam induced current microscopy. Importantly, the studies showed that, more than the step edges, the presence of terraces with low photoactivities primarily accounted for the efficiency losses in these materials. In an effort to expose the underlying reasons for the existence of low-performing terraces, local topographic and spectral response measurements were subsequently carried out. It was generally established that terraces, at the micro/nano-scale, are not uniform but textured and the texturing differs from one terrace to another. The results also showed that the local electronic structures of terraces differ from one another. Furthermore, compared to high photoactivity terraces, the low-performing terraces clearly indicated the presence of sub-bandgap surface states that could be acting as carrier traps. In summary, our study, employing spatially resolved techniques, has yielded significant phenomenological insights into the photoconversion process on the class of layered chalcogenide photoelectrode materials for solar water-splitting.
3:45 AM - J2.04
Development of Wide Bandgap Copper Chalcopyrite Thin Film Materials for Photoelectrochemical Hydrogen Production
Nicolas Gaillard 1 Alexander Deangelis 1 Marina Chong 1 Aiping Zeng 1
1University of Hawaii Honolulu United StatesShow Abstract
Photoelectrochemistry (PEC) is one of the most efficient methods to produce alternative fuels, although the efficiency, cost, and durability of lab-scale systems are currently not at the level required to make this technology economically feasible. The chalcopyrite material class, typically identified by its most popular alloy Cu(In,Ga)Se2, provides exceptionally good candidates to meet the requirements identified for cheap, sustainable solar fuels production. As we recently reported[i], co-evaporated 1.7 eV CuGaSe2 offers very high-saturated photocurrent densities (15 mA.cm-shy;2 in 0.5M H2SO4 under AM1.5G illumination), long durability (up to 400 hours), and high Faradaic efficiency (>85% for non-catalyzed systems). Although CuGaSe2 has the highest bandgap of the copper chalcopyrite class, its optical characteristics are still too close to that of amorphous silicon (a-Si), a low-cost material our research team has identified as an ideal photovoltaic driver in a monolithic hybrid photoelectrode device. Nevertheless, a solar-to-hydrogen efficiency of 3.7% was achieved using a co-planar integration scheme. In order to improve the water-splitting efficiency further, novel chalcopyrite alloys with bandgap greater than 1.7 eV must be developed.
In the present communication, we report on our effort to synthesize wide bandgap (1.8 eVG<2.2 eV) band-gap chalcopyrite materials for un-biased PEC water splitting using PEC/PV hybrid devices. Specifically, we investigate the impact of sulfur on the optical and photoelectrochemical characteristics of the copper chalcopyrite material class. Using co-evaporated 1 mu;m-thick CuGaSe2 as baseline system, we demonstrate that selenium can be substituted by sulfur using a simple annealing step. With this protocol, a dramatic change in optical properties was observed, with a bandgap increase from 1.7 eV (CuGaSe2) to 2.4 eV (CuGaS2), in good agreement with theoretical predictions[ii]. Then, by simply adjusting the indium content in the film during the initial growth process, red 2.0 eV CuIn0.3Ga0.7S2 was obtained. Mott-Schottky analysis indicated a 200 mV anodic shift of the flatband potential with increasing bandgap (300 meV), suggesting 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. Linear sweep voltammetry performed in 0.5M H2SO4 under AM1.5G simulated illumination revealed the excellent photo-conversion properties of 2.0 eV CuInGaS2, with photocurrent densities of 3.5 and 6.0 mA/cm2 at 0 VRHE and -0.4 VRHE, respectively, with negligible dark current from flatband potential (+0.5 VRHE) to photocurrent saturation (-0.5 VRHE). Preliminary results obtained on PV/CuInGaS2 hybrid structures will be also presented.
[i] N. Gaillard, D. Prasher, J. Kaneshiro, S. Mallory, and M. Chong, MRS Spring Meeting, Z2.07 (2013).
[ii] M. Bär, W. Bohne, J. Rohrich, E. Strub et al., Appl. Phys. Lett. 96, 3857 (2004).
4:15 AM - *J2.05
Single Junction Perovskite-BiVO4 Tandem Assembly for Solar Hydrogen Production
Prashant Kamat 1 2 3 Yong-Siou Chen 1 2 Joseph Manser 1 3
1University of Notre Dame Notre Dame United States2University of Notre Dame Notre Dame United States3University of Notre Dame Notre Dame United StatesShow Abstract
With the emergence of highly efficient perovskite materials, there is a need to understand the excited state behavior and charge separation events in these new materials[1,2]. The scientific issue related to the utilization of organic metal halide perovskite materials in solar fuels generation remains a scientific challenge because its susceptibility to degradation. The interaction of CH3NH3PbI3 with water vapor (90% humidity exposure) has allowed us to identify the transformations leading to the deterioration of the perovskite solar cells. The reaction with H2O at the surface of the perovskite film forms shallow traps in the band structure so that the portion of the CH3NH3PbI3 crystal which is pristine remains largely unaffected . In order to circumvent direct exposure of the perovskite film to water electrolysis, we have employed a tandem water splitting assembly composed of a BiVO4 photoanode and a single-junction CH3NH3PbI3 hybrid perovskite solar cell . This unique configuration allows efficient solar photon management, with the metal oxide photoanode selectively harvesting high energy visible photons and the underlying perovskite solar cell capturing lower energy visible-near IR wavelengths in a single-pass excitation. The sustained photovoltage and photoconversion efficiency of the single-junction CH3NH3PbI3 PV even under low energy excitation enables exceptional performance in tandem light-harvesting assemblies.
Manser, J. S.; Kamat, P. V., Band Filling with Charge Carriers in Organometal Halide Perovskites. Nature Photonics 2014, 8, 737-743.
Stamplecoskie, K. G.; Manser, J. S.; Kamat, P. V. Dual Nature of the Excited State in Organic-Inorganic Lead Halide Perovskites Energy Environ. Sci. 2015, 8, 208 - 215
Christians, J. A.; Miranda Herrera, P. A.; Kamat, P. V., Transformation of the Excited State and Photovoltaic Efficiency of CH3NH3PbI3 Perovskite upon Controlled Exposure to Humidified Air. J. Am. Chem. Soc. 2015, DOI: 10.1021/ja511132a
Chen, Y.-S.; Manser, J. S.; Kamat, P. V., All Solution-Processed Lead Halide Perovskite-BiVO4 Tandem Assembly for Photolytic Solar Fuels Production. J. Am. Chem. Soc., 2015, 137, 974-981.
4:45 AM - J2.06
Double-Deck Inverse Opal Photoanodes: Efficient Light Absorption and Charge Separation in Heterojunction
Ming Ma 1 Jong Hyeok Park 1 2
1Sungkyunkwan University Suwon Korea (the Republic of)2Sungkyunkwan University Suwon Korea (the Republic of)Show Abstract
For the first time, double-deck WO3/BiVO4 inverse opal photoanodes (DDIO-WO3/BiVO4) were prepared by swelling-shrinking mediated polystyrene template synthetic routes, and the use of the photoanodes in photoelectrochemical cells under simulated solar light was investigated. The double-deck photoanodes represented the compact interface between WO3 and BiVO4, inheriting the periodically ordered macroporous nanostructure. More significantly, the DDIO-WO3/BiVO4 inverse opal photoanodes prepared from the optimized fabrication condition demonstrated a photocurrent that was about 40 times higher than that of the pure inverse opal WO3 photoanodes at a bias of 1.23V vs. RHE. Even without an added catalyst, they produce an outstanding photocurrent density of about 3.3 mA/cm2 at a bias of 1.23V vs. RHE, which profits from improving the poor charge carrier mobility of BiVO4 by combining it with a WO3 skeleton and a shrouded bilayer inverse opal structure with a large surface area and good contact with the electrolyte.
5:00 AM - J2.07
Thermally-Enhanced Photoelectrochemical Activity of Bismuth Vanadate (BiVO4) Photoanode
Liming Zhang 1 Xiaofei Ye 1 Madhur Boloor 1 Nicolas A Melosh 1 William C. Chueh 1
1Stanford University Stanford United StatesShow Abstract
Solar-to-fuel efficiency in photoelectrochemical cells is often limited by the recombination rate of photo-excited charge carriers and the kinetic overpotential at the semiconductor/electrolyte interface. In small-polaron semiconductors such as BiVO4, carrier transport and electrocatalysis are thermally activated and benefit from higher operating temperatures. In this work, we studied the effect of heating in nanostructured BiVO4 in aqueous electrolyte. Our results have shown that while the open-circuit potential decreased with temperature at a rate of 2.1 mV/K, the photocurrent increased dramatically. By elevating the temperature from 9 #8451; to 42 #8451;, the saturation current density increased by 52%, from 2.1 to 3.2 mA/cm2. The stability under different temperatures was also explored. This work demonstrates that heating is a promising route to improve the photoelectrochemical activity of BiVO4, paving the way for further studies in solid-state cells.
5:15 AM - J2.08
Water Splitting Thin Film Transparent WO3 and Bivo4 Photoanodes by Sol Gel Process
Samantha Hilliard 3 1 2 Vincent Artero 2 Stephane Kressman 1 Christel Laberty-Robert 3
1Total Ramp;D Renewable Energies Paris France2Commissariat agrave; l'Eacute;nergie Atomique et aux Eacute;nergies alternatives (CEA) Grenoble France3Laboratoire de Chimie de la Matiegrave;re Condenseacute;e de Paris, Universiteacute; Pierre et Marie Curie (Paris VI), Collegrave;ge de France Paris FranceShow Abstract
Photoactive materials used in the oxygen evolution reaction for clean hydrogen water splitting technologies have been researched since the discovery of TiO2 as a photocatalytic material in 1972. Our current research is based on constructing a photoelectrocatalytic cell comprised of thin film photo-electrodes in a dual photo-system configuration which can thermodynamically attain the highest efficiency of any other cell architecture in a photocatalytic cell. This requires the photoelectrodes to be photo active, durable, low cost, scalable, possess varying band gaps, and requires one to be transparent. Electrodes are fabricated via sol-gel synthesis followed with thin film deposition by dip-coating; a technique which is low cost, easily scalable, and utilizes low temperature procedures. Our research is focused on the low temperature fabrication of transparent photoanodes made of WO3 and/or BiVO4. Tungsten trioxide, with its large internal quantum efficiency, band gap of 2.7eV, and its stability in acidic conditions make it a good candidate for water splitting technologies. However, for a system working in neutral conditions, bismuth vanadate with its band gap of 2.4eV is more stable at pH 7 and absorbs more visible light. These properties may allow BiVO4 to be used as a protective and n-tandem layer to coat WO3 and may as well increase charge carrier separation of the photoelectrode. Several approaches will be discussed in order to increase the efficiency of these materials; most notably: doping for increased conductivity and charge separation, nanostructuration for increased specific surface, and addition of catalysts to facilitate the water oxidation kinetics. WO3 and BiVO4, characterized by scanning electron and transmission electron microscopes, x-ray diffraction, UV-visible absorption and transmission, x-ray photoelectron spectroscopy, and photoelectrochemistry, are promising candidates for thin film photoelectrodes in a dual photo-system photoelectrocatalytic cell.
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5:30 AM - J2.09
Surface and Bulk Recombination in Spray-Deposited BiVO4 Photoanode: An Intensity Modulated Photocurrent Spectroscopy Study
Carolin Zachaeus 1 Fatwa Firdaus Abdi 2 Roel Van de Krol 3
1Helmholtz-Zentrum Berlin fuuml;r Materialien und Energie Berlin Germany2Helmholtz-Zentrum Berlin Berlin Germany3Helmholtz-Zentrum Berlin Berlin GermanyShow Abstract
Metal oxides have emerged as attractive candidates for photoelectrochemical water splitting, mainly due to their good stability in aqueous solutions, easy synthesis, and low cost. One of the most promising metal oxide photoanodes is bismuth vanadate (BiVO4)1. This material has been shown to evolve oxygen under illumination with visible light, and is stable in aqueous solutions with pH between 3 and 11. In addition, its conduction band edge is close to the reversible hydrogen electrode potential, which means that only little extra voltage is needed for evolution of hydrogen at the counter electrode in a BiVO4-based water splitting device. One of the challenges for BiVO4 is the efficient separation of electrons and holes. Recent work has shown that nanostructuring and doping are effective solutions for this problem2-4. Another issue that appears to limit the efficiency is slow oxygen evolution kinetics. The deposition of a co-catalyst, such as cobalt phosphate (CoPi), has been shown to greatly enhance the photocurrent and appears to address this issue1,5. However, the exact mechanism for this improvement is not yet fully clear.
In this study, we use Intensity Modulated Photocurrent Spectroscopy (IMPS) to examine the photocurrent kinetics of spray deposited BiVO4 photoanodes. An LED is used to illuminate the sample with a modulated intensity, and the real and imaginary parts of the opto-electrical impedance (i.e., the ratio between photocurrent and light intensity) are recorded. To interpret the resulting spectra, we used a model developed by Peter et al. that allows one to distinguish the rate constants for surface recombination and charge injection into the electrolyte.6 A comparison of bare and CoPi-catalyzed BiVO4 reveals that at modest applied potentials, the CoPi reduces the surface recombination rate by ~2 orders of magnitude. A similar surface passivation effect has been reported for CoOx-catalyzed hematite.6 More surprisingly, the CoPi seems to reduce the rate constant for charge injection into the electrolyte by a factor of ~10. Although the surface passivation effect still outweighs the reduction in charge transfer kinetics, resulting in higher photocurrents for CoPi-catalyzed BiVO4, the latter effect seems to contradict its function as an electrocatalyst. At more positive applied potentials (> 1.2 V vs RHE) the rate constants for surface recombination and charge transfer no longer depend on the illumination intensity, which implies that bulk recombination dominates in this potential regime. The implications of these intriguing observations will be discussed.
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5:45 AM - J2.10
Colloidal Chemistry for Ink-Based Doped and Undoped BiVO4 Photoanode
Anna Loiudice 1 Erin Creel 1 Jared James Lynch 2 Raffaella Buonsanti 3
1JCAP-LBNL Berkeley United States2Lawrence Berkeley National Lab Berkeley United States3Lawrence Berkeley National Laboratory/JCAP Berkeley United StatesShow Abstract
A complete control on morphology and microstructure of photoelectrodes is needed to establish precise correlations between structure and photocatalytic properties. Solution-based colloidal methods for preparing oxide semiconductors allow for the use of inexpensive and scalable processing techniques, like spin-coating or doctor-blades, and can give mesoporous thin films with controlled morphology at different lengtscales. The prototypical example of this approach is the mesoporous TiO2 used in dye-sensitized solar cells, where porosity, crystallite size and shape can be tuned independently. Recently, Sivula et al. have revealed the potentiality of using colloidal-based ink for hematite films to better understand the impact of the hematite structural properties on its performance as a photoanode for water oxidation. As for hematite, morphology seems to be key in the performance optimization of m-BiVO4 photoanodes. In fact, the state-of-art photocurrent reported so far for m-BiVO4 has been measured in films with nanoporous morphology. However, low temperature routes to nanocrystal-based ink for the B-V-O family have not been fully developed yet.
Herein, our recent work on colloidal nanocrystal-based inks for BiVO4 and Sb doped-BiVO4 photoanodes will be discussed.[3, 4]The nanocrystal composition and morphology was controlled through a one-pot seeded growth approach in which bismuth or bismuth antimony nanocrystals were reacted with the vanadium precursor in properly chosen conditions. After the synthesis, optically clear and colloidally stable of nanocrystals were used as ink to obtained monoclinic BiVO4 and Sb-BiVO4 upon annealing. In-situ XRD studies gave insight into the phase transformations occurring at different temperatures. Finally, charge carrier dynamics data and preliminary photoelectrochemical measuraments highlight the potential of these novel ink-based routes to the Bi-V-O photoanode family to build meaningful correlations between film structure and practical solar water oxidation functions.
1. (a) Brillet, J.; Gratzel, M.; Sivula, K Nano Lett. 2010, 10, 4155; (b) Sivula, K.; Le Formal, F.; Gratzel, M. ChemSusChem, 2011, 4, 432.
2. Kim, T. W.; Choi, K.-S. Science 2014, 343, 990.
3. Loiudice, A.; Cooper, J. K.; Mattox, T.; Sharp, I. D.; Buonsanti, R. "Colloidal Bi2O2.7/VOx nanocrystal heterodimers for ink-based BiVO4 films" submitted.
4. Loiudice, A.; Cooper, J. K.; Mattox, T.; Thao, T.; Drisdell, W.; Ma, J.; Wang, L-W; Yano, J.; Sharp, I. D.; Buonsanti, R. "Ink-based mesoporous Sb-BiVO4 films" in preparation.
Monday AM, April 06, 2015
Moscone West, Level 3, Room 3011
9:30 AM - *J1.01
Innovative Approaches to Fundamental Materials Challenges in Solar Water Splitting
Eric Lars Miller 1 Katie Randolph 1 David Peterson 1
1U.S. Department of Energy Washington United StatesShow Abstract
The US Department of Energy&’s (DOE) Fuel Cell Technologies Office has made significant progress in hydrogen and fuel cell technology advancement and cost reduction. With the expected rollouts of fuel-cell vehicles by major automotive manufacturers over the next several years, enabling technologies for the widespread production of affordable renewable hydrogen becomes increasingly important. Solar water-splitting processes, including photo-assisted electrolysis as well as the direct photoelectrochemical and thermochemical routes, can play a significant role. However, fundamental materials challenges remain which limit conversion efficiency and durability of these pathways, therefore preventing large-scale technoeconomic viability. Innovations are needed in the development of functional materials and interfaces which address the thermodynamic and kinetic limitations to performance and lifetime at the macro-, meso- and nano-scales. Researchers are increasingly adopting Clean Energy Materials Genome Initiative methodologies integrating advanced computation, experimentation and informatics to accelerate the materials discovery and development process. Examples of this approach specifically applied in the R&D of photoelectrochemical and thermochemical processes are discussed, and beneficial cross-cutting implications for all solar hydrogen pathways are highlighted.
10:00 AM - *J1.02
Sustainability Considerations for Solar Generation of Transportation Fuels
Frances Houle 1
1Lawrence Berkeley National Laboratory Berkeley United StatesShow Abstract
Although fully integrated devices for solar fuels are in the early stages, it is possible to consider how a future technology might look from what we know today. Recent studies by the Joint Center for Artificial Photosynthesis that assess the net energy balance for both a solar hydrogen device and a full facility find that the value is positive as long as sufficient efficiency and durability are achieved. JCAP&’s scientific and engineering programs are focused on improvements in materials systems and device designs to meet these two criteria. In this talk I will highlight our recent work on discovery and characterization of catalysts, light absorbers, corrosion barriers, membranes and architectures for hydrogen generation, and discuss the sensitivity of sustainability considerations to them.
10:30 AM - *J1.03
The Immobilization of a Molecular Catalyst on a p-GaInP2 Photoelectrode for Water Reduction
John A. Turner 1 Jing Gu 1 Yong Yan 1 James L. Young 2 1 Nathan R Neale 1
1National Renewable Energy Laboratory Golden United States2University of Colorado Boulder United StatesShow Abstract
The photoelectrochemical (PEC) process is an innovative approach for hydrogen production in which energy collection (solar absorber) and water electrolysis (catalysis) are mated into a single device. However, the bulk of the research over the last 40+ years has focused on metal oxides were the efficiency of PEC solar hydrogen generation is low due to the combined challenges of poor solar absorption and poor electronic properties. More suitable semiconductors for high efficiency solar photoconversion such as the III-Vs suffer from material instability and slow interfacial kinetics towards the water redox reactions. Integration of electrocatalysts to the semiconductor&’s surface is a necessary approach to both stabilize the interface and increase catalysis, thus enhancing the overall device performance. Stable catalysts for surface modification are particularly beneficial if they are also potentially low-cost, scalable, and transparent. Work on hydrogen evolution catalysts has been a very active area of research where numerous molecular, nanomaterial, and bulk catalysts have been developed. Noble metals, particularly platinum, are most commonly applied as they are the most active for the water redox reactions. These metals are neither earth abundant nor low-cost, so identifying catalytic systems that can match the activity and stability of noble metals but are based on earth abundant materials are clearly a high-priority area of research.
We will report on the immobilization of a cobaltoxime hydrogen evolution catalyst on a p-GaInP2 surface modified with a thin TiO2 layer via atomic layer deposition. Under illumination in pH 13 aqueous solution, the GaInP2-TiO2-cobaltoxime photocathode exhibited remarkable stability during 24 hours of continuous operation at 0 V vs. RHE. We observed a turnover number of 303,000 with a turnover frequency of 210 min-1 during that time. A high IPCE, up to ~70% through the visible range (<690nm), was measured for this structure, clarifying the advantage of a visible-light transparent hydrogen evolution catalyst.
11:15 AM - *J1.04
Catalysts, Protection Layers, and Semiconductors: The Challenge of Interfacing in the Tandem Design
Ib Chorkendorff 1
1Technical University of Denmark Kongens Lyngby DenmarkShow Abstract
Hydrogen is the simplest solar fuel to produce and in this presentation we shall give a short overview of the pros and cons of various tandem devices . The large band gap semiconductor needs to be in front, but apart from that we can chose to have either the anode in front or back using either acid or alkaline conditions. Since most relevant semiconductors are very prone to corrosion the advantage of using buried junctions and using protection layers offering shall be discussed [2-4]. Next we shall discuss the availability of various catalysts for being coupled to these protections layers and how their stability may be evaluated [5, 6]. Examples of half-cell reaction using protection layers for both cathode and anode will be discussed though some of recent examples under both alkaline and acidic conditions. Si is a very good low band gap semiconductor and by using TiO2 as a protection layer we can stabilize it for both H2 and O2 evolution [7, 8, 9, 10]. Notably NiOx promoted by iron is a material that is transparent, providing protection, and is a good catalyst for O2 evolution. We have also recently started searching for large band gap semicondutors like III-V based or pervoskite materials and follow the same strategy by using protection layers and catalysts .
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11:45 AM - J1.05
Bias-Free Photoelectrochemical Hydrogen Production Using Glutathione-Capped Gold Nanoclusters as Visible Photosensitizers
Yong-Siou Chen 1 2 Prashant V. Kamat 1 2
1University of Notre Dame Notre Dame United States2University of Notre Dame Notre Dame United StatesShow Abstract
Photoelectrochemical hydrogen production using sensitized photoelectrode as photoanode is a viable choice to store solar energy into chemical fuels. The systems that employ organic dyes or quantum dots as photosensitizers often require external bias or sacrificial reagents to overcome the charge transfer limitations at photoanode. We now report a bias-free photoelectrochemical hydrogen production system using glutathione-capped gold nanoclusters (Aux-GSH) as photosensitizer. These clusters provide a favorable reduction and oxidation window to induce hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). A two electrode photoelectrochemical cell (PEC) consisted of Aux-GSH sensitized TiO2 photoanode and Pt counter electrode was fabricated. When the photoanode was illuminated with visible light (> 420 nm), we observe a significant photocurrent activity under short circuit condition in an aqueous buffer (pH = 7). Over an hour long period of illumination, we detected 2 µmoles of hydrogen with a Faradaic efficiency of 65 %. By introducing EDTA as a sacrificial reagent in the electrolyte, around three times of hydrogen yield was observed. The capability of this newly discovered photosensitizer to conduct photoelectrochemical hydrogen production without applying external bias is superior to other photosensitizers. Our study paves the way to explore other nanoclusters with different physical and chemical properties as photosensitizers for hydrogen production from sunlight.
12:00 PM - J1.06
Design Considerations of Cost-Effective Solar-Hydrogen Generators
Miguel A. Modestino 1 Claudia A. Rodriguez 1 Christophe Moser 1 Demetri Psaltis 1
1EPFL Lausanne SwitzerlandShow Abstract
Electrochemical energy conversion devices that can directly capture and store solar energy in the form of fuels are a promising alternative to increase the share of renewables in our current energy landscape. Significant research efforts have been devoted towards the development of photoelectrochemical components (i.e. light absorbers and water splitting catalysts) that are able to spontaneously split water in the presence of solar irradiation. These efforts have led to major advances in the solar-fuels field, while only limited attention has been given to understanding the factors that drive economically viable solar-fuel generators. The study presented here introduces a system agnostic approach that allows for the understanding of economic factors behind the design of solar-hydrogen generators. Using this framework, we evaluate the underpinning effects of the material selection for the light absorption and water splitting components on the cost of the generated fuel ($/Kg of H2). Furthermore, designs for solar water-splitting devices are optimized for a given material&’s system in