A first-principles study: Structure and decomposition of mono-/bimetallic ammine borohydrides

Bimetallic ammine borohydrides have been demonstrated to be capable of improving the efficiency of dehydrogenation and purity of the released hydrogen as compared to monometallic AMBs. We have obtained the optimized structures, orbital, and decomposition thermodynamic properties of several metal ammine borohydrides (AMB) containing [Li(BH₄)_n]^1-n groups by performing a solid-state density functional theory calculation. The structures are abbreviated as M-Li(BH₄)_x(NH ₃)_y, where M means Li, Mg, Al, and Ca, respectively. [LiBH₄] segments in these compounds play a crucial role in suppressing borane emission. Additionally, it activates the B- H···H-N bonds and decreases the hydrogen removal energies. Furthermore, the strength of M-N bonds will dictate the impurity of the ammine from the decomposition. The stability of the AMBs can be found as follows from the results of orbitals: LiMg(BH₄)₃(NH₃) ₂ > Li₂Al(BH₄)₅(NH ₃)₄ > LiCa(BH₄)₃(NH ₃)₂ > [Li(BH₄)(NH₃)]₂. Finally, the [LiBH₄] group can polarize the molecule and improve the efficiency of dehydrogenation process and purity of released hydrogen from bimetallic ammine borohydrides (AMBs) as compared to monometallic AMBs, which is due to the different bond strengths of M-B and M-N bonds (M denotes different metal cations here).