Exceptional oxygen evolution reactivities on CaCoO₃ and SrCoO₃

We investigated the roles of covalent bonding, separation of surface oxygen, and electrolyte pH on the oxygen evolution reaction (OER) on transition metal oxides by comparing catalytic onset potentials and activities of CaCoO₃ and SrCoO₃. Both cubic, metallic perovskites have similar Co^IV intermediate spin states and onset potentials, but a substantially smaller lattice parameter and shorter surface oxygen separation make CaCoO₃ a more stable catalyst with increased OER activity. The onset potentials are similar, occurring where H⁺ is removed from surface -OH^-, but two competing surface reactions determine the catalytic activity. In one, the surface -O^- is attacked by electrolyte OH^- to form the surface -OOH^-; in the other, two -O^- form a surface peroxide ion and an oxygen vacancy with electrolyte OH^- attacking the oxygen vacancy. The second pathway can be faster if the surface oxygen separation is smaller.