Field effects explain the unintuitive potential response of electrochemical oxygen evolution in acid

Abstract

Electrochemical water oxidation (OER) is the most important electrode reaction in electrocatalysis, representing the default counter-reaction in the plethora of modern electroreductions. Given this prominent role in the electrochemistry-based green transition, improving its efficiency is of utmost importance. Here, identifying novel catalysts by means of computational screening necessitates clarity on the reaction mechanism as this is used to decide on appropriate activity descriptors. To date, however, the mechanism of OER, even on the most widely used catalyst in acid, IrO₂, is still debated, and the debate is fuelled by the consistent appearance of Tafel slopes indicative of a non-electrochemical rate-limiting step. Here, we employ density functional calculations and microkinetic modelling to analyse the mechanism of acidic OER on IrO₂(110), with an emphasis on the polarization of reaction intermediates. Introducing this degree of freedom shows that the electrostatic destabilization of surface-bound oxygen atoms with increasingly positive potentials increases the effective potential response of the reaction. Thus, a reaction mechanism through OOH*-formation could be confused with a non-electrochemical rate-limiting step. Furthermore, we highlight that a mechanism limited by the desorption of adsorbed O₂ is unlikely, as this step is facile at room temperature, but caution is needed in treating adsorbed O₂ in GGA-DFT. By incorporating these elements into our model, we simulate Tafel curves that reproduce the experimental potential response in both the low and high overpotential regions, pointing out that including surface dipole effects is essential for understanding and reproducing experiments.