Hydrogen evolution and entry in palladium at high current density

The kinetics of hydrogen evolution and entry into palladium membranes has been studied at high current densities using the electrochemical permeation technique. A two stage permeation transient was observed corresponding to diffusion in the α phase and mixed α + β phase, respectively. At low current densities the hydrogen evolution reaction exhibited a rate determining step of proton discharge followed by absorption of the adsorbed intermediate into the palladium lattice. At high current densities, after conversion of the surface layer to the β phase, a large increase in the recombination flux was observed with a concomitant decrease in the hydrogen entry flux. Since the composition of the entry surface was time dependent at constant charging current, the hydrogen evolution kinetics at the entry surface also exhibited a time dependence.