Study on Hydrogen Release Capacity of LiAlH4 Doped with CeO2

Study on Hydrogen Release Capacity of LiAlH4 Doped with CeO2

Rare Metal Materials and Engineering Volume 43, Issue 3, March 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 3, March 2014 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2014, 43(3): 0544-0547.

ARTICLE

Study on Hydrogen Release Capacity of LiAlH4 Doped with CeO2 Liu Shenglin1,

Ma Qiuhua2,

Lü Heng1,

Zheng Xueping1,

Feng Xin1,

Xiao Guo1,

Zheng Jiaojiao1 1

2

Chang’an University, Xi’an 710061, China; Henan University of Technology, Zhengzhou 450001, China

Abstract: This study mainly analyzed the effect of CeO2 on the hydrogen release capacity of LiAlH4 by PCT (Pressure-Composition-Temperature) device and SEM (Scanning Electron Microscope) analysis method. The results show that with CeO2 doping shortens markedly the time of hydrogen release of LiAlH4. Among all samples, the dehydrogenation time of the sample doped with 2 mol% CeO2 is the earliest. The study on the amount of hydrogen release finds that the sample doped with 1 mol% CeO2 presents the largest dehydrogenation amount. And with the increase of the doping amount from 1 mol% to 5 mol%, the total hydrogen release amount of the samples presents a decrease trend. Furthermore, the study on microstructures show that doping with CeO2 does not cause any obvious change of the microstructure of LiAlH4, and all samples show a kind of flocculent structure. Key words: hydrogen release amount; hydrogen release rate; LiAlH4; CeO2

Lightweight complex hydrides (e.g., alkali metal alanates), owing to their high volumetric and gravimetric capacities for hydrogen, are considered as a promising candidate for on-board hydrogen applications. For example, the hydrogen storage amounts of NaAlH4 and LiAlH4 can achieve 7.4 wt% and 10.5 wt%, respectively. In order to improve the hydrogen storage capacity of LiAlH4, a large number of study mainly focused on the adding of catalysts[1-5]. Mirna Resan et al[6] reported that the addition of TiCl3 and TiCl4 to LiAlH4 eliminated the first stage of hydrogen evolution and significantly lowered the decomposition temperature of the second step. Furthermore, some new catalysts have been studied, such as NH4Cl[7], TiF3[8], nanofiber[9], nano nickel[10], NH3[11] etc. However, the study on the rare earth compounds is very few. In this work, the study on the effect of CeO2 on hydrogen desorption of LiAlH4 was carried out in order to analyze the roles of rare earth compounds as catalysts.

1

Experiment LiAlH4, purity ≥ 98% was purchased from Tianjin Beidoux-

ing Fine Chemical Co., Ltd. LiAlH4 was used as the received with no additional purification. Appropriate amount of the reagents (usually 1 g) was put in the stainless steel vials, with stainless steel balls of 8 mm diameter and 4 mm diameter. The ratio of raw materials to balls was 1:32. The samples were handled and stored in a glove box with a dry argon atmosphere to prevent from contact with oxygen or moisture. The milling was carried out in a planetary ball mill for some time at a gyration rate of 400 r/min. The milling time was 30 min. Due to the unstable chemistry performance of LiAlH4, all the operations such as weighing, mixing, storage and preparing of samples were done under dry argon atmosphere in a glove box in order to prevent any reaction with water vapor and oxygen in air. Hydrogen-desorption experiments were carried out in the PCT apparatus. This can be operated up to 10 MPa and 400 °C. The pressure of hydrogen released in relation to volume was displayed by a pressure transducer. The experiments were done using a reactor. The samples were heated in vacuum at a heating rate of 2 °C/min. All the pressure and temperature data

Received date: March 18, 2013 Foundation item: National Natural Science Foundation of China (50806007); Central Key Projects of Scientific Research in Colleges and Universities Operating Costs (CHD2011ZD008) Corresponding author: Liu Shenglin, Ph. D., Associate Professor, School of Materials Science and Engineering, Chang’an University, Xi’an 710061, P. R. China, E-mail: [email protected] Copyright © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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Results and Discussion

This part of the experiment has mainly studied the hydrogen release properties of the samples doped with the CeO2 of different molar percentages. The CeO2 doping amount (mol%) are 0.5, 1, 2, 3, 4, 5 and 6. Fig.1 shows the relation curves on hydrogen release amount of the LiAlH4 samples doped with CeO2 and the time hydrogen release. It can be seen that the hydrogen release amount of the sample doped with 1 mol% CeO2 is much larger than those of the other doped samples comparatively, while the hydrogen release amount of the LiAlH4 doped with 0.5 mol% and 2 mol% CeO2 are also much larger than those of the other samples. And as the doping amount increases from 1 mol% to 5 mol%, the final hydrogen release amounts of the samples decrease, but when the doping amount is 6 mol%, the hydrogen release amount is increased. In addition, according to the slopes of the hydrogen release amount curves we can see that the hydrogen release rate variations of the doped samples in the whole hydrogen-release process are similar. Namely the starting hydrogen release speed is quick; with the extension of time, the hydrogen release rate increases rapidly, and then gradually decrease. Furthermore, it can be also seen that CeO2 doping obviously shortens the dehydrogenation time of LiAlH4 among all the samples, and the dehydrogenation time of the sample doped with 2 mol% CeO2 is the earliest. Fig.2 shows the relationship between the maximum hydrogen release amount and the doping amounts. It is easy to be seen that only the hydrogen release amounts of the samples doped with 0.5 mol% and 1 mol% are larger than that of the original sample. In addition, when the doping amount is among 1 mol%~5 mol%, with the increase of the CeO2 doping amount, the hydrogen release amounts of the samples decrease gradually, but when the doping amount is 6 mol%, the hydrogen release amount increases. When the doping amount is 1 mol%, the hydrogen release amount reaches the maximum value, and it reaches the minimum value at the doping amount of 5 mol%. Fig.3 shows the curves of the hydrogen release rate and the time of the samples doped with CeO2. It can be seen that the hydrogen release rate of the doped samples are higher than that of the original sample, and the hydrogen release starting time is much earlier, namely the hydrogen release rate peak values of the samples doped with 0.5 mol%~6 mol% CeO2 appear earlier than that of the original sample. The original sample and the samples doped with 0.5 mol%~6mol% CeO2 show that with the extension of time, their hydrogen release

6 5 4

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

3 2 1 0 –1 –20

20

60

100

140

Time/min Fig.1 Curves of the hydrogen release amount and the time of the LiAlH4 samples doped with 0 mol%~6 mol% CeO2

Hydrogen Release Amount/wt%

2

7

6.4 6.0 5.6 5.2 4.8 0

1

2

3

4

5

6

CeO2 Concentration/mol% Fig.2 Relationship between the hydrogen release amount of the LiAlH4 samples doped with 0 mol%~6 mol% CeO2 and the concentration of CeO2 Dehydrogenation Rate/wt%·min-1

were acquired during the heating process, and the dehydrogenation curves were drawn by special software. Besides, SEM samples were also prepared in the glove box under a dry argon atmosphere and scanning electron microscopy tests were performed with a S4800 SEM made in Japan.

Hydrogen Release Amount/wt%

Liu Shenglin et al. / Rare Metal Materials and Engineering, 2014, 43(3): 0544-0547

4

0.4

8 5 2

0.3

3 1

1 2 3 4 5 6 7 8

7 6

0.2

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

0.1 0.0 –20

20

60

100

140

Time/min Fig.3 Curves of the hydrogen release rate and the time of the LiAlH4 samples doped with 0 mol%~6 mol% CeO2

speeds mainly increase in the beginning, when the hydrogen release rate reaches to a certain value, they begin to decrease. Furthermore, the result indicates that the maximum hydrogen release rates of the samples doped with 0.5 mol%~4 mol% CeO2 are all higher than that of the original sample, while the maximum hydrogen release rates of the samples doped with 5 mol%~6 mol% CeO2 are lower than that of the original sample.

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Liu Shenglin et al. / Rare Metal Materials and Engineering, 2014, 43(3): 0544-0547

Hydrogen Release Amount/wt%

The hydrogen release rate of the sample doped with 2 mol% CeO2 is the highest. Fig.4 gives the amount of hydrogen release of the samples doped with 0.5 mol%~6 mol% CeO2 in the first and second steps. It is obvious that the hydrogen release amount of the sample doped with 1 mol% CeO2 is the largest in the first and second steps. The hydrogen release amount of the sample doped with 4 mol% CeO2 in the first step is the least, while the sample doped with 5 mol% CeO2 has the least amount of hydrogen release in the second step. Fig.5 gives the SEM images of LiAlH4 sample and the samples doped with CeO2. It can be found that the microstructures of all samples presents a kind of flocculent structure. Compared to the undoped sample, the microstructures of the

5 4 The first stage of hydrogen desorption

3

The second stage of hydrogen desorption

2 1 0

1

2

3

4

5

6

CeO2 Level/mol% Fig.4 Hydrogen release amount of the LiAlH4 samples doped with 0 mol%~6 mol% CeO2 in the first and second stage

a

b

c

d

0.5 μm

0.5 μm

0.5 μm

0.5 μm

e

f

g

h

0.5 μm

250 nm

0.5 μm

0.5 μm

Fig.5 SEM images of the LiAlH4 samples doped with different amounts of CeO2: (a) 0 mol%, (b) 0.5 mol%, (c) 1 mol%, (d) 2 mol%, (e) 3 mol%, (f) 4 mol%, (g) 5 mol%, and (h) 6 mol%

samples doped with CeO2 do not change obviously. In addition, a lot of pores are found on the surfaces of the powder of all samples. We consider that these pores may be left by over flowing hydrogen when the samples are milled.

1 Balema V P, Wiench J W, Dennis K W et al. Journal of Alloys and

3 Conclusions

3 Ismail M, Zhao Y, Yu X B et al. International Journal of Hydrogen

1) Doping with CeO2 shortens the time of the hydrogen release of LiAlH4. Among all samples, the dehydrogenation time of the sample doped with 2 mol% CeO2 is the earliest. 2) The sample doped with 1 mol% CeO2 presents the largest dehydrogenation amount. As the doping amount of CeO2 increase from 1 mol% to 5 mol%, the total hydrogen release amounts of the samples decreases except for the sample doped with 6 mol% CeO2. 3) Doping with CeO2 does not cause any obvious change of the microstructure of LiAlH4, and all samples show a kind of flocculent structure.

References

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