The origin of capacity loss in the pocket plate cadmium electrode

Electrical testing of negative limited mono-pocket plate cells, cyclic voltammetry and surface analysis by X-ray diffraction were used to investigate the origin of the second discharge plateau at the cadmium pocket plate electrode. The electrical testing results show that there is a direct coupling between the appearance of a second discharge plateau and a high carbonate concentration in combination with high discharge rates (1 C5A). An exchange to a carbonate free electrolyte results in recovering of the cell. Voltammetric measurements show that the reduction of CdCO3 takes place in the same potential region as the reduction of Cd(OH)2. Thus, the presence of CdCO3 in the discharged negative electrode cannot give rise to a second voltage plateau or capacity loss. However, a decrease in the hydroxide concentration results in a positive shift of the oxidation potential. These results are interpreted in the context of the porous structure of the pocket plate cadmium electrode. For a high carbonate concentration and large discharge rates, the hydroxide concentration within the pores are depleted leading to a positive potential shift and the appearance of a second discharge plateau.