Advancing solar-to-hydrogen conversion efficiency in COF heterostructures: From design principles to sacrificial-agent-free overall water splitting

초록

Covalent organic frameworks (COFs) have emerged as designable, crystalline organic semiconductors for solar-to-hydrogen conversion. Despite this promise, their overall efficiency is tightly limited by a triad of interdependent challenges: poor exciton dissociation, anisotropic charge-transport, and slow interfacial reaction kinetics. This review summarizes COF-based photocatalytic hydrogen evolution and overall water splitting (when reported), with a focus on hydrogen-evolution rates, apparent quantum yields (AQY), and solar-to-hydrogen (STH) efficiencies. After outlining mechanistic fundamentals and heterojunction charge-transfer modes of COF-based photocatalysts, we examine why the photocatalytic performance of these materials is governed by three structural engineering motifs: (1) intrinsic COF design (linkage chemistry, donor-acceptor (D-A) polarization, and heteroatom modulation), (2) noble-metal-integrated composites and semiconductor/carbon heterostructures (Type-II, Z-scheme, and S-scheme interfaces), and (3) metal-site frameworks (covalent metal-organic frameworks and metalated COFs). Across these classes, linkage robustness, shortened transport lengths, and built-in electric fields at well-coupled interfaces repeatedly emerge as dominant determinants of high activity and photon utilization. Field-driven Z-/S-scheme junctions provide reliable performance gains, exemplified by a COF/WO3 Z-scheme reaching 5.93 × 105 μmol h−1 g−1 with 56.1% AQY (420 nm), while overall water splitting has advanced to STH efficiencies approaching 0.67% via electronic modulation in linkage chemistry. Strategies advancing toward sacrificial-agent-free overall water splitting are highlighted, followed by guidelines for rigorous reporting and an assessment of opportunities in earth-abundant cocatalysts and scalable synthesis.

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