Electronic Structure and Stability of Mono- and Bimetallic Borohydrides and Their Underlying Hydrogen-Storage Properties: A Cluster Study

Using gradient corrected density functional theory and a cluster-based model we have studied the stability and hydrogen-storage properties of monometallic borohydrides, M(BH4)(3), and bimetallic borohydrides, KM(BH4)(4) (M = Al, Ga, Sc). A systematic study of BHx (x = 1-4), M(BH4)(n) (M = Al, Ga, Sc; n = 1-4), and KM(BH4)(4) reveals many interesting results. (i) The vertical detachment energy of BH4- is larger than that of any halogen atom; hence, BH4 can be classified as a superhalogen. (ii) When a metal atom, M, is surrounded with BH4 moieties whose number exceed the valence of the metal atom by one, a new class of highly electronegative molecules referred to as hyperhalogens can be formed. (iii) Both BH4- and M(BH4)(4)(-) can serve as the building blocks of super- and hyper-salts, respectively, when counterbalanced with a metal cation such as K+. (iv) The energy required to remove a hydrogen atom from a bimetallic borohydride such as KAl(BH4)(4) is found to be less than that from the corresponding monometallic borohydride, namely Al(BH4)(3), thus making bimetallic borohydrides potential candidates for hydrogen-storage materials. We hope these results will stimulate experimental investigation into new super- and hyperhalogens and their corresponding salts as potential hydrogen-storage materials.