Photoelectrocatalytic Materials for Solar Water Splitting

Photoelectrocatalytic water splitting offers a promising approach to convert sunlight into sustainable hydrogen energy. A thorough understanding of the relationships between the properties and functions of photoelectrocatalytic materials plays a crucial role in the design and fabrication of efficient photoelectrochemical systems for water splitting. This review presents the advances in the development of efficient photoelectrocatalytic materials. First, the fundamentals involved in the photoelectrocatalytic water splitting are elaborated. Then, the critical properties of photoelectrocatalytic materials are classified and discussed according to the associated photoelectrochemical processes, including light absorption, charge separation, charge transportation, and photoelectrocatalytic reactions. The importance of heterointerfaces in photoelectrodes is also mentioned in conjunction with the illustration of some functional interlayer materials. Finally, some strategies that can be employed in material screening and optimization for the construction of highly efficient photoelectrochemical devices for water splitting are also discussed.     

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.     

A Floating Sheet for Efficient Photocatalytic Water Splitting

Solar photocatalytic water splitting has been a promising way to provide clean hydrogen energy. There are two weaknesses in the typical photocatalytic process in which photocatalysts are generally dispersing in water under stir. One is the inadequate utilization of light energy and the other one is the cumbersome operation in the recycling procedure. This study demonstrates an efficient solar photocatalytic water splitting using a floating sheet with a novel WSe₂ cocatalyst. The sheet is fabricated by laser‐depositing WSe₂ film on a carbon foam (CF) substrate and drop‐casting of the synthesized nanodiamond‐embedded Cu₂O (NEC) photocatalysts. This is a new‐type artificial photocatalytic system that overcomes the above‐mentioned weaknesses of a powder‐dispersing system. The WSe₂ cocatalyst acts as an electron sink to promote electron–hole separation, resulting in the further improvement of photocatalytic performance. This floating NEC/WSe₂/CF structure achieves efficient water splitting upon simulated solar irradiation with an increased H₂ evolution rate, which is 13.2 times that of the powder‐dispersing system. This study offers a strategy for the design of new‐type photocatalytic system and discovery of alternative noble metal‐free cocatalysts.     

Bismuth Tantalum Oxyhalogen: A Promising Candidate Photocatalyst for Solar Water Splitting

As wide range of light absorption and suitable redox potentials are prerequisites for photocatalytic water splitting, exploring new semiconductor‐based materials with proper band structures for water splitting still calls for longstanding efforts. In this work, a series of photocatalysts, bismuth tantalum oxyhalide, Bi₄TaO₈X (X = Cl, Br), with valence band and conduction band positions at ≈?0.70 and ≈1.80 eV versus the reversible hydrogen electrode (RHE), respectively, are found to be capable for both water oxidation and reduction under visible light irradiation. Using flux synthetic methods, Bi₄TaO₈X (X = Cl, Br) with microplatelet morphology can be successfully prepared. The photocatalyst based on these materials shows an apparent quantum efficiency as high as 20% at 420 nm for water oxidation. In addition, a Z‐scheme system coupling Bi₄TaO₈Br with Ru/SrTiO₃:Rh is successfully achieved for overall water splitting with a stoichiometric ratio of H₂ and O₂ evolutions. This work demonstrates a new series of semiconductors Bi₄TaO₈X (X = Cl, Br) with the promising application in the field of solar energy utilization.     

Hybrid Photoelectrochemical Water Splitting Systems: From Interface Design to System Assembly

Photoelectrochemical (PEC) water splitting has attracted increasing attention due to its potential to mitigate energy and environmental issues. Hybrid PEC systems containing semiconductor photosensitizers and molecular catalysts are reported to be highly active and stable for water splitting with great potential for facilitating clean fuels production. In this review, following a showcasing of the fundamental details of hybrid PEC systems for water splitting, semiconductor/molecular catalyst interface designs are highlighted, with a focus on interfacial physicochemical interactions and binding, and interfacial energetics and dynamics for efficient charge transfer. Recent advances in hybrid system assemblies for PEC water splitting are also briefly introduced. Finally, future challenges and directions in the field of hybrid PEC water splitting for solar energy conversion are reviewed. The current review provides state‐of‐the‐art strategies for optimized interface design for creating highly active and stable PEC water splitting assemblies.     

Energetic Efficiency of Hydrogen Photoevolution by Algal Water Splitting

Absolute thermodynamic efficiencies of conversion of light energy into chemical-free energy of molecular hydrogen by intact microalgae have been measured with an original physical measuring technique using a tin-oxide semiconducting gas sensor. Thin films of microalgae comprising of 5 to 20 cellular monolayers have been entrapped on filter paper, thereby constraining them in a well-defined circular geometry. Based on absolute light absorption of visible polychromatic illumination in the low-intensity region of the light saturation curve, conversion efficiencies of 6 to 24% have been obtained. These values are the highest ever measured for hydrogen evolution by green algae.     

From Water Oxidation to Reduction: Homologous Ni–Co Based Nanowires as Complementary Water Splitting Electrocatalysts

A homologous Ni–Co based nanowire system, consisting of both nickel cobalt oxide and nickel cobalt sulfide nanowires, is developed for efficient, complementary water splitting. The spinel‐type nickel cobalt oxide (NiCo₂O₄) nanowires are hydrothermally synthesized and can serve as an excellent oxygen evolution reaction catalyst. Subsequent sulfurization of the NiCo₂O₄ nanowires leads to the formation of pyrite‐type nickel cobalt sulfide (Ni_0.33Co_0.67S₂) nanowires. Due to the 1D nanowire morphology and enhanced charge transport capability, the Ni_0.33Co_0.67S₂ nanowires function as an efficient, stable, and robust nonnoble metal electrocatalyst for hydrogen evolution reaction (HER), substantially exceeding CoS₂ or NiS₂ nanostructures synthesized under similar methods. The Ni_0.33Co_0.67S₂ nanowires exhibit low onset potential of ?65, ?39, and ?50 mV versus reversible hydrogen electrode, Tafel slopes of 44, 68, and 118 mV dec^?1 at acidic, neutral, and basic conditions, respectively, and excellent stability, comparable to the best reported non‐noble metal‐based HER catalysts. Furthermore, the homologous Ni_0.33Co_0.67S₂ nanowires and NiCo₂O₄ nanowires are assembled into an all‐nanowire based water splitting electrolyzer with a current density of 5 mA cm^?2 at a voltage as 1.65 V, thus suggesting a unique homologous, earth abundant material system for water splitting.     

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.     

A Perovskite Nanorod as Bifunctional Electrocatalyst for Overall Water Splitting

The development of highly efficient and low‐cost electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is paramount for water splitting associated with the storage of clean and renewable energy. Here, this study reports its findings in the development of a nanostructured perovskite oxide as OER/HER bifunctional electrocatalyst for overall water splitting. Prepared by a facile electrospinning method, SrNb_0.1Co_0.7Fe_0.2O_3–δ perovskite nanorods (SNCF‐NRs) display excellent OER and HER activity and stability in an alkaline solution, benefiting from the catalytic nature of perovskites and unique structural features. More importantly, the SNCF‐NR delivers a current density of 10 mA cm^?2 at a cell voltage of merely ≈1.68 V while maintaining remarkable durability when used as both anodic and cathodic catalysts in an alkaline water electrolyzer. The performance of this bifunctional perovskite material is among the best ever reported for overall water splitting, offering a cost‐effective alternative to noble metal based electrocatalysts.