Thermal decomposition behavior of calcium borohydride Ca(BH4)2

Article abstract of DOI:10.1016/j.jallcom.2007.07.097

The thermal decomposition behavior of adduct-free Ca(BH₄)₂, prepared by heating Ca(BH₄)₂·2THF powder under vacuum, was investigated by X-ray diffraction and thermal analyses. It has been found that Ca(BH₄)₂ undergoes a polymorphic transformation at 440 K and eventually decomposes in two steps between 620 and 770 K. CaH₂ and an unknown intermediate compound form after the first step, but CaH₂ is the only crystalline phase observed after the second step with a total weight loss of about 9.0 wt.%.

Full text of DOI:10.1016/j.jallcom.2007.07.097

Journal of Alloys and Compounds 461 (2008) L20–L22
Letter
Thermal decomposition behavior of calcium borohydride Ca(BH4)2
Jae-Hun Kima, Seon-Ah Jina,b, Jae-Hyeok Shima, Young Whan Choa,∗
a Materials Science and Technology Research Division, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea
b Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea
Received 19 June 2007; received in revised form 18 July 2007; accepted 18 July 2007
Available online 2 August 2007
Abstract
The thermal decomposition behavior of adduct-free Ca(BH4)2, prepared by heating Ca(BH4)2·2THF powder under vacuum, was investigated
by X-ray diffraction and thermal analyses. It has been found that Ca(BH4)2 undergoes a polymorphic transformation at 440 K and eventually
decomposes in two steps between 620 and 770 K. CaH2 and an unknown intermediate compound form after the first step, but CaH2 is the only
crystalline phase observed after the second step with a total weight loss of about 9.0 wt.%.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage materials; Phase transitions; Thermochemistry; X-ray diffraction; Thermal analysis
1. Introduction
In this study, we report the results on the thermodynamic
properties of adduct-free calcium borohydride Ca(BH4)2 in
detail for the first time. The thermal decomposition behavior of
Ca(BH4)2 has been investigated by means of the thermogravi-
metric analysis (TGA), differential scanning calorimetry (DSC)
coupled with mass spectrometry (MS) and in situ high tempera-
ture X-ray diffraction (XRD) as well as room temperature XRD
in order to understand the dehydrogenation reaction path.
Hydrogen is one of the most promising clean fuel of the
future, particularly for on-board applications for automobiles
[1]. As potential hydrogen storage materials, alkali and alkali-
earth metal borohydrides such as LiBH4, NaBH4, Mg(BH4)2
and Ca(BH4)2 have high gravimetric and volumetric hydrogen
densities [2–18]. Ca(BH4)2 is least studied among them and
very little is known about its thermochemistry, although it
has relatively high theoretical hydrogen storage capacity
(11.6 wt.%) and allegedly lower decomposition temperature
than that of LiBH4 [2,19]. Recently, Miwa et al. [18] have
characterized the crystal structure of adduct-free Ca(BH4)2 by
the Rietveld method and confirmed that the diffraction pattern
is in good agreement with the previous report [20]. Based on the
thermogravimetric analysis together with the results of ab-initio
calculations, they proposed that the desorption reaction of
Ca(BH4)2 is
2. Experimental
The starting material is Ca(BH4)2·2THF which has been purchased from
Aldrich. Adduct-free Ca(BH4)2 was prepared by heating the commercial powder
under vacuum at 473 K for 1 h. All materials were stored and handled in an argon-
filled glove box and both the water vapor and oxygen levels inside the glove box
were maintained below 1 ppm. The phase compositions and transformations of
the Ca(BH4)2 powders were characterized by XRD (Bruker D8 Advance) and
in situ high temperature XRD (PANalytical X’pert PRO MPD) with Cu K␣
radiation. The in situ high temperature XRD patterns were obtained every 50 K
up to 573 K under vacuum. The thermal decomposition behavior of Ca(BH4)2
was analyzed by DSC (Netzsch DSC 204 F1) coupled with MS (Netzsch QMS
403 C) and TGA (Netzsch TG 209 F1). The heating rate was fixed at 5 K/min
and the flow rate of pure Ar (99.9999%) gas was at 50 ml/min for both DSC/MS
and TGA measurements.
Ca(BH4)2 → 2/3CaH2 + 1/3CaB6 + 10/3H2 (9.6 wt.% H2)
(1)
although they mentioned the existence of an unknown interme-
diate compound. However, no detailed experimental data on the
thermal decomposition of Ca(BH4)2 have been disclosed so far
[19,20].
3. Results and discussion
Fig. 1 shows an XRD pattern of the vacuum dried and slow
cooled Ca(BH4)2 sample measured at room temperature. Since
the diffraction peaks match quite well with those of Ca(BH4)2
∗
Corresponding author. Tel.: +82 2 958 5465; fax: +82 2 958 5379.
E-mail address: oze@kist.re.kr (Y.W. Cho).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.07.097

J.-H. Kim et al. / Journal of Alloys and Compounds 461 (2008) L20–L22
L21
Fig. 1. XRD pattern of the Ca(BH4)2 sample dried under vacuum.
Fig. 3. In situ high temperature XRD patterns of Ca(BH4)2 during heating.
Solid circles and squares indicate low and high temperature phases of Ca(BH4)2,
respectively.
in the previous studies [18,20], it is thought to be a single phase
Ca(BH4)2 without any THF adduct. Typical DSC/MS curves
of this adduct-free Ca(BH4)2 are presented in Fig. 2(a). A small
endothermic peak appears at around 440 K and the second, sharp
endothermic peak at 640 K. The third, relatively broad endother-
mic peak finally appears between 670 and 770 K. According to
the MS data, there is no gas evolution at the first endothermic
reaction. However, both the second and third are accompanied
with hydrogen evolution and no other gas phase is detected by
MS. Fig. 2(b) shows a typical TGA curve and the weight loss
behavior matches quite well with the DSC data, except that there
is a slight weight decrease before 600 K. The total weight loss
is approximately 9.0% which is lower than that of the theoreti-
cal value based on the proposed decomposition reaction (1), but
higher than the following reaction:
Ca(BH4)2 → CaH2 + 2B + 3H2 (8.7 wt.% H2)
(2)
In order to find out the cause of the first endothermic reaction
which does not accompany with hydrogen evolution, an in situ
high temperature XRD analysis was carried out and the result
is presented in Fig. 3. As temperature increases up to 423 K,
the intensities of the diffraction peaks for Ca(BH4)2 gradually
decrease. At 473 K, unknown diffraction peaks start to appear
and only this unknown phase exists at 573 K. From this result
together with those of DSC, MS and TGA, the endothermic
reaction around 440 K is believed to be a polymorphic trans-
formation from low to high temperature phase of Ca(BH4)2. In
fact, the diffraction peaks of the high temperature phase were
observed together with the low temperature phase in the XRD
pattern of the air-cooled sample (not shown here).
Fig. 4 presents the XRD patterns of the Ca(BH4)2 samples
dehydrogenated up to 663 and 753 K under vacuum, together
with pure CaH2 and CaB6 for comparison. At 663 K, both
CaH2 and another unknown phase are observed. This unknown
phase seems to be an intermediate compound consisting of
calcium, boron and hydrogen. Similar cases during the dehy-
drogenation reaction of LiBH4 were reported previously [8–10].
When heated up to 753 K, the unknown peaks disappear and the
only CaH2 phase is observed while no crystalline B or CaB6
was observed. This indicates that the intermediate compound
decomposes into crystalline CaH2 and amorphous boron and/or
amorphous calcium boride. Not like the case of Zn(BH4)2 [21],
no borane gas has been detected by MS during dehydrogenation
in the present study.
According to the phase diagram [22], the only intermetallic
compound in the Ca–B binary system is CaB6. Moreover, ther-
modynamic calculation using the Thermo-Calc program [23]
indicates that the products of reaction (1) have Gibbs free energy
Fig. 2. (a) DSC and MS data (m/z = 2), and (b) TGA curve of the Ca(BH4)2
sample dried under vacuum.

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J.-H. Kim et al. / Journal of Alloys and Compounds 461 (2008) L20–L22
Acknowledgement
This work has been supported by the Hydrogen Energy R&D
Center, one of the 21st Century Frontier R&D Program, funded
by the Ministry of Science and Technology of Korea.
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In summary, we have investigated the thermal decomposi-
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a slow polymorphic transformation at around 440 K and eventu-
ally decomposes in two steps between 620 and 770 K. CaH2 and
an unknown intermediate compound form after the first step and
this intermediate compound finally decomposes into CaH2 and
amorphous B and/or amorphous calcium boride. Further stud-
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