Influence of titanium and nickel dopants on the dehydrogenation properties of Mg(AlH4)2: Electronic structure mechanisms

The structures and dehydrogenation properties of pure and Ti/Ni-doped Mg(AlH4)2 were investigated using the first-principles calculations. The dopants mainly affect the geometric and electronic structures of their vicinal AlH4 units. Ti and Ni dopants improve the dehydrogenation of Mg(AlH4)2 in different mechanisms. In the Ti-doped case, Ti prefers to occupy the 13-hedral interstice (TiiA) and substitute for the Al atom (TiAl), to form a high-coordination structure TiHn (n = 6, 7). The Ti 3d electrons hybridize markedly with the H 1s electrons in TiAl and with the Al 3p electrons in TiiA, which weakens the Al–H bond of adjacent AlH4 units and facilitates the hydrogen dissociation. A TiAl3H13 intermediate in TiiA is inferred as the precursor of Mg(AlH4)2 dehydrogenation. In contrast, Ni tends to occupy the octahedral interstice to form the NiH4 tetrahedron. The tight bind of the Ni with its surrounding H atoms inhibits their dissociation though the nearby Al–H bond also becomes weak. Therefore, Ti is the better dopant candidate than Ni for improving the dehydrogenation properties of Mg(AlH4)2 because of its abundant activated hydrogen atoms and low hydrogen removal energy.