Complex Hydrides Lisub2/subMHsub5/sub (M = B, Al) for Hydrogen Storage Application: Theoretical Study of Structure, Vibrational Spectra and Thermodynamic Properties

摘要

Gaseous lithium complex hydrides Lisub2/subMHsub5/sub (M = B, Al) have been studied using DFT/B3P86 and MP2 methods with 6-311++G(d,p) basis set. High content of hydrogen by these materials accord them with good candidacy as a class of hydrogen storage materials. The optimized geometrical parameters, vibrational spectra and thermodynamic properties of the hydrides and the subunits LiH, Lisub2/subHsup+/sup, Lisub2/subHsub2/sub, MHsub3/sub, MHsub4/subsup−/sup, and LiMHsub4/sub have been determined. For the LiBHsub4/sub the equilibrium configuration was tridentate of iC/isub3v/sub symmetry. For LiAlHsub4/sub two isomeric forms, bidentate (iC/isub2v/sub) and tridentate (iC/isub3v/sub), were confirmed to exist, and iC/isub2v/sub isomer was shown to dominate in saturated vapor. For complex hydrides Lisub2/subMHsub5/sub, different structural forms were considered but only one asymmetric form (iC/isub1/sub) appeared to be equilibrium. Several possible channels of dissociation of Lisub2/subMHsub5/sub were considered; the enthalpies and Gibbs free energies of the reactions were computed. The enthalpies of formation ?subf/subiH/i?(0) of the complex hydrides in gaseous phase were determined: ?60 ± 10 kJ?molsup?1/sup (Lisub2/subBHsub5/sub) and 33 ± 10 kJ?molsup?1/sup (Lisub2/subAlHsub5/sub). Heterophase decomposition of the gaseous Lisub2/subMHsub5/sub with solid products LiH and B/Al and hydrogen gas release was shown to be spontaneous at ambient temperature. Production of hydrogen gas via gaseous decomposition is highly endothermic and achievable at elevated temperatures. The complexes Lisub2/subMHsub5/sub are therefore proposed to be useful hydrogen storage materials under appropriate conditions.