Avoid the quasi-equilibrium assumption when evaluating the electrocatalytic oxygen evolution reaction mechanism by Tafel slope analysis

Tafel slope analysis is a powerful tool in comparing experimental data to a proposed reaction mechanism. Typically, in order to simplify the analysis, the non-rate-determining steps in the reaction mechanism are assumed to be in quasi-equilibrium. Here, Tafel analysis of the oxygen evolution reaction following the electrochemical oxide mechanism is performed using a full kinetic model. It is shown that this model (which uses the steady-state assumption) predicts a larger number of Tafel slopes than if the quasi-equilibrium assumption is used and provides and predicts the surface coverages which underpin these Tafel slopes. Importantly, this model predicts Tafel slopes of 30, 40, 60, and 120 mV, all of which are experimentally found on IrO2 and RuO2 anodes. Models using the quasi-equilibrium assumption fail to predict some Tafel regions, as these can occur when a non-rate-determining step is not at quasi-equilibrium.