Analysis for Reverse Temperature Dependence of Hydrogen Permeability through Pd-X (X = Y, Ho, Ni) Alloy Membranes Based on Hydrogen Chemical Potential

It is generally understood that the hydrogen permeability of Pd-Ag alloy membranes declines with decreasing temperature. However, recent studies have revealed that the hydrogen permeability of Pd-Ag alloy membranes inversely increases at a certain temperature range and reaches a peak. The peak behavior reflects the shape of pressure-composition isotherms (PCT curves). In order to elucidate the relationship between the reverse temperature dependence of hydrogen permeability and the PCT curves, the hydrogen permeability of pure Pd and Pd-X (X = Ho, Y, and Ni) alloy membranes were investigated. The pure Pd and Pd-5 mol%Ni alloy membranes, in which the alpha-alpha' phase transition occurs, exhibits more significant peak behaviors than Pd-5 mol%Ho, Pd-5 mol%Y, and Pd-23 mol%Ag alloy membranes, in which the alpha-alpha' phase transition is suppressed. Large differences in hydrogen solubility, at the hydrogen pressures above and below the plateau region or the inflection point, make the peak behaviors more significant. It is revealed that the peak temperature can be roughly predicted by the hydrogen pressure at the plateau regions or the inflection points in the PCT curves.