Implications of Nonelectrochemical Reaction Steps on the Oxygen Evolution Reaction: Oxygen Dimer Formation on Perovskite Oxide and Oxynitride Surfaces

According to the conventional understanding, the oxygen evolution reaction on metal oxide surfaces involves four proton-coupled electron transfer steps with *OH, *O, and *OOH reaction intermediates. Recently, several alternative reaction mechanisms with lower overpotentials were proposed for highly active catalysts. While for such reaction mechanisms additional intermediates leading to nonelectrochemical reaction steps could be considered, they are usually neglected when the thermodynamic overpotential of such mechanisms is investigated. We show here that this is a valid approximation for endothermic nonelectrochemical steps, which only affect the kinetics, while exothermic nonelectrochemical steps can also affect the thermodynamic overpotential. We show this on the basis of density functional theory calculations for one of those proposed mechanisms on surfaces of different perovskite oxides and oxynitrides. We find that for weakly binding surfaces the *O adsorbate spontaneously adopts a bidentate bridged dimer structure in a nonelectrochemical step with an energy gain in excess of 1 eV. This decrease in free energy needs to be compensated by an equivalent increase in magnitude of the electrochemical steps, which can affect the thermodynamic overpotential. This change may result in reaction mechanisms without nonelectrochemical steps having smaller thermodynamic overpotentials and thus being more favorable.