Engineering Covalent Organic Frameworks for Photocatalytic Overall Water Vapor Splitting

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.