Dynamics of hydrogen permeation across metallic membranes

Thin and supported palladium membranes can be coupled to gas reformers to produce purified hydrogen. Such systems can potentially be used in the automotive industry to feed PEM fuel cells. However, for such applications, the membrane design must be optimized to meet some specific requirements, in particular to allow fast accelerations. The purpose of this paper is to take advantage of the possibility offered by pneumato-chemical impedance spectroscopy to analyze the dynamics of hydrogen permeation in transient conditions of flow, to determine the conditions for which shifts in rate-determining step (rds) between surface and bulk rate contributions are observed. Results reported in this paper have been obtained using a Pd77Ag23 metallic membrane. The gas-phase impedance of this 50 μm thick membrane has been measured. A model has been developed to evaluate separately surface and bulk rate contributions. It is shown that in a typical permeation experiment performed in transient conditions of flow, the surface step is rate-determining in the early stages of the experiment (highly transient conditions of flow) whereas the bulk diffusion step becomes rate-determining at longer time (quasi-stationary conditions of flow). The relationship between membrane characteristics, experimental conditions and the time at which the shift in rds is observed are determined, opening the way to the development of customized membranes for operation in transient conditions of flow.