LiSc(BH₄)₄ as a Hydrogen Storage Material: Multinuclear High-Resolution Solid-State NMR and First-Principles Density Functional Theory Studies

Abstract

A lithium salt of anionic scandium tetraborohydride complex, LiSc(BH4)4, was studied both experimentally and theoretically as a potential hydrogen storage medium. Ball milling mixtures of LiBH4 and ScCl3 produced LiCl and a unique crystalline hydride, which has been unequivocally identified via multinuclear solid-state nuclear magnetic resonance (NMR) to be LiSc(BH4)4. Under the present reaction conditions, there was no evidence for the formation of binary Sc(BH4)3. These observations are in agreement with our first-principles calculations of the relative stabilities of these phases. A tetragonal structure in space group I4̅ (#82) is predicted to be the lowest energy state for LiSc(BH4)4, which does not correspond to structures obtained to date on the crystalline ternary borohydride phases made by ball milling. Perhaps reaction conditions are resulting in formation of other polymorphs, which should be investigated in future studies via neutron scattering on deuterides. Hydrogen desorption while heating these Li−Sc−B−H materials up to 400 °C yielded only amorphous phases (besides the virtually unchanged LiCl) that were determined by NMR to be primarily ScB2 and [B12H12]−2 anion containing (e.g., Li2B12H12) along with residual LiBH4. Reaction of a desorbed LiSc(BH4)4 + 4LiCl mixture (from 4LiBH4/ScCl3 sample) with hydrogen gas at ∼70 bar resulted only in an increase in the contents of Li2B12H12 and LiBH4. Full reversibility to reform the LiSc(BH4)4 was not found. Overall, the Li−Sc−B−H system is not a favorable candidate for hydrogen storage applications.