Hydrogen diffusion coefficient and mobility in palladium as a function of equilibrium pressure evaluated by permeation measurement

Hydrogen permeation flux is generally described by the square-root law, but deviation from the law has been widely reported. For more precise description, pressure-dependent permeability has been proposed. Based on this approach, the hydrogen diffusion coefficient in palladium was evaluated as a continuous function of equilibrium hydrogen pressure. Results showed a decreasing diffusion coefficient with pressure, in contrast to increasing permeability and solution coefficient. The pre-exponential factor and activation energy for the diffusion coefficient in the dilute limit, i.e., intrinsic diffusion coefficient, were, respectively, 2.40×10−7 m2/s and 21.1 kJ/mol H. This study did not assume constant hydrogen mobility, differently from most studies, but evaluated the mobility as a function of pressure. This precise analysis revealed slightly increasing mobility with pressure, because of less deep hydrogen potential at higher hydrogen concentrations. Finally, a guideline to develop membrane materials was provided. Materials with large deviation from Sievertsʹ law possibly have high permeability at practical pressures for permeation, i.e., atmospheric pressure or higher, even if the mobility in the dilute limit is low.