Frequency-domain analysis of hydrogen permeation across Pd77Ag23 metallic membranes

Hydrogen permeation across palladium-alloys membranes is an industrial process used for purification purposes. In state of the art systems, several tens of microns thick metallic membranes can be used and rate limitations generally come from atomic H diffusion. Cost considerations (for example for application in the automotive industry) require a reduction of the membrane thickness and operation at lower temperature. In the micron-thick range, surface contributions become increasingly rate-determining. To optimize permeation membranes, there is therefore a need to separately measure surface and bulk rate contributions to the overall permeation process. In this paper, pneumato-chemical impedance spectroscopy (PIS) is used to analyze the dynamics of hydrogen permeation across Pd77Ag23 membranes. Experimental pneumato-chemical transfer functions of the membrane are measured at different temperatures. Model impedances are calculated and fitted to the experimental ones, yielding microscopic rate parameter values such as surface resistance and hydrogen diffusion coefficient.