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Thesis - Oztek_Muzaffer_T_200508_MA

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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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