SUMMARY

Energy efficient purification of hydrogen is an important technological challenge. Pd/Agalloy membranes are particularly suited to this problem due to their high hydrogen permeation rate, thermal stability, and virtually infinite selectivity over all other gases. Over the past century these materials have been extensively studied and today they are commercially utilized for applications which demand high-purity hydrogen. In current systems, hydrogen flux is observed to be inversely proportional to membrane thickness which is indicative of the interstitial diffusion mechanism of hydrogen permeation. This observation, along with the high cost of palladium, has motivated continuous efforts to decrease membrane thickness. Theoretical modeling of membrane performance has predicted that as membrane thickness continues to decrease, eventually the permeation rate will no longer be limited by hydrogen diffusion through the bulk Pd but will become limited by desorption from the permeate surface. This is an vital transition to pinpoint due to the fact that below this thickness membrane performance will have a dramatically different dependence on temperature and hydrogen partial pressure, and no further performance enhancements will result from further decreasing membrane thickness. Additionally, it has been demonstrated that hydrogen permeation through bulk Pd depends on membrane microstructure, making deposition conditions and post-deposition thermal treatment important issues for repeatable performance. The interplay of these issues on the performance limitations of these ultra-thin hydrogen separating membranes will be experimentally investigated. The parametric dependence of a permeation regime transition will be determined and the effects of microstructure on this transition will be isolated. This transition will likely affect membrane transient response; therefore the transient response in the vicinity of this transition will also be investigated.