Tuning the electrochemical redox-mediated mechanism of oxygen evolution on cobalt sites by hydroxide ion coupling

Abstract

Heterogeneous molecular catalysts (HMCs) with cobalt (Co) active sites are potent for the electrochemical oxygen evolution reaction (OER) in energy conversion applications. Such catalysts typically operate through the classical redox-mediated mechanism, where dynamic equilibria of Co^2+/3+ and Co^3+/4+ redox states are present before and throughout the OER cycle. However, the generation of low-valent Co²⁺ sites is disadvantageous for catalysis. To address this, sulfate groups embedded in graphene were developed to link a model Co-2,2′-bipyridine complex, resulting in the synthesis of a novel Co-based HMC that generates a specific CoN₂O₄S₁ coordination moiety. These molecular Co sites were induced to oxidize from +2 to +3 oxidation state at open-circuit conditions, due to their proton-coupled electron transfer nature. This process ultimately eliminated the generation of the Co²⁺ state from its redox equilibrium and efficiently improved the turnover frequencies of Co sites toward OER, showing a two-order dependence on the concentrations of OH⁻ ions. This work provides a novel mechanistic perspective for the rational design of high-performance HMCs.