In this talk, we present new fundamental insights into the factors that control the kinetics of the OER on transition metal oxides, leading to a set of design criteria for the optimal active site that explicitly take into account the kinetics of the O-O bond formation step. Our results show that variations in the activation barrier for the water addition step between different active sites are not completely accounted for in terms of variations in the reaction energy by a standard Brønsted-Evans-Polanyi (BEP) relation. We find that this is related to the two-step nature of water addition – proton/electron transfer to form a hydroxyl radical followed by O-O bond formation. To account for this, we propose a modified “2D” BEP relation that more accurately captures the variations in activation barrier in terms of the energies of both of the comprising two sub-steps. Furthermore, the BEP relation turns out to be sensitive to the geometric structure of the active site, particularly the coordination of the reactive oxo anion. Interestingly, the particular form of the modified BEP relations indicates that electrochemical and non-electrochemical water addition pathways are kinetically equivalent on most active sites since the transition state occurs after the proton/electron transfer is complete. In addition to the BEP relation, the water addition reaction energy itself is related to the free energy of the preceding electrochemical step in which the site is oxidized to the active state, with a site having a higher redox potential for this step having a more exothermic water addition step. Together, these 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 the structure-sensitivity of the BEP relations, we also find that sites having a reactive μ<sup>3</sup>-oxo anion are more active than those with µ<sup>2</sup>- and η-oxo anions even when the redox potentials of the oxidation step are identical.

The importance of activation barriers in designing optimal active sites for the oxygen evolution reaction

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