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The time resolved mechanism of electrodeposition and the effect of this changing mechanism on the nucleation and growth of solid manganese dioxide has been investigated in both acidic and neutral electrolytes on the rotating ring disc electrode (RRDE). The fate of the Mn3+ intermediate is a key feature of this electrodeposition mechanism, the formation of which is dependent on the substrate, which in this case is either platinum, MnO2 or MnOOH. On the platinum surface, for all electrolytes, soluble Mn3+ is produced initially. The stability of this soluble Mn3+ species determines the initial morphology, and rate of change of mechanism for the process. In a neutral electrolyte, nucleation and growth of MnO2 occurs primarily through the precipitation of a 2D film of MnOOH on the platinum, which rapidly covers the surface. Nucleation in an acidic H2SO4 system occurs primarily via a disproportionation route which forms 3D MnO2 hemispheroids that cover the substrate slowly. Subsequent growth of MnO2 in both electrolytes then proceeds via formation of a MnOOH film, which is subsequently oxidized in the solid state to form MnO2. MnOOH oxidation to MnO2 appears kinetically limited, which is overall a limiting factor in the electrodeposition process.

In this work, the electrodeposition of manganese dioxide in the presence of citrate anions is characterized using a combination of in situ electrochemical quartz crystal microbalance (EQCM), small angle X-ray scattering (SAXS), quasi-in situ atomic force microscopy (AFM) and ex situ transmission electron spectroscopy (TEM). Manganese dioxide was deposited onto a platinum substrate at 1.3 V (vs SCE) from solutions containing 0.01 mM MnSO4, 0.1 M Na2SO4and manganese citrate concentrations of 0, 0.5, 2 or 4 mM. It was found that varying the citrate concentration changed the deposition mechanism by stabilising the Mn3+intermediate formed by the oxidation of Mn2+. The different stability of the Mn3+ion affects the rate of nucleation on the surface and subsequent growth of the film. This in turn, affects the morphology of the deposited material. Understanding the effects of the deposition mechanism on the resultant film morphology is necessary to optimise the properties of electrodeposited films for their various applications.

The time resolved mechanism of electrodeposition and the effect of this changing mechanism on the nucleation and growth of solid manganese dioxide has been investigated in both acidic and neutral electrolytes on the rotating ring disc electrode (RRDE). The fate of the Mn3+intermediate is a key feature of this electrodeposition mechanism, the formation of which is dependent on the substrate, which in this case is either platinum, MnO2or MnOOH. On the platinum surface, for all electrolytes, soluble Mn3+is produced initially. The stability of this soluble Mn3+species determines the initial morphology, and rate of change of mechanism for the process. In a neutral electrolyte, nucleation and growth of MnO2occurs primarily through the precipitation of a 2D film of MnOOH on the platinum, which rapidly covers the surface. Nucleation in an acidic H2SO4system occurs primarily via a disproportionation route which forms 3D MnO2hemispheroids that cover the substrate slowly. Subsequent growth of MnO2in both electrolytes then proceeds via formation of a MnOOH film, which is subsequently oxidized in the solid state to form MnO2. MnOOH oxidation to MnO2appears kinetically limited, which is overall a limiting factor in the electrodeposition process.