The electrochemistry of β-MnO2 and γ-MnO2 in alkaline electrolyte

The electrochemical reactions of β-MnO2 and γ-MnO2 in 7 M KOH have been studied using low-rate linear sweep voltammetry (≈25 μV/s) in conjunction with X-ray analysis of the reaction products. The reduction of β-MnO2 differs from γ-MnO2 in that there is no dilation of the β-MnO2 crystal lattice and Mn3O4 (hausmannite) is formed. The irreversibility of γ-MnO2 electrodes is related to the formation of Mn(OH)2. Manganese hydroxide is oxidized to δ-MnO2 and δ-MnO2 forms Mn3O4 on reduction. Hausmannite resists electrochemical oxidation. Poorly crystallized γ-MnOOH is formed as an intermediate in the oxidation of Mn(OH)2 to δ-MnO2. The formation of Mn3O4 is discussed in terms of the potential-pH diagram for manganese oxides and the thermodynamic properties of MnOOH.