Enhanced Catalytic Activity and Stability of the Oxygen Evolution Reaction on Tetravalent Mixed Metal Oxide

The oxygen evolution reaction (OER) is a crucial energy conversion reaction for achieving a sustainable society. Highly active OER catalysts were reported in Fe–Co mixed oxides such as perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and brownmillerite-type Ca2FeCoO5. The synergistic effects of Fe–Co mixing on the OER activity are expected in these oxides. However, the details have not been discussed yet because of their structure randomness. In this study, we examine Fe–Co mixing effects on OER catalytic activity for several perovskite and brownmillerite oxides with trivalent and tetravalent Fe and Co ions. Regardless of crystal structures and valence states, the Fe–Co mixing enhances the OER activities compared to the parent compounds. In particular, CaFe0.5Co0.5O3, a perovskite oxide consisting of Fe4+ and Co4+ ions, displays a significant increase in OER activity, which is comparable to BSCF, together with the enhancement of stability. The electronic state of CaFe0.5Co0.5O3 obtained by density-functional theory calculation with the special quasirandom structure model exhibits the existence of active sites with smaller charge-transfer energies than the parent compounds, CaFeO3 and CaCoO3. This finding proposes that the various structural and electronic environments around transition metal ions induced by random distributions of constituent metals contributes to dispersed electronic band structures, leading to highly active sites.