Kinetic study of a porous metal hydride electrode

A physio-chemical approach has been used to describe the behaviour of a porous metal hydride electrode. The metal hydride alloy MmNi(3.6)bCo(0.8)Mn(0.4)Al(0.3). was studied. Electrochemical impedance spectroscopy (EIS) and a potential ramp were used to study the applicability of the model close to and far from its equilibrium state. An EIS model was developed and took into account a finite conductivity in the solid or liquid phase together with a distribution in characteristic diffusion length, for the hydrogen diffusion, caused by a distribution in particle size. It was shown that the impedance in the low frequency region could not be described by an average particle size, if the particle size was distributed. The EIS model was used to determine different parameters as a function of relative concentration of hydrogen. The change within the parameters could not be explained by a single phase model. The parameters were also tested by a comparison with the result of a single particle's EIS measurement, and a good agreement was found. The potential ramp showed that the present model could not fully describe the transient behaviour of the respective material, far from its equilibrium state. The reason for this could be that the expression describing the surface phase and its connection to the bulk phase is too simplified. (C) 1999 Elsevier Science Ltd. All rights reserved.