Standard-state entropies and their impact on the potential-dependent apparent activation energy in electrocatalysis

The apparent activation energy, Eₐpp, is a common measure in thermal catalysis to discuss the activity and limiting steps of catalytic processes on solid-state materials. Recently, the electrocatalysis community adopted the concept of Eₐpp and combined it with the Butler-Volmer theory. Certain observations though, such as potential-dependent fluctuations of Eₐpp, are yet surprising because they conflict with the proposed linear decrease in Eₐpp with increasing overpotential. The most common explanation for this finding refers to coverage changes upon alterations in the temperature or the applied electrode potential. In the present contribution, it is demonstrated that the modulation of surface coverages cannot entirely explain potential-dependent oscillations of Eₐpp, and rather the impact of entropic contributions of the transition states has been overlooked so far. In the case of a nearly constant surface coverage, these entropic contributions can be extracted by a dedicated combination of Tafel plots and temperature-dependent experiments.