Redox processes at the manganese dioxide electrode .2. Slow-scan cyclic voltammetry

Slow-scan cyclic voltammetry was used to investigate the voltammetric behavior of electrolytic manganese dioxide (EMD), birnessite, chemically modified EMD (Bi-CMEMD), and birnessite (Bi-birnessite) electrodes under a variety of experimental conditions. During reduction, each electrode underwent a homogeneous reduction stage followed by a heterogeneous reduction stage. The composition at which the transition between these two stages occurred was dependent on the material under study. The initial cycle of the Bi-CMEMD electrode was similar to that of the EMD electrode. However, during the second cycle its behavior was similar to that of the Bi-birnessite electrode. This change in mechanism imparts rechargeable behavior to the Bi-CMEMD electrode. The end product of reduction was Mn(OH)(2), except for the birnessite electrode, where Mn3O4 was formed. The behavior of each electrode was also dependent on the graphite content used in the electrode, the electrolyte concentration, and the particle size of the manganese dioxide under study. The homogeneous reduction stage, which is a reaction involving a solid solution, favors a high proportion of graphite in the electrode and fine manganese dioxide particles. The heterogeneous reduction stage, which involves the formation of a soluble intermediate, was enhanced with concentrated KOH electrolytes and fine manganese dioxide particles. The influence of EMD surface area on electrode behavior was also investigated. The oxidation of Mn(OH)(2) in each electrode proceeded through a variety of solid Mn3+ intermediates to form a birnessite-like phase of manganese dioxide. The Bi3+ ions in the chemically modified electrodes were incorporated into the structure of the oxidation product.