Nucleation and Growth of Electrodeposited Manganese Dioxide for Electrochemical Capacitors

In this work the anodic electrodeposition of manganese dioxide from a solution of 0.01 M MnSO4 in 0.1 M H2SO4 onto a platinum substrate has been examined in order to determine the mechanism of deposition. Electrochemical data indicates that the dilute acid solution favours the formation of MnO2 via a MnOOH intermediate (hydrolysis). The electrodeposited material was examined further using AFM and TEM which showed that the oxidation of Mn2+ occurs in a number of steps. When a potential is applied, Mn2+ is electrochemically oxidised to soluble Mn3+. When the concentration of Mn3+ at the electrode reaches saturation, it chemically precipitates onto the electrode as MnOOH, causing an increase in the electroactive surface area. This stage of the deposition is characterised by the nucleation of MnOOH crystallites on the platinum surface. As the deposition proceeds, MnOOH undergoes solid state oxidation to MnO2. This later stage of the deposition favours the growth of existing MnO2 crystallites, leading to a decrease in the surface area of the electrodeposited material.