Thesis - Oztek_Muzaffer_T_200508_MA

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