Determination of the rate-limiting mechanism for permeation of hydrogen through microfabricated palladium-silver alloy membranes

For many years enhancement of the hydrogen permeation rate through I'd and Pd-alloy membranes has been accomplished by decreasing the membrane thickness. This approach is based on the idea that the permeation rate is limited by the diffusion process through the membrane bulk material. Theoretical modeling suggests that as membrane thickness is 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 mu m have been microfabricated and their hydrogen permeation behavior has been experimentally determined. At temperatures between 523 and 723 K, hydrogen permeation is limited by the diffusion process and is characterized by a membrane permeability pre-exponential factor of 5.51E-8 +/- 1.34E-8 mol/m/s/Pa-0.5 and an activation energy of 10.8 +/- 1.25 kJ/mol. At temperatures below 500 K, the activation energy increases appreciably. This transition is 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 is not observed under the operating conditions in this study. Additionally, two distinct failure modes have been observed for these microfabricated devices. The operating conditions leading to failure and cause of each failure mode are discussed. (C) 2009 Elsevier B.V. All rights reserved.