Volcano Activity Relationships for Proton-Coupled Electron Transfer Reactions in Electrocatalysis

This paper studies simple kinetic models for proton-coupled electron transfer reactions, and demonstrates that for reactions in which proton and electron do not transfer simultaneously, pH dependence of the overall reaction rate is expected. In particular, if the current is evaluated on the reversible hydrogen scale, this may lead to volcano-type activity relations as a function of pH. In case that an acid-base equilibrium is part of the mechanism, the optimal pH occurs close to the pK(a) of this equilibrium.