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showed the lowest plateau pressure and the highest cell volume among LaCo 5 , LaNi 5 ,<br />
LaCo 2 Ni 3 and several other compounds. As the extent of substitution increases, the cell<br />
volume expansion was greater [39]. Al was found to increase the cycle lifetime without<br />
significantly decreasing in hydrogen capacity. For LaNi 4.5 Al 0.5 and LaNi 4 Al, the heat of<br />
formation of the hydride (∆H) was found to be more exothermic by -38 and -46 kJ per<br />
mole, respectively [40]. In another study, increasing Al content was reported to have an<br />
adverse effect on the hydrogen capacity of LaNi 5-x Al x (x=0 to 0.5) prepared by arc<br />
melting, followed by crushing [41].<br />
One of the problems associated with the use of LaNi 5 for hydrogen storage is<br />
disproportionation upon cycling. The desired reversible reaction<br />
LaNi 5 + 3 H 2 LaNi 5 H 6<br />
is actually thermodynamically unfavorable relative to the less reversible<br />
disproportionation reaction<br />
LaNi 5 + H 2 → LaH 2 + 5Ni<br />
The disproportionation reaction requires the diffusion of metal atoms, which is a<br />
slow process at low temperature, and thus, the hydrogen desorption reaction<br />
predominates near room temperature [18, 42]. Despite the detectable disproportionation<br />
with extended cycling, good engineering lives were achieved for LaNi 4.7 Al 0.3 , which<br />
showed less than 5% H capacity loss after 1500 cycles at 85 ºC. The hydrogen capacity<br />
loss was 25% for LaNi 5 . It was found that the structural disintegration is reversible by<br />
heating the sample to 300 ºC for about 4 hours under vacuum to remove all bound<br />
hydrogen and reform LaNi 5 . The hydrogen capacity was completely restored to its<br />
original value by this process [43].<br />
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