Identification of the Dehydrogenated Product of Ca(BH4)2

We have investigated the decomposition path and reversibility of Ca(BH 4 ) 2 and Ca(BH 4 ) 2 + MgH 2 composite using X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, and Raman spectroscopy. Formation of CaB 6 during dehydrogenation of both systems was confirmed for the first time. CaB 6 appears as broad peaks in X-ray diffraction data, but Raman spectroscopy unambiguously captures the existence of CaB 6 . Reversibility of catalyzed Ca(BH 4 ) 2 was previously reported, and here we demonstrate reversibility of Ca(BH 4 ) 2 + MgH 2 composite. Dehydrogenated product of Ca(BH 4 ) 2 + MgH 2 is composed of CaH 2, CaB 6, and Mg. About 60% reversibility was achieved after rehydrogenation for 24 h under 90 bar of hydrogen pressure at 350 °C even without the help of catalysts, which makes a good contrast with the case of pure Ca(BH 4 ) 2 where almost negligible rehydrogenation occurs under the same conditions. To understand the difference, the role of Mg in rehydrogenation is worth further investigation. Formation of CaB 6 seems critical in the reversibility of Ca(BH 4 ) 2 containing systems; the case of other borohydrides is compared.