Dehydrogenation Properties of LiAlH4 Doped with Rare Earth Oxides

Dehydrogenation Properties of LiAlH4 Doped with Rare Earth Oxides

Rare Metal Materials and Engineering Volume 43, Issue 4, April 2014 Online English edition of the Chinese language journal Cite this article as: Rare ...

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Rare Metal Materials and Engineering Volume 43, Issue 4, April 2014 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2014, 43(4): 0799-0802.

ARTICLE

Dehydrogenation Properties of LiAlH4 Doped with Rare Earth Oxides Wang Xingang1, 1

Wang Liang2,

Zhang Huailong1,

Shi Bin1,

Zheng Xueping1

2

Chang’an University, Xi’an 710061, China; Shandong University of Science and Technology, Qingdao 266590, China

Abstract: The influences of Gd2O3 and Nd2O3 on the dehydrogenation properties of LiAlH4 were studied by PCT (pressure-composition-temperature) equipment and SEM (scanning electron microscope) analysis method. The results show that the samples doped with Gd2O3 and Nd2O3 have very good dehydrogenation properties under the same conditions. The studies on doping amount (0.5, 1, 2, 3, 4, 5, 6 mol%) demonstrate that the influences of Gd2O3 and Nd2O3 on the dehydrogenation amounts of LiAlH4 are very similar. With increasing of doping amount, the dehydrogenation amount of LiAlH4 shows a gradual decrease. Meanwhile, the starting time of hydrogen release for the doped LiAlH4 sample is shortened significantly. In addition, the influences of Gd2O3 and Nd2O3 doping on the microstructure of LiAlH4 are not obvious. Key words: dehydrogenation properties; doping amount; microstructure

The world economic development brings about a huge change of energy structure, because coal, oil and other non-renewable energy sources face drying up. Thus, the resulting competitions of various countries for limited resources in the global scope become increasingly fierce. Moreover, CO2, SO2 and other gases generated by burning the coal, oil and their downstream products have caused the greenhouse effect and acid rain, making the human face severe challenges of energy, resources and environmental crisis. In order to realize the sustainable development of human beings, looking for new environmentally friendly alternative energy sources has become extremely urgent[1]. Hydrogen energy, with its renewable and environmental-friendly characteristics, has become the most potential energy carrier. However, a problem does remain in regard to the storage and transportation of energies. In 1997, Bogdanovic and Schwichardi reported that NaAlH4 doped with Ti had very good reversible hydrogen storage performances[2]. Since then, the complex hydrides drew the attention of researchers with the high hydrogen storage capacity. For example, the hydrogen contents of NaAlH4 and LiAlH4 reached 7.4 wt% to 10.5 wt%, respectively, which has shown a good application prospect[3].

At present, a large number of researches have mainly focused on improving the hydrogen absorption and desorption properties of LiAlH4 by ball-milling process and adding catalysts[4-7]. The principal catalysts studied were elemental titanium, TiCl4, TiCl3, AlCl3, FeCl3, elemental iron, elemental nickel, vanadium, carbon nanofibres and rare metal and rare-earth metals including their compounds and nanoforms[8-10]. For example, Yoshitsugu Kojima studied the effects of various catalysts such as TiCl3, ZrCl4, VCl3, NiCl2 and ZnCl2 on the hydrogen release characteristics of LiAlH4. It was established that the catalytic activity defined by the hydrogen desorption capacity by a mechanochemical reaction decreased in following series TiCl3>ZrCl4>VCl3>NiCl2> ZnCl2[11]. In previous work, we studied the effect of LaCl3 on the hydrogen storage properties of LiAlH4. The results showed that doping with LaCl3 could improve the hydrogen storage properties of LiAlH4. Increasing the amount of LaCl3 from 1 to 6 mol% caused the marked changes in the behavior of hydrogen release of the LiAlH4 samples, and the total amount of hydrogen release increased first and then decreased[12]. We studied the influences of rare earth compounds Gd2O3 and Nd2O3 on the hydrogen release properties of LiAlH4.

Received date: September 18, 2013 Foundation item: the Fundamental Research Funds for the Central Universities (CHD2012ZD010) Corresponding author: Wang Xingang, Associate Professor, School of Materials Science and Engineering, Chang’an University, Xi’an 710061, P. R. China, Tel: 0086-29- 82337348, E-mail: [email protected] Copyright © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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Experiment

As-received LiAlH4 (93 wt% pure) was supplied by J&K Chemical Ltd. Gd2O3 (99 wt% purity) and Nd2O3 (99.9 wt% purity) were obtained from Sigma-Aldrich Ltd. All the raw materials were used without further purification so as to have a simple and economy process. According to the design and computation of this experiment, (0~6) mol% Gd2O3-LiAlH4 and Nd2O3-LiAlH4 samples, usually 1 g LiAlH4 admixed different mole amount of Gd2O3 and Nd2O3, were prepared. Due to the unstable chemistry performance of LiAlH4, all operations such as weighing, admixing, storage and preparation of samples were done under a dry argon atmosphere in a glove box in order to prevent any reaction with water vapor and oxygen in air. The prepared samples were introduced to a stainless steel vials containing stainless steel balls (the mass ratio of ball to powder was 40:1) which were sealed under the argon. The ball milling was carried out for 60 min by a Planetary Ball Mill. During this process, the ball milling was stopped every 20 min and the agate jars were turned up and down for making powder homogeneous mixing. After ball-milling, the samples were immediately transferred to 3 mL glass bottles in the glove box under the dry argon atmosphere. Hydrogen desorption experiments were performed in pressurecomposition-temperature (PCT) apparatus which can be operated up to the pressure of 10 MPa and the temperature of 400 °C. About 0.1 g sample was loaded in the vessel of the reactor and then inserted into a furnace which was programmed at a heating rate of 6 °C/min to 320 °C maintaining for 1 h. During the heating process, the corresponding data such as pressure and temperature were acquired and the curves of hydrogen desorption properties were monitored by special software. The mass fraction of desorbed hydrogen was calculated by comparing with the total weight of sample. Besides, SEM samples were also prepared in the glove box under the dry argon atmosphere and observed by a Field Emission Scanning Electron Microscope (Hitachi S4800).

2 2.1

Results and Discussion

Influence of doping Gd2O3 on the hydrogen release properties of LiAlH4 Fig.1 shows the hydrogen release curves of the LiAlH4 samples doped with (0~6) mol% Gd2O3. From the figure we can clearly observe that the starting hydrogen release time of all the doped samples is significantly shortened compared to that of the pure LiAlH4 sample. The initial hydrogen release time of the LiAlH4 sample doped with 1 mol% Gd2O3 is the earliest, followed by the sample doped with 6 mol%. The starting dehydrogenation time of the sample doped with 0.5 mol% Gd2O3 is minimum among all the doped samples. In addition, it can be also seen from the slope of curve in Fig.1 that the hydrogen release rates of all the doped samples are quicker than that of the pure sample.

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Time/min Fig.1 Hydrogen release curves of the LiAlH4 samples doped with Gd2O3

The study on the amount of hydrogen release indicates that the dehydrogenation amounts of all the doped samples are lower than that of the pure LiAlH4 sample except the sample doped with 1 mol% Gd2O3, as shown in Fig.2. Furthermore, it is clear that with the increase of Gd2O3 amount from 1 mol% to 6 mol%, the dehydrogenation amount of the sample presents a gradual decrease. Fig.3 shows the relation curves between the hydrogen release rate and time in the sample doped with Gd2O3. It is not difficult to find that doping with catalysts causes the increase of the hydrogen release rate and the shortening of the starting dehydrogenation time of the doped samples compared to the pure LiAlH4 sample. In addition, with extending of time, the hydrogen release rates of all samples at the initial stage speed up quickly. When the hydrogen release rates increase to a certain value, they begin to decrease. Compared with other samples, the sample doped with 1 mol% Gd2O3 has a high hydrogen release rate within the shortest time; however, the sample doped with 0.5 mol% Gd2O3 has the highest rate of hydrogen release. Fig.4 shows the maximum hydrogen release rate of the sample doped with Gd2O3 as a function of the concentration of Gd2O3. It is clearly indicated that the maximum dehydrogenation rate of the doped sample is higher than that of the Amount of Hydrogen Desorption/wt%

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Fig.2 Relation curve of the hydrogen release amount of the samples doped with Gd2O3

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7 6 5 4 0 mol% 0.5 mol% 1 mol% 2 mol% 3 mol% 4 mol% 5 mol% 6 mol%

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Fig.5 Hydrogen release curves of samples doped with Nd2O3 Hydrogen Release Amount/wt%

Max Dehydrogenation Rate/wt%·min-1

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Fig.3 Hydrogen release rate curves of samples doped with Gd2O3

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Gd2O3 Concentration/mol% Fig.4 Max hydrogen release rate curve of the LiAlH4 samples doped with Gd2O3

undoped sample. In addition, the maximum hydrogen release rate in the sample doped with 0.5 mol% is much higher than those of other samples. The variation trend of maximum hydrogen release rate for other doped samples is relatively undulate. 2.2 Influence of doping Nd2O3 on the hydrogen release properties of LiAlH4 Fig.5 shows the relationship between the amount and the time of the hydrogen release of the samples doped with (0~6) mol% Nd2O3. Compared with pure LiAlH4 sample, the starting hydrogen release time of all the doped samples is obviously earlier. At the initial step, the dehydrogenation time of the samples doped with 4 mol% and 5 mol% is the earliest, while that of the pure LiAlH4 sample is the latest. According to the slope of curve in Fig.5, it can be also seen that the release rate in all the doped samples are faster than that of the pure sample. Fig.6 shows the relationship between the maximum dehydrogenation amount of the sample and the concentration of Nd2O3. The dehydrogenation amounts of the samples doped with 0.5 mol% and 1 mol% are significantly greater than that of the pure sample. The total dehydrogenation amount of the sample doped with 2 mol% is only slightly higher than that of the pure sample, while the dehydrogenation amounts of the other doped samples are lower than that of the pure sample. With the concentration of Nd2O3 increasing from 0.5 mol% to 6 mol%, the dehydrogenation amount of the doped sample

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Nd2O3 Concentration/mol% Fig.6 Relation curve of the hydrogen release amounts of the samples doped with Nd2O3

decreases gradually except that the dehydrogenation amount of the sample doped with 4 mol%. The relation curve between the maximum hydrogen release rate of the sample doped with Nd2O3 and the concentration of Nd2O3, is shown in Fig.7. It can be seen that the maximum hydrogen release rates in all the doped samples are higher than that of the pure sample. In particular, for the sample doped with (0.5, 1, 2) mol% Nd2O3, their maximum hydrogen release rates are much higher than that of the pure sample. The maximum hydrogen release rate of the sample doped with 0.5 mol% is the highest, and the sample doped with 6 mol% has the lowest maximum hydrogen release rate. Except that the maximum hydrogen release rate of the sample doped with 3 mol% is lower than that of the sample doped with 4 mol%, the overall changing trend is that with the increase of the doping amount from 0.5 mol% to 6 mol%, the maximum hydrogen release rate of LiAlH4 decreases gradually. 2.3 SEM analysis The above studies demonstrate that the influences of different dopants and doping amounts on the dehydrogenation properties of LiAlH4 are significantly different. In order to study further the influencing factors of different dopants and doping amounts on the dehydrogenation properties of LiAlH4, the microstructures of the samples doped with different dopants and doping amounts were observed and analyzed by FE-SEM. Fig.8 shows the SEM images of pure LiAlH4 sample

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Wang Xingang et al. / Rare Metal Materials and Engineering, 2014, 43(4): 0799-0802

and the samples doped with 0.5 mol% and 1 mol% Gd2O3 and Nd2O3. The particle sizes of all doped samples are smaller than that of pure LiAlH4. Furthermore, the microstructures of the samples doped with different doping amounts and different dopants do not present any significant differences, which proves that the influence of the dopant on the microstructure is not obvious.

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Nd2O3 Concentration/mol% Fig.7 Max hydrogen release rate curve of the LiAlH4 samples doped with Nd2O3 a

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Conclusions

1) Doping with rare earth oxides Gd2O3 and Nd2O3 can significantly improve the hydrogen release properties of LiAlH4. The hydrogen release amount decreases markedly with the increases of the doping amount. 2) The LiAlH4 sample doped with 1 mol% Gd2O3 has the maximum hydrogen release amount of about 6.02 wt%. Meanwhile, its starting hydrogen release time is also earliest. 3) The LiAlH4 sample doped with 0.5 mol% Nd2O3 has the maximum hydrogen release amount of about 6.42 wt%. 4) Doping with Gd2O3 and Nd2O3 has no obvious influence on the microstructure of LiAlH4.

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d

e

5 Easton D S et al. J Alloy Compd[J], 2005, 398(1-2): 245 6 Sun T et al. J Hydrogen Energy[J], 200833(21): 6216 7 Xu Lou et al. Rare Metal Mater Eng[J], 2012, 41(5): 910 (in Chinese) 8 Mirna Resan et al. Int J Hydrogen Energy[J], 2005, 30: 1413

5 μm Fig.8

9 Blanchard D et al. Mater Sci Energy[J], 2004, B108: 54 10 Din Rafiud et al. J Phys Chem C[J], 2011, 115: 13 088

SEM images of pure LiAlH4 sample (a) and the samples of

11 Yoshitsugu Kojima et al. J Alloy Compd[J], 2008, 462: 275

LiAlH4 doped with 0.5 mol% Gd2O3 (b), 1 mol% Gd2O3 (c),

12 Zheng Xueping et al. In J Hydrogen Energy[J], 2007, 32: 4957

0.5 mol% Nd2O3 (d), and 1 mol% Nd2O3 (e)

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