Anodic behaviour of manganese in alkaline medium

Abstract

Voltammetry, Electrochemical Impedance Spectroscopy (EIS), Rotating Ring-Disk Electrode techniques (RRDE), Electrochemical Quartz Crystal Microbalance (EQCM) measurements, and in-situ Raman Spectroscopy were applied to investigate 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 species with the solution constitute the main characteristic of this electrode. When the Mn electrode is left in 1 M NaOH solution, it becomes spontaneously passive through two consecutive steps. In-situ Raman spectroscopy indicated that the electrode surface is covered by Mn 3O 4, Mn 2O 3, and MnO 2 as the potential is shifted towards more anodic values. The polarisation curves showed two anodic current peaks, in agreement with the two-step passivation process. EIS spectra exhibited the typical shape of passivation reactions with a large capacitive loop in the low frequency range. The double layer capacitance and the faradaic capacitance determined from EIS data indicate the increase in expanded surface area and bulk volume of the surface oxide with anodic potential. From RRDE measurements, the dissolution of Mn through Mn 2+ and Mn 3+ species were evaluated. EQCM measurements corroborated the growth of surface oxide species with the potential, and gave valuable information on the nature of the chemical species involved in the oxidation–reduction processes. A reaction mechanism of the Mn electrode in 1 M NaOH in a wide potential range is proposed.