ELECTROCHEMICAL OXIDATION OF MANGANESE(II) AT A PLATINUM-ELECTRODE

Electrochemical oxidation of Mn2+ in sulphuric acid to form MnO2 was studied using stationary and rotating platinum/platinum ring-disc electrodes. It appears that nucleation of MnO2 is governed by an equilibrium involving a Mn(III) intermediate. Growth of MnO2 involves the reduction of MnO2 surfaces by Mn2+ ions in the solution to form MnOOH intermediates. The subsequent electrochemical oxidation of MnOOH releases a hydrogen ion and results in the formation of MnO2. The rate constant of MnOOH oxidation to MnO2 was estimated to be 40 s-1. With a sufficient supply of Mn2+ ions, a layer of MnOOH is built up and the in-solid diffusion of hydrogen ions becomes the rate-determining-step. With a low Mn2+ concentration, diffusion of Mn2+ ions from bulk electrolyte to the MnO2/electrolyte interface is a factor controlling the growth of MnO2. The activation energy and the pre-exponential term of the diffusion coefficient of Mn2+ in 0.5 M sulphuric acid were determined to be 44.8 kJ mol-1 and 100 cm2 s-1, respectively.