A review of recent progress on Ni-based cocatalysts for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting represents a highly promising pathway for converting solar energy into clean hydrogen fuel. However, its practical application is hindered by challenges such as sluggish surface reaction kinetics, rapid charge carrier recombination, and the poor stability of photoelectrodes. The incorporation of cocatalysts onto photoelectrodes is a pivotal strategy to overcome these limitations. Among various cocatalysts, nickel-based materials have emerged as a focal point of research due to their earth abundance, cost-effectiveness, excellent electrocatalytic activity, and multifunctional roles. This review summarizes recent advances in Ni-based cocatalysts for PEC water splitting. The material families covered include nickel metals and alloys, oxides, hydroxides, sulfides, phosphides, and other emerging compounds such as borides and single-atom sites. The primary functions of these cocatalysts are elucidated, which encompass (i) providing abundant active sites for surface redox reactions (especially the kinetically demanding oxygen evolution reaction, OER), (ii) facilitating the extraction, separation, and transport of photo-generated charge carriers, (iii) reducing overpotentials and accelerating reaction kinetics, and (iv) enhancing the chemical and electrochemical stability of photoelectrodes. By analyzing the structure-property relationships and enhancement mechanisms across diverse photoelectrode platforms (e.g., BiVO 4 , Fe 2 O 3 , Si, TiO 2 ), this work highlights the design principles for effective cocatalyst integration. Finally, current challenges and future perspectives regarding the rational design of more efficient, durable, and scalable Ni-based cocatalysts are discussed to guide the development of practical PEC technologies for water splitting.