Identifying “Optimal” Electrocatalysts: Impact of Operating Potential and Charge Transfer Model

Abstract

Understanding the influence of potential on electrochemical surface reaction kinetics remains a challenge in identifying catalytic materials for numerous important reactions including water splitting (OER), hydrogen evolution (HER), and CO2 reduction, among others. Limitations in computational methods, complicated by the unique environment of the electrode–electrolyte interface, have compelled many studies to focus on the thermodynamics of reaction schemes and to generalize inferences about the kinetics of charge transfer. In instances where activation barrier estimates are available, they are typically assumed to follow the empirical Butler–Volmer (BV) model. In this Perspective, we illustrate that the relative magnitudes and potential-dependences of elementary barriers can have a marked effect on the properties of a catalyst deemed “optimal” for a given reaction. We use a simple pseudosteady-state analysis of two sequential surface-mediated charge transfers to assess the degree of rate control of each step as a function of the material and conditions. We compare BV kinetics to Marcus theory and also discuss more recent models that are specific to the interactions of an adsorbate with the electronic structure of a surface. Recent developments in the full simulation of charge transfer to surface species are also briefly discussed. Finally, we highlight the need for assessment of kinetics and identification of activity descriptors that are optimal at relevant operating conditions, and we conclude with an outlook on current research needs.