Diffusion and equilibrium properties of hydrogen in palladium measured by the stripping potentiostatic method

Theory and experiments on the (non-equilibrium) stripping potentiostatic method for evaluation of the diffusion coefficient and desorption isotherms of hydrogen in α-Pd are discussed. Diffusion coefficient of hydrogen D can be evaluated either under transient conditions, from the anodic stripping current (I_a) or under steady-state condition, from the anodic charge corresponding to the stripping of hydrogen held initially in Pd membrane (Q_a). Following data are reported: D = (3.60±0.06) × 10⁻⁷ cm² s⁻¹ (transient) and D = 3.41 × 10⁻⁷ cm² s⁻ (steady-state) at 20°C in 0.1 N H₂SO₄. Equilibrium concentration and equivalent pressure of hydrogen at the entrance side of Pd membrane can be calculated from the steady-state permeation current (I^∞_a or the anodic stripping charge (Q_a) and its open-circuit potential just after stopping hydrogen generation, E⁰_c. Reliable Sieverts' constants (K_s) have been calculated within 15 and 60°C, and the following thermodynamic values obtained: ΔH^o = −(2.6±0.1) kcal (mol H)⁻¹ and ΔS^o = −(14.5±0.4) cal K⁻¹ (mol H)⁻¹. These data agree well with those obtained in gas phase measurements.