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.