Hydrogen absorption and diffusion into and in palladium: ac-impedance analysis under impermeable boundary conditions

The hydrogen absorption reaction (har) and diffusion into and in palladium electrode has been investigated in 0.1 M NaOH solution under impermeable boundary conditions by using ac-impedance, open-circuit potential transient and current transient techniques. The ac-impedance measurements were carried out in the overpotential range of -0.10 to 0.25 V-rhe. Measured impedance spectra were analyzed by using complex non-linear least squares (CNLS) fitting method on the basis of Faradaic admittance equations for hydrogen absorption under impermeable boundary conditions. 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 beta-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 over-potential, it is indicated that the thin beta-phase palladium hydride layer acts as a barrier for hydrogen diffusion in the electrode. The formation of the thin beta-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 alpha- and beta-phase palladium hydride below 0.08 V-rhe.