Solar Water-splitting Devices Research Articles

Understanding the in-situ transformation of CuxO interlayers to increase the water splitting efficiency in NiO/n-Si photoanodes

The buried interface tens of nanometers beneath the solid-liquid junction is crucial for photocarrier extraction, influencing the overall efficiency of photoelectrochemical devices. Precise characterization of the interfacial properties is essential for device optimization but remains challenging. Here, we directly probe the in situ transformation of a CuxO interlayer at the NiO/n-Si interface by hard X-ray photoelectron spectroscopy. It is found that Cu(I) in the CuxO interlayer gradually transforms to Cu(II) with air exposure, forming an energetically more favorable interface and improving photoanode’s efficiency. Based on this finding, a reactive e-beam evaporation process is developed for the direct deposition of a CuO interlayer, achieving a half-cell solar-to-hydrogen efficiency of 4.56% for the optimized NiO/CuO/n-Si heterojunction photoanode. Our results highlight the importance of precision characterization of interfacial properties with advanced hard X-ray photoelectron spectroscopy in guiding the design of efficient solar water-splitting devices.

Synthesis and Tuning of Electronic, Optical, and Photoelectrochemical Properties of Copper Metavanadate Alloys via Alkaline Earth Metal Substitution

Unlike the well-studied and technologically advanced Group III-V and Group II-VI compound semiconductor alloys, alloys of ternary metal oxide semiconductors have only recently begun to receive widespread attention. Here, we describe the effect of alkaline earth metal substitution on the optical, electronic, and photoelectrochemical (PEC) properties of copper metavanadate (CuV2O6). As a host, the Cu-V-O compound family presents a versatile framework to develop such composition-property correlations. Alloy compositions of A0.1Cu0.9V2O6 (A = Mg, Ca) photoanodes were synthesized via a time and energy-efficient solution combustion synthesis (SCS) method. The effect of introducing alkaline earth metals (Mg, Ca) on the crystal structure, microstructure, electronic, and optical properties of copper metavanadates was investigated by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), transmission electron microscopy (TEM), and Raman spectroscopy. The PXRD, TEM, and Raman spectroscopy data demonstrated the polycrystalline powder samples to be mutually soluble, solid solutions of copper and alkaline earth metal metavanadates and not simple mixtures of these compounds. The DRS data showed a systematic decrease in the optical bandgap with Cu incorporation. These trends were corroborated by electronic band structure calculations. Finally, the PEC properties exhibited a strong dependence on the alloy composition, pointing to possible applicability in solar water splitting, heterogeneous photocatalysis, phosphor lighting/displays, and photovoltaic devices.

Modulated Electronic Structure of Co3O4 by Single Atoms for Efficient Anodic Oxygen Evolution in Acid.

The challenge of the practical application of a water electrolyzer system lies in the development of low-manufacturing cost, highly active, and stable electrocatalysts to replace the noble metal ones, in order to enable environmentally friendly hydrogen production on a large scale. Herein, a facile method is proposed for boosting the performance of Co3O4 through the incorporation of large-sized single atoms. Due to the larger ionic radius of rare earth metals than that of Co, the incorporation elongates the bond length of Co─O, resulting in the narrowed d-p band centers and the high spin configuration, which is favorable for the interaction and charge transfer with absorbent (*OH). As a result, the Ce-incorporated Co3O4 with the longest Co─O bond length exhibits the best oxygen evolution reaction (OER) performance, specifically, the turnover frequency is over 17 times higher than that of pristine Co3O4 nanosheet under an overpotential of 400mV. Powered by a commercial Si solar cell, a two-electrode solar water-splitting device combining Ce-incorporated Co3O4 and Pt delivers a solar-to-hydrogen conversion efficiency of 13.53%. The strategy could provide a new insight for improving the performance of OER electrocatalysts in acid toward practical applications.

Solar-Driven Upgrading of Biomass by Coupled Hydrogenation Using in Situ Photoelectrochemically Generated H2

Green H2 has been recognized as an important element in efforts to decarbonize our fossil fuel-dependent society. One approach to produce green H2 is solar water splitting in a photoelectrochemical (PEC) device. Solar-to-hydrogen (STH) efficiencies of up to 30% have been demonstrated1 but studies have shown that this approach still results in a relatively high levelized cost of hydrogen (LCOH) of ~10 US$ per kg of H2.2-3 This is ca. one order of magnitude higher than that of hydrogen produced via steam methane reforming (SMR), which forms the bulk of the currently produced H2. A possible solution is to incorporate an upgrading process of biomass feedstock that generates valuable chemicals into the solar water splitting device. This is expected to not only decrease the overall LCOH but also introduce an alternative renewable pathway in chemical manufacturing. In this study, we propose the concept of solar-driven hydrogenation of biomass-derived feedstock. Photoelectrochemically generated H2 in our solar water splitting device is coupled in situ with the homogenously catalyzed hydrogenation of itaconic acid (IA) to methyl succinic acid (MSA). IA has been identified by the US Department of Energy as one of the twelve building blocks that possess the potential to be transformed subsequently into several high-value bio-based chemicals or materials.4 MSA is a valuable chemical compound with an estimated global market size of up to ~15,000 tonnes, whose derivatives are ubiquitously used as solvents in cosmetics,5 polymer synthesis,6 binders in powder coatings,7 and organic synthesis, especially for pharmaceutical synthesis.8-9 Our coupled hydrogenation approach—performed in the PV-electrolyzer and PEC configurations using III-V PV cells and BiVO4-based photoelectrode, respectively—successfully demonstrate solar-driven H2-to-MSA conversion efficiencies as high as 60%. In comparison to the non-coupled approach (i.e., direct hydrogenation), our coupled system offers synergistic benefits in terms of prolonged durability and a higher degree of flexibility toward other important chemical transformation reactions. In addition, life-cycle net energy assessment and technoeconomic analysis results show that adding the coupled hydrogenation process significantly lowers the energy payback time and the LCOH, respectively, to a point that is competitive even with SMR-produced H2. Further implications and optimization potentials of the coupled PEC hydrogenation approach will be discussed. References Kim, J. H.; Hansora, D.; Sharma, P.; Jang, J.-W.; Lee, J. S., Toward practical solar hydrogen production – an artificial photosynthetic leaf-to-farm challenge. Chem. Soc. Rev. 2019, 48 (7), 1908-1971.Shaner, M. R.; Atwater, H. A.; Lewis, N. S.; McFarland, E. W., A comparative technoeconomic analysis of renewable hydrogen production using solar energy. Energy Environ. Sci. 2016, 9 (7), 2354-2371.Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F., Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry. Energy Environ. Sci. 2013, 6 (7), 1983-2002.Werpy, T.; Petersen, G. Top value added chemicals from biomass: volume I--results of screening for potential candidates from sugars and synthesis gas; National Renewable Energy Lab., Golden, CO (US): 2004.Richard, H.; Muller, B. Use of a 2-methylsuccinic acid diester derivative as solvent in cosmetic compositions; cosmetic compositions containing the same. WO2012119861A3, 2012.Verduyckt, J.; De Vos, D. Method for the production of methylsuccinic acid and the anhydride thereof from citric acid. WO2018065475A1, 2017.Mijolovic, D.; Szarka, Z. J.; Heimann, J.; Garnier, S. Powder coating useful as a coating agent, and for coating metallic- and non-metallic surfaces, comprises a binder comprising methyl succinic acid. DE102011080722A1, 2012.Okabe, M.; Lies, D.; Kanamasa, S.; Park, E. Y., Biotechnological production of itaconic acid and its biosynthesis in Aspergillus terreus. Appl. Microbiol. Biotechnol. 2009, 84 (4), 597-606.Willke, T.; Vorlop, K.-D., Biotechnological production of itaconic acid. Appl. Microbiol. Biotechnol. 2001, 56 (3), 289-295.

Tuning Interfacial Chemistry to Direct the Electrosynthesis of Metal Oxide Semiconductors.

ConspectusMetal oxide semiconductors have many features that make them attractive for both fundamental and applied studies. For example, these compounds contain elements (e.g., Fe, Cu, Ti, etc.) that are derived from minerals rendering them earth-abundant and, most often, are also not toxic. Therefore, they have been examined for possible applicability in a very diverse range of technological applications including photovoltaic solar cells, charge storage devices, displays, smart windows, touch screens, etc. The fact that metal oxide semiconductors have both n- and p-type conductivity makes them amenable for use as hetero- or homojunctions in microelectronic devices and as photoelectrodes in solar water-splitting devices. This Account presents a review of collaborative research on the electrosynthesis of metal oxides from our respective groups against the backdrop of key developments on this topic. The many variants that interfacial chemical modification schemes offer are shown herein to lead to the targeted synthesis of a wide array of not only simple binary metal oxides but also more complex chemistries involving multinary compound semiconductors and alloys.This Account presents our perspective on how parallel developments in the understanding of and ability to manipulate electrode-electrolyte interfaces have correspondingly enabled the innovation of a broad array of electrosynthetic strategies. These coupled with the advent of versatile tools to probe interfacial processes (undoubtedly, a child of the nanotechnology "revolution") afford an operando examination of how effective the strategies are to secure the targeted metal oxide product as well as the mechanistic nuances. Flow electrosynthesis, for example, removes many of the complications accruing from the accumulation of interfering side products─veritably, this is an Achilles heel of the electrosynthesis approach. Coupling flow electrosynthesis with downstream analysis tools based on spectroscopic or electroanalytical probes opens up the possibility of immediate process feedback and optimization. The combination of electrosynthesis, stripping voltammetry, and electrochemical quartz crystal nanogravimetry (EQCN), either in a static or in a dynamic (flow) platform, is shown below to offer intriguing possibilities for metal oxide electrosynthesis. While many of the examples below are based on our current and recent research and in other laboratories, unlocking even more potential will hinge on future refinements and innovations that surely are around the corner.

Hybrid Perovskite-Based Wireless Integrated Device Exceeding a Solar to Hydrogen Conversion Efficiency of 11.

A wireless solar water splitting device provides a means to achieve an inexpensive and highly distributed solar-to-fuel system owing to its portability, flexible scale, and simple design. Here, a highly efficient hydrogen-generating artificial leaf is introduced, which is a wireless configuration for converting solar energy into chemical energy, by integrating ahybrid perovskite (PSK) as the light absorber with catalysts for electrochemical reaction. First, a single integrated photoelectrochemical photocathode, and a spatially decoupled hydrogen evolution reaction catalyst, are fabricated. A decoupled geometry is adopted to enable the physical protection of the PSK layer from the electrolyte, thus allowing excellent stability for over 85h. Additionally, an efficient dual photovoltaicmodule photocathode is fabricatedto produce sufficient photovoltage to drive water splitting reactions, as well as a high photocurrent to achieve the applied-bias photoconversion efficiency (13.5%). To investigate the overall water splitting performance, a NiFe-OH catalyst is employed, and the device with a wired configuration achieves a photocurrent density of 9.35mA cm-2 , corresponding to a solar to hydrogen (STH) efficiency of 11.5%. The device with a fully integrated wireless artificial leaf configuration exhibited a similar STH efficiency of over 11%, demonstrating the effectiveness of this cell design.

Modeling the Photostability of Solar Water-Splitting Devices and Stabilization Strategies.

The practical implementation of photoelectrochemical devices for hydrogen generation is limited by their short lifetimes. Understanding the factors affecting the stability of the heterogeneous photoelectrodes is required to formulate degradation mitigation strategies. We developed a multiscale and multiphysics model to investigate and quantify the photostability of photoelectrodes. The model considers the photophysical processes in an electrocatalyst-coated semiconductor immersed in electrolyte, and the kinetics of the competing water-splitting and photocorrosion reactions. When applied to 12 promising compound semiconductors for use as photoanodes (GaAs, GaP, InP, GaN, SiC, AlP, AlAs, CdTe, CdS, CdSe, ZnS, and ZnSe), the semiconductor-electrocatalyst interfacial charge transfer rate constant was found to be the most significant parameter for stabilizing the photoelectrodes. Its increase induced a sigmoid-like increase in photostability, and its optimization made it possible to stabilize nine semiconductors. The semiconductor surface back-bond energy also increased the photostability in a sigmoid-like response, and its optimization allowed to stabilize five semiconductors. The increase of irradiance induced a logarithmic drop in photostability, and limiting it to 100 W/m2 could stabilize three semiconductors. We further observed that the photostability in a device of centimeter-scale can vary by more than 50%, showing stable zones and photocorrosion hotspots. The photoelectrode's length as well as the electrocatalyst and electrolyte conductivities were found to be relevant parameters to impact this heterogeneity in the photostability. Thus, the photostability is not only defined by material properties but also by the specific combination of materials and by the device architecture. The model presented in this study can also be applied to photocathodes and other PEC designs, and can serve as a design tool for understanding, quantifying, and improving the stability.

On the Effect of Thermal Processing on Sn Diffusion and Efficiency Enhancement in Hematite/FTO Photoanodes

The frequently underestimated effects of “in air” thermal treatment processing conditions such as temperature, duration, and heating and cooling rates in the design and efficiency of photoelectrodes fabricated for academic studies onto the most common commercial transparent conductive glass substrate i.e. fluorine-doped tin oxide (FTO) were investigated by XRD, XPS, SEM, conductive AFM, electrochemical impedance spectroscopy (EIS) as well as direct current (DC) and photoelectrochemical (PEC) measurements. The PEC response of Hematite photoanode thin films consisting of short nanorods thermally treated at 400 °C and 800 °C upon fast or extended time conditions is inhibited by factors such as crystallinity, Sn diffusion, or substrate integrity. A “fast” thermal treatment in air at 750 °C provided the best synergy between charge transfer resistance, Sn-diffusion from the FTO substrate, nanorod dimensions, reduced recombination, improved charge separation and minimized substrate damage. This study does offer valuable fundamental and practical insights for a better understanding of the benefits and drawbacks of photoelectrode thermal processing, which is critical for the improvement of the PEC performance-reproducibility relationship for FTO-based solar water splitting systems and devices.

Ruthenium-nickel-cobalt alloy nanoparticles embedded in hollow carbon microtubes as a bifunctional mosaic catalyst for overall water splitting

The development of efficient bifunctional catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is essential for reducing the cost of hydrogen production by water splitting. Herein, hollow microtubes composed of RuNi1Co1 alloy nanoparticles uniformly embedded in the carbon matrix (RuNi1Co1@CMT) are prepared through a simple impregnation followed by reduction. Benefiting from the unique mosaic structure and the synergistic effect between Ru and NiCo, RuNi1Co1@CMT achieves more exposed active sites and improved reaction kinetics. As a consequence, RuNi1Co1@CMT exhibits considerable catalytic activities with the overpotentials of 78 mV for HER and 299 mV for OER at 10 mA cm−2 in 1 M KOH. In addition, RuNi1Co1@CMT exhibits excellent stability for up to 30 h in both HER and OER processes at 20 mA cm−2, which is attributed to the protection of the RuNi1Co1 alloy particles by the carbon layer. Furthermore, the assembled RuNi1Co1@CMT || RuNi1Co1@CMT overall water splitting system shows a cell voltage of 1.58 V at 10 mA cm−2. The density functional theory (DFT) calculations indicate that the addition of Ru can optimize the hydrogen adsorption free energy of Ni and Co sites. Finally, a solar panel-driven water splitting device is built, which can realize green and sustainable hydrogen production. The fabrication of RuNi1Co1@CMT provides a new way for the preparation of effective alloy nanomaterials for energy storage and conversion.

Kesterite Cu2ZnSnS4 thin-film solar water-splitting photovoltaics for solar seawater desalination

Kesterite photovoltaic materials of Cu 2 ZnSnS 4 (CZTS) thin films present excellent photoelectrochemical (PEC) properties in solar water-splitting devices. It is found that photoelectrodes, including CZTS for solar water splitting, are suitable for application in solar seawater desalination due to the similarity of their operation principle and work conditions to photoelectrodes/solutions. Here we report the application of a CZTS-based photocathode from solar water splitting to solar seawater desalination. With the TiO 2 surface modification layer, the CZTS-based photocathode shows superior stability in the progress of seawater desalination and maintains desalination performance above 95% after testing for almost 500 h. Moreover, the device driven by solar energy does not require an additional electrical supply. The combination of redox flow and ion exchange membrane (IEM) for seawater desalination is a highly efficient and promising water treatment technique that can effectively alleviate the shortage of freshwater resources. Solar water-splitting photocathode with promising solar desalination performance Unbiased and stable solar desalination for almost 500 h is presented Intersection of research between water splitting and solar desalination The application of solar water-splitting photocathode for solar desalination Many regions have a shortage of freshwater, and the situation is getting worse with time. Here, Li et al. obtain freshwater from seawater using solar energy by exploiting a solar water-splitting photocathode.

Cu2O photocathodes for unassisted solar water-splitting devices enabled by noble-metal cocatalysts simultaneously as hydrogen evolution catalysts and protection layers

Here, we report a Cu2O-based photocathode applied with pulsed-laser-deposited overlayers with the structure of Ga2O3/AZO/TiO2 and 60 nm dense Pt simultaneously as a hydrogen evolution catalyst and protective layer under backside illumination from ITO substrate. And by depositing thin Au nanoparticles on the bottom of Cu2O, we observe the increase of light harvesting and the production of hot electrons from the surface plasmon resonance effect. The modified photocathode presents a photocurrent density of −4 mA cm−2 at 0 V versus reversible hydrogen electrode and a thermodynamically-based energy conversion efficiency of 0.27% in a weak acidic electrolyte. The stability is tremendously improved compared with other structures without the dense Pt layer. This enhancement is attributed to the formation of a p–n junction at the Cu2O and Ga2O3 interface and surface stabilization by TiO2 and Pt. To demonstrate the impact on the practical application of the photocathode, we fabricate the bias-free solar water-splitting tandem device using a transparent BiVO4 photoanode, which exhibits a stabilized peak photocurrent density of 0.1 mA cm−2 under continuous illumination.

Directly Photoexcited Oxides for Photoelectrochemical Water Splitting.

Artificial photosynthesis promises to become a sustainable way to harvest solar energy and store it in chemical fuels by means of photoelectrochemical (PEC) cells. Although it is intriguing to shift the fossil-fuel-based economy to a renewable carbon-neutral one, which will alleviate environmental problems, there is still a long way to go before it rivals traditional energy sources. Existing solar water-splitting devices can be sorted into three categories: photovoltaic-powered electrolysis, PEC water splitting, and photocatalysis (PC). PEC and PC systems hold the potential to further reduce the cost of devices due to their simple structures in which photoabsorbers and catalysts are closely integrated. PC is expected to be the least expensive approach; however, additional costs and concerns are brought about by the subsequent explosive gas separation. At the heart of all devices, semiconductor photoabsorbers should be efficient, robust, and cheap to satisfy the strict requirements on the market. Therefore, this Review intends to give readers an overview on PEC water splitting, with an emphasis on oxide material-based devices, which hold the potential to support global-scale production in the future.

Improving the Water Oxidation Efficiency with a Light-Induced Electric Field in Nanograting Photoanodes.

Severe charge recombination in solar water-splitting devices significantly limits their performance. To address this issue, we design a frustum of a cone nanograting configuration by taking the hematite and Au-based thin-film photoanode as a model system, which greatly improves the photoelectrochemical water oxidation activity, affording an approximately 10-fold increase in the photocurrent density at 1.23 V versus the reversible hydrogen electrode compared to the planar counterpart. The surface plasmon polariton-induced electric field in hematite plays a dominant role in efficiency enhancement by facilitating charge separation, thus dramatically increasing the incident photon-to-current efficiency (IPCE) by more than 2 orders of magnitude in the near band gap of hematite. And the relatively weak electric field caused by light scattering in the nanograting structure is responsible for the approximate maximum 20-fold increase in IPCE within a broadband wavelength range. Our scalable strategy can be generalized to other solar energy conversion systems.

A general strategy for the preparation of semiconductor-oxide-nanowire photoanodes

Semiconductor photoelectrodes for photoelectrochemical water splitting are at the heart of the area of renewable energy. Among various semiconductors as photoelectrode materials, metal oxide nanowires have attracted exceptional attention due to their favorable band-edge positions, wide distribution of bandgaps, low cost, and outstanding stability. In this study, we develop a general strategy for the preparation of semiconductor-oxide-nanowire photoanodes. Various metal oxide nanowires are grown directly from and on the metal or alloy substrates by thermal oxidation at ultra-low pressure using the corresponding metal or alloy foil as substrates. These metal oxide nanowires and their underlying conductive substrates are used directly as integrated photoanodes, which effectively avoids the detachment of the active light-harvesting materials from the substrates during the photoelectrochemical test. For example, a water photo-oxidation current density of the iron oxide photoanode reaches 0.71 mA/cm2 over a period of 20 h under 100 mW cm−2 illumination at the potential of 1.23 V versus the reversible hydrogen electrode. The successful preparation of these semiconductor-oxide-nanowire photoanodes have important implications for further development of photoelectrodes and highly efficient solar water-splitting devices.

Ordered Array of Metal Particles on Semishell Separated with Ultrathin Oxide: Fabrication and SERS Properties

Metal particles in gap cavities provide an interesting system to achieve hybrid local surface plasmon modes for local field enhancement. Here, we demonstrate a relatively simple method to fabricate Ag nanoparticles positioned on Ag semishells separated by a thin (~5 nm) dielectric layer. The obtained structure can provide strong local electric field enhancement for surface-enhanced Raman scattering (SERS). The fabrication of the ordered array structure was realized by nanosphere self-assembly, atomic layer deposition, and metal thin-film dewetting. Numerical simulation proved that, compared to the conventional metal semishell arrays, the additional Ag particles introduce extra hot spots particularly in the valley regions between adjacent Ag semishells. As a result, the SERS enhancement factor of the metal semishell-based plasmonic structure could be further improved by an order of magnitude. The developed novel plasmonic structure also shows good potential for application in plasmon-enhanced solar water-splitting devices.

Boosting the Efficiency of Photoelectrolysis by the Addition of Non-Noble Plasmonic Metals: Al & Cu.

Solar water splitting by semiconductor based photoanodes and photocathodes is one of the most promising strategies to convert solar energy to chemical energy to meet the high demand for energy consumption in modern society. However, the state-of-the-art efficiency is too low to fulfill the demand. To overcome this challenge and thus enable the industrial realization of a solar water splitting device, different approaches have been taken to enhance the overall device efficiency, one of which is the incorporation of plasmonic nanostructures. Photoanodes and photocathodes coupled to the optimized plasmonic nanostructures, matching the absorption wavelength of the semiconductors, can exhibit a significantly increased efficiency. So far, gold and silver have been extensively explored to plasmonically enhance water splitting efficiency, with disadvantages of high cost and low enhancement. Instead, non-noble plasmonic metals such as aluminum and copper, are earth-abundant and low cost. In this article, we review their potentials in photoelectrolysis, towards scalable applications.

Over 1% Efficient Unbiased Stable Solar Water Splitting Based on a Sprayed Cu2ZnSnS4 Photocathode Protected by a HfO2 Photocorrosion-Resistant Film

Surface passivation of the CdS/Cu2ZnSnS4 photocathode by a HfO2 layer was found to be very effective for enhancing the photoelectrochemical stability. The dependence of the photoelectrochemical performance, especially the stability of the Cu2ZnSnS4-based photocathode, on the thickness of the HfO2 film was systematically investigated. The thickness of the HfO2 layer obviously influenced the PEC stability and efficiency of the Cu2ZnSnS4-based photocathode. The CdS/Cu2ZnSnS4 photocathode modified with a 6 nm thick HfO2 layer showed long-term PEC photocurrent stability of over 10 h while still retaining high half-cell solar-to-hydrogen efficiency (HC-STH) of 2.7%. Finally, we fabricated an unbiased solar water splitting device based on the Pt/HfO2/CdS/Cu2ZnSnS4 photocathode in tandem with a BiVO4 photoanode, and this tandem device not only exhibited an unassisted STH conversion efficiency of 1.046% but also showed a high long-term stability of over 10 h.

Metal–Organic Framework-Derived CoWP@C Composite Nanowire Electrocatalyst for Efficient Water Splitting

Practical application of electrochemical water splitting demands durable, efficient, and non-noble metal catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, we report a new hydrogen evolution nanowire electrocatalyst, consisting of S-doped CoWP nanoparticles embedded in S- and N-doped carbon matrix (S-CoWP@(S,N)-C), which is in situ transformed from Hofmann-type (Co, W)-based metal–organic framework (MOF) nanowires. Because of S and N doping to the carbon matrix and the S doping to CoWP nanoparticles, the obtained S-CoWP@(S,N)-C catalyst reaches a current density of −10 mA cm–2 at −35 and −67 mV (vs RHE) in acidic and alkaline electrolytes, respectively. Powered by a lead halide perovskite solar cell, an unassisted two-electrode solar water-splitting device using MOF-derived S-CoWP@(S,N)-C HER electrocatalysts and S-CoW@(S,N)-C OER electrocatalysts displays a solar-to-hydrogen conversion efficiency of 10.98%. Our method is highly applicable for developing robu...

Boosting the performance of Cu2O photocathodes for unassisted solar water splitting devices

Although large research efforts have been devoted to photoelectrochemical (PEC) water splitting in the past several decades, the lack of efficient, stable and Earth-abundant photoelectrodes remains a bottleneck for practical application. Here, we report a photocathode with a coaxial nanowire structure implementing a Cu2O/Ga2O3-buried p–n junction that achieves efficient light harvesting across the whole visible region to over 600 nm, reaching an external quantum yield for hydrogen generation close to 80%. With a photocurrent onset over +1 V against the reversible hydrogen electrode and a photocurrent density of ~10 mA cm−2 at 0 V versus the reversible hydrogen electrode, our electrode constitutes the best oxide photocathode for catalytic generation of hydrogen from sunlight known today. Conformal coating via atomic-layer deposition of a TiO2 protection layer enables stable operation exceeding 100 h. Using NiMo as the hydrogen evolution catalyst, an all Earth-abundant Cu2O photocathode was achieved with stable operation in a weak alkaline electrolyte. To show the practical impact of this photocathode, we constructed an all-oxide unassisted solar water splitting tandem device using state-of-the-art BiVO4 as the photoanode, achieving ~3% solar-to-hydrogen conversion efficiency. The generation of hydrogen fuel from water and visible light requires photoelectrodes that are inexpensive, stable and highly active. Now, Luo, Gratzel and co-workers report Cu2O photocathodes that reach these goals. Incorporation into an unassisted solar water splitting device gives ~3% solar-to-hydrogen conversion efficiency.

Low Noble Metal Content Oxygen Evolution Reaction Catalyst for Solar Water Splitting

Water splitting for hydrogen production requires catalysts that are active for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). OER kinetics typically limit the efficiency of electrolysis, so improved OER catalysts are needed. This research has found an iridium doped nickel-iron based catalyst that is effective even at extremely low loadings. This low loading also allows the films to be transparent. This work shows an analysis of thin, transparent films of this catalyst. The stability, OER activity, transparency, loadings, composition, and efficiency in a solar water splitting device are all analyzed.