Engineering charge transfer by tethering halogens to covalent organic frameworks for photocatalytic sacrificial hydrogen evolution

Abstract

Covalent organic frameworks (COFs) are promising versatile organic semiconductors for the photocatalytic hydrogen evolution reaction (HER) but usually rely on the utilization of expensive Pt species as the co-catalyst and exhibit inferior activity in seawater relative to that in pure water. Herein, different halogen atoms (F, Cl, and Br) were integrated into a COF to optimize the photochemical properties, carrier thermodynamics, and kinetics. The Cl-containing COF TpPaCl exhibited effective co-catalyst-free HER performance under visible light irradiation, with a high HER rate of 16.3 mmol g⁻¹ h⁻¹ in artificial seawater (11.5 mmol g⁻¹ h⁻¹ in real seawater), outperforming that in pure water. The remarkable performance came from the fine and efficient adjustment of the local environment of the COF by the electronegative halogen atoms. Theoretical calculations and soft X-ray absorption spectroscopy (XAS) indicated that the carbon atoms adjacent to the halogen group exhibited strong interaction towards the key intermediate (H*) in the HER process and served as photoactive H₂ evolution sites. The strong polarization derived from the halogen also greatly improved the formation of long-life electrons and accelerated the successive charge transfer. The kinetics were enhanced after salt adsorption in seawater, contributing to the superior HER rate in seawater.