Electrochemical oxidation of manganese(II) at a platinum electrode

Abstract

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 ratedetermining-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 0.5m sulphuric acid were determined to be 44.8 kJ mol⁻¹ and 100 cm² s⁻¹, respectively.