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 boroh...