Progress in Developing Metal Oxide Nanomaterials for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting represents an environmentally friendly and sustainable method to obtain hydrogen fuel. Semiconductor materials as the central components in PEC water splitting cells have decisive influences on the device's solar‐to‐hydrogen conversion efficiency. Among semiconductors, metal oxides have received a lot of attention due to their outstanding (photo)‐electrochemical stability, low cost, favorable band edge positions and wide distribution of bandgaps. In the past decades, significant processes have been made in developing metal oxide nanomaterials for PEC water splitting. In this review, the recent progress using metal oxides as photoelectrodes and co‐catalysts for PEC water splitting is summarized. Their performance, limitations and potentials are also discussed. Last, the key challenges and opportunities in the development and implementation of metal oxide nanomaterials for PEC water splitting are discussed.     

Light‐Driven BiVO4–C Fuel Cell with Simultaneous Production of H2O2

Photoelectrochemical (PEC) systems have been researched for decades due to their great promise to convert sunlight to fuels. The majority of the research on PEC has been using light to split water to hydrogen and oxygen, and its performance is limited by the need of additional bias. Another research direction on PEC using light, is to decompose organic materials while producing electricity. In this work, the authors report a new type of unassisted PEC system that uses light, water and oxygen to simultaneously produce electricity and hydrogen peroxide (H₂O₂) on both the photoanode and cathode, which is essentially a light‐driven fuel cell with H₂O₂ as the main product at the two electrodes, meanwhile achieving a maximum power density of 0.194 mW cm^‐2, an open circuit voltage of 0.61 V, and a short circuit current density of 1.09 mA cm^‐2. The electricity output can be further used as a sign for cell function when accompanied by a detector such as a light‐emitting diode (LED) light or a multimeter. This is the first work that shows H₂O₂ two‐side generation with a strict key factors study of the system, with a clear demonstration of electricity output ability using low‐cost earth abundant materials on both sides, which represents an exciting new direction for PEC systems.     

MOF‐Based Transparent Passivation Layer Modified ZnO Nanorod Arrays for Enhanced Photo‐Electrochemical Water Splitting

This study introduces zeolitic imidazolate framework‐8 (ZIF‐8) as the first metal‐organic framework based transparent surface passivation layer for photo‐electrochemical (PEC) water splitting. A significant enhancement for PEC water oxidation is demonstrated based on the in situ seamless coating of ZIF‐8 surface passivation layer on Ni foam (NF) supported ZnO nanorod arrays photoanode. The PEC performance is improved by optimizing the ZIF‐8 thickness and by grafting Ni(OH)₂ nanosheets as synergetic co‐catalyst. With respect to ZnO/NF, the optimized Ni(OH)₂/ZIF‐8/ZnO/NF photoanode exhibits a two times larger photocurrent density of 1.95 mA cm^?2 and also a two times larger incident photon to current conversion efficiency of 40.05% (350 nm) at 1.23 V versus RHE (V_RHE) under AM 1.5 G. The synergetic surface passivation and the co‐catalyst modification contribute to prolonging the charge lifetime, to promoting the charge transfer, and to decreasing the overpotential for water oxidation.     

Metal Halide Perovskites for Solar‐to‐Chemical Fuel Conversion

This review article presents and discusses the recent progress made in the stabilization, protection, improvement, and design of halide perovskite‐based photocatalysts, photoelectrodes, and devices for solar‐to‐chemical fuel conversion. With the target of water splitting, hydrogen iodide splitting, and CO₂ reduction reactions, the strategies established for halide perovskites used in photocatalytic particle‐suspension systems, photoelectrode thin‐film systems, and photovoltaic‐(photo)electrocatalysis tandem systems are organized and introduced. Moreover, recent achievements in discovering new and stable halide perovskite materials, developing protective and functional shells and layers, designing proper reaction solution systems, and tandem device configurations are emphasized and discussed. Perspectives on the future design of halide perovskite materials and devices for solar‐to‐chemical fuel conversion are provided. This review may serve as a guide for researchers interested in utilizing halide perovskite materials for solar‐to‐chemical fuel conversion.     

Fullerene as a Photoelectron Transfer Promoter Enabling Stable TiO2‐Protected Sb2Se3 Photocathodes for Photo‐Electrochemical Water Splitting

Understanding the degradation mechanisms of photoelectrodes and improving their stability are essential for fully realizing solar‐to‐hydrogen conversion via photo‐electrochemical (PEC) devices. Although amorphous TiO₂ layers have been widely employed as a protective layer on top of p‐type semiconductors to implement durable photocathodes, gradual photocurrent degradation is still unavoidable. This study elucidates the photocurrent degradation mechanisms of TiO₂‐protected Sb₂Se₃ photocathodes and proposes a novel interface‐modification methodology in which fullerene (C₆₀) is introduced as a photoelectron transfer promoter for significantly enhancing long‐term stability. It is demonstrated that the accumulation of photogenerated electrons at the surface of the TiO₂ layer induces the reductive dissolution of TiO₂, accompanied by photocurrent degradation. In addition, the insertion of the C₆₀ photoelectron transfer promoter at the Pt/TiO₂ interface facilitates the rapid transfer of photogenerated electrons out of the TiO₂ layer, thereby yielding enhanced stability. The Pt/C₆₀/TiO₂/Sb₂Se₃ device exhibits a high photocurrent density of 17 mA cm^?2 and outstanding stability over 10 h of operation, representing the best PEC performance and long‐term stability compared with previously reported Sb₂Se₃‐based photocathodes. This research not only provides in‐depth understanding of the degradation mechanisms of TiO₂‐protected photocathodes, but also suggests a new direction to achieve durable photocathodes for photo‐electrochemical water splitting.     

Recent Development of Ni/Fe‐Based Micro/Nanostructures toward Photo/Electrochemical Water Oxidation

The oxygen evolution reaction (OER), as an important process involved in water splitting and rechargeable metal–air batteries, has drawn increasing attention in the context of clean energy generation and efficient energy storage. This review concerns the progress and new discoveries in the field of Ni/Fe‐based micro/nanostructures toward electrochemical and photo‐electrochemical (PEC) water oxidation during last few years. First, toward the design and construction of new electrocatalysis, different types of current Ni/Fe‐based compounds for OER are summarized. The mechanism studies of the active phases and positions of Ni/Fe‐based micro/nanostructures are further introduced to understand the properties of catalytic active sites, which could facilitate further improving the performance of Ni/Fe‐based OER electrocatalysts. Second, splitting water using sunlight with low overpotential is another important target in making solar‐to‐hydrogen micro/nanodevices, and thus attention is then focused on the development of several important Ni/Fe‐based PEC catalysts. Third, the recent theoretical calculations on the OER mechanism during water splitting and insights into electronic structures are analyzed; finally, the future trends and perspectives are also discussed briefly.     

Multifunctional Coatings from Scalable Single Source Precursor Chemistry in Tandem Photoelectrochemical Water Splitting

The straightforward and inexpensive fabrication of stabilized and activated photoelectrodes for application to tandem photoelectrochemical (PEC) water splitting is reported. Semiconductors such as Si, WO₃, and BiVO₄ can be coated with a composite layer formed upon hydrolytic decomposition of hetero­bimetallic single source precursors (SSPs) based on Ti and Ni, or Ti and Co in a simple single‐step process under ambient conditions. The resulting 3d‐transition metal oxide composite films are multifunctional, as they protect the semiconductor electrode from corrosion with an amorphous TiO₂ coating and act as bifunctional electrocatalysts for H₂ and O₂ evolution based on catalytic Ni or Co species. Thus, this approach enables the use of the same precursors for both photoelectrodes in tandem PEC water splitting, and SSP chemistry is thereby established as a highly versatile low‐cost approach to protect and activate photoelectrodes. In an optimized system, SSP coating of a Si photocathode and a BiVO₄ photoanode resulted in a benchmark noble metal‐free dual‐photoelectrode tandem PEC cell for overall solar water splitting with an applied bias solar‐to‐hydrogen conversion efficiency of 0.59% and a half‐life photostability of 5 h.     

Creation of 3D Textured Graphene/Si Schottky Junction Photocathode for Enhanced Photo‐Electrochemical Efficiency and Stability

This work presents a novel photo‐electrochemical architecture based on the 3D pyramid‐like graphene/p‐Si Schottky junctions. Overcoming the conventional transfer technique by which only planar graphene/Si Schottky junctions are currently available, this work demonstrates the 3D pyramid‐like graphene/p‐Si Schottky junction photocathode, which greatly enhances light harvesting efficiency and exhibits promising photo‐electrochemical performance for hydrogen generation. The formation of 3D pyramid‐like graphene/p‐Si Schottky junctions exhibits enhanced electrochemical activity and promotes charge separation efficiency compared with the bare pyramid Si surface without graphene. The inherent chemical inertness of graphene significantly improves the operational stability of 3D graphene/p‐Si Schottky junction photo‐electrochemical cells. The 3D pyramid‐like graphene/p‐Si Schottky junction photocathode delivers an onset potential of 0.41 V and a saturated photocurrent density of ?32.5 mA cm^?2 at 0 V (vs RHE) with excellent stability comparable to values reported for textured or nanostructured p‐Si photocathodes coated with ultrathin oxide layers by the conventional atomic layer deposition technique. These results suggest that the formation of graphene/Si Schottky junctions with a 3D architecture is a promising approach to improve the performance and durability of Si‐based photo‐electrochemical systems for water splitting or solar‐to‐fuel conversion.     

3D Porous Pyramid Heterostructure Array Realizing Efficient Photo‐Electrochemical Performance

Direct photo‐electrochemical (PEC) water splitting is of great practical interest for developing a sustainable energy systems, but remains a big challenge owing to sluggish charge separation, low efficiency, and poor stability. Herein, a 3D porous In₂O₃/In₂S₃ pyramid heterostructure array on a fluorine‐doped tin oxide substrate is fabricated by an ion exchange–induced synthesis strategy. Based on the synergistic structural and electronic modulations from density functional theory calculations and experimental observations, 3D porous In₂O₃/In₂S₃ photoanode by the protective layer delivers a low onset potential of ≈0.02 V versus reversible hydrogen electrode (RHE), the highest photocurrent density of 8.2 mA cm^?2 at 1.23 V versus RHE among all the In₂S₃ photoanodes reported to date, an incident photon‐to‐current efficiency of 76% at 400 nm, and high stability over 20 h for PEC water splitting are reported. This work provides an alternative promising prototype for the design and construction of novel heterostructures in robust PEC water splitting applications.     

Over 17% Efficiency Stand‐Alone Solar Water Splitting Enabled by Perovskite‐Silicon Tandem Absorbers

Realizing solar‐to‐hydrogen (STH) efficiencies close to 20% using low‐cost semiconductors remains a major step toward accomplishing the practical viability of photoelectrochemical (PEC) hydrogen generation technologies. Dual‐absorber tandem cells combining inexpensive semiconductors are a promising strategy to achieve high STH efficiencies at a reasonable cost. Here, a perovskite photovoltaic biased silicon (Si) photoelectrode is demonstrated for highly efficient stand‐alone solar water splitting. A p⁺nn⁺ ‐Si/Ti/Pt photocathode is shown to present a remarkable photon‐to‐current efficiency of 14.1% under biased condition and stability over three days under continuous illumination. Upon pairing with a semitransparent mixed perovskite solar cell of an appropriate bandgap with state‐of‐the‐art performance, an unprecedented 17.6% STH efficiency is achieved for self‐driven solar water splitting. Modeling and analysis of the dual‐absorber PEC system reveal that further work into replacing the noble‐metal catalyst materials with earth‐abundant elements and improvement of perovskite fill factor will pave the way for the realization of a low‐cost high‐efficiency PEC system.     

Enhancing Mo:BiVO4 Solar Water Splitting with Patterned Au Nanospheres by Plasmon‐Induced Energy Transfer

Plasmonic metal nanostructures have been extensively investigated to improve the performance of metal oxide photoanodes for photoelectrochemical (PEC) solar water splitting cells. Most of these studies have focused on the effects of those metal nanostructures on enhancing light absorption and enabling direct energy transfer via hot electrons. However, several recent studies have shown that plasmonic metal nanostructures can improve the PEC performance of metal oxide photoanodes via another mechanism known as plasmon‐induced resonant energy transfer (PIRET). However, this PIRET effect has not yet been tested for the molybdenum‐doped bismuth vanadium oxide (Mo:BiVO₄), regarded as one of the best metal oxide photoanode candidates. Here, this study constructs a hybrid Au nanosphere/Mo:BiVO₄ photoanode interwoven in a hexagonal pattern to investigate the PIRET effect on the PEC performance of Mo:BiVO₄. This study finds that the Au nanosphere array not only increases light absorption of the photoanode as expected, but also improves both its charge transport and charge transfer efficiencies via PIRET, as confirmed by time‐correlated single photon counting and transient absorption studies. As a result, incorporating the Au nanosphere array increases the photocurrent density of Mo:BiVO₄ at 1.23 V versus RHE by ≈2.2‐fold (2.83 mA cm^?2).     

(040)‐Crystal Facet Engineering of BiVO4 Plate Photoanodes for Solar Fuel Production

A (040)‐crystal facet engineered BiVO₄ ((040)‐BVO) photoanode is investigated for solar fuel production. The (040)‐BVO photoanode is favorable for improved charge carrier mobility and high photocatalytic active sites for solar light energy conversion. This crystal facet design of the (040)‐BVO photoanode leads to an increase in the energy conversion efficiency for solar fuel production and an enhancement of the oxygen evolution rate. The photocurrent density of the (040)‐BVO photoanode is determined to be 0.94 mA cm^?2 under AM 1.5 G illumination and produces 42.1% of the absorbed photon‐to‐current conversion efficiency at 1.23 V (vs RHE, reversible hydrogen electrode). The enhanced charge separation efficiency and improved charge injection efficiency driven by (040) facet can produce hydrogen with 0.02 mmol h^?1 at 1.23 V. The correlation between the (040)‐BVO photoanode and the solar fuel production is investigated. The results provide a promising approach for the development of solar fuel production using a BiVO₄ photoanode.     

Water Splitting Progress in Tandem Devices: Moving Photolysis beyond Electrolysis

Water photolysis is a sustainable technology to convert natural solar energy and water into chemical fuels and is thus considered a thorough solution to the forthcoming energy crises. Unassisted water splitting that could directly harvest solar light and subsequently split water in a single device has become an important research theme. Three types of tandem devices including photoelectrochemical (PEC), photovoltaic (PV) cell/PEC and PV/electrolyser tandem cells are proposed to realize water photolysis at different levels of integration and component. Recent progress in tandem water splitting devices is summarized, and crucial issues on device optimization from the perspective of each photo‐absorber functionalities in band edge potential, light absorptivity and transmittance are discussed. By increasing the performances of stand‐alone PEC or PV devices, a 20% solar to hydrogen efficiency is predicted that is a significant value towards further application in practice. Accordingly, the challenges for materials development and configuration optimization are further outlined.     

Overcoming Charge Collection Limitation at Solid/Liquid Interface by a Controllable Crystal Deficient Overlayer

Bulk and surface charge recombination of photoelectrode are two key processes that significantly hinder solar‐to‐fuel conversion of photoelectrochemical cell (PEC). In this study, the function of a “crystal‐deficient” overlayer is unveiled, which outperforms a traditionally used amorphous or crystalline overlayer in PEC water splitting by exhibiting a high conductivity and large electron diffusion length to enable unlimited electron collection. The optimized ≈2.5 nm thickness of the “crystal‐deficient” shell results in a depletion layer with a width of 3 nm, which overcomes the flat band limitation of the photovoltage and increases the light absorptivity in the wavelength range from 300 to 420 nm. In addition, a 50‐fold increase in the conductivity yields a one‐order‐of‐magnitude increase in the diffusion length of an electron (L_n )(≈20 μm), allowing for unlimited electron collection in the 1.9 μm TiO₂ nanowire array with the “crystal‐deficient” shell. The controllable “crystal‐deficient” overlayer in rutile TiO₂ nanowires photoanode achieves a photocurrent density greater than 2.0 mA cm^?2 at 1.23 V versus reversible hydrogen electrode (RHE), a 1.18% applied bias photon‐to‐current efficiency at 0.49 V versus RHE, a faradaic efficiency greater than 93.5% at 0.6 V versus Pt under air mass 1.5G simulated solar light illumination (100 mW cm^?2).     

Atomic Layer Deposition as a General Method Turns any 3D‐Printed Electrode into a Desired Catalyst: Case Study in Photoelectrochemisty

3D‐printing technologies have begun to revolutionize many manufacturing processes, however, there are still significant limitations that are yet to be overcome. In particular, the material from which the products are fabricated is limited by the 3D‐printing material precursor. Particularly, for photoelectrochemical (PEC) energy applications, the as‐printed electrodes can be used as is, or modified by postfabrication processes, e.g., electrochemical deposition or anodization, to create active layers on the 3D‐printed electrodes. However, the as‐printed electrodes are relatively inert for various PEC energy applications, and the aforementioned postfabrication processing techniques do not offer layer conformity or control at the Ångström/nano level. Herein, for the first time, atomic layer deposition (ALD) is utilized in conjunction with metal 3D‐printing to create active electrodes. To illustrate the proof‐of‐concept, TiO₂ is deposited by ALD onto stainless steel 3D‐printed electrodes and subsequently investigated as a photoanode for PEC water oxidation. Furthermore, by tuning the TiO₂ thickness by ALD, the activity can be optimized. By combining 3D‐printing and ALD, instead of other metal deposition techniques, i.e., sputtering, rapid prototyping of electrodes with controllable thickness of the desired material onto an as‐printed electrodes with any porosity can be achieved that can benefit a multitude of energy applications.     

Pseudomorphic Transformation of Interpenetrated Prussian Blue Analogs into Defective Nickel Iron Selenides for Enhanced Electrochemical and Photo‐Electrochemical Water Splitting

A significant methodology gap remains in the construction of advanced electrocatalysts, which has collaborative defective functionalities and structural coherence that maximizes electrochemical redox activity, electrical conductivity, and mass transport characteristics. Here, a coordinative self‐templated pseudomorphic transformation of an interpenetrated metal organic compound network is conceptualized into a defect‐rich porous framework that delivers highly reactive and durable photo(electro)chemical energy conversion functionalities. The coordinative‐template approach enables previously inaccessible synthesis routes to rationally accomplish an interconnected porous conductive network at the microscopic level, while exposing copious unsaturated reactive sites at the atomic level without electronic or structural integrity trade‐offs. Consequently, porous framework, interconnected motifs, and engineered defects endow remarkable electrocatalytic hydrogen evolution reaction and oxygen evolution reaction activity due to intrinsically improved turnover frequency, electrochemical surface area, and charge transfer. Moreover, when the hybrid is coupled with a silicon photocathode for solar‐driven water splitting, it enables photon assisted redox reactions, improved charge separation, and enhanced carrier transport via the built‐in heterojunction and additive co‐catalyst functionality, leading to a promising photo(electro)chemical hydrogen generation performance. This work signifies a viable and generic approach to prepare other functional interconnected metal organic coordinated compounds, which can be exploited for diverse energy storage, conversion, or environmental applications.     

Facile and Efficient Atomic Hydrogenation Enabled Black TiO2 with Enhanced Photo‐Electrochemical Activity via a Favorably Low‐Energy‐Barrier Pathway

Black TiO₂ has demonstrated a great potential for a variety of renewable energy technologies. However, its practical application is heavily hindered due to lack of efficient hydrogenation methods and a deeper understanding of hydrogenation mechanisms. Here, a simple and straightforward hot wire annealing (HWA) method is presented to prepare black TiO₂ (H–TiO₂) nanorods with enhanced photo‐electrochemical (PEC) activity by means of atomic hydrogen [H]. Compared to conventional molecular hydrogen approaches, the HWA shows remarkable effectiveness without any detrimental side effects on the device structure, and simultaneously the photocurrent density of H–TiO₂ reaches 2.5 mA cm^?2 (at 1.23 V vs reversible hydrogen electrode (RHE)). Due to the controllable and reproducible [H] flux, the HWA can be developed as a standard hydrogenation method for black TiO₂. Meanwhile, the relationships between the wire temperatures, structural, optical, and photo‐electrochemical properties are systematically investigated to verify the improved PEC activity. Furthermore, the density functional theory (DFT) study provides a comprehensive insight not only into the highly efficient mechanism of the HWA approach but also its favorably low‐energy‐barrier hydrogenation pathway. The findings will have a profound impact on the broad energy applications of H–TiO₂ and contribute to the fundamental understanding of its hydrogenation.     

Transition Metal Disulfides as Noble‐Metal‐Alternative Co‐Catalysts for Solar Hydrogen Production

The production of hydrogen fuels by using sunlight is an attractive and sustainable solution to the global energy and environmental problems. Platinum (Pt) is known as the most efficient co‐catalyst in hydrogen evolution reaction (HER). However, due to its high‐cost and limited‐reserves, it is highly demanded to explore alternative non‐precious metal co‐catalysts with low‐cost and high efficiency. Transition metal disulfides (TMDs) including molybdenum disulfide and tungsten disulfide have been regarded as promising candidates to replace Pt for HER in recent years. Their unique structural and electronic properties allow them to have many opportunities to be designed as highly efficient co‐catalysts over various photo harvesting semiconductors. Recent progress in TMDs as photo‐cocatalysts in solar hydrogen production field is summarized, focusing on the effect of structural matchability with photoharvesters, band edges tunability, and phase transformation on the improvement of hydrogen production activities. Moreover, recent research efforts toward the TMDs as more energy‐efficient and economical co‐catalysts for HER are highlighted. Finally, this review concludes by critically summarizing both findings and current perspectives, and highlighting crucial issues that should be addressed in future research activities.     

Design and Fabrication of a Precious Metal‐Free Tandem Core–Shell p+n Si/W‐Doped BiVO4 Photoanode for Unassisted Water Splitting

Tandem photoelectrochemical water splitting cells utilizing crystalline Si and metal oxide photoabsorbers are promising for low‐cost solar hydrogen production. This study presents a device design and a scalable fabrication scheme for a tandem heterostructure photoanode: p⁺n black silicon (Si)/SnO₂ interface/W‐doped bismuth vanadate (BiVO₄)/cobalt phosphate (CoPi) catalyst. The black‐Si not only provides a substantial photovoltage of 550 mV, but it also serves as a conductive scaffold to decrease charge transport pathlengths within the W‐doped BiVO₄ shell. When coupled with cobalt phosphide (CoP) nanoparticles as hydrogen evolution catalysts, the device demonstrates spontaneous water splitting without employing any precious metals, achieving an average solar‐to‐hydrogen efficiency of 0.45% over the course of an hour at pH 7. This fabrication scheme offers the modularity to optimize individual cell components, e.g., Si nanowire dimensions and metal oxide film thickness, involving steps that are compatible with fabricating monolithic devices. This design is general in nature and can be readily adapted to novel, higher performance semiconducting materials beyond BiVO₄ as they become available, which will accelerate the process of device realization.     

Recent Advancement of p‐ and d‐Block Elements,Single Atoms,and Graphene‐Based Photoelectrochemical Electrodes for Water Splitting

Solar‐assisted photoelectrochemical (PEC) water splitting to produce hydrogen energy is considered the most promising solution for clean, green, and renewable sources of energy. For scaled production of hydrogen and oxygen, highly active, robust, and cost‐effective PEC electrodes are required. However, most of the available semiconductors as a PEC electrodes have poor light absorption, material degradation, charge separation, and transportability, which result in very low efficiency for photo‐water splitting. Generally, a promising photoelectrode is obtained when the surface of the semiconductor is modified/decorated with a suitable co‐catalyst because it increases the light absorbance spectrum and prevents electron–hole recombination during photoelectrode reactions. In this regard, numerous p‐ and d‐block elements, single atoms, and graphene‐based PEC electrodes have been widely used as semiconductor/co‐catalyst junctions to boost the performances of PEC overall water splitting. This review enumerates the recent progress and applications of p‐ and d‐block elements, single atoms, and graphene‐based PEC electrodes for water splitting. The focus is placed on fundamental mechanism, efficiency, cells design, and various aspects that contribute to the large‐scale prototype device. Finally, future perspectives, summary, challenges, and outlook for improving the activity of PEC photoelectrodes toward whole‐cell water splitting are addressed.