Experimental requirements for measuring pneumatochemical impedances

Hydrogen storage remains a bottleneck process on the way to the hydrogen economy. For practical applications, metal hydride systems offer interesting features, in particular, the possibility of reversibly storing large amounts of hydrogen at low or moderate pressure. However, they still suffer from unfavorable specific energy, with mass-fraction values ranging from 0 up to 5 wt % whereas transport applications require 6 wt % and more. Besides this, higher sorption/desorption kinetics and better chemical stability over long-term cycling are also needed. This is why many studies are carried out in the research community on hydride-forming systems, to develop new materials meeting these requirements. Development and optimization of metal hydride reactors require coupled thermodynamic and kinetic characterization of metal-hydrogen systems. In particular, it is necessary to analyze the kinetics in terms of reaction mechanism, in order to identify the different steps of commonly observed multistep reaction paths, and to measure their individual rate parameters. By analyzing hydriding kinetics in the frequency (Fourier) domain, pneumatochemical impedance spectroscopy (PIS) now offers the possibility of measuring experimental impedances and identifying reaction steps. However, measurement of such impedances is indirect and nontrivial. The purpose of this paper is to detail the experimental requirements needed for correctly measuring gas-phase impedance diagrams. In particular, practical conditions of data sampling and data treatment are described. Experimental results obtained with the model LaNi5-H-2(g) system are presented to illustrate the potentialities of PIS analysis. (C) 2007 American Institute of Physics.