Covalent Organic Framework and Composite as Photocatalysts Toward Sustainable CO₂ Reduction

The intensifying climate emergency compels a rapid paradigm shift from fossil fuel-based energy systems toward sustainable, carbon-neutral alternatives. Among emerging strategies, the photocatalytic valorization of CO₂ into energy-dense fuels and commodity chemicals by suitable photocatalysts presents a straightforward and economically viable solution for both greenhouse gas mitigation and renewable energy storage. In this context, covalent organic frameworks (COFs) have emerged as a highly promising class of crystalline, porous semiconductor photocatalysts for CO₂ reduction reactions (CO₂RR), owing to their structural regularity, modularity, and optoelectronic tunability. In this review, we comprehensively outline the recent progress in three distinct categories of COF-based photocatalytic systems: metal-free COFs, single-metal-atom based COFs, and COF-based composites. Key strategies such as the judicious incorporation of donor-acceptor architectures, rational post-synthetic functionalization, and heterojunction engineering are discussed. Insights from in situ operando characterization and theoretical calculations are also presented to highlight the roles of exciton dynamics, charge separation, active site engineering, and structure-function relationship in CO₂RR. Finally, we propose future research directions for better utilization of COFs in solar fuel/chemical generation. Overall, this review aims to provide a comprehensive discussion on the advancement of COF-based photocatalysts and next-generation CO₂ valorization materials.