Linkage engineering in covalent organic frameworks as metal-free oxygen reduction electrocatalysts for hydrogen peroxide production

Covalent organic frameworks (COFs) are promising platforms with tailorable structures toward metal-free electrocatalytic oxygen reduction (ORR). Here, COFs with different linkages were constructed and explored in the selective two-electron ORR (2e- ORR) for hydrogen peroxide (H2O2) production. Interestingly, imine-linked Py-TD-COF delivers a remarkable H2O2 selectivity of 80–92 %, while amine-linked Py-TD-COF-NH exhibits relatively low H2O2 selectivity of 50–61 %. Experimental and theoretical results reveal that the donor-accepter property of Py-TD-COF enables proper activation of O2, while Py-TD-COF-NH shows relatively high activation of O2 due to the electronic modulation induced by the linkage transformation. Furthermore, the potential H-bonding between the amine group and the adsorbed oxygen molecules on Py-TD-COF-NH is proved to elongate the OO bond, thus accelerating the subsequent hydrogenation and lowering the barrier for the reduction of *OOH to *O intermediates. This work highlights the importance of suitable linkages and provides a guideline for designing metal-free COF catalysts for 2e- ORR.