Next-Generation Photocatalysts for Hydrogen Production: Innovations, Challenges, and Future Perspectives

Photocatalytic water splitting offers a direct pathway for converting sunlight into green hydrogen; however, progress remains limited by insufficient solar absorption, rapid electron–hole recombination, and slow surface redox kinetics. This review consolidates recent advances in heterojunction engineering (S- and Z-schemes), porous frameworks [metal organic frameworks (MOFs)/covalent organic frameworks (COFs)], carbon-based conductors, single-atom catalysts, and cocatalyst architectures developed to overcome these challenges. Three mechanistic principles are highlighted: Z-scheme architectures preserve strong oxidative and reductive potentials by directing low-energy carriers to internal recombination junctions, while retaining high-energy electrons and holes on separate components. S-scheme heterojunctions exploit Fermi-level equilibration and interfacial band bending to selectively remove low-energy carriers, while maintaining redox power. In contrast, type-II systems, although beneficial for charge separation, often reduce the driving force required for effective water splitting. The modular chemistry and porosity of MOFs/COFs enable precise band-edge tuning and site-specific activity but frequently require hybridization or conductivity enhancement to ensure efficient charge transport. Emerging strategies, including single-atom and dual-site catalysts, MXene quantum dots, Janus architectures, and MOF-derived conductive hybrids, are highlighted for integrating well-defined active sites with improved charge extraction. Persistent challenges in stability and interfacial charge transport are discussed, highlighting mechanistic validation and interface engineering as key to practical solar-to-hydrogen performance.