The present work is concerned with the electrochemical characteristics of hydrogen absorption into and evolution from hydride forming anodes.
In chapter Ⅲ, the hydrogen absorption reaction (HAR) into and hydrogen evolution reaction (HER) from Pd foil electrode have been investigated in 0.1 M NaOH solution by using electrochemical impedance spectroscopy (EIS), open-circuit potential transient and current transient techniques. Generalised Faradaic admittance for the indirect HAR into and HER from metal foil electrodes under the impermeable boundary conditions has been derived based upon Lim and Pyun's kinetic approach to HAR and HER. The ac-impedance measurements were carried out in the overpotential range of -0.10 to 0.25 $V_{RHE}$. Measured impedance spectra were analysed by using complex non-linear least squares (CNLS) fitting method on the basis of derived Faradaic admittance equation for hydrogen absorption under impermeable boundary conditions. Also, impedance spectra were theoretically calculated with different kinetic parameters for the HAR and HER having physical significance in order to characterise the influence of each kinetic parameter on the change of the impedance spectra in Pd foil electrode. From the occurrence of plateau region of the open-circuit potential transients and hydrogen content below 0.03 determined from the current transients it is suggested that a thin β-phase palladium hydride layer is formed beneath the electrode surface. The indirect to direct HAR transition in mode occurs at the overpotential of 0.08 $V_{RHE}$ below which the direct HAR is predominant. From the hydrogen diffusivity reduced with decreasing overpotential, it is indicated that the thin β-phase palladium hydride layer acts as a barrier for hydrogen diffusion in the electrode. The formation of the thin β-phase palladium hydride layer between the electrode surface and subsurface accounts for the predominant direct HAR mode and hydrogen diffusion impeded by the phase boundary between α-and β-phase palladium hydride below 0.08 $V_{RHE}$.
In chapter Ⅳ, the HER on 10 wt.% palladium - dispersed carbon (Pd/C) electrode in 0.1 M NaOH solution with reference to that on carbon (Vulcan XC-72) and Pd foil electrodes and the HAR into the α- and β-phase $LaNi_5H_x$ porous electrodes in 6 M KOH solution have been investigated by analysing the ac-impedance spectra combined with cyclic voltammograms. From the coincidence of the maximum charge transfer resistance and the minimum hydrogen evolution resistance for the HER at the respective applied cathodic potential for the Pd/C, carbon and Pd foil electrodes, it is suggested the HER from the Pd/C electrode takes place along with the hydrogen absorption/diffusion above -0.20 $V_{RHE}$, whereas the former dominates over the latter below -0.20 $V_{RHE}$. In the case of the Pd foil electrode the transition of the absorption/diffusion to evolution occurs at -0.06 $V_{RHE}$. In contrast to the Pd/C and Pd foil electrodes the HER simply proceeds from the carbon electrode below -0.30 $V_{RHE}$ strongly. Hydrogen evolution overvoltage on the Pd/C electrode is namely reduced by 0.10 V as compared to the carbon electrode due to the electrochemical active area increased by the finely dispersed Pd particles. On the other hand, the onset of the HER can be accounted for in terms of the transition of the Warburg impedance to capacitive loop at -0.048 $V_{RHE}$ for the α-phase porous electrode and at 0.012 $V_{RHE}$ for the β-phase porous electrode. The charge transfer resistance $R_{ct}$ values from the β-$LaNi_5H_x$ electrode are much smaller than those from the α-$LaNi_5H_x$ electrode in the whole applied potential range. The α to β-phase transition at -0.048 $V_{RHE}$ and the β- to α-phase transition at 0.012 $V_{RHE}$ were discussed in terms of applied potential dependence of $R_{ct}$ for the HAR.
In chapter Ⅴ, the hydrogen transport through Pd and $LaNi_5$ electrodes in the coexistence of two hydride phases (α- and β-phases) has been investigated by using current decay transient, cyclic polarisation and open-circuit potential transient techniques. The current decay transients were analysed by using new suggested method for the hydrogen transport through the electrode in the coexistence of two phases on the basis of the McNabb and Foster's physical model of hydrogen trapping. The occurrence of two staged current transient curves and hydrogen contents indicated that the β- PdH and β-$LaNi_5H_x$ layers are formed just beneath the electrode surface at the overpotentials of 0.08 $V_{RHE}$ and -0.02 $V_{RHE}$, respectively, and stable over the relatively narrow regions near the electrode surfaces at -0.10 $V_{RHE}$ for the Pd and -0.07 $V_{RHE}$ for the $LaNi_5$. From the appearances of two staged current transient curves, it was suggested that the hydrogen transport initially proceeds by up-hill diffusion of hydrogen from inner α-hydride regions toward outer β-hydride regions with simultaneous decomposition of the continuous β-phase hydrides formed beneath the electrode surface to the discontinuous β-hydrides. The more amounts of β-hydrides are, the faster are the decomposition rates.