Diffusion coefficient of hydrogen in α-phase palladium and palladium–platinum alloy

Abstract

Diffusion of hydrogen in the α-phase Pd– and Pd₈₁Pt₁₉–H systems was investigated at 298 K by electrochemical methods in an aqueous solution of H₂SO₄. Smooth or Pd-black covered specimens were used. The concentration of hydrogen in the metal matrix was gradually increased by control of the electrode potential; equilibrium concentrations were attained. The diffusion coefficient of hydrogen was evaluated by impedance spectroscopy close to equilibrium inside the specimen, at various hydrogen concentrations. The hydrogen evolution parallel to its absorption and diffusion was taken into account in the analysis of the experimental data. At the largest concentrations, the larger the concentration the smaller is the diffusion coefficient in both systems. This is especially evident for the Pd₈₁Pt₁₉ matrix. The diffusion coefficient evaluated from the rate of hydrogen stripping from specimens saturated to various concentrations of hydrogen was of lower reliability. The Pd-black coverage of the specimens decreased the reliability of estimates of the hydrogen diffusion coefficient and of its concentration. The temporary formation of β-PdH_n directly under the specimen surface during Pd-black deposition, resulting in the creation of hydrogen traps, is postulated.