Enhancement of the hydrogen storage capacity of Mg(AlH₄)₂ by excess electrons: a DFT study

The enhancement of hydrogen storage in complexes of magnesium alanate ([Mg(AlH₄)₂]_n⁻ where n = 2, 3) and their anions was investigated using the B3LYP/6-311++G** method. Hydrogen atoms were chemically absorbed on [Mg(AlH₄)₂]_n⁻ (n = 2, 3) with binding energies of −83 to –91.23 kcal mol⁻¹. It was determined that a maximum of two hydrogen atoms can be absorbed on [Mg(AlH₄)₂]_n⁻ (n = 2, 3) complexes at 10.51 and 10.21 wt%, respectively. Natural bond order analysis revealed that, in the absence of hydrogen, the excess electron population lies predominantly along the sigma bonds between Mg²⁺ atoms, while, upon absorption of the hydrogen atoms, the excess electron population lies on the absorbed hydrogen atoms. The vertical detachment energy showed that the excess electron is strongly bound to [Mg(AlH₄)₂]_n⁻ (n = 2, 3) complexes with energies of 78 to 153 kcal mol⁻¹. It can be seen from the stabilization energy values that the stability of hydrogen-absorbed complexes with excess electrons was significantly enhanced.