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

Abstract

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 cm 2 s −1 (transient) and D = 3.41 × 10 −7 cm 2 s − (steady-state) at 20°C in 0.1 N H 2SO 4. 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, E 0 c . Reliable Sieverts' constants ( K s ) have been calculated within 15 and 60°C, and the following thermodynamic values obtained: Δ H o = −(2.6±0.1) kcal (mol H) −1 and Δ S o = −(14.5±0.4) cal K −1 (mol H) −1. These data agree well with those obtained in gas phase measurements.