Photocatalytic water splitting for sustainable hydrogen: materials, mechanisms, and future directions

Abstract Photocatalytic water splitting offers a direct route for converting solar energy into hydrogen fuel, but practical implementation remains limited by low efficiencies, poor long-term stability, and challenges in scalability. This work is a critical mechanistic review rather than a formal systematic survey: it does not claim exhaustive literature coverage but instead develops a coherent framework for evaluating photocatalytic water-splitting systems through the limitations that govern their performance. We outline the essential requirements for overall water splitting and examine major photocatalyst classes, transition metal oxides, sulfides, nitrides and oxynitrides, metal–organic frameworks, and hybrid systems, through their intrinsic limitations: sluggish oxygen evolution kinetics, photocorrosion, bulk recombination, and interfacial instability. Charge separation strategies including type-II, Z-scheme, and S-scheme architectures are evaluated for their ability to suppress recombination while preserving redox driving force. Beyond bulk and interfacial electronic structure, we examine the roles of surface chemistry, cocatalyst integration, defect engineering, and passivation in determining achievable efficiencies and durability. We extend prior treatments by incorporating quantitative interface physics (Schottky barrier height and Fermi-level pinning), Gerischer-style thermodynamic criteria for photocorrosion, and a standardized testing protocol intended to improve cross-study comparability. Reported performance metrics including solar-to-hydrogen (STH) efficiency, apparent quantum yield (AQY), and applied-bias photon-to-current efficiency (ABPE) are assessed in light of testing conditions and stability considerations. Reactor design, gas separation, material abundance, and scalability are then used to distinguish laboratory-scale advances from deployable concepts. By integrating mechanistic analysis with benchmarking, we identify design principles for next-generation systems capable of moving beyond proof-of-concept demonstrations toward scalable solar hydrogen production.