Environmental Engineering Reference
In-Depth Information
FIGURE 6.7 Structure of LaNi 5 H 7 . The sizes of the atom are declined from La, to Ni, to H. Source :
Reproduced with permission from figure 2.52 in Sorensen [18]. (See color insert.)
chemical formula of A x B y H z , where A is the element in the first or second
group of the periodic table, and element B is either aluminum, nitrogen, or
boron. The corresponding complex hydrides are called alanates, nitrides, and
borohydrides, respectively. These materials are recently under intensive
investigation due to the discoveries of their reversible sorption capability [19,
20]. One specific example is sodium alanate (NaAlH 4 ), the dehydrogenation
of whcih involves two related reactions,
3
NaAlH
Na AlH
+
2
Al
+
3
H at
210
220
°
C
(6.13)
4
3
6
2
Na AlH
3
NaH Al
+ +
1 5
.
H at
250
°
C
.
(6.14)
3
6
2
This process is irreversible and the desorption kinetics is also slow, which
makes the material questionable for storage application. However, the dis-
covery of the reversible sorption of NaAlH 4 by Bogdanovic and Schwickardi
in 1997 by adding TiCl 3 into the structure has changed the view on complex
metal hydrides.[19] Ti-doped NaAlH 4 can desorb hydrogen at 120°C, be
rehydrogenated at 170°C, and has gravimetric density around 7.5 wt% [21].
The material has already been used in practical hydrogen storage units.
6.3.4
Improving Metal Hydride Performance
Although there are many advantages with using metal hydrides for onboard
vehicle applications, two major problems prevent them from practical
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