Superposition of dual electric fields in covalent organic frameworks for efficient photocatalytic hydrogen evolution

Abstract

Covalent organic frameworks (COFs) are promising materials for converting solar energy into green hydrogen. However, limited charge separation and transport in COFs impede their application in the photocatalytic hydrogen evolution reaction (HER). In this study, the intrinsically tunable internal bond electric field (IBEF) at the imine bonds of COFs was manipulated to cooperate with the internal molecular electric field (IMEF) induced by the donor-acceptor (D-A) structure for an efficient HER. The aligned orientation of IBEF and IMEF resulted in a remarkable H₂ evolution rate of 57.3 mmol·g^–1·h^–1 on TNCA, which was approximately 520 times higher than that of TCNA (0.11 mmol·g^–1·h^–1) with the opposing electric field orientation. The superposition of the dual electric fields enables the IBEF to function as an accelerating field for electron transfer, kinetically facilitating the migration of photogenerated electrons from D to A. Furthermore, theoretical calculations indicate that the inhomogeneous charge distribution at the C and N atoms in TNCA not only provides a strong driving force for carrier transfer but also effectively hinders the return of free electrons to the valence band, improving the utilization of photoelectrons. This strategy of fabricating dual electric fields in COFs offers a novel approach to designing photocatalysts for clean energy synthesis.