Strain gradient-induced diffusion of hydrogen in palladium and nickel membranes

The “uphill” diffusion of hydrogen during permeation through flat sheets of palladium and nickel has been studied by an electrochemical permeation method at 303 K. For both annealed and “as cold rolled” Pd samples, uphill diffusion effects on hydrogen absorption and desorption have been observed over a range of initial hydrogen contents from about H/Pd = 0.01, i.e. near or slightly less than the _max composition, up to H/Pd = 0.25–0.3. The occurrence of a non-Fickian component of permeation flux has been associated with temporary formation of lattice volume differences across the ( + β)/β and ( + β)/ interfaces during absorptions and desorptions, respectively. Influences of the magnitudes of galvanostatic hydrogen fluxes and of the membrane thickness on the uphill effects were examined. Analogous uphill effects were observed in similar studies with nickel membranes also in both annealed and “as cold rolled” states, which were much larger than those observed for palladium.