Hydride-forming electrode materials seen from a kinetic perspective

A model is proposed which includes the charge transfer reaction and the formation of adsorbed hydrogen at the electrode/electrolyte interface, absorption of hydrogen into the solid-state MH electrode, recombination of adsorbed hydrogen atoms into molecular hydrogen dissolved in the electrolyte and hydrogen in the gas phase. It was found that the rate of the charge transfer reaction, as represented by the exchange current, is dependent on the partial hydrogen pressure (PH2) in a rather complex way: a positive linear dependence between log I0 and log PH2 is found at relatively low partial pressures, which subsequently passes through a maximum, turning into a negative dependence at higher partial pressures. Both the slopes of the lines and the turning points are found to be strongly dependent on the rate constants of the various reactions. The theoretical model has been verified with both switchable mirror MH thin film electrodes and multicomponent MH electrode materials, nowadays widely applied in commercial NiMH batteries. The as-deduced theoretical relationship will form the basis for the modelling of complete NiMH batteries.