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Effect of MgCl2 additives on the H-desorption properties of Li–N–H system
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 0 3 e9 0 7
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Effect of MgCl2 additives on the H-desorption properties of LieNeH system Haiyan Leng a,b,*, Zhu Wu a, Weiyuan Duan a, Guanglin Xia a, Zhilin Li a a
Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences Shanghai, Changning Road, No. 856, Shanghai 200050, PR China b Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Shanghai 200072, PR China
article info
abstract
Article history:
LieNeH system, as a representative of metal-NeH system, is regarded as one of the most
Received 18 November 2010
promising hydrogen storage materials. Motivated by the ammonia mediated mechanism of
Received in revised form
LieNeH system, the effect of MgCl2 (which is an effective NH3 sorbent) on this system is
22 March 2011
systematically investigated in this work. Three different mechanisms are proposed to
Accepted 24 March 2011
explain the effects of MgCl2 with different amount in the improvement of LieNeH system.
Available online 12 May 2011
With small amount (<4 mol%), MgCl2 can improve the H-desorption properties of LieNeH system as an NH3 sorbent. When the amount of MgCl2 increases, the H-desorption prop-
Keywords:
erties of LieNeH system can be improved further though Mg2þ solid solution into LiNH2.
Hydrogen storage
With the amount of MgCl2 more than 25 mol%, the LieNeH system changes into
Complex metal hydride
LieMgeNeH system by the reaction between MgCl2 and LiNH2. The LieMgeNeH system
X-ray diffraction
formed by this reaction exhibits a much better property than those of LieMgeNeH system reported before. It may provide a new idea to improve the metal-NeH system to meet the requirement of practical application. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In order to develop high-performance solid-state hydrogen storage materials for on-board application, recently, much attention has been paid to complex metal hydride [1e8]. Among them, Metal-NeH system has been developed to be one of the most promising hydrogen storage materials within the last few years [8e12], since the pioneering work on LieNeH system reported by Chen et al. [8]. In LieNeH system, the reaction between LiNH2 and LiH with 1:1 M ratio shown in reaction 1, LiH þ LiNH2 4Li2 NH þ H2
(1)
theroretically releases 6.5 mass% of hydorgen in a reversible manner with a relatively low standard enthalpy change (DH w 45 kJ/mol H2) [8,13], which provides a practically viable hydrogen storage path if the release of hydrogen can proceed at around 100 C and the reabsorption can be realized at ambient temperature. Previous research shows that hydrogen releases from the mixture of LiNH2 and LiH in the temperature range of 200e450 C [14,15]. Ichikawa et al. found that an addition of 1 mol% of TiCl3 or nanosized Ti to the mixture can lower the hydrogen release temperature to a narrow range of 150e250 C and improve the hydrogen desorption kinetics [13,16]. To further lower the operating temperature and improve the
* Corresponding author. Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences Shanghai, Changning Road, No. 856, Shanghai 200050, PR China. Tel.: þ86 (0)21 62511070x8907; fax: þ86 (0)21 32200534. E-mail address: [email protected] (H. Leng). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.162
904
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 0 3 e9 0 7
LiNH2 /Li2 NH þ NH3
(2)
LiH þ NH3 /LiNH2 þ H2
(3)
Therefore, according to the ammonia mechanism, the transfer of NH3 from LiNH2 to LiH would crucially affect the H-desorption properties of LieNeH system. Hence the promotion of NH3 desorption could be one effective solution to improve the H-desorption properties of this system. It is expected that an accelerated emission of ammonia from the metal amide would significantly improve the H-desorption properties of LieNeH system. In fact, our previous work demonstrated that the metaleNeH system composed of Mg(NH2)2 and LiH, where LiNH2 is replaced by Mg(NH2)2 which is much more unstable and desorbs NH3 at much lower temperature, showed a quicker hydrogen desorption property at lower temperature than that of LiNH2 and LiH [11,20]. MgCl2 is well known as an NH3 absorbent in solid sorption heat transformer [21,22], which might promote the NH3 desorption from LiNH2. In this work, we will try to improve H-desorption properties of LieNeH system by adding MgCl2.
2.
Experimental
The starting material LiNH2 (95%), LiH (95%) MgCl2 (99.9%) and TiCl3 (76e78.5%, a mixture with AlCl3) were purchased from Alfa-Aesar Corp. and were used as received. The samples of the mixture of LiH and LiNH2 in 1:1 M ratio containing various amount of MgCl2 (0 mol%, 1 mol%, 4 mol%, 10 mol%, 25 mol% and 50 mol%), as well as the sample doped with 1 mol% TiCl3 were prepared by milling under Ar atomosphere for 2 h using a planetary ball milling apparatus (QM-1SP2) at 400 rpm. The weight ratio of ball-to-powder was about 50:1. The samples of LiNH2 mixed with different amount of MgCl2 were also prepared under the same condition. All the material handling was performed in a glove-box (MBraun Unilab) filled with argon to keep a low oxygen and water vapor concentration (less than 1 ppm) during the operation. The hydrogen desorption properties of the samples were measured in a Sieverts apparatus (Advanced Materials Corporation, USA) by volumetric measurement. About 100 mg sample was used in each experiment. The measurement was conducted in an automatic release mode, during which the whole system was first evacuated. The hydrogen was released into the vacuum. The reactor was heated at a ramp rate of 5 C/min from ambient to w500 C. The amounts of desorbed hydrogen of the sample were calculated from the hydrogen pressures measured in the calibrated volume by using the ideal gas law. Temperature programmed dehydrogenation
(TPD) curves were obtained by calculating the hydrogen desorption rate. Therefore, TPD measurement in this work was carried in a closed system compared to that carried under inert gas flow (open system). The samples with/without MgCl2 were also examined by the thermogravimetry- differential scanning calorimetry (TG-DSC) (Netzsch, STA449 F3) upon heating up to 500 C at a ramp rate of 5 C/min. The results obtained by volumetric methods combined with weight measurement by TG indicated that the amount of NH3 is negligible in the desorbed hydrogen from all samples. The NH3 emission behaviors of LiNH2 mixed with MgCl2 were examined by TG-DSC under Ar flow at a ramp rate of 5 C/min. X-ray diffraction (XRD) data were obtained with a Rigaku D/max 2200PC using Cu Ka radiation. Samples were mounted onto a glass board 1 mm in thickness in the Ar-filled glove box and sealed with an amorphous membrane airtight hood in order to avoid oxidation during the XRD measurement.
3.
Results and discussion
Fig. 1 shows the TPD profiles of the samples containing various amount of MgCl2 prepared by planetary ball milling for 2 h under argon atmosphere. As comparison, the sample with no additive and with 1 mol% TiCl3 were also measured under 0.2
a
No additive
b
1% TiCl3
c
1% MgCl2
d
4% MgCl2
e
10% MgCl2
f
25% MgCl2
0.1
Hydrogen desorption rate (mass%/min)
hydrogen desorption kinetics, the reaction mechanism between LiH and LiNH2 needs to be clearly understood. Two reaction mechanisms of LieNeH system, the polar mechanism proposed by Chen et al. [14] and the ammonia mediated mechanism proposed by Hu et al. [17] and Ichikawa et al. [18,19], are most widely accepted at present to explain the desorption behaviors of LieNeH system. According to the ammonia mediated mechanism, the reaction between LiNH2 and LiH is a two-step reaction as follows [17e19]:
0.0 0.2 0.1 0.0 0.2 0.1 0.0 0.2 0.1 0.0 0.2 0.1 0.0 0.2 0.1 0.0 0.2 0.1
A
g
50% MgCl2
0.0
100
200
300
400
500
o
Temperature ( C) Fig. 1 e Hydrogen desorption rate of (LiH D LiNH2) milled for 2 h with different amount of MgCl2 (a) no additive, (b) 1 mol% TiCl3, (c) 1 mol% MgCl2, (d) 4 mol% MgCl2 (e) 10 mol% MgCl2, (f) 25 mol% MgCl2, (g) 50 mol% MgCl2, respectively.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 0 3 e9 0 7
360 Region I
o
Peak temperature, Tp-H2 ( C)
the same condition. TiCl3 exhibits the best catalytic effect on LieNeH system that has ever been reported [13,16]. It can be seen that when 1 mol% MgCl2 was added, the main hydrogen desorption peak temperature (Tp) of the sample (267 C) decreased to the same level as that of 1 mol% TiCl3 doped sample (265 C). However, the shoulder peak of hydrogen desorption at higher temperature still remained. When the amount of MgCl2 increased to 4 mol%, there was no obviously further improvement in hydrogen desorption property (Tp ¼ 263 C). When 10 mol% MgCl2 was added, Tp decreased further to 245 C, which is evidently lower than that of LieNeH sample with 1 mol% TiCl3. When the amount of MgCl2 increased to 25 mol%, the shoulder peak of hydrogen desorption at higher temperature disappeared, instead, a small peak of hydrogen desorption appeared at lower temperature than that of the main peak. When the amount of MgCl2 increased up to 50 mol%, Tp decreased to 174 C which is about 100 C lower than that of LieNeH sample with 1 mol% TiCl3. It is worth noticing that the hydrogen desorption temperature of the sample with 50 mol% MgCl2 measured in the closed system is even lower than that of LieMgeNeH system measured in the open system reported so far [23,24]. In order to understand the effect of MgCl2 on the hydrogen storage properties, the relation between Tp and the amount of MgCl2 is plotted in Fig. 2. We can see that with the amount of MgCl2 increasing, TpH2 decreased. However, the plot can be clearly divided into three regions: from 0 to 4 mol% MgCl2, from 4 to 25 mol% MgCl2, and from 25 to 50 mol% MgCl2. When 1 mol% MgCl2 was added, TpH2 decreased 35 C. At region I, TpH2 shows no obvious change with the amount of MgCl2 increasing. At region II, the peak temperature decreased 20 C at first and became steady again. At region III, TpH2 decreased over 70 C. According to the above results, it could be deduced that the mechanisms of MgCl2 on the H-desorption properties of LieNeH system varied with the content of MgCl2. In order to find out what kinds of effect mechanisms of MgCl2 in LieNeH system, we examined the structural properties of all samples after ball milling by XRD analysis, as shown in Fig. 3. When MgCl2 < 25 mol% (at Region I and II), the samples are mainly composed of LiNH2 and LiH phases. When MgCl2 >¼ 25 mol% (at Region III), the samples are mainly
320
Region II
280
Region III
240
200
160 0
10
20
30
40
50
Content of MgCl2 (mol%) Fig. 2 e Relationship between hydrogen desorption peak temperature and the amount of MgCl2.
905
Fig. 3 e XRD profiles of the LieNeH samples with various amount of MgCl2 after ball-milling (a) No additive, (b) 1 mol % MgCl2, (c) 4 mol% MgCl2 (d) 10 mol% MgCl2 (e) 25 mol% MgCl2 (f) 50 mol% MgCl2, respectively.
composed of LiNH2 and LiH and LiCl phases. Furthermore, at Region I, the crystal lattice constants of LiNH2 showed almost no change, compared to that of the none-doped sample; at Region II and III, the lattice constants obviously increased compared to that of the none-doped sample. The relationship between the change of lattice constants and the amount of MgCl2 is plotted in Fig. 4 for the samples. At Region I, the lattice constant of the samples after ball milling changed little, which indicated that no Mg2þ dissolved into LieNeH samples. At Region II, the lattice constants increased quickly with the amount of MgCl2 increasing first, then reached a stable value when the amount of MgCl2 was larger than 25 mol%. It indicated that Mg2þ dissolved into LieNeH samples and the solution concentrations increased with the amount of MgCl2 increasing. The concentrations finally saturated when the amount of MgCl2 was over 25 mol%. As shown in Fig. 3, there are LiCl phases existed in the samples when the amount of MgCl2 was over 25 mol%, which indicated that when the solution concentration of Mg2þ saturated, the reaction between MgCl2 and LieNeH system occurred. There are two possible reactions between MgCl2 and LieNeH system as follows: 2LiH þ MgCl2 / 2LiCl þ MgH2
(4)
2LiNH2 þ MgCl2 /2LiCl þ MgðNH2 Þ2
(5)
In principle, both the reactions can be possible to proceed during milling or heating, as the enthalpy changes of the
906
o
2.5
Peak temperature, Tp-NH3 ( C)
Increment of Lattice constant of LiNH2 (%)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 0 3 e9 0 7
Region III
Region II
2.0 1.5 Region I
1.0 0.5 0.0
340 320 300 280 260 240 0
0
10
20
30
40
50
Content of MgCl2 (mol%) Fig. 4 e Relative increment of lattice constant of LiNH2 vs. the amount of MgCl2.
reaction 4 and 5 are calculated to be 69.8 kJ/mol MgCl2 and -133 kJ/mol MgCl2 (at least) respectively. If reaction 4 proceeds, then the sample added with 50 mol% MgCl2 will change its constitute into (0.5MgH2 þ LiNH2 þ LiCl); if reaction 5 proceeds, then the constitute of the sample will change into (LiH þ 0.5 Mg(NH2)2 þ LiCl). According to the enthalpy change values, the reaction 5 would be more preferable to occur. As reported by previous works [23e25], the hydrogen desorption properties of (2LiH þ Mg(NH2)2) are much better than that of (MgH2 þ 2LiNH2) Our results showed the H-desorption property of the sample added with 50 mol% MgCl2 measured in a closed system is even better than that of LieMgeNeH system measured in an open system, as shown in Fig. 1f.
No additive
DSC (a.u.)
1% MgCl2
4% MgCl2
10% MgCl2
25% MgCl2
50% MgCl2
0
100
200
300
400
500
10
20
30
40
50
Content of MgCl2 (mol%) Fig. 6 e Relationship between NH3 desorption peak temperature and the amount of MgCl2.
However, there are no trace of Mg(NH2)2 phase can be detected in the sample by XRD measurement which might be due to the nano/amorphous state of Mg(NH2)2 formed in this case since Mg(NH2)2 is easily converted into nano/amorphous state by ball milling [11]. The development of lattice constants and desorption properties of MgCl2 doped LieNeH samples against the amount of MgCl2 synchronize very well. According to above results, three different mechanisms of MgCl2 are proposed. At Region I (MgCl2 <¼ 4 mol%), MgCl2 may improve the hydrogen properties of LieNeH system through mainly playing an NH3 sorbent role. At Region II (MgCl2 > 4 mol%), Mg2þ can dissolve into LiNH2 to further improve the hydrogen desorption properties of LieNeH system. At Region III (MgCl2>¼ 25 mol%), MgCl2 can react with LiNH2 to form Mg(NH2)2 as described in reaction 5, which changes the LieNeH system into a LieMgeNeH system (LiH þ 0.5 Mg(NH2)2) with a much better hydrogen desorption property. Therefore, the effects of MgCl2 on LieNeH system would actually originate from the effect of MgCl2 on the decomposition of LiNH2. In order to confirm the effect of MgCl2 on the decomposition of LiNH2, the NH3 emission behaviors of LiNH2 mixed with various amount of MgCl2 were measured by TG-DSC equipment under Ar flow at a ramp rate of 5 C/min. As shown in Fig. 5, the decomposition temperatures of LiNH2 decreased obviously with the increase of MgCl2 amount. It confirmed that the addition of MgCl2 significantly influenced the Hdesorption property of LieNeH system by promoting the decomposition of LiNH2. Furthermore, the results of the main peak temperatures of NH3 desorption (Tp-NH3) were plotted in Fig. 6 against the content of MgCl2. The development of T p-NH3 is almost synchronized with that of Tp-H2 as shown in Fig. 2, which is consistent with the proposal of the three different mechanisms above.
o
Temperature ( C) Fig. 5 e DSC profiles of the LiNH2 with various amount of MgCl2 after ball-milling (a) No additive, (b) 1 mol% MgCl2, (c) 4 mol% MgCl2 (d) 10 mol% MgCl2 (e) 25 mol% MgCl2 (f) 50 mol% MgCl2, respectively.
4.
Conclusions
We studied the effects of various amount of MgCl2 on the hydrogen desorption properties of LieNeH system. The results showed that the hydrogen desorption properties of
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 0 3 e9 0 7
LieNeH system can be significantly improved by the addition of MgCl2. However, the hydrogen desorption doesn’t improve linearly with the increase of MgCl2 amount. Based on the TPD and XRD analysis, three different mechanisms of MgCl2 on LieNeH system are proposed, i.e., as a NH3 sorbent (MgCl2 < 4 mol%), Mg2þ solid solution (MgCl2 > 4 mol%) and reaction with LiNH2 (MgCl2 > 25 mol%). Through reaction with LiNH2, the addition of MgCl2 can lead to the best improvement of H-desorption property by changing the LieNeH system into a LieMgeNeH system (LiH þ 0.5 Mg(NH2)2).
Acknowledgements This work was financial supported by the Project of the Shanghai Committee of Science and Technology, China (Grant No. 09dz1206800) and the National Natural Science Foundation of China (Grant No. 21071149).
references
[1] Bogdanovic B, Schwickardi M. Ti-doped alkali metal aluminium hydrides as potential novel reversible hydrogen storage materials. J Alloys Comp 1997;253:1e9. [2] Chen Y, Wu CZ, Wang P, Cheng HM. Structure and hydrogen storage property of ball-milled LiNH2/MgH2 mixture. Int J Hydrogen Energy 2006;31:1236e40. [3] Fang ZZ, Kang XD, Wang P. Improved hydrogen storage properties of LiBH4 by mechanical milling with various carbon additives. Int J Hydrogen Energy 2010;35:8247e52. [4] Wu CZ, Miyaoka H, Ichikawa T, Kojima Y. Hydrogen desorption properties of LieBNeH system synthesized by mechanical milling. Int J Hydrogen Energy 2008;33:3128e31. [5] Dai HB, Kang XD, Wang P. Ruthenium nanoparticles immobilized in montmorillonite used as catalyst for methanolysis of ammonia borane. Int J Hydrogen Energy 2010;35:10317e23. [6] Vajo JJ, Skeith SL, Mertens F. Reversible storage of hydrogen in destabilized LiBH4. J Phys Chem B 2005;109:3719e22. [7] Ichikawa T, Fujii H, Isobe S, Nabeta K. Rechargeable hydrogen storage in nanostructured mixtures of hydrogenated carbon and lithium hydride. Appl Phys Lett 2005;86: 241914e3. [8] Chen P, Xiong Z, Luo J, Lin J, Tan KL. Interaction of hydrogen with metal nitrides and imides. Nature 2002;420:302e4.
907
[9] Luo W. (LiNH2eMgH2): a viable hydrogen storage system. J Alloys Comp 2004;381:284e7. [10] Xiong Z, Wu G, Hu J, Chen P. Ternary imides for hydrogen storage. Adv Mater 2004;16:1522e5. [11] Leng HY, Ichikawa T, Hino S, Hanada N, Isobe S, Fujii H. New Metal-N-H system composed of Mg(NH2)2 and LiH for hydrogen storage. J Phys Chem B 2004;108:8763e6. [12] Nakamori Y, Kitahara G, Miwa K, Towata S, Orimo S. Reversible hydrogen-storage functions for mixtures of Li3N and Mg3N2. Appl Phys A 2005;80:1e3. [13] Ichikawa T, Isobe S, Hanada N, Fujii H. Lithium nitride for reversible hydrogen storage. J Alloys Compd 2004;365:271e6. [14] Chen P, Xiong ZT, Luo JZ, Lin JY, Tan KL. Interaction between lithium amide and lithium hydride. J Phys Chem B 2003;107: 10967e70. [15] Yao JH, Shang C, Zinsou KFA, Guo ZX. Desorption characteristics of mechanically and chemically modified LiNH2 and (LiNH2þ LiH). J Alloys Compd 2006;432:277e82. [16] Isobe S, Ichikawa T, Hanada N, Leng HY, Fichtner M, Fuhr O, et al. Effect of Ti catalyst with different chemical form on LieNeH hydrogen storage properties. J Alloys Compd 2005; 404:439e42. [17] Hu YH, Ruckenstein E. Ultrafast reaction between LiH and NH3 during H2 storage in Li3N. J Phys Chem A 2003;107: 9737e9. [18] Ichikawa T, Hanada N, Isobe S, Leng HY, Fujii H. Mechanism of novel reaction from LiNH2 and LiH to Li2NH and H2 as a promising hydrogen storage system. J Phys Chem.B 2004; 108:7887e92. [19] Isobe S, Ichikawa T, Hino S, Fujii H. Hydrogen desorption mechanism in a Li-N-H system by means of the isotopic exchange technique. J Phys Chem B 2005;109:14855e9. [20] Leng HY, Ichikawa T, Hino S, Hanada N, Isobe S, Fujii H. Synthesis and decomposition reactions of metal amides in metaleNeH hydrogen storage system. J Power Sour 2006;156:166e70. [21] Haije WG, Veldhuis JBJ, Smeding SF, Grisel RJH. Solid/vapour sorption heat transformer: design and performance. Appl Thermal Eng 2007;27:1371e6. [22] Jacobsen HS, Hansen HA, Andreasen JW, Shi Q, Andreasen A, Feidenhans’l R, et al. Nanoscale structural characterization of Mg(NH3)6Cl2 duringNH3 desorption: an in situ small angle X-ray scattering study. Chem Phys Lett 2007;441:255e60. [23] Leng HY, Ichikawa T, Fujii H. Hydrogen storage properties of LieMgeNeH systems with different ratios of LiH/Mg(NH2)2. J Phys Chem B 2006;110:12964e8. [24] Hu JJ, Fichtner M. Formation and stability of ternary imides in the Li-Mg-N-H hydrogen storage system. Chem Mater 2009;21:3485e90. [25] Zinsou KFA, Yao J, Guo ZX. Reaction paths between LiNH2 and LiH with effects of nitrides. J Phys Chem B 2007;111: 12531e6.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effect of MgCl2 additives on the H-desorption properties of LieNeH system Haiyan Leng a,b,*, Zhu Wu a, Weiyuan Duan a, Guanglin Xia a, Zhilin Li a a
Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences Shanghai, Changning Road, No. 856, Shanghai 200050, PR China b Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Shanghai 200072, PR China
article info
abstract
Article history:
LieNeH system, as a representative of metal-NeH system, is regarded as one of the most
Received 18 November 2010
promising hydrogen storage materials. Motivated by the ammonia mediated mechanism of
Received in revised form
LieNeH system, the effect of MgCl2 (which is an effective NH3 sorbent) on this system is
22 March 2011
systematically investigated in this work. Three different mechanisms are proposed to
Accepted 24 March 2011
explain the effects of MgCl2 with different amount in the improvement of LieNeH system.
Available online 12 May 2011
With small amount (<4 mol%), MgCl2 can improve the H-desorption properties of LieNeH system as an NH3 sorbent. When the amount of MgCl2 increases, the H-desorption prop-
Keywords:
erties of LieNeH system can be improved further though Mg2þ solid solution into LiNH2.
Hydrogen storage
With the amount of MgCl2 more than 25 mol%, the LieNeH system changes into
Complex metal hydride
LieMgeNeH system by the reaction between MgCl2 and LiNH2. The LieMgeNeH system
X-ray diffraction
formed by this reaction exhibits a much better property than those of LieMgeNeH system reported before. It may provide a new idea to improve the metal-NeH system to meet the requirement of practical application. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In order to develop high-performance solid-state hydrogen storage materials for on-board application, recently, much attention has been paid to complex metal hydride [1e8]. Among them, Metal-NeH system has been developed to be one of the most promising hydrogen storage materials within the last few years [8e12], since the pioneering work on LieNeH system reported by Chen et al. [8]. In LieNeH system, the reaction between LiNH2 and LiH with 1:1 M ratio shown in reaction 1, LiH þ LiNH2 4Li2 NH þ H2
(1)
theroretically releases 6.5 mass% of hydorgen in a reversible manner with a relatively low standard enthalpy change (DH w 45 kJ/mol H2) [8,13], which provides a practically viable hydrogen storage path if the release of hydrogen can proceed at around 100 C and the reabsorption can be realized at ambient temperature. Previous research shows that hydrogen releases from the mixture of LiNH2 and LiH in the temperature range of 200e450 C [14,15]. Ichikawa et al. found that an addition of 1 mol% of TiCl3 or nanosized Ti to the mixture can lower the hydrogen release temperature to a narrow range of 150e250 C and improve the hydrogen desorption kinetics [13,16]. To further lower the operating temperature and improve the
* Corresponding author. Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences Shanghai, Changning Road, No. 856, Shanghai 200050, PR China. Tel.: þ86 (0)21 62511070x8907; fax: þ86 (0)21 32200534. E-mail address: [email protected] (H. Leng). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.162
904
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 0 3 e9 0 7
LiNH2 /Li2 NH þ NH3
(2)
LiH þ NH3 /LiNH2 þ H2
(3)
Therefore, according to the ammonia mechanism, the transfer of NH3 from LiNH2 to LiH would crucially affect the H-desorption properties of LieNeH system. Hence the promotion of NH3 desorption could be one effective solution to improve the H-desorption properties of this system. It is expected that an accelerated emission of ammonia from the metal amide would significantly improve the H-desorption properties of LieNeH system. In fact, our previous work demonstrated that the metaleNeH system composed of Mg(NH2)2 and LiH, where LiNH2 is replaced by Mg(NH2)2 which is much more unstable and desorbs NH3 at much lower temperature, showed a quicker hydrogen desorption property at lower temperature than that of LiNH2 and LiH [11,20]. MgCl2 is well known as an NH3 absorbent in solid sorption heat transformer [21,22], which might promote the NH3 desorption from LiNH2. In this work, we will try to improve H-desorption properties of LieNeH system by adding MgCl2.
2.
Experimental
The starting material LiNH2 (95%), LiH (95%) MgCl2 (99.9%) and TiCl3 (76e78.5%, a mixture with AlCl3) were purchased from Alfa-Aesar Corp. and were used as received. The samples of the mixture of LiH and LiNH2 in 1:1 M ratio containing various amount of MgCl2 (0 mol%, 1 mol%, 4 mol%, 10 mol%, 25 mol% and 50 mol%), as well as the sample doped with 1 mol% TiCl3 were prepared by milling under Ar atomosphere for 2 h using a planetary ball milling apparatus (QM-1SP2) at 400 rpm. The weight ratio of ball-to-powder was about 50:1. The samples of LiNH2 mixed with different amount of MgCl2 were also prepared under the same condition. All the material handling was performed in a glove-box (MBraun Unilab) filled with argon to keep a low oxygen and water vapor concentration (less than 1 ppm) during the operation. The hydrogen desorption properties of the samples were measured in a Sieverts apparatus (Advanced Materials Corporation, USA) by volumetric measurement. About 100 mg sample was used in each experiment. The measurement was conducted in an automatic release mode, during which the whole system was first evacuated. The hydrogen was released into the vacuum. The reactor was heated at a ramp rate of 5 C/min from ambient to w500 C. The amounts of desorbed hydrogen of the sample were calculated from the hydrogen pressures measured in the calibrated volume by using the ideal gas law. Temperature programmed dehydrogenation
(TPD) curves were obtained by calculating the hydrogen desorption rate. Therefore, TPD measurement in this work was carried in a closed system compared to that carried under inert gas flow (open system). The samples with/without MgCl2 were also examined by the thermogravimetry- differential scanning calorimetry (TG-DSC) (Netzsch, STA449 F3) upon heating up to 500 C at a ramp rate of 5 C/min. The results obtained by volumetric methods combined with weight measurement by TG indicated that the amount of NH3 is negligible in the desorbed hydrogen from all samples. The NH3 emission behaviors of LiNH2 mixed with MgCl2 were examined by TG-DSC under Ar flow at a ramp rate of 5 C/min. X-ray diffraction (XRD) data were obtained with a Rigaku D/max 2200PC using Cu Ka radiation. Samples were mounted onto a glass board 1 mm in thickness in the Ar-filled glove box and sealed with an amorphous membrane airtight hood in order to avoid oxidation during the XRD measurement.
3.
Results and discussion
Fig. 1 shows the TPD profiles of the samples containing various amount of MgCl2 prepared by planetary ball milling for 2 h under argon atmosphere. As comparison, the sample with no additive and with 1 mol% TiCl3 were also measured under 0.2
a
No additive
b
1% TiCl3
c
1% MgCl2
d
4% MgCl2
e
10% MgCl2
f
25% MgCl2
0.1
Hydrogen desorption rate (mass%/min)
hydrogen desorption kinetics, the reaction mechanism between LiH and LiNH2 needs to be clearly understood. Two reaction mechanisms of LieNeH system, the polar mechanism proposed by Chen et al. [14] and the ammonia mediated mechanism proposed by Hu et al. [17] and Ichikawa et al. [18,19], are most widely accepted at present to explain the desorption behaviors of LieNeH system. According to the ammonia mediated mechanism, the reaction between LiNH2 and LiH is a two-step reaction as follows [17e19]:
0.0 0.2 0.1 0.0 0.2 0.1 0.0 0.2 0.1 0.0 0.2 0.1 0.0 0.2 0.1 0.0 0.2 0.1
A
g
50% MgCl2
0.0
100
200
300
400
500
o
Temperature ( C) Fig. 1 e Hydrogen desorption rate of (LiH D LiNH2) milled for 2 h with different amount of MgCl2 (a) no additive, (b) 1 mol% TiCl3, (c) 1 mol% MgCl2, (d) 4 mol% MgCl2 (e) 10 mol% MgCl2, (f) 25 mol% MgCl2, (g) 50 mol% MgCl2, respectively.
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360 Region I
o
Peak temperature, Tp-H2 ( C)
the same condition. TiCl3 exhibits the best catalytic effect on LieNeH system that has ever been reported [13,16]. It can be seen that when 1 mol% MgCl2 was added, the main hydrogen desorption peak temperature (Tp) of the sample (267 C) decreased to the same level as that of 1 mol% TiCl3 doped sample (265 C). However, the shoulder peak of hydrogen desorption at higher temperature still remained. When the amount of MgCl2 increased to 4 mol%, there was no obviously further improvement in hydrogen desorption property (Tp ¼ 263 C). When 10 mol% MgCl2 was added, Tp decreased further to 245 C, which is evidently lower than that of LieNeH sample with 1 mol% TiCl3. When the amount of MgCl2 increased to 25 mol%, the shoulder peak of hydrogen desorption at higher temperature disappeared, instead, a small peak of hydrogen desorption appeared at lower temperature than that of the main peak. When the amount of MgCl2 increased up to 50 mol%, Tp decreased to 174 C which is about 100 C lower than that of LieNeH sample with 1 mol% TiCl3. It is worth noticing that the hydrogen desorption temperature of the sample with 50 mol% MgCl2 measured in the closed system is even lower than that of LieMgeNeH system measured in the open system reported so far [23,24]. In order to understand the effect of MgCl2 on the hydrogen storage properties, the relation between Tp and the amount of MgCl2 is plotted in Fig. 2. We can see that with the amount of MgCl2 increasing, TpH2 decreased. However, the plot can be clearly divided into three regions: from 0 to 4 mol% MgCl2, from 4 to 25 mol% MgCl2, and from 25 to 50 mol% MgCl2. When 1 mol% MgCl2 was added, TpH2 decreased 35 C. At region I, TpH2 shows no obvious change with the amount of MgCl2 increasing. At region II, the peak temperature decreased 20 C at first and became steady again. At region III, TpH2 decreased over 70 C. According to the above results, it could be deduced that the mechanisms of MgCl2 on the H-desorption properties of LieNeH system varied with the content of MgCl2. In order to find out what kinds of effect mechanisms of MgCl2 in LieNeH system, we examined the structural properties of all samples after ball milling by XRD analysis, as shown in Fig. 3. When MgCl2 < 25 mol% (at Region I and II), the samples are mainly composed of LiNH2 and LiH phases. When MgCl2 >¼ 25 mol% (at Region III), the samples are mainly
320
Region II
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Region III
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Content of MgCl2 (mol%) Fig. 2 e Relationship between hydrogen desorption peak temperature and the amount of MgCl2.
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Fig. 3 e XRD profiles of the LieNeH samples with various amount of MgCl2 after ball-milling (a) No additive, (b) 1 mol % MgCl2, (c) 4 mol% MgCl2 (d) 10 mol% MgCl2 (e) 25 mol% MgCl2 (f) 50 mol% MgCl2, respectively.
composed of LiNH2 and LiH and LiCl phases. Furthermore, at Region I, the crystal lattice constants of LiNH2 showed almost no change, compared to that of the none-doped sample; at Region II and III, the lattice constants obviously increased compared to that of the none-doped sample. The relationship between the change of lattice constants and the amount of MgCl2 is plotted in Fig. 4 for the samples. At Region I, the lattice constant of the samples after ball milling changed little, which indicated that no Mg2þ dissolved into LieNeH samples. At Region II, the lattice constants increased quickly with the amount of MgCl2 increasing first, then reached a stable value when the amount of MgCl2 was larger than 25 mol%. It indicated that Mg2þ dissolved into LieNeH samples and the solution concentrations increased with the amount of MgCl2 increasing. The concentrations finally saturated when the amount of MgCl2 was over 25 mol%. As shown in Fig. 3, there are LiCl phases existed in the samples when the amount of MgCl2 was over 25 mol%, which indicated that when the solution concentration of Mg2þ saturated, the reaction between MgCl2 and LieNeH system occurred. There are two possible reactions between MgCl2 and LieNeH system as follows: 2LiH þ MgCl2 / 2LiCl þ MgH2
(4)
2LiNH2 þ MgCl2 /2LiCl þ MgðNH2 Þ2
(5)
In principle, both the reactions can be possible to proceed during milling or heating, as the enthalpy changes of the
906
o
2.5
Peak temperature, Tp-NH3 ( C)
Increment of Lattice constant of LiNH2 (%)
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Region III
Region II
2.0 1.5 Region I
1.0 0.5 0.0
340 320 300 280 260 240 0
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Content of MgCl2 (mol%) Fig. 4 e Relative increment of lattice constant of LiNH2 vs. the amount of MgCl2.
reaction 4 and 5 are calculated to be 69.8 kJ/mol MgCl2 and -133 kJ/mol MgCl2 (at least) respectively. If reaction 4 proceeds, then the sample added with 50 mol% MgCl2 will change its constitute into (0.5MgH2 þ LiNH2 þ LiCl); if reaction 5 proceeds, then the constitute of the sample will change into (LiH þ 0.5 Mg(NH2)2 þ LiCl). According to the enthalpy change values, the reaction 5 would be more preferable to occur. As reported by previous works [23e25], the hydrogen desorption properties of (2LiH þ Mg(NH2)2) are much better than that of (MgH2 þ 2LiNH2) Our results showed the H-desorption property of the sample added with 50 mol% MgCl2 measured in a closed system is even better than that of LieMgeNeH system measured in an open system, as shown in Fig. 1f.
No additive
DSC (a.u.)
1% MgCl2
4% MgCl2
10% MgCl2
25% MgCl2
50% MgCl2
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Content of MgCl2 (mol%) Fig. 6 e Relationship between NH3 desorption peak temperature and the amount of MgCl2.
However, there are no trace of Mg(NH2)2 phase can be detected in the sample by XRD measurement which might be due to the nano/amorphous state of Mg(NH2)2 formed in this case since Mg(NH2)2 is easily converted into nano/amorphous state by ball milling [11]. The development of lattice constants and desorption properties of MgCl2 doped LieNeH samples against the amount of MgCl2 synchronize very well. According to above results, three different mechanisms of MgCl2 are proposed. At Region I (MgCl2 <¼ 4 mol%), MgCl2 may improve the hydrogen properties of LieNeH system through mainly playing an NH3 sorbent role. At Region II (MgCl2 > 4 mol%), Mg2þ can dissolve into LiNH2 to further improve the hydrogen desorption properties of LieNeH system. At Region III (MgCl2>¼ 25 mol%), MgCl2 can react with LiNH2 to form Mg(NH2)2 as described in reaction 5, which changes the LieNeH system into a LieMgeNeH system (LiH þ 0.5 Mg(NH2)2) with a much better hydrogen desorption property. Therefore, the effects of MgCl2 on LieNeH system would actually originate from the effect of MgCl2 on the decomposition of LiNH2. In order to confirm the effect of MgCl2 on the decomposition of LiNH2, the NH3 emission behaviors of LiNH2 mixed with various amount of MgCl2 were measured by TG-DSC equipment under Ar flow at a ramp rate of 5 C/min. As shown in Fig. 5, the decomposition temperatures of LiNH2 decreased obviously with the increase of MgCl2 amount. It confirmed that the addition of MgCl2 significantly influenced the Hdesorption property of LieNeH system by promoting the decomposition of LiNH2. Furthermore, the results of the main peak temperatures of NH3 desorption (Tp-NH3) were plotted in Fig. 6 against the content of MgCl2. The development of T p-NH3 is almost synchronized with that of Tp-H2 as shown in Fig. 2, which is consistent with the proposal of the three different mechanisms above.
o
Temperature ( C) Fig. 5 e DSC profiles of the LiNH2 with various amount of MgCl2 after ball-milling (a) No additive, (b) 1 mol% MgCl2, (c) 4 mol% MgCl2 (d) 10 mol% MgCl2 (e) 25 mol% MgCl2 (f) 50 mol% MgCl2, respectively.
4.
Conclusions
We studied the effects of various amount of MgCl2 on the hydrogen desorption properties of LieNeH system. The results showed that the hydrogen desorption properties of
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 9 0 3 e9 0 7
LieNeH system can be significantly improved by the addition of MgCl2. However, the hydrogen desorption doesn’t improve linearly with the increase of MgCl2 amount. Based on the TPD and XRD analysis, three different mechanisms of MgCl2 on LieNeH system are proposed, i.e., as a NH3 sorbent (MgCl2 < 4 mol%), Mg2þ solid solution (MgCl2 > 4 mol%) and reaction with LiNH2 (MgCl2 > 25 mol%). Through reaction with LiNH2, the addition of MgCl2 can lead to the best improvement of H-desorption property by changing the LieNeH system into a LieMgeNeH system (LiH þ 0.5 Mg(NH2)2).
Acknowledgements This work was financial supported by the Project of the Shanghai Committee of Science and Technology, China (Grant No. 09dz1206800) and the National Natural Science Foundation of China (Grant No. 21071149).
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