Photocathode For Solar Hydrogen Production Research Articles

Porous silicon-nanowire-based electrode for the photoelectrocatalytic production of hydrogen

Building a porous photocathode for solar hydrogen production with earth-abundant materials: silicon nanowires to harvest light, molybdenum sulfide to turn protons into hydrogen.

An Integrated Metal‐Free Modification Method to Construct Efficient and Durable Bulk Heterojunction Photocathode for Solar Hydrogen Production

AbstractPolymer semiconductor with bulk heterojunction (BHJ) structure has attracted increasing attention to fabricate highly efficient photoelectrode for converting solar energy and water into hydrogen, thanks to its high photocurrent output and positive onset potential beyond 0.6 V. However, BHJ‐based photoelectrodes demonstrate poor anticorrosion against irradiation in aqueous environment, thus the photoelectrochemical (PEC) stability is a very intractable problem to solve. Herein, an inside and outside integrated modification method is developed to help BHJ‐based photocathode withstand PEC erosion during hydrogen production in acidic solution. The obtained BHJ (PBDB‐T:ITIC:PC71BM)‐based photocathode with surface carbon protective layer allows sustained and fast PEC hydrogen evolution (an average photocurrent density up to 13.5 mA cm−2) for a duration up to 15 h, which is among the best results of BHJ‐based photocathodes. The internal modification with fullerene derivative of PC71BM boosts the photogenerated electron transfer to balance against relative fast hole transfer, which largely alleviates electron accumulation and improves PEC intrinsic stability as a result. The surface carbon layer effectively resists the permeation of aqueous electrolyte without hindering interfacial charge transfer. This metal‐free protective method is very promising toward construction of highly robust BHJ‐based photoelectrodes for sustainable water splitting.

WS2/p-Si-based photocathodes with high activity originated from the unique vertical geometry of the 2D WS2 nanoplatelets

Earth-abundant two-dimensional (2D) materials-based photocathodes for solar hydrogen production may replace conventional noble metal-based ones when they can be grown with a proper orientation and dimension for efficient charge transfer and catalytic reactions. Here, we show that WS2 films can be grown as wafer-scale films with unique vertically aligned 2H-WS2 nanoplatelets using a single-step vacuum deposition process directly on substrates such as Si wafer and F-doped tin oxide (FTO)/glass. The as-grown WS2/p-Si shows enhanced photocathode performances that are attributed to the unique vertical WS2 nanoplatelet structure, which endows excellent antireflection properties (reflectance as low as ∼ 4%) for enhanced absorption, high photocurrent density (ca. 37 mA cm−2) at low overpotential optimized by controlled vertical length, and catalytically active WS2 edge-sites exposed to the surface for excellent catalytic hydrogen evolution reaction (HER). The results suggest that the nanostructured WS2/p-Si heterostructures are promising multi-functional photocathodes for solar hydrogen production with excellent stability for long-term operation in ambient condition.

Efficient hydrogen evolution from water over thin film photocathode composed of solid solutions between ZnSe and Cu(In, Ga)Se2 with composition gradient

Solid solutions between ZnSe and Cu(In, Ga)Se2 (ZnSe:CIGS) have promising properties as photocathodes for solar hydrogen production from water, such as a long absorption edge of about 900 nm and a large driving force for reaction, >0.9 V, originated by a deep valence band maximum of 1.0–1.1 V vs normal hydrogen electrode (NHE). However, their performance is limited with an incident photon-to-current conversion efficiency (IPCE) of 65% at 400 nm and a half-cell solar-to-hydrogen energy conversion efficiency (HC-STH) of 3.6% so far. Te addition during ZnSe:CIGS thin film deposition by vacuum co-evaporation clearly decreased the optimal deposition temperature from 450 to 380 °C and resulted in columnar shaped grains of submicrometer size in diameter and with almost no grain boundary between the film surface and the backside electrode of Mo, which is significantly larger than the case of without Te addition. Interestingly, a Ga/In compositional ratio gradient was further introduced to the depth profile, which can facilitate charge separation. Structural characterizations using XRD and cross-sectional transmission electron microscopy revealed that the composition gradient was mainly formed by the diffusion of In through grain boundaries in the Ga-rich layer, and thus, the mixing between the Ga-rich and In-rich layers was more significant at a lower deposition temperature owing to the smaller grain size. The photocathode fabricated from the ZnSe:CIGS thin film with the composition gradient showed a very high IPCE of 89% at 540 nm and 0 V vs the reversible hydrogen electrode, and HC-STH of 3.7%, which is higher than values reported thus far. In conclusion, it is clarified that the potential gradient, which can be introduced by composition gradient, is beneficial for photoelectrodes and photocatalysts to achieve higher performance.

Enabling Solar Hydrogen Production over Selenium: Surface State Passivation and Cocatalyst Decoration

As the first material ever used in a solid-state solar cell, selenium (Se) has outstanding merits of light absorption, carrier mobility, intrinsic environmental stability, and straightforward film preparation, making it an attractive light-absorbing semiconductor for photovoltaic applications. However, the exploration of selenium for photoelectrochemical (PEC) cells remains vastly unreported. Here, we successfully enable selenium as an absorber in the photocathode for solar hydrogen production. A rapid thermal annealing process is adopted to prepare a tetragonal Se film with a preferred [100] orientation. Using a thin layer of TiO2 buffer and platinum decoration, a photocurrent density up to −7.2 mA/cm2 is achieved over the optimal Se/TiO2/Pt photocathode at 0 VRHE under simulated one sun illumination. The roles of surface state passivation by TiO2 and cocatalyst decoration by Pt in charge separation and transfer were thoroughly investigated by capacitance–voltage profiling, photoassisted Kelvin probe force microscopy, photoelectrochemical impedance spectroscopy, and also transient photovoltage decay. Our work provides Se as a promising candidate for solar hydrogen production and also highlights the effectiveness of surface state passivation and cocatalyst decoration in the fabrication of efficient PEC devices.

Solution-Processed Synthesis of Copper Oxide (Cu x O) Thin Films for Efficient Photocatalytic Solar Water Splitting.

This article reports a solution-processed synthesis of copper oxide (CuxO) to be used as a potential photocathode for solar hydrogen production in the solar water-splitting system. CuxO thin films were synthesized through the reduction of copper iodide (CuI) thin films by sodium hydroxide (NaOH), which were deposited by the spin coating method from CuI solution in a polar aprotic solvent (acetonitrile). The phase and crystalline quality of the synthesized CuxO thin films prepared at various annealing temperatures were investigated using various techniques. The X-ray diffraction and energy dispersive X-ray spectroscopy studies confirm the presence of Cu2O, CuO/Cu2O mixed phase, and pure CuO phase at annealing temperatures of 250, 300, and 350 °C, respectively. It is revealed from the experimental findings that the synthesized CuxO thin films with an annealing temperature of 350 °C possess the highest crystallinity, smooth surface morphology, and higher carrier density. The highest photocurrent density of −19.12 mA/cm2 at −1 V versus RHE was achieved in the photoelectrochemical solar hydrogen production system with the use of the CuxO photocathode annealed at a temperature of 350 °C. Therefore, it can be concluded that CuxO synthesized by the spin coating method through the acetonitrile solvent route can be used as an efficient photocathode in the solar water-splitting system.

Establishing Stability in Organic Semiconductor Photocathodes for Solar Hydrogen Production.

As organic semiconductors attract increasing attention to application in the fields of bioelectronics and artificial photosynthesis, understanding the factors that determine their robust operation in direct contact with aqueous electrolytes becomes a critical task. Herein we uncover critical factors that influence the operational stability of donor:acceptor bulk heterojunction photocathodes for solar hydrogen production and significantly advance their performance under operational conditions. First, using the direct photoelectrochemical reduction of aqueous Eu3+ and impedance spectroscopy, we determine that replacing the commonly used fullerene-based electron acceptor with a perylene diimide-based polymer drastically increases operational stability and identify that limiting the photogenerated electron accumulation at the organic/water interface to values of ca. 100 nC cm-2 is required for stable operation (>12 h). These insights are extended to solar-driven hydrogen production using MoS3, MoP, or RuO2 water reduction catalyst overlayers where it is found that the catalyst morphology strongly affects performance due to differences in charge extraction. Optimized performance of bulk heterojunction photocathodes coated with a MoS3:MoP composite gave 1 Sun photocurrent density up to 8.7 mA cm-2 at 0 V vs RHE (pH 1). However, increased stability was gained with RuO2 where initial photocurrent density (>8 mA cm-2) deceased only 15% or 33% during continuous operation for 8 or 20 h, respectively, thus demonstrating unprecedented robustness without a protection layer. This performance represents a new benchmark for organic semiconductor photocathodes for solar fuel production and advances the understanding of stability criteria for organic semiconductor/water-junction-based devices.

Highly efficient and stable Si photocathode with hierarchical MoS2/Ni3S2 catalyst for solar hydrogen production in alkaline media

Designing highly efficient and stable Si photocathodes for solar hydrogen production in alkaline electrolytes is especially challenging due to the sluggish surface kinetics for hydrogen evolution reaction (HER) and unsuccessful contact between the catalyst and Si substrate. Herein, a facile electrodeposition process is reported to integrate the hierarchical MoS2/Ni3S2 structure as a catalyst layer onto the surface of Ni protected Si photocathode. Rough surface morphology and obvious electronic interactions between Ni3S2 and MoS2 were observed experimentally, while the theoretical calculation illustrated that the atomic mixed MoS2/Ni3S2 interfaces enhance the hydrogen-adsorption of MoS2 and hydroxide adsorption of Ni3S2 and thus accelerate both the Volmer and Tafel steps in HER process. An excellent photoelectrochemical (PEC) performance with an onset potential of 0.54 V vs. reversible hydrogen electrode and an applied bias photon-to-current efficiency of 11.2% were obtained in MoS2/Ni3S2/Ni/n+np+-Si photocathode under simulated AM1.5G illumination in 1 M KOH aqueous solution. Furthermore, comparing with the hybrid MoS2@Ni3S2 catalyst on Ni/Si with an inferior stability, the hierarchical MoS2/Ni3S2 structure releases the stress between the layers and leads to the stability of the photocathode with over 172-h PEC operation under 41.5 mA/cm2. This finding represents a potential low-cost and scalable approach toward making high performance, precious metal-free Si photocathodes for solar hydrogen production in alkaline media.

How photocorrosion can trick you: a detailed study on low-bandgap Li doped CuO photocathodes for solar hydrogen production.

The efficiency of photoelectrochemical tandem cells is still limited by the availability of stable low band gap electrodes. In this work, we report a photocathode based on lithium doped copper(ii) oxide, a black p-type semiconductor. Density functional theory calculations with a Hubbard U term show that low concentrations of Li (Li0.03Cu0.97O) lead to an upward shift of the valence band maximum that crosses the Fermi level and results in a p-type semiconductor. Therefore, Li doping emerged as a suitable approach to manipulate the electronic structure of copper oxide based photocathodes. As this material class suffers from instability in water under operating conditions, the recorded photocurrents are repeatedly misinterpreted as hydrogen evolution evidence. We investigated the photocorrosion behavior of LixCu1-xO cathodes in detail and give the first mechanistic study of the fundamental physical process. The reduced copper oxide species were localized by electron energy loss spectroscopy mapping. Cu2O grows as distinct crystallites on the surface of LixCu1-xO instead of forming a dense layer. Additionally, there is no obvious Cu2O gradient inside the films, as Cu2O seems to form on all LixCu1-xO nanocrystals exposed to water. The application of a thin Ti0.8Nb0.2Ox coating by atomic layer deposition and the deposition of a platinum co-catalyst increased the stability of LixCu1-xO against decomposition. These devices showed a stable hydrogen evolution for 15 minutes.

H2-Evolving Dye-Sensitized Photocathode Based on a Ruthenium–Diacetylide/Cobaloxime Supramolecular Assembly

The development of NiO-based molecular photocathodes is attracting more interest in the field of dye-sensitized photoelectrochemical cells for efficient conversion of sunlight into fuel. For this purpose, different strategies are developed to assemble the molecular components together to build functional devices. Here, an original dye/catalyst supramolecular assembly was designed and obtained via axial coordination of a cobalt-based H2-evolving catalyst, i.e., a cobaloxime complex, to a pyridyl-functionalized ruthenium–diacetylide photosensitizer. The new supramolecular assembly was successfully employed for the construction of efficient NiO-based photocathodes for solar hydrogen production. We report a joint experimental and theoretical study of the new photocatalytic system, including electrochemical and X-ray photoelectron spectroscopy analyses. Photoelectrochemical generation of H2 under pertinent aqueous conditions eventually led to a faradaic efficiency of 27%.

Enhancing the Performance of Si-Based Photocathodes for Solar Hydrogen Production in Alkaline Solution by Facilely Intercalating a Sandwich N-Doped Carbon Nanolayer to the Interface of Si and TiO2.

Photoelectrochemical (PEC) water splitting is a promising but immensely challenging technology for sustainable production of hydrogen. To this end, highly active, durable, and inexpensive photocathodes that operate under conditions compatible with those for photoanodes are desired. Herein, Si-based composite photocathodes were constructed by coating the front surface of Si with an N-doped carbon nanolayer and then a TiO2 protective layer, followed by decorating the electrode surface with Ni and Ni-Mo catalysts. The carbon nanolayer, denoted as CPDA, was formed directly on the Si surface by in situ self-polymerization of dopamine, followed by carbonization of the polydopamine (PDA) coating. The performance of the as-fabricated Si photocathodes with and without the CPDA layer was comparatively studied for PEC hydrogen evolution reaction (HER) inalkaline electrolytes to evaluate the effect of the sandwich CPDA layer in between the Si substrate and the TiO2 layer on the photoelectrocatalytic behaviors of Si-based electrodes. The photocathodes containing the CPDA layer demonstrated lower electron transfer resistance, higher built-in photovoltage, and larger band bending relative to the analogous electrodes without the CPDA layer. Accordingly, the short-circuit photocurrents of the Ni and Ni-Mo-decorated photocathodes with the CPDA layer were enhanced by a factor of 2.8-3.3, and their open-circuit photovoltages were enlarged by 0.14-0.22 V, compared to those of the corresponding electrodes without the CPDA layer in 1 M KOH under simulated 1 sun illumination. Moreover, the photocathodes with the CPDA layer also exhibited an improved stability for PEC HER in alkaline solutions, with a faradaic efficiency of 90-97% in the initial hour.

Plasmonic enhanced Cu2O-Au-BFO photocathodes for solar hydrogen production

A novel Cu2O-Au-BFO heterostructure photocathode was constructed which significantly improved the efficiency of photo-generated carrier transfer for solar hydrogen production. A BiFeO3 (BFO) ferroelectric film was synthesized on top of a Cu2O layer by a sputtering process. The BFO layer acted to protect the Cu2O layer from photochemical corrosion, increasing photoelectrochemical (PEC) stability. The p–n heterojunction between Cu2O and BFO layers enhanced the PEC properties by suppressing charge recombination and improved interfacial charge transfer efficiency. When Cu2O and BFO are interfaced by Au Nanoparticles (NPs) the PEC performance was further enhanced, due to hot-electron transfer at the plasmonic resonance. After positive poling, the depolarization field across the whole volume of BFO film drove electrons into the electrolyte solution, inducing a significant anodic shift, Vop of 1.01 V vs. RHE, together with a significantly enhanced photocurrent density of −91 μA/cm2 at 0 V vs. RHE under 100 mW/cm2 illumination. The mechanism was investigated through experimental and theoretivcal calculations.

Thiourea-assisted coating of dispersed copper electrocatalysts on Si photocathodes for solar hydrogen production

Abstract Photoelectrochemical water splitting can convert solar energy into clean hydrogen energy for storage. It is desirable to explore non-precious electrocatalysts for practical applications of a photoelectrode in a large scale. Here, we developed a facile spin-coating and in-situ photoelectrochemical reduction method to prepare a dispersed Cu electrocatalyst on a Si photocathode, which improves the performance remarkably. We find that thiourea in the precursor solution for spin-coating plays an important role in obtaining dispersed Cu particles on the surface of a Si photoelectrode. With thiourea in the precursor, the Cu/Si photocathode shows higher performance than the one without thiourea. Moreover, the Cu/Si photocathode also indicates good stability after 16 h illumination.

Improving the Stability and Efficiency of CuO Photocathodes for Solar Hydrogen Production through Modification with Iron.

Cupric oxide (CuO) is considered as a promising photocathode material for photo(electro)chemical water splitting because of its suitable band gap, low cost related to copper earth abundancy, and straightforward fabrication. The main challenge for the development of practical CuO-based photocathodes for solar hydrogen evolution is to enhance its stability against photocorrosion. In this work, stable and efficient CuO photocathodes have been developed by using a simple and cost-effective methodology. CuO films, composed of nanowires and prepared by chemical oxidation of electrodeposited Cu, develop relatively high photocurrents in 1 M NaOH. However, this photocurrent appears to be partly associated with photocorrosion of CuO. It is significant though that, even unprotected, a faradaic efficiency for hydrogen evolution of ∼45% is attained. The incorporation of iron through an impregnation method, followed by a high-temperature thermal treatment for promoting the external phase transition of the nanowires from CuO to ternary copper iron oxide, was found to provide an improved stability at the expense of photocurrent, which decreases to about one-third of its initial value. In contrast, a faradaic efficiency for hydrogen evolution of ∼100% is achieved even in the absence of co-catalysts, which is ascribable to the favorable band positions of CuO and the iron copper ternary oxide in the core-shell structure of the nanowires.

Novel p-n heterojunction copper phosphide/cuprous oxide photocathode for solar hydrogen production

A Copper phosphide (Cu3P) micro-rod (MR) array, with coverage by an n-Cu2O thin layer by electrodeposition as a photocathode, has been directly fabricated on copper foil via simple electro-oxidation and phosphidation for photoelectrochemical (PEC) hydrogen production. The morphology, structure, and composition of the Cu3P/Cu2O heterostructure are systematically analyzed using a scanning electron microscope (SEM), X-ray diffraction and X-ray photoelectron spectra. The PEC measurements corroborate that the p-Cu3P/n-Cu2O heterostructural photocathode illustrates efficient charge separation and low charge transfer resistance to achieve the highest photocurrent of 430 μA cm−2 that is greater than other transition metal phosphide materials. In addition, a detailed energy diagram of the p-Cu3P/n-Cu2O heterostructure was investigated using Mott-Schottky analysis. Our study paves the way to explore phosphide-based materials in a new class for solar energy applications.

Earth-Abundant Tin Sulfide-Based Photocathodes for Solar Hydrogen Production.

Tin‐based chalcogenide semiconductors, though attractive materials for photovoltaics, have to date exhibited poor performance and stability for photoelectrochemical applications. Here, a novel strategy is reported to improve performance and stability of tin monosulfide (SnS) nanoplatelet thin films for H2 production in acidic media without any use of sacrificial reagent. P‐type SnS nanoplatelet films are coated with the n‐CdS buffer layer and the TiO2 passivation layer to form type II heterojunction photocathodes. These photocathodes with subsequent deposition of Pt nanoparticles generate a photovoltage of 300 mV and a photocurrent density of 2.4 mA cm−2 at 0 V versus reversible hydrogen electrode (RHE) for water splitting under simulated visible‐light illumination (λ > 500 nm, P in = 80 mW cm−2). The incident photon‐to‐current efficiency at 0 V versus RHE for H2 production reach a maximum of 12.7% at 575 nm with internal quantum efficiency of 13.8%. The faradaic efficiency for hydrogen evolution remains close to unity after 6000 s of illumination, confirming the robustness of the heterojunction for solar H2 production.

Humidity sensitive polymers In solution processed adjustable pore-volume Cu(In,Ga)S2 photocathodes for solar hydrogen production

P-type Cu(In,Ga)S2 semiconductors are promising candidates to be used as photocathodes for solar water splitting. Porous structures have been widely used to improve the performances of photoelectrodes due to good minority carrier transport. However, a porous photoelectrode has longer transport distance of majority carriers, which limits its performance. Controlling pore volume of a photoelectrode can balance minority and majority carrier transport and improve the performance. Here, a porous Cu(In,Ga)S2 film is prepared by facile spin-coating method. The pore volume of Cu(In,Ga)S2 film is controlled by adjusting relative humidity (RH) of air during spin-coating process. Further studies suggest that polyvinyl acetate (PVAc) in precursor solution is a humidity sensitive polymer and plays a key role to form different pore volume. The 40% RH sample has the best performance due to its optimum pore volume. After further coated with CdS surface passivation layer and Pt electrocatalyst on the surface, a 40% RH Cu(In,Ga)S2 photocathode indicates a photocurrent density of 8.6 mA cm−2 at 0 VRHE, which is one of the highest photocurrents of Cu(In,Ga)S2 photocathodes. This new strategy for adjusting pore volume is also suitable to prepare other solution-processed inorganic materials.

Photoelectrochemical deposition of CoP on cuprous oxide photocathodes for solar hydrogen production

Photoelectrochemical (PEC) water splitting is an attractive and sustainable energy conversion method. In this work, cobalt phosphide (CoP) is photoelectrochemically deposited on a p-type cuprous oxide (p-Cu2O) photocathode for solar hydrogen production. Under visible light irradiation, the PEC assembly is active for hydrogen evolution, generating a current density of up to −5.3mAcm−2 and −4.2mAcm−2 in acidic and basic conditions at 0V vs. the reversible hydrogen electrode (RHE).

Engineering Molybdenum Disulfide Protected Silicon Photocathodes for Corrosion Resistance in Acid

Photoelectrochemical (PEC) water splitting devices offer a promising path to sustainable production of hydrogen.1 Silicon shows great promise as a small band gap photocathode in tandem photoelectrochemical (PEC) water splitting due to its appropriate band structure, energetic alignment for driving hydrogen evolution, and its band gap of 1.1 eV.2 However, some challenges remain to ensure that silicon photoelectrodes possess the required catalytic activity and stability for use in highly effect PEC water splitting devices. Molybdenum disulfide (MoS2) is a low permeability material known for its excellent stability and HER activity in acid. We have previously demonstrated MoS2 protected silicon photocathodes which operate for 100 hrs without significant loss of activity.3 Here, we further this study to design strategies, such as the utilization of a molybdenum silicide interface layer, for improved activity and over 600 hour stability with unchanged photoelectrochemical performance (onset potential, fill factor and light limited photocurrent density), which is among the best performance of any precious metal free photocathode. Critical to developing future electrodes with enhanced stability, we probe the failure mechanism of the MoS2 protected photocathodes. By XPS we observe that the MoS2 is fully absent post stability testing. Detailed failure mechanisms of the photocathodes and electrolyte are uncovered by electrochemical methods, SEM, Raman microscopy and ICP. Furthermore, we implement increased light intensity (10 suns), as well as light and dark cycling to probe accelerated durability testing protocols utilizing MoS2 protected photocathodes. From such analyses we uncover a feedback loop that enables insightful design of future corrosion resistant protection schemes. (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q. X.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446–6473. (2) Seitz, L. C.; Chen, Z.; Forman, A. J.; Pinaud, B. A.; Benck, J. D.; Jaramillo, T. F. Modeling Practical Performance Limits of Photoelectrochemical Water Splitting Based on the Current State of Materials Research. ChemSusChem 2014, 7 (5), 1372–1385. (3) Benck, J. D.; Lee, S. C.; Fong, K. D.; Kibsgaard, J.; Sinclair, R.; Jaramillo, T. F. Designing Active and Stable Silicon Photocathodes for Solar Hydrogen Production Using Molybdenum Sulfide Nanomaterials. Adv. Energy Mater. 2014, 4 (18), 1400739.

Molybdenum Disulfide As a Protection Layer and Catalyst for Gallium Indium Phosphide Solar Water Splitting Photocathodes

Gallium indium phosphide (GaInP2) is a semiconductor with a nearly ideal bandgap for solar water-splitting as the top absorber in a dual junction tandem absorber device. It has been used in conjunction with a gallium arsenide (GaAs) bottom absorber in an overall water splitting cell with 12.4% solar-to-hydrogen (STH) efficiency, one of the highest STH efficiencies for an integrated photoelectrochemical (PEC) water-splitting device reported to date. However, GaInP2 suffers from one of the biggest challenges facing the field: instability due to electrochemical corrosion in aqueous electrolytes. Molybdenum disulfide (MoS2) nanomaterials can be used to both protect GaInP2 and significantly improve its catalytic ability since it is resistant to corrosion and also possesses high activity for the hydrogen evolution reaction (HER). In this work, we demonstrate that GaInP2 photocathodes coated with thin MoS2 surface protecting layers exhibit excellent activity and stability for solar hydrogen production and we probe the details of failure mechanisms using novel flow cell microscopic and spectroscopic techniques. Our GaInP2 photocathodes demonstrated no loss in performance (photocurrent onset potential, fill factor, and light limited current density) until 60 hours of operation which represents a five-hundred fold increase in stability compared to bare p-GaInP2 samples tested in identical conditions. We believe this to be one of the first successful attempts to stabilize GaInP2 using a thin film protection layer scheme. Furthermore, as this protection scheme has previously been used successfully on silicon photocathodes, this work highlights the potential for MoS2 to be used as a thin film protection layer for many different semiconductor water splitting devices that are unstable in acid. Using a custom-designed flow cell coupled with various microscopic and spectroscopic techniques (optical, Raman, FT-IR), we gained a greater understanding of the failure mechanisms of MoS2 as a thin-film protection layer. We discovered that pinhole formation in the MoS2 layer exposes the GaInP2 substrate, which readily corrodes in the acidic conditions, ultimately leading to device degradation. The flow cell further allowed us to capture the time scale of this pinhole formation. These insights represent a deeper understanding of MoS2 as a protection layer and can be leveraged to improve the stability of thin film protected semiconductor water splitting devices. References Khaselev, O.; Turner, J. A., A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production Via Water Splitting. Science 1998, 280, 425-427. Benck, J. D.; Lee, S. C.; Fong, K. D.; Kibsgaard, J.; Sinclair, R.; Jaramillo, T. F., Designing Active and Stable Silicon Photocathodes for Solar Hydrogen Production Using Molybdenum Sulfide Nanomaterials. Advanced Energy Materials 2014, 4, 1-8 Britto R.J., Benck J.D., Young J.L., Hahn C., Deutsch, T.G., Jaramillo T.F. In Review (2016) Figure 1