(126c) The Importance of Activation Barriers in Designing Optimal Active Sites for the Oxygen Evolution Reaction

Computational catalyst design provides an efficient means for discovering improved catalysts for important electrocatalytic reactions such as the oxygen evolution reaction (OER). However, the most commonly employed approach for computational OER electrocatalyst design [1] is limited by a neglect of kinetic activation barriers, which can be quite large for the elementary step forming the O-O bond. In this talk, we present new fundamental insights into the factors that control the kinetics of the OER on transition metal oxides. Importantly, we find that the existence of significant activation barriers for one or more elementary steps leads to an entirely different set of design criteria for the optimal active site than those based on thermodynamics alone, a result that likely bears significance for other electrocatalytic reactions as well. Our findings suggest that the active site functions by storing the energy supplied by the external potential in the form of a high energy hole on the metal cation of the active site. This energy is then released to kinetically drive the rate limiting water addition step. Accordingly, the optimal active site at a given electrode potential will have a redox potential equal to the electrode potential, thereby enabling the site to store all of the energy supplied by the external potential while still existing in the active oxidized state. Due to structure-sensitivity we observe in the Brønsted-Evans-Polanyi relations for the water addition step, we also find that sites having a reactive μ³-oxo anion are more active than those with μ²- and η-oxo anions even when the redox potentials of the oxidation step are identical. Interestingly, it is these latter two less active geometries that are typically proposed as the OER active site in many theoretical and experimental studies.

1. Rossmeisl et al., J. Electroanal. Chem. 607, 83 (2007).