Identification of the Dehydrogenated Product of Ca(BH₄)₂

Abstract

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.

Chemical State, Distribution, and Role of Ti- and Nb-Based Additives on the Ca(BH4)2 System

“…4,5 As a consequence, a decomposition temperature lower than 100°C at 1 bar H 2 pressure is estimated. However, several decomposition paths for pure Ca(BH 4 57.3, 10 31.09, 9 and 39.2 6 or 31.34 9 kJ mol −1 H 2 for reactions 1, 2, 4, and 5, respectively. These values are calculated at 300 K and 1 bar H 2 pressure.…”

“…14,15 Unsuccessful reversibility in the pure Ca(BH 4 ) 2 system is likely related to formation of both boron (reluctant to react even in harsh conditions) and [B 12 H 12 ] 2− phases upon desorption and slow mass transport of boron-containing phases upon rehydrogenation although a high hydrogen pressure was applied.…”

“…9 Rehydrogenation attempts on the decomposition products of undoped Ca(BH 4 ) 2 did not lead to its reversible formation. 14,15 Unsuccessful reversibility in the pure Ca(BH 4 ) 2 system is likely related to formation of both boron (reluctant to react even in harsh conditions) and [B 12 H 12 ] 2− phases upon desorption and slow mass transport of boron-containing phases upon rehydrogenation although a high hydrogen pressure was applied. 14−16 Transition-and light-metal based additives have shown to be beneficial on the sorption reaction kinetics of complex hydrides.…”

Tetrahydroborates: Development and Potential as Hydrogen Storage Medium

“…is the thermodynamically favorable formation of the M(B 12 H 12 ) n compounds (reaction (29)). On the one hand, they are considered as intermediate compounds [85][86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][101][102][103]. However, on the other hand, their high stability and highly negative enthalpy of formation suggest that they are final decomposition products of M(BH 4 ) n [92,[102][103][104][105][106].…”

Metal hydrides for concentrating solar thermal power energy storage

“…Of most interest for thermal storage, from a cost point of view, are Ca4Mg3H14, Ca19Mg8H54 and NaMgH3. The H2 desorption from Ca4Mg3H14 and Ca19Mg8H54 have not been characterised but it is likely these materials initially decompose to CaH2 and Mg [71]. This would limit their practical H2 capacity to 2.44 and 1.60 wt.%, respectively.…”