Mechanochemical synthesis and thermal decomposition of zinc borohydride

Article abstract of DOI:10.1016/j.jallcom.2005.11.045

Zinc borohydride Zn(BH₄)₂ (8.5 wt% theoretical hydrogen storage capacity) has been synthesized by mechanochemical reaction between NaBH₄ and ZnCl₂ and its structure was analyzed by XRD and FT-IR. The thermal stability of Zn(BH₄)₂ in the presence of inert by-product NaCl was studied by TG/MS and DSC under flowing argon. It melts at about 85 °C and starts to decompose almost at the same time producing diborane B₂H₆ and hydrogen gases between 85 and 140 °C. The decomposition reaction is endothermic and the remaining solid product after decomposition is Zn with a trace of B.

Full text of DOI:10.1016/j.jallcom.2005.11.045

Journal of Alloys and Compounds 422 (2006) 273–275
Mechanochemical synthesis and thermal
decomposition of zinc borohydride
∗
Eun Jeon, YoungWhan Cho
Nano-Materials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Seoul 130-650, Republic of Korea
Received 24 October 2005; received in revised form 15 November 2005; accepted 15 November 2005
Available online 6 January 2006
Abstract
4 2 4
Zinc borohydride Zn(BH ) (8.5 wt% theoretical hydrogen storage capacity) has been synthesized by mechanochemical reaction between NaBH
and ZnCl
2
and its structure was analyzed by XRD and FT-IR. The thermal stability of Zn(BH
4
)
2
in the presence of inert by-product NaCl was
◦
studied by TG/MS and DSC under flowing argon. It melts at about 85 C and starts to decompose almost at the same time producing diborane B
and hydrogen gases between 85 and 140 C. The decomposition reaction is endothermic and the remaining solid product after decomposition is
Zn with a trace of B.
2
H
6
◦
©
2005 Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage materials; Complex metal hydride; Zinc borohydride; Mechanochemical reaction; Thermal decomposition
1
. Introduction
[8]. It can be prepared by metathesis reaction between NaBH4
and ZnCl2 in an appropriate solvent such as diethyl ether [9].
It is, however, difficult to remove solvent completely without
causing decomposition of Zn(BH4)2 itself. Therefore, the ther-
mal decomposition behavior of adduct-free Zn(BH4)2 could not
be properly investigated in the past. In this paper, we have
investigated the stability and thermal decomposition behavior
of Zn(BH4)2 prepared by mechnochemical reaction between
NaBH4 and ZnCl2 at room temperature without using any sol-
vent.
Hydrogen is one of the most promising clean fuel of the
future, particularly for on-board applications for automobiles.
The combustion of hydrogen, either physical or chemical, does
not produce carbon dioxide like hydrocarbons and clean water
is the only product, which can be permanently recycled with-
out causing any atmospheric pollution. To be practical, a safe,
light, compact, and economical mean of hydrogen storage is
required. So far, the available hydrogen storage techniques are:
low temperature liquefaction, high-pressure compression, and
absorption into metallic solids (intermetallic hydrides). They
are neither economical (liquefaction), compact (compression),
nor lightweight (absorption) [1–3]. As the first two problems
are fundamental, the main research activities are now focused
on solid-state hydrogen storage materials with high theoretical
capacities such as complex metal hydrides [4,5].
2
. Experimental procedures
Sodium borohydride (NaBH4, 99%) from Sigma-Aldrich was used as
received. Zinc chloride (ZnCl , 98%) from Kanto chemical was used after vac-
2
◦
uum drying at 200 C for 24 h. All materials were stored and handled in a glove
box (MBraun LABstar) filled with pure argon (99.9999%) and both the water
vapor and oxygen levels inside the glove box were maintained below 1 ppm. A
mixture of ZnCl2 (0.681 g, 5 mM) and NaBH4 (0.378 g, 10 mM) was charged
with seventeen Cr-steel balls (7.9 mm in diameter) into a hardened steel vial
(70 cc internal volume) inside the globe box and sealed with a lid having a Viton
O-ring. The ball-to-powder weight ratio (BPR) was approximately 35:1. The
high-energy ball milling was performed by a SPEX-8000 Mixer/Mill (SPEX
Industries Inc.) for 30 min at room temperature. The phase compositions of
the milled powders were characterized by X-ray diffractometry (XRD) using
a Bruker D8 Advance diffractometer with Cu K␣ radiation. FT-IR spectra of
the milled samples were measured using a Thermo Mattson IR 300. The ther-
mal decomposition behavior of Zn(BH4)2 in the presence of inert by-product
Zn(BH4)2 belongs to complex metal hydrides which are rep-
resented as Mx(BH4)y (M = Li, Na, K, Rb, Cs, Zn, etc.), in which
there is current interest as potential hydrogen storage materials
[
6,7]. It has high theoretical hydrogen capacity (8.5%) and is
◦
reported to have low decomposition temperature (50–120 C)
∗
Corresponding author. Tel.: +82 2 958 5465; fax: +82 2 958 5379.
E-mail address: oze@kist.re.kr (Y. Cho).
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.11.045

2
74
E. Jeon, Y. Cho / Journal of Alloys and Compounds 422 (2006) 273–275
Fig. 1. XRD patterns of Zn(BH4)2(+NaCl). The asterisk indicates a diffraction
peak of NaCl.
NaCl was analyzed by TG/MS using a NETSCH TG209 thermogravimetric ana-
lyzer coupled with a NETSCH QMS403C massspectrometer and DSC using a
NETSCH DSC204 differential scanning calorimeter. The heating rate was fixed
◦
at 10 C/min and the flow rate of pure Ar gas was at 50 ml/min for both DSC
and TG/MS measurements.
3
. Results and discussion
The XRD pattern of the product powder after ball milling for
0 minispresentedin Fig. 1. Althoughinformationonthecrystal
Fig. 3. (a) TG and hydrogen MS curves of Zn(BH4)2(+NaCl); (b) TG and DSC
curves of Zn(BH ) (+NaCl).
3
4
2
structure of Zn(BH4)2 is not available at present, the remaining
peaks, after excluding NaCl peaks, were found to match quite
well with those of the previous studies [10]. Neither ZnCl2 nor
NaBH4 peaks were observed, which indicates that the following
reaction is completed after milling for 30 min.
bending at 1127 cm 1 [10]. Strong bands due to B–H
−
t(terminal)
−
1
stretching between 2400 and 2600 cm , one from bidentate
structure and ionic structure B–H stretching between
t(terminal)
−
1
2
200 and 2300 cm are detected. As well as, two bands from
−1
ZnCl2 + 2NaBH4 → Zn(BH4)2 + 2NaCl
(1)
B–Hb(bridge) stretching at 1614 and 2083 cm , a Zn–H stretch-
−
1
−1
ing at 1409 cm , and a BH2 deformation at 1127 cm are
detected, which are typical bands observed in metal borohy-
drides [9]. This result also indicates that Zn(BH4)2 was indeed
formed by mechanochemical reaction.
Fig. 2 shows the FT-IR spectra of Zn(BH4)2(+NaCl) together
with milled ZnCl2 and NaBH4. The characteristic Zn–Cl stretch-
−
1
ing appears at 1670 cm . The ionic structure of NaBH4 should
exhibit three bands in the infrared spectrum, a B–H stretching at
The result of TG/MS measurement of Zn(BH4)2(+NaCl) is
presented in Fig. 3a. The decomposition reaction started at about
− −1
1
2
200–2400 cm , a Na-H stretching at 1634 cm , and a BH2
◦ ◦
5 C and completed at about 140 C. It was accompanied by the
8
evolution of hydrogen. As soon as the decomposition reaction
started, a sharp endothermic peak was recorded between 85 and
◦
1
05 C in DSC curve as shown in Fig. 3b. This peak is attributed
tothemeltingofZn(BH4)2. Infact, themeltingphenomenonwas
visually confirmed by heating the sample contained in an argon
filledvialunderastereoscopicmicroscope. Abroadendothermic
peak due to relatively slow decomposition reaction was also
◦
detected between 85 and 140 C and it matches well with the
weight loss curve (Fig. 3b).
As the reaction product after milling contains 2 mol of NaCl
for every 1 mol of Zn(BH4)2, the total weight loss after the
completion of dehydrogenation reaction is expected to be 3.8%
rather than 8.5% of pure Zn(BH4)2 if hydrogen is the only evolv-
ing gas during thermal decomposition. However, the measured
weight loss was 12.1%, which is much higher than the expected
3.8%. After careful examination of MS spectrum, it was con-
Fig. 2. FT-IR spectra of milled ZnCl2, milled NaBH4, and Zn(BH4)2(+NaCl).

E. Jeon, Y. Cho / Journal of Alloys and Compounds 422 (2006) 273–275
275
◦
Fig. 5. XRD pattern of Zn(BH4)2(+NaCl) after heating for 3 days at 150 C.
The asterisk indicates a diffraction peak of NaCl.
4. Conclusions
We have synthesized Zn(BH4)2 by mechanically activated
metathesis reaction at room temperature and its thermal decom-
position behavior has been investigated by TG/MS and DSC.
◦
Zn(BH4)2 melts at about 85 C under flowing Ar condition and
almost at the same time decomposes slowly producing diborane
B2H and hydrogen gases as well as metal Zn and a trace of B
6
◦
between 85 and 140 C. Although the decomposition reaction
Fig. 4. (a) TG and diborane (B2H6) MS curves of Zn(BH4)2(+NaCl); (b) DSC
and diborane (B2H6) MS curves of Zn(BH4)2(+NaCl).
is endothermic, it is confirmed that Zn(BH4)2 even decomposes
at room temperature. It is, therefore, concluded that Zn(BH4)2
seems not suitable materials for reversible hydrogen storage for
on-board applications.
firmed that a significant amount of diborane B2H [11] was also
6
produced as shown in Fig. 4. The calculated boron content in
the Zn(BH4)2(+NaCl) mixture is 10.2% and the predicted total
weight loss (hydrogen + diborane) is, therefore, 14.0% which is
slightly higher than the measured value. It was later confirmed
by ICP-AES that the residual boron content in the final decom-
position product was 0.8%. Considering a possibility of partial
decomposition of Zn(BH4)2 during mechanochemical reaction
process, it is concluded that the measured weight loss agrees
with the calculated one rather well.
Acknowledgements
This work has been financially supported by the Hydrogen
Energy R&D Center under the 21st Century Frontier R&D Pro-
gram and by Korea Science and Engineering Foundation under
the International Cooperative Research Program of the Ministry
of Science and Technology, Republic of Korea.
References
The XRD pattern of the Zn(BH4)2(+NaCl) sample heated at
◦
1
50 C for 3 days is presented in Fig. 5. Metal Zn was observed
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3
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[
Zn(BH4)2 (solid) → Zn(BH4)2 (liquid)
(2)
(3)
[
[
Zn(BH4)2 (solid or liquid) → Zn + B2H + H2
6