Materials for solar fuels and chemicals.

The conversion of sunlight into fuels and chemicals is an attractive prospect for the storage of renewable energy, and photoelectrocatalytic technologies represent a pathway by which solar fuels might be realized. However, there are numerous scientific challenges in developing these technologies. These include finding suitable materials for the absorption of incident photons, developing more efficient catalysts for both water splitting and the production of fuels, and understanding how interfaces between catalysts, photoabsorbers and electrolytes can be designed to minimize losses and resist degradation. In this Review, we highlight recent milestones in these areas and some key scientific challenges remaining between the current state of the art and a technology that can effectively convert sunlight into fuels and chemicals.

[1]  Nathan S Lewis,et al.  Highly active electrocatalysis of the hydrogen evolution reaction by cobalt phosphide nanoparticles. , 2014, Angewandte Chemie.

[2]  Mohammad Khaja Nazeeruddin,et al.  Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts , 2014, Science.

[3]  Kimberly M. Papadantonakis,et al.  A taxonomy for solar fuels generators , 2015 .

[4]  Thomas Olsen,et al.  Computational screening of perovskite metal oxides for optimal solar light capture , 2012 .

[5]  Jakob Kibsgaard,et al.  Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis. , 2012, Nature materials.

[6]  J. Nørskov,et al.  Nanoscale limitations in metal oxide electrocatalysts for oxygen evolution. , 2014, Nano letters.

[7]  M. Pourbaix.  Atlas of Electrochemical Equilibria in Aqueous Solutions , 1974 .

[8]  Yao Zheng,et al.  Hydrogen evolution by a metal-free electrocatalyst , 2014, Nature Communications.

[9]  Guosong Hong,et al.  MoS2 nanoparticles grown on graphene: an advanced catalyst for the hydrogen evolution reaction. , 2011, Journal of the American Chemical Society.

[10]  Nathan S. Lewis,et al.  An analysis of the optimal band gaps of light absorbers in integrated tandem photoelectrochemical water-splitting systems , 2013 .

[11]  H. Jónsson,et al.  Density functional theory calculations for the hydrogen evolution reaction in an electrochemical double layer on the Pt(111) electrode. , 2007, Physical chemistry chemical physics : PCCP.

[12]  Nathan S. Lewis,et al.  Evaluation of Pt, Ni, and Ni–Mo electrocatalysts for hydrogen evolution on crystalline Si electrodes , 2011 .

[13]  Alexander J. Cowan,et al.  Dynamics of photogenerated holes in surface modified α-Fe2O3 photoanodes for solar water splitting , 2012, Proceedings of the National Academy of Sciences.

[14]  Matthew W. Kanan,et al.  Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper , 2014, Nature.

[15]  Thomas F. Jaramillo,et al.  New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces , 2012 .

[16]  J. Nørskov,et al.  Computational high-throughput screening of electrocatalytic materials for hydrogen evolution , 2006, Nature materials.

[17]  Matthew R. Shaner,et al.  Stabilization of Si microwire arrays for solar-driven H2O oxidation to O2(g) in 1.0 M KOH(aq) using conformal coatings of amorphous TiO2 , 2015 .

[18]  E. L. Miller,et al.  Photoelectrochemical water splitting , 2013 .

[19]  Charles C. L. McCrory,et al.  Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. , 2015, Journal of the American Chemical Society.

[20]  T. Jaramillo,et al.  Building an appropriate active-site motif into a hydrogen-evolution catalyst with thiomolybdate [Mo3S13]2- clusters. , 2014, Nature chemistry.

[21]  A. Fujishima,et al.  Electrochemical Photolysis of Water at a Semiconductor Electrode , 1972, Nature.

[22]  A. Nozik,et al.  Solar conversion efficiency of photovoltaic and photoelectrolysis cells with carrier multiplication absorbers , 2006 .

[23]  Frances A. Houle,et al.  Particle suspension reactors and materials for solar-driven water splitting , 2015 .

[24]  Kristin A. Persson,et al.  First principles high throughput screening of oxynitrides for water-splitting photocatalysts , 2013 .

[25]  A. Vojvodić,et al.  Homogeneously dispersed multimetal oxygen-evolving catalysts , 2016, Science.

[26]  T. Morikawa,et al.  A monolithic device for CO2 photoreduction to generate liquid organic substances in a single-compartment reactor , 2015 .

[27]  Chengxiang Xiang,et al.  Modeling, simulation, and fabrication of a fully integrated, acid-stable, scalable solar-driven water-splitting system. , 2015, ChemSusChem.

[28]  S. Boettcher,et al.  Nickel-iron oxyhydroxide oxygen-evolution electrocatalysts: the role of intentional and incidental iron incorporation. , 2014, Journal of the American Chemical Society.

[29]  Kyoung-Shin Choi,et al.  Efficient and stable photo-oxidation of water by a bismuth vanadate photoanode coupled with an iron oxyhydroxide oxygen evolution catalyst. , 2012, Journal of the American Chemical Society.

[30]  R. T. Ross,et al.  Limits on the yield of photochemical solar energy conversion , 1977 .

[31]  Jacob Bonde,et al.  Biomimetic hydrogen evolution: MoS2 nanoparticles as catalyst for hydrogen evolution. , 2005, Journal of the American Chemical Society.

[32]  Juan Bisquert,et al.  Water oxidation at hematite photoelectrodes: the role of surface states. , 2012, Journal of the American Chemical Society.

[33]  J. Brennecke,et al.  Switching the reaction course of electrochemical CO₂ reduction with ionic liquids. , 2014, Langmuir : the ACS journal of surfaces and colloids.

[34]  James R. McKone,et al.  Solar water splitting cells. , 2010, Chemical reviews.

[35]  Thomas F. Jaramillo,et al.  Catalyzing the Hydrogen Evolution Reaction (HER) with Molybdenum Sulfide Nanomaterials , 2014 .

[36]  Egill Skúlason,et al.  Modeling the electrified solid-liquid interface , 2008 .

[37]  A. Asthagiri,et al.  Selectivity of CO(2) reduction on copper electrodes: the role of the kinetics of elementary steps. , 2013, Angewandte Chemie.

[38]  Thomas F. Jaramillo,et al.  Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts , 2007, Science.

[39]  T. Jaramillo,et al.  Hydrogen Evolution on Supported Incomplete Cubane-type (Mo3S4) 4+ Electrocatalysts , 2008 .

[40]  Thomas F. Jaramillo,et al.  Gold-supported cerium-doped NiOx catalysts for water oxidation , 2016, Nature Energy.

[41]  Mietek Jaroniec,et al.  Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles. , 2012, Journal of the American Chemical Society.

[42]  Y. Shao-horn,et al.  Orientation-Dependent Oxygen Evolution Activities of Rutile IrO2 and RuO2. , 2014, The journal of physical chemistry letters.

[43]  D. Piron,et al.  Study of Electrodeposited Nickel‐Molybdenum, Nickel‐Tungsten, Cobalt‐Molybdenum, and Cobalt‐Tungsten as Hydrogen Electrodes in Alkaline Water Electrolysis , 1994 .

[44]  K. Sivula,et al.  Semiconducting materials for photoelectrochemical energy conversion , 2016 .

[45]  Fuding Lin,et al.  Adaptive semiconductor/electrocatalyst junctions in water-splitting photoanodes. , 2014, Nature materials.

[46]  Jens K Nørskov,et al.  Modeling CO2 reduction on Pt(111). , 2013, Physical chemistry chemical physics : PCCP.

[47]  Nathan S. Lewis,et al.  Simulations of the irradiation and temperature dependence of the efficiency of tandem photoelectrochemical water-splitting systems† , 2013 .

[48]  D. Gamelin.  Water splitting: Catalyst or spectator? , 2012, Nature chemistry.

[49]  Y. Hori,et al.  Electrochemical CO 2 Reduction on Metal Electrodes , 2008 .

[50]  Hubert A. Gasteiger,et al.  A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. , 2012 .

[51]  G. Scuseria,et al.  Hybrid functionals based on a screened Coulomb potential , 2003 .

[52]  T. Jaramillo,et al.  Core-shell MoO3-MoS2 nanowires for hydrogen evolution: a functional design for electrocatalytic materials. , 2011, Nano letters.

[53]  H. Hwang,et al.  Band Edge Engineering of Oxide Photoanodes for Photoelectrochemical Water Splitting: Integration of Subsurface Dipoles with Atomic‐Scale Control , 2016 .

[54]  Thomas F. Jaramillo,et al.  Electrocatalytic conversion of carbon dioxide to methane and methanol on transition metal surfaces. , 2014, Journal of the American Chemical Society.

[55]  Fuding Lin,et al.  Semiconductor-Electrocatalyst Interfaces: Theory, Experiment, and Applications in Photoelectrochemical Water Splitting. , 2016, Accounts of chemical research.

[56]  Adam C. Nielander,et al.  Interface engineering of the photoelectrochemical performance of Ni-oxide-coated n-Si photoanodes by atomic-layer deposition of ultrathin films of cobalt oxide , 2015 .

[57]  H. Queisser,et al.  Detailed Balance Limit of Efficiency of p‐n Junction Solar Cells , 1961 .

[58]  Zhenhai Xia,et al.  A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. , 2015, Nature nanotechnology.

[59]  Frances A. Houle,et al.  Opportunities to improve the net energy performance of photoelectrochemical water-splitting technology , 2016 .

[60]  Joseph H. Montoya,et al.  Theoretical evaluation of the surface electrochemistry of perovskites with promising photon absorption properties for solar water splitting. , 2015, Physical chemistry chemical physics : PCCP.

[61]  Andrew A. Peterson,et al.  Activity Descriptors for CO2 Electroreduction to Methane on Transition-Metal Catalysts , 2012 .

[62]  S. Boettcher,et al.  Solution-cast metal oxide thin film electrocatalysts for oxygen evolution. , 2012, Journal of the American Chemical Society.

[63]  K. Sivula.  Metal Oxide Photoelectrodes for Solar Fuel Production, Surface Traps, and Catalysis. , 2013, The journal of physical chemistry letters.

[64]  M. Dignam,et al.  Efficiency of Splitting Water with Semiconducting Photoelectrodes , 1984 .

[65]  Jens K Nørskov,et al.  Electrochemical Barriers Made Simple. , 2015, The journal of physical chemistry letters.

[66]  Frances A. Houle,et al.  Life-cycle net energy assessment of large-scale hydrogen production via photoelectrochemical water splitting , 2014 .

[67]  S. Schuldiner.  Hydrogen Overvoltage on Bright Platinum , 1952 .

[68]  Masakazu Sugiyama,et al.  A 24.4% solar to hydrogen energy conversion efficiency by combining concentrator photovoltaic modules and electrochemical cells , 2015 .

[69]  Andrew J. Medford,et al.  Departures from the Adsorption Energy Scaling Relations for Metal Carbide Catalysts , 2014 .

[70]  Kyle L. Gunther,et al.  Critical role of interfacial effects on the reactivity of semiconductor-cocatalyst junctions for photocatalytic oxygen evolution from water , 2016 .

[71]  Desheng Kong,et al.  Synthesis of MoS2 and MoSe2 films with vertically aligned layers. , 2013, Nano letters.

[72]  O. Hansen,et al.  Iron-Treated NiO as a Highly Transparent p-Type Protection Layer for Efficient Si-Based Photoanodes. , 2014, The journal of physical chemistry letters.

[73]  John Kitchin,et al.  Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces , 2011 .

[74]  Jan Rossmeisl,et al.  Universality in Oxygen Evolution Electrocatalysis on Oxide , 2011 .

[75]  Michael Grätzel,et al.  Cathodic shift in onset potential of solar oxygen evolution on hematite by 13-group oxide overlayers , 2011 .

[76]  Yoshiaki Nakano,et al.  Characteristics of hydrogen generation from water splitting by polymer electrolyte electrochemical cell directly connected with concentrated photovoltaic cell , 2013 .

[77]  Joseph H. Montoya,et al.  Direct Water Decomposition on Transition Metal Surfaces: Structural Dependence and Catalytic Screening , 2016, Catalysis Letters.

[78]  Xiaoming Ge,et al.  Molybdenum phosphide as an efficient electrocatalyst for the hydrogen evolution reaction , 2014 .

[79]  Nathan S. Lewis,et al.  Modeling, simulation, and design criteria for photoelectrochemical water-splitting systems , 2012 .

[80]  K. Sivula,et al.  Surface modification of semiconductor photoelectrodes. , 2015, Physical chemistry chemical physics : PCCP.

[81]  H. Gasteiger,et al.  Hydrogen Oxidation and Evolution Reaction Kinetics on Platinum: Acid vs Alkaline Electrolytes , 2010 .

[82]  P. Kenis,et al.  Ionic Liquid–Mediated Selective Conversion of CO2 to CO at Low Overpotentials , 2011, Science.

[83]  Yijie Huo,et al.  Solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen efficiency over 30% , 2016, Nature Communications.

[84]  Y. Ping,et al.  Energetics and Solvation Effects at the Photoanode/Catalyst Interface: Ohmic Contact versus Schottky Barrier. , 2015, Journal of the American Chemical Society.

[85]  Arnold J. Forman,et al.  Modeling practical performance limits of photoelectrochemical water splitting based on the current state of materials research. , 2014, ChemSusChem.

[86]  Charlie Tsai,et al.  Designing an improved transition metal phosphide catalyst for hydrogen evolution using experimental and theoretical trends , 2015 .

[87]  Karen Chan,et al.  Molybdenum Sulfides and Selenides as Possible Electrocatalysts for CO2 Reduction , 2014 .

[88]  J. Nørskov,et al.  Hydrogen evolution on nano-particulate transition metal sulfides. , 2008, Faraday discussions.

[89]  Stuart Licht,et al.  Efficient Solar Water Splitting, Exemplified by RuO2-Catalyzed AlGaAs/Si Photoelectrolysis , 2000 .

[90]  A. Bard,et al.  Screening of transition and post-transition metals to incorporate into copper oxide and copper bismuth oxide for photoelectrochemical hydrogen evolution. , 2013, Physical chemistry chemical physics : PCCP.

[91]  Michel Dupuis,et al.  Frontiers, opportunities, and challenges in biochemical and chemical catalysis of CO2 fixation. , 2013, Chemical reviews.

[92]  Brian D. James,et al.  Technoeconomic Analysis of Photoelectrochemical (PEC) Hydrogen Production , 2009 .

[93]  E. Baerends,et al.  Self-consistent approximation to the Kohn-Sham exchange potential. , 1995, Physical review. A, Atomic, molecular, and optical physics.

[94]  Jinghua Guo,et al.  Direct Observation of Two Electron Holes in a Hematite Photoanode during Photoelectrochemical Water Splitting , 2012 .

[95]  Alexis T. Bell,et al.  Thermodynamic and achievable efficiencies for solar-driven electrochemical reduction of carbon dioxide to transportation fuels , 2015, Proceedings of the National Academy of Sciences.

[96]  Alexis T. Bell,et al.  An investigation of thin-film Ni-Fe oxide catalysts for the electrochemical evolution of oxygen. , 2013, Journal of the American Chemical Society.

[97]  Kyoung-Shin Choi,et al.  Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting , 2014, Science.

[98]  Thomas F. Jaramillo,et al.  New cubic perovskites for one- and two-photon water splitting using the computational materials repository , 2012 .

[99]  Antonio Abate,et al.  Efficient photosynthesis of carbon monoxide from CO2 using perovskite photovoltaics , 2015, Nature Communications.

[100]  Jan Rossmeisl,et al.  Beyond the volcano limitations in electrocatalysis--oxygen evolution reaction. , 2014, Physical chemistry chemical physics : PCCP.

[101]  Eric L. Miller,et al.  Photoelectrochemical production of hydrogen : Engineering loss analysis , 1997 .

[102]  Anubhav Jain,et al.  New Light‐Harvesting Materials Using Accurate and Efficient Bandgap Calculations , 2015 .

[103]  V. Jović,et al.  Electrodeposition of Ni-Mo alloy coatings and their characterization as cathodes for hydrogen evolution in sodium hydroxide solution , 2008 .

[104]  Kristin A. Persson,et al.  Commentary: The Materials Project: A materials genome approach to accelerating materials innovation , 2013 .

[105]  Kristian Sommer Thygesen,et al.  BANDGAP CALCULATIONS AND TRENDS OF ORGANOMETAL HALIDE PEROVSKITES , 2014 .

[106]  Matthew W. Kanan,et al.  Aqueous CO2 reduction at very low overpotential on oxide-derived Au nanoparticles. , 2012, Journal of the American Chemical Society.

[107]  Todd G. Deutsch,et al.  Sunlight absorption in water – efficiency and design implications for photoelectrochemical devices , 2014 .

[108]  W. Jaegermann,et al.  Chemical and electronic characterization of methyl-terminated Si(111) surfaces by high-resolution synchrotron photoelectron spectroscopy , 2005 .

[109]  H. Jónsson,et al.  Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode , 2004 .

[110]  S. Dahl,et al.  Hydrogen production using a molybdenum sulfide catalyst on a titanium-protected n(+)p-silicon photocathode. , 2012, Angewandte Chemie.

[111]  Ib Chorkendorff,et al.  Crystalline TiO2: A Generic and Effective Electron-Conducting Protection Layer for Photoanodes and -cathodes , 2015 .

[112]  T. Jaramillo,et al.  Engineering Cobalt Phosphide (CoP) Thin Film Catalysts for Enhanced Hydrogen Evolution Activity on Silicon Photocathodes , 2016 .

[113]  Thomas W. Hamann,et al.  Splitting water with rust: hematite photoelectrochemistry. , 2012, Dalton transactions.

[114]  R. T. Ross,et al.  Some Thermodynamics of Photochemical Systems , 1967 .

[115]  G. N. Baum,et al.  Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry , 2013 .

[116]  Turner,et al.  A monolithic photovoltaic-photoelectrochemical device for hydrogen production via water splitting , 1998, Science.

[117]  Yohan Park,et al.  Atomic layer-deposited tunnel oxide stabilizes silicon photoanodes for water oxidation. , 2011, Nature materials.

[118]  K. Fujii,et al.  Solar to Chemical Energy Conversion , 2016 .

[119]  Kazuhiro Sayama,et al.  High-throughput screening using porous photoelectrode for the development of visible-light-responsive semiconductors. , 2007, Journal of combinatorial chemistry.

[120]  Juan Bisquert,et al.  Photoelectrochemical and impedance spectroscopic investigation of water oxidation with "Co-Pi"-coated hematite electrodes. , 2012, Journal of the American Chemical Society.

[121]  Paul J. A. Kenis,et al.  Effect of Cations on the Electrochemical Conversion of CO2 to CO , 2013 .

[122]  J. Nørskov,et al.  Bifunctional alloys for the electroreduction of CO2 and CO. , 2016, Physical chemistry chemical physics : PCCP.

[123]  Leone Spiccia,et al.  Renewable fuels from concentrated solar power: towards practical artificial photosynthesis , 2015 .

[124]  M. Field,et al.  Cyclopentadienyl ruthenium-nickel catalysts for biomimetic hydrogen evolution: electrocatalytic properties and mechanistic DFT studies. , 2009, Chemistry.

[125]  Dong Sung Choi,et al.  Molybdenum sulfide/N-doped CNT forest hybrid catalysts for high-performance hydrogen evolution reaction. , 2014, Nano letters.

[126]  G. Ceder,et al.  Efficient band gap prediction for solids. , 2010, Physical review letters.

[127]  Matthew R. Shaner,et al.  Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting , 2015 .

[128]  Svetlozar Nestorov,et al.  The Computational Materials Repository , 2012, Computing in Science & Engineering.

[129]  T. Jaramillo,et al.  Designing Active and Stable Silicon Photocathodes for Solar Hydrogen Production Using Molybdenum Sulfide Nanomaterials , 2014 .

[130]  João E. Benedetti,et al.  Synthesis and characterization of a quaternary nanocomposite based on TiO2/CdS/rGO/Pt and its application in the photoreduction of CO2 to methane under visible light , 2015 .

[131]  Thomas F. Jaramillo,et al.  Accelerating materials development for photoelectrochemical hydrogen production: Standards for methods, definitions, and reporting protocols , 2010 .

[132]  M. F. Weber,et al.  Splitting water with semiconducting photoelectrodes—Efficiency considerations , 1986 .

[133]  Kimberly M. Papadantonakis,et al.  Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films , 2015, Proceedings of the National Academy of Sciences.

[134]  Thomas Bligaard,et al.  Density functional theory in surface chemistry and catalysis , 2011, Proceedings of the National Academy of Sciences.

[135]  Jakob Kibsgaard,et al.  Molybdenum phosphosulfide: an active, acid-stable, earth-abundant catalyst for the hydrogen evolution reaction. , 2014, Angewandte Chemie.

[136]  Thomas F. Jaramillo,et al.  Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production: Insights into the Origin of their Catalytic Activity , 2012 .

[137]  Kristian Sommer Thygesen,et al.  Electrochemical CO2 and CO reduction on metal-functionalized porphyrin-like graphene , 2013 .

[138]  James R. Bolton,et al.  Limiting and realizable efficiencies of solar photolysis of water , 1985, Nature.

[139]  J. Nørskov,et al.  Chemical activity of the nitrogenase FeMo cofactor with a central nitrogen ligand: density functional study. , 2004, Journal of the American Chemical Society.

[140]  Joseph H. Montoya,et al.  Improving Oxygen Electrochemistry through Nanoscopic Confinement , 2015 .

[141]  Sasha Omanovic,et al.  Characterization of Ni, NiMo, NiW and NiFe electroactive coatings as electrocatalysts for hydrogen evolution in an acidic medium , 2005 .

[142]  Shoushan Fan,et al.  Grain-boundary-dependent CO2 electroreduction activity. , 2015, Journal of the American Chemical Society.

[143]  K. Domen,et al.  Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. , 2014, Chemical Society reviews.

[144]  Andrew A. Peterson,et al.  How copper catalyzes the electroreduction of carbon dioxide into hydrocarbon fuels , 2010 .

[145]  M. Grätzel,et al.  Covalent Immobilization of a Molecular Catalyst on Cu2O Photocathodes for CO2 Reduction. , 2016, Journal of the American Chemical Society.

[146]  Charles C. L. McCrory,et al.  Benchmarking heterogeneous electrocatalysts for the oxygen evolution reaction. , 2013, Journal of the American Chemical Society.

[147]  G. Nagasubramanian,et al.  On the role of surface states in semiconductor electrode photoelectrochemical cells , 1980 .

[148]  A. Vojvodić,et al.  New design paradigm for heterogeneous catalysts , 2015 .

[149]  Brian E. Conway,et al.  Modern Aspects of Electrochemistry , 1974 .

[150]  L. Peter,et al.  Photoelectrochemical water splitting at semiconductor electrodes: fundamental problems and new perspectives. , 2014, Chemphyschem : a European journal of chemical physics and physical chemistry.

[151]  Y. Shao-horn,et al.  Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. , 2012, The journal of physical chemistry letters.

[152]  John M. Gregoire,et al.  Mn 2 V 2 O 7 : An Earth Abundant Light Absorber for Solar Water Splitting , 2015 .

[153]  T. T. Rantala,et al.  Kohn-Sham potential with discontinuity for band gap materials , 2010, 1003.0296.

[154]  J. Gregoire,et al.  Mn2V2O7: An Earth Abundant Light Absorber for Solar Water Splitting , 2015 .