Discovering Competing Electrocatalytic Mechanisms and Their Overpotentials: Automated Enumeration of Oxygen Evolution Pathways

Electrocatalytic processes involving several successive electron-transfer steps are ubiquitous. Given the many possible reaction intermediates that could exist on the surface of an electrocatalyst, the determination of the precise operative mechanism for a given overall reaction is a challenging task. Indeed, multiple possible pathways could give rise to the final product in an electrocatalytic cycle, although some of these pathways could be more likely to occur than others. In this work, we formulate an optimization framework to discover all possible reaction mechanisms leading to a desired electrochemical product and to sort the mechanisms according to their thermodynamic reaction overpotentials. To this end, we elucidate the basic physical constraints that underlie any electrochemical transformation. We use these constraints to enumerate all potential reaction mechanisms for the specific case of the oxygen evolution reaction (OER) involved in water splitting on β-nickel oxyhydroxide, a promising alkaline-medium electrocatalyst. Our analysis reveals several competing OER mechanisms on pure and iron-doped forms of this material, via both single- and two-site pathways and consisting of varying numbers of elementary steps. Our work underscores the complexity of characterizing the mechanism of a given electrocatalytic process, while providing a methodology to address this complexity.