The activation of LaNi5-H

Hydrogen absorption by LaNi5 was first studied by Neumann in the late 1960’s , and its potential as a hydrogen storage material was subsequently recognized by van Vucht et al. . Justi et al. later suggested using the compound as a battery electrode material, although there are a number of practical drawbacks to the use of pure LaNi5 for this application, including the poor long term cycling stability of the pure alloy and the relatively high absorption/desorption plateau pressure. These problems were later largely solved by the partial substitution of the La and the Ni, and the resultant materials are now widely used in commercial Ni-MH batteries. There is therefore a large body of research on LaNi5 and its derivatives, and the material is still of interest as a model hydrogen storage alloy. Recent developments include the partial substitution of the Ni by Sn to produce a material with excellent long term gas phase cycling stability . LaNi5 and hydrogen storage LaNi5 reacts readily with hydrogen gas at ambient temperature and above. The compound shows hydrogen absorption and desorption behaviour that is typical of many metal-hydrogen systems. At lower hydrogen pressures a relatively small amount of hydrogen is absorbed following a relatively large increase in hydrogen pressure. This is the α phase region, in which atomic hydrogen diffuses randomly between interstitial sites in the metal lattice. At a slightly higher pressure, at the onset of the so-called plateau region, an ordered hydride phase, known as the β phase, forms. Over a relatively small pressure range, the hydrogen content increases dramatically as the β phase grows through the α phase matrix. Once the host lattice has been converted to the β phase, at the end of the plateau region, a relatively small amount of further hydrogen uptake occurs over a relatively large pressure range. It is this plateau region that is exploited in the compound’s use as a hydrogen storage material and the practical reversible hydrogen storage capacity is defined approximately by the width of the plateau .