The present work is concerned with the hydrogen absorption into hydride forming foil and porous electrodes such as Pd, $LaNi_4.7Al_0.3$ and $LaNi_5$.
In chapter III, the effects of poison thiourea on hydrogen absorption reaction (HAR) into Pd electrode have been investigated by using ac-impedance spectroscopy supplemented by cyclic voltammetry and hydrogen permeation techniques. The ac-impedance measurements were carried out in the overpotential range of -0.05 to 0.30 V_RHE in 0.1 M NaOH solution containing various concentrations of thiourea. Measured impedance spectra were analyzed by using complex non-linear least squares (CNLS) fitting method on the basis of Faradaic admittance equation for hydrogen absorption under impermeable boundary condition. The shape of impedance spectra was changed from diffusional Warburg impedance to capacitive arc, indicating that the diffusion-controlled HAR mainly takes place on the Pd foil electrode in 0.1 M NaOH solution containing below $10^{-7} M$ thiourea above which the interface-controlled HAR dominantes over the diffusional-controlled HAR. From the thiourea concentration dependence of charge transfer resistance, peak current density in cyclic voltammogram and the steady state permeation current density, the poison thiourea acts as a hydrogen absorption inhibitor and blocks the (-phase Pd hydride formation.
In chapter IV, the performance of $LaNi_4.7Al_0.3$ and $LaNi_5$ porous electrodes used as anodes for nickel-metal hydride secondary batteries has been evaluated by analysis of current decay transients. From the measured three-staged current decay transients for the hydrogen transport through the both electrodes in the coexistence of two hydride phases, the discharge capacity and beta- to alpha-phase transition time were determined. The optimum charging condition and velocity of alpha/beta phase boundary movement were discussed with respect to the experimentally obtained discharge capacity and transition time. The potentiostatic current decay transient technique is more conveniently employed to locate the optimum charging condition, as compared with the usual galvanostatic charge-discharge technique.
From the results of chapter III and IV, the hydrogen absorption reaction into the hydride forming foil and porous electrodes such as Pd, $LaNi_4.7Al_0.3$ and $LaNi_5$ is significantly affected by the properties of the electrode surface and presence of the adsorbant in the electrolyte.
