Hydrogen storage properties of LiBH4 destabilized by SrH 2

Article abstract of DOI:10.1016/j.jallcom.2012.09.138

In this work, we have succeeded in destabilizing LiBH₄ by the addition of SrH₂, via the reaction 6LiBH₄ + SrH ₂ → SrB₆ + 6LiH + 10H₂ with a theoretical hydrogen capacity of 9.1 wt.%. According to the van't Hoff and Arrhenius equations, the dehydrogenation enthalpy change and activation energy for the LiBH₄/SrH₂ system were experimentally determined to be 48 kJ/mol H₂ and 64 kJ/mol, respectively. Both are remarkably reduced in comparison with the pristine LiBH₄, which is responsible for the improved dehydrogenation property of the LiBH₄/SrH₂ system. The dehydrogenated products SrB₆ + 6LiH can be rehydrogenated to form LiBH₄ and LiSrH₃ at 723 K under an initial hydrogen pressure of 8.0 MPa.

Full text of DOI:10.1016/j.jallcom.2012.09.138

Journal of Alloys and Compounds 551 (2013) 8–11
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Hydrogen storage properties of LiBH4 destabilized by SrH2
D.M. Liu a,b, W.J. Huang a, T.Z. Si a, Q.A. Zhang a,
⇑
a School of Materials Science and Engineering, Anhui University of Technology, Maanshan, Anhui 243002, PR China
b State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, we have succeeded in destabilizing LiBH4 by the addition of SrH2, via the reaction 6LiBH4 + -
SrH2 ? SrB6 + 6LiH + 10H2 with a theoretical hydrogen capacity of 9.1 wt.%. According to the van’t Hoff
and Arrhenius equations, the dehydrogenation enthalpy change and activation energy for the LiBH4/
SrH2 system were experimentally determined to be 48 kJ/mol H2 and 64 kJ/mol, respectively. Both are
remarkably reduced in comparison with the pristine LiBH4, which is responsible for the improved dehy-
drogenation property of the LiBH4/SrH2 system. The dehydrogenated products SrB6 + 6LiH can be rehy-
drogenated to form LiBH4 and LiSrH3 at 723 K under an initial hydrogen pressure of 8.0 MPa.
Ó 2012 Elsevier B.V. All rights reserved.
Received 20 August 2012
Received in revised form 26 September
2012
Accepted 26 September 2012
Available online 5 October 2012
Keywords:
Hydrogen storage materials
Lithium borohydride
Destabilized hydride system
Hydrogen storage properties
1. Introduction
tion to form LiBH4. Following this strategy, the LiBH4/CaH2 system
with a small amount of catalyst was also investigated [7,21–24]. It
Hydrogen, as a highly efficient and clean secondary energy car-
rier, is an ideal substitute for conventional fossil fuels in the future.
For large-scale utilization of hydrogen as an energy carrier, hydro-
gen storage systems with high efficiency and safety should be devel-
oped. Various solid-state materials, such as metal hydrides, alanates
and borohydrides, have been widely investigated for this purpose.
Among them, LiBH4 is one of the most promising candidates for
on-board hydrogen storage, due to its high gravimetric (18.4 wt.%)
and volumetric (121 kg/m3) hydrogen density [1–3]. However,
LiBH4 alone has a reaction enthalpy change as high as 74 kJ/mol
H2 for its decomposition into LiH and B, and therefore a dehydroge-
nation temperature above 370 °C is required under 0.1 MPa hydro-
gen pressure [4]. Furthermore, the dehydrogenation process of
LiBH4 is rather sluggish and the rehydrogenation is hardly possible
under mild temperature and pressure conditions [4,5].
was found that a dehydrogenation enthalpy change of 56.5 kJ/mol
H2 could be obtained by incorporating LiBH4 with CaH2, based on
the formation of calcium hexaboride CaB6 [21].
In view of the previous results mentioned above, a question
arises as to whether other binary alkaline-earth metal hydrides
than MgH2 and CaH2 can be used as the destabilizing additives
for LiBH4. Stimulated by this question, the dehydrogenation and
rehydrogenation properties, as well as the destabilization mecha-
nism involved for the LiBH4/SrH2 system have been investigated
in this work. No catalysts were doped into the LiBH4/SrH2 system,
which enables the investigation of the pure effect from SrH2.
2. Experimental details
2.1. Sample preparation
During the last decade, LiBH4 multicomponent reactive systems
have been intensively investigated to destabilize LiBH4 and en-
hance its dehydrogenation kinetics [6–20]. For example, Vajo
et al. [6] reported that LiBH4 could release hydrogen with a re-
duced reaction enthalpy by the addition of MgH2, because of the
change in reaction pathway as described below:
Commercial LiBH4 powder (95%, Alfa Aesar) was used as-received without fur-
ther purification. SrH2 powder was synthesized by reacting metallic Sr scraps (99%,
Alfa Aesar) with hydrogen (99.999%). The 6LiBH4 + SrH2 mixture was ball-milled
under 0.5 MPa hydrogen pressure at a rotation speed of 400 rpm for only 2 h by
using a QM-1SP planetary mill. Stainless steel vials (250 mL in volume) and balls
(10 mm in diameter) were used. The ball to sample weight ratio was 20:1. To avoid
air-exposure, all sample handling was carried out in an Ar-filled glove box equipped
with a purification system keeping the typical O2/H2O levels below 1 ppm.
2LiBH4 þ MgH2 ! MgB2 þ 2LiH þ 4H2
ð1Þ
2.2. Sample characterization
Moreover, the formation of MgB2 could overcome the chemical
inertness of pure boron, thus greatly improving the rehydrogena-
Dehydrogenation and rehydrogenation properties of the samples were exam-
ined based on the volumetric method by using a carefully calibrated Sieverts-type
apparatus (Suzuki Shokan Co., Ltd., Japan). The temperature dependence of dehy-
drogenation was determined by heating the sample from ambient temperature to
⇑
Corresponding author. Tel.: +86 555 2311891; fax: +86 555 2311570.
E-mail address: zhang03jp@yahoo.com.cn (Q.A. Zhang).
0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.09.138

D.M. Liu et al. / Journal of Alloys and Compounds 551 (2013) 8–11
873 K at a heating rate of 2 K/min. Isothermal dehydrogenation was performed by
9
quickly heating and then keeping the sample at a given temperature. The hydrogen
back pressure for the above temperature ramp and isothermal dehydrogenation
examinations was below 0.1 MPa. Pressure–composition (P–C) isotherms were
measured at 693, 723 and 753 K to investigate the thermodynamic property of
the LiBH4/SrH2 system in the dehydrogenation process. The rehydrogenation exper-
iment was carried out at 723 K under an initial hydrogen pressure of 8.0 MPa.
To elucidate the phase components of the ball-milled, dehydrogenated and
rehydrogenated 6LiBH4 + SrH2 mixtures, X-ray diffraction (XRD) measurements
SrH2
LiBH4
SrB6
LiH
(b) Dehydrogenated
were performed using a Rigaku D/Max 2500VL/PC diffractometer with Cu Ka radi-
ation at 50 kV and 150 mA. The XRD samples were loaded and sealed in a special
holder that can keep the sample under argon atmosphere in the course of
measurement.
(a) As-milled
3. Results and discussion
20 25 30 35 40 45 50 55 60 65 70 75
Two Theta (degree)
3.1. Dehydrogenation behavior
3.1.1. Thermal behavior upon dehydrogenation
Fig. 2. XRD patterns of the LiBH4/SrH2 system: (a) as-milled and (b) dehydroge-
For understanding the dehydrogenation stage of the LiBH4/SrH2
system, Fig. 1 gives the amount of hydrogen desorbed as a function
of temperature for the as-milled 6LiBH4 + SrH2 mixture at a heating
rate of 2 K/min. For comparison, the hydrogen desorption curve of
the pristine LiBH4 sample ball-milled for 2 h was also measured. It
is observed that dehydrogenation of the LiBH4/SrH2 system starts
around 490 K, and the majority of hydrogen is released in the
temperature range of 660–760 K. A dehydrogenation amount of
8.7 wt.% was totally achieved. In contrast, the pristine LiBH4 re-
leased hydrogen sluggishly, and the amount of hydrogen desorbed
up to 873 K is only 6.8 wt.%. This value is much lower than the the-
oretical one (13.8 wt.%). Evidently, the thermal stability of LiBH4 can
be remarkably reduced by using SrH2 as a destabilizing additive.
It should be noted from Fig. 1 that dehydrogenation of the
LiBH4/SrH2 system proceeds in a single step, i.e., through a direct
reaction between LiBH4 with SrH2. In comparison with the two-
step dehydrogenation in the LiBH4/MgH2 system [25], such a fea-
ture enables the LiBH4/SrH2 system to release hydrogen prior to
the decomposition of SrH2. That is, LiBH4 is also effective in lower-
ing the stability of SrH2, which is similar to the results observed in
the LiBH4/CaH2 and LiBH4/CaF2 systems [7,21,26].
nated at 723 K.
6LiBH4 þ SrH2 ! SrB6 þ 6LiH þ 10H2
The theoretical hydrogen amount desorbed from this reaction
can be calculated to be 9.1 wt.%, which is consistent with the mea-
sured value of 8.7 wt.% (see Fig. 1).
ð2Þ
3.1.2. Dehydrogenation thermodynamics
In order to evaluate the dehydrogenation thermodynamics of
the LiBH4/SrH2 system, P–C isotherms (see Fig. 3) were measured
at 693, 723 and 753 K, respectively. It can be seen that the dehy-
drogenation isotherms have a well-defined plateau region, and
the equilibrium pressures vary from 0.49 MPa at 693 K to
0.94 MPa at 753 K. It is well known that the enthalpy change
(D
H) and entropy change (
DS) for a dehydrogenation reaction
can be calculated from the van’t Hoff equation:
ln P ¼ ½ꢀ H=ðRTÞꢁ þ S=R
D
D
ð3Þ
where P is the equilibrium pressure at absolute temperature T, and
R is the gas constant. Fig. 4 gives the van’t Hoff plot of the LiBH4/
SrH2 system by using the dehydrogenation equilibrium pressures
at 4.0 wt.% (see Fig. 3). According to the equation shown in Fig. 4,
the enthalpy and entropy changes for the dehydrogenation of the
LiBH4/SrH2 system can be determined to be 48 kJ/mol H2 and 82
Fig. 2 presents the XRD patterns of the 6LiBH4 + SrH2 samples
after ball-milling and dehydrogenation at 723 K. It can be seen that
the as-milled sample consists mainly of LiBH4 and SrH2, indicating
that no definite reaction occurred between the starting materials
during milling. After dehydrogenation, as indicated in Fig. 2b,
SrB6 and LiH can be formed without the remaining LiBH4 or SrH2.
The XRD results suggest that the LiBH4/SrH2 system is dehydroge-
nated according to the following reaction:
J/(mol K) H2, respectively. The
DS value here is rather different from
the typical value of ꢂ130 J/(mol K) H2 for most metal hydrides. This
difference can be mainly attributed to the higher entropy of LiBH4
10
10
9
753 K
723 K
693 K
8
7
6
6LiBH4+SrH2
5
4
1
LiBH4
800
3
2
1
0
300
0.1
400
500
600
700
900
0
1
2
3
4
5
6
7
8
9
Temperature (K)
Hydrogen content (wt.%)
Fig. 1. Temperature dependence of dehydrogenation for the 2 h ball-milled
Fig. 3. Dehydrogenation P–C isotherms of the LiBH4/SrH2 system at different
6LiBH4 + SrH2 mixture and the pristine LiBH4 at a heating rate of 2 K/min.
temperatures.

10
D.M. Liu et al. / Journal of Alloys and Compounds 551 (2013) 8–11
than LiH [27], and the lower
DS value will result in a higher equilib-
9
8
7
6
5
4
3
2
1
0
rium dehydrogenation temperature under 0.1 MPa hydrogen pres-
sure, T(0.1 MPa). For the LiBH4/SrH2 system, extrapolation of the
linear behavior shown in Fig. 4 gives the T(0.1 MPa) value of 582 K.
Compared with the pristine LiBH4 [4], the addition of SrH2 low-
ers the enthalpy change of dehydrogenation by 26 kJ/mol H2.
Hence, the stability of the LiBH4/SrH2 system decreases signifi-
cantly. Note that the dehydrogenation enthalpy change (48 kJ/
mol H2) in the present case is lower than that for the LiBH4/CaH2
system (56.5 kJ/mol H2 [21]), indicating that SrH2 exhibits a some-
what stronger destabilization effect on LiBH4 than its analog CaH2.
753 K
723 K
693 K
3.1.3. Dehydrogenation kinetics
Fig. 5 presents the isothermal dehydrogenation curves for the
LiBH4/SrH2 system at 693, 723 and 753 K. It is clear that the LiBH4/
SrH2 system desorbed hydrogen quickly at these temperatures. The
amounts of hydrogen desorbed within 3600 s are 6.3, 8.0 and
8.5 wt.% at 693, 723 and 753 K, respectively. To further understand
the dehydrogenation rate-limiting step of the LiBH4/SrH2 system,
the isothermal curves were analyzed by the kinetic modeling,
0
1000 2000 3000 4000 5000 6000 7000
Time (s)
Fig. 5. Isothermal dehydrogenation curves of the LiBH4/SrH2 system at different
temperatures.
which can be generally expressed as the following equation:
0.7
Z
g(α) = 1.6E-4 t - 6.2E-3
gð
aÞ ¼
da=fð
aÞ ¼ kt
ð4Þ
0.6
g(α) = 2.3E-4 t + 6.2E-3
0.5
where
a
is the reacted fraction at time t, g(
a
) and f(
a
) are the func-
0.4
tions representing the reaction mechanism of diffusion, nucleation
and nuclei growth, or phase boundary, and so on, and k is the rate
constant. In the present work, the reacted fraction was calculated
by dividing the hydrogen amount desorbed at time t by the satu-
rated dehydrogenation amount (8.7 wt.%). Then various mechanism
functions, as described in the previous works [28,29], were tried to
α
0.3
g(α) = 9.5E-5 t - 2.9E-3
0.2
753 K (α < 0.87)
723 K (α < 0.95)
693 K (α < 0.82)
0.1
0.0
fit the experimental data shown in Fig. 5. As a result, the function
1/3
1 ꢀ (1 ꢀ
a)
gives the best linearity over a broader range of a for
0
1000 2000 3000 4000 5000 6000
Time (s)
each measurement, with the correlation coefficient of R2 > 0.998.
In Fig. 6, one can observe the linear relationships between
1/3
Fig. 6. Plots of 1 ꢀ (1 ꢀ
a)
1/3 vs. t for the LiBH4/SrH2 system dehydrogenated at 693,
723 and 753 K, respectively.
1 ꢀ (1 ꢀ
a)
and t at 693, 723 and 753 K, respectively. This result
suggests that the dehydrogenation process of the LiBH4/SrH2 sys-
tem in the investigated temperature range is mainly rate-limited
by the three-dimension phase boundary migration.
where k0 is the pre-exponential factor, R is the gas constant, and T is
the absolute temperature. As shown in Fig. 7, a good linear relation-
ship exists between lnk and 1/T. From the slope (ꢀEa/R) of the
straight line, Ea was calculated to be 64 kJ/mol for the LiBH4/SrH2
system. This value is much lower than that of the pristine LiBH4
(111–156 kJ/mol [5,30,31]). The decrease in activation energy can
be regarded as another reason contributing to the improved dehy-
drogenation property of the LiBH4/SrH2 system.
The rate constants at different temperatures were extracted
from the slopes of the fitted straight lines shown in Fig. 6. As a re-
sult, the apparent activation energy Ea for dehydrogenation can be
determined according to the Arrhenius equation:
k ¼ k0 exp½ꢀEa=ðRTÞꢁ
ð5Þ
2.4
2.2
2.0
1.8
1.6
3.2. Rehydrogenation behavior
After dehydrogenation, the LiBH4/SrH2 system was subjected to
rehydrogenation at 723 K under an initial hydrogen pressure of
8.0 MPa. Fig. 8a gives the isothermal rehydrogenation curve, indi-
cating that 6.0 wt.% of hydrogen could be reabsorbed within 10 h.
The XRD pattern for the rehydrogenated sample (shown in
Fig. 8b) demonstrates that LiBH4 was regenerated upon hydrogena-
tion, even though the reverse reaction proceeded incompletely un-
der the present experimental conditions. Moreover, LiSrH3 is
obviously observed in Fig. 8b. According the XRD results, the rehy-
drogenation reaction for the LiBH4/SrH2 system can be deduced as:
lnP = -5765.3/T + 9.9069
1.4
SrB6 þ 6LiH þ 17=2H2 ! 5LiBH4 þ LiSrH3 þ B
In the recent work on the LiBH4/CaF2 system [26], it was consid-
ered that the rehydrogenation product CaB6 could make the forma-
tion of LiB12H12 or LiBH4 easier than that from B, owing to that CaB6
ð6Þ
1.30
1.35
1.40
1.45
1.50
-1
1000/T (K
)
Fig. 4. Van’t Hoff plot of the LiBH4/SrH2 system.

D.M. Liu et al. / Journal of Alloys and Compounds 551 (2013) 8–11
11
9.1 wt.%. The dehydrogenation enthalpy change for the LiBH4/
SrH2 system is 48 kJ/mol H2, which is reduced by 26 kJ/mol in com-
parison with pristine LiBH4. The dehydrogenation process of the
LiBH4/SrH2 system exhibits reduced activation energy of 64 kJ/
mol. Moreover, the dehydrogenation process is rate-limited by a
three-dimension phase boundary mechanism. Upon rehydrogena-
tion at 723 K under an initial hydrogen pressure of 8.0 MPa, LiBH4
can be regenerated with the formation of LiSrH3.
-8.2
-8.4
-8.6
-8.8
-9.0
-9.2
-9.4
lnk = -7707.9/T + 1.8779
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 50901001), the Natural Science
Foundation of Anhui Province (No. 1208085ME83) and the Scien-
tific Research Foundation of Education Department of Anhui Prov-
ince of China (No. KJ2010A044).
1.30
1.35
1.40
1.45
1.50
-1
1000/T (K
)
Fig. 7. Arrhenius plot for the dehydrogenation of the LiBH4/SrH2 system.
References
[1] H.W. Li, Y. Yan, S. Orimo, A. Züttel, C.M. Jensen, Energy 4 (2011) 185.
[2] C. Li, P. Peng, D.W. Zhou, L. Wan, Int. J. Hydrogen Energy 36 (2011) 14512.
[3] Y. Bouhadda, S. Djellab, M. Bououdina, N. Fenineche, Y. Boudouma, J. Alloys
Compd. 534 (2012) 20.
[4] P. Mauron, F. Buchter, O. Friedrichs, A. Remhof, M. Bielmann, C.N. Zwicky, A.
Züttel, J. Phys. Chem. B 112 (2008) 906.
7
(a)
6
5
4
[5] A. Züttel, S. Rentsch, P. Fischer, P. Wenger, P. Sudan, P. Mauron, C.
Emmenegger, J. Alloys Compd. 356–357 (2003) 515.
3
Rehydrogenation at 723 K
[6] J.J. Vajo, S.L. Skeith, F. Mertens, J. Phys. Chem. B 109 (2005) 3719.
[7] F.E. Pinkerton, M.S. Meyer, J. Alloys Compd. 464 (2008) L1.
[8] J. Mao, Z. Guo, X. Yu, H. Liu, J. Alloys Compd. 509 (2011) 5012.
[9] Y. Jiang, B.H. Liu, J. Alloys Compd. 509 (2011) 9055.
[10] Y. Zhang, Q. Tian, Int. J. Hydrogen Energy 36 (2011) 9733.
[11] M. Meggouh, D.M. Grant, G.S. Walker, J. Phys. Chem. C 115 (2011) 22054.
[12] D.M. Liu, Q.Q. Liu, T.Z. Si, Q.A. Zhang, F. Fang, D.L. Sun, L.Z. Ouyang, M. Zhu,
Chem. Commun. 47 (2011) 5741.
2
1
0
0
6000 12000 18000 24000 30000 36000
Time (s)
[13] F.C. Gennari, Int. J. Hydrogen Energy 36 (2011) 15231.
[14] J.H. Shim, Y.S. Lee, J.Y. Suh, W. Cho, S.S. Han, Y.W. Cho, J. Alloys Compd. 510
(2012) L9.
(b)
LiBH4
LiSrH3
SrB6
[15] X. Wu, X. Wang, G. Cao, S. Li, H. Ge, L. Chen, M. Yan, J. Alloys Compd. 517 (2012)
127.
[16] Y. Zhou, Y. Liu, W. Wu, Y. Zhang, M. Gao, H. Pan, J. Phys. Chem. C 116 (2012)
1588.
Rehydrogenated
[17] J. Shao, X. Xiao, L. Chen, X. Fan, S. Li, H. Ge, Q. Wang, J. Mater. Chem. 22 (2012)
20764.
[18] C. Luo, H. Wang, T. Sun, M. Zhu, Int. J. Hydrogen Energy 37 (2012) 13446.
[19] S. Deng, X. Xiao, L. Han, Y. Li, S. Li, H. Ge, Q. Wang, L. Chen, Int. J. Hydrogen
Energy 37 (2012) 6733.
[20] J. Chen, Y. Zhang, Z. Xiong, G. Wu, H. Chu, T. He, P. Chen, Int. J. Hydrogen Energy
37 (2012) 12425.
20 25 30 35 40 45 50 55 60 65 70 75
Two Theta (degree)
[21] J.H. Lim, J.H. Shim, Y.S. Lee, Y.W. Cho, J. Lee, Scripta Mater. 59 (2008) 1251.
[22] Y. Zhou, Y. Liu, Y. Zhang, M. Gao, H. Pan, Dalton Trans. 41 (2012) 10980.
[23] K. Jiang, X. Xiao, L. Chen, L. Han, S. Li, H. Ge, Q. Wang, J. Alloys Compd. 539
(2012) 103.
Fig. 8. (a) Isothermal rehydrogenation curve at 723 K and (b) XRD pattern of the
rehydrogenated product for the LiBH4/SrH2 system.
[24] D. Liu, J. Yang, J. Ni, A. Drews, J. Alloys Compd. 514 (2012) 103.
[25] U. Bösenberg, S. Doppiu, L. Mosegaard, G. Barkhordarian, N. Eigen, A.
Borgschulte, T.R. Jensen, Y. Cerenius, O. Gutfleisch, T. Klassen, M. Dornheim,
R. Bormann, Acta Mater. 55 (2007) 3951.
[26] P.P. Yuan, B.H. Liu, B.J. Zhang, Z.P. Li, J. Phys. Chem. C 115 (2011) 7067.
[27] G. Walker, in: G. Walker (Ed.), Solid-State Hydrogen Storage: Materials and
Chemistry, Woodhead Publishing Limited, Cambridge, 2008.
[28] Y. Zhang, Q.F. Tian, J. Zhang, S.S. Liu, L.X. Sun, J. Phys. Chem. C 113 (2009)
18424.
has a structure close to the polyhedral structure in [B12H12]
2ꢀ. Sim-
ilar effect is believed to exist in the present case in view of the fact
that SrB6 has a CaB6-type cubic structure [32].
4. Conclusions
[29] Y. Li, G. Zhou, F. Fang, X. Yu, Q. Zhang, L. Ouyang, M. Zhu, D. Sun, Acta Mater. 59
(2011) 1829.
[30] A.F. Gross, J.J. Vajo, S.L. Van Atta, G.L. Olson, J. Phys. Chem. C 112 (2008) 5651.
[31] S. Kato, M. Bielmann, A. Borgschulte, V. Zakaznova-Herzog, A. Remhof, S.
Orimo, A. Züttel, Phys. Chem. Chem. Phys. 12 (2010) 10950.
[32] C.H. Chen, T. Aizawa, N. Lyi, A. Sato, S. Otani, J. Alloys Compd. 366 (2004) L6.
In this paper, we report the dehydrogenation and rehydrogena-
tion properties of a new reactive hydride system LiBH4/SrH2. It was
found that the thermal stability of LiBH4 can be remarkably re-
duced by the addition of SrH2, via the reaction 6LiBH4 + SrH2 ? -
SrB6 + 6LiH + 10H2 with
a theoretical hydrogen capacity of