(614c) Electrode Potential Dependent Activation Barriers Approximated With Density Functional Theory

Density functional theory (DFT) studies of electrocatalytic systems are widespread, but the calculation of activation barriers for elementary steps involving electron and ion transfer remains challenging.  A simple and transferable DFT approach to estimate these barriers for inner sphere electrochemical reactions, focusing on approximating the electron transfer coefficient, will be presented.  Our approach follows from the assumption that an electrocatalytic elementary step is inherently an inner-sphere reaction, for which attainment of the transition state requires nuclei rearrangement local to an adsorbed species, and electron transfer is rapid once the transition state is attained.  The Arrhenius concept (transition state theory) of the reaction rate then holds, and the Born-Oppenheimer approximation allows for evaluation of reaction rates without explicit consideration of electron conduction. We replace the need to locate a transition state for an electrochemical reaction (reduction of an adsorbed species by a proton/electron pair, for example) with the location of a transition state for an analogous non-electrochemical reaction (hydrogenation).  It is assumed that the transition states for these two reactions are identical at one specific electrode potential.  Once the transition state is located, the activation free energy, G_act⁰, may be assigned to the potential, U⁰, at which the chemical potential of the H* species is equivalent to the true H⁺ + e^- reactant. The activation barrier can be extrapolated as a function of electrode potential using Butler-Volmer theory, with the symmetry factor estimated by examination of the transition state and reaction coordinate.

The talk will further detail this approach and demonstrate that the method produces activation barriers consistent with experimental electrokinetics, and also yields values of the electron transfer coefficient that make sense and fit well with the other data.  The first elementary step of CO₂ reduction will be used as a detailed example case, with application to other reactions discussed.