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

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.