Environmental Engineering Reference
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FIGURE 6.6 Desorption curves of MgH 2 at 573 K in vacuum with Nb 2 O 5 catalyst and milled for 2, 5,
10, 20, 50, and 100 hours. Source : Reproduced with permission from Barkhordarian et al. [15].
or an alkaline metal and can form a stable hydride, while element B is a
transition metal and forms an unstable hydride. The resultant alloy or inter-
metallic compound tends to form a hydride of intermediate stability. Some
well-known intermetallic compounds are with x   =  0.5, 1, 2, and 5. Most
intermetallics have low gravimetric density, and some require high sorption
temperature. Therefore, they are not yet suitable for onboard applications. A
well-studied AB 5 intermetallic is LaNi 5 , which can form a hydride (LaNi 5 H 7 )
under moderate hydrogen pressures and ambient temperatures (see Fig. 6.7)
[16]. Its enthalpy of formation is −15.7 kJ·(mol·H) −1 and enthalpy of decom-
position is −15.1  kJ·(mol·H) −1 [8]. Its reversible gravimetric density is
1.25 wt% [17]. For all the AB 5 intermetallics, the gravimetric storage capac-
ity is substantially lower than the current U.S. DOE target for mobile hydro-
gen storage applications. However, the AB 5 intermetallics have some
remarkable cycling performance and high volumetric storage density, and
are therefore a prime example of practically effective reversible hydrogen
storage materials.
6.3.3 Complex Metal Hydrides
Complex hydrides have the highest hydrogen storage density among all the
metal hydrides. They are salt-like materials in which hydrogen is either ioni-
cally or covalently bound to the storage material. Complex hydrides have the
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