Beyond the Pre-Equilibrium Approximation: Consequences of Elementary Step (Ir)reversibility on the Mechanistic...

The relationship between electrochemical potential and reaction rate-or Tafel slope-is fundamental to the study of multi-step charge transfer reactions. However, despite its importance and ubiquitous use, Tafel slope is seldom interpreted outside of "cardinal" values. The mechanistic interpretation of cardinal Tafel slopes is predicated on the pre-equilibrium approximation (PEA): that the path between the (catalyst) resting state and rate-determining step is in equilibrium. This stringent approximation severely limits opportunities to elicit mechanistic information from electrochemical processes. In this Scientific Perspective, we broaden the existing framework for mechanistic interpretation of Tafel slope through a simple, universal equation that generally describes Tafel slope in terms of elementary-step symmetry factors and approach-to-equilibrium (i.e., approach to PEA accuracy). The predictiveness and mechanistic utility of these theoretical developments are showcased through analysis of experimental data available in the literature for a broad range of electrochemical and thermochemical catalytic reactions, including O₂, H₂, Cl₂, and CO redox. The learnings accrued in these case studies inform kinetic studies of all multi-step charge transfer reactions and are particularly relevant for mixed-potential-driven mechanisms of thermochemical catalysis, which in recent years have been shown to be preponderant at metal-liquid interfaces.