Abstract
Gaseous lithium complex hydrides Li2MH5 (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, Li2H+, Li2H2, MH3, MH4−, and LiMH4 have been determined. For the LiBH4 the equilibrium configuration was tridentate of C3v symmetry. For LiAlH4 two isomeric forms, bidentate (C2v) and tridentate (C3v), were confirmed to exist, and C2v isomer was shown to dominate in saturated vapor. For complex hydrides Li2MH5, different structural forms were considered but only one asymmetric form (C1) appeared to be equilibrium. Several possible channels of dissociation of Li2MH5 were considered; the enthalpies and Gibbs free energies of the reactions were computed. The enthalpies of formation ∆fH(0) of the complex hydrides in gaseous phase were determined: 60 ± 10 kJmol1 (Li2BH5) and 33 ± 10 kJmol1 (Li2AlH5). Heterophase decomposition of the gaseous Li2MH5 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 Li2MH5 are therefore proposed to be useful hydrogen storage materials under appropriate conditions.
Highlights
Utilization of hydrogen as fuel energy is limited by lack of viable hydrogen storage materials [1]
The results obtained with both density functional theory (DFT) and MP2 methods are in good agreement between each other as well as with the available reference data [22]
The results obtained by DFT and MP2 methods are in good correspondence between each other; good accordance was observed in the reference data available for lower species
Summary
Utilization of hydrogen as fuel energy is limited by lack of viable hydrogen storage materials [1]. Application of hydrogen in fuel cells has an advantage due to the fact that hydrogen is environmentally benign with water as the only byproduct, renewable and has very high energy density compared to any known conventional fuel sources [2, 3]. Materials suitable for hydrogen storage should have the characteristics such as high capacity to store large weight percent and volumetric fraction of hydrogen, good desorption/adsorption kinetics as a reversible mechanism [5, 6]. Thermal stability and reaction reversibility of these hydrides becomes the key barrier for the growth of hydrogen powered fuel cells [9]. In 1996, Bogdanovic and Schwickardi [10] first reported on the adsorption and desorption isotherms of catalyzed NaAlH4 at the temperature of around 180°C to 210°C, this has unlocked the researchers’ interest towards
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