Anodic Evolution of Oxygen on Solid Solutions with Spinel-type Structure

Abstract

The electrodes of solid solutions with spinel-type structure, Pt/CozFe3_x04, Pt/MnxFe3-x04and Pt/NizFe8, 04, were prepared by thermally decomposing metal nitrate mixtures on a platinum substrate at 1150°C in air, and the relationship between the oxygen overvoltage and magnetic property of the resulting electrodes was investigated. The effective Bohr magneton which corresponds directly to the number of unpaired d electrons per molecule was calculated from the saturation magnetization measured at room temperature. In the case of the Ptl CosFe3_x04 electrodes, both the overvoltage and effective Bohr magneton decreased wi th increasing X value up to 2.0. On the other hand, in the case of the Pt/MnxFe3_x04 and PtiNizFe, 0, electrodes, \ he x dependencies of oxygen overvoltage and effective Bohr magnton were reverse to each other. That is, the oxygen overvoltage increased in the case of the former electrodes whereas it decreased in the case of the latter two kinds of electrodes as the effective Bohr magneton increased. From the kinetic studies, it was presumed that the ratedetermining step was the second step or the chemical reaction of OH radical with 0H ior in the former electrodes but it was the first step or the discharge reaction of OH- ion at the latter two kinds of electrodes. The above relationship between the oxygen overvoltage an effective Bohr magneton was therefore explained from a point of view that the rate of the first step increased but that of the second step decreased with an increase of the number oi unpaired d electrons due to the increase of the stability of the reaction intermediate or OF radical.