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.
 F. F. Abdi, R. van de Krol, J. Phys. Chem. C, 2012, 116, 9398.
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 L. M. Peter, J. Solid State Electrochem., 2013, 17, 315.
5:45 AM - J2.10
Colloidal Chemistry for Ink-Based Doped and Undoped BiVO4 Photoanode