Electrochemical oxidation of Mn(OH)(2) in alkaline media

Abstract

Redox transformations of the Mn(II)/Mn(III) and Mn(II)/Mn(IV) pairs have been studied by transient electrochemical, spectroelectrochemical, and impedance measurements in alkaline solutions at a manganese electrode. The Mn(O)/Mn(II) and Mn(II)/Mn(III) pairs, which have not been clearly identified hitherto in the literature, have been observed using these techniques. A species observed at 285 nm during oxidation of the Mn(OH)(2)-covered manganese electrode was assigned to MnOOH. This species eventually led to passive films whose major component is MnO2 absorbing at about 400 nm. An accumulation of the passive films was observed at longer electrolysis times. Electrochemical impedance measurements were used to construct a Tafel plot for the manganese oxidation. The polarization resistance reached a minimum at about +0.17 V vs. AglAgCl (saturated KCl) without oxygen evolution. Some kinetic information including resistance-capacitance time constants for the oxidation reaction and an exchange current density are also reported.