(An) investigation to the electorchemical characteristics of hydrogen absorption into and evolution from hydride forming anodes

Abstract

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...