Composition dependent synergistic effects on oxygen evolution reaction catalysis for CaFe1_xMnxO3

We investigate oxygen evolution reaction (OER) catalytic activity for CaFe1_xMnxO3. The OER activity of CaFe1_xMnxO3 decreases almost monotonically with increasing Mn content x. No any compositions do not exceeded CaFeO3 in OER activity, but we observe that the OER activity is gently decreased except for Fe-rich side (x = 0.1) in comparison with the weighted average OER activity between the endmembers of highly active CaFeO3 (x = 0) and less active CaMnO3 (x = 1). We discuss probable electronic effects of the Fe4+-Mn4+ couple on OER activity based on the density functional theory calculation using the special quasirandom structure models. The local electronic states of Mn ions were deactivated by Fe substitution on the Fe-rich side (x = 0.25), whereas the Fe ions are constantly activated by Mn substitution in the entire composition range. This finding proposes that local electronic states are variously changed by the kind of couples of multiple elements, which may contribute to the design of mixed transition metal oxides for OER catalysts.