Intramolecular Double Activation by Biligands Sharing a Single Metal Atom for Preferred Two-Electron Oxygen Reduction

Abstract

It is challenging to selectively promote the two-electron oxygen reduction reaction (2e-ORR) since highly ORR-active electrocatalysts are not satisfied with 2e-ORR and are most likely to go all the way to 4e-ORR, completely reducing dioxygen to water. Recently, however, the possibility of a 2e-ORR preference over 4e-ORR was raised by extensively considering multiple ORR mechanisms and employing a potential-dependent activity measure for constructing volcano plots. Here, we realized the preferred 2e-ORR via an intramolecular double activation of the peroxide intermediate (*OOH) by allowing the intermediate to be easily desorbed before proceeding to 4e-ORR. Dioxygen was transformed to *OOH on a carbon atom of the imidazole ligand of zeolitic imidazolate framework-8 (ZIF-8). When an amine group was introduced via ligand exchange, the selectivity of 2e-ORR was enhanced by 11%. The added amine attracted the oxygen atom of *OOH via a hydrogen bond to weaken the binding strength of *OOH to the carbon active site (double activation). The amine-decorated ZIF-8 exhibited H2O2 faradaic efficiency at 98.5% at ultrahigh-rate production at 625 mg cm-2 h-1 by 1 A cm-2 in a flow cell.