Engineering electron delocalization of ultrathin covalent organic framework nanosheets to elevate photocatalytic hydrogen evolution in seawater

Photocatalytic hydrogen (H₂) evolution reaction (HER) using seawater is one of the most attractive pathways for H₂ production, but it remains one challenge to design efficient catalyst exhibiting even better performance in seawater than in pure water. Herein, ultrathin covalent organic frameworks (COFs) nanosheets favoring high dispersion in the HER medium were synthesized through the Schiff base condensation of aldehyde monomers and benzidine by using a relatively high monomer concentration in a rapid microwave-assisted solvothermal route and their electron delocalization properties were facilely engineered by varying the amount of hydroxyl groups in the aldehyde monomers. With the monomer 2,4,6-trihydroxy-1,3,5-benzenetricarbaldehyde, highly delocalized COF's matrix TPBD was fabricated by integrating abundant C = O groups, significantly accelerating the charge separation and transfer, as demonstrated by combined theoretical and experimental investigation. The visible light driven HER rate over TPBD was as high as 72.5 mmol g^-1h⁻¹ in pure water. The salt adsorption in seawater further strengthened the electron delocalization and thus the better charge transfer, elevating the HER rate to 106.9mmol g^-1h⁻¹ in artificial seawater and 86.4mmol g^-1h⁻¹ in real seawater over TPBD.