The self-discharge mechanism of AB5AB5-type hydride electrodes in Ni/MH batteries

A new strategy in the study of the self-discharge mechanism of metal-hydride electrodes has been developed. The self-discharge behavior of a LaNi3.55Co0.75Mn0.4Al0.3LaNi3.55Co0.75Mn0.4Al0.3 electrode in alkaline solution, with and without ZnO additive, was investigated. Experimental results showed that the self-discharge rate (hydrogen desorption) within the single-phase region is controlled by the difference between the equilibrium hydrogen partial pressure at the electrode and the actual hydrogen partial pressure in the cell. Dissolved oxygen was also found to exert a strong influence on the self-discharge rate in the single-phase region. In the two-phase region, the self-discharge is limited by the rate of phase transformation. A four-step mechanism for the self-discharge process is proposed.