Simulation of Potential-Dependent Activation Energies in Electrocatalysis: Mechanism of O-O Bond Formation on RuO2

Theoretical assessment of potential-dependent activation energies of electrochemical reactions is of critical importance but remains challenging. In this work, we present two computational tools to tackle this long-standing challenge. First, we implement a general computational framework for constant-potential saddle searches including both atomic positions and number of electrons as variables. Second, we develop a novel correction method to determine potential-dependent activation energies based on conventional zero-charge calculations. Different from existing capacitance-only schemes, our correction method takes into account the potential-dependent structural relaxation. This correction allows a significant reduction in computational overhead, but is still quite accurate, supplementing the direct constant-potential simulation. With these tools, we study the O-O bond formation reaction on the RuO2(110) surface. A number of new mechanistic insights are developed from the constant-potential calculations. The proton-coupled electron transfer pathway for the O-O bond formation has been determined to be favorable over the water dissociation mechanism. It is found that both H atoms strip from H2O successively to directly form adsorbed O-2 without forming a stable intermediate OOH. With this example, we also demonstrate that the charge-structure coupling effect is not always negligible and can be captured by the full-Hessian correction. We expect the methods developed here to become useful tools in electrocatalysis.