Symposium ES11—Advanced Low Temperature Water-Splitting for Renewable Hydrogen Production via Electrochemical and Photoelectrochemical Processes

Currently over 95% of hydrogen is made from fossil fuels through natural gas reforming or coal gasification. A major part of reducing carbon emissions must therefore include sources of renewable hydrogen. In addition, capture of existing carbon dioxide sources and subsequent conversion to higher value chemicals also requires a hydrogen source. Using hydrogen from traditional sources for this purpose does not make environmental sense, since the net effect is just shifting where the carbon dioxide is emitted geographically. Large scale, cost competitive sources of renewable hydrogen are critical in moving to more sustainable processes worldwide.

Low temperature electrolysis has decreased significantly in cost while increasing in scale over the last several years, and there continue to be large opportunities for additional cost reduction and performance improvements. With the advent of low cost renewable electricity, particularly in times of excess capacity when storage is most needed, electrolysis is reaching the production and cost scale to provide a viable solution. However, as research activities have increased, differences in testing conditions, reporting comparisons, cell designs, and other factors convolute data interpretation and can lead to misleading conclusions. In addition, insufficiently controlled procedures can mask effects. This talk will describe some of the complex interactions that need to be considered, and current efforts in developing community-wide tests ranging from ex-situ material screening protocols to realistic device testing. This work is part of an ongoing program across all of the major advanced water splitting technologies, the subject of this symposium and the parallel high temperature water splitting symposium.

Cost competitive hydrogen generation at scale using sunlight and water can play a significant role in decarbonization of our society. In recent years, photoelectrochemical (PEC) water splitting has made significant progress in materials discovery, mechanistic understanding, as well as in achieving un-precedent solar to hydrogen conversion efficiencies in the laboratory scale. To further advance this technology and reach the bench scale (0.1 kg/day) and sub-scale (2 kg/day) operational platforms, development of standard protocols and best practices for benchmarking materials and devices as a community is critical. Standardized methodologies and best practices also provide significant benefit in accelerated RD&D of advanced materials and cross-cutting efforts.

In this presentation, I will talk about recent community-based efforts in defining targets, best practices and gaps in PEC hydrogen production in HydroGEN Energy Material Network (EMN) consortium. Specific challenges related to PEC will be discussed including scale up challenges, integration challenges at the component level, lack of performance data and test protocols for photoelectrodes under real-world operating conditions and lack of fundamental understanding of photoelectrode corrosion mechanisms and accelerated test protocols. In addition, I will also present a briefly summary and status reports on lab EMN nodes/capabilities for PEC water-splitting. Key takeaways from a recent HydroGEN Benchmarking&Protocols Workshop and DOE PEC Working Group Meeting will also be discussed.

Hydrogen production via water electrolysis is considered to be an efficient way to smoothen the fluctuating power output of renewable energy sources and avoid oversupplies. This presentation will provide the status of the standards and protocols development for high temperature electrolysis (HTE) technology within the US DOE HydroGEN Energy Materials Network Consortium. The current material standards, advanced characterization techniques, best practices and testing protocols from screening of fundamental material properties to device level benchmarking will be discussed to address key challenges and leverage technical advancements across the different water splitting technologies.

This presentation will provide the status of standards and protocol development within the US DOE HydroGEN Energy Materials Network (EMN) Consortium for advanced water splitting using concentrated solar thermochemical redox cycles, generally referred to as STCH, solar thermochemical hydrogen,.

Cost competitive water splitting to produce hydrogen at scale using concentrated sunlight can contribute significantly to a net CO₂ neutral society. In recent years, STCH has made significant progress. Working as a community will further advance STCH technology with a goal of reaching demonstration scales of multiple kilowatts with promising redox active materials. The development of standard protocols, definitions, metrics, best practices, and roadmaps for benchmarking materials and reactor designs is critical to accelerate progress and lower barriers for new researchers.

In this presentation, we will discuss recent HydroGEN EMN community-based efforts in defining metrics, best practices, and gaps in STCH. In addition, we will present a brief summary of EMN nodes/capabilities as well as key conclusions from a recent questionnaire and from a recent HydroGEN Benchmarking & Protocols Workshop specifically for STCH.

Energy storage in chemical bonds has more than an order of magnitude larger energy density than even the most powerful batteries (~142MJ/kg H2 vs ~9MJ/kg Li-ion batteries). The former, therefore, promises to be essential for the cost-effective development of renewable energy sources, like solar and wind. The main challenges for practical water splitting devices that we have worked towards resolving are: 1) electrocatalysts made from earth-abundant materials, 2) lowering the anode overpotential for the O2 evolving reaction at practical current densities (> 100 mA/cm2), 3) the creation of thin films of these electrocatalysts for application to integrated photoelectrochemical cells operating at 1 to 10 suns; and 4) side-by-side direct comparison of High Performance (GaInP2/GaAs tandem) vs. High Value (SrNbO2N) photovoltaic semiconductors interfaced to earth-abundant catalysts.
Towards goal #1, we have developed two families of electrocatalysts for the two half-reactions: anode OERi: Cubic LiCoO2 and LiCo2O4 have out-performed commercial Iridium and Ruthenium oxides in independent testing in alkaline media up to 400 mA/cm2; and, HERii,iii: Ni5P4 (and to a lesser extent Ni3P) which on a geometric area basis equals the platinum benchmark and are both highly robust in acidic and alkaline media. Toward goal #2, we have developed optically thin films of the Ni5P4 catalyst on silicon and III-V substrates by Physical Vapor Deposition (PVD) that exhibits excellent performance at 1 sun continuous operation (> 120 h). This advance was made possible by including a thin protection layer of TiN to prevent both interdiffusion during high temperature synthesis and preventing surface oxidation. We have developed fabrication methods to prepare HP photocathodes using GaInP2/GaAs absorber and Ni5P4 which achieves 11.5% STH efficiency, at the state-of-the-art for this configuration. For OER we have developed a dark anode/OER catalyst interface comprised of spinel LiCo2O4 electrodeposited on Ti/TiO2 which avoids formation of unstable Co3O4. This interface has operated in electrolysis mode for 13 days continuously without failure at 430 mV overpotential (~7 mA/cm2). Reducing the OER overpotential for operation at higher current densities is a future thrust. Research supported by joint NSF-CBET/DOE-EERE water splitting, and DOE-EERE-HydroGEN.
i Gardner, G.; Al-Sharab, J.; Danilovic, N.; Go, Y. B.; Ayers, K. E.; Greenblatt, M.; Dismukes, G. C. Energy Environ. Sci. 2015, 9, 3–9.
ii Laursen, A. B.; Patraju, K. R.; Whitaker, M. J.; Retuerto, M.; Sarkar, T.; Yao, N.; Ramanujachary, K. V.; Greenblatt, M.; Dismukes, G. C. Energy Environ. Sci. 2015, 8 (3), 1027–1034.
iii Laursen, A. B.; Wexler, R. B.; Whitaker, M. J.; Izett, E. J.; Calvinho, K. U. D.; Hwang, S.; Rucker, R.; Wang, H.; Li, J.; Garfunkel, E.; Greenblatt, M.; Rappe, A. M.; Dismukes, G. C. ACS Catal. 2018, 8 (5), 4408–4419.

Mesoporous ZnFe₂O₄ and LiFe₅O₈ thin film photoanodes were fabricated by sol-gel synthesis,[1,2] using a polymer-templating approach. By means of different amphiphilic diblock- and triblock copolymers, large and partly ordered mesopores are obtained after dip-coating by evaporation-induced self-assembly followed by heat treatment. Scanning electron microscopy (SEM) confirms the porous morphology, and Raman spectroscopy and XRD Rietveld analysis revealed phase pure spinel ferrite photoanodes.
Furthermore, photocurrent and Mott-Schottky measurements were performed at different pH values to determine the flat band potential and photocurrent density of the thin film electrodes calcined at various temperatures. By varying the block-copolymers, altered pore structures of the photoanodes and their influence on photoelectrochemical performance will be discussed.

[1] K. Kirchberg, S. Wang, L. Wang, R. Marschall, Chem. Phys. Chem. 19 (2018) 2313-2320.
[2] S. Waitz, C. Suchomski, T. Brezesinski, R. Marschall, Chem. Photo. Chem (2018), DOI: 10.1002/cptc.201800154

Nanostructured forms of TiO₂ have been extensively investigated as photoactive materials for energy-related applications, such as dye-sensitized solar cells, photocatalysis and water splitting. Annealing in reducing atmospheres has been frequently employed as an approach to increase TiO₂ performance; in this way, defects such as oxygen vacancies and Ti³⁺ sites are formed, leading to increased light absorption, higher n-type conductivity, and new catalytic sites with enhanced reactivity [1]. These defects often induce a series of discrete electronic states in the bandgap with similar energies and their characterization is of pivotal importance for a deep understanding of their role in enhancing the material performance; this aspect is still an open topic in the current research. A powerful tool to probe such defect states is photoluminescence (PL) spectroscopy. For anatase TiO₂, in particular, a broad visible emission has been reported, with two main components in the green (green PL) and in the red (red PL) regions associated with holes and electrons in deep traps, respectively [2]. However, a definitive framework is still lacking: PL properties depend on the material porosity/morphology and detailed investigations concerning the effect of reducing thermal treatments and the comparison between different excitation wavelengths (i.e. above and below the bandgap energy) are very limited.

This work presents a detailed PL spectroscopy investigation on thin nanostructured TiO₂ films prepared by Pulsed Laser Deposition. To selectively discern the luminescent traps, the effects of i) different degrees of defectivity, related to oxidizing (air) and reducing (Ar/H₂, vacuum and pure H₂) post-deposition annealing treatments, ii) above- and below-bandgap excitation energies, and iii) surrounding medium (air, ethanol and vacuum) are considered. All the films exhibit the pure anatase crystalline phase, as shown by Raman spectroscopy and X-ray diffraction, and a low concentration of Ti³⁺ sites, as evidenced by X-ray photoelectron spectroscopy and Electron Paramagnetic Spectroscopy. However, differences between the films emerge in PL spectra. First, the effect of the excitation wavelength with air as surrounding medium is considered. With UV excitation (325 nm) all the films exhibit a wide PL band centered around 1.8-2.0 eV; with all the visible excitations (457 nm, 514.5 nm and 633 nm), a similar PL emission is found for the films treated in reducing atmospheres, but not for the air-annealed film. These emissions are associated deep Ti³⁺ states (red PL mechanism) and the different behavior of the air-annealed film is ascribed to its low concentration of Ti³⁺ ions. Second, the effect of the surrounding media in PL measurements is considered. With both UV and green excitation, the PL emission measured in vacuum do not substantially change from that measured in air for all the films; conversely, only with UV excitation and in ethanol, the PL of the air-annealed film is quenched and that of the other films exhibits a blueshift, compatible with the green PL mechanism. Finally, a tentative interpretative model considering all these observations is proposed.

The present findings confirm how PL spectroscopy can efficiently probe defect states in reduced TiO₂, further developing the results presented in previous reports and suggesting a complex relationship between PL and photoactivity. Follow-up studies on defects in TiO₂ materials are foreseen, considering different phases (i.e. brookite) or morphologies (i.e. nanotubes), as well as on other photoactive materials; such studies may allow a deeper understanding on the relationship between defects and photoactivity, yielding to higher material performance.

[1] X. Chen et al. Chem. Soc. Rev. 2015, 44, 1861; L. Mascaretti et al. Sol. Energy Mater. Sol. Cells 2017, 169, 19.
[2] F. J. Knorr et al. J. Phys. Chem. C 2008, 112, 12786; D. K. Pallotti et al. J. Phys. Chem. C 2017, 121, 9011.

Atmospheric oxygen that we live on is the result of a fundamental evolutionary step in photosynthesis which occurred on Earth 2-3 billion years ago [Biello 2009]. The water oxidation in natural photosystem II accounts for 50% of this oxygen. The man-made analog to this natural process is water oxidation in photoelectrochemical cells - one of the most complex processes in physical chemistry. I will showcase how modern x-ray spectroscopy methods have assisted in the understanding of the molecular processes which occur in natural <!--[endif]---->[Bora 2013, Ralston 2000, Visser 2001] and in man-made "photosystems". The operando and in situ analyses of the aforementioned processes is still a major technical challenge. A most recent example is the element and orbital specific identification of transient electron holes in iron oxide [Braun 2012], chemical surface intermediates [Bora 2011] and changes in the water molecules in the electrochemical double layer during photoelectrochemical water oxidation [Braun 2016]. I will also demonstrate how the electronic structure evolution is quantitatively paralleled by the electric transport properties of the electrodes during operation.

[Biello 2009] Biello D: The Origin of Oxygen in Earth's Atmosphere. In Scientific American 2009.
[Bora 2011] Bora DK et al. Journal of Physical Chemistry C 2011, 115:5619-5625.
[Bora 2013] Bora DK et al. J. Electr. Spec. Rel. Phenom. 2013, 190:93-105.
[Braun 2012] Braun A et al. Journal of Physical Chemistry C 2012, 116:16870-16875.
[Braun 2016] Braun A et al. Catalysis Today 2016, 260:72-81.
[Ralston 2000] Ralston CY et al.Journal of the American Chemical Society 2000, 122:10553-10560.
[Visser 2001] Visser H et al. Journal of the American Chemical Society 2001, 123:7031-7039.

The development of an efficient, stable photoanode to provide protons and electrons to the (photo)cathode remains a primary materials challenge in the establishment of a scalable technology for artificial photosynthesis. However, the absolute and relative stability and performance of different OER catalyst compositions vary with electrolyte pH. The operational conditions for the solar fuels generator is often dictated by the pH stability range of the light absorber, not the conditions optimal for OER catalysis. We have evaluated the activity and stability against corrosion of over 10,000 multi-metal oxide compositions from pH 1-13 using high throughput electrochemical methods. The experimental and data processing methods and characteristic results are described. The Mn-Sb-O system contained compositions and structures with high acid stability and OER catalytic performance. The characterization of performance and structure of the Mn-rich alloys in the rutile oxide structure will be described. The typical photoanode architecture consists of a semiconductor light absorber coated with a metal oxide that serves a combination of functions, including corrosion protection, electrocatalysis, light trapping, hole transport, and elimination of deleterious surface recombination sites. We describe a high throughput investigation of the variation in performance and photo-response of integrated photoanodes consisting of BiVO₄ coated with a sputter deposited Fe-Ce oxide film as a function of composition and loading at pH 9 and 13.

Photoelectrochemical (PEC) water splitting is one of the potential approaches for the generation of hydrogen which can fulfill the global energy requirement [1]. In developing economical and efficient photo-catalysts for water oxidation, n-type monoclinic BiVO₄ has been widely reported due to its favorable band gap [2]. But the photo conversion efficiency of BiVO₄ catalyst is relatively low due to poor charge transport, fast surface charge recombination and sluggish water oxidation kinetics [3]. In current report, BiVO₄ photocatalyst doped with yttrium(Y) was prepared by ultrasonic spray coating technique to overcome poor charge transport. Various percentage of Y (1,2,3,4,5 wt %) is doped in pristine BiVO₄ to optimize the PEC performance. The 4wt % Y doped BiVO₄ photo-anode has shown highest PEC photo current 2.20 mA/cm² at 1.23 V RHE and 4.5 mA/cm² at 2 V RHE under simulated one sunlight illumination in K₂HPO₄ electrolyte. However, the photocatalytic activity of the other Y: BiVO₄ photo anodes were suppressed as the Y doping level decreased or increased, which could be attributed to the formation of photogenerated charge carrier recombination centers at the Y substituting sites. Here, Y³⁺ dopant replaces Bi³⁺ sites in BiVO₄ lattice to provide expanded light absorption wavelength region from 400-600nm. Further, Y: BIVO4 photoanode were investigated by XRD, XPS and SEM. It was observed from XRD that Y-doping didn’t generate any impurity phases in pristine BiVO₄. XPS analysis showed the Y-peaks 3d3/2 and 3d5/2 at the binding energies 165eV and 160eV respectively, which confirm the oxidation state of Y as +3. The electrochemical impedance and cyclic voltametry measurements showed that the Y doping lowered the interfacial resistance, charge transfer resistance and increased the charge donor/surface state density. Further PEC performance was corroborated with changes in band gap, crystal lattice, surface grain morphology occurred due to Y doping in BiVO₄.
Reference:
Tanel Kaambre, Martins Vanags, Rainer Pärna, Vambola Kisand, Reinis Ignatans, Janis Kleperis, Andris Šutkaa, “Yttrium-doped hematite photoanodes for solar water splitting: Photoelectrochemical and electronic properties” Ceramics International 44 (2018) 13218–13225
Hye Won Jeong, Tae Hwa Jeon, Jum Suk Jang, Wonyong Choi, and Hyunwoong Park, “Strategic Modification of BiVO4 for Improving Photoelectrochemical Water Oxidation Performance” J. Phys. Chem. C 2013, 117, 9104−9112
Tae Woo Kim, Yuan Ping, Giulia A. Galli, Kyoung-Shin Choi “Simultaneous enhancements in photon absorption and charge transport of bismuth vanadate photoanodes for solar water splitting” 10.1038/ncomms9769.

BiVO₄ is an attractive candidate as a photoanode for photoelectrochemcial (PEC) water splitting due to its suitable valence band edge position and chemical stability under neutral electrolyte condition. A metal organic decomposition is a facile route to fabricate BiVO₄ photoanodes; however, it often experiences a reproducibility issue. We identified aging duration of vanadium precursor solution, vanadyl acetylacetonate in methanol, as a factor profoundly affecting reproducibility. Substantial changes in structural, optical, and electrical properties of BiVO₄ films are observed upon varying aging time of vanadium precursor solutions, which subsequently impacts PEC water oxidation and sulfite oxidation reactions. With an optimum length of aging days, some deficiency of oxygen is observed, which is accompanied by increase in carrier concentration and reduced charge transfer resistance of the PEC device, leading to a highest PEC performance, comparable to the state-of-the-art undoped BiVO₄ photoanodes. Our findings point to the importance of understanding solution chemistry and utilization of the understanding for fine adjustment of composition of BiVO₄ films in order to produce highly reproducible and efficient BiVO₄ photoanodes.

Electrochemical generation of hydrogen and oxygen during photoelectrochemical water splitting leads to the formation of gas bubbles when the concentration of dissolved gases exceeds a critical concentration in solution. Gas bubbles inhibit the performance of solar-to-fuels devices by reflecting incident sunlight and restricting mass transfer to and from catalysts. Membrane embedded microwire arrays provide a path to fully integrated solar-to-fuels devices, but the removal of gas bubbles from the underside of the device is challenging, as gravity will direct bubbles towards the photoelectrode surface. Studies on silicon-based cathodes in a variety of orientations relative to the gravitational vector will aid in understanding the opportunities and challenges associated with unassisted photoelectrochemical water-splitting at scale.


Arrays of silicon microwires were formed via photolithographic patterning and reactive ion etching and then coated with a sputtered platinum electrocatalyst layer to form cathodes which were stable in acidic media and operated at low catalytic overpotentials for hydrogen evolution. Devices were characterized via electrochemical measurements in 0.5 M H₂SO₄ (aq) coupled with high speed photography. The effects of the wire diameter and packing fraction on the nucleation density and break-off diameter of hydrogen bubbles as a function of current density have been characterized. All electrode designs were found to exhibit significant hydrogen bubble coverage at current densities in excess of 75 mA cm^-2, but at current densities expected for solar-to-fuels devices driven by unconcentrated sunlight—less than 30 mA cm^-2—bubble coverage could be suppressed by appropriate microwire array designs. The operational overpotentials of large area cathodes have been compared as a function of tilt in a downward facing orientation to assess the ability of microwire arrays to remove bubbles without assistance under operating conditions.

Photo-electrochemical water splitting systems at present show low performance compared to theoretical predictions. One of the reasons for this low performance is the inefficiency of oxygen evolution reaction (OER), which occurs at the semiconductor (photoanode)-electrolyte interface. Hence, for improving the performance of such water splitting systems, we need to improve the performance of the semiconductor-electrolyte interface. However, it is still unclear, what the performance limiting processes are at the interface. We present a new approach that allows for simulating - from an atomistic model - the same electrochemical data that is measured experimentally.[1] We develop a micro kinetic model for the OER by combining density functional theory (DFT) and state-space modeling. By comparing theory and experiments, the performance limiting processes at the interface can be identified.
In literature, micro kinetic modeling has been successfully used for metallic electrodes, using Butler-Volmer theory for calculation of reaction rates.[2] However, Butler-Volmer theory is only valid for metal-electrolyte interface and does not describe the charge transfer reactions at the semiconductor-electrolyte interface during water splitting process.[3] We therefore use the Gerischer theory for semiconductors in this study. Hematite (Fe₂O₃) – water interface is the model system used here. We choose the four step OER mechanism proposed by Rossmeisl et al.[4] as the electrochemical model. Based on this mechanism, mass conservation and charge balance equations are formulated. The resulting set of differential equations are represented as a state-space model (SSM); the applied potential is the input and the current density is the output.
The energy of formation of the four OER intermediates at hematite surface are calculated by DFT [5] and are used to calculate the reaction rates. With this input and hematite specific parameters from the literature, electrochemical data such as current-voltage plots and electrochemical impedance spectra are simulated from the state-space model. We can also simulate the surface coverages of all intermediate species as a function of applied potential. This data is experimentally not available, but is of utmost importance for experimental studies on identification of OER intermediates. The simulated impedance from the electrochemical reactions are compared to measured impedance data from in-house experiments.[1] The simulation results are in close agreement with the experimental measurements and capture the dynamics well. Identification of performance limiting processes by comparison with experiments will be discussed. The model is generic and can be extended to study other materials and other interfaces. From a multi-scale modeling perspective, the developed model is a continuum model, which connects first principle calculations and experimental data for semiconductor-electrolyte interfaces. Further, predicting the effect of doping and cocatalyst loading on the surface of the semiconductor electrode using the model will also be discussed.
References
[1] K. George, M. Van Berkel, X. Zhang, R. Sinha and A. Bieberle-Hütter, Submitted to J. Phys. Chem. C.
[2] H. A. Hansen, V. Viswanathan and J. K. Nørskov, J. Phys. Chem. C, 2014, 118, 6706–6718.
[3] L. M. Peter, J. Solid State Electrochem., 2013, 17, 315–326.
[4] J. Rossmeisl, Z. W. Qu, H. Zhu, G. J. Kroes and J. K. Nørskov, J. Electroanal. Chem., 2007, 607, 83–89.
[5] X. Zhang, P. Klaver, R. Van Santen, M. C. M. Van De Sanden and A. Bieberle-Hütter, J. Phys. Chem. C, 2016, 120, 18201–18208.

Despite their outstanding photovoltaic performance and record-high solar-to-hydrogen efficiency, the high production cost and limited photoelectrochemical stability of III–V compound semiconductors prohibit their practical application in solar-driven photoelectrochemical water splitting. Here we present a novel strategy for III-V photoelectrodes that can overcome these obstacles via printing-based assemblies of surface-engineered, epitaxially-grown nanomembranes. Ultrathin (i.e. emitter + base = 300 nm) GaAs photocathodes having a buried pn-junction are released from the growth wafer and printed onto a non-native, transparent substrate to form integrated bifacial photocathodes for the hydrogen evolution half-reaction of solar water splitting. Hexagonally periodic TiO₂ nanoposts are incorporated into the light absorbing interface of ultrathin GaAs to maximize the light absorption, while the n-type ohmic metal, protection layer, and co-catalyst are deposited on the reactive interface to optimize the catalytic efficiency and stability. The resulting bifacial GaAs photocathodes with nanophotonic light management exhibit significantly enhanced saturated current density (from -11.7 mA/cm² to -16.3 mA/cm²) and diagnostic electrode efficiency (from ~8.6% to ~11.3%) compared to the bare GaAs photocathodes without TiO₂ nanoposts, under AM1.5G solar illumination in aqueous sulfuric acid (0.5 M H₂SO₄).

To date, there has been intense research in zinc oxide (ZnO) nanostructures in terms of the variety of morphologies that can be achieved and the availability of low-cost synthesis methods. ZnO is a versatile photo-responsive material which can be used as photocatalyst and photoelectrode for solar cells (e.g. perovskite solar cells). The photocatalytic activity of ZnO can be further enhanced by the extra polarization from the piezoelectric operation under external mechanical forces. Due to their low stability against mechanical stress one-dimensional ZnO such as nanowires and nanorods have inherent limitations for sono-assisted water splitting applications. In this work, we investigate pyramidal ZnO structures for piezoelectric-photoelectrochemical devices by utilizing their higher mechanical stability and photocatalytic activity. The surface of pyramidal ZnO with (112) crystal plane has higher surface energy and photocatalytic activity.
To achieve the ZnO nanopyramid we utilize a modified crystal growth method by adjusting the reaction kinetics and fluid dynamics during chemical vapor deposition (CVD). Synthesis under different experimental parameters such as reaction temperature, reactor design, reaction pressure, and gas ratio are carried out. The morphology is controlled by adjusting experimental parameter and the effects of crystal growth conditions on the piezo-photocatalytic properties are also investigated. The morphology, crystal structure, and optical/electrical properties of synthesized ZnO are characterized by the scanning electron microscopy, X-ray diffraction analysis, and optical spectroscopies, respectively. The piezoelectric and photocatalytic properties is characterized by piezoelectric force microscopy and potentiostat measurements, respectively. The enhanced performance of photoelectrochemical water splitting is tested under the presence of mechanical forces in the form of ultrasonic waves.

The HydroGEN consortium brings together unique National Laboratory experts and tools (nodes) with researchers developing advanced water splitting materials in a variety of technologies: Low and High -Temperature Electrolysis (LTE/HTE), Photoelectrochemical (PEC) and Solar-Thermochemical (STCH) Water Splitting. Lawerence Berkely National Laboratory is a HydroGEN partner lab and has participated in 9 projects since 2018. This poster will highlight:
-Berkley Lab capabilities and expertise available to researchers in LTE, HTE, PEC and STCH fields
-Our collaborative work performed with our partners in LTE, HTE and PEC in the last year
-Benchmarking LTE, PEC and HTE materials performance
-Tackling big scale problems in LTE, PEC and HTE through multi-node, multi-institution projects "supernodes"

CuGa₃Se₅ is a promising absorber material for photoelectrochemical (PEC) water splitting due to its suitable wide band gap and durability in electrolyte solution. Zn_1-xMg_xO is a potential buffer layer for the CuGa₃Se₅ absorber, because of its tunable conduction band offset as a function of Mg concentration. Zn_1-xMg_xO thin film is deposited on the evaporated CuGa₃Se₅ absorber using combinatorial RF sputtering. Prior to the buffer layer deposition, the absorber is treated in Cd²⁺ containing solution. The quality of the CuGa₃Se₅/Zn_1-xMg_xO interfaces as a function of varying Mg is studied based on solid state photovoltaic device performance. PEC performance of the device is characterized in three electrode configuration with SiO₂/Mo/CuGa₃Se₅/Zn_1-xMg_xO as the photocathode, Pt counter electrode, and a reference electrode. To quantify the device performance in relation to the Mg content in the buffer, chopped-light current−voltage (CLIV) measurement is performed using a scanning droplet cell measurement system. The results of these experiments facilitate the understanding of CuGa₃Se₅/Zn_1-xMg_xO photocathode interfaces, and application of CuGa₃Se₅ absorbers for PEC water splitting.
This work was performed as part of the HydroGEN EMN between National Renewable Energy Laboratory (NREL) and University of Hawaii, with support from the Combinatorial (A. Zakutayev), Chalcopyrite (C. Muzzillo), and PEC (T. Deutsch) nodes at NREL.

In this work, we study the performance of Cu₂O as a photocathode for photoelectrochemical water splitting. The morphology of the Cu₂O is engineered to improve the hydrogen generation efficiency through the enlargement of the surface area and increased light absorbance. The nanostructured Cu₂O is synthesized by a two-step method. First, a Cu layer is deposited by e-beam evaporator onto FTO. Second, the Cu₂O nanostructure film is obtained via a solution and thermal oxidation method under nitrogen environment. Due to the p-type semiconductor properties of Cu₂O, the synthesize film can be used as a catalyst for the reduction reaction. This research shows improvement in the photoelectrochemical efficiency of Cu₂O photocathodes by preventing the formation of unreacted Cu layer between Cu₂O and conductive substrate (FTO). Specifically, the effect of unreacted Cu layer is investigated. Cu₂O samples are synthesized with the same surface morphology, but exhibit different unreacted Cu layer thickness between Cu₂O and FTO. The fully reacted Cu₂O photocathode shows an enhanced photocurrent density and improved efficiency in hydrogen evolution compared to Cu₂O with unreacted Cu. The enhanced photoelectrochemical performance of the photocathode without a trace of unreacted Cu layer is attributed to the reduction in charge recombination resulting from the full oxidation of the Cu layer. Experimentally, it is confirmed that unreacted Cu significantly reduces carrier lifetime and negatively affects the photoelectrochemical performance of the photocathode. The elimination of the unreacted Cu layer leads to higher photocurrent density. Morphology of Cu₂O is characterized by scanning electron microscopy. Crystallinity of the samples is analyzed by X-ray diffraction. Absorbance is measured by ultraviolet-visible-near infrared spectrophotometry. The electrochemical properties of the synthesized materials in the electrolyte are analyzed by measuring current density versus voltage and electrochemical impedance spectroscopy.

One proposed integrated solar water splitting device involves two ion-exchange membrane-embedded, semiconducting micro-/nanowire arrays electrically connected as a tandem. Such an electrical connection must be optically transparent, ion permeable, and adhere to each membrane-embedded array. A composite Nafion-Poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) film possesses excellent adhesion and charge transport properties while being more transparent than PEDOT:PSS alone. Understanding the phase-separation behavior between conductive and non-conductive domains via PeakForce Tunneling Atomic Force Microscopy (PF-TUNA) enables insight towards the Nafion-PEDOT interactions that explain bulk electrical properties. A percolative network of conductive PEDOT domains held by a Nafion/PSS bulk enable electron conduction while maintaining the mechanical stability and proton conducting properties of Nafion. Addition of DMSO reduces these domain sizes from μm to nm scale, producing a dramatic >10⁴ fold increase in lateral conductivity. Nanoscale current-voltage sweeps probe the spatial variations of ohmic behavior that lead to macroscale contact resistance at composite/p-Si interfaces. The measured composite properties are input into a sensitivity analysis that guides device design via load-line analysis of a model tandem water splitting device.

High aspect ratio, microstructured photoelectrodes allow for the integration of reflective and opaque electrocatalysts with semiconducting light absorbers, while avoiding losses to the maximum photocurrent density. Microwire and microcone arrays have been fabricated from p-type Si and decorated with thick films of electrochemically deposited CoP or sputtered Pt to prepare photocathodes for the generation of H₂ from 0.5 M H₂SO₄ (aq). P-Si/CoP microwire and microcone arrays produced photocurrent densities of -10 mA cm^-2 at potentials that were 130 mV more positive than optimized planar p-Si/CoP devices. Champion p-Si/CoP microcone array devices exhibited ideal regenerative cell solar-to-hydrogen efficiencies > 3%, and were primarily limited by the photovoltage of the p-Si/CoP junction. Thick (~16 nm) layers of Pt or CoP deposited onto n⁺p-Si microcone arrays yielded light-limited photocurrent densities of ~ 32 mA cm^-2, constituting a reduction in light-limited photocurrent density of 6% relative to bare Si µ-cone-array photocathodes, while maintaining high fill factors and low overpotentials for H₂ production.

For practical hydrogen production via photoelectrochemical (PEC) water splitting, low-cost photoelectrodes that can efficiently operate in neutral electrolytes should be developed. Herein, we present novel solution-based synthesis method enabling the morphology variation of Sb₂Se₃ light absorber, which are nontoxic, low bandgap and earth-abundant materials. The morphology of Sb₂Se₃ films varies from dense particulate planar film to 1-dimensional nanowire-stacked film upon modulating the Sb and Se molar ratio in the precursor ink. The resulting Sb₂Se₃ films have identical composition and bandgap value regardless of initial precursor ratios. The formation mechanism was elucidated by analyzing Se-Sb precursor ink using liquid Raman spectroscopy. The effects of morphology and crystallographic orientation on the electrical and consequently the charge transport properties of Sb₂Se₃ photocathodes are investigated by conductive atomic force microscopy. Sequential deposition of CdS as a buffer layer with TiO₂ and Pt on the morphology controlled Sb₂Se₃ film enables us to reduce a band offset between the Sb₂Se₃ and TiO₂, achieving an enhanced onset potential. An onset potential of 0.55 V versus reversible hydrogen (RHE) electrode was obtained with 11 mA cm^–2 at 0 V versus RHE under 1.5 global illumination in a neutral electrolyte (pH 6.5). Furthermore, stable hydrogen production over 3 h was observed during the course of photocurrent generation by gas chromatography, demonstrating the promising potential of the proposed Sb₂Se₃ photocathodes as efficient PEC water-splitting devices.

The vast majority of fuels and chemicals that are produced and consumed across the globe today are derived from fossil fuels: oil, coal, and natural gas. This is true of H₂ as well, with annual production rate of over 60 billion kg/year, predominantly used in petroleum refining and in fertilizer production. Developing new, sustainable routes to H₂ production using renewable resources (e.g. wind and solar) could play an important role in decarbonizing energy, and motivate new uses of H₂ as a fuel itself and/or reducing agent for lower-carbon footprint processes. Solar-based water-splitting is a promising route to sustainable H₂. This talk will focus on photoelectrochemical (PEC) water-splitting processes, with a focus on photocathodes. To achieve a cost-effective process, ambitious targets involving efficiency, durability, and low-cost must be reached. This talk will describe efforts to develop catalyst materials and associated coatings capable of providing high activity and extended durability to photovoltaic-grade solar-absorbing materials, including silicon, chalcopyrite semiconductors, and III-V materials.

Achieving efficient hydrogen evolution in water using solar light in a stand-alone system without any external bias is a highly desirable but difficult target. Ever since the first report of sunlight induced water splitting using a TiO₂ electrode four decades ago, photoelectrochemical (PEC) water splitting has been intensely investigated using various semiconductor photoelectrodes. However, the main requirement for a photoelectrode in a PEC cell to induce spontaneous hydrogen evolution is the availability of a semiconductor with an appropriate bandgap of approximately 2.5 eV, having a conduction band that is sufficiently negative for H₂evolution and a valence band that is sufficiently positive for O₂ evolution. Unfortunately, devices capable of spontaneous gas evolution are inefficient because the photoelectrodes with a bandgap of around 2.5 eV absorb only a limited portion of the solar spectrum and require an additional bias for overcoming overpotentials.^1,2 One approach to solve this problem of single semiconductor PEC cells is the use of a tandem configuration. In particular, a photoelectrode combined with an appropriate photovoltaic (PV) cell with complementary absorption ability can harvest a broad part of the solar spectrum and deliver a high solar to hydrogen conversion efficiency.³
After introducing the PV-PEC approach for unbiased hydrogen evolution, I will present our work on perovskite solar cell assisted CdS photoanode. The tandem PV-PEC device consisting CdS/TiO₂ photoanode as a top absorber and encapsulated perovskite solar cell as a bottom absorber was developed on a double-side conducting glass using a facile and integrated fabrication process. The fabrication of the integrated device has been achieved using a combination of solution processing and atomic layer deposition techniques. A remarkable highest photocurrent density of 8.2 mA/cm² was obtained for unassisted solar hydrogen generation under AM1.5 G illumination using 0.25 M Na₂S and 0.35 M Na₂SO₃ as an electrolyte.⁴
Next, I will present our recent work on tandem perovskite photovoltaic assisted silicon photoelectrodes. Highly efficient buried junction n-Si based photocathodes were developed by decoupling light harvesting and hydrogen evolution reactions and breaking the tradeoff between light harvesting and light blocking catalysts. Highly efficient semitransparent perovskite cells with appropriate bandgap were developed as a top absorber. The proposed device satisfies the critical requirement of an efficient tandem configuration by providing complementary light absorption with high optical transparency to long wavelength photons and strong PEC conversion from short wavelength photons. Using this configuration, we achieved an unprecedented >17% solar-to-hydrogen conversion efficiency for unbiased solar water splitting.⁵

References:
[1] A. Fujishima, K. Honda, “Electrochemical Photolysis of Water at a Semiconductor Electrode” Nature 238(1972) 37–38.
[2] M. Walter, E. Warren, J. McKone, S. Boettcher, Q. Mi, E. Santori and N. S. Lewis, “Solar Water Splitting Cells”, Chem. Rev., 2010, 110, 6446–6473.
[3] Zhang K., Ma M., Li P., Wang D. H., Park J. H, “Water Splitting Progress in Tandem Devices: Moving Photolysis beyond Electrolysis” Adv. Energy Mater., 6: 1600602.
[4] S. K. Karuturi, H. Shen, T. Duong, P.R. Reddy Narangari, R. Yew, J.W. Leung, K. Catchpole, H.H. Tan, C. Jagadish, “Perovskite Photovoltaic Integrated CdS/TiO2 Photoanode for Unbiased Photoelectrochemical Hydrogen Generation”, ACS Appl. Mater. Interfaces 2018, 10, 28, 23766-23773.
[5] S. K. Karuturi, H. Shen, T. Duong, P.R. Reddy Narangari, H.H. Tan, C. Jagadish, K. Catchpole, “Perovskite-Silicon Tandem Absorbers for High Efficiency Unbiased Solar Water Splitting ”, manuscript in preparation.

Photoelectrochemical (PEC) water splitting has the potential to become an efficient method for renewable hydrogen production. However, the efficiency, projected cost, and durability of lab-scale systems are not yet at the level required to make this technology economically feasible. Amongst all materials studied to date, the chalcopyrite class is arguably one of the best classes for PEC water splitting, as it has already demonstrated low-cost and high photoconversion capabilities as a photovoltaic material. In the context of PEC, our group has shown that co-evaporated 1.65 eV bandgap (E_G) CuGaSe₂ is capable of evolving hydrogen with Faradaic efficiency greater than 85% and generate photocurrent densities over 15 mA/cm2, as measured in a 3-electrode configuration in 0.5M H₂SO₄ under simulated AM1.5G illumination. Unfortunately, CuGaSe₂’s narrow E_G limits its integration as top absorber into a dual junction stacked PEC device (also known as hybrid photoelectrode, HPE). In the present communication, we report on our latest efforts to synthesize wide-E_G (1.8-2.0 eV) chalcopyrites, compatible with the HPE integration scheme, and capable of generating saturated photocurrent densities greater than 10 mA/cm². We present specifically results on E_G tunable Cu(In,Ga)(S,Se)₂, Cu(In,Ga)S₂, CuGa(S,Se)₂ and CuGa₃Se₅. We also discuss some of the strategies developed to improve their surface energetics for the hydrogen evolution reaction, including the use of In₂S₃ n-type buffer layers. We also include recent durability testing of chalcopyrite photocathodes coated with metal oxide protection layers. Finally, we present solid-state techniques and theoretical modeling used by our team to identify possible pitfalls in these material systems and discuss potential paths for improvement.

Converting solar energy to storable chemical fuels such as hydrogen via photoelectrochemical (PEC) water splitting is a promising way to reduce the dependence on fossil fuels and mitigate global climate changes. A chalcogenide compound, Cu(In,Ga)Se₂ (CIGS) has been regarded as a promising candidate for a photocathode due to its proper band position against hydrogen evolution potential. However, for unassisted (i.e., unbiased) water splitting, CIGS photocathode needs to be coupled with another light absorber for additional photovoltages. Here, we investigated PEC performance and the application to bias-free water splitting cells of the ZnS/CdS double-buffered CIGS photocathode. Insertion of the ZnS into a conventional CIGS/CdS structure led to band grading within the CIGS absorber, which was found to be beneficial for the separation and transport of photoexcited charge carries. With the double-buffered CIGS photocathode, a photocurrent density of -35.1 mA/cm² at 0 V_RHE and an onset potential of 0.66 V_RHE were achieved, which are among the highest numbers reported in literature. By forming a tandem device consisting of the CIGS photocathode and a metal halide perovskite photovoltaic cell, we demonstrated solar-to-hydrogen conversion efficiency over 5%. Details of materials, devices characterization, and analysis will be presented.

Intermediate-band oxides with tunable transport energy levels can serve as chemically-stable, transparent conductive protective layers for photoelectrochemical water splitting at scale. They have shown to promote charge separation and/or charge transport to surface-attached co-catalysts or intermediates at specific electrochemical potentials, practically interfacing the otherwise unstable light absorbers with strong acid or base and protecting them against corrosion. The initial re-discovery of “leaky” TiO₂ coatings utilize their Ti³⁺- defect bands to stabilize an entire class of Si, GaP, GaAs, CdTe, BiVO₄ semiconductors, as well as structured Si pillars and GaAs wires for 1000s-hour operation under sunlight. Recently, a ternary protective coating of manganese-alloyed titanium oxide, (Ti,Mn)O_x, had achieved the incorporation of air-stable, highly oxidized cation impurities, e.g. Mn³⁺, into an otherwise insulating TiO₂, with Mn compositions up-to 70% which is 100-fold beyond the thermodynamic solubility of Mn in TiO₂.

The individual redox of the Mn cations composes a continuous Mn-impurity band. On the one hand, this intermediate band enabled charge injection to acid stable catalysts. It lifts the efficiency-stability trade-off of stabilized III-V photoelectrodes along with their corrosion mechanism at various stages of protection, which will be demonstrated with in-situ microscopy and spectroscopy. On the other hand, the intermediate band induced electronic interactions at the nano-interface with underlying (photo-)electrodes, for charge separation and activity improvement. Two new phenomena were observed at the interfaces of few-nm ALD oxides and their coated electrocatalysts: i) three dimensional active sites help stabilize key intermediates and reduce barrier for O-O coupling for water oxidation; and ii) local electron accumulation at the buried interfaces boosted hydrogen evolution activity whilst ALD coatings are porous enough for transport of hydrogen evolution reactants and products.

Semiconductors perform a critical role in photoelectrochemical processes for the conversion of solar energy into fuels. The III-V semiconductors, such as GaP, typically have high mobilities and have resulted in some of the highest efficiency photoelectrodes. However, these materials are generally very susceptible to corrosion as photoanodes in strong acidic or alkaline solutions under illumination or at applied bias. Herein, we investigate and quantify corrosion of n-GaP and a related novel ternary alloy, GaSbP, at applied bias and under illumination in strong acidic electrolyte. In situ UV/vis spectroscopy measurements, along with inductively coupled plasma mass spectroscopy (ICP-MS) were utilized to detect the dissolved semiconductor species at various operating conditions and quantify the faradaic efficiency of the corrosion reaction. Ultimately, surface protection strategies like atomic layer deposition of TiO₂ will be applied to probe their consequences on the semiconductor corrosion as a function of the deposition parameters.

Among many functional materials, amorphous metal oxides, particularly the thin-film morphology synthesized by atomic layer deposition (ALD), are critical components in many modern energy systems, such as batteries,^[1,2] catalysts,^[3–5] solar cells,^[6,7] and photoelectrochemical (PEC) electrodes.^[8,9] One representative example is the use of ALD amorphous TiO₂ films to protect Si PEC electrodes for fuel generation. However, a wide variety of performances have been obtained from ALD TiO₂ by varying the Ti precursor, growth temperature, and thickness despite a longstanding, yet likely overlooked expectation that ALD films are homogeneous.^[10] Our recent observations have shown structural homogeneities in amorphous ALD TiO₂ films identified as metastable intermediates: kinetically trapped structures between the metastable amorphous phase and the equilibrium crystalline phase.^[11,12] These intermediates enable new and enhanced properties by which their presence significantly change the performance of the overall film.
Metastable intermediates represent a nonequilibrium state of matter that impose profound impacts to materials properties beyond our understandings of monolithic and equilibrium systems. We have discovered hidden metastable intermediates in amorphous TiO₂ thin films and their critical role in electrochemical damage. These intermediates have a non-bulk crystal-like structure and exhibit significantly higher electrical conductivity than both the amorphous and the crystalline phases. When these TiO₂ films are applied to protect Si PEC photoanodes, the intermediates can induce localized high electrochemical currents that largely accelerate the etching of the TiO₂ film and the Si electrode underneath. The intermediates can be effectively suppressed by raising their nucleation barrier via reducing the film thickness from 24 to 2.5 nm. The homogeneous amorphous TiO2-film-coated Si photoanodes achieved more than 500 h of PEC water oxidation at a steady photocurrent density of over 30 mA/cm².
These results reveal an additional pathway for optimizing amorphous oxides grown by ALD: controlling the phase composition of the amorphous and intermediate phases is crucial for optimizing the performance of oxide protected electrodes. In addition to thickness, manipulating other deposition parameters such as temperature, precursor choice, pulse and purge times, and doping offer multiple avenues for achieving greater protection performance by impeding the formation of intermediates.

References:
[1] X. Han, Y. Gong, K. (Kelvin) Fu, X. He, G. T. Hitz, J. Dai, A. Pearse, B. Liu, H. Wang, G. Rubloff, Y. Mo, V. Thangadurai, E. D. Wachsman, L. Hu, Nat. Mater. 2016, 16, 572.
[2] A. C. Kozen, C.-F. Lin, A. J. Pearse, M. A. Schroeder, X. Han, L. Hu, S.-B. Lee, G. W. Rubloff, M. Noked, ACS Nano 2015, 9, 5884.
[3] X. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science 2011, 331, 746.
[4] B. J. O’Neill, D. H. K. Jackson, J. Lee, C. Canlas, P. C. Stair, C. L. Marshall, J. W. Elam, T. F. Kuech, J. A. Dumesic, G. W. Huber, ACS Catal. 2015, 5, 1804.
[5] X. Yan, L. Tian, M. He, X. Chen, Nano Lett. 2015, 15, 6015.
[6] C.-Y. Chang, K.-T. Lee, W.-K. Huang, H.-Y. Siao, Y.-C. Chang, Chem. Mater. 2015, 27, 5122.
[7] B. R. Sutherland, S. Hoogland, M. M. Adachi, P. Kanjanaboos, C. T. O. Wong, J. J. McDowell, J. Xu, O. Voznyy, Z. Ning, A. J. Houtepen, E. H. Sargent, Adv. Mater. 2014, 27, 53.
[8] S. Hu, M. R. Shaner, J. A. Beardslee, M. Lichterman, B. S. Brunschwig, N. S. Lewis, Science 2014, 344, 1005.
[9] Y. Yu, Z. Zhang, X. Yin, A. Kvit, Q. Liao, Z. Kang, X. Yan, Y. Zhang, X. Wang, Nat. Energy 2017, 2, 17045.
[10] S. M. George, Chem. Rev. 2010, 110, 111.
[11] J. Shi, Z. Li, A. Kvit, S. Krylyuk, A. V. Davydov, X. Wang, Nano Lett. 2013, 13, 5727.
[12] Y. Yu, C. Sun, X. Yin, J. Li, S. Cao, C. Zhang, P. M. Voyles, X. Wang, Nano Lett. 2018, 18, 5335.

We report for the first time the fabrication of a photoelectrode using a thin film GaN homostructure loaded with Fe₂O₃ as a cocatalyst and its successful photoelectrochemical (PEC) hydrogen gas generation from water. For the PEC characterization of the Fe₂O₃/GaN photoelectrode, it was irradiated using a Xe arc lamp with a power density of 100 mW/cm² for three hours. After this characterization, the photoelectrode did not show signals of photocorrosion or degradation. A photoelectrode with the same GaN homostructure but using NiO as cocatalyst [Hayashi, Ohkawa, et al., Jpn. J. Appl. Phys. 51, 112601 (2012)] was also characterized under the same conditions to compare the PEC performance of both structures. The Fe₂O₃/GaN photoelectrode exhibited a stable H₂ generation rate of 40.6 μmol/(cm²h) and a light-to-hydrogen energy conversion efficiency of 2.6%, which is higher than the value of 2.3% found for the NiO/GaN photoelectrode. In addition, the surface of the Fe₂O₃/GaN photoelectrode was characterized using electron microscopy and energy-dispersive X-ray spectroscopy (EDS).
The thin film GaN homostructure was grown on a c-plane patterned sapphire substrate by metalorganic vapor phase epitaxy and consisted of (top to bottom) unintentionally doped (uid)-GaN (100 nm)/n-GaN (Si-doped, n=3x10¹⁸ cm^-3, 3 µm)/uid-GaN (2 µm). The Fe₂O₃ cocatalyst was deposited through the spin coating of a metalorganic decomposition solution containing 3 wt% of Fe₂O₃. The PEC characterization of the photoelectrode was performed using a Pt wire as a cathode, with both electrodes immersed in 1 M NaOH solution and without applying any external electrical bias between the electrodes. The gas volume generated at the cathode and the photocurrent between the electrodes were recorded during the PEC characterization. The gas was characterized using gas chromatography to verify its H₂ content.
The observations performed through scanning electron microscopy allowed us to identify the presence of circular structures on the Fe₂O₃/GaN surface with diameters between 400 to 600 nm. Utilizing transmission electron microscopy and EDS we confirmed that the Fe₂O₃ deposition formed particles with a “shrub” shape, not a thin film.
The results of this work represent another step towards the efficient generation of H₂ using nitride-based photocatalysts. We demonstrated that Fe₂O₃ improves the photocatalytic energy conversion efficiency and prevents the photocorrosion of GaN thin film structures when they are used as photoelectrodes in alkaline solutions. Thus, Fe₂O₃ can be added to the list of GaN cocatalysts, which expands the technological applications possibilities of nitride-based semiconductors.

In photoelectrochemical (PEC) systems, realization of near-limiting photocurrent densities at the thermodynamic potential for water splitting demands careful design of the optical and electronic properties of the surface films exposed to the electrolyte. We report unassisted water splitting in a photocathode device utilizing tandem heterojunction III-V semiconductor photoelectrodes with absorber bandgaps of 1.78 and 1.26 eV. An initial solar-to-hydrogen efficiency of 19.3 % was obtained in acid electrolytes and an efficiency of 18.5 % was achieved at neutral pH conditions under simulated sunlight¹. Our device approaches 85% of the theoretical limit of realistic water splitting for the light absorber bandgap combination employed in our device. The achievement of high solar-to-hydrogen efficiencies, relative to limiting efficiencies, derives from the ability to tailor light absorption and electron transport in a protective TiO₂ coating that serves as the reactive catalyst support as well as the electrolyte interface. High photocurrent densities require the combination of the antireflection properties of the anatase TiO₂ layer, with the use of an optically transparent Rh nanoparticle surface layer. To achieve optimal light coupling, we carefully simulated the optical properties of the Rh nanoparticles, including parasitic absorption, in combination with a TiO₂ thickness chosen to minimize reflection and verified these parameters through experimental measurements. In addition, favorable band alignment between the semiconductor conduction band and the energy for water reduction, was found to facilitate energy-efficient electron transport at the cathode/water electrolyte interface, is necessary. This improvement in integrated PEC device efficiency constitutes a significant step towards achievement of a target 25% solar-to-hydrogen efficiency needed to fulfill the techno-economic goals for hydrogen production from photoelectrochemical water splitting, though the photoelectrode stability and the origins of activity losses for this configuration will be discussed, and an outlook for designs with extended durability will be discussed.

1. Cheng, W.-H. et al. Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency. ACS Energy Lett.3, 1795–1800 (2018).

Direct photoelectrochemical (PEC) hydrogen production aims to provide a clean and cost-effective solar fuel. Solar-to-hydrogen (STH) conversion efficiency is central to evaluating and comparing research results, and it largely establishes the prospect for successfully introducing commercial solar water-splitting systems. Present measurement practices do not follow well-defined standards, and common methods potentially impact research results and their implications. We demonstrate underestimated influence factors and experimental strategies for improved accuracy[1].

Our focus is tandem devices that have the prospect for greater STH efficiency[2], but increased complexity that requires more careful consideration of characterization practices. We perform measurements on an advanced version of the classical GaInP/GaAs design[3] while considering (i) calibration and adjustment of the illumination light-source; (ii) confirmation of the consistency of results by incident photon-to-current efficiency (IPCE), and (iii) definition and confinement of the active area of the device.

We propose applying the following standards for future PEC performance reporting: (i) traceable disclosure of the illumination-source configuration (lamp, filters, optics, PEC configuration) and/or its measured spectral distribution; (ii) thorough device-area definition including confinement of the illumination area and avoidance of indirect light paths; (iii) complementary IPCE confirmation of the solar-generation potential; (iv) proper consideration of faradaic efficiency. We will also give a brief overview of the PEC measurements and capabilities available at NREL to support academic and industry research.

[1] H. Doescher, J. L. Young, J. F. Geisz, J. A. Turner, and T. G. Deutsch, “Solar to hydrogen efficiency: Shining light on phoelectrochemical device performance,” Energy Environ. Sci. 2015.
[2] H. Doescher, J. F. Geisz, T. G. Deutsch, and J. A. Turner, “Sunlight absorption in water – efficiency and design implications for photoelectrochemical devices,” Energy Environ. Sci. 2014.
[3] O. Khaselev and J. A. Turner, “A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting,” Science. 1998.

Measurements of photoelectrode durability in three-electrode (half-cell) configurations are typically not representative of the results obtained for nominally the same electrodes/materials when tested in a two-electrode (full-cell) configuration. While full-cell measurements are the best proxy to predict performance in a deployed photoelectrochemical water-splitting system, there are few materials that can drive both the water reduction and oxidation half-reactions. Component-level or half-cell testing is the only option for materials unable to perform unassisted water splitting. During this talk the results of multiple durability tests, some with and some without a reference electrode, will be used to evaluate key differences between the two types of testing in an effort to elucidate what leads to the incongruity between half- and full-cell measurements. It is anticipated that with this understanding, the photoelectrochemical water-splitting community can begin to move towards an accepted protocol for long-term durability testing that will have more predictive relevance to realistic device configurations.

Electrochemical interfaces are central to water splitting technologies. Knowledge of the chemical mechanisms occurring at these interfaces is invaluable for materials discovery efforts, guiding synthesis for improved catalytic performance or resistance to corrosion. But accurately characterizing these complex buried interfaces under operating conditions is a significant challenge. The high penetration depth and elemental selectivity of synchrotron X-ray methods are ideal for accessing electrochemical interfaces without significantly limiting the chemistry. In this talk I provide an overview of strategies for operando synchrotron characterization of electrochemical interfaces, covering established techniques as well as new capabilities. These include X-ray absorption spectroscopy (XAS), ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and a recently developed system for grazing incidence XAS and X-ray diffraction (XRD) to probe catalyst surfaces under high current conditions. I include examples of these methods applied to water splitting catalysis as part of the Joint Center for Artificial Photosynthesis (JCAP), and discuss potential applications for future studies as well as areas where further capability development is still needed.

Degradation of catalysts and light absorbers during photoelectrochemical (PEC) water splitting is becoming a critical limitation of PEC devices as efficiencies improve. Inductively coupled plasma mass spectrometry (ICP-MS) is currently the gold standard for detection of corrosion products in such systems, but limits throughput and offers little mechanistic information due to its high sampling time. Here, we demonstrate the use of in situ anodic stripping voltammetry using mercury ultramicroelectrode to detect corrosion products of transition metal catalysts for water splitting. Mercury ultramicroelectrodes can be easily fabricated in any electrochemistry lab and offer faster sampling rates than ICP-MS. Finally, we demonstrate that the improved temporal resolution enables rapid testing of corrosion rates at a range of intermediate current densities applicable to solar driven water splitting.

HydroGEN (https://www.h2awsm.org/) EMN is a six-lab consortium, led by the National Renewable Energy Laboratory (NREL). It currently comprises six core national laboratories: NREL, Sandia National Laboratory (SNL), Lawrence Berkeley National Laboratory (LBNL), Idaho National Laboratory (INL), Lawrence Livermore National Laboratory (LLNL), and Savannah River National Laboratory (SRNL). HydroGEN aims to accelerate the discovery and development advanced water splitting materials (AWSM) for sustainable, large-scale hydrogen production, in order to more effectively enable the widespread commercialization of hydrogen and fuel cell technologies. With the rollouts of fuel cell electric vehicles (FCEVs) by major automotive manufacturers underway, enabling AWS technologies for the widespread production of affordable, sustainable hydrogen becomes increasingly important.

The HydroGEN Consortium offers an extensive collection of materials research capabilities for addressing RD&D challenges in efficiency, durability and cost. Leveraging the HydroGEN Consortium’s staff of leading technical experts and broad collection of resource capabilities is expected to advance the maturity and technology readiness levels in all the advanced water splitting technologies, including advanced electrolysis (low and high temperature), photoelectrochecmical (PEC) and solar thermochemical (STCH) routes. Currently, there are 20 HydroGEN seedling projects, and one project focused on benchmarking advanced water splitting technologies. These 21 new projects utilized over 40 unique capabilities across the six HydroGEN core labs, from computational modeling to material synthesis to advanced characterization. HydroGEN is indeed a national innovation ecosystem that comprises 11 national labs, 7 companies, and 30 universities. This presentation will provide an overview of the HydroGEN EMN consortium and highlight some advanced water electrolysis and PEC projects as well as a few unique HydroGEN capability nodes that the community can leverage.

Converting solar energy efficiently into hydrogen is a key element to develop a sustainable and affordable hydrogen economy. The presentation will give an insight in how concentrated solar radiation can be coupled into hydrogen production processes. It will discuss the benefits and challenges of using the sunlight directly instead of converting it into other energy vectors.
The main focus will be on technologies with the perspective of large scale production at very high temperatures. Therefore solar tower systems for such production processes will be presented. Also the different components like concentrator, receiver, and reactor of the solar production plants will be described, possible locations will be discussed, and synergies with other R&D efforts on using high temperature heat will be shown. Hybrid solutions e.g. from the sulfur industry will demonstrate how concentrated solar radiation can contribute even today to actual industrial business models.
As many of the addressed processes have to be operated continuously high temperature heat storage will also be introduced. Especially thermochemical heat storage has the potential for being the ideal technology for heat provision in high temperature production processes.
The European Union is supporting the development in its research framework programs since the 1980s. However until 2002 only a few projects were carried out. Since then a continuous improvement took place which was accelerated by the implementation of the European Fuel Cell and Hydrogen Joint Undertaking in 2008. This private public partnership lead by industry is in charge to bring technologies faster into the market than research programs lead by the European Commission would. The presentation will give an overview of the actions carried out so far and the perspective of the European efforts.

Widespread adoption of inexpensive renewable-electricity sources brings with it many exciting challenges and opportunities, the most significant being an overabundance of low-cost electricity from renewables as well as cheap photovoltaics. Thus, there is an opportunity to use virtually free electrons and photovoltaics to produce fuels and chemicals using electrochemical and photoelectrochemical devices, respectively with water as a reactant. Electrochemical hydrogen production using water electrolysis is a commercial technology, barriers remain in terms of cost and efficiency improvements for its widespread adoption; while alternative water-splitting technologies that have the long-term potential to produce inexpensive hydrogen include photoelectrochemical (PEC) approaches, are still in their infancy. In either case materials development, interface development and cell design are critical in advancing the efficiency and cost of water splitting devices. Benchmarking protocols help to identify promising materials, facilitate translation of materials from one technology readiness level to another, and can provide clear technology crosscutting opportunities. This talk will cover benchmarking activities performed within the HydroGEN Energy Materials Network on: low temperature electrolysis (LTE) electrocatalysts, development of benchmarking cells for photoelectrochemical (PEC) water splitting devices, and crosscutting opportunities between LTE and PEC with respect to the hydrogen evolution and oxygen evolution catalysts activity and durability.

The energy materials network HydroGEN consists of six laboratories that have core capabilities related to hydrogen generation in four technology areas: Photoelectrochemical (PEC), Solar Thermochemical (STCH), High- and Low-Temperature Electrolysis (HTE & LTE). The core capabilities are described as nodes and are available to support projects selected by means of a competitive process through personnel, equipment, expertise, capability, materials, and data exchange and sharing platforms. Starting in the current funding period supernodes were created that combine the unique abilities of the HydroGEN nodes to perform relevant and ground-breaking research outside of the funded projects.
To date, materials advances are required to improve the cost, performance, and durability of LTE and hybrid electrolysis devices. Most materials advances are screened through ex-situ testing protocols due to ease of measurement and throughput. However, properties measured ex-situ are not always relevant for device operation and can be influenced by a number of factors, including synthesis/processing parameters and operating conditions of the individual tests. The optimized integration of novel materials, and the verification that ex-situ results have merit for operating systems is an important detail for accelerating research and development in the electrolysis space.
Savannah River National Laboratory (SRNL), Lawrence Berkeley National Laboratory (LBNL) and National Renewable Energy Laboratory (NREL) have joined several nodes together to probe the connection between materials, electrode composition and processing, and device performance for low temperature electrolysis (LTE)/hybrid electrolysis. This supernode combines/integrates nodes that focus on ex-situ materials characterization, electrode processing, in-situ device characterization, and modeling, so that ex-situ characterization approaches can be validated for their applicability to device performance and durability.
Initial results of the supernode will be presented on typical LTE materials sets that are supposed withstand for example high oxidation potential and high differential pressure. Results may give insights into mass transport challenges and resistivity losses. The presented work is intended to lead up to linking the inherent properties of materials and their composition and processing to observed device performance and durability. These efforts directly support LTE efforts and the low temperature aspects of the combined hybrid cycle.

We report on the electrochemical activity and surface properties of Ni/Au overlayers during the hydrogen evolution reaction. Ni/Au overlayers were prepared using chronopotentiometric deposition of well-defined Ni thin films on Au (111) surfaces from dilute NiSO₄ solutions in 0.1 M H₂SO₄. Using this method, we prepared Ni overlayers with varying thicknesses from 10 nm to 500 nm on Au (111) substrates. X-ray diffraction and scanning electron microscopy analysis of films is used for characterizing the deposited Ni films. Our electrochemical analysis of these films exhibit that the overpotential required for hydrogen evolution decreases as the thickness of the Ni overlayer increases from ca. 10 nm to 100 nm. Thicker Ni overlayers exhibit higher overpotentials (as high as ca. 150 mV). Interestingly, thicknesses greater than 50nm show additional reductive features in the ca. -50 mV to 400 mV (vs. RHE) potential range. Ni overlayers also suppress features associated with sulfate interactions with Au surfaces. Such high activity of Ni/Au overlayers are significant, especially considering the stability region of Ni in acidic media. We will present results from surface enhanced Raman spectroscopy analysis of these surfaces in electrochemical solutions, and optical in-situ surface stress measurements to probe adsorbate effects during hydrogen evolution reactions on Ni/Au overlayers.

In this talk we will give an overview of the HydroGEN photoelectrochemical (PEC) water-splitting 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 bundled to collaboratively develop an understanding of integration issues and degradation mechanisms that emerge as PEC devices move from the current research scale, typically 1 cm², to 10s of cm² and greater that are necessary to consider a PEC module or panel. Larger devices are expected to introduce new component integration challenges and pathways for degradation that this PEC Supernode project seeks to identify while suggesting routes to take that should maximize success in the upscaling process.

The PEC Supernode project is an alliance of synthesis, characterization, and modeling nodes housed at the National Renewable Energy Laboratory and Lawrence Berkeley National Laboratory whose tasks include: 1) Photoreactor chassis fabrication and baselining, 2) development, fabrication, and testing of larger area PEC devices based on tandem III-V absorbers, 3) development of in-situ morphology and corrosion analysis techniques and characterization, and 4) analysis of durability mechanisms and impacts using life cycle analysis and techno-economic analysis. We will also describe a new photoreactor chassis that has been designed to facilitate indoor and on-sun efficiency and durability benchmarking measurements. The design has been validated in a preliminary set of multi-institution round robin testing toward understanding and harmonizing measurement conditions and protocol, which will be elaborated upon here.

We report on the surface and subsurface hydrogen interactions on well-defined platinum surfaces in electrochemical environments during the hydrogen evolution reaction (HER). We are particularly focused on the Pt-Cu binary system. Well-defined surfaces of Pt and Cu/Pt are prepared using thermal evaporation and electrodeposition, respectively. An optical laser method is used to perform in-situ surface stress measurements to track early stages of catalyst activation through adsorbate-induced effects. Surface stress changes on Pt surfaces show a distinctive compressive feature as hydrogen is adsorbed onto the Pt surfaces. The magnitude of this compressive feature decreases with increasing scan rates, which implies fast surface activation of hydrogen. Interestingly, the magnitude and potential dependence of the hydrogen-specific surface stress changes do not change as the solution pH changes (between pH = 1, 7 and 14). However, surface stresses change significantly in the presence of H₂ gas.

Additionally, hydrogen-specific stress changes are completely suppressed in the presence of ultra-thin copper overlayers. Our analysis shows that the adsorption of active species, i.e. H⁺ at the catalyst's active sites, causes structural changes and clearly demonstrates that the early stages of adsorbate formation could describe electrocatalysis and surface/subsurface structural effects. We will further discuss surface tuning of the Cu-Pt system for improved hydrogen evolution reactivity using insight from our in-situ analysis.

Chalcopyrite alloy materials class is an attractive candidate for photovoltaic (PV) and photoelectrochemical (PEC) devices due to its high bandgap tunability and presence of low cost synthesis pathways. However, these advantages are also known to be associated with challenges in the materials synthesis as well as the device assembly processes. For example, certain chalcopyrite alloys exhibit strong tendency of developing secondary phase(s) during it’s synthesis, which affects on the device performance in unpredictable manner: it could be beneficial or detrimental depending on the alloy composition and the synthesis condition.
In this presentation, we first review the issue of secondary phases, such as ordered vacancy compound (OVC), reported in literature as well as our recent findings on sulfur containing chalcopyrite materials synthesis, where sulfur was incorporated in order to achieve an optimal band alignment for a PEC H₂production device. The electronic property of OVC interacts with both the photoabsorber (chalcopyrite) and the buffer, where the defect properties of these materials play crucial roles in the functionality of hetero junction (ex. charge carrier separation efficiency, open circuit voltage). In order to find the synthesis routes that would minimize the detrimental effect of OVC formation and its intrinsic defects to the performance of complex hetero-junction (absorber-OVC-buffer), we have been developing thermodynamic alloy phase diagram of candidate photoabsorber materials that includes OVCs as well as the corresponding defect property database. We will discuss how these data may be used to predict the device performance (ex. open circuit voltage) behavior, which in turn, may be used to strategize the PEC/PV device development.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office.

The HydroGEN consortium brings together unique DOE National Laboratory experts and tools (nodes) with researchers developing advanced water splitting materials in a variety of technologies: Low and High -Temperature Electrolysis (LTE/HTE), Photoelectrochemical (PEC) and Solar-Thermochemical (STCH) Water Splitting. Lawerence Livermore National Laboratory is a HydroGEN partner lab and has participated in 4 projects since 2018. This poster will highlight:
-Livermore Lab capabilities and expertise available to researchers in LTE, HTE, PEC and STCH fields
-Our collaborative work performed with our partners in LTE and PEC in FY18
-Tackling big scale problems in PEC, LTE, HTE and STCH through multi-node, multi-institution projects "supernodes"
-Come and ask questions as to how we could help your water splitting research

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office.

The open circuit voltage of a conventional H₂-O₂ fuel cell is limited to ~1.23 V under atmospheric pressure, which limits the efficiency of the system. Here, we reported a dual-electrolyte microfluidic fuel cell system (DEFC), which could stably deliver an open circuit voltage up to 1.76 V at room temperature under atmospheric pressure, and a peak power density of 145 mW/cm², improved by 3 times compared with that in single electrolyte mode. When operated in the reverse mode, the microfluidic cell can be used for water electrolysis, demonstrating a water split voltage of ~0.74 V and a round trip voltage efficiency of 83.5% at a current density of 100 mA/cm². The neutralization energy of the two electrolytes, which could be wasted as heat, can be directly utilized to produce electricity or split water with high efficiency. Given the DEFC's features of high open-circuit voltage, low water splitting voltage, and room-temperature operation condition, the reported DEFCs technology presents a superior route for high-efficiency energy conversion and storage system that could revolutionize the fields of large-scale energy storage and portable power systems.

Improving both the efficiency and productivity of water electrolysis is necessary for making the electrochemical generation of hydrogen economically competitive. The efficiency depends on the surface area and catalytic activity of the electrode, whereas the productivity depends on the ease with which gas can be removed from the electrode surface. Flow-through electrodes are widely used to improve the efficiency and productivity of electrochemical processes, but studies of their use in alkaline water electrolysis are relatively rare. We hypothesized that the use of a flow-through electrode could enhance the rate of bubble removal from the electrode surface, thereby improving productivity, but it was not clear what length scale of the electrode would maximize the surface area available for electrolysis without hindering bubble removal. To determine how the efficiency and productivity of the flow-through electrode depends on the electrode length scale, we compared the performance of the oxygen evolution reaction with flow-through electrodes consisting of a Ni foam, Ni microfibers (Ni MFs, d = 1.2 mm), and Ni-coated Cu nanowires (NiCu NWs, d = 350 nm). Although the turnover frequency (TOF) for the Ni foam was 2.4 times higher than the Ni MF electrode and 42 times higher than the NiCu NW electrode, its surface area was more than 90 times lower than the other electrodes, resulting in the lowest overall current density. Although the NiCu NW electrode had a higher surface area than the Ni MF electrode, it had a lower overall current density because of its low TOF. We attribute the high performance of the Ni MF electrode to the fact that the mean pore size of the Ni MF electrode (5-10 mm) is on the order of the mean bubble size (20 mm) that was measured by examining the bubbles generated from a mesh electrode under an optical microscope. The performance of the Ni MF electrode was further improved by coating it with a Ni-Fe layered double hydroxide (LDH) catalyst, resulting in an OER overpotential of 247 mV at 0.1 A cm^-2 and a Tafel slope of 26 mV dec^-1. These values are among the lowest reported for Ni-Fe LDH catalysts. The LDH-coated Ni MF anode was paired with a cathode coated with a Ni-Mo alloy to measure the overall productivity for water splitting. The maximum current density for water splitting was 20 A cm^-2 at 3.9 V in 30% KOH, which is 20 times greater than the highest current densities previously reported for alkaline water electrolysis. This current density and voltage could be sustained for at least 8 hours with no signs of degradation. This high current density was attributed to the ability of the flowing fluid to promote bubble removal and keep the hot electrode surface from boiling water. This work demonstrates that flow-through microfibrous electrodes can improve the efficiency and productivity of alkaline water electrolysis.

In order to generate hydrogen, which is promising fuel in the future, electrochemical water splitting through hydrogen evolution reaction (HER) have taken center stage. Although, existing catalysts for HER such as platinum group metals have been considered the best catalyst, they have problems with high cost and scarcity. Recently, transition metal boride (TMBs) has been considered potential HER electrocatalysts and various covalent networks of boron in the crystal structure have meaningful relation with electrocatalytic activity for HER. In this work, we synthesize vanadium borides which have different boron chains by arc melting; VB (zig-zag chains), V₃B₄ (double chains), and VB₂ (flat hexagonal network). Electrochemical measurements show that VB₂ requires the lower overpotentials (-0.360 V) than that of VB (-0.430 V) and V₃B₄ (-0.416 V) in 0.5 M H₂SO₄ to achieve a current density of 100 mA/cm². Besides, the activity of VB₂ is remained after 5000 cyclic voltammetry measurements. These results indicate that graphene-like flat boron layers in VB₂ give positive effect for increasing HER activity due to its electrical conductivity and ΔG_H value close to 0 V.

Plasmas have a number of unique properties which researchers may be able to leverage to enhance desired electrocatalyst material properties and steer electrochemical reaction performance. Two examples of the recent application of plasmas for advancements in electrochemical fuel production are presented. In the first, the nonequilibrium reaction environment of an air plasma enabled the synthesis of homogeneous phases of metastable compositions of a mixed W_1-xIr_xO_3-δ alloy. For a low-platinum-group-metal (PGM) composition of only 1% Ir, plasma-synthesized W_0.99Ir_0.01O_3-δ was observed to be an acid-stable oxygen evolution reaction (OER) catalyst of competitive overpotential. Moreover, the plasma-synthesized alloy was observed to have an OER overpotential ~ 570 mV less than that of a thermally oxidized alloy of the same composition. In the second example of a plasma enhancement, a nitrogen plasma was coupled into the cathode of a proton exchange membrane electrolyzer to provide energetically activated N₂ species to the electrocatalyst surface and demonstrate a proof-of-concept synergistic enhancement in the rate of ammonia production. At an applied bias of ~3.5 V across the electrolyzer, plasma-assisted operation was observed to produce 47% more ammonia than the combination of plasma-without-bias and bias-without-plasma conditions.

Due to the extreme conditions encountered during typical use; solar thermochemical (STCH) water splitting materials that do not begin in a low energy state will evolve during cycling. Evolution during cycling may take many forms, including Oswald ripening of grains or slow conversion of impurity phases, all of which will have some effect on the performance of the material during cycling. Thus, creating phase pure materials with stable grain sizes is of utmost importance to evaluating STCH materials. We report on the methods of designing high quality STCH materials from the modeling stage onwards, the synthesis of phase-pure and pre-ripened materials, and characterization used to determine the stability of the material over many cycles.

Next generation perovskite-oxide based materials for thermochemical water splitting (aka STCH) are key to developing a massively scalable technology that efficiently converts solar energy to hydrogen. The challenge is to design optimal perovskites that achieve DOE performance targets, such as high efficiency operation at low thermal reduction temperature. Current research is mired in a heuristic approach to probing the enormous array of possible material compositions that manifest water splitting activity in perovskites. A comprehensive molecular-level picture of the factors that influence redox cycle behavior is missing. We report on a HydroGEN Advanced Water Splitting Materials (H2 AWSM) consortium effort to develop a fundamental understanding of how the unique crystallographic structures induced by the Mn-O ligand bond found in layered perovskites influences properties critical to favorable material behavior. This is accomplished by examining perovskite electronic structures resulting from redox transitions using advanced spectral microscopies and, in conjunction with atomistic theory, deriving correlations between electronic configurations and thermodynamic properties that manifest water splitting material behavior.

Making easy-to-make, cost-efficient, and durable bifunctional electrocatalysts for overall water splitting to produce oxygen and hydrogen is of paramount significance for future renewable energy system but still a challenge. Herein, we explored earth-abundant and non-toxic delafossites with a CuMO₂-type general composition, where ‘M’ is trivalent metals of Al, Ga, Cr, Co, Fe, etc. First, we report that nanocrystalline CuGaO₂ hexagons showed enhanced electro-catalytic activity as compared to the sub-micron sized and micron sized particles of CuGaO₂ counterparts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in 0.5 M KOH versus Ag/AgCl. Then, based on a widely used strategy to increase the surface area of functional materials, we synthesized delafossite CuGaO₂ nanoflakes. Their electrochemical (EC) and photoelectrochemical (PEC) performance in 0.5 M KOH electrolyte versus Ag/AgCl along with their stability were studied. The developed electrolyzers are very efficient to produce H₂, cheaper, offers no complications in synthesis of materials and fabrication of electrolyzers, which can greatly enable the realization of clean renewable energy infrastructure.

Pacific Northwest National Laboratory’s (PNNL) experience in solid oxide electrolysis cell (SOEC) technology and related high temperature electrochemical devices extends back to 1987 and has been documented in over 150 peer-reviewed technical papers and numerous patents. PNNL’s experimental and modeling experience and capabilities in high temperature electrochemistry encompass all aspects of the technology, from materials development and characterization to design, fabrication, and testing of cells, stacks, and systems. In addition to performing work for the U.S. Department of Energy and other agencies, PNNL has collaborated successfully with a number of industrial developers.

PNNL’s high temperature electrochemistry laboratories contain all the materials synthesis, processing, fabrication and testing equipment necessary to develop and test solid oxide electrolyzers, stacks, and complete power systems. The experimental work is being supported by computational modeling at the cell, stack, and system levels. PNNL also hosts a national user facility, the Environmental Molecular Sciences Laboratory, with a large suite of physical and chemical characterization tools.

Advanced in super-proton conducting materials are being made under the DOE HydroGEN Energy Material Network. The desired goal is to reduce the operating temperature of high temperature steam electrolysis to around 600 C or less to reduce degradation and fouling of the electrode/electrolyte, interconnections, and associated stack containment and gas handling materials. Four projects funded under the DOE HydroGEN program are focusing on various aspects of materials development and testing. Methods for fabricating these materials are being developed and tested by various experimental capabilities (nodes) within the designated HydroGEN core labs. Initial testing is focusing on the performance of these materials.

With the gradual consumption of fossil fuels and rigorously increasing pressures from environment pollutions, it is important and necessary to develop and harvest clean renewable energy in large scales to fulfill the sustainable development in economy and society. Hydrogen generated from electrochemical water splitting can not only replace fossil fuels as clean and efficient energy carrier, but also realize the large-scale conversion and storage of sustainable energy-produced electricity. Generally, electrocatalytic water splitting consists of two half reactions, namely hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), while an extra additional potential is required for the implementation because of the limitation in slow cathodic and anodic kinetics. Non-noble metal-based electrocatalysts (such as manganese, iron, cobalt, and nickel) are of huge advantages in the low cost and rich abundance as compared with noble metals, but their activity and stability are still not satisfactory for practical applications. In this regard, rational design of the structures and composites of hierarchical electrocatalysts using various strategies are widely explored to aim for reducing the overpotentials for efficient water splitting. Typical methods include controlling the morphology, employing the conductive substrates as supports, regulating the ratios of various constituent components, engineering the defects, doping with the foreign elements, as well as surface coating and decoration in order to enhance the catalytic activity and stability of electrocatalysts. Recently, more attentions are paid on regulating the electrocatalytic activity of catalysts by incorporating with rare earth elements. Because of the stable chemical property and unique electronic structure of rare earth elements, their integration with hierarchical transition metal-based electrocatalysts is developed as a novel method to modify the structure and property of electrocatalysts. In this presentation, cerium as a typical rare earth element, is selected and introduced into the transition metal-based electrocatalysts to improve their HER and OER properties. The role of cerium-based species and their influences on the structures and properties of electrocatalysts are also investigated.

Fuel cells and electrochemical water splitting are promising techniques for clear energy conversion to feed energy demand without environmental concerns. Their commercialization of these techniques requires the earth-abundant efficient and durable electrocatalysts for electrocatalytic reactions, including oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and oxygen evolution reaction (OER). The performance of these electrocatalysts are closely related to the intrinsic activity and the number of the accessible catalytic sites in the electrocatalysts. This presentation will focus on the development of earth-abundant efficient electrocatalysts for ORR, HER and OER by synergistically engineering the electronic structure and morphologies of electrocatalysts to boost their electrocatalytic performance. Several reasonable strategies will be introduced for rationally modulating the morphological structure, electronic structure, and crystalline structure of electrocatalysts to not only improve the intrinsic activity of catalytic sites but also increase the number of accessible catalytic sites. [1-7] Simultaneously, the electron transportation and mass transfer can be enhanced by the rational design of electrode architecture. Hopefully, these results may give new insights for the rational design and bottom-up synthesis of cost-effective and high-performance electrocatalysts for fuel cells and sustainable hydrogen production through electrochemical water splitting.

References:
[1] L.B. Huang, L. Zhao, Y. Zhang, Y.Y. Chen, Q.H. Zhang, H. Luo, X. Zhang, T. Tang, L. Gu, J.S. Hu, Adv. Energy Mater. 2018, 8, 1800734.
[2] T. Tang, W.-J. Jiang, S. Niu, N. Liu, H. Luo, Q. Zhang, W. Wen, Y.-Y. Chen, L.-B. Huang, F. Gao, J.-S. Hu, Adv. Funct. Mater. 2018, 28, 1704594.
[3] W.J. Jiang, S. Niu, T. Tang, Q.H. Zhang, X.Z. Liu, Y. Zhang, Y.Y. Chen, J.H. Li, L. Gu, L.J. Wan and J.S. Hu. Angew. Chem. Int. Ed. 2017, 56, 6572-6577.
[4] T. Tang, W.J. Jiang, S. Niu, N. Liu, H. Luo, Y.Y. Chen, S.F. Jin, F. Gao, L.J. Wan, and J.S. Hu, J. Am. Chem. Soc., 2017, 139, 8320–8328.
[5] Y.Y. Chen, Y. Zhang, X. Zhang, T. Tang, H. Luo, S. Niu, Z. Dai, L.J. Wan and J.S. Hu. Adv. Mater. 2017, 29, 1703311.
[6] T.Q. Zhang, J. Liu, L.B. Huang, X.D. Zhang, Y.G. Sun, X.C. Liu, D.S. Bin, Xi Chen, A.M. Cao, J.S. Hu, and L.J. Wan. J. Am. Chem. Soc., 2017, 139, 11248–11253.
[7] W.-J. Jiang, L. Gu, L. Li, Y. Zhang, X. Zhang, L.-J. Zhang, J.-Q. Wang, J.-S. Hu, Z. Wei, L.-J. Wan, J. Am. Chem. Soc. 2016, 138, 3570-3578.

Anion exchange membrane (AEM) electrolyzers are an attractive alternative for producing hydrogen due to the potential stability and activity of non-precious metal catalysts in alkaline environments. Recently, AEM electrolyzers have been demonstrated using various membranes, ionomers, porous transport layers, and catalysts [1]. The influence of these materials and their properties on AEM electrolyzer performance, however, is underdeveloped.

In this work, we use a coupled, nonisothermal, multiphase model of an AEM electrolyzer to understand the role of material properties in improving device performance. We use the model to identify performance bottlenecks and provide guidance for next-generation electrolyzer materials and designs by exploring sensitivity to membrane and catalyst-layer properties. Finally, we explore the effects of various electrolyte solution feeds and operating paradigms on cell performance.

Acknowledgements
This work was funded under the HydroGEN Consortium, by the Office of Energy Efficiency and Renewable Energy (EERE), of the U.S. Department of Energy under contract number DE-AC02-05CH11231. This work was supported in part by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Science Undergraduate Laboratory Internship (SULI) program. This work was also supported in part by Workforce Development & Education at Berkeley Lab.

1. I. Vincent and D. Bessarabov, Renew. Sustain. Energy Rev., 81, 1690–1704 (2018).

Molecular hydrogen is the most promising candidate for clean, renewable and economical energy carrier to address the every increasing demand for clean energy. However bottleneck for large scale commercialization of the water alkali electrolyzer is the requirement of costly Pt electrocatalyst. Search for efficient and cheaper alternatives based on Earth abundant elements is limited by the slow kinetics of the Volmer step for adsorption of hydrogen and subsequent Tafel step for generation of molecular hydrogen from recombination of adsorbed H atoms. In a recent work MoNi4 electrocatalyst supported on MoO2 is synthesized from thermal reduction of NiMoO4. Very high efficiency for HER with 0 mV onset over potential tafel slope ~30 mV/decade has been reported which originated from drastically accelerated Volmer and Tafel steps. However detail understanding for microstructural evolution, alloy formation and phase separation during thermal reduction of NiMoO4 along with the role of MoO2 is required for further improvement of the catalyst. In the present work detail Transmission electron microscopic studies including in-situ heating, nanobeam diffraction and STEM-EDS is used for comprehensive understanding of the growth mechanism of the electrocatalyst. Role of each of the co components of the electrocatalyst and their stabilities are evaluated through impedance spectroscopy and array of other electro chemical techniques

Nanoscale catalytic materials are key active components of electrolytic and photoelectrochemical devices for storing and converting electrical energy. Advances in their knowledge-driven, rational design requires insight into their geometric and chemical structure under operating conditions.

In this talk, I will highlight recent work on the understanding of electrocatalytic Ir oxide and Ir-Ni oxide-based core-shell nanocatalysts. I will outline their preparation, ex-situ and in-situ characterization, and catalytic performance and discuss new insights into the unique geometric and electronic structure of oxygen ligands and metal ions in hole-doped IrOx lattices during the oxygen evolution reaction process.

Water electrolysis at low temperatures could provide the ideal means of storing intermittent energy from renewables, in the form of hydrogen. The efficiency is limited by oxygen evolution at the anode. In the current contribution, I will present the results of model experiments to elucidate the factors controlling the activity and stability of these oxide catalysts used to catalyse oxygen evolution.
Under the operating conditions of traditional alkaline electrolysers,NiFe_xO_yH_z constitute the most active catalysts. However, the origin of their catalytic activity is under intense debate. Thus far, there was no consensus on whether the O₂ evolved is derived from the surface or the lattice. In order to answer this question, we fabricated model electrodes based on size-selected Ni-Fe nanoparticles, produced using a magnetron nanoparticle source.¹We isotopically labelled the oxygen in the reactant water as well as the lattice of the particles, to determine the source of the O₂ evolved. We detected the O₂ in operando and in real time with a novel micromachined mass spectrometry device. We probed the presence of oxygen in the core of the particles ex-situ with low energy ion scattering. Our experiments definitively showed that the O₂ originates from the surface. Consequently, we propose that further improvements to the catalyst activity are contingent on the properties of the surface—as opposed to the bulk —chemistry.
Relative to traditional alkaline electrolysers, polymer electrolyte membrane electrolysers are more suitable for fast start up and shut down; they can also operate at much higher current densities. However, the acidic and oxidising conditions at the anode set severe constraints on the choice of catalyst material. All compounds, apart from IrO_x and RuO_x, are catalytically inactive or unstable. Even IrO_x and RuO_x corrode at a rate significant enough to degrade overall electrode performance.
Most earlier studies on Ru based catalysts tested the performance over a wide composition, ranging from catalysts that were metallic in the core to oxides in the core; those reports suggested that activity and stability are intrinsically related. However, in the current contribution, we probed this relationship on catalysts exclusively based on RuO₂, including nanoparticles, a Ru(110) single crystal and oriented thin films.² In contrast to the prior studies, we found that there was no obvious relationship between activity and stability. Our finding suggests that dissolution occurs from distinct surface sites from those that are responsible for O₂ evolution. This understanding corroborates our earlier work, which showed that surface Ir stabilises RuO_x³ and surface Ti stabilises MnO_x,⁴ without compromising activity. On this basis, we propose there should be ample opportunities for engendering further improvements to the stability of oxygen evolution catalysts under acidic conditions.



1 Roy, C., Sebok, B., Scott, S. B., Fiordaliso, E. M., Sørensen, J. E., Bodin, A., Trimarco, D. B., Damsgaard, C. D., Vesborg, P. C. K., Hansen, O., Stephens, I. E. L., Kibsgaard, J. & Chorkendorff, I. Nature Catalysis, (in press).
2 Roy, C., Rao, R. R., Stoerzinger, K. A., Hwang, J., Rossmeisl, J., Chorkendorff, I., Shao-Horn, Y. & Stephens, I. E. L. ACS Energy Letters, (2018).
3 Pedersen, A. F., Escudero-Escribano, M., Sebok, B., Bodin, A., Paoli, E., Frydendal, R., Friebel, D., Stephens, I. E. L., Rossmeisl, J., Chorkendorff, I. & Nilsson, A. The Journal of Physical Chemistry B, (2017).
4 Frydendal, R., Paoli, E. A., Chorkendorff, I., Rossmeisl, J. & Stephens, I. E. L. Advanced Energy Materials 5, 1500991, (2015).

While it was shown that metastability can increase a catalyst’s water-oxidation activity (1,2) it is still unclear what the underlying mechanism is. Using density functional theory calculations, we analyze the oxygen evolution reaction (OER) on (001) surfaces of three different materials chosen for their very different electronic properties. To account for the metastability, we study the OER on a large variety of different reaction sites by sampling many defective surface structures. Totally, we compute the OER overpotential on 770 symmetrically inequivalent reaction sites. For all three materials, we find a large range of overpotentials for different reaction sites including also overpotentials at the top of the activity volcano. This implies that the structural variety of reaction sites on dissolving metastable heterogeneous catalysts will, for some sites, lead to overpotentials close to the top of the activity volcano. Under the assumption that reaction sites with low overpotentials will dominate a catalyst's apparent activity, this implies that, independent of the electronic properties of the catalyst, operating a heterogeneous catalyst at the border of its stability region can lead to an increased apparent activity due to the large geometric variety of different reaction sites.

(1) SH Chang et al. Faraday Discussions 176 (2015): 125-133.
(2) N Danilovic et al. The Journal of Physical Chemistry Letters 5.14 (2014): 2474-2478.


Funding Acknowledgement
Swiss National Science Foundation PP00P2_157615

One-dimensional-arrays material possesses high surface area, oriented electron transfer pathway and facile mass transfer pathway, which shows prominent potential for electrocatalyst. IrO₂ nano-arrays (IrO₂ NAs) performs excellent activity in oxygen evolution reaction in electrochemical water splitting. In this work, TiO2 nanotube arrays (TNTA) with a length of 1500 nm is applied as template to study electrodepositing process of IrO₂ on conductive nanotube substrate through cyclic voltammetry (CV). By conducting different scan rate of CV and concentration of Ir precursor in electrodeposition, IrO₂ NAs, nanotubes or hollow nanowires, with different length is obtained, which is determined by the distribution of deposited IrO₂ under different applied conditions.
With fixed concentration of Ir precursor, there is a positive-related linear range between scan rate and the length of IrO₂ NAs that is the depth of TNTA where deposited IrO₂ could construct constant layer on the tube wall. A possible mechanism is proposed. As the whole tube wall of TNTA is conductive, the electrodeposition of IrO₂ will take place throughout the tube. Due to the competition effect of the consuming of Ir precursor by depositing and supplementing through diffusion from bulk electrolyte, concentration gradient of Ir precursor would be gradually formed along the tube and leads to uneven depositing. And this phenomenon would be exacerbated with the prolongation of depositing time. In electrodepositing of CV, each cycle includes the depositing period and non-depositing period in which Ir precursor could be sufficiently diffused throughout the TNTA. And with a slower scan rate of CV the uneven depositing will be more significant. Besides, as the CV depositing goes on, a cover layer of IrO₂ on the top surface of TNTA, which would hinder the diffusion of Ir precursor form the bulk electrolyte to inner zone of TNTA, would be formed as we have reported. As the depositing rate near the mouth of TNTA will be hardly affected by the prolongation of depositing period of each cycle, the formation of cover layer is mainly determined by the total depositing time, while the deposited amount of IrO₂ in the depth of TNTA depends on the cycles of CV. Briefly, the length of obtained IrO₂ NAs is determined by the number of depositing cycles before the cover layer is formed. The deposition with different concentration of Ir precursor and fixed scan rate has been studied, too. With lower concentration, the required numbers to form cover layer increases, and results in longer IrO₂ NAs, which is consistent to our proposed mechanism.

In commercial electrolysis, the cost of electricity input drives the cost of hydrogen production. Electrolysis, therefore, is typically run at high catalyst loading and constant power input over long periods of time. Lowering water-splitting hydrogen production costs to a level comparable to steam methane reformation requires: coupling electrolysis with low-cost power input (wind, solar) to reduce feedstock costs; and minimizing capital cost at lower capacity.[1,2] Part of reducing capital cost includes reducing catalyst loading and while minimal durability loss is seen with thick catalyst layers, losses become more apparent with loading reductions.[3]
Presented efforts include evaluating load-follow operation, start-stop operation, and contaminants, studying relevant mechanisms for loss in half-cell tests and how these losses are observed in single-cell tests. Electrolyzer durability was evaluated at low iridium-anode loading (0.1-0.5 mg cm^-2) and with different power inputs (potentials, intermittency) to focus on systems aspects needed to significantly reduce hydrogen production costs. Within load-follow operation, iridium loss was driven by dissolution. Although higher potential accelerated single-cell loss, cycling (square/triangle wave) also increased the rate of performance decay since rapidly increasing input resulted in localized potential spikes within the catalyst layer. While start-stop operation accelerates loss through a variety of pathways (dry operation on membrane, pressure cycling on membrane and hydrogen crossover), these efforts focused on the effect of potential excursions on the catalyst. Simulated hard-stops were more detrimental to iridium durability than load-follow, likely due to surface and near-surface reduction increasing dissolution rates. The effect of contaminants was also evaluated based on likely metals added with water input. Contaminant effects in half-cell tests were observed but were relatively small and may have been due to the large amount of free electrolyte and desorption on iridium at high potential. Losses were more significant in single-cells and may have been exacerbated by contaminants blocking within the membrane and plating the cathode.
Several catalyst types have been tested, including metals (metal/hydroxide) and oxides with supports, high surface area, multicomponent, and alloys. These materials have been studied in half-cells for the activity improvement mechanism and in single-cells to evaluate potential advantages/disadvantages in electrolysis performance and durability. By including different catalyst types, we are looking to establish performance/durability guidelines and perspectives on how catalyst development and system controls factor into low temperature electrolysis at low loading and with intermittency. These tests have significant implications with a shift toward low-cost hydrogen production through electrolysis coupled with renewable power inputs.

[1] Alia, S. M., H2@Scale: Experimental Characterization of Durability of Advanced Electrolyzer Concepts in Dynamic Loading. Department of Energy, U. S., Ed. https://www.hydrogen.energy.gov/pdfs/review18/tv146_alia_2018_p.pdf, 2018.

[2] Denholm, P.; O’Connell, M.; Brinkman, G.; Jorgenson, J. Overgeneration from Solar Energy in California: A Field Guide to the Duck Chart; Vol. NREL/TP-6A20-65023; National Renewable Energy Laboratory: Golden, CO, 2015. Available at the following: http://www.nrel.gov/docs/fy16osti/65023.pdf.

[3] Alia, S. M.; Rasimick, B.; Ngo, C.; Neyerlin, K. C.; Kocha, S. S.; Pylypenko, S.; Xu, H.; Pivovar, B. S. J. Electrochem. Soc. 2016, 163, F3105−F3112.

The utilization of renewable energy has substantially driven more attention into electrolysis technologies. An electrolyzer can utilize “off peak” electricity from solar or wind farms to produce hydrogen or other fuels. These fuels can then be operated in a fuel cell mode to generate electricity when needed or used for other industrial applications. Further, the establishment of H2@scale consortium by DOE has presented unprecedented opportunities for renewable H2. However, current hydrogen production from electrolysis comprises only a small fraction of the global hydrogen market due to the high costs that results from expensive materials even if “free” electricity from renewable energy can be acquired.

Alkaline membrane water electrolysis enables to use non-precious metals (or oxides) as the catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The commercial OER catalyst IrOx for proton exchange membrane (PEM) electrolysis is very expensive; more importantly, the global storage and production of iridium is very scarce, tremendously limiting the mass production and deployment of PEM electrolyzers. There have been a large number of non-precious metal based OER and HER studies. However, most of these studies have been performed on the rotating disk electrodes (RDEs) under alkaline solutions. Their applications in real alkaline membrane electrolyzers have been very limited.

This work will present the primary challenges associated with the realization of alkaline membrane electrolysis, which include catalyst durability, electrode design and alkaline membrane/ionomer development. First, many OER or HER catalysts demonstrating good activity in the RDE do not work for the real electrolyzer cell. Second, an optimal interaction between the ionomer and the catalyst in the electrode has not been established. Third, current commercial alkaline membrane and ionomer cannot meet the stability requirement; they decay rapidly under high operating voltage and high temperature. Furthermore, water transport in alkaline membrane electrolysis is much more complex compared to PEM water electrolysis.

The pursuit of alkaline membrane/ionomer with various chemistries and structures by a few researcher groups has rejuvenated alkaline membrane electrolysis. They will provide a better platform to test the OER or HER catalyst to enable alkaline membrane electrolysis operation.

Recently, significant progress has been made in photoelectrochemical water splitting and solar hydrogen generation. For practical application, it is essential that the constituting semiconductor photoelectrodes can perform unassisted solar water splitting both efficiently and stably. To date, much of the research has been focused on metal-oxide, Si, and III-V semiconductor photoelectrodes. A double-junction photoelectrode, consisting of a top light absorber (energy bandgap ~1.7 eV) and a bottom light absorber (energy bandgap ~1.1 eV) promises a solar-to-hydrogen conversion efficiency up to 30%. While Si is ideally suited to serve as the bottom light absorber, it has remained challenging to identify a 1.7-2.0 eV top light absorber that is stable in harsh photocatalysis conditions. Recent studies suggest that metal-nitride photoelectrodes, e.g. InGaN possess excellent potential for unassisted solar water splitting. InGaN with an indium composition ~50% exhibits an energy bandgap ~1.75 eV and is thermodynamically stable in photoelectrochemical water splitting. In this context, we have performed a detailed investigation of the design, synthesis, and photoelectrochemical performance of InGaN/Si double-junction photocathodes. We have demonstrated that InGaN/Si photocathodes can perform unassisted solar water splitting with relatively high efficiency in 0.5 M sulfuric acid under AM1.5G one-sun illumination. Long-term stable operation (>1,000 hrs) has been demonstrated for GaN/Si photocathodes without using any extra surface protection.

In this work, GaN and InGaN nanowires are grown directly on n⁺-p Si wafer by plasma-assisted molecular beam epitaxy. The surfaces of InGaN nanostructures are tuned to be N-rich, not only for their top c-plane surfaces but also for their nonpolar lateral surfaces, which can effectively protect against corrosion. We have discovered that the conduction band offset between GaN and Si is essentially negligible, which enables the efficient extraction of photo-generated electrons from the underlying Si wafer. The GaN/Si photocathode exhibits an applied bias photon-to-current efficiency of 10.5%, which is among the highest values reported for a Si photocathode. With the use of GaN nanostructures as a surface protection layer, we have demonstrated that GaN/Si photocathodes can exhibit long-term stable operation (>1,000 hr) without using any other extra surface protection. To realize double-junction photoelectrodes, p-type InGaN nanowires are integrated on n⁺-p Si wafer through a polarization enhanced p⁺⁺/n⁺⁺ GaN nanowire junction, which enables the transport of photoexcited holes from the p-InGaN to the n-GaN/Si. The top InGaN nanowire arrays with an indium composition ~50% has an energy bandgap ~1.75 eV and is designed to absorb sunlight in the visible spectrum, whereas the rest of the photons with wavelengths <1.1 μm are absorbed by the underlying planar Si p-n junction. The double junction InGaN/Si photocathode has V_on of ~ 2.3 V vs RHE in 0.5 M sulfuric acid. A solar-to-hydrogen conversion efficiency ~5% is measured under AM1.5G one-sun illumination. Work is currently in progress to achieve solar-to-hydrogen conversion efficiency >10%, which, together with the long-term stability analysis will be performed and reported.

Achieving high current densities while maintaining a high energy conversion efficiency is one of the main challenges for enhancing the competitiveness of solar fuel producing photo-electrochemical devices. I will discuss and show how thermal integration allows for an operational device at current densities and efficiencies which otherwise could not be supported. I will quantify the increase in the theoretical maximum efficiencies at given current densities of photo-electrochemical devices resulting from thermal synergies using detailed balance calculations coupled to electrochemical load curves and energy conservation equations. I will then discuss device implementation of such an approach and show how more realistic device models (multi-dimensional, multi-scale, multi-physics) can be used to support the device implementation and its operational understanding. I will elaborate how the coupled heat, mass and charge transport in such continuous flow devices can be used to dynamically control the specific operating point of the device and how this approach can be exploited to deal with unintended or uncontrollable variations in the operating conditions, resulting, for example, from degradation of the materials over the lifetime and from the daily and seasonal changes in incoming irradiation. I will finish with discussing pathways for enhanced thermal integration.

One of the main challenges for the development of photoelectrochemical cells that could potentially meet the DOE target of $2/kg of H2 has been the lack of low-cost materials, which exhibits high-efficiency (photovoltage and photocurrent) at low-cost. While solution-based materials thin-film technologies have offered promise of cost, they have been plagued by low efficiency (photovoltage insufficient to drive the two half-cell reactions of H2 and O2 generation) and photo-oxidation (or reduction). In contrast, classical semiconductor based tandem photocells (Turner Cell) have been demonstrated to efficiently split water at an efficiency exceeding 15%, but high cost and photo-corrosion have emerged as key challenges.

Solution-processed halide hybrid (inorganic-organic) perovskites have demonstrated an extraordinary potential for clean sustainable energy technologies and low-cost optoelectronic devices such as solar cells; light emitting diodes, detectors, sensors, ionic conductors etc. Single junction photovoltaic efficiencies have sky rocketed to 23.3% and attempts are being made to fabricate high efficiency perovskite-perovskite tandems cells. A unique advantage of halide hybrid perovskites over other photo-absorbers is that a single junction can produce >1.1 V. Thus, by combining perovskites with different band-gaps, it should be possible to achieve >1.8 V photovoltage, enough drive the energy intensive water-splitting reaction without the need for an external battery. However, for classical PECs, degradation against aqueous and corrosive electrolytes will require the use of effective barriers, which prevent degradation and are electronically transparent.
In this talk, I will describe our recent work on halide perovskites single junction cells and perovskite-perovskite tandems as a potential photo-absorbers for PECs. I will also discuss out initial studies on using protective barriers that test the feasibility of using the hybrid perovskites in combination of HER and OER catalysts for driving the half reactions. We anticipate that successful attempts to create a proof-of-concept PEC system based on hybrid perovskites and earth abundant catalysts will create a disruptive platform to address a long-standing problem and may also open opportunities to further understand the physics of soft matter and liquid interfaces.

Solar-driven water splitting is a promising approach that can potentially realize cost-effective hydrogen production from readily available resources on earth. Among various candidate materials, gallium indium phosphide (GaInP₂) represents an enabling material to serve as a top-junction in monolithically grown multijunction photoelectrodes for ultrahigh efficiency solar water splitting. In such tandem photoelectrode systems, GaInP₂ has multiple roles such as light absorption, electrocatalysis, and corrosion-protection as it is in direct contact with an electrolyte, all of which are critically important to realize high performance solar water splitting. Here we present an approach to simultaneously enhance both efficiency and stability of GaInP₂ photocathodes in the hydrogen-evolution half-reaction of solar water splitting. The key aspect is to incorporate nanostructured density-graded surface of GaInP₂ photocathodes in conjunction with sulfur-based tailoring of surface atomic composition. The resulting surface-tailored GaInP2 photocathodes not only significantly suppressed reflection losses at the semiconductor/electrolyte interface with enhanced saturated current density from -13.3 mA/cm² to -15.2 mA/cm² but also provides a dramatically boosted electrochemical stability with lifetime over 124 hours in 0.5 M H₂SO₄ under simulated AM1.5G illumination.

The hydrogen production will play a major role for the energy transition. Recently, for water splitting context [1], [2], a demonstration of the efficiency enhancement of BiVO4 photoanodes has been shown in PEC devices[3] by simply a texturation of surfaces. Moreover, in the study of the semiconductor photocatalyst materials[4], GaP semiconductor appears to be a good candidate for photoelectrode in PEC devices[5]. One strong argument is to have at least 1.73 eV photopotential requirements for water splitting and its bandgap is larger and about 2.26 eV.
In this aim of water splitting applications, we propose a surface energy engineering for a large scale textured GaP template monolithically integrated on Si. Based on experimental analysis and theory, the stability of the {114} facets is scrutinized by scanning tunneling microscopy images and also supported by density functional theory calculations. We then show that change of the surface energy for experimentally promoting the GaP(114) surface texturation can be achieved through (i) destabilizing the GaP(001) surface by using a vicinal Si substrate or through (ii) favoring the {114} facets formation by changing the group-V atmosphere above the surface on a miscut-free GaP substrate.

This work is supported by the French National Research Agency project ANTIPODE (Grant no. 14-CE26-0014-01) and Région Bretagne. The ab initio simulations have been performed on HPC resources of CINES under the allocation 2017-[x2017096724] made by GENCI (Grand Equipement National de Calcul Intensif).

[1] M. G. Walter et al., ‘Solar Water Splitting Cells’, Chem. Rev., vol. 110, no. 11, pp. 6446–6473, Nov. 2010.
[2] A. Fujishima and K. Honda, ‘Electrochemical Photolysis of Water at a Semiconductor Electrode’, Nature, vol. 238, no. 5358, p. 37, Jul. 1972.
[3] J. Zhao et al., ‘High-Performance Ultrathin BiVO4 Photoanode on Textured Polydimethylsiloxane Substrates for Solar Water Splitting’, ACS Energy Lett., vol. 1, no. 1, pp. 68–75, Jul. 2016.
[4] A. Kudo and Y. Miseki, ‘Heterogeneous photocatalyst materials for water splitting’, Chem. Soc. Rev., vol. 38, no. 1, pp. 253–278, 2009.
[5] E. E. Barton, D. M. Rampulla, and A. B. Bocarsly, ‘Selective Solar-Driven Reduction of CO2 to Methanol Using a Catalyzed p-GaP Based Photoelectrochemical Cell’, J. Am. Chem. Soc., vol. 130, no. 20, pp. 6342–6344, May 2008.

Corrosion is a ubiquitous natural process that impacts all engineering systems, occurs through many mechanisms, and has been studied extensively, especially for metals and non-metals commonly used in construction, pipelines, and shipping. Corrosion is particularly challenging for photoelectrochemical (PEC) water-splitting devices which call for immersion of multiple conductive materials (semiconductors, catalysts, membranes) in electrolytes saturated with either hydrogen or oxygen. In this talk, I will compare and contrast the typical and best practices used in photoelectrochemical stability testing with those developed by corrosion engineers, to identify practices that can be beneficially adopted by the PEC water-splitting community. I will also discuss these practices with a focus on mechanistic studies, corrosion prevention, failure analysis, and lifetime predictions for materials in corrosive environments.

Photoelectrochemical (PEC) device is one of the promising solutions to the increasing global energy demand by using semiconductors to store solar energy in a form of hydrogen/hydrocarbon fuels. However, the operating conditions for these PEC devices, either strong alkaline or strong acid, are typically corrosive to the semiconductors. These semiconductors also need to provide sufficient photovoltage to drive the desire chemical reactions. Among them, II-VI semiconductors such as CdTe, ZnTe are promising candidates for PEC photoelectrodes because of their favorable band positions and potential thermodynamic stability over a wide range of operating pH. Herein, we first describe the corrosion behaviors of these materials and the interfaces between the semiconductors and the electrolytes in both strong acid and strong alkaline media. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectroscopy (ICP-MS) and electrochemical techniques are employed to study the chemical and physical changes of the materials. The corrosion kinetics of the materials are quantified at different operating conditions. We also discuss possible protection schemes to increase the stability of the materials under operation in PEC devices.

The generation of hydrogen from water and sunlight through photoelectrochemical cells (PECs) offers a promising approach for producing scalable and sustainable carbon-free energy. However, the design of high-performance PECs requires a detailed understanding of physicochemical properties of the interface between semiconductor photoelectrode materials and liquid water, which is often difficult to probe under operating conditions. In this talk, we investigate the surface chemistry of representative III-V semiconductor electrodes, including GaN and Ga(In)P, at the interface with water using a combination of first-principles and experimental techniques. We show that coupling first-principles molecular dynamics simulations with advanced electronic structrure methods beyond DFT, and in situ experiments allows one to link electronic properties with local interfacial chemistry, and to probe chemical and morphological changes of photoelectrode surfaces in contact with water. We demonstrate how subtle features in the surface mophology can significantly affect the electronic structures of the semiconductors, including the band edge positions of the materials. Implications for the efficiency and stability of the III-V semiconductor photoelectrodes will be discussed.

This work was supported by the U.S. Department of Energy at the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Recently, cuprous oxide (Cu₂O) have been prepared as a promising candidate for low cost solar energy conversion on account of a suitable bandgap (~ 2.1 eV) to absorb a large portion of the solar spectrum(λ ≤ 590 nm), and favorable band positions to enable carrier injection to water splitting process. Notably, worldwide reserves of copper is estimated 720 million tons compared with 66 000 tons for all platinum group metals combined. However, despite such potential, Cu₂O is liable to suffer oxidative or reductive photocorrosion to form CuO or Cu from its operating conditions (e.g. electrolyte at benign pH). The critical requirements especially for photoanode are the electrical properties of it and the interface between a photoactive material and the outermost layer, to deliver all of the photoelectrons to water splitting process rather than photocrossion process. Recently, our group demonstrated optical excitation at their localized surface plasmon resonance (LSPR) frequency and induced local enhancement of electromagnetic (EM) fields near the nanoparticles surface that can be markedly boosted using metal-dielectric hybrid-structure geometry. The dual functional plasmonic metal layers which act as a passivation and also plasmonic resonator encourages us to develop brand-new design for water splitting photoelectrode.