Compensation Effect in H2 Permeation Kinetics of PdAg Membranes

Abstract

Knowledge about the (inter)dependence of permeation kinetic parameters on the stoichiometry of H2-selective alloys is still rudimentary, although uncovering the underlying systematic correlations will greatly facilitate current efforts into the design of novel high-performance H2 separation membranes. Permeation measurements with carefully engineered, 2–7 μm thick supported Pd100–xAgx membranes reveal that the activation energy and pre-exponential factor of H2 permeation laws vary systematically with alloy composition, and both kinetic parameters are strongly correlated for x ≤ 50. We show that this permeation kinetic compensation effect corresponds well with similar correlations in the hydrogen solution thermodynamics and diffusion kinetics of PdAg alloys that govern H2 permeation rates. This effect enables the consistent description of permeation characteristics over wide temperature and alloy stoichiometry ranges, whereas hydrogen solution thermodynamics may play a role, too, as a yet unrecognized source of kinetic compensation in, for example, H2-involving reactions over metal catalysts or hydrogenation/dehydrogenation of hydrogen storage materials.