Hydrogen desorption kinetics of the destabilized LiBH4AlH3 composites

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Hydrogen desorption kinetics of the destabilized LiBH4eAlH3 composites Haizhen Liu a,*, Li Xu a, Peng Sheng a, Shuangyu Liu a, Guangyao Zhao a, Bo Wang a, Xinhua Wang b,c,**, Mi Yan b,c a

State Key Laboratory of Advanced Transmission Technology, Global Energy Interconnection Research Institute, State Grid Corporation of China, Beijing 102209, PR China b State Key Laboratory of Silicon Materials, School of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, PR China c Key Laboratory of Novel Materials for Information Technology of Zhejiang Province, Zhejiang University, Hangzhou 310027, PR China

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abstract

Article history:

LiBH4 can be destabilized by AlH3 addition. In this work, the hydrogen desorption kinetics

Received 23 October 2016

of the destabilized LiBH4eAlH3 composites were investigated. Isothermal hydrogen

Received in revised form

desorption studies show that the LiBH4 þ 0.5AlH3 composite releases about 11.0 wt% of

29 November 2016

hydrogen at 450  C for 6 h and behaves better kinetic properties than either the pure LiBH4

Accepted 10 December 2016

or the LiBH4 þ 0.5Al composite. The apparent activation energy for the LiBH4 decomposi-

Available online xxx

tion in the LiBH4 þ 0.5AlH3 composite estimated by Kissinger's method is remarkably lowered to 122.0 kJ mol1 compared with the pure LiBH4 (169.8 kJ mol1). Besides, AlH3 also

Keywords:

improves the reversibility of LiBH4 in the LiBH4 þ 0.5AlH3 composite. For the LiBH4 þ xAlH3

Hydrogen storage materials

(x ¼ 0.5, 1.0, 2.0) composites, the decomposition kinetics of LiBH4 are enhanced as the AlH3

Lithium borohydride

content increases. The sample LiBH4 þ 2.0AlH3 can release 82% of the hydrogen capacity of

Aluminium hydride

LiBH4 in 29 min at 450  C, while only 67% is obtained for the LiBH4 þ 0.5AlH3 composite in

Decomposition kinetics

110 min. JohnsonMehlAvrami (JMA) kinetic studies indicate that the reaction

Kinetic model

LiBH4 þ Al / ‘LieAleB’ þ AlB2 þ H2 is controlled by the precipitation and subsequently growth of AlB2 and LieAleB compounds with an increasing nucleation rate. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction With a high hydrogen density of 18.5 wt%, lithium borohydride (LiBH4) has draw much attention for on-board hydrogen storage during the last decade [1e14]. However, LiBH4 is thermodynamically stable and only liberate hydrogen (Eq. (1)) at a temperature higher than 400  C at 1 bar H2 [1]. What's

more, the recovery of LiBH4 following the reverse reaction of Eq. (1) requires a temperature over 600  C and a hydrogen pressure higher than 155 bar H2 [2]. 1

2LiBH4 42LiH þ 2B þ 3H2 DH ¼ 67 kJ mol

H2




(1)

The metallic aluminium (Al) or Al-containing compounds (e.g., LiAlH4, Li3AlH6) have been employed as a destabilization

* Corresponding author. State Key Laboratory of Advanced Transmission Technology, Global Energy Interconnection Research Institute, State Grid Corporation of China, Beijing 102209, PR China. Fax: þ86 10 66601688. ** Corresponding author. Fax: þ86 571 87952716. E-mail addresses: [email protected] (H. Liu), [email protected] (X. Wang). http://dx.doi.org/10.1016/j.ijhydene.2016.12.083 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Liu H, et al., Hydrogen desorption kinetics of the destabilized LiBH4eAlH3 composites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.083

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agent to improve the hydrogen desorption/absorption properties of LiBH4 [15e27]. The Al-containing compounds like LiAlH4 or Li3AlH6 will first undergo decomposition to generate Al, which then reacts with LiBH4 based on Eq. (2) to form AlB2, thus destabilize LiBH4. It was theoretically estimated by Cho et al. that the decomposition temperature of Eq. (2) is only 188  C at 1 bar H2, which is significantly lower than that of the pure LiBH4 (403  C) [15]. 2LiBH4 þ Al42LiH þ AlB2 þ 3H2

(2)

In our previous work, aluminium hydride (AlH3) was employed as a novel Al source to destabilize LiBH4 [16]. It was found that AlH3 is better as an Al source than the metallic Al. This is due to both that AlH3 is a brittle metal hydride which can be easily refined by ball milling and that AlH3 can generate oxide-free Al after decomposition. The hydrogen desorption process of the LiBH4 þ 0.5AlH3 composite when heating from room temperature to 500  C is composed of three steps. The first step is the self-decomposition of AlH3 forming Al (Eq. (3)) and the second step is the subsequent decomposition of LiBH4 reacting with Al and forming AlB2 and LieAleB compounds (Eq. (4)). When the reaction Eq. (4) proceed to a certain extent, LiBH4 will no longer react with Al due to the kinetic barrier of the formed AlB2 and LieAleB layers covering the Al particle surfaces. In this case, the residual LiBH4 tends to undergo a self-decomposition forming LiH and B and releasing hydrogen, which corresponds to the third decomposition step (Eq. (1)). However, the hydrogen desorption kinetics of the LiBH4eAlH3 systems and the detailed reaction mechanism between LiBH4 and Al are still unclear. AlH3 /Al þ H2

(3)

LiBH4 þ Al/'Li  Al  B' þ AlB2 þ H2

(4)

In the present work, in order to gain an insight into the hydrogen desorption kinetics of the LiBH4eAlH3 systems and also into the kinetic model of the reaction between LiBH4 and Al, the hydrogen desorption kinetics of the LiBH4 þ 0.5AlH3 composite were first studied compared with the LiBH4 þ 0.5Al composites. Then, the hydrogen desorption properties of the LiBH4 þ xAlH3 (x ¼ 0.5, 1, 2) composites are investigated to demonstrated the effect of AlH3 addition of various contents on the hydrogen desorption kinetics of LiBH4. Finally, the kinetic model of the reaction between LiBH4 and Al is preliminarily revealed and discussed.

Experimental details LiBH4 (Acros, 95%) and Al (Sinopharm Group, 99%) were used as received. AlH3 was synthesized by a wet chemical method summarized by Brower et al. [28]. The detailed synthesis process can be found in other literatures [29e31]. The LiBH4, LiBH4 þ 0.5Al, LiBH4 þ 0.5AlH3, and the LiBH4 þ xAlH3 (x ¼ 0.5, 1, 2) samples were prepared by ball milling using a planetary ball mill (QM-3SP4, Nanjing Nanda Instrument Plant). Experimentally, 1 g samples and 50 g stainless steel balls were sealed in a stainless steel vial with an internal volume of 100-mL in an argon-filled glovebox. The

milling process was carried out at 400 rpm for 30 min. During the milling process, it was paused every 6 min for cooling to prevent temperature rising of local areas resulting from longterm milling. The hydrogen desorption/absorption measurements were carried out on a Sieverts-type apparatus. For the nonisothermal hydrogen desorption measurements, the samples were heated from room temperature to 500  C with a heating rate of 4  C min1 and then kept at 500  C for 1 h. For the isothermal hydrogen desorption measurements, the samples were rapidly heated to 450  C and held at 450  C. For the hydrogen absorption measurements, the dehydrogenated samples were first heated to 400  C and hold at this temperature. Then hydrogen of about 5 MPa was filled into the sample chamber. The hydrogen absorption process proceeded for 5 h. The thermal analysis was carried out on a differential scanning calorimeter (DSC, Netzsch STA449F3). The samples were heated from room temperature to 500  C with a set heating rate, during which an argon atmosphere was flowed at 50 mL min1 to prevent oxidation of the samples. The powder X-ray diffraction (XRD) was carried out on a PANalytical X-ray diffractometer (X'Pert Pro, Cu Ka, 40 kV, 40 mA). During the XRD measurements, the samples were sealed with an amorphous membrane to avoid oxidation. A field emission scanning electronic microscopy (SEM, FEI SIRION 100) was used to study the morphologies of the samples.

Results and discussion The hydrogen desorption kinetics of the LiBH4, LiBH4 þ 0.5Al and LiBH4 þ 0.5AlH3 samples were first studied by isothermal hydrogen desorption measurements. The samples were rapidly heated to 450  C and kept at 450  C in a chamber with an initial pressure of vacuum. Fig. 1a shows the isothermal hydrogen desorption curves of the samples at 450  C plotted as the hydrogen desorption capacity of the sample versus desorption duration, while Fig. 1b displays the hydrogen desorption extent of LiBH4 in each sample versus desorption duration. It can be observed from Fig. 1a that the three samples behave different decomposition process. The decomposition of the pure LiBH4 shows an acceleration-decelerationacceleration-deceleration process and release hydrogen of 9.8 wt%. The LiBH4 þ 0.5Al composite presents a sigmoidtype process and release hydrogen of 7.0 wt%, which is lower than that of the pure LiBH4. This is because Al in the LiBH4 þ 0.5Al composite do not contain any hydrogen. As for the LiBH4 þ 0.5AlH3 composite, it can be seen that the decomposition proceed through three steps, which is very similar to the non-isothermal decomposition process reported in the previous work [16]. The first step of the isothermal decomposition of LiBH4 þ 0.5AlH3 in Fig. 1a is related to the first self-decomposition of AlH3 generating Al and releasing hydrogen of 3.5 wt% (Eq. (3)). The second decomposition step is related to the reaction between LiBH4 and the previously formed Al forming AlB2 and LieAleB compounds and releasing hydrogen of 1.4 wt% (Eq. (4)). When the reaction Eq. (4) proceeds to a certain degree, it tends to

Please cite this article in press as: Liu H, et al., Hydrogen desorption kinetics of the destabilized LiBH4eAlH3 composites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.083

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temperature (188  C) of reaction Eq. (4) at 1 bar H2 is much lower than that of reaction Eq. (1) (403  C). Therefore, the destabilization of LiBH4 by AlH3 addition may have contributed to the improvement of the hydrogen desorption kinetics of LiBH4. The hydrogen desorption kinetics of LiBH4 in each sample were further studied by calculating the apparent activation energy (Ea) for the decomposition reaction of LiBH4 using the Kissinger's method [32]. It was suggested that the apparent activation energy can be determined by Eq. (5): ln

Fig. 1 e Isothermal hydrogen desorption curves of LiBH4, LiBH4 þ 0.5Al, and LiBH4 þ 0.5AlH3 samples at 450  C plotted as hydrogen desorption capacity (a) and hydrogen desorption extent of LiBH4 in the samples (b) versus hydrogen desorption time.

slow down or even stop due to the kinetic barrier that the reaction products (AlB2 or LieAleB compounds) will cover on the surfaces of the Al particles and prevent LiBH4 from contacting and reacting with Al particles further. As a consequence, LiBH4 prefers to undergo a self-decomposition and then release hydrogen of 6.1 wt%, which related to the third step. After hydrogen desorption at 450  C for 6 h, the LiBH4 þ 0.5AlH3 composite totally release about 11.0 wt% of hydrogen, which is higher than either the pure LiBH4 (9.8 wt %) or the LiBH4 þ 0.5Al composite (7.0 wt%). From the hydrogen desorption extent curves of LiBH4 in each sample in Fig. 1b, it can be seen that after hydrogen desorption at 450  C for 6 h, the pure LiBH4 releases about 53% of its capacity and the LiBH4 in the LiBH4 þ 0.5Al composite releases about 61% of its capacity. As for the LiBH4 þ 0.5AlH3 composite, LiBH4 releases nearly 69% of its capacity. This indicate that both the as-received metallic Al and the asprepared AlH3 can improve the hydrogen desorption kinetics of LiBH4. However, AlH3 is much better than the as-received metallic Al as an Al source to enhance the hydrogen desorption properties of LiBH4. Fig. 1b also shows that the isothermal hydrogen desorption curves of the LiBH4 of each sample possess different shapes, which means there may exist different hydrogen desorption mechanism among these three samples. As for the LiBH4 þ 0.5AlH3 composite, the hydrogen desorption process of LiBH4 in the composite is composed of two steps. The hydrogen desorption rate of the first step is much faster than the second step. The first step is related to reaction Eq. (4) and the second step is related to reaction Eq. (1). It has been calculated by Cho et al. [15] that the decomposition

c Ea ¼ þA RTP T2P

(5)

where c is the heating rate used in the DSC measurement, Tp is the peak temperature of the LiBH4 decomposition, R is the universal gas constant, and A is also a constant. The DSC curves of the samples at various heating rate are shown in Fig. 2aec. The peak temperatures of the LiBH4 decomposition are extracted from the DSC curves and listed in Table 1. Fig. 2d displays the Kissinger's plot and the estimated apparent activation energies are listed in Table 1. The activation energy of the pure LiBH4 decomposition was calculated to be 169.8 kJ mol1 and the metallic Al slightly lowers the activation energy of LiBH4 decomposition to 166.8 kJ mol1. As for the LiBH4 þ 0.5AlH3 composite, the activation energy of LiBH4 decomposition is only 122.0 kJ mol1, which is significantly lower than either the pure LiBH4 or the LiBH4 þ 0.5Al composite. It is believed that the reduction of the activation energy of the LiBH4 decomposition directly contributes to the improvement of the hydrogen desorption kinetics of LiBH4. Fig. 3 shows the SEM images of the as-received metallic Al and the as-synthesized AlH3. As can be seen that the particle size of the as-received Al is generally over 100 mm, which is at least one order larger than the as-synthesized AlH3 (~10 mm or smaller). It is worth to note that AlH3 is a brittle metal hydride and can be easily refined by ball milling, while the metallic Al is a ductile metal and may tend to form agglomeration or undergo cold welding when subjected to ball milling. It is believed that the particle size of Al is a key factor in the reaction between LiBH4 and Al. The smaller particle size of AlH3 over Al is the one of the reasons why the LiBH4 þ 0.5AlH3 composite shows better hydrogen desorption kinetics than the LiBH4 þ 0.5Al composite. In order to study the cycling hydrogen desorption properties, the three dehydrogenated samples were first rehydrogenated under conditions of 400  C and 5 MPa H2 for 5 h and then dehydrogenated at 450  C for 6 h. Fig. 4a shows the isothermal hydrogen desorption capacity curves of the three re-hydrogenated samples. As can be seen that after hydrogen desorption for 1 h, the re-hydrogenated pure LiBH4 and the LiBH4 þ 0.5Al release about 1.8 wt% and 1.4 wt% of hydrogen, respectively, while it is 2.7 wt% for the rehydrogenated LiBH4 þ 0.5AlH3 composite. Totally, the rehydrogenated pure LiBH4, LiBH4 þ 0.5Al composite, and LiBH4 þ 0.5AlH3 composite release about 3.1 wt%, 1.8 wt%, and 3.2 wt% of hydrogen respectively after hydrogen desorption for 6 h. This suggests that the LiBH4 þ 0.5AlH3 composite show better reversibility than either the pure LiBH4 or the LiBH4 þ 0.5Al composite. Fig. 4b shows the

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Fig. 2 e DSC curves of the LiBH4 (a), LiBH4 þ 0.5Al (b), and LiBH4 þ 0.5AlH3 (c) samples with various heating rate. (d) Estimation of the apparent activation energies (Ea) for the first desorption step of LiBH4 using the Kissinger's method with the parameters obtained from DSC measurements.

isothermal hydrogen desorption extent of each sample. Since Al or AlH3 in the composite does not uptake any hydrogen under the present hydrogenation conditions, the curves in Fig. 4b were obtained by divided the hydrogen desorption capacity shown in Fig. 4a by the theoretical hydrogen absorption capacity of each sample assuming that Al or AlH3 did not uptake any hydrogen and only LiBH4 was recovered. It is observed from Fig. 4b that after hydrogen desorption for 1 h, the LiBH4 þ 0.5AlH3 composite releases the largest quantity of hydrogen of about 25%, which is nearly double the capacity of the LiBH4 þ 0.5Al composite and about 2.5 times the capacity of the pure LiBH4. After hydrogen desorption for 6 h, the re-hydrogenated LiBH4 þ 0.5AlH3 composite release about 29.4% of its theoretical reversible capacity, which is almost double the capacity (~16%) of the re-hydrogenated pure LiBH4 or LiBH4 þ 0.5Al composite. These results indicate that AlH3 addition improves not only the hydrogen desorption kinetics but also the reversibility of LiBH4. However, the LieAleBeH composite still suffers from severe capacity loss, which is the same as reported in other work [18,24,26,27]. The LiBH4 þ xAlH3 (x ¼ 0.5, 1, and 2) composites were studied to demonstrate the effect of AlH3 addition of various contents on the hydrogen desorption kinetics of LiBH4. The composites were prepared by ball milling and their hydrogen desorption properties were investigated. Fig. 5 displays the XRD patterns of the as-prepared samples. It can be seen that the as-milled composites (Fig. 5bed) generally contains the

starting materials (Fig. 5a and e), which suggests that LiBH4 did not react with AlH3 during the milling process. Fig. 6 shows the non-isothermal hydrogen desorption curves of the LiBH4 þ xAlH3 (x ¼ 0.5, 1, and 2) composites. The temperature was gradually increased from room temperature to 500  C and kept at this temperature for 1 h. The hydrogen desorption process of each composite is generally composed of three steps, which is similar with the isothermal desorption curves shown in Fig. 1. The relevant reaction of each step is

Table 1 e Peak Temperatures of the LiBH4 decomposition at various heating rates extracted from the DSC Curves in Fig. 2aec. Then apparent activation energies of the LiBH4 decomposition for each sample are also listed. Sample

Peak Temperature (TP) ( C) 





433.2 427.7 419.3

444.2 445.0 434.4

464.0 463.2 466.0



4 C 8 C 16 C 2 C min1 min1 min1 min1 LiBH4 LiBH4 þ 0.5Al LiBH4 þ 0.5AlH3

482.3 478.0 482.2

Activation energy (Ea) (kJ mol1) 169.8 ± 13.2 166.8 ± 7.3 122.0 ± 13.7

Fig. 3 e SEM images of the as-received metallic Al and the as-synthesized AlH3.

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Fig. 4 e Isothermal hydrogen desorption curves of the rehydrogenated LiBH4, LiBH4 þ 0.5Al, and LiBH4 þ 0.5AlH3 composite plotted as hydrogen desorption capacity (a) and hydrogen desorption extent of LiBH4 in the samples (b) versus desorption duration.

Fig. 6 e Non-isothermal hydrogen desorption curves of the LiBH4 þ xAlH3 (x ¼ 0.5, 1, and 2) composites plotted as hydrogen desorption capacity (b) and hydrogen desorption extent of LiBH4 in the composite (c) versus hydrogen desorption time. (a) shows the heating process.

indicated in Fig. 6b and c. Table 2 lists the hydrogen desorption capacity of each step extracted from the hydrogen desorption curves in Fig. 6b. As can be seen that with the AlH3 content increasing, hydrogen released from Step 1 (AlH3 / Al þ H2) and Step 2 (LiBH4 þ Al / AlB2 þ ‘LieAleB’ þ H2) both increases. This is because the reaction between LiBH4 and Al occurs generally on the surfaces of Al particles [16]. More AlH3 content will lead to more Al particle surfaces, thus results in more hydrogen released from Step 1 and 2. Consistently, the

Table 2 e Hydrogen desorption capacity of each step for the LiBH4 þ xAlH3 (x ¼ 0.5, 1, and 2) composites. Sample LiBH4 þ 0.5AlH3 LiBH4 þ 1.0AlH3 LiBH4 þ 2.0AlH3 a

Fig. 5 e XRD patterns of the pure LiBH4, the as-synthesized AlH3 and the ball milled LiBH4 þ xAlH3 (x ¼ 0.5, 1, and 2) composites.

Capacity of Step 1

Capacity of Step 2

Capacity of Step 3

3.3 wt% 4.7 wt% 6.0 wt%

2.2 wt% (20%)a 2.5 wt% (31%) 3.7 wt% (75%)

6.2 wt% (56%) 3.6 wt% (48%) 0.4 wt% (8%)

Data in parenthesis represent the percent of hydrogen released from LiBH4, which are obtained by dividing the hydrogen releasing capacity of each step by the theoretical hydrogen capacity of LiBH4 (18.5 wt%).

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hydrogen released from Step 3 decreases. For example, as shown in Table 2, the extents of hydrogen released from LiBH4 for the Step 2 and Step 3 are 20% and 56% respectively for the LiBH4 þ 0.5AlH3 composite, while they are 75% and 8% respectively for the LiBH4 þ 2.0AlH3 composite. Therefore, the content of the AlH3 addition affects greatly the hydrogen desorption properties of LiBH4. To study the hydrogen desorption kinetics of the LiBH4 þ xAlH3 (x ¼ 0.5, 1, and 2) composites, the isothermal hydrogen desorption measurements were carried out at 450  C and the isothermal hydrogen desorption curves are shown in Fig. 7. Fig. 7a displays the hydrogen desorption capacity of the composites versus time, while Fig. 7b displays the hydrogen desorption extent of LiBH4 in each composite versus time. As can be seen from Fig. 7a that all composites undergo first rapid AlH3 decomposition (within 3 min) and then slow LiBH4 decomposition. From Fig. 7b, the decompositions of LiBH4 in the three composites behave different processes. The decomposition of the LiBH4 þ 0.5AlH3 first accelerates within 10 min and then turns into a slow process within the following 100 min with a nearly constant hydrogen releasing rate. After hydrogen desorption for 110 min, a total hydrogen capacity of 67% is obtained from the LiBH4 þ 0.5AlH3. In contrast, the decomposition of the LiBH4 þ 1.0AlH3 first accelerates within 15 min, followed by a slow process for about 15 min, and again accelerates for about 10 min. A total hydrogen capacity of 76% is obtained after hydrogen desorption for 44 min. As for the LiBH4 þ 2.0AlH3, the decomposition presents a sigmoid-type process and proceeds rapidly. A total hydrogen capacity of 82% is obtained after hydrogen desorption for only 29 min.

Table 3 lists the hydrogen desorption capacities and rates of LiBH4 in the three LiBH4 þ xAlH3 (x ¼ 0.5, 1, and 2) composites. As can be seen that AlH3 addition significantly accelerates the decomposition of LiBH4. The decomposition rate of LiBH4 increases with the addition content of AlH3 increasing. The rate for the LiBH4 þ 2.0AlH3 (2.83% min1) is more than 4 times as that for the LiBH4 þ 0.5AlH3 (0.61% min1). It should be noted that the hydrogen desorption extent of LiBH4 with 1.0AlH3 or 2.0AlH3 addition have exceeded the theoretical desorption extent (75%) of Eq. (1); this is because a part of product LiH have decomposed to release hydrogen further and formed a LieAleB compound. To gain an insight into the hydrogen desorption kinetics of the AlH3-added LiBH4, the hydrogen desorption kinetic curves in Fig. 7b were further studied by the JohnsonMehlAvrami (JMA) equation [33,34]: a ¼ 1  expðktn Þ

(6)

where a is the extent of a reaction in time (t), k is the rate constant, and n is the Avrami exponent, which reflects the nucleation and growth morphology. Eq. (6) can be rewritten as follows (Eq. (7)): ln½lnð1  aÞ ¼ lnðkÞ þ nlnðtÞ

(7)

According to Eq. (7), the plot of ln[ln(1  a)] versus ln(t) should be a straight line at a given constant temperature, and the Avrami exponent, n, can be estimated from the slope of the line. The hydrogen desorption extent curves of the three AlH3-added LiBH4 plotted as ln[ln(1  a)] versus ln(t) are shown in Fig. 8. It can be seen that the decomposition of the

Table 3 e Hydrogen desorption properties of the LiBH4 in the three LiBH4 þ xAlH3 (x ¼ 0.5, 1, and 2) composites. Sample LiBH4 þ 0.5AlH3 LiBH4 þ 1.0AlH3 LiBH4 þ 2.0AlH3

Fig. 7 e Isothermal hydrogen desorption curves of the LiBH4 þ xAlH3 (x ¼ 0.5, 1, and 2) composites at 450  C plotted as hydrogen desorption capacity (a) and hydrogen desorption extent of LiBH4 in the composite (b) respectively versus hydrogen desorption time.

Total hydrogen Total time Average released/% taken/min rate/(% min1) 67 76 82

110 44 29

0.61 1.73 2.83

Fig. 8 e Hydrogen desorption extent (a) of LiBH4 in the three LiBH4 þ xAlH3 (x ¼ 0.5, 1, and 2) composites plotted as ln [¡ln(1 ¡ a)] versus ln(t/min).

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Fig. 9 e Schematic picture showing the nucleation and growth process (aed) of the LieAleB and AlB2 layers along the interfaces between molten LiBH4 and Al particles. t0 < t1
Conclusions The hydrogen desorption kinetics of LiBH4 were significantly improved in the present work by the addition of AlH3 in that more hydrogen desorption capacity, lower activation energy and better reversibility of LiBH4eAlH3 can be achieved compared with the pure LiBH4 and the LiBH4eAl. In the LiBH4e AlH3 composites, increasing the content of AlH3 addition

accelerates the hydrogen desorption kinetics of LiBH4 due to more Al particle surfaces and thus larger extent of the reaction between LiBH4 and the previously decomposed Al. The reaction between LiBH4 and Al is controlled by the precipitation and growth of AlB2 and LieAleB compounds with an increasing nucleation rate. The formed AlB2 and LieAleB layers covering on the surfaces of Al particles are the kinetic barrier to the reaction between LiBH4 and Al. Therefore, the cracking of the barrier seems being able to further improve the hydrogen desorption properties of the LieAleBeH composites.

Acknowledgements This work was supported by State Grid Corporation of China (No. SGRIDGKJ[2016]123), National Natural Science Foundation of China (Nos. 51471149 and 51171168), and Public Project of Zhejiang Province (No. 2015C31029).

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Please cite this article in press as: Liu H, et al., Hydrogen desorption kinetics of the destabilized LiBH4eAlH3 composites, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2016.12.083