Synergistic Effect between Fe⁴⁺ and Co⁴⁺ on Oxygen Evolution Reaction Catalysis for CaFe_1–xCo_xO₃

Abstract

Chemical substitution is an effective way to improve electrocatalytic properties in transition metal oxides. We investigate the synergistic effect between Fe⁴⁺ and Co⁴⁺ ions on the catalytic activity for oxygen evolution reaction (OER) in the Fe-Co-mixed perovskite oxide CaFe_1–xCo_xO₃. The OER activity of CaFe_1–xCo_xO₃ is substantially increased by small amounts of Co (Fe) doping into CaFeO₃ (CaCoO₃), leading to the superiority compared to the pure Fe and Co perovskite oxides. The x dependences of the OER overpotential and specific activity for CaFe_1–xCo_xO₃ (0.05 ≦ x ≦ 0.95) are expressed by constant offset from the weighted average between CaFeO₃ and CaCoO₃, which can be interpreted to be the synergistic effect between Fe⁴⁺ and Co⁴⁺ ions on OER activity. The absence of the optimum x for the highest activity for CaFe_1–xCo_xO₃ contrasts with the volcano-like plots reported in various mixed-metal oxides. First-principle calculations using the special quasirandom structure models on CaFe_1–xCo_xO₃ (x = 0.03–0.5) demonstrate that about half the amount of Fe⁴⁺ is electronically activated to possess smaller charge-transfer energies, corroborating the enhancement of catalytic activity in CaFe_1–xCo_xO₃. These findings provide new insight into the synergistic effects in complex transition metal oxide catalysts.