Oxygen-Evolving Covalent Organic Frameworks via Phosphonate Ylide-Engineering for Enhanced Photocatalytic Overall Water Splitting.

Abstract

Covalent organic frameworks (COFs) exhibit significant promise for photocatalytic overall water splitting to hydrogen generation. However, the high electron density distribution at aromatic carbon in COFs results in inert oxygen evolution, significantly hindering photocatalytic overall water splitting activity. Here, a universal strategy is developed for localized electron density manipulation by utilizing the reactivity of unsaturated carbon at the linkers in the COFs to construct phosphonate ylide polar sites, featuring positively charged phosphorus and negatively charged carbon. Under photoexcitation, this local electron distribution generates a polaron effect, enhances photogenerated exciton dissociation, prevents radiative relaxation, accelerates photogenerated charge separation, and induces an extremely low oxygen evolution barrier at the phosphorus sites. The results show the phosphonate ylide COF has achieved H₂ and O₂ evolution rates of 24.7 and 12.0 µmol h^-1 under visible light irradiation, with the 62 times increase over the pristine COF. Furthermore, this strategy has been successfully validated in several other COFs, demonstrating its broad applicability. To further validate its practical utility, a large-scale outdoor device of 4 m² using this catalyst is fabricated, which achieved a hydrogen production rate exceeding 300 mmol day^-1, highlighting its excellent potential for practical applications.