Immobilization of a Molecular Copper Complex and a Carboxylate-Terminated Cocatalyst on a Metal Oxide Electrode for Enhanced Electrocatalytic Oxygen Reduction

Abstract

Positioning basic/acidic groups near a molecular catalyst has become a popular strategy to modulate proton-coupled electron transfer reactions in energy conversion and storage. However, the detailed understanding of the real role played by basic/acidic groups remains lacking. Here, we report the immobilization of a copper(II) dipicolylamine catalyst bearing a phosphonic anchor (CuL) on a metal oxide electrode to investigate the activity of oxygen reduction reaction (ORR) in aqueous solution. The catalytic current of ORR by the CuL catalyst was significantly enhanced by three times as a carboxylate-terminated ligand was integrated onto the metal oxide surfaces to act as a cocatalyst. The CuL-coated metal oxide electrode with a carboxylate-terminated cocatalyst exhibits turnover frequency values higher than the one with an amine-terminated cocatalyst by 2.5-fold and the CuL-only case by 9.6-fold. Extensive experimental and theoretical studies reveal that a hydrogen bond formed between the proximal carboxylate of the cocatalyst and a Cu-OOH intermediate generated via protonation of a Cu-O2 adduct by the bulk buffer solution that aids O–O bond cleavage, thereby promoting the four-electron four-proton reduction of dioxygen into water reaching a Faradaic efficiency of 97.6%. Our finding is in stark contrast to the typical assignment on the role of basic groups near a catalyst that features proton shuttling to facilitate protonation of a metal-O2 intermediate forming a metal-OOH intermediate. This work offers a facile strategy to enhance simultaneously the activity and selectivity of surface-supported non-precious metal electrocatalysts that are important toward realizing alternative energy conversion schemes.