Theoretical Study of AAlH4 (A=Li, Na, K, Rb, Cs) Alanates as a Hydrogen Storage Material

The high percentage of hydrogen in the inherent alanates structures, give them the potential for being good hydrogen storage materials. In this study the geometric and electronic properties of alkali alanates are investigated by means of density functional theory. The least and most Al-H bonding lengths are 1.62 and 1.71Å for LiAlH4 and RbAlH4, respectively. Increment of cation size, from Li to Cs, would lead to the increment of A-H bond lengths and coordination number of alkali ions. The density of states plots clearly shows insulator characteristic of all structures with the band gaps reach to 5.1eV. The Lithium alanate band gap is the narrowest one among all, which is a good reason for instability of this compound at room temperature. All Partial-DOS plots show Al-p and H-s states in the same range of energies to make sp orbital. Moreover, the conduction bands distributions are mainly dominated by elemental states of the final composites, so these bands are generally filled by alkali and Al orbitals. Besides, comparison between elemental and compositional DOS of alkali parts, reveals the exchange of one electron from alkali toward the AlH4 blocks. The covalent bonds are expected between Aluminum and Hydrogen atoms with non-covalent bonds for alkali positively charged elements

Keywords:

hydrogen absorption , alanate , density functional theory , Aluminum complex hydrides