In situ synchrotron X-ray diffraction study on the dehydrogenation behavior of LiAlH4–MgH2 composites

In situ synchrotron X-ray diffraction study on the dehydrogenation behavior of LiAlH4–MgH2 composites

Journal of Alloys and Compounds 599 (2014) 164–169 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

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Journal of Alloys and Compounds 599 (2014) 164–169

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

In situ synchrotron X-ray diffraction study on the dehydrogenation behavior of LiAlH4–MgH2 composites Wei-Che Hsu a, Cheng-Hsien Yang a, Chia-Yen Tan a, Wen-Ta Tsai a,b,⇑ a b

Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan, Taiwan

a r t i c l e

i n f o

Article history: Received 18 December 2013 Received in revised form 12 February 2014 Accepted 12 February 2014 Available online 20 February 2014 Keywords: LiAlH4 MgH2 Dehydrogenation reaction In situ synchrotron X-ray diffraction

a b s t r a c t Dehydrogenation behavior of LiAlH4–MgH2 composites was investigated by using thermal gravimetric analysis (TGA) and in situ synchrotron X-ray diffraction (XRD) technique. The experimental results showed that MgH2 could play a catalytic role in lowing the initial dehydrogenation temperature of LiAlH4. Besides, MgH2 could be destabilized by the dehydrogenation reaction products of LiAlH4. The initial dehydrogenation temperature of LiAlH4–MgH2 composites was as low as 145 °C and MgH2 could decompose below 300 °C. The compounds such as LiMgAlH6, Al3Mg2, Al12Mg17 and Li0.92Mg4.08 formed during dehydrogenation process suggested some mutual reactions proceeded between LiAlH4 and MgH2. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Among the solid state hydrogen storage methods, complex metal hydrides have recently become a group of promising hydrogen storage materials owing to their high hydrogen storage capacity compared with conventional metal hydrides [1–3]. Complex metal hydrides can be categorized into borohydrides, alanates (metal aluminum hydrides), amides and inides, etc. Lithium aluminum hydride (LiAlH4) is a widely studied metal aluminum hydride and attracted much attention due to its high gravimetric hydrogen density. The theoretical hydrogen capacity is up to 7.9 wt%, which is calculated from the first two dehydrogenation reactions of LiAlH4, as shown in the following [4–7]:

LiAlH4ðlÞ ! 1=3Li3 AlH6ðsÞ þ 2=3AlðsÞ þ H2 ð160  180  C; 5:3 wt% H2 Þ ð1Þ 1=3Li3 AlH6ðsÞ ! LiHðsÞ þ 1=3AlðsÞ þ 1=2H2 ð180  220  C; 2:6 wt% H2 Þ ð2Þ Dehydrogenation reaction of LiAlH4 can proceed not only from liquid state but also from solid state as reported in our recent investigation [7]. However, the dehydrogenation temperatures of the first (regardless from liquid or solid state) and the second steps are not low enough for the target set by the U.S. Department of En⇑ Corresponding author address: Department of Materials Science and Engineering, National Cheng Kung University, 1, Ta Hsueh Road, Tainan 701, Taiwan. Tel.: +886 62757575x62927; fax: +886 62754395. E-mail address: [email protected] (W.-T. Tsai). http://dx.doi.org/10.1016/j.jallcom.2014.02.064 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

ergy (DOE) for light-duty vehicle [8]. Further improvements on the dehydrogenation properties of LiAlH4 are necessary for practical applications. In order to lower the dehydrogenation temperature, several methods such as particle size reduction by mechanical milling [6,9], doping metal-based additives [10,11] and adding carbon-based materials to LiAlH4 [12–14] were confirmed feasible to improve hydrogen storage properties. Although some additives are beneficial to facilitate the dehydrogenation of complex metal hydrides, they cannot contribute to the total amount of released hydrogen. Instead, a reduction of the total hydrogen storage capacity may happen. Therefore, the idea of destabilization of complex metal hydrides by admixing with other metal hydrides is introduced. For example, Vajo et al. [15] demonstrated the effects of MgH2 on destabilizing LiBH4 by the formation of MgB2 during thermal dehydrogenation. Based on this fact, there are many studies investigating the destabilized effect between mixed hydrides, such as NaAlH4–MgH2 [16,17], NaAlH4–Mg(AlH4)2 [18,19], and LiBH4–LiNH2 [20]. Zhang et al. [21] firstly proposed that MgH2 was effectively in destabilizing LiAlH4. Chen et al. [22] revealed that the formation of Al12Mg17 alters the reaction pathway of LiAlH4–MgH2 and therefore could deteriorate the thermal stability of MgH2. Similar observations on the mutual destabilization between LiAlH4 and MgH2 have also been reported most recently by several research groups [23–25]. However, the specific role of MgH2 on assisting dehydrogenation of LiAlH4 is not clear yet. The mechanisms and reaction steps of LiAlH4–MgH2 system during thermal dehydrogenation are of interest and need to be further examined. In this study, in situ synchrotron XRD technique was employed to investigate the

W.-C. Hsu et al. / Journal of Alloys and Compounds 599 (2014) 164–169

dehydrogenation mechanism of LiAlH4–MgH2 composites. The role of MgH2 in the dehydrogenation of LiAlH4 was also discussed 2. Experimental Lithium aluminum hydride (LiAlH4, Chemetall, 97% purity) was mixed with magnesium hydride (MgH2, Aldrich, 95% purity) by mechanical milling. The precursors were preserved in a N2-purified glove box, in which the moisture and oxygen concentrations were maintained below 1 ppm. The mole ratio of LiAlH4 to MgH2 varied from 4:1, 2:1, 1:1, 1:2 and 1:4. The samples were designated as 4Li1Mg, 2Li1Mg, 1Li1Mg, 1Li2Mg and 1Li4Mg, respectively. In each batch, the hydrides mixture was loaded in a 75-ml cylindrical vessel made of stainless steel together with specific stainless steel balls. The ball-to-powder weight ratio was maintained at 10:1. Finally, mechanical mixing was performed using a high energy ball-milling machine (SPEX 8000) under 1700 rpm. for 0.5 h Dehydrogenation behavior of the above various powders was evaluated using a thermogravimetric analyzer (TGA) incorporating a high-pressure microbalance (Cahn D-110). The amount of H2 released and the dehydrogenation temperature were measured and determined. In each test, the sample with an initial weight of ca. 200 mg was loaded in a quartz crucible and transferred into the TGA chamber. Subsequently, the chamber was evacuated to 1  104 torr level, followed by the introduction of H2 gas (99.999% purity) to ambient pressure. Until the microbalance system was stabilized, the TGA was executed from room temperature to target temperature at a heating rate of 5 °C min1. The crystal structures of various powder mixtures used in this study, before and after dehydrogenation, were identified by an X-ray diffractometer (Rigaku MiniFlex II, Cu Ka radiation). To gain inside the dehydrogenation reaction mechanisms simultaneously during heating, in situ synchrotron X-ray diffraction (in situ XRD) analysis was performed with the assistance of the synchrotron radiation facility (beamline 01C2 in National Synchrotron Radiation Research Center in Hsinchu, Taiwan). In each analysis, the sample was loaded in a 1-mm diameter glass capillary tube, and then mounted to the specimen holder. One end of the tube was introduced with a dynamic N2 gas flow, and the other end was open to the atmosphere. During diffraction analysis, the sample was uniformly heated from room temperature to 405 °C at a ramping rate of 5 °C min1 using a hot-air blower. Wavelength of the synchrotron X-ray was 1.033209 Å. Every 2-D diffraction pattern was successively collected with the exposure time of 132 s and a total acquiring time of 4 min by a Mar345 imaging plate. The 2-D diffraction pattern was then converted to 1-D pattern by the Fit2D software. The high temperature transition of the crystal structure of a synthesized powder during dehydrogenation was thus analyzed.

3. Results and discussion 3.1. Preparation of LiAlH4–MgH2 composites Fig. 1 shows the XRD patterns of various LiAlH4–MgH2 powders, revealing that the diffraction peaks of LiAlH4 and MgH2 appeared

(a)

165

in all composite powders. No intermetallic compounds were found after milling, indicating the energy produced during mechanical milling was insufficient to induce any reaction between LiAlH4 and MgH2. As expected, the diffraction peak intensity of LiAlH4 decreased with an increasing amount of MgH2, in the mixture. 3.2. Thermogravimetric analysis Dehydrogenation temperatures and the amount of hydrogen released from the as-milled LiAlH4, MgH2 and LiAlH4–MgH2 composites were evaluated quantitatively by TGA. As seen in Fig. 2, LiAlH4 and MgH2 (both ball-milled for 0.5 h) began to dehydrogenate at 175 °C and 385 °C, respectively. The former exhibited two-step dehydrogenation process as described in reactions (1) and (2) mentioned above, while single-step H2 discharging according to the following reaction was observed:

MgH2 ! Mg þ H2

ð3Þ

For all LiAlH4–MgH2 composites, three-step dehydrogenation characteristics were observed. At temperature roughly below 270 °C, their TGA curves revealed a feature similar to that of plain LiAlH4. These results suggested that the first and the second steps were associated with the successive dehydrogenation reactions of LiAlH4 in the mixtures. It was evident that the first dehydrogenation temperature was lowered comparing with that of plain LiAlH4, which also decreased with increasing MgH2 content in the mixtures. These results indicated that MgH2 played an effective role in catalyzing H2 discharging of LiAlH4. For LiAlH4–4MgH2 composite, the first dehydrogenation temperature was 145 °C, which was 30 °C lower than that of pure LiAlH4 (175 °C). As seen in Fig. 2, the amount of hydrogen released in the first step (mainly from LiAlH4) varied with the composition of the composite, which decreased with increasing MgH2 content in the mixture. A plateau region was observed in each TGA curve of the LiAlH4– MgH2 composites, as shown in Fig. 2, indicating the completion of the second step dehydrogenation reaction of LiAlH4. The on-set of the third step dehydrogenation was mainly associated with the dissociation of MgH2. It is worth noting that a substantial decrease in the dissociation temperature of MgH2 (from 385 °C to 265–305 °C) was observed, which was mainly attributed to the catalytic effects of the reaction products formed in the first and/or

(b)

Fig. 1. XRD patterns of various ball-milled LiAlH4–MgH2 composites (LiAlH4:MgH2 = 4:1, 2:1, 1:1, 1:2, 1:4).

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Hydrogen desorption (wt%)

0 a

-2

b

c

-4 0.5 h ball-milled LiAlH4

a b c d e

-6

0

d

0.5 h ball-milled MgH2

LiAlH 4-4MgH 2

e

LiAlH 4-2MgH 2 LiAlH 4-MgH2 2LiAlH4-MgH 2 4LiAlH4-MgH 2

100

200

300

400

500

Temperature (o C) Fig. 2. TGA results of various ball-milled LiAlH4, MgH2 and LiAlH4–MgH2 composites (LiAlH4:MgH2 = 4:1, 2:1, 1:1, 1:2, 1:4).

Table 1 TGA results showing the dehydrogenation temperature and the amounts of hydrogen released from LiAlH4, MgH2 and their composites. Powders

Initial dehydrogenation temperature (°C)

LiAlH4 MgH2 4Li1Mg 2Li1Mg 1Li1Mg 1Li2Mg 1Li4Mg

175

LiAlH4

155 155 150 150 145

Total amount of H2 released (wt%)

MgH2 385 265 290 285 300 305

et al. [23]. The dependence of the desorption temperature of a hydride which has the higher desorption temperature on the composition of the composite might be described according to the composites rule-of-mixtures (ROM) as proposed by Czujko et al. [23] The TGA results clearly indicated that LiAlH4 and MgH2 could catalyze each other to lower their respective dehydrogenation temperatures. The initial dehydrogenation temperatures and the total amounts of H2 released from LiAlH4, MgH2 and their composites as measured from TGA analyses are summarized in Table 1. The XRD results shown in Fig. 3(a) and (b) demonstrated the final products formed in the Li-rich and Mg-rich composites after TGA tests. For Li-rich composites such as 4Li1 Mg and 2Li1 Mg, the formation of Al3Mg2 phase suggests that there had certain reactions occurred between MgH2 and Al which was one of the products after dehydrogenation of LiAlH4. For equal molar (1Li1 Mg) and Mg-rich composites (1Li2 Mg and 1Li4 Mg), two additional intermetallic compounds such as Al12Mg17 and Li0.92Mg4.08 were formed during the dehydrogenation process. Table 2 lists the crystal structure data from ICCD (The International Centre for Diffraction Data) for compounds involving in the dehydrogenation reaction of LiAlH4–MgH2 composites. The detailed dehydrogenation reaction mechanisms of LiAlH4–MgH2 composites will be discussed latter based on the results of in situ synchrotron XRD analysis.

3.3. In-situ synchrotron XRD analysis

6.64 6.30 6.42 5.78 5.77 5.31 5.27

Note: All ball-milled for 0.5 h.

the second step dehydrogenation reaction of LiAlH4, as will be discussed latter. Moreover, the dissociation temperature of MgH2 decreased with increasing amount of LiAlH4 present in the mixture. Similar observations were reported by Varin et al. [16] and Czujko

(a)

During in situ synchrotron XRD analysis, the powders were heated at a rate of 5 °C min1 from room temperature up to 405 °C in order to induce dehydrogenation reactions. The diffraction patterns were then collected every 20 °C. Fig. 4(a) displays the in situ synchrotron XRD patterns of the 1Li1 Mg composite. When increasing the temperature to 165 °C, the peaks corresponding to Li3AlH6 and Al appeared in the diffraction patterns, indicating the onset of the first step dehydrogenation reaction as described in reaction (1). Heating the powders continuously to about 225 °C led to the gradual decomposition of Li3AlH6 to form LiH and Al, in accordance with the second step dehydrogenation reaction as described in reaction (2).

(b)

Fig. 3. XRD patterns of (a) Li-rich composites, and (b) Mg-rich composites after dehydrogenation at 400 °C.

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W.-C. Hsu et al. / Journal of Alloys and Compounds 599 (2014) 164–169 Table 2 List of crystal structure data from ICCD for compounds involving in the phase transformation of 1Li1Mg and 1Li4Mg samples. Compound

System

Space group

Group number

Lattice parameter (Å)

Ref

LiAlH4 b-MgH2 Li3AlH6 Al3Mg2 Al12Mg17 Li0.92Mg4.08 LiMgAlH6

Monoclinic Tetragonal Monoclinic Cubic Cubic Hexagonal Hexagonal

P21/n P42/mnm P21/m Fd-3m I-43m P63/mmc P321

14 136 11 227 217 194 150

a = 4.845, b = 7.536, c = 7.826 b = 103.94° a = 4.517, b = 4.517, c = 3.0205 a = 7.892, b = 8.096, c = 5.65 b = 91.85° a = 28.39, b = 28.39, c = 28.39 a = 10.549, b = 10.549, c = 10.549 a = 3.192, b = 3.192, c = 5.132 a = 7.896, b = 7.896, c = 4.379

[26] [27] [28] [29] [30] [31] [32]

(a)

(b)

Fig. 4. (a) In-situ synchrotron XRD patterns of the LiAlH4–MgH2 composite (1Li1 Mg) heated from room temperature to 405 °C; (b) variation of the highest peak intensity with temperature for the major species appeared during heating.

Raising the temperature above 305 °C, MgH2 and Al gradually decreased in their intensities along with the appearance of Al3Mg2 and Al12Mg17 peaks, indicating the possible occurrence of the following two additional reactions:

2MgH2 þ 3Al ! Al3 Mg2 þ 2H2

ð4Þ

intensity is also summarized in Fig. 5(b). The results showed that LiAlH4 started to decompose at 125 °C. Interestingly, a new phase, namely LiMgAlH6, was formed beside Li3AlH6 with the expense of LiAlH4. In the Mg-rich composite, it was likely that a reaction between LiAlH4 and MgH2 occurred according to the following reaction:

17MgH2 þ 12Al ! Al12 Mg17 þ 17H2

ð5Þ

LiAlH4 þ MgH2 ! LiMgAlH6

As a result, premature decomposition of MgH2 with the help of the above two reactions gave rise to lowering its dehydrogenation temperature. Since Al was one of the dehydrogenation products of reactions (1) and (2), the presence of LiAlH4 indirectly catalyzed the dehydrogenation reaction of MgH2. The progressive transformation of reaction products in view of the changes of their intensities with temperature, derived from Fig. 4(a), is manifested in Fig 4(b). The reaction steps and their corresponding temperatures in dehydrogenation are clearly demonstrated, which are consistent with the TGA analysis. Fig. 5(a) shows the in situ synchrotron XRD patterns of 1Li4 Mg composite. The temperature-dependent X-ray diffraction peak

ð6Þ

Continued heating to 165 °C, the peak intensity of LiMgAlH6 disappeared but accompanied with increasing in the intensities of Al, Li3AlH6 and MgH2, indicating the decomposition of LiMgAlH6 according to the following reaction:

LiMgAlH6 ! 1=3Li3 AlH6 þ MgH2 þ 2=3Al þ H2

ð7Þ

The decomposition of LiMgAlH6 to release H2 might start at temperature as low as 145 °C, as revealed in TGA results shown in Fig. 2 and Table 1. The formation of this intermediate phase seemed beneficial for H2 desorption from LiAlH4, though through an indirect pathway and with the aid of MgH2. In other words, MgH2 could also play catalytic role in destabilizing LiAlH4.

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(a)

(b)

Fig. 5. (a) In situ synchrotron XRD patterns of the LiAlH4–4 MgH2 composite (1Li4Mg) heated from room temperature to 405 °C; (b) variation of the highest peak intensity with temperature for the major species appeared during heating.

Further raising the temperature to 205 °C, Li3AlH6 gradually decomposed to form Al and LiH according to reaction (2). The absence of LiH peaks in Fig. 5 was due to the low concentration of LiAlH4 present in the 1Li4 Mg composite. As the temperature was increased to 325 °C, the peaks corresponding to Al12Mg17 and Li0.92Mg4.08 intermetallic compounds appeared. These two compounds were likely formed according to reaction (5) and/or the following reaction:

4:08MgH2 þ 0:92LiH ! Li0:92 Mg4:08 þ 4:54H2

ð8Þ

The XRD patterns shown in Fig. 5 revealed that Al12Mg17 and/or Li0.92Mg4.08 might be formed at temperature below 325 °C, which was lower than that of the dehydrogenation temperature of MgH2 according to reaction (3). Furthermore, TGA results also indicated that the desorption of H2 from the LiAlH4–MgH2 composites could start at temperature around 300 °C. Thus, assisted dehydrogenation of MgH2 through reactions (5) and (8) could occur. Since Al and LiH participated in reactions (5) and (8) were the intermediate products formed during dehydrogenation of LiAlH4, the catalytic role of LiAlH4 regarding dehydrogenation of MgH2 was quite clear. 4. Conclusions The dehydrogenation behavior of LiAlH4–MgH2 composites with various compositions was investigated employing TGA and in situ X-RD analyses. TGA results showed that all LiAlH4–MgH2 composites studied revealed three-step dehydrogenation processes, with the first and the second steps corresponded to the decomposition of LiAlH4, while the third step associated with MgH2. The initial dehydrogenation temperature of LiAlH4 and that of MgH2 in all composites were lower than their pure counterparts. Both LiAlH4 and MgH2 exhibited mutual catalytic effects in assisting their dehydrogenation reaction through indirect pathways.

Intermediate reaction products such as Al, Li3AlH6, LiMgAlH6, LiH, Al3Mg2, Al12Mg17, and Li0.92Mg4.08 were confirmed by performing in situ XRD analysis. Moreover, the specific reaction steps during dehydrogenation of LiAlH4–MgH2 composites in the temperature range of 25–405 °C were identified. Acknowledgements The authors gratefully acknowledge the financial support from the National Science Council of the Republic of China (Taiwan) under Grant NSC 100-2221-E-006-245-MY2 and the Research Center for Energy Technology and Strategy, National Cheng Kung University under Grant D100-23003. References [1] F. Schüth, B. Bogdanovic´, M. Felderhoff, Chem. Commun. (2004) 2249. [2] I.P. Jain, P. Jain, A. Jain, J. Alloys Comp. 503 (2010) 303. [3] B. Sakintuna, F. Lamari-Darkrim, M. Hirscher, Int. J. Hydrogen Energy 32 (2007) 1121. [4] T.N. Dymova, V.N. Konoplev, D.P. Aleksandrov, A.S. Sizareva, T.A. Silina, Russ. J. Coord. Chem. 21 (1995) 165. [5] V.P. Balema, K.W. Dennis, V.K. Pecharsky, Chem. Commun. (2000) 1665. [6] J.R. Ares, K.F. Aguey-Zinsou, M. Porcu, J.M. Sykes, M. Dornheim, T. Klassen, R. Bormann, Mater. Res. Bull. 43 (2008) 1263. [7] Wei-Che Hsu, Cheng-Hsien Yang, Wen-Ta Tsai, Int. J. Hydrogen Energy 3 (9) (2014) 927. [8] US Department of Energy, Targets for onboard hydrogen storage systems for light-duty vehicles, 2009 September (retrieved 10 5 2012). [9] A. Andreasen, T. Vegge, A.S. Pedersen, J. Solid State Chem. 178 (2005) 3672. [10] M. Resan, M.D. Hampton, J.K. Lomness, D.K. Slattery, Int. J. Hydrogen Energy 30 (2005) 1417. [11] J.R. Ares Fernandez, F. Aguey-Zinsou, M. Elsaesser, X.Z. Ma, M. Dornheim, T. Klassen, R. Bormann, Int. J. Hydrogen Energy 32 (2007) 1033. [12] L. Hima Kumar, B. Viswanathan, S. Srinivasa Murthy, Int. J. Hydrogen Energy 33 (2008) 366. [13] M.S.L. Hudson, H. Raghubanshi, D. Pukazhselvan, O.N. Srivastava, Int. J. Hydrogen Energy 35 (2010) 2083. [14] M. Ismail, Y. Zhao, X.B. Yu, A. Ranjbar, S.X. Dou, Int. J. Hydrogen Energy 36 (2011) 3593.

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