On the Reversibility of Hydrogen-Storage Reactions in Ca(BH4)2: Characterization via Experiment and Theory

With 11.6 wt % H2, Ca(BH4)2 is a promising hydrogen-storage candidate material provided the issue of reversibility can be addressed. We investigate theoretically and experimentally the structure and reversibility of Ca(BH4)2 in (de)hydrogenation reactions, including an intermediate CaB12H12 phase. From our first-principles calculations, we predict several polymorphs of CaB12H12 that compete in energy in dehydrogenation (within 1 kJ/mol-H2), indicating no long-range ordered state is likely. Our experimental microchemical analysis and structural characterization of the dehydrogenated Ca(BH4)2 show that the intermediate phase is amorphous-like. Theoretically determined X-ray diffraction patterns for all coexisting Ca(BH4)2, CaH2, and CaB12H12 polymorphic phases reproduce the observed diffraction peaks. The calculated reaction formation enthalpies versus H-content reveal limited reversibility, as CaB12H12 is energetically very favorable. Our results suggest the (de)hydrogenation process for Ca(BH4)2 via CaB12H12 intermediates is reversible, but, due to its stability, full release of H2 between Ca(BH4)2 and CaH2 + CaB6 (or B) is not possible except at high temperature. Thus, Ca(BH4)2 has limited viability as a reversible on-board storage material for vehicular applications, although it may hold some promise as a chemical hydride.