[NH₄⁺][BH₃^-] (x = 1-5): Role of Polynuclear Superhalogens in High-Capacity Hydrogen Storage

Superhalogen anions are characterized by a higher vertical detachment energy (VDE) than those of halides. Ammonium borohydride, [NH₄⁺][BH₄^-], is a potential candidate for high-capacity hydrogen storage but is not practically used due to its instability against dissociation to ammonia borane. Interestingly, BH₄^- is a superhalogen anion, and therefore, we use polynuclear BH₃^- superhalogen anions and study [NH₄⁺][BH₃^-] complexes for x = 2-5 using density functional theory. The gravimetric hydrogen density of these complexes is smaller than that of [NH₄⁺][BH₄^-] only slightly. We calculate the dehydrogenation energy and the Gibbs free energy of [NH₄⁺][BH₃^-] complexes through ammonia borane. We notice that these complexes are more stable than [NH₄⁺][BH₄^-], whose stability increases with an increase in x. The enhanced stability of [NH₄⁺][BH₃^-] complexes appears as a consequence of the increase in the VDE of polynuclear BH₃^- superhalogen anions. We believe that these findings might attract experimentalists to synthesize these complexes.