Degree of span control to determine the impact of different mechanisms and limiting steps: Oxygen evolution reaction over Co3O4(001) as a case study

Abstract

Oxygen evolution reaction (OER) is the limiting process in electrolyzers for the production of green hydrogen. Although computational studies using density functional theory calculations provide insights into reaction mechanisms and limiting steps of OER catalysts, our mechanistic understanding is still limited even for state-of-the-art OER catalysts. This finding can be related to the fact that most computational studies rely on the approximation of the electrocatalytic activity by a single reaction mechanism and the limiting reaction step is solved by thermodynamic considerations, thereby assuming that the reaction rate is determined by a single step only. In this article, we present a framework to account for the mechanistic diversity in the formation of gaseous oxygen, using the example of a Co₃O₄ model catalyst due to the use of cobalt oxide-based materials in alkaline electrolysis. In addition to traditional reaction mechanisms, we consider Walden-like pathways in the analysis and show that multiple reaction mechanisms compete under OER conditions. To gain unprecedented insight into the limiting reaction steps, we introduce the concept of degree of span control, a thermodynamic representation based on Campbell’s generalized degree of rate control from thermal catalysis but aimed at the study of electrocatalytic processes. We demonstrate that in the OER over Co₃O₄(001), different reaction steps contribute to the OER current density to a different extent when the applied electrode potential is modulated. The degree of span control framework is considered useful for screening reaction mechanisms and limiting steps of catalytic processes at electrified solid–liquid interfaces before performing kinetic studies of selected elementary steps.