Electrocatalytic properties of spinel-type MnxFe3 –x O4synthesized below 100 °C for oxygen evolution in KOH solutions

Abstract

Some spinel ferrites (Mn_xFe_{3 –x}O₄; x= 0, 0.5, 1.0, 1.5) have been synthesized at 70 °C by the method of precipitation, for electrochemical characterization as oxygen anodes. The electrodes were prepared as films on a nickel substrate by painting with an oxide-slurry, followed by sintering at 400 °C. The electrocatalytic activity for O₂ evolution was found to be a maximum with the MnFe₂O₄ electrode. Based on the observed current density at 0.625 V, the activity of MnFe₂O₄ was 20 times higher than that of Fe₃O₄. The reaction followed approximately second-order kinetics in [OH^–] at the pure and manganese-substituted ferrite film electrodes. The Tafel slope values were ca. RT/F on Fe₃O₄ and Mn_0.5Fe_2.5O₄ and ca. 2RT/3F on MnFe₂O₄ and Mn_1.5Fe_1.5O₄. Based on the electrode kinetic results, a reaction mechanism for oxygen evolution is suggested.