Why approximating electrocatalytic activity by a single free-energy change is insufficient

Progress in the area of electrocatalysis has been spurred by theoretical predictions, using the free energies of reaction intermediates within the electrocatalytic cycle as a measure to assess electrocatalytic activity. Most commonly, the framework of the thermodynamic overpotential, eta(TD), is applied to study activity trends of electrodes in a class of materials. The concept of eta(TD), however, relies on the evaluation of a single free-energy change at the equilibrium potential of the reaction, which may explain that the notion of eta(TD) does not always capture activity trends correctly. To compensate this shortcoming, the electrochemical-step symmetry index (ESSI) was introduced, which accounts for all free-energy changes at the equilibrium potential among the mechanistic description. Yet, both eta(TD) and the ESSI do not consider overpotential and kinetic effects in the analysis, motivating the introduction of an overpotential-dependent activity descriptor for multiple-electron processes, G(max(eta)). In this manuscript, these three descriptors to approximate electrocatalytic activity in a heuristic fashion are compared, elaborating that the assessment of activity by a single free-energy change is too simplistic. (C) 2021 Elsevier Ltd. All rights reserved.