Tailored NiOx/Ni Cocatalysts on Silicon for Highly Efficient Water Splitting Photoanodes via Pulsed Electrodeposition

Metal-insulator-semiconductor photoelectrodes for enhanced photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting provides a scalable and integrated platform to harness renewable solar energy for green hydrogen production. The practical implementation of PEC systems hinges on addressing three critical challenges: enhancing energy conversion efficiency, ensuring long-term stability, and achieving economic viability. Metal-insulator-semiconductor (MIS) heterojunction photoelectrodes have gained significant attention over the last decade for their ability to efficiently segregate photogenerated carriers and mitigate corrosion-induced semiconductor degradation. This review discusses the structural composition and interfacial intricacies of MIS photoelectrodes tailored for PEC water splitting. The application of MIS heterostructures across various semiconductor light-absorbing layers, including traditional photovoltaic-grade semiconductors, metal oxides, and emerging materials, is presented first. Subsequently, this review elucidates the reaction mechanisms and respective merits of vacuum and non-vacuum deposition techniques in the fabrication of the insulator layers. In the context of the metal layers, this review extends beyond the conventional scope, not only by introducing metal-based cocatalysts, but also by exploring the latest advancements in molecular and single-atom catalysts integrated within MIS photoelectrodes. Furthermore, a systematic summary of carrier transfer mechanisms and interface design principles of MIS photoelectrodes is presented, which are pivotal for optimizing energy band alignment and enhancing solar-to-chemical conversion efficiency within the PEC system. Finally, this review explores innovative derivative configurations of MIS photoelectrodes, including back-illuminated MIS photoelectrodes, inverted MIS photoelectrodes, tandem MIS photoelectrodes, and monolithically integrated wireless MIS photoelectrodes. These novel architectures address the limitations of traditional MIS structures by effectively coupling different functional modules, minimizing optical and ohmic losses, and mitigating recombination losses.

Partially oxidized inter-doped RuNi alloy aerogel for the hydrogen evolution reaction in both alkaline and acidic media

A facile reduction and doping process is employed with the supercritical ethanol drying method to form RuNi alloy aerogels. The optimized heterostructure comprising RuNi metal, RuO2, and NiO phases is synthesized through partial oxidation. When applied to the surface of Ni foam, the multiphase aerogels form a morphology of highly porous 0D colloidal aerogel networks on the surface. RuNi alloy-Ni foam oxidized at 350 °C (RuNi-350@NF) has an overpotential of 89 and 61 mV in 1 M KOH and 0.5 M H2SO4 media at 50 mA cm-2, as well as satisfactory long-term stability. Additionally, the Tafel slopes in alkaline and acidic media are found to be 34 and 30.9 mV dec-1, respectively. Furthermore, it exhibits long-term stability (35 h) in alkaline and acidic media at high current densities of 50 mA cm-2, respectively. This study presents a novel strategy for developing exceptionally efficient and free-standing 3D porous aerogel electrocatalysts with potential applications in hydrogen production.

Porous 2D Catalyst Covers Improve Photoelectrochemical Water-Oxidation Performance

Confined catalysis under the cover of 2D materials has emerged as a promising approach for achieving highly effective catalysts in various essential reactions. In this work, a porous cover structure is designed to boost the interfacial charge and mass transfer kinetics of 2D-covered catalysts. The improvement in catalytic performance is confirmed by the photoelectrochemical oxidation evolution reaction (OER) on a photoanode based on an n-Si substrate modified with a NiOx thin-film model electrocatalyst covered with a porous graphene (pGr) monolayer. Experimental results demonstrate that the pGr cover enhances the OER kinetics by balancing the charge and mass transfer at the photoanode and electrolyte interface compared to the intrinsic graphene cover and cover-free control samples. Theoretical investigations further corroborate that the pore edges of the pGr cover boost the intrinsic catalytic activity of active sites on NiOx by reducing the reaction overpotential. Furthermore, the optimized pores, which can be easily controlled by plasma bombardment, allow oxygen molecules produced in the OER to pass through without peeling off the pGr cover, thus ensuring the structural stability of the catalyst. This study highlights the significant role of the porous cover structure in 2D-covered catalysts and provides new insight into the design of high-performance catalysts.

Electrochemical Synthesis of Hollow Nanoparticles via Anodic Transformation of Metastable Core-Shell Precursors

Recent advances in electrosynthesis of nanomaterials expanded structural and compositional variations accessible by the electrochemical method; however, reliably synthesizable morphological variety fall shy of that available by conventional solvothermal synthesis. In this communication, electrochemical preparation of surfactant-free hollow nanoparticles is demonstrated. By anodic conversion of core-shell precursors with metastable cores, hollowed nickel nanoparticles with uniform dimensions were synthesized and characterized. Implementation of TEM grids as the working electrodes, identical location tracking of the morphological evolution of single particles to anodic stimulus has been demonstrated. The synthesized nanoparticles were employed as catalysts for the alkaline hydrogen evolution reaction and exhibited catalytic rates that compare favorably to the Pt/C benchmark. This marks the first pure electrochemical synthesis of hollow nanoparticles and shall contribute to the structural diversification of electrosynthesized nanomaterials.

Near-IR Photoinduced Electrochemiluminescence Imaging with Structured Silicon Photoanodes

Infrared (IR) imaging devices that convert IR irradiation (invisible to the human eye) to a visible signal are based on solid-state components. Here, we introduce an alternative concept based on light-addressable electrochemistry (i.e., electrochemistry spatially confined under the action of a light stimulus) that involves the use of a liquid electrolyte. In this method, the projection of a near-IR image (λexc = 850 or 840 nm) onto a photoactive Si-based photoanode, immersed into a liquid phase, triggers locally the photoinduced electrochemiluminescence (PECL) of the efficient [Ru(bpy)3]2+-TPrA system. This leads to the local conversion of near-IR light to visible (λPECL = 632 nm) light. We demonstrate that compared to planar Si photoanodes, the use of a micropillar Si array leads to a large enhancement of local light generation and considerably improves the resolution of the PECL image by preventing photogenerated minority carriers from diffusing laterally. These results are important for the design of original light conversion devices and can lead to important applications in photothermal imaging and analytical chemistry.

Cocatalysts-Photoanode Interface in Photoelectrochemical Water Splitting: Understanding and Insights

Sluggish oxygen evolution reactions on photoanode surfaces severely limit the application of photoelectrochemical (PEC) water splitting. The loading of cocatalysts on photoanodes has been recognized as the simplest and most efficient optimization scheme, which can reduce the surface barrier, provide more active sites, and accelerate the surface catalytic reaction kinetics. Nevertheless, the introduction of cocatalysts inevitably generates interfaces between photoanodes and oxygen evolution cocatalysts (Ph/OEC), which causes severe interfacial recombination and hinders the carrier transfer. Recently, many researchers have focused on cocatalyst engineering, while few have investigated the effect of the Ph/OEC interface. Hence, to maximize the advantages of cocatalysts, interfacial problems for designing efficient cocatalysts are systematically introduced. In this review, the interrelationship between the Ph/OEC and PEC performance is classified and some methods for characterizing Ph/OEC interfaces are investigated. Additionally, common interfacial optimization strategies are summarized. This review details cocatalyst-design-based interfacial problems, provides ideas for designing efficient cocatalysts, and offers references for solving interfacial problems.

Engineering the Inhomogeneity of Metal-Insulator-Semiconductor Junctions for Photoelectrochemical Methanol Oxidation

Si-based inhomogeneous metal-insulator-semiconductor (MIS) junctions with a discontinuous metal nanostructure on the Si/insulator layer are expected to be efficient photoelectrodes for solar energy conversion. However, the formation of a metal nanostructure with an optimized arrangement on semiconductors for efficient charge carrier collection is still a big challenge. Herein, we report a method for the in situ formation of an n-Si inhomogeneous MIS junction with well-dispersed metal nanocontacts through a self-assembly process during photoelectrochemical (PEC) methanol oxidation. The photovoltage shows a strong dependence on the inhomogeneity of the n-Si MIS junction, which can be precisely tuned by the applied electrode potential and operation time. The appropriate inhomogeneity of the Schottky junction as well as the high barrier regions induced by the metal oxide/(oxy)hydroxide layer synergistically produces a large photovoltage of 500 mV for the n-Si inhomogeneous MIS junction. Finally, the n-Si-based photoanode is coupled with a CO2-to-formate reaction to realize the production of formate at both electrodes, resulting in a high faradic efficiency (FE) of 86 and 93% for anode and cathode reactions at an operational current of 30 mA/cm2, respectively. These findings provide important insights into the design of highly efficient inhomogeneous MIS junctions through an in situ self-assembly route for solar energy conversion and storage.

Enhancing the Photoelectrochemical Performance of a Nanoporous Silicon Photocathode through Electroless Nickel Deposition

Renewable energy sources, particularly solar energy, are key to our efforts to decarbonize. This study investigates the photoelectrochemical (PEC) behavior of nanoporous silicon (NPSi) and its Ni-coated hybrid system. The methods involve the application of a Ni coating to NPSi, a process aimed at augmenting catalytic activity, light absorption, and carrier transport. Scanning electron microscopy was used to analyze the morphological changes on NPSi surfaces due to the Ni coating. Results demonstrate that the Ni coating creates unique structures on NPSi surfaces, with peak PEC performance observed at 15 min of coating time and 60 °C. These conditions were found to promote electron-hole pair separation and uniform Ni coverage. A continuous 50-min white light illumination experiment confirmed stable PEC fluctuations, showing the interplay of NPSi's characteristics and Ni's catalytic effect. This study provides practical guidance for the design of efficient water-splitting catalysts, contributing to the broader field of renewable energy conversion.

Atomically dispersed iridium catalysts on silicon photoanode for efficient photoelectrochemical water splitting

Stabilizing atomically dispersed single atoms (SAs) on silicon photoanodes for photoelectrochemical-oxygen evolution reaction is still challenging due to the scarcity of anchoring sites. Here, we elaborately demonstrate the decoration of iridium SAs on silicon photoanodes and assess the role of SAs on the separation and transfer of photogenerated charge carriers. NiO/Ni thin film, an active and highly stable catalyst, is capable of embedding the iridium SAs in its lattices by locally modifying the electronic structure. The isolated iridium SAs enable the effective photogenerated charge transport by suppressing the charge recombination and lower the thermodynamic energy barrier in the potential-determining step. The Ir SAs/NiO/Ni/ZrO₂/n-Si photoanode exhibits a benchmarking photoelectrochemical performance with a high photocurrent density of 27.7 mA cm^-2 at 1.23 V vs. reversible hydrogen electrode and 130 h stability. This study proposes the rational design of SAs on silicon photoelectrodes and reveals the potential of the iridium SAs to boost photogenerated charge carrier kinetics.

Surface-Tailored Medium Entropy Alloys as Radically Low Overpotential Oxygen Evolution Electrocatalysts

Numerous studies have explored new materials for electrocatalysts, but it is difficult to discover materials that surpass the catalytic activity of current commercially available noble metal electrocatalysts. In contrast to conventional transition metal alloys, high-entropy alloys (HEAs) have immense potential to maximize their catalytic properties because of their high stability and compositional diversity as oxygen evolution reactions (OERs). This work presents medium-entropy alloys (MEAs) as OER electrocatalysts to simultaneously satisfy the requirement of high catalytic activity and long-term stability. The surface of MEA electrocatalyst is tailored to suit the OER via anodizing and cyclic voltammetry activation methods. Optimized electrical properties and hydrophilicity of the surface enable an extremely low overpotential of 187 mV for achieving the current density of 10 mA cm^-2 alkaline media. Furthermore, a combined photovoltaic-electrochemical system with MEA electrocatalyst and a perovskite/Si tandem solar cell exhibits a solar-to-hydrogen conversion efficiency of 20.6% for an unassisted hydrogen generation system. These results present a new pathway for designing sustainable high efficiency water splitting cells.

Controlled Band Offsets in Ultrathin Hematite for Enhancing the Photoelectrochemical Water Splitting Performance of Heterostructured Photoanodes

Formation of type II heterojunctions is a promising strategy to enhance the photoelectrochemical performance of water-splitting photoanodes, which has been tremendously studied. However, there have been few studies focusing on the formation of type II heterojunctions depending on the thickness of the overlayer. Here, enhanced photoelectrochemical activities of a Fe₂O₃ film deposited-BiVO₄/WO₃ heterostructure with different thicknesses of the Fe₂O₃ layer have been investigated. The Fe₂O₃ (10 nm)/BiVO₄/WO₃ heterojunction photoanode shows a much higher photocurrent density compared to the Fe₂O₃ (100 nm)/BiVO₄/WO₃ photoanode. The Fe₂O₃ (10 nm)/BiVO₄/WO₃ trilayer heterojunction anodes have sequential type II junctions, while a thick Fe₂O₃ overlayer forms an inverse type II junction between Fe₂O₃ and BiVO₄. Furthermore, the incident-photon-to-current efficiency measured under back-illumination is higher than those measured under front-illumination, demonstrating the importance of the illumination sequence for light absorption and charge transfer and transport. This study shows that the thickness of the oxide overlayer influences the energy band alignment and can be a strategy to improve solar water splitting performance. Based on our findings, we propose a photoanode design strategy for efficient photoelectrochemical water splitting.

Electrochemical synthesis of core-shell nanoparticles by seed-mediated selective deposition

Conventional solvothermal synthesis of core-shell nanoparticles results in them being covered with surfactant molecules for size control and stabilization, undermining their practicality as electrocatalysts. Here, we report an electrochemical method for the synthesis of core-shell nanoparticles directly on electrodes, free of surfactants. By implementation of selective electrodeposition on gold cores, 1st-row transition metal shells were constructed with facile and precise thickness control. This type of metal-on-metal core-shell synthesis by purely electrochemical means is the first of its kind. The applicability of the nanoparticle decorated electrodes was demonstrated by alkaline oxygen evolution catalysis, during which the Au-Ni example displayed stable catalysis with low overpotential.

A strategy for preparing high-efficiency and economical catalytic electrodes toward overall water splitting

Electrolyzing water technology to prepare high-purity hydrogen is currently an important field in energy development. However, the preparation of efficient, stable, and inexpensive hydrogen production technology from electrolyzed water is a major problem in hydrogen energy production. The key technology for hydrogen production from water electrolysis is to prepare highly efficient catalytic, stable and durable electrodes, which are used to reduce the overpotential of the hydrogen evolution reaction and the oxygen evolution reaction of electrolyzed water. The main strategies for preparing catalytic electrodes include: (i) choosing cheap, large specific surface area and stable base materials, (ii) modulating the intrinsic activity of the catalytic material through elemental doping and lattice changes, and (iii) adjusting the morphology and structure to increase the catalytic activity. Based on these findings, herein, we review the recent work in the field of hydrogen production by water electrolysis, introduce the preparation of catalytic electrodes based on nickel foam, carbon cloth and new flexible materials, and summarize the catalytic performance of metal oxides, phosphides, sulfides and nitrides in the hydrogen evolution and oxygen evolution reactions. Secondly, parameters such as the overpotential, Tafel slope, active site, turnover frequency, and stability are used as indicators to measure the performance of catalytic electrode materials. Finally, taking the material cost of the catalytic electrode as a reference, the successful preparations are comprehensively compared. The overall aim is to shed some light on the exploration of high-efficiency and economical electrodes in energy chemistry and also demonstrate that there is still room for discovering new combinations of electrodes including base materials, composition lattice changes and morphologies.

Engineering Nanostructure-Interface of Photoanode Materials Toward Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) water oxidation based on semiconductor materials plays an important role in the production of clean fuel and value-added chemicals. Nanostructure-interface engineering has proven to be an effective way to construct highly efficient PEC water oxidation photoanodes with good light capture, carrier transport, and water oxidation kinetics. However, from theoretical and application perspectives, the relationship between the nanostructure and interface of photoanode materials and their PEC performance remains unclear. In this review, the PEC water oxidation reaction mechanism and evaluation criteria are briefly presented. The theoretical basis and research status of the nanostructure-interface engineering on constructing high-performance PEC water oxidation photoanodes are summarized and discussed. Finally, the current challenges and the future opportunities of nanostructure-interface engineering for the PEC reactions are pointed out.

Direct Synthesis of Molybdenum Phosphide Nanorods on Silicon Using Graphene at the Heterointerface for Efficient Photoelectrochemical Water Reduction

MoP nanorod-array catalysts were directly synthesized on graphene passivated silicon photocathodes without secondary phase. Mo-O-C covalent bondings and energy band bending at heterointerfaces facilitate the electron transfer to the reaction sites. Numerous catalytic sites and drastically enhanced anti-reflectance of MoP nanorods contribute to the high solar energy conversion efficiency. Transition metal phosphides (TMPs) and transition metal dichalcogenides (TMDs) have been widely investigated as photoelectrochemical (PEC) catalysts for hydrogen evolution reaction (HER). Using high-temperature processes to get crystallized compounds with large-area uniformity, it is still challenging to directly synthesize these catalysts on silicon photocathodes due to chemical incompatibility at the heterointerface. Here, a graphene interlayer is applied between p-Si and MoP nanorods to enable fully engineered interfaces without forming a metallic secondary compound that absorbs a parasitic light and provides an inefficient electron path for hydrogen evolution. Furthermore, the graphene facilitates the photogenerated electrons to rapidly transfer by creating Mo-O-C covalent bondings and energetically favorable band bending. With a bridging role of graphene, numerous active sites and anti-reflectance of MoP nanorods lead to significantly improved PEC-HER performance with a high photocurrent density of 21.8 mA cm^-2 at 0 V versus RHE and high stability. Besides, low dependence on pH and temperature is observed with MoP nanorods incorporated photocathodes, which is desirable for practical use as a part of PEC cells. These results indicate that the direct synthesis of TMPs and TMDs enabled by graphene interlayer is a new promising way to fabricate Si-based photocathodes with high-quality interfaces and superior HER performance.

Grain Boundaries Boost Oxygen Evolution Reaction in NiFe Electrocatalysts

In a polycrystalline material, the grain boundaries (GBs) can be effective active sites for catalytic reactions by providing an electrodynamically favorable surface. Previous studies have shown that grain boundary density is related to the catalytic activity of the carbon dioxide reduction reaction, but there is still no convincing evidence that the GBs provide surfaces with enhanced activity for oxygen evolution reaction (OER). Combination of various electrochemical measurements and chemical analysis reveals the GB density at surface of NiFe electrocatalysts directly affects the overall OER. In situ electrochemical microscopy vividly shows that the OER occurs mainly at the GB during overall reaction. It is observed that the reaction determining steps are altered by grain boundary densities and the meaningful work function difference between the inside of grain and GBs exists. High-resolution transmission electron microscopy shows that extremely high index planes are exposed at the GBs, enhancing the oxygen evolution activity. The specific nature of GBs and its effects on the OER demonstrated in this study can be applied to the various polycrystalline electrocatalysts.

Custom plating of nanoscale semiconductor/catalyst junctions for photoelectrochemical water splitting

Photoelectrochemical water splitting under harsh chemical conditions can be promoted by dispersed transition metal nanoparticles electrodeposited on n-Si surfaces, without the need for classical protection layers. Although this method is simple, it only allows for poor control of metal morphology and geometry on the photoanode surface. Herein, we introduce templated nanoscale electrodeposition on photoactive n-Si for the customization of nanoscale inhomogeneous Schottky junctions and demonstrate their use as stable photoanodes. The photoelectrochemical properties of the so-manufactured photoanodes exhibit a strong dependence on the photoanodes' geometrical features, and the obtained experimental trends are rationalized using simulation.

Three-step method with self-sacrificial Co to prepare a uniform 5 nm-scale Pt catalyst for the oxygen reduction reaction

Simple and rapid preparation method for a highly dispersed and small-sized CoPt catalyst.

Electrodeposition of (hydro)oxides for an oxygen evolution electrode

Electrochemical water splitting is a promising technology for hydrogen production and sustainable energy conversion, but the electrolyzers that are currently available do not have anodic electrodes that are robust enough and highly active for the oxygen evolution reaction (OER). Electrodeposition provides a feasible route for preparing freestanding OER electrodes with high active site utilization, fast mass transport and a simple fabrication process, which is highly attractive from both academic and commercial points of view. This minireview focuses on the recent electrodeposition strategies for metal (hydro)oxide design and water oxidation applications. First, the intrinsic advantages of electrodeposition in comparison with traditional technologies are introduced. Then, the unique properties and underlying principles of electrodeposited metal (hydro)oxides in the OER are unveiled. In parallel, illustrative examples of the latest advances in materials structural design, controllable synthesis, and mechanism understanding through the electrochemical synthesis of (hydro)oxides are presented. Finally, the latest representative OER mechanism and electrodeposition routes for OER catalysts are briefly overviewed. Such observations provide new insights into freestanding (hydro)oxides electrodes prepared via electrodeposition, which show significant practical application potential in water splitting devices. We hope that this review will provide inspiration for researchers and stimulate the development of water splitting technology.

A high-performance silicon photoanode enabled by oxygen vacancy modulation on NiOOH electrocatalyst for water oxidation

Silicon (Si) is an attractive photoanode material for photoelectrochemical (PEC) water splitting. However, Si photoanode towards the oxygen evolution reaction (OER) is highly challenged due to its poor stability and catalytic inactivity. The integration of highly active electrocatalysts with Si photoanodes has been considered to be an effective strategy to improve their OER performance by accelerating the reaction kinetics and inhibiting Si photocorrosion. In this work, ultra-small NiFe nanoparticles are deposited onto the n-Si/Ni/NiOOH surface to improve the activity and stability of Si photoanodes by engineering the electrocatalyst and Si interface. Ultra-small NiFe nanoparticles can introduce oxygen vacancies via modulating the local electronic structure of Ni hosts in NiOOH electrocatalysts for fast charge separation and transfer. Besides, NiFe nanoparticles can also serve as a co-catalyst exposing more active sites and as a protection layer preventing Si photocorrosion. The as-prepared n-Si/Ni/NiOOH/NiFe photoanode exhibits excellent OER activity with an onset potential of 1.0 V versus reversible hydrogen electrode (RHE) and a photocurrent density of ∼25.2 mA cm^-2 at 1.23 V versus RHE. This work provides a promising approach to design high-performance Si photoanodes by surface electrocatalyst engineering.

Electroless Plating of NiFeP Alloy on the Surface of Silicon Photoanode for Efficient Photoelectrochemical Water Oxidation

N-type silicon is a kind of semiconductor with a narrow band gap that has been reported as an outstanding light-harvesting material for photoelectrochemical (PEC) reactions. Decorating a thin catalyst layer on the n-type silicon surface can provide a direct and effective route toward PEC water oxidation. However, most of catalyst immobilization methods for reported n-type silicon photoanodes have been based on energetically demanding, time-consuming, and high-cost processes. Herein, a high-performance NiFeP alloy (NiFeP)-decorated n-type micro-pyramid silicon array (n-Si) photoanode (NiFeP/n-Si) was prepared by a fast and low-cost electroless deposition method for light-driven water oxidation reaction. The saturated photocurrent density of NiFeP/n-Si can reach up to ∼40 mA cm^-2, and a photocurrent density of 15.5 mA cm^-2 can be achieved at 1.23 V_RHE under light illumination (100 mW cm^-2, AM1.5 filter), which is one of the most promising silicon-based photoanodes to date. The kinetic studies showed that the NiFeP on the silicon photoanodes could significantly decrease the interfacial charge recombination between the n-type silicon surface and electrolyte.

Strategies for Semiconductor/Electrocatalyst Coupling toward Solar-Driven Water Splitting

Hydrogen (H2) has a significant potential to enable the global energy transition from the current fossil-dominant system to a clean, sustainable, and low-carbon energy system. While presently global H2 production is predominated by fossil-fuel feedstocks, for future widespread utilization it is of paramount importance to produce H2 in a decarbonized manner. To this end, photoelectrochemical (PEC) water splitting has been proposed to be a highly desirable approach with minimal negative impact on the environment. Both semiconductor light-absorbers and hydrogen/oxygen evolution reaction (HER/OER) catalysts are essential components of an efficient PEC cell. It is well documented that loading electrocatalysts on semiconductor photoelectrodes plays significant roles in accelerating the HER/OER kinetics, suppressing surface recombination, reducing overpotentials needed to accomplish HER/OER, and extending the operational lifetime of semiconductors. Herein, how electrocatalyst coupling influences the PEC performance of semiconductor photoelectrodes is outlined. The focus is then placed on the major strategies developed so far for semiconductor/electrocatalyst coupling, including a variety of dry processes and wet chemical approaches. This Review provides a comprehensive account of advanced methodologies adopted for semiconductor/electrocatalyst coupling and can serve as a guideline for the design of efficient and stable semiconductor photoelectrodes for use in water splitting.

Nanoscale semiconductor/catalyst interfaces in photoelectrochemistry

Semiconductor structures (for example, films, wires, particles) used in photoelectrochemical devices are often decorated with nanoparticles that catalyse fuel-forming reactions, including water oxidation, hydrogen evolution or carbon-dioxide reduction. For high performance, the catalyst nanoparticles must form charge-carrier-selective contacts with the underlying light-absorbing semiconductor, facilitating either hole or electron transfer while inhibiting collection of the opposite carrier. Despite the key role played by such selective contacts in photoelectrochemical energy conversion and storage, the underlying nanoscale interfaces are poorly understood because direct measurement of their properties is challenging, especially under operating conditions. Using an n-Si/Ni photoanode model system and potential-sensing atomic force microscopy, we measure interfacial electron-transfer processes and map the photovoltage generated during photoelectrochemical oxygen evolution at nanoscopic semiconductor/catalyst interfaces. We discover interfaces where the selectivity of low-Schottky-barrier n-Si/Ni contacts for holes is enhanced via a nanoscale size-dependent pinch-off effect produced when surrounding high-barrier regions develop during device operation. These results thus demonstrate (1) the ability to make nanoscale operando measurements of contact properties under practical photoelectrochemical conditions and (2) a design principle to control the flow of electrons and holes across semiconductor/catalyst junctions that is broadly relevant to different photoelectrochemical devices.

5.1% efficiency of Si photoanodes for photoelectrochemical water splitting catalyzed by porous NiFe (oxy)hydroxide converted from NiFe oxysulfide

A porous NiFe (oxy)hydroxide catalyst fabricated on n+pp+-Si/Ni/NiOx, which is converted from an electrodeposited NiFe oxysulfide, allows a silicon photoanode for water splitting to hit a record 5.1% efficiency with good stability of up to 135 h under 40 mA cm-2 in 1.0 M NaOH.

All-Solution-Processed WO3/BiVO4 Core-Shell Nanorod Arrays for Highly Stable Photoanodes

Tungsten oxide (WO₃) and bismuth vanadate (BiVO₄) are one of the most attractive combinations to construct an efficient heterojunction for photoelectrochemical (PEC) applications. Here, we report an all-solution-processed WO₃/BiVO₄ heteronanostructure photoanode with highly enhanced photoactivity and stability for sustainable energy production. The vertically aligned WO₃ nanorods were synthesized on a fluorine-doped tin oxide/glass substrate by the hydrothermal method without a seed layer and BiVO₄ was deposited by pulsed electrodeposition for conformal coating. Owing to the long diffusion lengths of charge carriers in the WO₃ nanorods, the ability to absorb the wider range of wavelengths, and appropriate band-edge positions of the WO₃/BiVO₄ heterojunction for spontaneous PEC reaction, the optimum WO₃/BiVO₄ photoanode has a photocurrent density of 4.15 mA/cm² at 1.23 V versus RHE and an incident-photon-to-current efficiency of 75.9% at 430 nm under front illumination, which are a double and quadruple those of pristine WO₃ nanorod arrays, respectively. Our work suggests an environment-friendly and low-cost all-solution process route to synthesize high-quality photoelectrodes.

A porous Ni-O/Ni/Si photoanode for stable and efficient photoelectrochemical water splitting

In this study a porous Ni-O/Ni/Si photoanode with superior photoelectrochemical activity and stability was obtained.