ELECTROCHEMICAL STUDY OF HYDROGEN ABSORPTION IN POLYCRYSTALLINE PALLADIUM

The hydrogen reactions on polycrystalline Pd in 0.1 M NaOH at 25-degrees-C have been studied by using transients at constant potential, and by impedance spectroscopy and X-ray diffraction techniques. At potentials, E(s), more positive than the reversible potential, E(r), for the H-2 evolution reaction, the current-time response and the impedance data indicate H atom diffusion into the bulk Pd. The X-ray diffraction pattern of electrodes cathodized during 20 min at these potentials are similar to those obtained for Pd. At E(s) < E(r), the current transients exhibit a current maximum which increases as E(s) moves in the negative direction. The Nyquist plot for the rising part of the transients indicates the H atom diffusion into the bulk metal and H-2 evolution on the Pd surface at high frequencies. The X-ray diffraction pattern of the electrodes cathodized at E(s) < E(r) shows the presence of the beta-PdH phase and Pd. The experimental results indicate that different reactions take place simultaneously in this potential range: (i) H-2 evolution, (ii) H diffusion into the bulk Pd, (iii) nucleation and diffusion-controlled growth of the beta-PdH phase. Taking into account the contribution of these reactions, a model, which is able to reproduce the experimental current transients, is presented.