Home >> Research Overview >> Research by Faculty Member >> Will Medlin
Medlin Research Group
Sulfur tolerance of palladium alloy membranes
 A major limitation of modern fuel cell technology is the low tolerance of fuel cell catalysts to even ppm levels of carbon monoxide and sulfur-containing gases. For this reason, highly selective separation of the hydrogen fuel from reformate gas streams is necessary to obtain a useable feed gas; this requirement can be especially demanding for high-sulfur fuels. Perhaps the greatest challenge is that any hydrogen purification technology must itself be tolerant of reactive gases. A particularly promising method for hydrogen purification is the use of palladium and palladium-alloy thin film membranes. Although these membranes are highly efficient for exclusion of contaminant gases, sulfur compounds can block surface sites necessary for hydrogen adsorption, sometimes drastically lowering the overall permeation rate.
 Exposure to sulfides has also been found to result in dramatic changes in the structural properties of the membrane film. These structural changes often compromise the stability of the film, leading to total membrane failure. Intriguingly, recent studies show that membrane stability is strongly dependent on the metal composition of the film. To prepare hydrogen purification systems for lower-quality fuel streams, a better understanding is needed of the relationship between metal composition and membrane performance/durability in the presence of sulfide poisons.
In research funded by the Office of Naval Research, both experimental and theoretical methods are being used to study how film composition affects the tolerance of membranes to the model sulfide H2S. For the experimental component, membrane films are prepared using electron-beam evaporation, tested in a membrane flow cell for hydrogen permeation under various H2S concentrations, and characterized both before and after operation using a suite of spectroscopic and microscopic techniques.
 Complementary first-principles computations are being used to construct molecular models that explain the effects of metal alloy composition on sulfide poisoning of metal surfaces. The main advantages of the research approach are the following: (1) e-beam evaporation permits exceptional control over composition; (2) characterization of membrane films both before and after extended operation will identify reasons for observed membrane performance changes; and (3) theoretical calculations of sulfide poisoning interactions will provide molecular-level justifications for observed membrane performance. Ultimately, this work will help to identify optimal membrane compositions for maximum robustness in complex gas feeds.
Research Personnel
Esther Wilcox, Melissa Farbod, Freya Kugler, Era Krauss
Other areas of research in the Medlin Research Group:
|
