A cyclic voltammetric study of the kinetics and mechanism of electrodeposition of manganese dioxide

Abstract

The kinetics and mechanism of electrooxidation of Mn²⁺ ions to MnO₂ (EMD) has been studied in electrolytes comprising MnSO₄ and H₂SO₄ by cyclic voltammetry at 80° C. The voltammogram of a Pt electrode cycled between 0.6 and 1.6V vs SCE exhibits an anodic current peak at about 1.3V vs SCE resulting in the deposition of MnO₂ on the electrode, while a cathodic peak appears at 0.8V vs SCE. It is shown that the pair of peaks do not correspond to a single reversible reaction but represent two separate irreversible electrode processes. The cyclic voltammetric peak current for the deposition of EMD is found to be proportional to the square root of Mn²⁺ ion concentration in the electrolyte and independent of acid concentration. Based on these results, a mechanism for the formation of EMD involving diffusion of Mn²⁺ ions to the electrode surface, oxidation of Mn²⁺_surface to Mn³⁺_ads, and H₂O to OH_ads as the primary oxidation steps is invoked. Mn³⁺_ads ions dissociate disproportionately into Mn²⁺_ads and Mn⁴⁺_ads ions at the electrode surface. The Mn²⁺_ads and Mn⁴⁺_ads ions, respectively, react with OH_ads and H₂O resulting in the formation of EMD.