Anomalous Temperature Dependence of Hydrogen Permeability through Palladium-Silver Binary Alloy Membrane and Its Analysis Based on Hydrogen Chemical Potential

The hydrogen permeability of Pd-Ag alloy membranes has been investigated over a wide temperature range between 100 degrees C and 500 degrees C. The hydrogen permeation coefficient, Phi, for Pd-23mol%Ag decreases with decreasing temperature above 300 degrees C, in good agreement with the previous literature. However, Phi starts to increases below 250 degrees C, and a peak is observed at around 180 degrees C. Considering the silver concentration and operating temperature, the alpha-alpha' phase transition never occurs in this condition. In other words, the alpha-alpha' phase transition is not the reason for the anomalous peak behavior of Pd-23mol%Ag alloy at low temperature. In addition, it is confirmed that the diffusion limiting hydrogen permeation reaction takes place from room temperature up to 500 degrees C. To understand the reason for the peak appearance, the hydrogen permeability has been analyzed in view of the new description of hydrogen permeation based on hydrogen chemical potential. As a result, it is found that the temperature dependence of the PCT factor,f(PCT), is dominant for the peak appearance, meaning that the corresponding pressure composition isotherms (PCT curves) are essential for the understanding of hydrogen permeability of the alloy. Dependences of the pressure condition and silver concentration on the peak behavior have also been investigated. The peak temperature increases with increasing the hydrogen pressure at feed side. In addition, the peak appears at lower temperature and becomes remarkable with decreasing silver concentration of Pd-Ag alloy membrane. In other words, the composition of Pd-Ag alloy membranes must be designed based on the operating temperature or pressure condition. Thus, this study suggests new possibilities of alloy design for Pd-Ag alloy membranes.