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The long-term stability in acidic or alkaline aqueous electrolytes of p-Ga0.52In0.48P photocathodes, with a band gap of ∼1.8 eV, for the solar-driven hydrogen-evolution reaction (HER) has been evaluated from a thermodynamic, kinetic, and mechanistic perspective. At either pH 0 or pH 14, etched p-GaInP electrodes corroded cathodically under illumination and formed metallic In0 on the photoelectrode surface. In contrast, under the same conditions, electrodeposition of Pt facilitated the HER kinetics and stabilized p-GaInP/Pt photoelectrodes against such cathodic decomposition. When held at 0 V versus the reversible hydrogen electrode, p-GaInP/Pt electrodes in either pH = 0 or pH = 14 exhibited stable current densities (J) of ∼-9 mA cm-2 for hundreds of hours under simulated 1 sun illumination. During the stability tests, the current density-potential (J-E) characteristics of the p-GaInP/Pt photoelectrodes degraded due to pH-dependent changes in the surface chemistry of the photocathode. This work provides a fundamental understanding of the stability and corrosion mechanisms of p-GaInP photocathodes that constitute a promising top light absorber for tandem solar-fuel generators.

The long-term stability for the hydrogen-evolution reaction (HER) of homojunction pn

Photoanodes containing a WO3/BiVO4 heterojunction have demonstrated promising photoelectrochemical water splitting performance, but the ability to effectively passivate the WO3/BiVO4 interface has limited charge transport and collection. Here, the WO3/BiVO4 interface is passivated with a sulfur-modified Bi2O3 interfacial layer with a staggered band edge alignment to facilitate charge transfer and lifetime. Additionally, BiVO4 was co-doped with Ga3+ at Bi3+ sites and W6+ at V5+ sites (i.e., (Ga,W):BiVO4) to improve the light absorption and photogenerated charge carrier concentration. The optimized WO3/S:Bi2O3/(Ga,W):BiVO4 photoanode exhibited a photocurrent density of 4.0 ± 0.2 mA cm−2 compared to WO3/(Ga,W):BiVO4 with 2.8 ± 0.12 mA cm−2 at 1.23 VRHE in K2HPO4 under simulated AM 1.5G illumination. Time-resolved photoluminescence spectroscopic analysis of the WO3/S:Bi2O3/(Ga,W):BiVO4 electrode validated the enhanced interfacial charge transfer kinetics. In operando femto- and nano-second transient absorption spectroscopy confirmed the presence of long-lived photogenerated charge carriers and revealed lower recombination initially due to rapid charge separation of WO3/S:Bi2O3/(Ga,W):BiVO4. The distribution and role of sulfur was further investigated using EDAX, XPS and TOF-SIMS depth profiling. Finally, a Co-Pi co-catalyst layer was added to achieve a photocurrent of 5.1 ± 0.25 mA cm−2 and corresponding H2 generation rate of 67.3 μmol h−1 cm−2 for the WO3/S:Bi2O3/(Ga,W):BiVO4/Co-Pi photoanode.

Computational studies are employed to construct our understanding of systems and to reduce the monetary and temporal demands of research and development. Proton exchange membrane water electrolysis (PEMWE) devices, with their intricate two-phase transport phenomena and numerous realizable applications, provide an excellent opportunity to utilize computational fluid dynamics (CFD). Toghyani et al.1 modeled the performance of a high-temperature, vapor-fed PEMWE device using CFD simulations, finding fair agreement with prior experimental studies. For low-temperature electrolysis, Nie and Chen2 used a mixture model to study the flow regimes in liquid-fed channels. We employed a CFD model to simulate a PEMWE device, exploring the viability of isothermal and non-isothermal mixture models for a liquid-fed cell. The flow rate was altered to study the change in cell temperature and performance, and the model results were compared to experimental data. Additionally, bulk properties of the porous transport layer at the anode were altered to examine performance effects; such effects were expected to be virtually non-existent in light of experimental observations.3,4 The results demonstrate when a continuum assumption is best utilized to predict electrolysis efficiency. References: S. Toghyani, E. Afshari, and E. Baniasadi, Journal of Thermal Analysis and Calorimetry, 135, 1911–1919 (2019). J. Nie and Y. Chen, International Journal of Hydrogen Energy, 35, 3183–3197 (2010). T. Schuler, T. J. Schmidt, and F. N. Büchi, Journal of The Electrochemical Society, 166, F555–F565 (2019). J. Lopata et al., Journal of The Electrochemical Society, 167, 064507 (2020). Figure 1

While photoelectrochemical (PEC) solar-to-hydrogen efficiencies have greatly improved over the past few decades, advances in PEC durability have lagged behind. Corrosion of semiconductor photoabsorbers in the aqueous conditions needed for water splitting is a major challenge that limits device stability. In addition, a precious-metal catalyst is often required to efficiently promote water splitting. Herein, we demonstrate unassisted water splitting using a nonprecious metal molybdenum disulfide nanomaterial catalytic protection layer paired with a GaInAsP/GaAs tandem device. This device was able to achieve stable unassisted water splitting for nearly 12 h, while a sibling sample with a PtRu catalyst was only stable for 2 h, highlighting the advantage of the nonprecious metal catalyst. In situ optical imaging illustrates the progression of macroscopic degradation that causes device failure. In addition, this work compares unassisted water splitting devices across the field in terms of the efficiency and stability, illustrating the need for improved stability.

GaInP

Photoelectrochemical (PEC) water splitting, which utilizes sunlight and water to produce hydrogen fuel, is potentially one of the most sustainable routes to clean energy. One challenge to success is that, to date, similar materials and devices measured in different labs or by different operators lead to quantitatively different results, due to the lack of accepted standard operating procedures and established protocols for PEC efficiency testing. With the aim of disseminating good practices within the PEC community, we provide a vetted protocol that describes how to prepare integrated components and accurately measure their solar-to-hydrogen (STH) efficiency (ηSTH). This protocol provides details on electrode fabrication, ηSTH test device assembly, light source calibration, hydrogen evolution measurement, and initial material qualification by photocurrent measurements under monochromatic and broadband illumination. Common pitfalls in translating experimental results from any lab to an accurate STH efficiency under an AM1.5G reference spectrum are discussed. A III–V tandem photocathode is used to exemplify the process, though with small modifications, the protocol can be applied to photoanodes as well. Dissemination of PEC best practices will help those approaching the field and provide guidance for comparing the results obtained at different lab sites by different groups.

The long-term stability in acidic or alkaline aqueous electrolytes of p-Ga

Durable performance and high efficiency in solar-driven water splitting are great challenges not yet co-achieved in photoelectrochemical (PEC) cells. Although photovoltaic cells made from III–V semiconductors can achieve high optical–electrical conversion efficiency, their functional integration with electrocatalysts and operational lifetime remain great challenges. Herein, an ultra-thin TiN layer was used as a diffusion barrier on a buried junction n+p-GaInP2 photocathode, to enable elevated temperatures for subsequent catalyst growth of Ni5P4 as nano-islands without damaging the GaInP2 junction. The resulting PEC half-cell showed negligible absorption loss, with saturated photocurrent density and H2 evolution equivalent to the benchmark photocathode decorated with PtRu catalysts. High corrosion-resistant Ni5P4/TiN layers showed undiminished photocathode operation over 120 h, exceeding previous benchmarks. Etching to remove electrodeposited copper, an introduced contaminant, restored full performance, demonstrating operational ruggedness. The TiN layer expands the synthesis conditions and protects against corrosion for stable operation of III–V PEC devices, while the Ni5P4 catalyst replaces costly and scarce noble metal catalysts.

Catalytic MoSx thin films have been directly photoelectrodeposited on GaInP2 photocathodes for stable photoelectrochemical hydrogen generation. Specifically, the MoSx deposition conditions were controlled to obtain 8-10 nm films directly on p-GaInP2 substrates without ancillary protective layers. The films were nominally composed of MoS2, with additional MoOxSy and MoO3 species detected and showed no long-range crystalline order. The as-deposited material showed excellent catalytic activity toward the hydrogen evolution reaction relative to bare p-GaInP2. Notably, no appreciable photocurrent reduction was incurred by the addition of the photoelectrodeposited MoSx catalyst to the GaInP2 photocathode under light-limited operating conditions, highlighting the advantageous optical properties of the film. The MoSx catalyst also imparted enhanced durability toward photoelectrochemical hydrogen evolution in acidic conditions, maintaining nearly 85% of the initial photocurrent after 50 h of electrolysis. In total, this work demonstrates a simple method for producing dual-function catalyst/protective layers directly on high-performance, planar III-V photoelectrodes for photoelectrochemical energy conversion.

Monolithic perovskite/perovskite tandem solar cells have attracted substantial attention owing to their promise for high power conversion efficiency (PCE) and low fabrication cost. Conventional all-perovskite tandem solar cells employ a mixed tin (Sn)-lead (Pb) low-bandgap subcell, which suffers from instability issues in the ambient air. Here, we fabricate two-terminal perovskite/perovskite tandem solar cells consisting of two solution-processed perovskite subcells based on the more airstable pure Pb-based perovskites. The champion perovskite tandem cell exhibits a PCE of 19.2% with a high open-circuit voltage of 2.15 V. We further demonstrate photovoltaicelectrolysis (PV-EC) water-splitting applications using these high-photovoltage all-perovskite tandem solar cells.

In this talk, I will first give an overview of some recent work in photoelectrochemical (PEC) materials and device development as part of the HydroGEN Consortium. Within this consortium, I serve as the NREL lead and a capability expert for the PEC water splitting technology. Our group supports several, university-led seedling projects in developing various PEC materials and plays the central role in a national laboratory collaboration focused on improving PEC device durability. I will highlight the development and demonstration of our flagship capability for PEC device benchmarking and on-sun testing. In the second part of this talk, I will present some efforts in low-temperature electrolysis (LTE) research as part of the H2NEW Consortium and a Technology Commercialization Fund project. My work focuses on ex-situ characterization of membrane electrode assembly (MEA) performance and development of porous transport layer (PTL) materials and surface modifications. The ex-situ characterization approaches are used in concert with in-situ testing to understand and improve LTE materials and interface properties. Finally, I will give an overview of NREL’s recent E2M electrochemical device testbed development for CO2 and N2 reduction research. This includes powerful, inline product analysis systems and custom hardware to study electrocatalyst integration, evaluate cell architectures, and conduct spatial and combinatorial studies.

The production of hydrogen fuels, via water splitting, is of practical relevance for meeting global energy needs and mitigating the environmental consequences of fossil-fuel-based transportation. Water photoelectrolysis has been proposed as a viable approach for generating hydrogen, provided that stable and inexpensive photocatalysts with conversion efficiencies over 10% can be discovered, synthesized at scale, and successfully deployed (Pinaud et al., Energy Environ. Sci., 2013, 6, 1983). While a number of first-principles studies have focused on the data-driven discovery of photocatalysts, in the absence of systematic experimental validation, the success rate of these predictions may be limited. We address this problem by developing a screening procedure with co-validation between experiment and theory to expedite the synthesis, characterization, and testing of the computationally predicted, most desirable materials. Starting with 70 150 compounds in the Materials Project database, the proposed protocol yielded 71 candidate photocatalysts, 11 of which were synthesized as single-phase materials. Experiments confirmed hydrogen generation and favorable band alignment for 6 of the 11 compounds, with the most promising ones belonging to the families of alkali and alkaline-earth indates and orthoplumbates. This study shows the accuracy of a nonempirical, Hubbard-corrected density-functional theory method to predict band gaps and band offsets at a fraction of the computational cost of hybrid functionals, and outlines an effective strategy to identify photocatalysts for solar hydrogen generation.

Gallium indium phosphide (GaInP2) is a semiconductor with promising optical and electronic properties to serve as the large bandgap, top junction in a dual absorber tandem solar water splitting device. Poor intrinsic catalytic ability and surface corrosion in aqueous electrolyte remain key obstacles. Significant progress has been made developing thin-film protection layers and active catalysts for photoelectrochemical devices, but combining these into a catalytic protection layer that can provide long-term stability without sacrificing performance has proven difficult due, in large part, to challenges in developing active and stable interfaces. In this work, we demonstrate that a nanoscale molybdenum disulfide (MoS2) film functions both as an effective protection layer and excellent hydrogen evolution catalyst for GaInP2 photocathodes, with only a ∼10% loss in initial light-limited current density after 100 h, and a photocurrent onset potential better than that of the same state-of-the-art device with a platinum–ruthenium catalyst. Using transient photoreflectance spectroscopy, we probed the carrier dynamics of these photocathodes and show that the MoS2 coated device exhibits improved electron transfer at the surface interface compared to the PtRu catalyzed device. These MoS2 protected devices are among the most active and stable single-absorber photocathodes for solar water splitting to date and offer a promising pathway towards generating hydrogen with high efficiency and significant longevity.

Starting from the classical GaInP/GaAs tandem photoelectrochemical water splitting device, higher solar-to-hydrogen efficiencies can be pursued by extending photon absorption to longer wavelengths. We incorporate strain-balanced GaInAs/GaAsP quantum wells into the bottom GaAs junction, to increase the range of photon absorption. The inclusion of 1.34 eV quantum wells in the depletion region of the bottom cell extends the absorption edge to 930 nm. With a corresponding increase in the thickness of the top cell for current matching, the light-limiting photocurrent increases by >8%. The estimated solar-to-hydrogen efficiency is 13.6 ± 0.5%, and we show a pathway to further improvement. With the semiconductor device remaining on the growth substrate, this quantum well architecture may enable improved stability and durability of the photoelectrochemical electrodes.

Correction for ‘Optimizing accuracy and efficacy in data-driven materials discovery for the solar production of hydrogen’ by Yihuang Xiong et al., Energy Environ. Sci., 2021, 14, 2335–2348; DOI: 10.1039/D0EE02984J.

Catalytic MoS

While III-V semiconductors have achieved the highest photo-electrochemical solar-to-hydrogen conversion efficiencies, they are remarkably unstable during operation in a harsh electrolyte. The first part of this talk will focus on the degradation mechanism of inverted metamorphic mulijunction (IMM) III-V cells and surface modification strategies aimed at protecting them from photocorrosion. We applied noble metal catalysts, oxide coatings by atomic layer deposition, and MoS2 in an effort to protect the GaInP2 surface that was in contact with acidic electrolyte. We also grew epitaxial capping layers from III-V alloys that should be more intrinsically stable than GaInP2. The ability of the various modifications to protect the IMM’s surface was evaluated by operating at each electrode at short circuit for extended periods of time. The second part of this talk will identify the challenges encountered while scaling the IMM III-V absorber areas of from ~0.15 cm2 up to 16 cm2 and incorporating them in a photoreactor capable of generating 3 standard liters of hydrogen in 8 hours under natural sunlight. To successfully scale photo-electrochemical water-splitting technologies from bench to demonstration size requires addressing predictable and unpredictable complications. Despite using Comsol multiphysics to model our photoreactor and identify suitable specifications for a prototype, several practical issues were uncovered during testing that led to multiple iterations of photoreactor design between the initial and final generations. Several bottlenecks that ranged from counter electrode composition and orientation to bubble management needed redress in order to meet our performance targets. Ultimately, the demonstration-scale system was able to generate nearly twice the target volume of hydrogen in an 8-hour outdoor trial.

Expanding upon our prior experimental work, we constructed a three-dimensional model of a polymer electrolyte membrane water electrolyzer using computational fluid dynamics. We applied the assumption of pseudo-two-phase flow, the flow of two phases with equal velocity. Experimental data were used to obtain parameters and to determine the conditions under which this model was valid. Anodic distributions of current density, temperature, liquid saturation, and relative humidity were obtained at various flow rates. The overall current density and temperature difference from inlet to outlet at the anode agreed strongly with experimental measurements under most circumstances. This verification allowed us to further examine the apparent gas coverage calculated from experimental and model temperature data. Results suggested a low liquid saturation and low relative humidity at the anode due to the consumption of liquid water and water vapor. However, we questioned the accuracy of the pseudo-two-phase assumption at low water feed rates. We concluded that the model was applicable to systems with liquid water feed rates greater than 0.6 ml min−1 cm−2. Therefore, it is a fair screening method that can advise which operating conditions lead to excessive temperatures or drying at the anode, thereby promoting the longevity of the membrane and catalyst.

Earth−abundant Copper-Barium-Thiostannate Cu2BaSnS4 (CBTS)-based thin films have recently been reported to exhibit the optoelectronic and defect properties suitable as absorbers for photoelectrochemical (PEC) water splitting and the top cell of tandem photovoltaic solar cells. However, the photocurrents of CBTS-based PEC devices are still much lower than the theoretical value, partially due to ineffective charge collection at CBTS/water interface and instability of CBTS in electrolytes. Here, we report on overcoming these issues by employing overlayer engineering. We find that CdS/ZnO/TiO2 overlayers can significant-ly improve the PEC performance, achieving saturated cathodic photocurrents up to 7.8 mA cm−2 at the potential of -0.10 V versus reversible hydrogen electrode (RHE) in a neutral electrolyte solution, which is much higher than the best bare CBTS film attaining a photocurrent of 4.8 mA cm−2 at the potential of -0.2 V versus RHE. Our results suggest a viable approach for improving the performance ...

Efficient water splitting using light as the only energy input requires stable semiconductor electrodes with favorable energetics for the water-oxidation and proton-reduction reactions. Strategies to tune electrode potentials using molecular dipoles adsorbed to the semiconductor surface have been pursued for decades but are often based on weak interactions and quickly react to desorb the molecule under conditions relevant to sustained photoelectrolysis. Here, we show that covalent attachment of fluorinated, aromatic molecules to p-GaAs(1 0 0) surfaces can be employed to tune the photocurrent onset potentials of p-GaAs(1 0 0) photocathodes and reduce the external energy required for water splitting. Results indicate that initial photocurrent onset potentials can be shifted by nearly 150 mV in pH -0.5 electrolyte under 1 Sun (1000 W m-2 ) illumination resulting from the covalently bound surface dipole. Though X-ray photoelectron spectroscopy analysis reveals that the covalent molecular dipole attachment is not robust under extended 50 h photoelectrolysis, the modified surface delays arsenic oxide formation that results in a p-GaAs(1 0 0) photoelectrode operating at a sustained photocurrent density of -20.5 mA cm-2 within -0.5 V of the reversible hydrogen electrode.

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

The photoluminescence (PL) intensity of semiconductors can be modulated by their ambient. We show GaInP2 to respond reversibly to water vapor, irreversibly to oxygen, and with a time dependence to air. We characterize the reversible PL response to water vapor in a set of steady-state measurements that reveal a systematic dependence on pressure. We derive a model for this behavior based on Langmuir adsorption and Shockley–Read–Hall recombination principles to describe how partial pressure controls luminescence. The expression for the GaInP2/water vapor system shows excellent agreement to measurements. Combined, the PL monitoring technique and model demonstrate a quantitative approach for probing gas/surface defect interactions of semiconductor processing steps, luminescence-based sensors, and photocatalytic surfaces.

CuGa3Se5 is a promising candidate material with wide band gap for top cells in tandem photovoltaic and photoelectrochemical (PEC) devices. However, traditional CdS contact layers used with other chalcopyrite absorbers are not suitable for CuGa3Se5 due to the higher position of its conduction band (CB) minimum. Mg x Zn1− x O (MZO) is a transparent oxide with adjustable band gap and CB position as a function of magnesium composition, but its direct application is hindered by CuGa3Se5 surface oxidation. Here, MZO is investigated as a contact (n-type ‘buffer’ or ‘window’) material to CuGa3Se5 absorbers pretreated in Cd2+ solution, and an onset potential close to 1 V vs reversible hydrogen electrode in 10 mM hexaammineruthenium (III) chloride electrolyte is demonstrated. The Cd2+ surface treatment changes the chemical composition and electronic structure of the CuGa3Se5 surface, as demonstrated by photoelectron spectroscopy measurements. The performance of CuGa3Se5 absorber with Cd2+ treated surface in the solid-state test structure depends on the Zn/Mg ratio in the MZO layer. The measured open circuit voltage of 925 mV is promising for tandem PEC water splitting with CuGa3Se5/MZO top cells.

Photoelectrochemical (PEC) water-splitting systems utilize sunlight to directly split water to hydrogen and oxygen, providing a storable chemical fuel.1 While a variety of semiconductor material systems have demonstrated unassisted solar hydrogen production, III-V semiconductors have shown the highest reported solar-to-hydrogen (STH) conversion efficiencies and best device longevities.2–4 However, to date, work in the field has focused mostly on laboratory conditions, with few PEC systems having been tested outdoors using natural sunlight. In this work, we design III-V semiconductor systems with a molybdenum disulfide catalyst layer and demonstrate unassisted PEC hydrogen generation on-sun. First, a window and capping layer in conjunction with the MoS2 catalytic protection layer are shown to improve the photovoltage and durability of a single-junction pn-GaInP2 photocathode over a MoS2/pn-GaInP2 photocathode. Translating this top cell architecture to a tandem absorber device allows for more meaningful outdoor measurements and improved unassisted water-splitting performance. Specifically, we employ a tandem, lattice-matched GaInP2/GaAs (1.8/1.4 eV bandgaps) system that has been developed with high quality fabrication and yields>10% STH efficiency.2,3 An on-sun photoreactor platform was designed for PEC benchmarking and material testing that incorporates a 2-axis solar tracker and gas collection system.5 Insolation and environmental instrumentation at the co-located NREL Solar Radiation Research Laboratory provide a full record of the solar irradiance and environmental conditions, allowing for more accurate benchmarking of on-sun performance. PEC device characteristics were probed via current-voltage and chronoamperometry measurements under both natural and simulated sunlight illumination, while material structure is probed by XPS and SEM. The MoS2-protected tandem absorbers exhibit >10% STH efficiency under both laboratory and on-sun conditions. During a day of PEC testing under natural sunlight, >11 standard mL of hydrogen is generated with no applied bias. We will discuss challenges in catalyst and semiconductor stability and in cell design that limit PEC durability and hydrogen yield. References: (1) 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. (2) Young, J. L.; Steiner, M. A.; Döscher, H.; France, R. M.; Turner, J. A.; Deutsch, T. G. Direct Solar-to-Hydrogen Conversion via Inverted Metamorphic Multi-Junction Semiconductor Architectures. Nat. Energy 2017, 2 (4), 17028. (3) Khaselev, O.; Turner, J. A. A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting. Science 1998, 280 (5362), 425–427. (4) Cheng, W.-H.; Richter, M. H.; May, M. M.; Ohlmann, J.; Lackner, D.; Dimroth, F.; Hannappel, T.; Atwater, H. A.; Lewerenz, H.-J. Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. ACS Energy Lett. 2018, 3 (8), 1795–1800. (5) HydroGEN Consortium. On-Sun Photoelectrochemical Solar-to-Hydrogen Benchmarking https://www.h2awsm.org/capabilities/sun-photoelectrochemical-solar-hydrogen-benchmarking.

Porous transport layers (PTLs) play an important role in proton exchange membrane water electrolysis (PEMWE) cells. The PTL facilitates water and gas transport, as well as thermal and electrical conduction, and is required to sustain good contact with adjacent components. It is expected that using PTLs with variations in material properties such as structure, composition, thickness and wettability results in performance changes of the PEMWE. In this study, a general mathematical PEMWE model is developed that separates and analyzes the contributions of ohmic, activation, diffusion and Nernst potentials. For model validation, three inherently different anode PTL structures (carbon paper, sintered titanium particles, and titanium felt) are operated over a range of conditions. Additionally, the effects of PTL wettability were used to verify the model using Polytetrafluoroethylene (PTFE) treated Toray papers with PTFE loading ranging from 0% to 20%. The modeling results of both PTFE treated and untreated materials show good agreement with the experimental data. Mass transport or diffusion loss is the primary reason for performance differences between PTFE treated and untreated PTLs. Sintered titanium PTLs with thicknesses above 1 mm suffer from up to 33% increased ohmic losses without indicating any obvious changes in activation and diffusion losses when compared to untreated PTLs. The losses of the cell increase when using PTFE treated Toray paper. Individual contributions are quantified and assigned to increased ohmic, activation, and diffusion losses. In conclusion, the proposed model offers insights into the overpotential contributions of a PEMWE. It is a useful tool for predicting performance of various PTL materials and can be applied for PTL development and optimization efforts.

Catalytic interface of semiconductor photoelectrodes is critical for high-performance photoelectrochemical solar water splitting because of its multiple roles in light absorption, electrocatalysis, and corrosion protection. Nevertheless, simultaneously optimizing each of these processes represents a materials conundrum owing to conflicting requirements of materials attributes at the electrode surface. Here we show an approach that can circumvent these challenges by collaboratively exploiting corrosion-resistant surface stoichiometry and structurally-tailored reactive interface. Nanoporous, density-graded surface of ‘black’ gallium indium phosphide (GaInP2), when combined with ammonium-sulfide-based surface passivation, effectively reduces reflection and surface recombination of photogenerated carriers for high efficiency photocatalysis in the hydrogen evolution half-reaction, but also augments electrochemical durability with lifetime over 124 h via strongly suppressed kinetics of corrosion. Such synergistic control of stoichiometry and structure at the reactive interface provides a practical pathway to concurrently enhance efficiency and durability of semiconductor photoelectrodes without solely relying on the development of new protective materials., Photoelectrochemical water splitting presents an integrated means storing sunlight into fuels, yet high optical losses and corrosion limit device performance. Here, authors boost absorption and durability in gallium-indium phosphide photocathodes via a sulfurized and nanostructured protection layer.

Illumination characteristics from artificial sources strongly influence the experimental performance of solar water-splitting devices, with the highest impact on tandem structures designed for optimum conversion efficiency. We highlight quantitative and qualitative flaws of common characterization techniques, discuss their impact on research results and strategy, and demonstrate approaches toward advanced measurement accuracy.

We report on the remarkable stability of unmodified, epitaxially grown GaAs photocathodes during hydrogen evolution at −15 mA cm−2 in 3 M sulfuric acid electrolyte. Contrary to the perception regarding instability of III–V photoelectrodes, results here show virtually no performance degradation and minimal etching after 120 hours.

Solar photoconversion in semiconductors is driven by charge separation at the interface of the semiconductor and contacting layers. Here we demonstrate that time-resolved photoinduced reflectance from a semiconductor captures interfacial carrier dynamics. We applied this transient photoreflectance method to study charge transfer at p-type gallium-indium phosphide (p-GaInP2) interfaces critically important to solar-driven water splitting. We monitored the formation and decay of transient electric fields that form upon photoexcitation within bare p-GaInP2, p-GaInP2/platinum (Pt), and p-GaInP2/amorphous titania (TiO2) interfaces. The data show that a field at both the p-GaInP2/Pt and p-GaInP2/TiO2 interfaces drives charge separation. Additionally, the charge recombination rate at the p-GaInP2/TiO2 interface is greatly reduced owing to its p-n nature, compared with the Schottky nature of the p-GaInP2/Pt interface.

The p-type semiconductor GaInP2 has a nearly ideal bandgap (∼1.83 eV) for hydrogen fuel generation by photoelectrochemical water splitting but is unable to drive this reaction because of misalignment of the semiconductor band edges with the water redox half reactions. Here, we show that attachment of an appropriate conjugated phosphonic acid to the GaInP2 electrode surface improves the band edge alignment, closer to the desired overlap with the water redox potentials. We demonstrate that this surface modification approach is able to adjust the energetic position of the band edges by as much as 0.8 eV, showing that it may be possible to engineer the energetics at the semiconductor/electrolyte interface to allow for unbiased water splitting with a single photoelectrode having a bandgap of less than 2 eV.

Our goal is to improve solar-to-hydrogen (STH) efficiency from just over 10% to over 20% via novel tandem semiconductor materials and configurations. Our primary focus is to develop inverted metamorphic multijunction (IMM) III-V semiconductors that have bandgaps optimized for water splitting. We will also discuss measurement challenges in appraising STH efficiency, some of which are specific to tandem absorbers. Using more stringent measurement standards, we have confirmed 16.2% STH on our most advanced IMM devices.

Solar water splitting via multi-junction semiconductor photoelectrochemical cells provides direct conversion of solar energy to stored chemical energy as hydrogen bonds. Economical hydrogen production demands high conversion efficiency to reduce balance-of-systems costs. For sufficient photovoltage, water-splitting efficiency is proportional to the device photocurrent, which can be tuned by judicious selection and integration of optimal semiconductor bandgaps. Here, we demonstrate highly efficient, immersed water-splitting electrodes enabled by inverted metamorphic epitaxy and a transparent graded buffer that allows the bandgap of each junction to be independently varied. Voltage losses at the electrolyte interface are reduced by 0.55 V over traditional, uniformly p-doped photocathodes by using a buried p–n junction. Advanced on-sun benchmarking, spectrally corrected and validated with incident photon-to-current efficiency, yields over 16% solar-to-hydrogen efficiency with GaInP/GaInAs tandem absorbers, representing a 60% improvement over the classical, high-efficiency tandem III–V device. Solar water-splitting efficiency can be enhanced by careful bandgap selection in multi-junction semiconductor structures. Young et al. demonstrate a route that allows independent bandgap tuning of each junction in an immersed water-splitting device, enabling a solar-to-hydrogen efficiency of over 16%.

The properties of porous transport layers (PTL) in electrolysis devices and their effects on cell performance have been studied extensively in recent literature. This paper provides a detailed analysis with regards to the transport in the catalyst layer (CL). The work demonstrated that the catalyst loading affects the sensitivity of electrolysis performance to PTL properties, particularly those of the PTL surface in contact with the CL. It was demonstrated that upon reducing catalyst loading, PTL properties had an increased effect on the performance of PEMWE cells. While we observed mild performance variations among PTLs when using a high anode catalyst loading, strong correlations between PTL surface properties and cell performance existed at a low catalyst loading. PTL properties affected performance by influencing the in-plane conductivity and permeability of the CL. The variation of apparent exchange current density and apparent CL bubble coverage with the stoichiometric flow rate was studied at low anode feed rates. This led to the emergence of a PTL grain size effect on apparent bubble coverage at high catalyst loading. We provide a descriptive analysis of the phenomena causing voltage losses in PEMWE devices. These findings are important for electrochemical modeling and designing the PTL/CL interface.

Polymer electrolyte membrane water electrolysis (PEMWE) is a promising technology for the integration and utilization of renewable energy and hydrogen as an energy carrier. Advancements in catalysis have made the commercial viability outlook very encouraging. Nonetheless, more work is necessary to advance the technology from niche markets towards broad market penetration. Through this study, we increase understanding of transport and interfacial phenomena, which can be used to design more efficient electrolyzers and improve cell performance. Preventing excessive oxygen accumulation in the catalyst layer (CL) by adjusting the geometry and properties of flow channels, the porous transport layer (PTL), and the CL itself may result in increased catalyst utilization, efficiency, and performance. Controlling feed water to allow a significant increase in CL temperature may also improve efficiency. In order to optimize performance without operating under conditions that may affect cell durability, PEMWE scale-up relies on robust models that accurately predict current and temperature distributions. Such models, particularly 3-D flow models, provide the opportunity to improve electrolysis efficiency through process engineering. Electrochemical models have been commonly used to fit known relationships to the measured performance of PEMWE devices. Computational modeling is a lean approach to enhancing technology without the cost and duration of experimentation, albeit via the quantitative understanding of prior empirical observations. This work investigated the effects of experimental parameters on transport, highlighting the need to accurately model temperature elevation and gas inhabitation in the CL. This was done in order to develop and validate a 3-D PEMWE model that predicts current distribution and overall cell efficiency using computational fluid dynamics (CFD) simulations. The following experiments examined a PEMWE device under a wide range of conditions in order to uncover information about multiscale phenomena that may improve the model. The experimental parameter space was designed minding the importance of the through-plane pressure drop in the PTL and the temperature rise in the CL. It included variation of current density, inlet water flow rate, temperature, and properties of cell components. Polarization curves and complimentary Nyquist plots from electrochemical impedance spectroscopy (EIS) were used in addition to inlet/outlet temperatures and cell pressure drop to observe mass and heat transport. A summary of the findings from the main validation experiments and their interpretations will be discussed.

Hydrogen production through proton exchange membrane water electrolysis (PEMWE) is regarded as a very promising technology that offers a highly efficient and robust path to store electrical energy in form of hydrogen and further enables the integration of renewable energy into the electrical grid 1-3. The total cell voltage of a single PEM water electrolyzer cell is composed of the open circuit voltage, activation overpotential, diffusion overpotential, and ohmic loss overpotential 4. The total voltage of the cell can be expressed as Eq(1) 5. V=VOCV+Vact+Vohm+Vdiff (1) Where VOCV is the Nernst potential which is also called the reversible voltage and can be calculated from the Nernst equation or Gibbs free energy, Vact is the activation overpotential due to the electrochemical reaction and it represents the overpotential to drive the electron transfer and electrochemical reaction kinetics as described by the Butler-Volmer model, Vohm is the ohmic overpotential caused by the resistances of the electronic/ionic components and interfacial contact resistances, and Vdiff is the diffusion overpotential related to mass transport. Porous transport layers (PTLs) are key components in PEMWE. They facilitate mass transport, thermal and electrical conduction, and are required to sustain a good contact with adjacent components 6-8. It is expected that various PTLs with different structures and wettability exhibit nonidentical performance in PEMWE. In this study, a general model is established to investigate the PTL parameters of four inherently different PTLs, including non-treated Toray paper, PTFE treated Toray paper, sintered titanium particles and Ti felt. SEM images highlighting the structures of these PTL materials are shown in Figure 1. The materials, which differed in thickness, porosity, material and composition were evaluated with identical MEAs over a range of operating conditions for model validation. The resulting data indicate that the hydrophobic additives in the Toray paper will decrease the PEMWE performance due to increased mass transport loss and ohmic loss. Ti felt with similar thickness and porosity as non-treated Toray paper showed slightly reduced performance due to its higher ohmic loss. Sintered Ti particle PTLs exhibit much higher ohmic resistance than Ti felt and nontreated Toray paper, which may be attributed to its significantly larger thickness and/or an increased contact resistance. The experimental data was employed to validate the MATLAB/Simulink model which enables the simulation of the contribution of each overpotential. The model was used to successfully describe the effects of PTFE loading on PEMWE performance, as shown in Figure 2. The hydrophobic treatment resulted, as expected, in detrimental performance effects. The diffusion loss of a 20% PTFE treated Toray paper is significantly higher than that of the nontreated PTLs (Figure 2). The modeling results also indicate that the change of the wettability of the PTLs by hydrophobic additives/agents, impacted not only the diffusion loss significantly, but also affected the ohmic loss and activation loss. This was specifically detrimental to PEMWE cell performance when operating at high current densities. The model that was validated in this study can be used to predict the effects of key PTL parameters and utilized to optimize PEMWE performance. Reference Bender, G.; Carmo, M.; Smolinka, T.; Gago, A.; Danilovic, N.; Mueller, M.; et al, Initial approaches in benchmarking and round robin testing for proton exchange membrane water electrolyzers. International Journal of Hydrogen Energy 2019, 44 (18), 9174-9187. Pivovar, B.; Rustagi, N.; Satyapal, S., Hydrogen at Scale (H2@ Scale): Key to a Clean, Economic, and Sustainable Energy System. The Electrochemical Society Interface 2018, 27 (1), 47-52. Kang, Z.; Yang, G.; Mo, J.; Li, Y.; Yu, S.; Cullen, D. A.; et al, Novel thin/tunable gas diffusion electrodes with ultra-low catalyst loading for hydrogen evolution reactions in proton exchange membrane electrolyzer cells. Nano Energy 2018, 47, 434-441. Rahim, A. A.; Tijani, A. S.; Kamarudin, S. K.; Hanapi, S., An overview of polymer electrolyte membrane electrolyzer for hydrogen production: Modeling and mass transport. Journal of Power Sources 2016, 309, 56-65. Han, B.; Steen III, S. M.; Mo, J.; Zhang, F.-Y., Electrochemical performance modeling of a proton exchange membrane electrolyzer cell for hydrogen energy. International Journal of Hydrogen Energy 2015, 40 (22), 7006-7016. Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., A comprehensive review on PEM water electrolysis. International journal of hydrogen energy 2013, 38 (12), 4901-4934. Lettenmeier, P.; Kolb, S.; Sata, N.; Fallisch, A.; Zielke, L.; Thiele, S.; et al, Comprehensive investigation of novel pore-graded gas diffusion layers for high-performance and cost-effective proton exchange membrane electrolyzers. Energy & Environmental Science 2017, 10 (12), 2521-2533. Kang, Z., Development of Novel Thin Membrane Electrode Assemblies (MEAs) for High-Efficiency Energy Storage. 2018. Figure 1

This talk will give an overview of the HydroGEN Photoelectrochemistry (PEC) Supernode and present recent progress. This Supernode project is a collaboration of nine PEC capability nodes that are part of the HydroGEN Advanced Water Splitting Materials (AWSM) consortium. These nodes are contributing to a common objective of developing an understanding of integration issues and emergent degradation mechanisms of PEC devices upon scale up above 1cm2 active areas. Larger devices are expected to introduce new component integration challenges and pathways for degradation that this PEC Supernode project will identify and propose pathways for future progress. The objective of this Supernode includes: 1) Photoreactor chassis fabrication and baselining, 2) development, fabrication, and testing of larger area PEC devices, 3) development of in-situ morphology and corrosion analysis techniques and characterization, and 4) analysis of durability mechanisms and impacts using modeling LCA and TEA. A new photoreactor chassis will be presented that has been designed to facilitate indoor and on-sun efficiency and durability benchmarking measurements. The design will be validated in a preliminary set of multi-institution round robin measurements toward understanding and harmonizing measurement conditions and protocol. Furthermore, we will present initial STH, durability and degradation characterization results upon scale up to 8 cm2 active area of integrated vapor fed PEC cell.

Hydrogen generation via proton exchange membrane water electrolysis is one technology that offers a pathway to store large amounts of electrical energy in the form of hydrogen and enables the integration of renewables into the grid. In order to realize a large market penetration of carbon free hydrogen, produced through PEM water electrolyzers, high capital and operating costs need to be reduced via rapid and successful research and development. This is only possible if a meaningful comparison of performance across laboratories can be achieved and demonstrated. As shown in Figure 1, performance data in the literature of nominally identical systems vary significantly. For example, up to 200 mV deviation was reported at current densities of 1 A/cm2for cells that employed Nafion 117 membranes (Figure 1) [1]. These deviations may originate from using different materials, hardware, test equipment, and operating conditions and procedures. The data confirm an obvious need for standardization of materials and testing protocols to enable such a comparison in the low temperature electrolysis field. Herein, we address this need by reporting on the latest results of a round robin test effort conducted by contributors to the IEA Electrolysis Annex 30. Building upon a method and equipment framework developed and verified in an iterative process, the work recently expanded to include titanium based porous transport layer materials and pressure capable hardware. As shown in Figure 2, initial results with these material sets between laboratories (Figure 2, right) indicate a similar if not reduced deviation between laboratories when compared to the results of the first study (Figure 2, left). The presentation will address the method and thought process selected for harmonizing low temperature electrolysis performance experiments and defining equipment requirements. Latest results and progress of the round robin phase II effort will be reported and discussed and further action summarized. References: Bender, M. Carmo, T. Smolinka, A. Gago, N. Danilovic, M. Mueller, F. Ganci, A. Fallisch, P. Lettenmeier, K.A. Friedrich, K. Ayers, B. Pivovar, J. Mergel, D. Stolten, „Initial approaches in benchmarking and round robin testing for proton exchange membrane water electrolyzers”, International Journal of Hydrogen Energy, Vol. 44, Issue 18, 2019, P. 9174-9187, https://doi.org/10.1016/j.ijhydene.2019.02.074 Lickert, M. Carmo, G. Bender, B. Pivovar, T. Smolinka, S. Fischer, J. Young, A. Kang, P. Kadam, A. Fallisch, „Round Robin Testing Activities – Phase 2”, IEA Annex 30 Workshop, Hannover, March 2019 Figure 1

This talk will give an overview of the HydroGEN Photoelectrochemistry (PEC) Supernode and present recent progress. This Supernode project is collaboration of nine PEC capability nodes that are part of the HydroGEN Advanced Water Splitting Materials consortium. These nodes are contributing to a common objective of developing understanding of integration issues and emergent degradation mechanisms of PEC devices beyond the current research scales that typically does not exceed 1 cm2 active areas. Larger devices are expected to introduce new component integration challenges and pathways for degradation that this PEC Supernode project will identify and propose pathways for future progress. The PEC Supernode project tasks include: 1) Photoreactor chassis fabrication and baselining, 2) development, fabrication, and testing of larger area PEC devices, 3) development of in-situ morphology and corrosion analysis techniques and characterization, and 4) analysis of durability mechanisms and impacts using modeling LCA and TEA. A new photoreactor chassis will be presented that has been designed to facilitate indoor and on-sun efficiency and durability benchmarking measurements. The design will be validated in a preliminary set of multi-institution round robin measurements toward understanding and harmonizing measurement conditions and protocol. This preliminary protocol and initial set of round robin results will be presented.