INFLUENCE OF KOH CONCENTRATION ON THE GAMMA-MNO2 REDOX MECHANISM

The redox mechanism of gamma-MnO2 was studied in KOH electrolytic solutions of increasing concentration ranging from 1 to 9N. The reduction process of the first cycle, carried out at i = 0.33 Am-2, shows a one-stage reduction in KOH 1N and two stage-reduction in [KOH] greater than or equal to 3N. In all cases the first stage of reduction is the H+/e(-) insertion process. For [KOH] greater than or equal to 3N, an intermediate oxyhydroxide, tentatively identified by electrochemical measurement as MnO1.69 was formed during this process. The flat part of the E versus capacity curve observed for [KOH] greater than or equal to 3N is attributed to the Mn(III) dissolution mechanism. The H+/e(-) insertion process decreases while the second heterogeneous stage increases, with increasing KOH concentration. The oxidation process of the first cycle and the cycling behaviour was studied by Step Potential ElectroChemical Spectroscopy (SPECS). In KOH 1N, one main anodic peak is observed in the voltammogram of the first oxidation process. For [KOH] greater than or equal to 3N, two main oxidation peaks are observed. XRD and chronoamperometric data indicate that these are different steps of the oxidation process. During the redox cycling, different electrochemical behaviour is observed depending on the KOH concentration. In 1N KOH, the voltamperometric and XRD data show that the redox mechanism of the gamma-MnO2 can be described as a H+/e(-) insertion/desinsertion process, with good reversibility. For 3N greater than or equal to [KOH], after the first cycle, a different redox mechanism is observed and a loss of electrochemical activity of gamma-MnO2 is also noticed.