(594f) First Principles Study of Dehydrogenation Reaction Intermediates of Complex Metal Borbohydrides

Complex metal borohydrides are the promising materials for onboard reversible hydrogen storage. In particular, materials of the form M(BH₄)₂, where M= Ca or Mg have received considerable attention due to its high gravimetric capacity > 11 wt % H. The experiments have shown that the first step in the dehydrogenation is

6M(BH₄)₂ -> 5MH₂+MB₁₂H₁₂+13H₂,

which involves formation of an intermediate phase, MB₁₂H₁₂, M= Mg and Ca. It should be noted that this intermediate is amorphous in nature and has not been properly understood.

Using the first-principles study we propose that MB₁₂H₁₂ has a wealth of nearly isoenergetic states that are structurally distinct. The predicted polymorphs are characterized by different orientations of the closo-borane groups and location of the cations. The results of our study are used to shed light on the nature of the intermediate phase MB₁₂H₁₂ with special reference to the energetics, the reactions they undergo, thermodynamics and the XRD-pattern.