Engineering Covalent Organic Frameworks for Photocatalytic Overall Water Vapor Splitting

Abstract

Photocatalytic overall water vapor splitting (OWVS) into H₂ and O₂ not only owns the potential of avoiding the backward reaction of O₂ reduction reaction reforming H₂O, but also realizes H₂ production without available liquid water. However, this attempt is still a blank due to the weak absorption of photocatalysts to water vapor. Herein, we report the first example of visible-light-driven OWVS by combining the water-adsorbing ability and photocatalytic activity of covalent organic frameworks (COFs). The overall water splitting (OWS) activity of Tp-COF skeleton was realized by introducing tripyridyltriazine segment. The Pt@Tp-TAPyT-COF achieves high visible-light-driven H₂ and O₂ evolution rates (HER and OER) of 148.4 and 74.8 µmol g⁻¹ h⁻¹, respectively. Under water vapor conditions with diverse relative humidities (RHs), the Pt@Tp-TAPyT-COF could drive OWVS even without backward reaction. By further optimizing the structure of β-ketoamine section, it was found that the Pt@DHTA-TAPyT-COF showed optimal OWVS activity, with the H₂ and O₂ evolution rate of 51.2 and 25.6 µmol g⁻¹ h⁻¹ under RH = 88%, respectively. The advantage of OWVS compared to traditional solid–liquid OWS was further confirmed by a continuous activity test of 45 h. Further experiments and theoretical calculations indicated that carbonyl-O and pyridine-N atoms in COFs serve as water-absorbing sites, and the absorbed water molecules could promote water-splitting activity of active sites in COFs simultaneously.