A simple method to approximate electrode potential-dependent activation energies using density functional theory

Abstract

Density functional theory (DFT) examination of electrocatalytic mechanisms are useful in fuel cell/electrolysis development, but the calculation of potential-dependent activation barriers for elementary steps involving electron and ion transfer remains challenging. A simple and transferable DFT approach to estimate these constant potential barriers for inner sphere electrochemical reactions is presented. The challenge of finding the transition state for an electrochemical reaction step (A*+H++e−→AH*, where * denotes surface-adsorbed species) is met by using an equivalent analogous non-electrochemical reaction (A*+H*→AH*). The transition state of the non-electrochemical step is referenced to an equilibrium potential (U0), at which the analogous non-electrochemical state μ(H*) is in equilibrium with its equivalent electrochemical state μ(H++e−), allowing for the barrier to be referenced to the chemical potential of the ion in the bulk electrolyte. The potential-dependence is incorporated by extrapolating the activation energy using Marcus theory. The first elementary step of CO2 electroreduction to COOH* is used as a detailed example case for illustrating the method. Additional elementary reduction reactions involving CH, OH and NH bond formations are included to demonstrate the transferability of the method. The method is simple and easy to implement to approximate potential-dependent activation energies at the computational cost of a singly hydrogenation barrier calculation and can aid in the development of more active and selective catalysts for electrochemical reactions.