On the reversibility of hydrogen storage in Ti- and Nb-catalyzed Ca(BH4)2

The hydrogen sorption properties of calcium borohydride (Ca(BH4)2) catalyzed with a small amount of TiF3, TiCl3, NbF5 or NbCl5 are investigated using thermal analyses and X-ray diffraction. NbF5 exhibits the best performance among all the catalysts; it causes a decrease in the hydrogen desorption temperature which leads to hydrogen absorption at practical temperature and pressure conditions. The hydrogen content of Ca(BH4)2 with NbF5 reaches about 5.0 wt.% after hydrogen absorption at 693 K for 24 h under 90 bar of hydrogen. The main dehydrogenation product of Ca(BH4)2 with NbF5 is a CaH2−xFx solid solution with a CaF2(C1) structure, while pure Ca(BH4)2 produces CaH2 after hydrogen desorption.