A first-principles simulation of the metal borohydride ammonia borane complex (LiBH4)2(NH3BH3) and the decomposition reaction pathway for hydrogen storage

Abstract

In this work, we predict a range of favorable functional properties of (LiBH4)2NH3BH3, a relatively new member of the boron-containing metal borohydride ammonia borane family, by means of ab initio calculations, as well as its parent compounds, LiBH4 and NH3BH3, for comparisons. Both the mechanical and dynamical stabilities of this new compound have been demonstrated theoretically for the first time. Results from elastic modulus calculations show that the mechanical properties of (LiBH4)2NH3BH3 are remarkably improved compared with its parent compounds. Secondly, Electronic structure results show that it remains to an insulator with large band gap typical of the boron-containing hydrogenous family, but the band gap can be tuned by the compositions of NH3BH3 and LiBH4. Charge analysis demonstrates that charge transfers in individual layers of LiBH4 and NH3BH3 are similar to LiBH4 and NH3BH3, respectively. A measurable amount of charge transfers from the LiBH4 layers to the NH3BH3 layers result in enhanced activation properties for hydrogenation and dehydrogenation in (LiBH4)2NH3BH3. Thirdly, Free energies of six possible dehydrogenation reactions have been calculated from 0 K to 700 K, and the results show that combination of NH3BH3 and LiBH4 can reduce the dehydrogenation energies compared with the parent compounds, a result consistent with recent experiments. Meanwhile, the N–B bond strengths increase and thereby borazine and diborane formation are reduced upon dehydrogenation.