Critical parameters and essential strategies in designing photoanodes to overcome the sluggish water oxidation reaction

Photoelectrochemical (PEC) water splitting is a promising technology to generate green hydrogen energy from solar light. However, the water oxidation process limits the efficiency of overall water splitting. To overcome the limitation, it needs to understand some critical parameters in a PEC water-splitting process and precise strategies to select, design, prepare, and modify the materials for photoelectrode. A fundamental consideration in selecting materials for a PEC cell should include a thermodynamic requirement, appropriate band structure, high crystallinity and surface area, high stability, and low cost. The interested photoanode semiconductors with low cost and toxicity are discussed with various modification strategies, such as surface modification with nanostructuring, co-catalyst, and surface passivation. The strategies with heterojunction, Z-scheme charge transfer, and gradient doping with cations and anions are also presented to improve the charge transfer and lower its recombination rate. In addition, the recent application of ternary oxide-based photoanodes is briefly reviewed. Finally, the challenges and prospects for the future development of photoanodes are presented.