The rate-determining step of the photoelectrochemical (PEC) water splitting is the water oxidation reaction at the photoanode, which is 4 orders of magnitude slower than the water reduction reaction at the photocathode. In a conventional process to accelerate the water oxidation, oxygen evolution cocatalysts (OECs) are usually used on the surface of a photoanode. As an alternative strategy, we considered employing a composite photoanode made up of a semiconductor and carbon spheres, for it is expected that the photogenerated electrons on the semiconductor particles can be captured by the carbon spheres as electron reservoirs and leave the photogenerated holes on the surface of the semiconductor particles, which promotes the effective separation of photogenerated electrons and holes. More importantly, the holes accumulated in the valence band accelerate the water oxidation reaction rate with the rapid release of electrons stored on the carbon sphere. Therefore, the composite photoanode achieves a higher photocurrent at a lower applied bias. We provide a proof of concept for this strategy by preparing a composite photoanode by combining bismuth vanadium oxide BiVO4 with carbon spheres and find that the resulting photoanode displays a remarkable enhancement in the rate of the photoanode water oxidation.

sunlight-driven photoelectrochemical (PEC) water splitting is believed to be a promising technique for solar-to-hydrogen (STH) conversion owing to the advantages of high efficiency, clean and lowcost. [6][7][8] However, overall water splitting is a thermodynamically uphill reaction, and the sluggish kinetics of four-electron water oxidation at the photoanode severely restricts the reaction rate of PEC water splitting. [9][10][11] Therefore, it is highly desirable to explore a new PEC H 2 production system with lower thermodynamic energy consumption and more favorable anode reaction kinetics than overall water splitting.Inspired by the traditional microbial fuel cells technology, utilizing PEC systems to deal with the low-cost organic wastes and produce electricity or H 2 is an effective way to realize the efficient energy conversion and utilization, which is called PEC fuel cell and can solve the problems of low power output and low efficiency in microbial fuel cells. [12,13] Meanwhile, replacing sluggish water oxidation with thermodynamically more favorable organic wastes oxidation is a desirable strategy to enhance the PEC H 2 production performance. However, in almost all the reported PEC fuel cells, the organic wastes were always used as hole scavengers and the PEC H 2 production performances were entirely determined by the photoelectric conversion of photoelectrodes. [13,14] It is well known that the oxidation of organic wastes is a spontaneous exothermic reaction, but the energy in organic wastes cannot be effectively converted into the corresponding electric energy or H 2 during the oxidation process in almost all the reported PEC fuel cells, which is a matter of great concern. Moreover, the H 2 production activities in most reported PEC fuel cells are still low, which can't meet the requirement of practical application.Generally, the traditional electrochemical fuel cell can effectively convert the energy in organic fuels into electricity, and the strong adsorption of organic fuels and reaction intermediates on the surface of electrode is a prerequisite for the anode reactions. [15,16] Therefore, we speculate that a strong interaction between the organic fuel and photoelectrode in PEC fuel Utilizing a photoelectrochemical (PEC) fuel cell to replace difficult water oxidation with facile oxidation of organic wastes is regarded as an effective method to improve the H 2 production efficiency. However, in most reported PEC fuel cells, their PEC activities are still low and the energy in organic fuels cannot be effectively utilized. Here, a unique BiVO 4 PEC fuel cell is successfully developed by utilizing the low-cost biomass, tartaric acid, as an organic fuel. Thanks to the strong complexation between BiVO 4 and tartaric acid, a bridge for the charge and energy transfer is successfully constructed, which not only improves the photoelectric conversion efficiency of BiVO 4 , but also effectively converts the chemical energy of biomass into H 2 . Remarkably, under AM1.5G illumination, the optimal nanoporous BiVO...

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With the exhaustion of conventional fossil fuels, the exploration of green and sustainable energy will become an important topic of social development. Hydrogen is considered a clean and effective energy source, and its combustion produces only water, which is harmless to the environment. Photocatalytic water splitting, which utilizes solar energy and produces H 2 and O 2 , can become a very important reaction for alleviating energy shortages and environmental pollution. Water splitting includes the reduction and oxidation half-reactions, among which the oxidation half-reaction is the rate-determining process. Even though current studies mainly focus on the H 2 or O 2 evolution reactions in the presence of sacrificial agents, overall water splitting remains a challenging problem. Metal-organic frameworks (MOFs) and their precursors have been attracting increasing attention as photocatalysts for water splitting. This paper reviews the research progress in MOFs for photocatalytic overall water splitting and discusses the development prospects and challenges of MOFs. In this study, the research progress in MOF-based watersplitting catalysts for photocatalysis and electrocatalysis is systematically reviewed. Herein, MOF-based catalysts are classified into MOFs, MOF composites, and MOF-derived photocatalysts. We also analyze the prospects and challenges in the preparation of efficient and stable MOF photocatalysts for overall water splitting and propose the construction of new efficient MOFs with double active sites, aiming to improve the efficiency of photocatalytic hydrogen and oxygen evolution to achieve the overall water splitting.