Probing the structure, stability and hydrogen storage properties of calcium dodecahydro-closo-dodecaborate

Calcium borohydride can reversibly store up to 9.6 wt% hydrogen; however, the material displays poor cyclability, generally associated with the formation of stable intermediate species. In an effort to understand the role of such intermediates on the hydrogen storage properties of Ca(BH4)2, calcium dodecahydro-closo-dodecaborate was isolated and characterized by diffraction and spectroscopic techniques. The crystal structure of CaB12H12 was determined from powder XRD data and confirmed by DFT and neutron vibrational spectroscopy studies. Attempts to dehydrogenate/hydrogenate mixtures of CaB12H12 and CaH2 were made under conditions known to favor partial reversibility in calcium borohydride. However, up to 670 K no notable formation of Ca(BH4)2 (during hydrogenation) or CaB6 (during dehydrogenation) occurred. It was demonstrated that the stability of CaB12H12 can be significantly altered using CaH2 as a destabilizing agent to favor the hydrogen release.

Calcium dodecahydro-closo-dodecaborate, CaB12H12 (1), was isolated by dehydration/desolvation of [Ca(H2O)7][B12H12]·H2O (2) or [Ca(H2O)5(MeCN)2][B12H12] (3). The crystal structure of 1 was determined by powder X-ray diffraction and confirmed by neutron vibrational spectroscopy and first-principles calculations. Hydrogen storage properties of 1 in the presence of calcium hydride were elucidated.Figure optionsDownload as PowerPoint slide