An RDE and RRDE study into the electrodeposition of manganese dioxide

Electrodeposition of manganese dioxide has been examined using a combination of rotating disk electrode (RDE) and rotating ring–disk electrode (RRDE) voltammetry, with the goal of developing an understanding of the electrodeposition mechanism. Experimental variables that have been examined include electrolyte composition (combined changes within the ranges 0.1–1.0 M MnSO4 and 0.1–5.0 M H2SO4), rotation rate (1000–4000 rpm) and temperature (22–98 °C). Voltammetric data (current peak instead of sigmoidal response, and non-proportional current–concentration data) indicates that already deposited manganese dioxide is a poorer catalytic surface compared to Pt. The overall electrodeposition process revolves on the formation of a Mn(III) intermediate, and whether it is soluble for extended periods of time, as in concentrated H2SO4 (>1.0 M), or whether it hydrolyzes rapidly to precipitate as a solid Mn(III) species (e.g. MnOOH) as in more dilute H2SO4 solutions (<1.0 M). In the more concentrated acid electrolytes most of the Mn(III) was lost to the bulk electrolyte through convection, with what little manganese dioxide that was formed resulting from chemical disproportionation. However, in dilute acid electrolytes, evidence suggests that the solid hydrolysis product underwent solid state oxidation to manganese dioxide. Activation energies extracted from temperature studies supports the different mechanism under different acid concentrations. Experiments examining the effect of rotation rate also indicate that the overall electrodeposition process is not mass transport limited.