Advances in Cu2O-based Photocathodes for Photoelectrochemical Water Splitting

Owing to the growing consumption of non-renewable resources and increased environmental pollution, significant attention has been directed toward developing renewable and environmentally friendly energy sources. Hydrogen has emerged as a clean energy carrier and is considered an ideal chemical for power generation via fuel cells. Using renewable energy to power hydrogen production is an attractive prospect, and hydrogen production through photoelectrochemical water splitting is considered a promising area of interest; consequently, significant research is being conducted on rationally designed photoelectrodes. Generally, a photocathode for hydrogen evolution must have a conduction band that is more negative than the reduction potential of hydrogen. Numerous photocathode materials have been developed based on this premise; these include p-Si, InP, and GaN. Compared with other photocathode materials, Cu-based compounds are advantageous owing to their low preparation costs and diverse chemical states. For example, Cu 2 O is a non-toxic p-type metal oxide semiconductor material with an appropriate band structure for water splitting and a direct band gap of 1.9–2.2 eV. Furthermore, the production of Cu 2 O is facile, and the required materials are abundant; thus, it has attracted significant interest as a material for photocathodes. However, Cu 2 O suffers from rapid recombination of photogenerated carriers and severe photo-corrosion, leading to unsatisfactory efficiency and poor stability. Intrinsically, the poor photo-stability of Cu 2 O can be attributed to the location of the redox potential of Cu 2 O within its bandgap, owing to which photoelectrons tend to preferentially reduce Cu 2 O to Cu rather than reduce water to reduction. Therefore, Cu 2 O itself is not an ideal hydrogen evolution catalyst. Thus, co-catalysts are necessary to improve its hydrogen evolution activity and photostability. In addition to co-catalysts, combining Cu 2 O with tailored n-type semiconductors to generate built-in electric fields of p-n junctions has attracted extensive attention owing to its ability of increasing the separation of photogenerated carriers. Similarly, applying a hole transfer layer on the substrate can promote photocarrier separation. Furthermore, considering that water is indispensable for Cu 2 O reduction, one effective approach to improve the stability of Cu 2 O is the addition of a protective/passivation layer to isolate Cu 2 O from water in aqueous electrolytes. In this review, we provide a brief overview of the mechanism of photoelectrochemical water splitting and the band structure of Cu2O; preparation methods of Cu 2 O photocathodes; strategies to improve the efficiency and stability of Cu 2 O photocathodes, including the construction of p-n junctions, integration with co-catalysts, and modifications using hole transport layers; advanced photoelectrochemical characterization techniques; and a discussion regarding the direction of future photocathode research. Cu 2 O is a promising photocathode for water splitting; however, its use is limited owing to the recombination of photogenerated carriers and photo-corrosion. The preparation methods of Cu 2 O, strategies to enhance its photoactivity and stability, and its characterization techniques are reviewed.