Distinct oxygen reduction pathways for solar H2O2 production by regulating unsaturated bonds in covalent organic frameworks

Abstract

Photocatalytic H₂O₂ generation via the two-electron oxygen reduction reaction (2e⁻ ORR) is a highly sustainable approach, capable of proceeding via either a one-step or two-step 2e⁻ ORR route. Nonetheless, precise regulation of the 2e⁻ ORR pathways still remains a formidable challenge. Herein, for the first time, we modulate the 2e⁻ ORR pathway through unsaturated bond control in covalent organic frameworks (COFs). We synthesize a pair of isostructural COFs distinguished only by their unsaturated bonds. The alkyne-containing TY-COF favors the two-step 2e⁻ ORR route, whereas the alkene-containing TE-COF follows the one-step 2e⁻ ORR route. Without any sacrificial agents in O₂, the TY-COF and TE-COF display impressive H₂O₂ production rates of 6455 and 4804 μmol g⁻¹ h⁻¹, respectively. Further theoretical results reveal that the regulation of unsaturated bonds alters the electron–hole distribution along the COF skeletons, prompting the reorganization of the catalytic centers for ORR (the benzene ring in TY-COFs and the triazine in TE-COFs), which leads to divergent ORR pathways. Additionally, free-standing TY-COF and TE-COF membranes, fabricated via the interfacial polymerization method, are also able to drive H₂O₂ photosynthesis. The present work offers a new strategy and valuable inspiration for modulating 2e⁻ ORR pathways via strategic architectural engineering of COFs.