Co-Catalyst Engineering toward Efficient and Scalable Photoelectrochemical Water Splitting: From Functional Roles to Integration Strategies

Abstract

Intensive research has positioned photoelectrochemical (PEC) water splitting as a promising paradigm for sustainable solar-to-hydrogen conversion. Despite their less complex scale-up requirements, PEC systems remain constrained by sluggish surface reaction kinetics that limit their overall efficiency. In this context, co-catalysts are no longer viewed merely as surface modifiers for enhancing oxygen and hydrogen evolution reactions (OER/HER), but are now recognized as key components governing multiple functional roles of photoelectrodes. Thus, this review highlights how the intrinsic properties of co-catalysts and their interactions influence photoelectrode performance by examining three major aspects, namely light management, charge transfer, and catalytic activity. Beyond material selection, practical efficiency of PEC systems also depends on how co-catalysts are integrated, as their fabrication route can differ from those of photoelectrodes. Therefore, particular emphasis is also placed on various key fabrication strategies, encompassing vapor processes, including physical and chemical vapor deposition, as well as wet processes such as immersion-and droplet-based deposition techniques. Focus is further directed toward the scalability of these methods to demonstrate their feasibility for practical implementation. Finally, perspectives on advancing scalable co-catalyst deposition methods are presented. Ultimately, this work serves as a valuable reference for bridging co-catalyst research from laboratory-scale studies to large-scale applications.