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) h as been studied in electrolytes comprising MnSO4 and H2SO4 by cyclic voltam metry 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 re sulting in the deposition of MnO2 on the electrode, while a cathodic peak a ppears at 0.8 V vs SCE. It is shown that the pair of peaks do not correspon d to a single reversible reaction but represent two separate irreversible e lectrode processes. The cyclic voltammetric peak current for the deposition of EMD is found to be proportional to the square root of Mn2+ ion concentr ation in the electrolyte and independent of acid concentration. Based on th ese results, a mechanism for the formation of EMD involving diffusion of Mn 2+ 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 e lectrode surface. The Mn-ads(2+) and Mn-ads(4+) ions, respectively, react w ith OHads and H2O resulting in the formation of EMD.