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 .
