Anodic behaviour of manganese in alkaline medium

Voltammetry, Electrochemical Impedance Spectroscopy (EIS), Rotating Ring-Di sk Electrode techniques (RRDE), Electrochemical Quartz Crystal Microbalance (EQCM) measurements, and in-situ Raman Spectroscopy were applied to invest igate the anodic behaviour of Mn in 1 M NaOH solution over a wide potential range. Prior to these experiments, for EQCM, an improved plating bath was designed for coating the thin gold electrode of the quartz sensor with Mn. The results obtained revealed clearly that various oxides, depending on the electrode potential, cover the electrode surface. The oxidation-reduction processes between these different oxides and the associated exchange of spe cies with the solution constitute the main characteristic of this electrode . When the Mn electrode is left in 1 M NaOH solution, it becomes spontaneou sly passive through two consecutive steps. In-situ Raman spectroscopy indic ated that the electrode surface is covered by Mn3O4, Mn2O3, and MnO2 as the potential is shifted towards more anodic values. The polarisation curves s howed two anodic current peaks, in agreement with the two-step passivation process. EIS spectra exhibited the typical shape of passivation reactions w ith a large capacitive loop in the low frequency range. The double layer ca pacitance and the faradaic capacitance determined from EIS data indicate th e increase in expanded surface area and bulk volume of the surface oxide wi th anodic potential. From RRDE measurements, the dissolution of Mn through Mn2+ and Mn3+ species were evaluated. EQCM measurements corroborated the gr owth of surface oxide species with the potential, and gave valuable informa tion on the nature of the chemical species involved in the oxidation-reduct ion processes. A reaction mechanism of the Mn electrode in 1 M NaOH in a wi de potential range is proposed. (C) 2001 Elsevier Science Ltd. All rights r eserved.