Study on dehydrogenation properties of the LiAlH4–NH4Cl system

Study on dehydrogenation properties of the LiAlH4–NH4Cl system

Journal of Alloys and Compounds 551 (2013) 508–511 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepa...

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Journal of Alloys and Compounds 551 (2013) 508–511

Contents lists available at SciVerse ScienceDirect

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

Study on dehydrogenation properties of the LiAlH4–NH4Cl system Zheng Xueping a,⇑, Zheng Jiaojiao a, Ma Qiuhua b, Liu Shenglin a, Feng Xin a, Lin Xiaobin a, Xiao Guo a a b

School of Materials Science and Engineering, Chang’an University, Xi’an 710061, China Materials Science and Engineering Institute, Henan University of Technology, Zhengzhou 450001, China

a r t i c l e

i n f o

Article history: Received 27 September 2012 Received in revised form 8 November 2012 Accepted 9 November 2012 Available online 19 November 2012 Keywords: LiAlH4–NH4Cl system Dehydrogenation properties Microstructure

a b s t r a c t In this work, the dehydrogenation properties of the LiAlH4–NH4Cl system and the LiAlH4–NH4Cl doped with CeO2 were analyzed by PCT (pressure–composition–temperature) equipment and SEM (Scanning Electron Microscope) analysis method. The result showed that the LiAlH4–NH4Cl system presented the fastest rate and the earliest starting time of hydrogen release when the molar ratio of LiAlH4 to NH4Cl was 1:1, and the dehydrogenation amount of the sample which the molar ratio of LiAlH4 to NH4Cl is 1:1 was very high and reached to 6.6 wt%. The study on doping with CeO2 showed that doping with CeO2 had further raised the rate and the amount of hydrogen release of the LiAlH4–NH4Cl system and shortened the starting time of hydrogen release. Furthermore, the result showed that doping with CeO2 made the flocculent microstructure of the LiAlH4–NH4Cl system more obvious and looser. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Compared with metal hydride hydrogen storage alloy, complex hydride hydrogen storage alloy obviously has higher hydrogen storage amount. For example: the hydrogen storage amount of NaAlH4 and LiAlH4 has achieved, respectively 7.4 and 10.5 wt%. Up to now, complex hydride hydrogen storage alloy as a new type of hydrogen storage alloy have a rapid development, but the research on reaction mechanism of complex hydride hydrogen storage alloy is still in a primary exploration stage. A great deal of research has focused on improving the hydrogen absorption and release properties of complex hydride hydrogen storage alloy, and the main method is to find better catalyst. For example, Blanchard et al. [1] have studied the Ti-based and V-based compounds. Resan et al. [2] and other scientists have studied TiH2, TiCl3, TiCl4, AlCl3, FeCl3, Fe, Ni, V, Ti and C. In addition, some new catalysts have been studied, such as NH4Cl [3], TiF3 [4], nanofiber [5], nano nickel [6], NH3 [7], etc. In addition, some researchers have studied the sample preparation technology, and hope to improve the hydrogen absorption and release properties of the complex hydride hydrogen storage alloys by changing the preparation technology. Recently, a series of composite complex hydrides as hydrogen storage system have been reported, such as: LiAlH4–MgH2–TiF3, LiBH4–MgH2, NaBH4/MH2, etc. [8–10]. Under this background, this study has chosen LiAlH4–NH4Cl as the study object to analyze the dehydrogenation properties of the LiAlH4–NH4Cl system and the effect of CeO2 on the hydrogen storage properties of the LiAlH4– NH4Cl system. ⇑ Corresponding author. Tel.: +86 13991873613. E-mail address: [email protected] (X. Zheng). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.051

2. Experimental method LiAlH4 (P98 wt% pure) was used as received with no additional purification. In order to avoid the influence of vapor attached to the surface of NH4Cl and CeO2 powder on the test results, NH4Cl and CeO2 powder was dried firstly at 100 °C. Furthermore, all samples were weighed in the glove box filled with dry argon. In order to obtain the LiAlH4–NH4Cl sample with the optimum molar ratio, the experiment firstly studied the hydrogen release properties of the LiAlH4–NH4Cl samples with molar ratios of LiAlH4 to NH4Cl (1:0.5, 1:1, 1:1.5 and 1:2), the purpose was to select the sample with the optimum molar ratio, on this basis, the influence of CeO2 on the hydrogen release properties of LiAlH4–NH4Cl composite system was discussed. The doping amount was 1, 2, 3, 4 and 5 mol%. In order to make the raw materials be uniformly mixed, the experiment firstly used a planetary ball mill to mill LiAlH4 and NH4Cl for 1 h, respectively, and then mixed the ball-milled powders for 5 min. At last, the ball-milled sample was put into a glove box filled with argon for loading. Hydrogen desorption experiments were carried out in pressure–composition– temperature (PCT) apparatus. This can be operated up to 10 MPa and at 400 °C. The pressure of hydrogen release in relation to volume was displayed by a pressure transducer. The experimental studies were done by a reactor. This consisted of two parts: heater and sample vessel. The former was used to connect with the pressure transducer and thermocouple. It had a 2.2 cm outside diameter (OD), 0.5 cm wall and 20 cm internal length. It was loaded with the sample vessel (1 cm OD, 0.1 cm wall and 5 cm internal length). The sample vessel was loaded with about 0.1 g of LiAlH4. The reactor was heated with an air furnace. During heating, the hydrogen released overflowed from the sample vessel firstly into the heater and then into the transit pressure transducer. The value of hydrogen pressure can be clearly read. According to the computing formula, the weight percentage of release hydrogen is obtained. However, it is necessary to be pointed out that the hydrogen was cooled to room temperature by the cool water after it overflowed from the sample vessel.

3. Results and discussion 3.1. Study on the molar ratio of LiAlH4 to NH4Cl In this work, the different molar ratios of LiAlH4 to NH4Cl (1:0.5, 1:1, 1:1.5, and 1:2) were studied. The samples were firstly heated

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to 200 °C and then kept for about 180 min under this temperature. According to the amount and the time of hydrogen release of all samples, the sample with the best properties of hydrogen release was selected out. Fig. 1 has showed the relation curves on the dehydrogenation amount and the dehydrogenation time of LiAlH4–NH4Cl system which the molar ratios of LiAlH4 to NH4Cl are 1:0.5, 1:1, 1:1.5, 1:2, respectively. It can be seen obviously when molar ratio of LiAlH4 to NH4Cl is 1:0.5, the dehydrogenation amount of the sample is the largest, followed by the sample which molar ratio of LiAlH4 to NH4Cl is 1:1. Their hydrogen release amount reaches to about 6.7 and 6.6 wt%, respectively. However, the starting dehydrogenation time of the sample which molar ratio of LiAlH4 to NH4Cl is 1:1 is the earliest among all samples. Comparatively, when the molar ratio of LiAlH4 to NH4Cl is 1:1.5 and 1:2, the dehydrogenation amount and the dehydrogenation time of the samples decrease markedly. By comparing the overall four curves it can be seen that, with the increase of the amount of NH4Cl, the hydrogenation amount of the LiAlH4–NH4Cl system decreases gradually, the time of hydrogen release is also delayed except for the LiAlH4– NH4Cl system which the molar ratio of LiAlH4 to NH4Cl is 1:1. Fig. 2 has given the relation curves of the dehydrogenation speed and the dehydrogenation time of the LiAlH4–NH4Cl samples which the molar ratio of LiAlH4 to NH4Cl are 1:0.5, 1:1, 1:1.5 and 1:2, respectively. It can be seen that, when the molar ratio of LiAlH4 to NH4Cl is 1:1, the rate of hydrogen release appears two obvious peak values at about 25 and 50 min, respectively. In addition, compared to other samples, the sample which molar ratio of LiAlH4 to NH4Cl is 1:1 has the fastest dehydrogenation rate. With increase of amount of NH4Cl, the maximum rate of hydrogen release decreases obviously. According to the above results, it is not difficult to be found that, when the molar ratio of LiAlH4 to NH4Cl is 1:1, the LiAlH4–NH4Cl system has the shortest time and the fastest rate of hydrogen release. The amount of hydrogen release is also very near to the largest amount of hydrogen release, while it is significantly higher than the dehydrogenation amount of other samples. Therefore, in this work, the LiAlH4–NH4Cl system with molar ratio of 1:1 is selected as the basal sample to study further the effect of catalyst on this system. 3.2. The influence of CeO2 on the dehydrogenation property of the LiAlH4–NH4Cl system

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Fig. 2. The relation curves on the rate and the time of hydrogen release of the LiAlH4–NH4Cl samples with different molar ratios of LiAlH4 to NH4Cl.

Fig. 3. The relation curves on the amount and the time of hydrogen release of the LiAlH4–NH4Cl samples doped with CeO2.

In this part work, the LiAlH4–NH4Cl sample doped with CeO2 was heated to 200 °C and kept for 90 min under this temperature.

Fig. 1. The relation curves on the amount and the time of hydrogen release of the LiAlH4–NH4Cl samples with different molar ratios of LiAlH4 to NH4Cl.

Fig. 4. The relation curves on the rate and the time of hydrogen release of the LiAlH4–NH4Cl samples doped with CeO2.

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X. Zheng et al. / Journal of Alloys and Compounds 551 (2013) 508–511

Fig. 5. SEM analysis on the LiAlH4–NH4Cl samples. (a) 1:0.5 mol; (b) 1:1 mol; (c)1 :1.5 mol; (d) 1:2 mol.

Fig. 6. SEM analysis on the LiAlH4–NH4Cl samples doped with CeO2. (a) Doped with 1 mol% CeO2; (b) Doped with 2 mol% CeO2; (c) Doped with 3 mol% CeO2; (d) Doped with 4 mol% CeO2; (e) Doped with 5 mol% CeO2.

X. Zheng et al. / Journal of Alloys and Compounds 551 (2013) 508–511

The result shows that the dehydrogenation starting time of the samples doped with 1–5 mol% CeO2 is earlier than that of the undoped sample (Fig. 3). Among them, the sample doped with 1 mol% CeO2 has the earliest starting time of hydrogen release, followed by the sample doped with 2 mol% CeO2. In addition, the dehydrogenation amount of the sample doped with 1 mol% CeO2 is obviously larger than that of the undoped LiAlH4–NH4Cl sample. However, with increasing of the amount of CeO2, the dehydrogenation amount of the LiAlH4–NH4Cl system presents a gradual decrease trend, and except for the sample doped with 1 mol% CeO2, the dehydrogenation amount of other samples is obviously lower than that of the undoped sample. Fig. 4 has given the relation curves on the dehydrogenation rate and the dehydrogenation time of the samples doped with CeO2. Compared to the other doped samples, the sample doped with 1 mol% CeO2 has the maximum rate of hydrogen release in the first 20 min, as prolonging time, the dehydrogenation rate of the sample doped with 2 mol% CeO2 increases. When the dehydrogenation time reaches 38 min, the sample doped with 3 mol% CeO2 shows the maximum hydrogen release rate, followed by the sample doped with 2 mol% CeO2. 3.3. SEM analysis In order to further study the effect CeO2 on the LiAlH4–NH4Cl system, the microstructures of the LiAlH4–NH4Cl system and the LiAlH4–NH4Cl system doped with CeO2 are analyzed by SEM (Figs. 5 and 6). The result shows that the microstructure of all samples presents a kind of flocculent structure. According to Fig. 5, it is very easy to be found that the microstructure of the LiAlH4–NH4Cl system has not presented markedly difference when increasing the molar ratio of LiAlH4 to NH4Cl from 1:0.5 to 1:2. While, compared with the undoped samples, the flocculent microstructure of the samples doped with CeO2 looks more obvious and looser and the powder particles of these samples looks more uniform and smaller (Fig. 6). In addition, the result from Fig. 6 shows that the flocculent microstructure of the samples doped with CeO2 changes more and more obvious and the powder particles of these samples also changes smaller and smaller with the increase of the mount of CeO2. Furthermore, among all the samples doped with CeO2, the sample doped with 5 mol% CeO2 has the most loose microstructure and the powder particles of this sample are also the most uniform. 4. Conclusions In this study, the dehydrogenation properties of the LiAlH4– NH4Cl system and the LiAlH4–NH4Cl doped with CeO2 are analyzed

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by PCT and SEM methods. The result shows that the LiAlH4–NH4Cl system has the maximum rate and the earliest starting time of hydrogen release when the molar ratio of LiAlH4 to NH4Cl is 1:1. Importantly, the dehydrogenation amount of the sample which the molar ratio of LiAlH4 to NH4Cl is 1:1 is very high and reaches to about 6.6 wt%. On this base, doping with CeO2 has further raised the dehydrogenation rate of the LiAlH4–NH4Cl system and shortened the starting time of hydrogen release. In addition, compared to other doped samples, the LiAlH4–NH4Cl doped with 1 mol% CeO2 has the maximum dehydrogenation amount, about 7 wt%. Furthermore, the study on microstructure finds that the microstructure of the LiAlH4–NH4Cl system and the LiAlH4–NH4Cl doped with CeO2 presents a kind of flocculent structure. However, compared to the LiAlH4–NH4Cl system, the flocculent microstructure of the LiAlH4–NH4Cl doped with CeO2 presents more obvious and looser and the powder particles of these doped samples look more uniform. Acknowledgements This research has received financial support from the Project financially supported by National Natural Science Foundation funded projects (50806007) and Central key projects of scientific research in colleges and universities operating costs (CHD2011ZD008). References [1] D. Blanchard, H.W. Brinks, B.C. Hauback, P. Norby, Mater. Sci. Eng. B108 (2004) 54–59. [2] M. Resan, M.D. Hampton, J.K. Lomness, D.K. Slattery, Int. J. Hydrogen Energy 30 (2005) 1413–1416. [3] Huajun Zhang, Yook Si Loo, Hans Geerlings, Jianyi Lin, Wee Shong Chin, Int. J. Hydrogen Energy 35 (2010) 176–180. [4] Shu-Sheng Liu, Li-Xian Sun, Yao Zhang, Fen Xu, Jian Zhang, Hai-Liang Chu, MeiQiang Fan, Tao Zhang, Xiao-Yan Song, Jean Pierre Grolier, Int. J. Hydrogen Energy 34 (2009) 8079–8085. [5] L. Hima Kumara, B. Viswanathan, S. Srinivasa Murthy, Int. J. Hydrogen Energy 33 (2008) 366–373. [6] Robert A. Varin, Leszek Zbroniec, Tomasz Czujko, Zbigniew S. Wronski, Int. J. Hydrogen Energy 36 (2011) 1167–1176. [7] Weifang Luo, Donald Cowgill, Ken Stewart, Vitalie Stavila, J. Alloys Comp. 497 (2010) L17–L20. [8] Jianfeng Mao, Zaiping Guo, Xuebin Yu, Mohammad Ismail, Huakun Liu, Int. J. Hydrogen Energy 36 (2011) 5369–5374. [9] Pattaraporn Sridechprasat, Yindee Suttisawat, Pramoch Rangsunvigit, Boonyarach Kitiyanan, Santi Kulprathipanja, Int. J. Hydrogen Energy 36 (2011) 1200–1205. [10] S. Garroni, C. Milanese, A. Girella, A. Marini, G. Mulas, E. Menéndez, C. Pistidda, M. Dornheim, S. Suriñach, M.D. Baró, Int. J. Hydrogen Energy 35 (2010) 5434– 5441.