For many years enhancement of the hydrogen permeation rate through Pd and Pd-alloy membranes was accomplished by decreasing the membrane thickness. This approach was based on the idea that the permeation rate was limited by the diffusion process through the membrane bulk material. Theoretical modelling suggests that as membrane thickness was reduced into the micrometer range the rate-limiting permeation mechanism may shift to desorption from the permeate surface. In order to test the model predictions, free-standing Pd–Ag alloy membranes (23 wt% Ag) with a thickness on the order of 1 μm were microfabricated and their hydrogen permeation behavior was experimentally determined. At temperatures between 523 and 723K, hydrogen permeation was limited by the diffusion process and was characterized by a membrane permeability pre-exponential factor of 5.51 x 10-8mol/m/s/Pa0.5 and an activation energy of 10.8kJ/mol. At temperatures below 500K, the activation energy increases appreciably. This transition was consistent with diffusion-limited permeation in the presence of non-ideal absorption behavior of hydrogen in the Pd–Ag alloy. A transition into a desorption-limited permeation regime was not observed under the operating conditions in this study. Additionally, two distinct failure modes were observed for these microfabricated devices. The operating conditions leading to failure and cause of each failure mode were discussed.

Determination of the Rate-Limiting Mechanism for Permeation of Hydrogen through Microfabricated Palladium–Silver Alloy Membranes. L.S.McLeod, F.L.Degertekin, A.G.Fedorov: Journal of Membrane Science, 2009, 341[1-2], 225-32