A hydrophilic fully conjugated covalent organic framework for photocatalytic CO₂ reduction to CO nearly 100% using pure water

Designing stable eco-friendly materials that use abundant H₂O as the hydrogen and electron source for photocatalytic conversion of CO₂ presents a promising route to confronting climate change, but it remains challenging. In this work, we demonstrated that the strategy of converting imine-linkers into 4-carboxyl-quinoline linkages in covalent organic frameworks (COFs) could be used to prepare efficient crystalline porous polymeric photocatalysts for CO₂ reduction using H₂O as the electron donor. In particular, QL-COF, featuring hydrophilic 4-carboxyl-quinoline linkages, was found to show improved adsorption of H₂O and CO₂ compared with LZU1-COF which was constructed via imine linkages. Theoretical studies indicated that hydrophilic -COOH groups presented strong binding with H₂O molecules. Importantly, the pre-adsorbed H₂O molecules were found to further enhance the binding of CO₂ molecules in the pores. The fully conjugated framework of QL-COF improved the separation and transfer of photogenerated charge carriers, leading to excellent activity and photostability for photoreduction of CO₂ with gaseous H₂O. Ultimately, QL-COF reached a high selectivity of 99.3% for CO generation (156 μmol g⁻¹ h⁻¹) under simulated sunlight irradiation.