Electrocatalytic properties of spinel-type MnxFe3-x O-4 synthesized below 100 degrees C for oxygen evolution in KOH solutions

Some spinel ferrites (MnxFe3-xO4; x = 0, 0.5, 1.0, 1.5) have been synthesized at 70 degrees 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 degrees C. The electrocatalytic activity for O-2 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 Mn0.5Fe2.5O4 and ca. 2RT/3F on MnFe2O4 and Mn1.5Fe1.5O4. Based on the electrode kinetic results, a reaction mechanism for oxygen evolution is suggested.