A MATHEMATICAL-MODEL OF THE SELF-DISCHARGE OF A NI-H2 BATTERY

A simple mathematical model is presented and used to characterize the self-discharge of a nickel oxyhydroxide (NiOOH) electrode in a hydrogen environment. This model includes diffusion of dissolved hydrogen in an electrolyte film which covers a flooded electrode, electrochemical oxidation of hydrogen, reduction of nickel oxyhydroxide, and changes of surface area and of porosity of the electrode during the self-discharge process. Although the self-discharge process is complicated, the predictions of the model are consistent with experimental results reported in the literature, which include linear relationships between the logarithm of hydrogen pressure and time and between the logarithm of the capacity remaining and time. The model predictions indicate that hydrogen oxidation takes place predominantly near the front side of the electrode, but the reduction of nickel oxyhydroxide to nickel hydroxide takes place uniformly throughout the electrode.