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

Abstract

With 11.6 wt % H 2, Ca(BH 4 ) 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(BH 4 ) 2 in (de)hydrogenation reactions, including an intermediate CaB 12 H 12 phase. From our first-principles calculations, we predict several polymorphs of CaB 12 H 12 that compete in energy in dehydrogenation (within 1 kJ/mol-H 2 ), indicating no long-range ordered state is likely. Our experimental microchemical analysis and structural characterization of the dehydrogenated Ca(BH 4 ) 2 show that the intermediate phase is amorphous-like. Theoretically determined X-ray diffraction patterns for all coexisting Ca(BH 4 ) 2, CaH 2, and CaB 12 H 12 polymorphic phases reproduce the observed diffraction peaks. The calculated reaction formation enthalpies versus H-content reveal limited reversibility, as CaB 12 H 12 is energetically very favorable. Our results suggest the (de)hydrogenation process for Ca(BH 4 ) 2 via CaB 12 H 12 intermediates is reversible, but, due to its stability, full release of H 2 between Ca(BH 4 ) 2 and CaH 2 + CaB 6 (or B) is not possible except at high temperature. Thus, Ca(BH 4 ) 2 has limited viability as a reversible on-board storage material for vehicular applications, although it may hold some promise as a chemical hydride.