Excess electrons in LiAlH₄ clusters: Implication for hydrogen storage

The structures, frontier molecular orbitals, electron populations, stabilization energies, vertical detachment energies, and hydrogen interactions with nearby atoms of the anionic LiAlH₄ (LAH) clusters and their hydrogenated species are investigated with density functional theory. For the hydrogenated anionic LAH dimers and trimers, there are two states, the dipole-bound state and the surface state. The importance of excess electrons (EE) for hydrogen storage is addressed because of the calculated results. The hydrogen storage capacity of the LAH clusters (diads and triads) is improved with the existence of EE as evidenced by the enhanced weight percent. The head-to-head trimers shrink with the addition of hydrogen atoms because of the strong coupling between the central hydrides and Li cations. Furthermore, the anionic LAH cluster size and the coupling mode between Li⁺ and the ATH₄^- fragment are important factors influencing the hydrogen storage capacity.