Formation of a thin MnIII oxide film on noble metal electrodes from a manganese solution

MnII–glycine complex in slightly alkaline solution is oxidized at a less positive potential (+0.25 to +0.6 V vs. Ag ∣ AgCl) where free Mn2+ ion is not oxidizable, leading to the formation of a thin uniform film containing MnIII oxide. The film prepared by continuous cyclic scans in the potential region between +0.2 and +0.6 V exhibited an electron spin resonance (ESR) peak attributed to MnII(H2O)6, but this resonance peak disappeared when the lowest potential was shifted to 0 V. From the electrochemical quartz crystal microbalance (EQCM) measurements, the film deposited anodically can be represented as Mn2IIIO6[MnII(H2O)6]3 which corresponds to a structure where MnII in Mn5O6 (Mn2IIIMn3IIO6) is not bound to the oxygen atom in the MnIIIO network but is coordinated with water molecules. The MnII(H2O)6 in the film was suggested to be released during the reduction of the oxide matrix and oxidized to form the MnIIIO bond at more positive potentials than +0.6 V.