Manganese dioxide electrodeposition in sulphate electrolytes: the influence of ferrous ions

In the development of a process for recovering manganese from spent batteries as electrolytic manganese dioxide (EMD), it has been found that the acid leach contains manganese, zinc, and smaller concentrations of diverse electroactive metallic ions. The influence of ferrous ion in the anodic electrodeposition of EMD was investigated by cyclic voltammetry on electrodes of glassy carbon, and graphite fibre cloth, as a function of Fe2+ concentration, potential scan rate and convection. The charge under the MnO2 reduction peak was used to quantify the amount of oxide deposited on the electrode. On the glassy carbon electrode, the peak current density decreases with increasing Fe2+ ion concentration. The peak current density decrease is more notable at low scan rates or with electrode rotation. The effect may be ascribed to the oxidation of ferrous ion by MnO2 and the intermediate MnOOH. This reaction, controlled by the transport of Fe2+ ion toward the interface, is sufficiently rapid to cause the dissolution of the oxide, even at potentials where it should be anodically protected. With static graphite fibre electrodes the effect is less marked, because the Fe2+ ions do not penetrate into the material, being oxidised on the external surface of the electrode. However, when forced convection is introduced a significant reduction in the quantity of EMD formed is observed.