ABSTRACT
Introduction Metal-hydride systems are increasingly studied for hydrogen-storage applications. In particular, many intermetallic compounds (IMCs) such as the Haucke (AB5) phases oer several interesting features, in terms of specic energy, volumetric energy, and kinetics. Practical applications require the determination of both thermodynamic and kinetic properties which are, in general, measured separately. Pneumatochemical impedance spectroscopy (PIS) [1] now oers the possibility of analyzing hydriding/dehydriding reaction mechanisms, raw kinetic data being measured using a classical Sieverts experimental setup [2]. Using PIS, kinetic information is not only a measure of a global reaction rate (limited by any ratedetermining step): the dierent steps involved in the sorption reaction are put into evidence and the values of individual rate parameters are inferred. Results obtained so far on various IMCs indicate that most hydriding mechanisms involve two microscopic reaction steps: (i) the surface chemisorption of molecular hydrogen which is dissociated into two surface ad-atoms; (ii) the diusion-controlled transport of hydrogen atoms to bulk regions where they can either accumulate (in solid-solution domains) or precipitate to form metal hydrides (in two-phase domains). PIS is therefore an in situ spectroscopic tool which can be used to analyze surface and bulk processes in solid-gas systems.
