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Effects of fluoride additives on dehydrogenation behaviors of 2LiBH4 + MgH2 system
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 e1 0 2 6
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effects of fluoride additives on dehydrogenation behaviors of 2LiBH4 D MgH2 system Huaqin Kou, Xuezhang Xiao*, Jinxiu Li, Shouquan Li, Hongwei Ge, Qidong Wang, Lixin Chen* Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
article info
abstract
Article history:
Various additives such as NbF5, TiF3, CeF3, LaF3 and FeF3 were used to dope 2LiBH4 þ MgH2
Received 30 September 2010
system and their dehydriding properties were investigated systematically. Among the
Received in revised form
different additives, NbF5 exhibited the most prominent behavior in terms of fast kinetics
26 February 2011
and lower the dehydrogenation temperature of 2LiBH4 þ MgH2 system. Differential scan-
Accepted 5 March 2011
ning calorimetry (DSC) shows that 2LiBH4 þ MgH2 system milled with 5 mol% NbF5 lowered
Available online 3 April 2011
the dehydrogenation temperature by 30 C and 58 C for the two-step dehydrogenation compared to neat 2LiBH4 þ MgH2 system. Temperature programmed desorption (TPD)
Keywords:
indicates that 2LiBH4 þ MgH2 þ 0.05NbF5 dehydrided completely below 450 C with 8.1 wt.%
Complex hydride
hydrogen released. Fourier transform infrared spectrometer (FTIR) and X-ray diffraction
LiBH4
(XRD) analysis evidence that MgB2 was fully formed after dehydrogenation. Experimental
MgH2
results show that the 2LiBH4 þ MgH2 þ 0.05NbF5 system indeed has good reversible
Fluoride additive
hydriding/dehydriding properties.
Hydrogen storage
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Complex hydrides with high theoretical hydrogen densities hold as potential solid state hydrogen storage materials for the wide spread use of hydrogen-powered vehicles [1e3]. LiBH4, which owns the highest reversible hydrogen storage capacity, has gained considerable interest [4e9]. However, LiBH4 is thermodynamically stable and kinetically slow for practical use, and it is required for extremely rigorous temperature and pressure conditions to reverse [10,11]. A variety of approaches have been developed to address the thermodynamics and kinetics limitations of LiBH4, such as: incorporation LiBH4 into mesoporous scaffolds [12e14], partial cation substitution [15e17], thermodynamic destabilization of LiBH4 by metal hydride [18e20] and addition of catalysts [21e23]. Among these, the last two approaches display great
potential for adjusting thermodynamics and enhance dehydrogenation/rehydrogenation kinetics. Vajo et al. developed a hydrogen storage system composed of LiBH4 and MgH2, in which LiBH4 was destabilized and its reversibility was improved effectively [24,25]. The reversible dehydrogenation/rehydrogenation reaction is expressed as follows: 2LiBH4 þ MgH2 4MgB2 þ 2LiH þ 4H2
(1)
Moreover, the theoretical hydrogen capacity of 2LiBH4 þ MgH2 system is still as high as 11.8 wt.%. However, the kinetics of the pure 2LiBH4 þ MgH2 system is still slow and the dehydrogenation temperature is too high [26]. Therefore, many efforts have been focused on improving hydrogen sorption rate and reducing reaction temperature for 2LiBH4 þ MgH2 system. Wang et al. found that TiF3 and single-
* Corresponding authors. Tel./fax: þ86 571 8795 1152. E-mail addresses: [email protected] (X. Xiao), [email protected] (L. Chen). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.027
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 e1 0 2 6
walled carbon nanotubes appear good catalytic effect on LieMgeBeH system [27,28]. The catalytic effect of Nb2O5 on the LiBH4eMgH2 system was investigated by Sun et al. [29]. In this work, we investigated the dehydriding properties of 2LiBH4 þ MgH2 system with various additives. These results provide some fundamental insights into the effect of various additives on 2LiBH4 þ MgH2 system.
2.
Experimental
LiBH4 (95%), LaF3 (99.9%) were obtained from Acros Organics Corp. MgH2 (98%), CeF3 (99.9%) were purchased from Alfa Aesar Corp. NbF5 (98%), TiF3 (99%), FeF3 (98%), MgB2 (96%) and boron (amorphous) were produced by SigmaeAldrich Corp. All materials were used as received in powder form. Mixtures of 2LiBH4 þ MgH2 with 5 mol% NbF5/CeF3/LaF3/FeF3 or 3 mol% TiF3 were mechanically milled under 1 MPa hydrogen pressure in a Planetary mill at 400 rpm for 2 h. The ball to powder ratio was in around 40:1. All sample operations were performed in a glovebox under continuous purified argon atmosphere. Hydrogen desorption/absorbtion behaviors of the samples were monitored with a Sievert’s type apparatus. Typical dehydriding performance started from a 4 bar initial hydrogen pressure. Each time, 150w250 mg sample was put into a closed large reaction sample volume (820 ml), which resulted in 0.2w0.3 bar pressure change after dehydrogenation. Because the pressure increase during dehydrogenation process is small, the initial dehydrogenation pressure almost presents the reaction pressure circumstance. For consideration of practical purpose, the weight of various additives was taken into account in calculation of hydrogen capacity. The differential scanning calorimetry (DSC) and mass spectrometer (MS) experiments were performed by synchronous thermal analysis (Netzch 449C Jupiter/QMS 403C). High purity Ar was used as purge gas, and the heating rate was 10 C/min for all measurements. Fourier transform infrared spectrometer (FTIR) spectrum of species was recorded at ambient condition in air by Bruker Tensor27 FTIR spectrometer. The identification of the samples was carried out by Xray diffraction (XRD, X’Pert-PRO, Cu Ka radiation). To prevent the H2O and O2 contamination during the measurements, a special sample holder was used.
3.
Fig. 1 e DSC profiles of 2LiBH4 D MgH2 system with various additives.
lowered by adding NbF5, corresponds to decomposition of LiBH4 (reaction (3)) rather than reaction of Mg and LiBH4 (reaction (4)) under Ar atmosphere [30]. Small sharp satellite peaks at 450 C in most experimental samples are presumably due to inhomogeneity distribution of MgH2 or impurity interfused during ball milling rather than endothermic peak of LiBH4, which was verified in the DSC curve of milled MgH2 sample in our additional experiments. It can be found that NbF5 exhibits the maximum effect in promoting the whole dehydrogenation of 2LiBH4 þ MgH2 system, resulting in 30 C and 58 C decreases for the two-step dehydrogenation respectively from neat 2LiBH4 þ MgH2 sample. MgH2 /Mg þ H2
(2)
LiBH4 /LiH þ B þ 3=2 H2
(3)
Mg þ 2LiBH4 /MgB2 þ 2LiH þ 3H2
(4)
Results and discussion
Fig. 1 presents DSC curves of 2LiBH4 þ MgH2 with various additives. All the curves exhibit four distinct endothermic peaks. The first peak around 110 C corresponds to the phase transformation of LiBH4 (the orthorhombic to hexagonal structure) and the second one between 260 and 280 C corresponds to melting of LiBH4 [10]. It is interesting to find that the doping of NbF5, TiF3 and FeF3 slightly decreases the melting temperature of LiBH4. The third peak between 300 and 400 C, which is broad and lowest in the case of LaF3 as additive, corresponds to the dehydrogenation reaction of MgH2 (reaction (2)) [30]. Among these additives, NbF5 appears to be the second effective in reducing the dehydrogenation temperature of MgH2. The fourth peak around 450 C, which is largely
Fig. 2 e MS profiles for the hydrogen evolution from 2LiBH4 D MgH2 system with various additives.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 e1 0 2 6
Fig. 2 displays the MS results for the hydrogen evolution from 2LiBH4 þ MgH2 with various additives. For all curves, a two-step desorption process, which corresponds to decomposition of MgH2 and LiBH4, is observed to be in good agreement with DSC results. It can be seen that 2LiBH4 þ MgH2 þ 0.05NbF5 starts releasing hydrogen from 350 C and desorption completes below 450 C. The temperature of the second hydrogen desorption is not affected obviously by doping LaF3 in 2LiBH4 þ MgH2, though the hydrogen release of MgH2 is reduced greatly to low temperature. The two-step desorption process of neat 2LiBH4 þ MgH2 sample completes below 500 C, with a temperature onset at approximately 370 C. Similar two-step desorption behaviors have been observed for 2LiBH4 þ MgH2 with other additives. In addition, no B2H6 was detected during the dehydrogenation, which is not shown here. To confirm the effect of additives on reaction (2) and reaction (3), hydrogen desorption behaviors of 2LiBH4 þ MgH2 with various additives under 0.01 bar dehydrogenation hydrogen pressure were examined, shown in Fig. 3. It is obviously found that the sample of 2LiBH4 þ MgH2 þ 0.05NbF5 exhibits the fastest kinetics. More than 7 wt.% hydrogen is obtained when heating up to 430 C by doping NbF5, while the neat 2LiBH4 þ MgH2 sample releases 3.4 wt.% hydrogen. The average dehydriding rate of the sample with NbF5 additive is over 2 times higher than that of the neat 2LiBH4 þ MgH2 sample. However, reference [24] showed that the reversibility of 2LiBH4 þ MgH2 system after dehydrogenation under such low hydrogen pressure (reaction (5)) has very poor kinetics and the system was not fully rehydrogenated. And our previous experiments showed that a hydrogen back-pressure of 4 bar is necessary for dehydrogenation according to the reaction (1) [31]. 2LiBH4 þ MgH2 /Mg þ 2LiH þ 2B þ 4H2
(5)
Fig. 4 shows hydrogen desorption curves of 2LiBH4 þ MgH2 with various additives under 4 bar dehydrogenation hydrogen pressure. For all samples, a typical two-step dehydrogenation is observed. An incubation plateau appears during desorption
Fig. 3 e Hydrogen desorption curves of 2LiBH4 D MgH2 with various additives under 0.01 bar dehydrogenation hydrogen pressure. The dot curve represents the temperature profile, and the temperature ramping rate is 5 C/min.
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Fig. 4 e Hydrogen desorption curves of 2LiBH4 D MgH2 with various additives under 4 bar dehydrogenation hydrogen pressure. The dot curve represents the temperature profile, and the temperature ramping rate is 5 C/min.
process except sample of 2LiBH4 þ MgH2 þ 0.05NbF5. Compared to other additives, a pronounced kinetics improvement is obtained by using NbF5 as additive in 2LiBH4 þ MgH2 system, with 7.5 wt.% hydrogen released before temperature holding at 450 C. The rate of hydrogen release is exceedingly slow by doping FeF3, being unable to reach full desorption after dehydriding for 10 h. Above experimental results indicate that not only reaction (1) but also reaction (5) dehydrogenation process of 2LiBH4 þ MgH2 system can be promoted significantly by doping NbF5. And it appears that the catalytic effect of these additives on the dehydrogenation behavior of 2LiBH4 þ MgH2 system increase in order of FeF3 < CeF3 < LaF3 < TiF3 < NbF5. The XRD patterns of 2LiBH4 þ MgH2 with various additives after dehydrogenation under 4 bar hydrogen pressure are shown in Fig. 5. The concurrent existence of both MgB2 and Mg are detected in the dehydrided products of 2LiBH4 þ MgH2 with various additives except that of NbF5 doped 2LiBH4 þ MgH2 sample. It indicates that NbF5 significantly promoting the fully formation of MgB2 instead of Mg retained, resulting in great potential of fully rehydrogenation for 2LiBH4 þ MgH2 system. Unfortunately, no diffraction peak relevant to NbF5 is observed in the dehydrided products, and similar results appeared in TiF3 and FeF3 doped 2LiBH4 þ MgH2 samples. However, La4H12.19 and LaB6 are produced in 2LiBH4 þ MgH2 þ 0.05LaF3 sample; CeH2.51 and CeB6 are produced in 2LiBH4 þ MgH2 þ 0.05CeF3 sample. Fig. 6 shows the dehydriding curves of 2LiBH4 þ MgH2 þ 0.05NbF5 and neat 2LiBH4 þ MgH2 samples from room temperature to 400 C. It can be seen that there is a long dehydriding plateau in neat 2LiBH4 þ MgH2 sample. And the amount of hydrogen desorbed almost maintains at 3 wt.%. No trend of massive hydrogen to release appears until temperature increases to 450 C, even though dehydriding plateau extends to 48 h at 400 C. In contrast, the 2LiBH4 þ MgH2 þ 0.05NbF5 sample can release 8.1 wt.% hydrogen within only 4.5 h. It is clearly that the NbF5 additive markedly increased the desorption kinetics of
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 e1 0 2 6
Fig. 5 e XRD patterns of 2LiBH4 D MgH2 with various additives (NbF5, TiF3, CeF3, LaF3, FeF3) and neat 2LiBH4 D MgH2 sample after dehydrogenation from room temperature to 450 C under 4 bar hydrogen pressure.
Fig. 7 e TPD curves of 2LiBH4 D MgH2 D 0.05NbF5 and neat 2LiBH4 D MgH2 samples. The temperature heating rate is 2 C/min.
the host materials. The TPD data generated by Sieverts apparatus (see Fig. 7) show that the additive NbF5 effectively reduced the temperature of hydrogen desorption in contrast with neat 2LiBH4 þ MgH2 sample. 2LiBH4 þ MgH2 þ 0.05NbF5 sample released 8.2 wt.% hydrogen from 350 C and completed below 450 C, which is consistent with the MS results mentioned above. Nevertheless, neat 2LiBH4 þ MgH2 sample started releasing hydrogen at 370 C and only 5.6 wt.% hydrogen was obtained even up to 500 C. The results of TPD show that addition of NbF5 effectively reduces the dehydriding temperature of 2LiBH4 þ MgH2 system. The elemental boron produced after dehydrogenation of LiBH4-relevant, which is usually amorphous, can be hardly detected by XRD measurement [18]. So, the FTIR spectra are used to characterize the frequencies of BeB spectra in amorphous. Fig. 8 shows the FTIR spectra of 2LiBH4 þ MgH2 þ
0.05NbF5 sample after dehydrogenation at 400 C. It is found that matched vibration signal corresponding to boron was hardly detected in the IR spectrum of dehydrided sample. The absorption peak at 1633 cm1 belongs to OeH band [32]. On the other hand, the IR spectrum for the dehydrided sample was almost the same as the MgB2. It indicates that the formation of the boron phase was effectively inhibited in the 2LiBH4 þ MgH2 system by using NbF5 as additive, and NbF5 plays an important role in the fully formation of MgB2. Dehydriding curves of 2LiBH4 þ MgH2 þ 0.05NbF5 under 4 bar hydrogen pressure at 400 C during the initial 5 cycles are shown in Fig. 9. It can be seen that no obvious degradation tendency emerged in the cycling properties. The cycling retentivity should be supported by the full formation of MgB2 after dehydrogenation (shown in the XRD results of Fig. 9 inset), which
Fig. 6 e Dehydriding curves of 2LiBH4 D MgH2 D 0.05NbF5 and neat 2LiBH4 D MgH2 samples from room temperature to 400 C. The dot curve represents the temperature profile, and the temperature ramping rate is 5 C/min.
Fig. 8 e FTIR spectra for (a) dehydrided sample of 2LiBH4 D MgH2 D 0.05NbF5 after dehydrogenation; (b) MgB2; (c) Boron.
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improvement in reducing dehydrogenation temperature and enhancing dehydrogenation rate. Compared to neat 2LiBH4 þ MgH2 sample, a lower dehydrogenation temperature by 30 C and 58 C respectively for the two-step dehydrogenation was observed in the 2LiBH4 þ MgH2 þ 0.05NbF5 sample under Ar flow. And over 8.1 wt.% hydrogen can be released from 2LiBH4 þ MgH2 þ 0.05NbF5 below 450 C under a 4 bar initial hydrogen pressure, with an average dehydriding rate over 2 times higher than that of the neat 2LiBH4 þ MgH2 sample. The results of XRD confirm that MgB2 was fully formed instead of Mg retained after dehydrogenation and 2LiBH4 þ MgH2 þ 0.05NbF5 shows a great performance of full reversibility.
Fig. 9 e Dehydriding curves of 2LiBH4 D MgH2 D 0.05NbF5 under 4 bar hydrogen pressure at 400 C during the initial 5 cycles. Rehydrogenation was performed under 70 bar hydrogen pressure at 400 C after dehydrogenation. XRD patterns of 2LiBH4 D MgH2 D 0.05NbF5 after (a) the 1st dehydrogenation and (b) the 5th dehydrogenation are displayed in the inset.
ensured the favorable rehydrogenation. Furthermore, it is interesting to note that dehydriding rate was enhanced after the first dehydrogenation/rehydrogenation cycle. The dehydriding enhancement may just be caused by the catalytic component produced from the addition NbF5 to 2LiBH4 þ MgH2 system, which was always not identified in this studying stage. However, it is confirmed that 2LiBH4 þ MgH2 þ 0.05NbF5 indeed has an excellent reversibility. The evidently positive influence of NbF5 additive on the hydrogen storage properties of 2LiBH4 þ MgH2 system is exhibited in above experimental results. Unfortunately, the catalytic component is not confirmed by XRD analysis. The catalytic effect of NbF5 on the 2LiBH4 þ MgH2 system may come from substitution of H by rich F anion in NbF5 during hydrogen desorption process, which has been predicted to be thermodynamic favorable by theoretical calculations [33]. Another possibility is that the crystallite size of the catalytic component is so fine that it can not easily detected by XRD measurement, which is being investigated in our proceeding work. The full formation of MgB2 after dehydrogenation appeared in the sample of 2LiBH4 þ MgH2 with NbF5, indicating that the loss of H-capacity can be avoided upon subsequent rehydrogenation. And excellent reversible hydrogen storage property can be potentially obtained. However, studies for further reducing dehydrogenation temperature are needed in order to make this system more practical.
4.
Conclusions
We have studied the effect of various additives on the hydrogen desorption characteristics of 2LiBH4 þ MgH2 system. Among these dopants examined, NbF5 exhibits the best
Acknowledgments The authors wish to acknowledge the financial support of this research from the National Basic Research Program of China (Grant No. 2010CB631300 and 2007CB209701), the National Natural Science Foundation of China (Grant No. 51001090, 50871099), the Program for New Century Excellent Talents in Universities (Grant No. NCET-07-0741), the University Doctoral Foundation of the Ministry of Education (Grant No. 20090101110050), and the China Postdoctoral Science Foundation (200902622).
references
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[12] Gross AF, Vajo JJ, Van Atta SL, Olson GL. Enhanced hydrogen storage kinetics of LiBH4 in nanoporous carbon scaffolds. J Phys Chem C 2008;112:5651e7. [13] Yu XB, Wu Z, Chen QR, Li ZL, Weng BC, Huang TS. Improved hydrogen storage properties of LiBH4 destabilized by carbon. Appl Phys Lett 2007;90:034106. [14] Zhang Y, Zhang WS, Wang AQ, Sun LX, Fan MQ, Chu HL, et al. LiBH4 nanoparticles supported by disordered mesoporous carbon: hydrogen storage performances and destabilization mechanisms. Int J Hydrogen Energy 2007;32:3976e80. [15] Orimo S, Nakamori Y, Zu¨ttel A. Material properties of MBH4 (M ¼ Li, Na, and K). Mater Sci Eng B 2004;108:51e3. [16] Miwa K, Ohba N, Towata SI, Nakamori Y, Orimo SI. Firstprinciples study on copper-substituted lithium borohydride, (Li1xCux)BH4. J Alloys Comp 2005;404e406:140e3. [17] Nakamori Y, Miwa K, Ninomiya A, Li H, Ohba N, Towata SI, et al. Correlation between thermodynamical stabilities of metal borohydrides and cation electronegativites: first-principles calculations and experiments. Phys Rev B 2006;74:045126. [18] Au M, Jurgensen A. Modified Lithium Borohydrides for reversible hydrogen storage. J Phys Chem B 2006;110:7062e7. [19] Au M, Jurgensen A, Zeigler K. Modified lithium borohydrides for reversible hydrogen storage (2). J Phys Chem B 2006;110: 26482e7. [20] Yang J, Sudik A, Wolverton C. Destabilizing LiBH4 with a metal (M ¼ Mg, Al, Ti, V, Cr, or Sc) or metal hydride (MH2 ¼ MgH2, TiH2, or CaH2). J Phys Chem C 2007;111: 19134e40. [21] Guo YH, Yu XB, Gao L, Xia GL, Guo ZP, Liu HK. Significantly improved dehydrogenation of LiBH4 destabilized by TiF3. Energy Environ Sci 2010;3(4):465e70. [22] Xu J, Cao JY, Yu XB, Zou ZQ, Akins DL, Yang H. Enhanced catalytic hydrogen release of LiBH4 by carbon-supported Pt nanoparticles. J Alloys Comp 2010;490(1e2):88e92.
[23] Yu XB, Grant DM, Walker GS. Dehydrogenation of LiBH4 destabilized with various oxides. J Phys Chem C 2009;113(41): 17945e9. [24] Vajo JJ, Skeith SL, Mertens F. Reversible storage of hydrogen in destabilized LiBH4. J Phys Chem B 2005;109(9):3719e22. [25] Vajo JJ, Olson GL. Hydrogen storage in destabilized chemical systems. Scripta Materialia 2007;56(10):829e34. [26] Bo¨senberg U, Doppiu S, Mosegaard L, Barkhordarian G, Eigen N, Borgschulte A, et al. Hydrogen sorption properties of MgH2eLiBH4 composites. Acta Materialia 2007;55(11):3951e8. [27] Wang P, Ma LP, Fang ZZ, Kang XD, Wang PJ, Cheng HM. Improved hydrogen storage property of LieMgeBeH system by milling with titanium trifluoride. Energy Environ Sci 2009; 2(1):120e3. [28] Wang P, Fang ZZ, Ma LP, Kang XD, Wang PJ. Effect of SWNTs on the reversible hydrogen storage properties of LiBH4eMgH2 composite. Int J Hydrogen Energy 2008;33(20):5611e6. [29] Fan MQ, Sun LX, Zhang Y, Xu F, Zhang J, Chu HL. The catalytic effect of additive Nb2O5 on the reversible hydrogen storage performances of LiBH4eMgH2 composite. Int J Hydrogen Energy 2008;33(1):74e80. [30] Nakagawa T, Ichikawa T, Hanada N, Kojima Y, Fujii H. Thermal analysis on the LieMgeBeH systems. J Alloys Comp 2007;446-447:306e9. [31] Kou HQ, Xiao XZ, Chen LX, Li SQ, Wang QD. Study on the formation mechanism of MgB2 in 2LiBH4 þ MgH2 system for reversible hydrogen storage. T Nonferr Metal Soc, accepted. [32] Bukka K, Miller JD, Shabtai J. FTIR study of deuterated montmorillonites: structural features relevant to pillared clay stability. Clays Clay Miner 1992;40(1):92e102. [33] Yin LC, Wang P, Fang ZZ, Cheng HM. Thermodynamically tuning LiBH4 by fluorine anion doping for hydrogen storage: a density functional study. Chem Phys Lett 2008;450(4e6): 318e21.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effects of fluoride additives on dehydrogenation behaviors of 2LiBH4 D MgH2 system Huaqin Kou, Xuezhang Xiao*, Jinxiu Li, Shouquan Li, Hongwei Ge, Qidong Wang, Lixin Chen* Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
article info
abstract
Article history:
Various additives such as NbF5, TiF3, CeF3, LaF3 and FeF3 were used to dope 2LiBH4 þ MgH2
Received 30 September 2010
system and their dehydriding properties were investigated systematically. Among the
Received in revised form
different additives, NbF5 exhibited the most prominent behavior in terms of fast kinetics
26 February 2011
and lower the dehydrogenation temperature of 2LiBH4 þ MgH2 system. Differential scan-
Accepted 5 March 2011
ning calorimetry (DSC) shows that 2LiBH4 þ MgH2 system milled with 5 mol% NbF5 lowered
Available online 3 April 2011
the dehydrogenation temperature by 30 C and 58 C for the two-step dehydrogenation compared to neat 2LiBH4 þ MgH2 system. Temperature programmed desorption (TPD)
Keywords:
indicates that 2LiBH4 þ MgH2 þ 0.05NbF5 dehydrided completely below 450 C with 8.1 wt.%
Complex hydride
hydrogen released. Fourier transform infrared spectrometer (FTIR) and X-ray diffraction
LiBH4
(XRD) analysis evidence that MgB2 was fully formed after dehydrogenation. Experimental
MgH2
results show that the 2LiBH4 þ MgH2 þ 0.05NbF5 system indeed has good reversible
Fluoride additive
hydriding/dehydriding properties.
Hydrogen storage
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Complex hydrides with high theoretical hydrogen densities hold as potential solid state hydrogen storage materials for the wide spread use of hydrogen-powered vehicles [1e3]. LiBH4, which owns the highest reversible hydrogen storage capacity, has gained considerable interest [4e9]. However, LiBH4 is thermodynamically stable and kinetically slow for practical use, and it is required for extremely rigorous temperature and pressure conditions to reverse [10,11]. A variety of approaches have been developed to address the thermodynamics and kinetics limitations of LiBH4, such as: incorporation LiBH4 into mesoporous scaffolds [12e14], partial cation substitution [15e17], thermodynamic destabilization of LiBH4 by metal hydride [18e20] and addition of catalysts [21e23]. Among these, the last two approaches display great
potential for adjusting thermodynamics and enhance dehydrogenation/rehydrogenation kinetics. Vajo et al. developed a hydrogen storage system composed of LiBH4 and MgH2, in which LiBH4 was destabilized and its reversibility was improved effectively [24,25]. The reversible dehydrogenation/rehydrogenation reaction is expressed as follows: 2LiBH4 þ MgH2 4MgB2 þ 2LiH þ 4H2
(1)
Moreover, the theoretical hydrogen capacity of 2LiBH4 þ MgH2 system is still as high as 11.8 wt.%. However, the kinetics of the pure 2LiBH4 þ MgH2 system is still slow and the dehydrogenation temperature is too high [26]. Therefore, many efforts have been focused on improving hydrogen sorption rate and reducing reaction temperature for 2LiBH4 þ MgH2 system. Wang et al. found that TiF3 and single-
* Corresponding authors. Tel./fax: þ86 571 8795 1152. E-mail addresses: [email protected] (X. Xiao), [email protected] (L. Chen). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.03.027
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walled carbon nanotubes appear good catalytic effect on LieMgeBeH system [27,28]. The catalytic effect of Nb2O5 on the LiBH4eMgH2 system was investigated by Sun et al. [29]. In this work, we investigated the dehydriding properties of 2LiBH4 þ MgH2 system with various additives. These results provide some fundamental insights into the effect of various additives on 2LiBH4 þ MgH2 system.
2.
Experimental
LiBH4 (95%), LaF3 (99.9%) were obtained from Acros Organics Corp. MgH2 (98%), CeF3 (99.9%) were purchased from Alfa Aesar Corp. NbF5 (98%), TiF3 (99%), FeF3 (98%), MgB2 (96%) and boron (amorphous) were produced by SigmaeAldrich Corp. All materials were used as received in powder form. Mixtures of 2LiBH4 þ MgH2 with 5 mol% NbF5/CeF3/LaF3/FeF3 or 3 mol% TiF3 were mechanically milled under 1 MPa hydrogen pressure in a Planetary mill at 400 rpm for 2 h. The ball to powder ratio was in around 40:1. All sample operations were performed in a glovebox under continuous purified argon atmosphere. Hydrogen desorption/absorbtion behaviors of the samples were monitored with a Sievert’s type apparatus. Typical dehydriding performance started from a 4 bar initial hydrogen pressure. Each time, 150w250 mg sample was put into a closed large reaction sample volume (820 ml), which resulted in 0.2w0.3 bar pressure change after dehydrogenation. Because the pressure increase during dehydrogenation process is small, the initial dehydrogenation pressure almost presents the reaction pressure circumstance. For consideration of practical purpose, the weight of various additives was taken into account in calculation of hydrogen capacity. The differential scanning calorimetry (DSC) and mass spectrometer (MS) experiments were performed by synchronous thermal analysis (Netzch 449C Jupiter/QMS 403C). High purity Ar was used as purge gas, and the heating rate was 10 C/min for all measurements. Fourier transform infrared spectrometer (FTIR) spectrum of species was recorded at ambient condition in air by Bruker Tensor27 FTIR spectrometer. The identification of the samples was carried out by Xray diffraction (XRD, X’Pert-PRO, Cu Ka radiation). To prevent the H2O and O2 contamination during the measurements, a special sample holder was used.
3.
Fig. 1 e DSC profiles of 2LiBH4 D MgH2 system with various additives.
lowered by adding NbF5, corresponds to decomposition of LiBH4 (reaction (3)) rather than reaction of Mg and LiBH4 (reaction (4)) under Ar atmosphere [30]. Small sharp satellite peaks at 450 C in most experimental samples are presumably due to inhomogeneity distribution of MgH2 or impurity interfused during ball milling rather than endothermic peak of LiBH4, which was verified in the DSC curve of milled MgH2 sample in our additional experiments. It can be found that NbF5 exhibits the maximum effect in promoting the whole dehydrogenation of 2LiBH4 þ MgH2 system, resulting in 30 C and 58 C decreases for the two-step dehydrogenation respectively from neat 2LiBH4 þ MgH2 sample. MgH2 /Mg þ H2
(2)
LiBH4 /LiH þ B þ 3=2 H2
(3)
Mg þ 2LiBH4 /MgB2 þ 2LiH þ 3H2
(4)
Results and discussion
Fig. 1 presents DSC curves of 2LiBH4 þ MgH2 with various additives. All the curves exhibit four distinct endothermic peaks. The first peak around 110 C corresponds to the phase transformation of LiBH4 (the orthorhombic to hexagonal structure) and the second one between 260 and 280 C corresponds to melting of LiBH4 [10]. It is interesting to find that the doping of NbF5, TiF3 and FeF3 slightly decreases the melting temperature of LiBH4. The third peak between 300 and 400 C, which is broad and lowest in the case of LaF3 as additive, corresponds to the dehydrogenation reaction of MgH2 (reaction (2)) [30]. Among these additives, NbF5 appears to be the second effective in reducing the dehydrogenation temperature of MgH2. The fourth peak around 450 C, which is largely
Fig. 2 e MS profiles for the hydrogen evolution from 2LiBH4 D MgH2 system with various additives.
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Fig. 2 displays the MS results for the hydrogen evolution from 2LiBH4 þ MgH2 with various additives. For all curves, a two-step desorption process, which corresponds to decomposition of MgH2 and LiBH4, is observed to be in good agreement with DSC results. It can be seen that 2LiBH4 þ MgH2 þ 0.05NbF5 starts releasing hydrogen from 350 C and desorption completes below 450 C. The temperature of the second hydrogen desorption is not affected obviously by doping LaF3 in 2LiBH4 þ MgH2, though the hydrogen release of MgH2 is reduced greatly to low temperature. The two-step desorption process of neat 2LiBH4 þ MgH2 sample completes below 500 C, with a temperature onset at approximately 370 C. Similar two-step desorption behaviors have been observed for 2LiBH4 þ MgH2 with other additives. In addition, no B2H6 was detected during the dehydrogenation, which is not shown here. To confirm the effect of additives on reaction (2) and reaction (3), hydrogen desorption behaviors of 2LiBH4 þ MgH2 with various additives under 0.01 bar dehydrogenation hydrogen pressure were examined, shown in Fig. 3. It is obviously found that the sample of 2LiBH4 þ MgH2 þ 0.05NbF5 exhibits the fastest kinetics. More than 7 wt.% hydrogen is obtained when heating up to 430 C by doping NbF5, while the neat 2LiBH4 þ MgH2 sample releases 3.4 wt.% hydrogen. The average dehydriding rate of the sample with NbF5 additive is over 2 times higher than that of the neat 2LiBH4 þ MgH2 sample. However, reference [24] showed that the reversibility of 2LiBH4 þ MgH2 system after dehydrogenation under such low hydrogen pressure (reaction (5)) has very poor kinetics and the system was not fully rehydrogenated. And our previous experiments showed that a hydrogen back-pressure of 4 bar is necessary for dehydrogenation according to the reaction (1) [31]. 2LiBH4 þ MgH2 /Mg þ 2LiH þ 2B þ 4H2
(5)
Fig. 4 shows hydrogen desorption curves of 2LiBH4 þ MgH2 with various additives under 4 bar dehydrogenation hydrogen pressure. For all samples, a typical two-step dehydrogenation is observed. An incubation plateau appears during desorption
Fig. 3 e Hydrogen desorption curves of 2LiBH4 D MgH2 with various additives under 0.01 bar dehydrogenation hydrogen pressure. The dot curve represents the temperature profile, and the temperature ramping rate is 5 C/min.
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Fig. 4 e Hydrogen desorption curves of 2LiBH4 D MgH2 with various additives under 4 bar dehydrogenation hydrogen pressure. The dot curve represents the temperature profile, and the temperature ramping rate is 5 C/min.
process except sample of 2LiBH4 þ MgH2 þ 0.05NbF5. Compared to other additives, a pronounced kinetics improvement is obtained by using NbF5 as additive in 2LiBH4 þ MgH2 system, with 7.5 wt.% hydrogen released before temperature holding at 450 C. The rate of hydrogen release is exceedingly slow by doping FeF3, being unable to reach full desorption after dehydriding for 10 h. Above experimental results indicate that not only reaction (1) but also reaction (5) dehydrogenation process of 2LiBH4 þ MgH2 system can be promoted significantly by doping NbF5. And it appears that the catalytic effect of these additives on the dehydrogenation behavior of 2LiBH4 þ MgH2 system increase in order of FeF3 < CeF3 < LaF3 < TiF3 < NbF5. The XRD patterns of 2LiBH4 þ MgH2 with various additives after dehydrogenation under 4 bar hydrogen pressure are shown in Fig. 5. The concurrent existence of both MgB2 and Mg are detected in the dehydrided products of 2LiBH4 þ MgH2 with various additives except that of NbF5 doped 2LiBH4 þ MgH2 sample. It indicates that NbF5 significantly promoting the fully formation of MgB2 instead of Mg retained, resulting in great potential of fully rehydrogenation for 2LiBH4 þ MgH2 system. Unfortunately, no diffraction peak relevant to NbF5 is observed in the dehydrided products, and similar results appeared in TiF3 and FeF3 doped 2LiBH4 þ MgH2 samples. However, La4H12.19 and LaB6 are produced in 2LiBH4 þ MgH2 þ 0.05LaF3 sample; CeH2.51 and CeB6 are produced in 2LiBH4 þ MgH2 þ 0.05CeF3 sample. Fig. 6 shows the dehydriding curves of 2LiBH4 þ MgH2 þ 0.05NbF5 and neat 2LiBH4 þ MgH2 samples from room temperature to 400 C. It can be seen that there is a long dehydriding plateau in neat 2LiBH4 þ MgH2 sample. And the amount of hydrogen desorbed almost maintains at 3 wt.%. No trend of massive hydrogen to release appears until temperature increases to 450 C, even though dehydriding plateau extends to 48 h at 400 C. In contrast, the 2LiBH4 þ MgH2 þ 0.05NbF5 sample can release 8.1 wt.% hydrogen within only 4.5 h. It is clearly that the NbF5 additive markedly increased the desorption kinetics of
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 e1 0 2 6
Fig. 5 e XRD patterns of 2LiBH4 D MgH2 with various additives (NbF5, TiF3, CeF3, LaF3, FeF3) and neat 2LiBH4 D MgH2 sample after dehydrogenation from room temperature to 450 C under 4 bar hydrogen pressure.
Fig. 7 e TPD curves of 2LiBH4 D MgH2 D 0.05NbF5 and neat 2LiBH4 D MgH2 samples. The temperature heating rate is 2 C/min.
the host materials. The TPD data generated by Sieverts apparatus (see Fig. 7) show that the additive NbF5 effectively reduced the temperature of hydrogen desorption in contrast with neat 2LiBH4 þ MgH2 sample. 2LiBH4 þ MgH2 þ 0.05NbF5 sample released 8.2 wt.% hydrogen from 350 C and completed below 450 C, which is consistent with the MS results mentioned above. Nevertheless, neat 2LiBH4 þ MgH2 sample started releasing hydrogen at 370 C and only 5.6 wt.% hydrogen was obtained even up to 500 C. The results of TPD show that addition of NbF5 effectively reduces the dehydriding temperature of 2LiBH4 þ MgH2 system. The elemental boron produced after dehydrogenation of LiBH4-relevant, which is usually amorphous, can be hardly detected by XRD measurement [18]. So, the FTIR spectra are used to characterize the frequencies of BeB spectra in amorphous. Fig. 8 shows the FTIR spectra of 2LiBH4 þ MgH2 þ
0.05NbF5 sample after dehydrogenation at 400 C. It is found that matched vibration signal corresponding to boron was hardly detected in the IR spectrum of dehydrided sample. The absorption peak at 1633 cm1 belongs to OeH band [32]. On the other hand, the IR spectrum for the dehydrided sample was almost the same as the MgB2. It indicates that the formation of the boron phase was effectively inhibited in the 2LiBH4 þ MgH2 system by using NbF5 as additive, and NbF5 plays an important role in the fully formation of MgB2. Dehydriding curves of 2LiBH4 þ MgH2 þ 0.05NbF5 under 4 bar hydrogen pressure at 400 C during the initial 5 cycles are shown in Fig. 9. It can be seen that no obvious degradation tendency emerged in the cycling properties. The cycling retentivity should be supported by the full formation of MgB2 after dehydrogenation (shown in the XRD results of Fig. 9 inset), which
Fig. 6 e Dehydriding curves of 2LiBH4 D MgH2 D 0.05NbF5 and neat 2LiBH4 D MgH2 samples from room temperature to 400 C. The dot curve represents the temperature profile, and the temperature ramping rate is 5 C/min.
Fig. 8 e FTIR spectra for (a) dehydrided sample of 2LiBH4 D MgH2 D 0.05NbF5 after dehydrogenation; (b) MgB2; (c) Boron.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 e1 0 2 6
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improvement in reducing dehydrogenation temperature and enhancing dehydrogenation rate. Compared to neat 2LiBH4 þ MgH2 sample, a lower dehydrogenation temperature by 30 C and 58 C respectively for the two-step dehydrogenation was observed in the 2LiBH4 þ MgH2 þ 0.05NbF5 sample under Ar flow. And over 8.1 wt.% hydrogen can be released from 2LiBH4 þ MgH2 þ 0.05NbF5 below 450 C under a 4 bar initial hydrogen pressure, with an average dehydriding rate over 2 times higher than that of the neat 2LiBH4 þ MgH2 sample. The results of XRD confirm that MgB2 was fully formed instead of Mg retained after dehydrogenation and 2LiBH4 þ MgH2 þ 0.05NbF5 shows a great performance of full reversibility.
Fig. 9 e Dehydriding curves of 2LiBH4 D MgH2 D 0.05NbF5 under 4 bar hydrogen pressure at 400 C during the initial 5 cycles. Rehydrogenation was performed under 70 bar hydrogen pressure at 400 C after dehydrogenation. XRD patterns of 2LiBH4 D MgH2 D 0.05NbF5 after (a) the 1st dehydrogenation and (b) the 5th dehydrogenation are displayed in the inset.
ensured the favorable rehydrogenation. Furthermore, it is interesting to note that dehydriding rate was enhanced after the first dehydrogenation/rehydrogenation cycle. The dehydriding enhancement may just be caused by the catalytic component produced from the addition NbF5 to 2LiBH4 þ MgH2 system, which was always not identified in this studying stage. However, it is confirmed that 2LiBH4 þ MgH2 þ 0.05NbF5 indeed has an excellent reversibility. The evidently positive influence of NbF5 additive on the hydrogen storage properties of 2LiBH4 þ MgH2 system is exhibited in above experimental results. Unfortunately, the catalytic component is not confirmed by XRD analysis. The catalytic effect of NbF5 on the 2LiBH4 þ MgH2 system may come from substitution of H by rich F anion in NbF5 during hydrogen desorption process, which has been predicted to be thermodynamic favorable by theoretical calculations [33]. Another possibility is that the crystallite size of the catalytic component is so fine that it can not easily detected by XRD measurement, which is being investigated in our proceeding work. The full formation of MgB2 after dehydrogenation appeared in the sample of 2LiBH4 þ MgH2 with NbF5, indicating that the loss of H-capacity can be avoided upon subsequent rehydrogenation. And excellent reversible hydrogen storage property can be potentially obtained. However, studies for further reducing dehydrogenation temperature are needed in order to make this system more practical.
4.
Conclusions
We have studied the effect of various additives on the hydrogen desorption characteristics of 2LiBH4 þ MgH2 system. Among these dopants examined, NbF5 exhibits the best
Acknowledgments The authors wish to acknowledge the financial support of this research from the National Basic Research Program of China (Grant No. 2010CB631300 and 2007CB209701), the National Natural Science Foundation of China (Grant No. 51001090, 50871099), the Program for New Century Excellent Talents in Universities (Grant No. NCET-07-0741), the University Doctoral Foundation of the Ministry of Education (Grant No. 20090101110050), and the China Postdoctoral Science Foundation (200902622).
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