We have investigated the decomposition path and reversibility of Ca(BH4)2 and Ca(BH4)2 + MgH2 composite using X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, and Raman spectroscopy. Formation of CaB6 during dehydrogenation of both systems was confirmed for the first time. CaB6 appears as broad peaks in X-ray diffraction data, but Raman spectroscopy unambiguously captures the existence of CaB6. Reversibility of catalyzed Ca(BH4)2 was previously reported, and here we demonstrate reversibility of Ca(BH4)2 + MgH2 composite. Dehydrogenated product of Ca(BH4)2 + MgH2 is composed of CaH2, CaB6, 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(BH4)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 CaB6 seems critical in the reversibility of Ca(BH4)2 containing systems; the case of other borohydrides is compared.

LiBH 4 is one of the promising candidates for hydrogen storage materials because of its high gravimetric and volumetric hydrogen capacity. However, its high dehydrogenation temperature and limited reversibility has been a hurdle for its use in real applications. In an effort to overcome this barrier and to adjust the thermal stability, we make a composite system LiBH 4 -Ca(BH 4 ) 2 . In order to fully characterize this composite system we study xLiBH 4 + (1 -x)Ca(BH 4 ) 2 for several x values between 0 and 1, using differential scanning calorimetry, in situ synchrotron X-ray diffraction, thermogravimetric analysis, and mass spectrometry. Interestingly, this composite undergoes a eutectic melting at ca. 200°C in a wide composition range, and the eutectic composition lies between x ) 0.6 and 0.8. The decomposition characteristics and the hydrogen capacity of this composite vary with x, and the decomposition temperature is lower than both the pure LiBH 4 and Ca(BH 4 ) 2 at intermediate compositions, for example, for x ≈ 0.4, decomposition is finished below 400°C releasing about 10 wt % of hydrogen. Partial reversibility of this system was also confirmed for the first time for the case of a mixed borohydride composite.