(A) study on the electrochemical characteristics of hydrogen(deuterium) absorption on palladium and the mechanism of palladium-electrodeposition

Abstract

The behaviour of hydrogen adsorption on palladium has been investigated in chapter III by ac impedance measurements in 0.1 M NaOH solution of pH = 13 at 298 K. The electrode impedance has been measured by superimposing an ac voltage of 5mV amplitude ranging between $10^{-1}$ and $10^4$ Hz on a dc potential range -0.76 to 0.44 V(standard hydrogen electrode(she)). The equivalent circuit representing the hydrogen adsorption and diffusion into palladium deposit is proposed from the analysis of the frequency dependence of electrode admittance. The capacitance, arising from the adsorbed hydrogen, has been determined as a function of applied potential by complex non-linear least squares (CNLS) curve fitting method based on the proposed equivalent circuit. Detailed analysis of the Warburg impedance could not be given due partly to the lack of data in low frequencies and partly to the complicated boundary condition for hydrogen bulk kiffusion, however, the diffusion process associated with the adsorbed hydrogen was confirmed. In chapter IV, Faradaic admittance of hydrogen absorption reaction (h.a.r.) on metal membrane electrode has been derived on the basis of "two models" of the h.a.r. under the permeable boundary condition by using an extension of Armstrong``s kinetic approach. In the case of the h.a.r through adsorbed phase (model A), two limiting cases of the Faradaic admittance are considered depending upon the magnitude of rate of hydrogen transfer from bulk to metal surface. One involves the diffusion-controlled h.a.r.for fast rate of hydrogen transfer and the other is the interface-controlled h.a.r. for slow rate of hydrogen transfer from bulk to metal surface. Depressing and/or tailing of the second semicircle, often found in the complexplane impedance spectra for hydrogen evolution reaction (h.a.r), is fairly explaned by the diffusion-controlled h.a.r. In the case of the direct h.a.r. without passing through the adsorbed phase (model B), the Faradaic admittanc...