Recent developments in aluminum-based hydrides for hydrogen storage
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phase is α-AlH3, which crystallizes in the trigonal space group, R-3c, with a hexagonal unit cell (Figure 2a).7 The structure consists of corner-connected AlH6 octahedra with H bridging bonds and is similar to the other polymorphs (α’, β, γ), which typically consist of corner-connected and, in some cases, edge-sharing AlH6 octahedra. Although the structures of the AlH3 polymorphs appear strikingly similar to the structures of hexahydroaluminates (Figure 2), the AlH3 phases are missing the stabilizing cations (e.g., alkali metals) that balance the structure by donating some of their charge to the AlH6 complexes. The general formula for the tetrahydroaluminates is Mx+Ny+ (AlH4)x+y, where M and N are typically alkali (x, y = 1) or alkaline earth metals (x, y = 2), or a combination thereof. In some cases, transition metal elements will also form alanates
Tetrahydrides LiAlH4 NaAlH4 KAlH4 Mg(AlH4)2
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(e.g., Ti(AlH4)4),9 but these compounds are typically unstable and only form at low temperatures (e.g., –80°C). Examples of a few tetrahydroaluminates with high gravimetric hydrogen capacity include LiAlH4 (10.6 wt%), LiMg(AlH4)3 (9.7 wt%), Mg(AlH4)2 (9.3 wt%), Ca(AlH4)2 (7.9 wt%), and NaAlH4 (7.5 wt%). In these materials, the H atoms are covalently bonded to Al in well-separated, isolated (AlH4)– tetrahedra that are connected via the cations (M and N), as shown in the representative structures of Ca(AlH4)2 and LiMg(AlH4)3 of Figure 3.7,10 The tetrahedra in NaAlH4 have perfect symmetry, but in most other cases (Li-, K-, Mg- and Ca-alanates), the tetrahedra are slightly disordered, with average Al–H (or Al–D) distances in the range of 1.605–1.644 Å. The coordination number of the cations increases with the corresponding ionic radius (typically 5–10 for the known tetrahydroaluminates) along with a similar increase in the distance to the nearest H atom. There exist several other alanates composed of alkali and alkaline earth elements and AlH6 octahedra, which have the general formula Max+Nby+AlH3+ax+by. These include (1) the mixed alkali hexahydroaluminates, such as Na2LiAlH6 (Figure 2b), K2NaAlH6, and K2LiAlH6; (2) the alkaline earth hydrides, such as CaAlH5, BaAlH5, Ba2AlH7, and Sr2AlH7; and (3) the mixed alkali-alkaline earth hydrides, such as LiMgAlH6. It is interesting to note that many of the hexahydroaluminates form as intermediates during hydrogen desorption from the tetrahydroaluminates, as shown: Ca(AlH 4 ) 2 → CaAlH5 + Al + 3/2 H 2 → CaH 2 + 2Al + 3H 2 (1) LiMg(AlH 4 )2 → LiMgAlH 6 + Al + H 2 → MgH 2 + 2Al + 3H 2 . (2)
In the hexahydroaluminates, the hydrogen is covalently bonded to Al in (AlH6)3– octahedra that are linked in a number of different ways. In Li3AlH6, the structure forms a three-dimensional network that can be described as a distorted bcc structure of (AlH6)3– units with all tetrahedral sites filled with Li. In the mixed alkali alanates, such as Na2LiAlH6, the structure consists of corner-sharing (AlH6)3– and (LiH6)3– octahedra, where each Table I. Experimentally determined structures of various ala
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