Multiple Factors on Catalytic Activity for Oxygen Evolution Reaction in Magnetoplumbite Fe–Co Oxide BaFe_12–xCo_xO₁₉

Complex metal oxides consisting of multiple transition-metal elements exhibit a higher electrocatalytic performance than simple transition-metal oxides. In this study, we demonstrate the physical and electrochemical properties of BaFe12–xCoxO19 (x = 0–12), a solid solution of magnetoplumbite-structured BaFe12O19 and BaCo12O19. Single-phase samples in the whole composition range are successfully obtained by using high-pressure and high-temperature conditions of 6.5 GPa and 1373 K. Co- and Fe-doping into BaFe12O19 and BaCo12O19, respectively, efficiently increases the catalytic activity for the oxygen evolution reaction (OER). The OER activity of BaFe12–xCoxO19 is maximized at two compositions of x = 5 and 10, whereas the nonsystematic change in the OER activity for the intermediate compositions between x = 5 and 10 indicates a competition of positive and negative factors on the OER activity such as the dopant as an active site, electrical conductivity, spin polarization, and charge-transfer resistance. We eventually found a characteristic relationship between specific activity and charge-transfer resistance reflecting the mechanistic difference for BaFe12–xCoxO19.