Lithium borohydride-melamine complex as a promising material for chemical hydrogen storage

Article abstract of DOI:10.1016/j.jallcom.2012.09.100

A complex hydride, 3LiBH₄C₃N₆H ₆, crystallizing in monoclinic structure with lattice parameters of a=17.4701 ?, b = 17.6332 ?, c = 4.1749?, β = 99.7453° and V= 1267.54 ?³, was synthesized by solid reaction between LiBH₄ and C₃N₆H₆. 3LiBH ₄C₃N₆H₆ starts to evolve hydrogen at 100 °C, which is 190 °C lower than that of pristine LiBH⁴. Combination of protic hydrogen of [BH₄] and hydridic hydrogen of NH in 3LiBH₄C₃N₆H₆ may greatly improve the dehydrogenation properties, and totally 7.83 wt.% H₂ can be released from 3LiBH₄C₃N₆H₆ in the temperature range of 100-340 °C.

Full text of DOI:10.1016/j.jallcom.2012.09.100

Journal of Alloys and Compounds 552 (2013) 98–101
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Lithium borohydride–melamine complex as a promising material for chemical
hydrogen storage
⇑
Lin Liu, Daqiang Hu, Teng He, Yao Zhang, Guotao Wu, Hailiang Chu, Peikun Wang, Zhitao Xiong ,
Ping Chen
Dalian National Laboratory for Clear Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 June 2012
Received in revised form 21 September 2012
Accepted 21 September 2012
Available online 13 October 2012
A complex hydride, 3LiBH ꢀC N H , crystallizing in monoclinic structure with lattice parameters of
4
3
6
6
3
a = 17.4701 Å, b = 17.6332 Å, c = 4.1749 Å, b = 99.7453° and V = 1267.54 Å , was synthesized by solid
reaction between LiBH
90 °C lower than that of pristine LiBH
of NH in 3LiBH
ꢀC
can be released from 3LiBH
4
and C
3
N H
6 6
. 3LiBH
4
ꢀC
3
N
6
H
6
starts to evolve hydrogen at 100 °C, which is
ꢁ
1
4
. Combination of protic hydrogen of [BH
4
]
and hydridic hydrogen
4
3
N H
6 6
may greatly improve the dehydrogenation properties, and totally 7.83 wt.% H
ꢀC in the temperature range of 100–340 °C.
Ó 2012 Elsevier B.V. All rights reserved.
2
4
3 6 6
N H
Keywords:
Chemical hydrogen storage
Lithium borohydride–melamine complex
hydride
Protic hydrogen
Hydridic hydrogen
1
. Introduction
Hydrogen storage is one of the enabling technologies for the
4
hydrides. A variety of combinations of LiBH and metal hydrides
such as LiBH
[28], LiBH –YH
[33,34], LiBH –Mg(AlH
MgH –LaH
shown that most of these additives could effectively reduce the
4
–MgH
2
[24–26], LiBH
4
–CaH
2
[27,28], LiBH
[31,32], LiBH –Li
NiH [36] and LiBH
4
–CeH
2
4
3
[29,30], LiBH
4
–LiAlH
4
4
3
AlH
6
development of hydrogen-based energy system. Chemical com-
pounds or complexes with high hydrogen contents and superior
storage performances are urgently needed by onboard hydrogen
4
4
)
2
[35], LiBH
4
–Mg
2
4
4
–
2
3
[37,38] have been investigated. Such studies have
fuel cell for next generation vehicles [1–5]. LiBH
4
possessing a
4
dehydrogenation temperatures of LiBH and enhance the dehydro-
gravimetric density of 18.3% is always considered as one of the
most promising candidates for hydrogen storage. Though extensive
investigations have been paid on this borohydride in the past dec-
ades, there are still several drawbacks to be overcome before utiliz-
ing it for practical usage [6]. For example, to reach 0.1 MPa
genation kinetics.
A novel system, LiBH
Aoki et al. and Pinkerton et al. during the last several years [39–41].
Since then, a variety of systems such as LiBH –LiNH –MgH
[42,43], LiBH –Mg(NH –LiH [43] have been developed for im-
prove hydrogen storage performance. Recently, a novel hydrogen
4 2
–LiNH , was developed independently by
4
2
2
4
2 2
)
hydrogen equilibrium pressure LiBH
ature above 380 °C due to its overhigh thermal stability, which is
far above the operation temperature of Proton Exchange
Membrane fuel cell (PEM fuel cell). Furthermore, even at such high
temperature dehydrogenation of LiBH is still kinetically sluggish
6–8].
Doping LiBH
ordered mesoporous carbon & carbon nanotube [13,14], metal ha-
4
has to be heated to a temper-
+
ꢁ
storage system LiBH4ꢀn[C(NH
tual dehydrogenation improvement, and >10.0 wt.% of H
released below 250 °C [44]. Effects of metal chlorides (MgCl
and AlCl ) on the decomposition of LiBH were investigated.
ꢀNH
Hydrogen release from mixtures of Li(NH )BH and metal chlorides
may be directly from M(BH (M = Mg, Zn or Al, x is the
ꢀxNH
chemical valence of M) due to the metathesis reaction between
LiBH and those metal chlorides [45]. Combination of LiBH with
NH molecule resulted in complex hydrides LiBH xNH , and
17.8 wt.% of hydrogen could be released from the Li(NH
in the temperature range of 135–250 °C with Co catalyst [46].
Hydrazine was also combined with LiBH to form LiBH NH NH
and around 13.0 wt.% hydrogen can be released from Fe-catalyzed
LiBH NH at 140 °C [47]. In these cases, the strong potential in
ꢀNH
the combination of protic H (NH) and hydridic H (BH) atoms
2
)
3
] [BH
4
]
complexes exhibit a mu-
can be
, ZnCl
a
2
2
2
4
3
4
3
[
3
4
4
with metals (Mg, Al, Ti, V, Cr, Ni, etc.) [9–12], dis-
4
)
x
3
lides (TiCl
3
, TiF
3
, ZnF
2
, AlF
3
, ZnCl
, TiO
2
, MnF
, V
2
, CeCl
, ZrO
3
, VCl
, SnO
3
, LiI etc.) [15–
, etc.) [6,21–
4
4
2
2
0], and metal oxides (SiO
2
2
2
O
5
2
2
3
4
3
3] have been widely attempted to enhance hydrogen desorption
. Also reported in literature is that LiBH can
be destabilized with a variety of metal hydrides and metal complex
3
)
4/3ꢀBH
4
properties of LiBH
4
4
4
4
2
2
,
⇑
Corresponding author. Tel./fax: +86 411 84379583.
E-mail address: xzt@dicp.ac.cn (Z. Xiong).
4
2
2
d+
dꢁ
0
925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.09.100

L. Liu et al. / Journal of Alloys and Compounds 552 (2013) 98–101
99
and the establishment of a strong B–N bond should be the driving
forces for the favorable dehydrogenation of these B–N–H compos-
d+
ites. Therefore, combining LiBH
NH) could be a feasible approach for lowering the dehydrogena-
tion temperature of LiBH
In this study, melamine (C
lid organic amine with 1,3,5-triazine skeleton and containing
.76 wt.% of protic Hd+, was employed to interact with LiBH
. A
complex hydride of 3LiBH was formed upon ball milling
ꢀC
a mixture of LiBH and C with a molar ratio of 3/1. 3LiBH
starts to liberate hydrogen at ca. 100 °C, which is 190 °C
lower than that of LiBH . Within the temperature range of 100–
40 °C, it can evolve 7.83 wt.% or 7.5 equivalent of hydrogen. The
great difference between the desorption profiles of 3LiBH
ꢀC
and LiBH was believed to be mainly due to the combination of H
NH ) and H (BH
4
with hydrides with protic H
(
4
.
3
6 6
N H ), a commercially available so-
4
4
4
3 6 6
N H
4
3
N
H
6 6
4
ꢀ
3 6 6
C N H
4
3
4
3 6 6
N H
d+
4
dꢁ
(
2
4
) in 3LiBH
4
ꢀC N H .
3 6 6
Fig. 1. XRD patterns of LiBH
4
, C
3
N
6
H
6
and the post-milled LiBH
4 3
–C N
6
H
6
samples.
2
. Experimental section
LiBH
China) were commercial products and used without further purification. A Retsch
PM400 planetary mill was used to ball mill LiBH –C mixtures of different
LiBH /C molar ratios (6/1, 3/1, 3/2, and 1/1) at 150 rpm for 2 h. To reduce
the heat effect caused by mechanical milling, the mill was set to revolve for 60 s
in one direction and pause 15 s, and then to revolve in reversal direction. To avoid
oxygen and moisture contaminations, all the sample loadings were conducted in-
side a glove box that was filled with purified argon. To understand the mechanism
4 3 6 6
(95%) (Alfa Aesar, USA) and C N H (99%) (Kermel chemicals of Tianjin,
monoclinic structure with its lattice parameters of a = 17.4701 Å,
b = 17.6332 Å, c = 4.1749 Å, b = 99.7453° and V = 1267.54 Å .
3
4
3 6 6
N H
Raman spectra of LiBH
shown in Fig. S1. It can be seen that C
istic N–H stretches at 3420 and 3471 cm , while the internal
vibration of BH anion in LiBH can be detected at 2273 and
299 cm . Raman characterization shows that 3LiBH –1C
4
3
, C N
H
6 6
4 3 6 6
, and 3LiBH –1C N H , are
4
3 6 6
N H
3 6 6
N H presents its character-
ꢁ1
4
4
ꢁ1
2
4
3 6 6
N H
of dehydrogenation of 3LiBH
three desorption steps (the corresponding ending temperatures for three steps were
4
ꢀC
3 6 6
N H , intermediates were collected at the end of
ꢁ1
presents typical N–H stretches (3375 and 3453 cm ) and B–H
stretches (2287 and 2323 cm ), respectively. The red shifts of
ꢁ1
1
65, 210 and 340 °C, respectively) and then subject to some characterizations.
Temperature Programmed Desorption (TPD) was performed over a home-made
N–H stretches and the blue shifts of B–H stretches of in 3LiBH
4
–
B
system consisting of a micro-reactor and a Hidden Mass Spectrometer (MS, HPR20).
Ca. 20 mg sample was heated in argon stream at 2 °C/min and the tail gas was con-
ducted to MS for analysis. Differential Scanning Calorimetry (DSC) measurements
were carried out on a Netzsch STA449C thermal analysis system. A heating rate
of 2 °C/min was applied to 5 mg sample. Volumetric release measurements were
performed on a commercial gas reaction controller (Advanced materials Co. USA).
Ca. 100 mg sample was firstly purged in vacuum for 5 min and then heated to a de-
sired temperature at a ramping rate of 2 °C/min.
11
1
3
C N
H
6 6
4 2
prove the interaction of BH group and NH groups.
NMR characterization observed the only presence of BH
4
resonance
at d = ꢁ40.2 ppm and the single boron chemical environment also
indicated the formation of a single phase. In comparison with BH4
in LiBH
4
(d = ꢁ41.2 ppm) the subtle downfield shift (1 ppm) hinted
that boron in the new structure might not receive electron directly
from nitrogen (Fig. S2). Therefore, on the basis of these character-
ization results, solid–solid reaction between LiBH4 and C N H in
Powder X-ray diffraction (XRD) patterns were recorded over a 2h range of 5°–
9
0° on an X’Pert Pro (PANAnalytical) diffractometer with Cu Ka radiation at 40 kV
3
6
6
and 40 mA. N–H and B–H stretches in the reactants were monitored by using a Ra-
man spectroscopy (Renishaw, UK) equipping with a CCD detector and a He/Ne laser
generator emitting at a wavelength of 514 nm. FT-IR measurements were con-
ducted on a Varian 3100 unit in DRIFT mode. Powdery samples were mixed with
KBr and then loaded into an in situ cell for measurement. B solid-state magic-
angle-spinning (MAS) NMR experiments were conducted on a Bruker Advance III
the ball milling process can be xpressed as:
LiBH ! 3LiBH ꢀ C
þ C
In fact, solid–solid reaction between LiBH
3
4
3
N
6
H
6
4
3 6 6
N H
1
1
4
and C
3 6 6
N H seems to
be fairly easy. The milled LiBH and C
gether and gently ground using a agate pestle and a motar. In ten
minutes, there appeared the 3LiBH phase (Fig. S3).
ꢀC
Dehydrogenation properties of the prepared 3LiBH
ꢀC
4
3 6 6
N H powders were put to-
5
4
00 NMR spectrometer working at a frequency of 128.28 MHz and owning a
1
1
mm MAS NMR probe. The chemical shift of B resonance was referenced to BF3-
O at 0 ppm.
ꢀEt
2
4
3 6 6
N H
4
3 6 6
N H
complex hydride were investigated by means of TPD-MS, DSC
and volumetric release measurement. TPD-MS result in Fig. 2a
3
. Results and discussion
shows that 3LiBH
4
ꢀC
3 6 6
N H started to evolve hydrogen at ca.
No gas emission was observed during the process of ball milling
LiBH –C N H mixture with different molar ratios. XRD character-
4 3 6 6
100 °C, and three hydrogen peaks were observed at 159, 205 and
300 °C, respectively. No other gaseous products were detected by
MS during the whole TPD measurement (Fig. S4). In contrast,
izations on these milling residues, however, revealed the appear-
ance of a set of new diffraction peaks (Fig. 1). Clearly, it indicated
apparent emission of hydrogen from LiBH
peratures above 290 °C, which is 190 °C higher than the onset tem-
perature for hydrogen desorption from 3LiBH DSC
ꢀC
measurements on 3LiBH showed two exothermic signals
corresponding to the 1st and 3rd desorption steps, and the 2nd
hydrogen peak between them was mild endothermic. As generally
accepted, an exothermic or very mild endothermic desorption
should not be thermodynamically favored in its reverse absorption
4
was observed at tem-
the occurrence of solid-solid reaction between LiBH
to form new phase. To determine the stoichiometry of the reaction,
LiBH /C mixtures with molar ratios of 6/1, 3/1, 3/2 and 1/1
were ball milled under the same conditions. It was found that
LiBH phase persisted in the post-milled 6LiBH –1C sample,
while C phase was observed in the post-milled 3LiBH
and 1LiBH –1C samples (Fig. 1). In the case of
–1C , only the above mentioned set of diffraction peaks
and C were com-
pletely disappeared (Fig. 1). It suggested that LiBH would interact
with C at a stoichiometry of 3/1 gave rise to the formation of
4 3 6 6
and C N H
4
3 6 6
N H .
4
3
N
H
6 6
4 3 6 6
ꢀC N H
4
4
3 6 6
N H
3
N
6
H
6
4
–
2
3
C N
H
6 6
4
3 6 6
N H
3
LiBH
4
3
N
6
H
6
process. In this sense, hydrogen evolved by 3LiBH
4
ꢀC
3 6 6
N H will not
were presented and all the peaks of LiBH
4
3
N
H
6 6
be rechargeable. In fact, almost all the investigated B–N based
hydrogen storage systems were found releasing hydrogen exother-
mically and this is understandable from the fact that chemical
bonding between B and N will be formed and strengthened follow-
ing hydrogen desorption and the dehydrogenation products are
4
3 6 6
N H
a new complex hydride. After indexing with the TREOR program,
the new set of diffraction peaks was found matching well with a

1
00
L. Liu et al. / Journal of Alloys and Compounds 552 (2013) 98–101
Fig. 3. 11B MAS NMR spectra of (a) 3LiBH
intermediates collected at (b) 165 °C, (c) 210 °C and (d) 340 °C. Spinning side band
4
ꢀC
3 6 6
N H and its dehydrogenation
was marked with asterisk.
0
ppm correspond to tetra-coordinated BIV atoms [48,49]. There-
fore, the resonances at ꢁ25.5 and 15.2 ppm can be assigned to
tetra-coordinated BNH site and tri-coordinated BNH site respec-
tively [48,49]. As heating in progress to 210 °C, two overlapped
peaks at about 18.8 and 23.5 ppm appeared, which should corre-
spond to the formation of BN and/or BN H. In the meantime,
persistence of BH resonance further evidenced itself a characteris-
tic signal of LiBH . Thermal decomposition of LiBH will only occur
at temperatures above 290 °C. At the end of whole dehydrogena-
tion the BH
signal was vanished and those two overlapped 11
3
2
11
B
3
2
4
4
4
Fig. 2. TPD-H
2
, DSC and volumetric release measurements on 3LiBH
4
ꢀC
3 6 6
N H .
4
B
peaks intensified to become the dominant peaks. FT-IR character-
izations on the dehydrogenation intermediates revealed that the
used to be highly stable materials, such as BN and Li
ric release measurements indicated that the first two desorption
steps both evolved 3 equiv. of H and in the last step 1.5 equiv. of
was desorbed. In total, 7.5 equiv. or 7.83 wt.% of H can be
evolved by 3LiBH at temperatures below 340 °C. It is rea-
ꢀC
sonable to speculate from the chemical composition viewpoint
that there are three BH and three NH anions in 3LiBH
ꢀC
) will lead to an evo-
3 2
BN . Volumet-
CN heterocycles in 3LiBH
the 2nd desorption step because at end of 1st step its characteristic
stretching band in the range of 1200–1600 cm
4
ꢀC
3 6 6
N H structure was broken during
2
H
2
2
ꢁ1
could still be
4
3 6 6
N H
observed, but afterward the band disappeared from the spectra
of dehydrogenation intermediate collected at 210 °C (Fig. 4).
From above results, we can conclude that complexation
4
d+
2
dꢁ
4
3 6 6
N H
and a combination of H (NH
2 4
) and H (BH
between C
process, resulting in B–H and N–H bonds that are in contact on a
molecular scale in 3LiBH complex. The interaction be-
ꢀC
) in the structure of 3LiBH
ꢀC
complex tends to promote hydrogen release, and leads to desorp-
tion of H at much low temperature. It is worth of mentioning that
3 6 6 4
N H and LiBH take place during the solid reaction
d+
lution of 3 equiv. of H ; as two protic H residing in NH , such
2
2
desorption process proceeds twice. The last desorption step may
3 6 6
N H
dꢁ
4
relate to a different mechanism since only hydridic H is left in
d+
dꢁ
tween H (NH
2
) and H (BH
4
4
3 6 6
N H
structure. Considering that the 1st desorption step was exothermic
and the emitted reaction heat would help to drive the 2nd desorp-
2
tion step, we therefore heated 3LiBH
4
ꢀC
3
N
6
H
6
isothermally at
1
80 °C and found that ca. 5.7 equivalent of H
2
or 6.0 wt.% of hydro-
gen was released in 2 h (Fig. S5).
No phase was identified in the intermediates collected at the
end of 1st and 2nd dehydrogenation steps (Fig. S6). However, we
can still speculate from the absence of LiBH
4
pattern at the first
checking point that the dehydrogenation was likely through the
d+
dꢁ
combination of H (NH
2
) and H
4
(BH ). Diffraction peaks of
Li CN are detected in the final dehydrogenation product, and its
2
2
appearance indicates the decomposition of CN heterocycle of mel-
amine. 1 B MAS NMR characterization on the dehydrogenation
product of step one showed a dominant peak at ꢁ41.3 ppm and
other three minor peaks at ꢁ25.5, 0.6, and 15.2 ppm, respectively
1
(
Fig. 3). The d = ꢁ41.3 ppm signal was ascribed to LiBH
4
that is
structure during the heating
diffraction peaks in XRD pattern sug-
gests that its content must be very low at this stage. However, due
to the relatively high response ratio of BH anion even small
amount of LiBH will result in a strong NMR signal. As generally
likely isolated from 3LiBH
process. The absence of LiBH
4 3 6 6
ꢀC N H
4
4
4
Fig. 4. FT-IR spectra of LiBH
4
,
C
3
N
6
H
6
,
3LiBH
4
ꢀC
3 6 6
N H and dehydrogenated
accepted, chemical shifts ranging from 0 to 40 ppm correspond
to tri-coordinated B atoms, while those in the range of ꢁ40 to
3LiBH ꢀC N H samples collected after heating at 165, 210, and 340 °C in vacuum
4
3
6
6
III
mode.

L. Liu et al. / Journal of Alloys and Compounds 552 (2013) 98–101
101
decomposition of C
00 °C. Here, the release of NH
suppressed due to the interaction between H (NH
and no NH
from TPD-MS analysis. According to the FT-IR and solid B MAS
NMR results, the dehydrogenation mechanism of 3LiBH
3
N
H
6 6
releases only NH
3
at temperatures above
[11] P. Ngene, M. van Zwienen, P.E. de Jongh, Chem. Commun. 46 (2010) 8201–
203.
8
3
3
from 3LiBH
4
ꢀC
3
N
6
H
6
complex was
[
[
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d+
dꢁ
2
) and H (BH
4
),
3
signal could be observed in the desorption process
1
1
[14] X.B. Yu, Z. Wu, Q.R. Chen, Z.L. Li, B.C. Weng, T.S. Huang, Appl. Phys. Lett. 90
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(
ꢀC
4 3
N
6
H
6
[
[
[
[
[
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hydrogen release occurs through the combination of protonic H
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dꢁ
(
2 4 2
NH ) and hydridic H (BH ) into H . Supplementary material .
4
. Conclusions
8
299–8305.
3
LiBH
4
ꢀC
3
N
6
H
6
complex has been synthesized through solid
and C . Pyrolysis of 3LiBH was com-
[
20] I. Llamas-Jansa, N. Aliouane, S. Deledda, J.E. Fonnelop, C. Frommen, K.
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reaction of LiBH
4
3
N
H
6 6
4
ꢀC N H
3 6 6
posed of three steps and more than 7.83 wt.% of hydrogen can be
evolved at temperatures below 340 °C. The dehydrogenation of
LiBH
[
[
d+
3
4
ꢀC
3
N
6
H
6
may be relevant to a combination of protic H in
and hydridic H in LiBH
309.
dꢁ
NH in C
2
3
N
6
H
6
4
, thus yeilding hydrogen
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as the only gaseous product. It is our expectation that progress
made in this study would help to open a promising approach for
tuning the hydrogen storage properties of other existing borohy-
drides or amines.
[
[
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[
[
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The authors acknowledge the financial support from the 973
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20971120, 50901070, 51101144, and 21273229).
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