INFLUENCE OF KOH CONCENTRATION ON THE GAMMA-MNO2 REDOX MECHANISM

The redox mechanism of gamma-MnO2 was studied in KOH electrolytic solu tions 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 tha n 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 inte rmediate oxyhydroxide, tentatively identified by electrochemical measu rement 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 3 N is attributed to the Mn(III) dissolution mechanism. The H+/e(-) inse rtion process decreases while the second heterogeneous stage increases , with increasing KOH concentration. The oxidation process of the firs t cycle and the cycling behaviour was studied by Step Potential Electr oChemical Spectroscopy (SPECS). In KOH 1N, one main anodic peak is obs erved in the voltammogram of the first oxidation process. For [KOH] gr eater 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 electroche mical behaviour is observed depending on the KOH concentration. In 1N KOH, the voltamperometric and XRD data show that the redox mechanism o f 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.