Deciphering potential-dependent oxygen reduction kinetics in spinel manganese ferrite

Accurate evaluation of oxygen reduction reaction (ORR) catalysts, especially transition metal oxides, requires careful control of testing parameters. Here, we report a potential-window-dependent ORR activity for spinel manganese ferrite (MnFe₂O₄). The ORR activity exhibits a volcano-shaped dependence on the lower potential limit during cathodic scans. Combined experimental analysis and joint density-functional theory (JDFT) calculations reveal that changes in surface Mn site density, Mn oxidation state, irreversible Fe segregation, and potential-dependent reaction free energies contribute to this behavior. Our findings highlight that sweeping potential windows can critically influence ORR performance and must be carefully optimized for the accurate assessment of transition metal oxide catalysts.