Electrochemical oxidation of manganese(II) at a platinum electrode |
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Authors: | W -H Kao V J Weibel |
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Affiliation: | (1) RAYOVAC Corporation, 53711 Madison, WI, USA;(2) Johnson Controls, Inc., 53201 Milwaukee, WI, USA |
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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. |
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