Hydrogen permeation across metallic membranes is an industrial process used for purification purposes. Palladium-based alloys are generally used as permeation materials, at operating temperatures above the critical value so that the metal-H system remains single phased and diffusion proceeds at a sufficient rate. In state of the art systems, metallic membranes with typical thickness of a few tens of microns are used and rate limitation are generally attributed to atomic H diffusion. Because of cost considerations, it is necessary to reduce the thickness of these membranes. In the micron-thick range, surface contributions become rate determining. Further, when the membrane is used on the exit side of a gas reformer to directly extract hydrogen, corrosion problems are expected to appear on the upstream side of the membrane. There is therefore a need to separately probe surface and bulk rate contributions.
In this communication, pneumato-chemical impedance spectroscopy is used to analyze the dynamics of hydrogen permeation across metallic membranes such as Pd77Ag23. Pneumato-chemical transfer functions of the membrane are measured and modelled, showing that the different steps of the permeation mechanism can be individually measured, i.e. surface resistances related to hydrogen dissociation (upstream side) and recombination (downstream side) processes, and bulk diffusion impedance.