Single-atom-anchored covalent organic frameworks for photocatalytic hydrogen evolution: design principles, dynamic mechanisms, and emerging opportunities.

Photocatalytic water splitting is a promising route for solar-to-hydrogen conversion, but organic photocatalysts are often limited by strong exciton binding, rapid charge recombination, inefficient carrier transport, and sluggish proton-reduction kinetics. Covalent organic frameworks (COFs) offer molecularly tunable structures and electronic properties, yet pristine COFs generally lack efficient catalytic sites. Incorporating single-atom sites (SA sites) into COFs can simultaneously provide active hydrogen-evolution centers and regulate local charge-transfer pathways. This Review summarizes recent advances in single-atom catalysts supported on COFs for photocatalytic hydrogen production. It discusses how COFs stabilize SA sites and how these sites promote exciton dissociation, charge separation and migration, electron accumulation, and interfacial proton reduction. Top-down and bottom-up synthetic strategies are compared, with emphasis on the roles of metal identity, ligand environment, coordination structure, geometry, and metal-support interactions. Representative systems are categorized as noble-metal, earth-abundant-metal, dual-site, and cluster-assisted catalysts, while challenges in cross-study performance comparison are critically assessed. Finally, key opportunities in operando characterization, scalable synthesis, standardized evaluation, machine-learning-assisted screening, and device integration are highlighted. This Review establishes atomic-level structure-charge-transfer-activity relationships to guide the rational design of efficient COF-based photocatalysts for solar hydrogen production.