Hydrogen diffusion and solubility in palladium thin films

Palladium thin films (220, 460 and 1350 Å thick) were fabricated by physical vapor deposition and characterized electrochemically. The reversible hydrogen storage capacity of the films was determined by electrochemical cycling. The hydrogen solubility of the thin films was larger in the α-phase but smaller in the β-phase than that of the bulk. The hydrogen diffusion coefficient, D_H, in the Pd films was measured by the electrochemical stripping method in the Pd-H solid solution (α-phase) at the temperatures ranging from 280 to 330 K. A well-defined potential range was found for applying the electrochemical stripping technique. The D_H decreases with decreasing film thickness and is 2-3 orders of magnitude smaller than that in the bulk. The D_H vs 1/T plots show that all the thin films follow the Arrhenius behavior, D_H = D₀ exp(-E_a/RT), with approximately the same activation energy E_a and a decreasing prefactor D₀ as the film thickness decreases.