Improved dehydrogenation properties of Ca(BH4) 2-LiNH2 combined system

Article abstract of DOI:10.1039/c0dt00294a

Ca(BH₄)₂-LiNH₂ combined system is shown to release hydrogen at much lower temperature compared to the pure Ca(BH ₄)₂. The improved dehydrogenation in this system can be ascribed to a combination reaction between [BH₄] and [NH₂] based on the reaction mechanism of positive H and negative H. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0dt00294a

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Improved dehydrogenation properties of Ca(BH4)2-LiNH2 combined system†
Hailiang Chu, Zhitao Xiong, Guotao Wu, Jianping Guo, Teng He and Ping Chen*
Received 12th April 2010, Accepted 28th August 2010
DOI: 10.1039/c0dt00294a
Ca(BH4)2-LiNH2 combined system is shown to release hy-
drogen at much lower temperature compared to the pure
Ca(BH4)2. The improved dehydrogenation in this system can
be ascribed to a combination reaction between [BH4] and
[NH2] based on the reaction mechanism of positive H and
negative H.
Metal borohydrides have attracted much attention as hydrogen
storage materials, which exhibit higher weight capacity for hydro-
gen and are therefore obvious candidates to meet the capacity
requirements for storing hydrogen.1 Among the borohydrides,
LiBH4, with a gravimetric capacity of 18.3%, is the most ex-
tensively studied.2 However, this compound is too stable to be
used in hydrogen storage applications. More recently, Ca(BH4)2,
which has more favorable thermodynamics (32 kJ mol-1 H2,
estimated in theory)3 than LiBH4, while maintaining attractive
hydrogen capacity (11.4 wt.%), has been acknowledged as one of
the potential candidates for hydrogen storage materials.4
It was observed that a-Ca(BH4)2, heated under an inert or
hydrogen atmosphere, was first transformed to the b-phase of
◦
Ca(BH4)2 at around 170 C. Hydrogen release started at around
◦
350 C in two endothermic steps, and 9.4 wt.% weight loss was
detected up to 530 ◦C. The decomposition of Ca(BH4)2 was
assumed to be the following: Ca(BH4)2 → 2/3 CaH2 + 1/3
CaB6 + 10/3 H2. However, Wang et al. found that CaB12H12 was
in the dehydrogenated products by theoretical and experimental
studies,5 and the presence of the dodecaborane closomolecule
(B12H12) has been observed after decomposition of LiBH4 and
Mg(BH4)2.6 To further improve the dehydrogenation of Ca(BH4)2,
Kim et al. recently investigated the effects of ball milling and
additives on the dehydriding behavior of Ca(BH4)2 and found that
the initial dehydrogenation temperature of Ca(BH4)2 is reduced
by the addition of NbF5.7 Herein, we report a reactive binary
mixture, Ca(BH4)2-LiNH2, that presents a significant decrease in
the dehydrogenation temperature as compared to the constituent
compounds.
Ca(BH4)2 was synthesized by means of the metathesis of CaCl2
and NaBH4 in tetrahydrofuran (THF) solution according to our
previous report: CaCl2 + 2 NaBH4 → Ca(BH4)2 + 2 NaCl.8 Fig. 1
shows the TPD-MS and volumetric release results for Ca(BH4)2-
LiNH2 samples compared with the pure Ca(BH4)2. For Ca(BH4)2,
two main peaks of hydrogen evolution at 361 and 435 ◦C are
observed in the TPD-MS curve. The above results are comparable
with a previous report for Ca(BH4)2 decomposed under argon
as a carrier gas.9 In the case of the Ca(BH4)2-2LiNH2 mixture,
Fig. 1 TPD-MS (top) and volumetric release (bottom) results for
(i) Ca(BH4)2, (ii) Ca(BH4)2-LiNH2, (iii) Ca(BH4)2-2LiNH2 and (iv)
Ca(BH4)2-3LiNH2.
two distinct hydrogen release events were observed with peak
temperatures at 306 and 396 ◦C, which are much lower than
those for the pure Ca(BH4)2. However, an apparent NH3 release
appeared from 50 to 300 ◦C. To further clarify the different
decomposition properties, volumetric release measurements were
conducted for these two samples, as shown in Fig. 1(b). The
results for the pure Ca(BH4)2 show about 8.1 wt.% volumetric
◦
hydrogen release by 480 C, which is slightly lower than the TG
results (more than 9 wt.%) for Ca(BH4)2 prepared by drying the
commercial Ca(BH4)2·2THF adduct.10 It should be noted that
hydrogen released from the samples is gradually accumulated
in the sample chamber. At the end of the measurement the
sample reactor is filled with hydrogen of ca. 45 psi. It is likely
due to the equilibrium pressure limitation that not all hydrogen
can be released from the samples. For the Ca(BH4)2-2LiNH2
sample, an apparent two step decomposition is also observed,
Dalian Institute of Chemical Physics, Dalian, 116023, China. E-mail:
pchen@dicp.ac.cn; Fax: +86 411 84685940; Tel: +86 411 84379583
† Electronic supplementary information (ESI) available: Experimental
details, FTIR, XRD patterns of Ca(BH4)2-LiNH2 samples and TG-DSC
of Ca(BH4)2-2LiNH2. See DOI: 10.1039/c0dt00294a
◦
with onset temperature for dehydrogenation at 230 C, which is
about 100 ◦C lower than that for the pure Ca(BH4)2, indicating
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significant improvement in the dehydrogenation temperature. The
total dehydrogenation capacity is 7.2 wt.% up to 480 C for the
observed in the spectra of the as-prepared samples with the B–H
absorption band at about 2300 cm-1. However, the features for the
N–H bond that can be ascribed to LiNH2, with its characteristic
bands at 3263 and 3325 cm-1 have shifted to lower wave number,
even though no hydrogen or ammonia was released from the
mixture during preparation. Similar results are observed in FT-
IR spectra (Fig. S3, ESI†). This suggests that Ca(BH4)2-LiNH2
is not a simple physical mixture, but rather that new phases may
be formed during the preparation. The red-shift of N–H bonds
in Ca(BH4)2-LiNH2 may result from its complexation with B–H
bonds, to which similar results have been found in LiBH4-LiNH2
system,13 but it is different from Mg(BH4)2-LiNH2 system, in
which the characteristic bands of N–H bond almost disappeared
after ball milling treatment.11 The XRD patterns (Fig. S4, ESI†)
also give evidence for the formation of new phases. The as-
prepared Ca(BH4)2-LiNH2 samples are different from each other
with no LiNH2 and Ca(BH4)2 phases observed, although they
do not have crystallinity as good as the starting materials. In the
case of Ca(BH4)2-2LiNH2 and Ca(BH4)2-3LiNH2 samples, similar
results from XRD, FT-IR and Raman are observed. It should be
noted that B–H stretching modes of post-milled samples in FT-IR
results seem to be narrower and shift relative to Ca(BH4)2 to the
lower frequencies, especially for Ca(BH4)2-2LiNH2 and Ca(BH4)2-
3LiNH2 (Fig. S3, ESI†).
The activation energy, Ea, during the dehydrogenation for the
two steps is estimated by Kissinger’s method.14 The TPD-MS
profiles at heating rates of 1.5, 2, 2.5, and 3 K min-1 as a
function of temperature are measured for the Ca(BH4)2-LiNH2
(mole ratio 1 : 2) sample, as shown in Fig. 3. From the peak
temperature, TP, observed at the heating rate, b, the Kissinger
plots, i.e., ln [b/TP2] as a function of the inverse of TP, are given
in the inset of the Fig. 3. From the slope of the straight line, the
activation energy Ea for the first and second step dehydrogenation
is determined to be about 77.3 and 93.2 kJ mol-1, respectively. To
clarify the chemical reactions before and after dehydrogenation,
XRD for samples collected after volumetric release measurements
at different temperatures, i.e., 320 ◦C, 400 ◦C and 480 ◦C,
are performed for Ca(BH4)2-2LiNH2 shown in Fig. 4. After
dehydrogenation to 320 ◦C, the XRD patterns correspond to CaO
and LiCa4(BN2)3,8 which indicates that LiCa4(BN2)3 is formed in
◦
Ca(BH4)2-2LiNH2 sample. Quantitative analysis of evolved gas
after volumetric release measurement indicates that there is only
5900 ppm NH3 in the exhaust gas. Thus the weight loss due
to NH3 contribution is as low as about 0.36 wt.%, which can
be neglected. Furthermore, the onset decomposition temperature
in TPD is slightly higher than that in volumetric release. These
differences may be related to the fact that the decomposition
reaction is performed under different pressures in these two cases.
The TPD measurement is conducted under dynamic argon gas in
open system, while the volumetric release is run under vacuum in
closed reactor.
Further investigation of Ca(BH4)2-LiNH2 with mole ratios of
1 : 1 and 1 : 3 were monitored by TPD and volumetric release
measurements. The temperature dependences of H2 and ammonia
signals detected by MS and quantitative H2 evolution are also
illustrated in Fig. 1(a) and (b), respectively. It revealed that,
from volumetric release measurements, the onset temperature for
hydrogen desorption shifted to higher temperature with increased
LiNH2 content, although it was still lower than that for the pure
Ca(BH4)2. Clearly it can release about 8.0 wt.% and 7.6 wt.%
hydrogen by 480 ◦C for Ca(BH4)2-LiNH2 and Ca(BH4)2-3LiNH2,
respectively. TPD-MS curves show that the peak temperature for
the first-step dehydrogenation increases with increased LiNH2
content, and some ammonia is evolved at temperature range from
50 to 300 ◦C for these two samples. Quantitative analysis of evolved
gas evidenced that the amount of ammonia, i.e., 207 ppm and
13 100 ppm for Ca(BH4)2-LiNH2 and Ca(BH4)2-3LiNH2 samples,
increases with increased LiNH2 content. Analogous results were
observed for Mg(BH4)2-LiNH2 system.11 Compared with the pure
LiNH2,12 the NH3 release temperature from the Ca(BH4)2-LiNH2
mixtures decreases significantly, suggesting that, probably due to
the formation of Ca–N bonds between the two compounds, the
Ca(BH4)2 may weaken the Li–N bonds in LiNH2, causing them to
break at lower temperature. This suggests that Ca ions in Ca(BH4)2
may play a crucial role in determining the amount of ammonia
release.
To gain further insight into the chemical reactions occurring in
the ball milling process, Raman, FT-IR and XRD measurements
were carried out. Fig. 2 shows Raman spectra of the samples before
and after ball milling. Typical features of the [BH4] group can be
Fig. 2 Raman spectra of (i) LiNH2, (ii) Ca(BH4)2, (iii) Ca(BH4)2-LiNH2,
Fig. 3 TPD-MS results for Ca(BH4)2-2LiNH2 at various heating rates.
(iv) Ca(BH4)2-2LiNH2 and (v) Ca(BH4)2-3LiNH2.
The inset shows the Kissinger plots.
10586 | Dalton Trans., 2010, 39, 10585–10587
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Ca(BH4)2-LiNH2 material is regarded as a potential candidate for
hydrogen storage if the evolution of ammonia evolution during
hydrogen desorption will be suppressed and the dehydrogenation
temperature will be further decreased.
The authors acknowledge the financial support from the
Hundred Talents Projects of CAS (KGCX2-YW-806) and Knowl-
edge Innovation Program of CAS (KJCX2-YW-H21), Science
and Technology Plan of Dalian (2009J22DW016), 863 Project
(2009AA05Z108), 973 Project (2010CB631304), and National
Natural Science Foundation of China (20971120, 10979051 and
20973162).
Notes and references
1 P. Wang and X. D. Kang, Dalton Trans., 2008, 5400; J. H. Kim, S. A.
Jin, J. H. Shim and Y. W. Cho, J. Alloys Compd., 2008, 461, L20; G.
Severa, E. Ronnebro and C. M. Jensen, Chem. Commun., 2010, 46, 421;
T. Jaron and W. Grochala, Dalton Trans., 2010, 39, 160.
2 J. J. Vajo, S. L. Skeith and F. Mertens, J. Phys. Chem. B, 2005, 109,
3719; J. Xu, X. B. Yu, J. Ni, Z. Q. Zou, Z. L. Li and H. Yang, Dalton
Trans., 2009, 8386.
3 K. Miwa, M. Aoki, T. Noritake, N. Ohba, Y. Nakamori, S. Towata,
A. Zu¨ttel and S. Orimo, Phys. Rev. B: Condens. Matter Mater. Phys.,
2006, 74, 155122.
4 J. H. Kim, S. A. Jin, J. H. Shim and Y. W. Cho, Scr. Mater., 2008, 58,
481; E. Ronnebro and E. H. Majzoub, J. Phys. Chem. B, 2007, 111,
12045.
5 L. L. Wang, D. D. Graham, I. M. Robertson and D. D. Johnson, J.
Phys. Chem. C, 2009, 113, 20088.
6 S. J. Hwang, B. C. Jr. Bowman, J. W. Reiter, J. R. Rijssenbeek, G. L.
Soloveichik, J. C. Zhao, H. Kabbour and C. C. Ahn, J. Phys. Chem.
C, 2008, 112, 3164; H. W. Li, K. Miwa, N. Ohba, T. Fujita, T. Sato, Y.
Yan, S. Towata, M. W. Chen and S. Orimo, Nanotechnology, 2009, 20,
204013.
7 J. H. Kim, J. H. Shim and Y. W. Cho, J. Power Sources, 2008, 181, 140.
8 H. L. Chu, Z. T. Xiong, G. T. Wu, J. P. Guo, X. L. Zheng, T. He, C. Z.
Wu and P. Chen, Chem.–Asian J., 2010, 5, 1594.
9 M. Aoki, K. Miwa, T. Noritake, N. Ohba, M. Matsumoto, H. W. Li, Y.
Nakamori, S. Towata and S. Orimo, Appl. Phys. A: Mater. Sci. Process.,
2008, 92, 601.
10 Y. Kim, D. Reed, Y. S. Lee, J. Y. Lee, J. H. Shim, D. Book and Y. W.
Cho, J. Phys. Chem. C, 2009, 113, 5865.
11 X. B. Yu, Y. H. Guo, D. L. Sun, Z. X. Yang, A. Ranjbar, Z. P. Guo, H.
K. Liu and S. X. Dou, J. Phys. Chem. C, 2010, 114, 4733.
12 J. H. Yao, C. Shang, K. F. Aguey-Zinsou and Z. X. Guo, J. Alloys
Compd., 2007, 432, 277.
13 P. A. Chater, W. I. F. David, S. R. Johnson, P. P. Edwards and P. A.
Anderson, Chem. Commun., 2006, 2439.
14 H. E. Kissinger, Anal. Chem., 1957, 29, 1702.
Fig. 4 XRD results for Ca(BH4)2-2LiNH2 samples after volumetric
release at (i) 320 ◦C, (ii) 400 ◦C, and (iii) 480 ◦C.
the first step for hydrogen desorption. When the heat temperature
is increased to 400 and 480 ◦C, CaH2 is observed in XRD patterns,
and the peak intensity of LiCa4(BN2)3 decreases with increased
temperature, which indicates that LiCa4(BN2)3 may participate
in the second reaction for hydrogen desorption. In addition, we
run TG-DSC measurements of post-milled samples. For example,
TG-DSC results of post-milled Ca(BH4)2-2LiNH2 sample (Fig.
S5, ESI†) show that there are two main peaks, i.e., an exothermic
peak at 302 ◦C and an endothermic peak at 390 ◦C. This implies
that there could be some reversibility in this system. However, our
preliminary attempt of rehydrogenating the post-dehydrogenated
powder under a H2 pressure of 50 bar in the temperature range of
20–300 ◦C was unsuccessful.
The improved decomposition in the Ca(BH4)2-LiNH2 system
might be due to a combination reaction of [BH4] with [NH2], in
which the [BH4] consists of negatively charged hydrogen, while
positively charged hydrogen is bonded to N atoms in [NH2] group.
This assumption is experimentally confirmed by formation of
HD when we heated the post-milled Ca(BD4)2-2LiNH2 sample
(Fig. S6, ESI†). In this case, the reaction of Hd+ + Hd- → H2
might be one of the driving forces making the chemical reaction
take place, which has been demonstrated in the Mg(NH2)2/LiH,
LiNH2/CaH2, LiNH2BH3, and Ca(NH2BH3)2 systems,15 as well
as the reported result on Mg(NH3)nCl2/LiBH4 system.16
15 Z. T. Xiong, J. J. Hu, G. T. Wu, P. Chen, W. F. Luo, K. Gross and J.
Wang, J. Alloys Compd., 2005, 398, 235; H. L. Chu, Z. T. Xiong, G.
T. Wu, T. He, C. Z. Wu and P. Chen, Int. J. Hydrogen Energy, 2010,
35, 8317; Z. T. Xiong, C. K. Yong, G. T. Wu, P. Chen, W. Shaw, A.
Karkamkar, T. Autrey, M. O. Jones, S. R. Johnson, P. P. Edwards and
W. I. F. David, Nat. Mater., 2008, 7, 138; H. V. K. Diyabalanage, R. P.
Shrestha, T. A. Semelsberger, B. L. Scott, M. E. Bowden, B. L. Davis
and A. K. Burrell, Angew. Chem., Int. Ed., 2007, 46, 8995.
16 L. Gao, Y. H. Guo, G. L. Xia and X. B. Yu, J. Mater. Chem., 2009, 19,
7826.
In summary, hydrogen desorption of Ca(BH4)2-LiNH2 binary
system is released at lower temperature compared with the pure
Ca(BH4)2. For example, Ca(BH4)2-3LiNH2 has a desorption
capacity of 7.2 wt.% held at ~ 300 ◦C for 3 h, with the onset
temperature of dehydrogenation at 200 ◦C. It is proposed that
the improved dehydrogenation in Ca(BH4)2-LiNH2 results from
a combination reaction between the [BH4] and [NH2]. Then the
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 10585–10587 | 10587
©