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

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 degrees C. The voltammogram of a Pt electrode cycled between 0.6 and 1.6 V vs SCE exhibits an anodic current peak at about 1.3 V vs SCE resulting in the deposition of MnO2 on the electrode, while a cathodic peak appears at 0.8 V 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 Mn-surface(2+) to Mn-ads(3+),and H2O to OHads as the primary oxidation steps is invoked. Mn-ads(3+) ions dissociate disproportionately into Mn-ads(2+) and Mn-ads(4+) ions at the electrode surface. The Mn-ads(2+) and Mn-ads(4+) ions, respectively, react with OHads and H2O resulting in the formation of EMD.