Proton-Coupled Electron Transfer Catalyst: Heterogeneous Catalysis. Application to an Oxygen Evolution Catalyst

Proton coupled electron transfer (PCET) catalysts are investigated in the framework of cyclic voltammetry (CV). We analyze heterogeneous catalysts and provide a detailed kinetic analysis of the various responses expected in the case of a PCET catalyst following either stepwise or concerted pathways. Both buffered solution and nonbuffered aqueous media are considered. In the first case, we show that, besides possible limitation by buffer diffusion, the PCET sequence may be rate-limiting. CV wave shape, position, and intensity are discussed in terms of thermodynamic and kinetic penalties associated with each mechanism. When water is the proton acceptor in aqueous solution, diffusion of hydronium ions away from the electrode surface is a key parameter, and we show that, for stepwise processes, the equilibrated PCET is not the optimal catalytic response. Finally, we illustrate this formal analysis with an electrodeposited cobalt oxide behaving as a PCET catalyst for the oxygen evolution reaction in phosphate buffer. The PCET is at equilibrium, and the rate-determining step is a chemical step, presumably O-O bond formation, but the CV is controlled by buffer diffusion in the investigated conditions.