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LiBH4·NH3 modified by metal hydrides for advanced dehydrogenation
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 8 1 0 1 e1 8 1 0 7
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NH3BH3/LiBH4$NH3 modified by metal hydrides for advanced dehydrogenation Yingbin Tan a,b, Ziwei Tang a, Shaofeng Li a, Qian Li b,**, Xuebin Yu a,* a b
Department of Materials Science, Fudan University, Shanghai 200433, China Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Shanghai 200072, China
article info
abstract
Article history:
The significantly enhanced dehydrogenation performance of binary complex system,
Received 18 June 2012
NH3BH3/LiBH4$NH3, were achieved through a chemical modification of LiH to form ternary
Received in revised form
composites of x (LiHeNH3BH3)/LiBH4$NH3. Among the studied composites, 3LiHe3NH3BH3/
5 September 2012
LiBH4$NH3 released ca. 10 wt. % high-pure hydrogen (>99.9 mol%) below 100 C with fast
Accepted 10 September 2012
kinetics, while less than 8 wt. % hydrogen, accompanied with a fair number of volatile
Available online 10 October 2012
byproducts, were released from 3NH3BH3/LiBH4$NH3 at the same conditions. Further
Keywords:
composites is based on the combination mechanism of Hdþ and Hd through the interac-
investigations revealed that the hydrogen emission from x (LiHeNH3BH3)/LiBH4$NH3 Dehydrogenation properties
tion between LiHeNH3BH3 and NH3 group in LiBH4$NH3, in which the controllable protic
Binary complex system
hydrogen source from the stabilized NH3 group played a crucial role in providing optimal
Kinetics
stoichiometric ratio of Hdþ and Hd, and thus leading to the significant improvement of
Protic hydrogen
dehydrogenation capacity and purity. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The development of a safe and efficient storage medium for hydrogen is vital to its use as an alternative energy carrier [1]. Decades of extensive efforts on metal/alloy hydrides [2,3], nanostructured carbon materials [4], and complex hydrides [5] have been conducted to solve this problem. However, the multiple target criteria accepted as necessary for the successful application of such stores have not yet been achieved by any single material [6e8]. Recently, ammonia borane, NH3BH3 (AB), owning a high hydrogen-storage capacity (19.6 wt. %) and relatively favorable thermal stability, has been regarded as one of the most promising candidates for chemical hydrogen storage [9e15]. However, the practical
application of AB as on-board hydrogen source has been hindered due to the following several problems: (1) the hydrogen release kinetics from solid AB is comparably slow with a long induction period [16,17]; (2) the release of volatile by-products such as borazine, diborane, ammonia and monomeric aminoborane [18,19], which are undesired in combination with a fuel cell; (3) serious foaming during the decomposition process of AB that complicates the apparatus design [20]. To solve these problems, several approaches, including adoption of transition metals catalysts [21e23], nanoscaffolds [24e26], ionic liquids [27e29] and additives [30] etc., have been developed to improve the dehydrogenation properties of AB in terms of the improved dehydrogenation kinetics, eliminated induction period and suppressed volatile
* Corresponding author. Tel.: þ86 21 5566 4581. ** Corresponding author. Tel./fax: þ86 21 56338065. E-mail addresses: [email protected] (Q. Li), [email protected] (X. Yu). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.09.069
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by-products release. However, from a practical point of view, these approaches are still far from ideal for hydrogen storage applications, owing to the reduction of hydrogen capacity with these non-hydrogen foreign materials added. Therefore, development of new strategies to achieve a generation of high-purity hydrogen from AB without significant decrease on the system’s hydrogen capacity still remains a huge challenge. We have previously reported that the hydrogen-enriched AB/LiBH4$NH3 eutectic system presented improved decomposition properties via an interaction of AB and the NH3 group in the LiBH4$NH3, in which LiBH4 worked as a carrier of ammonia and played a crucial role in promoting the interaction between the NH3 group and AB [31]. However, along with the hydrogen evolution, release of a large amount of volatile by-products (ammonia, diborane and borazine) has also occurred [32], which promotes us to pursue various modifications on this complex system so as to further improve its dehydrogenation properties. In this paper, we report our latest results on the study of the AB/LiBH4$NH3 modified via metal hydrides, in which the LiH was found to be the most effective additive in enhancing the dehydrogenation properties of AB/LiBH4$NH3. Our results showed that 3LiHe3AB/LiBH4$NH3 could release ca. 10 wt % high-pure hydrogen at 100 C within 3 h, suggesting the potential for this complex system as a promising candidate for chemical hydrogen storage.
2.
Experimental
2.1.
Reagents and synthesis
H2 ðCH2 Þ and NH3 ðCNH3 Þ can be calculated from the follow two equations, CH2 þ CNH3 ¼ 1 CH2 2:02 þ CNH3 17:03 Mp ¼ Wp
2.2.
(2)
Decomposition behavior of the sample was also studied by the powder X-ray diffraction (XRD, Rigaku D/max 2400, Cu Ka radiation, 16 kW) to confirm the phase structure. Amorphous polymer tape was used to cover the surface of the powder to avoid sample oxidation during the XRD measurement. Solid-state infrared spectra of the samples (as KBr pellets) were recorded with Nicolet Nexus 470 in the range of 500e4000 cm1. During the IR measurement (KBr pellets), samples were loaded into one closed tube with CaF2 windows. Raman spectra were collected at room temperature in backscattering geometry using the 632.8 nm line of an Arþ-ion gas laser with a power of 100 mW, and the samples were mounted with a thin cover glass to isolate them from air. Solid-state MAS NMR spectra were measured using a Bruker Avance 400 MHz spectrometer, using a Doty CP-MAS probe with no probe background. The powder samples collected after decomposition reaction was spun at 5 kHz, using 4 mm ZrO2 rotors filled up in purified argon atmosphere glove boxes. A 0.55 ms single-pulse excitation was employed, with repetition times of 1.5 s.
3. The starting materials, LiBH4 (95%), LiH (95%) and NH3BH3 (97%), were purchased from SigmaeAldrich and used in asreceived form without further purification. NH3 was purified by soda lime before using. LiBH4$NH3 was synthesized by exposing LiBH4 to NH3 atmosphere at room-temperature, LiHeAB/LiBH4$NH3, 2LiHe2AB/LiBH4$NH3 and 3LiHe3AB/ LiBH4$NH3 were prepared by grinding LiH-AB and LiBH4$NH3 for 10 min at mole ratios of 1:1, 2:1 and 3:1, respectively. All samples were handled in an argon-filled glove box. Heat treatments were carried out in a close test tube under an argon atmosphere, and hydrogen was released into a carrier stream of argon through a T-joint with a thin connection tube to maintain the argon atmosphere over the samples.
(1)
Results and discussion
The initial investigation of metal hydrides addition on the dehydrogenation of 2AB/LiBH4$NH3 was conducted [31]. The temperature-programmed desorption (TPD) results for the MHe2AB/LiBH4$NH3 (MH ¼ LiH, NaH, MgH2, and CaH2, the mole ratio of MH to AB and LiBH4$NH3 is 1:2:1) composites are shown in Fig. 1. It shows that, except the MgH2, the addition of LiH, NaH and CaH2 significantly lowered the decomposition
Instrumentation and analyses
Hydrogen release property measurements were performed by eliminating ammonia temperature programmed desorption (EATPD) and temperature programmed desorption (TPD) performed on a Sieverts-type apparatus, connected with a reactor filled with sample (w0.1 g) under argon atmosphere (1 bar) with a heating rate of 5 C min1. EATPD instrument with molecular sieves was used to eliminate the ammonia during thermal decomposition. The contents of H2 and NH3 in the emission gas were determined using gravimetric and volumetric results. Firstly, the mass percent (Wp) and mole per gram (Mp) of gas released from the samples were calculated from the weights of the samples and volumetric results, then the mole proportion of
Fig. 1 e TPD results for 2LiHe2AB/LiBH4$NH3, 2NaHe2AB/ LiBH4$NH3, CaH2e2AB/LiBH4$NH3, MgH2e2AB/LiBH4$NH3 and 2AB/LiBH4$NH3. The heating rate is 5 C minL1.
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Fig. 2 e EATPD results for (a) x (LiHeAB)/LiBH4$NH3 (x [ 1, 2, 3) samples. The x AB/LiBH4$NH3 (x [ 1, 2, 3) samples were also measured for comparison. (b) A comparison of TPD results for 3LiHe3AB/LiBH4$NH3 with LiH/LiBH4$NH3, LiH/AB and 3AB/LiBH4$NH3. The heating rate is 5 C minL1.
temperature of 2AB/LiBH4$NH3 sample, to which the onset temperature was decreased from 100 C to 50, 50 and 75 C, respectively. Particularly, for the LiH addition, which has the most reactive and negative charged hydrogen among these metal hydrides, a significant improvement on the dehydrogenation temperature, while without decrease on the overall gravimetric hydrogen yield of the mixture by 200 C, was observed. Given that LiH presented the most effective activity on the dehydrogenation improvement of the composites above, further investigations on the x (LiHeAB)/LiBH4$NH3 samples with various molar ratios were conducted. Fig. 2a shows the TPD results for the dehydrogenation of x (LiHeAB)/LiBH4$NH3 (x ¼ 1, 2, 3) samples with a heating rate of 5 C min1. Clearly, compared to the AB/LiBH4$NH3 system, addition of LiH leads to significant improvement in terms of both hydrogen release temperature and capacity. During decomposition, the AB/ LiBH4$NH3 system released hydrogen in a two-step decomposition feature with the onset temperature at ca. 100 C (Fig. 2a), while the hydrogen release temperature of LiHeAB/ LiBH4$NH3 system was lowered to ca. 50 C, and the majority of hydrogen was released at temperature below 100 C. Changing the LiH content did not have significant effect on the initial dehydrogenation properties, while the total dehydrogenation capacity by 250 C was increased with the increasing LiH content. In addition, an apparent second-step dehydrogenation was observed for the composites with a high ratio of LiH. Notably, the dehydrogenation properties of x (LiHeAB)/ LiBH4$NH3 are also better than other two binary systems, i.e. LiH/AB and LiH/LiBH4$NH3 (Fig. 2b), indicating the dehydrogenation advantages of the multi-component composites [33]. More importantly, the addition of LiH was found to eliminate the detrimental gas impurities significantly. A summary of the gravimetric and volumetric measurement resulted from the decomposition of AB/LiBH4$NH3 and x (LiHeAB)/LiBH4$NH3 composites after heating to 250 C are listed in Table 1. It can be seen that the volatile by-products concentration was reduced from 19.12 mol % of AB/LiBH4$NH3 to 1.2 mol % for LiHeAB/LiBH4$NH3. Furthermore, the by-products concentration was decreased to 0.88 mol % and 0.11 mol % for 2LiHe2AB/ LiBH4$NH3 and 3LiHe3AB/LiBH4$NH3, respectively, suggesting that the increased LiHeAB content, which results in the
decrease of LiBH4$NH3 ratio in the composites, may further contribute to the volatile by-products suppression. The dehydrogenation behavior for the 3LiHe3AB/ LiBH4$NH3 sample was further investigated by using isothermal volumetric EATPD measurements [31] at various temperatures as shown in Fig. 3. In agreement with the literature [34], the neat AB produced negligible hydrogen below 80 C, and only approximately 2.1 wt % and 4.2 wt % of H2 was released at 90 and 100 C within 60 min, respectively. Meanwhile, the 3AB/LiBH4$NH3 started to release hydrogen at above 80 C with sluggish kinetics, releasing 3.4, 5.6 and 7.3 wt % hydrogen at 80, 90 and 100 C within 1 h, respectively. As for the LiH added sample, a significantly increased H2-release rate was present even at 70 C, releasing more than 6 wt% hydrogen with 30 min. At elevated temperatures (>90 C), the majority of hydrogen release was completed in a few minutes. Particularly, at 90 and 100 C, 7.8 wt % and 9.5 wt% H2 were generated within 10 min, respectively. Note that in all the temperature range, the dehydrogenation properties of 3LiHe3AB/LiBH4$NH3 is advanced compared with the LiAB, a recently reported appearing hydrogen storage material [18]. Clearly, the fast dehydrogenation rate and high hydrogen capacity and purity displayed by the 3LiHe3AB/LiBH4$NH3 in the temperature below 100 C enable this material to be a promising hydrogen storage candidate.
Table 1 e Summary of gas evolution for the x AB/ LiBH4$NH3 and x (LiHeAB)/AB/LiBH4$NH3 mixtures. Samples
AB/LiBH4$NH3 2AB/LiBH4$NH3 3AB/LiBH4$NH3 LiHeAB/LiBH4$NH3 2LiHe2AB/LiBH4$NH3 3LiHe3AB/LiBH4$NH3
Total weight loss (wt. %)
Gas desorption (mol/g)
H-purity (mol %)
23.96 25.98 28.39 10.34 12.29 13.56
0.049 0.059 0.066 0.0469 0.0574 0.065
80.88 84.14 84.80 98.8 99.12 99.89
Assumed that the released gases consisted of hydrogen and ammonia.
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Fig. 3 e Time dependences of hydrogen release from 3LiHe3AB/LiBH4$NH3 at 100 C, 90 C, 80 C and 70 C. The neat AB, LiAB and LiBH4$NH3/3AB at 90 C were also measured for comparison.
In order to understand the thermal decomposition mechanism of the x (LiHeAB)/LiBH4$NH3 composites, X-ray diffraction (XRD), 11B solid state nuclear magnetic resonance (11B NMR), Fourier transform infrared (FTIR) and Raman spectrometry measurements were conducted for the 3LiHe3AB/LiBH4$NH3 sample. Fig. 4 shows the FTIR data of the sample before and after dehydrogenation. Notably, the position of the LieH vibration is observed in the as-prepared 3LiHe3AB/LiBH4$NH3 (Fig. 4 and Fig. S1), suggesting that there is no chemical reaction occurred before heat treatment. After dehydrogenation at 150 C, FTIR analysis reveals that the intensity of BeH bands at 1169 and 1073 cm1 is dramatic decay accompanied by the distinct reduction of the NeH stretch (3200e3400 cm1), resulting in the formation of BeN
bending vibration peak at 799 cm1. Meanwhile, disappearance of the LieH bond was also observed, indicating the occurrence of the interaction among the LiH, AB and LiBH4$NH3. The Raman spectra (Fig. 5) show that the NeH vibration attributed to AB (at 3246 cm1) is clearly identified, while the NeH vibration ascribed to LiBH4$NH3 (at 3293 cm1) is disappeared, accompanied by the disappearance of the BeH bending (at 1575 cm1), which suggests that the protic NeH bonds in LiBH4$NH3 is more active than that in AB to combine with the hydridic BeH bonds [35]. Moreover, it was observed that the NeH bending ascribed to AB (at 3246 cm1) is also disappeared, and a new vibration BeN centered at 1326 cm1 emerged. The results above illustrate that the hydrogen released from 3LiHe3AB/LiBH4$NH3 at below 150 C may
Fig. 4 e FTIR spectra of (a) 3LiHe3AB/LiBH4$NH3 at 25 C and its products after heating to 150 C and 250 C. The neat AB and LiBH4$NH3 were also measured for comparison. (b) LiHeAB/LiBH4$NH3, 2LiHe2AB/LiBH4$NH3 and 3LiHe3AB/ LiBH4$NH3$dehydrogenated to 250 C. FTIR spectra of LiBH4 after heating to 250 C were also measured for comparison.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 8 1 0 1 e1 8 1 0 7
Fig. 5 e Raman spectra of 3LiHe3AB/LiBH4$NH3 at 25 C and its products after heating to 150 C and 250 C. The neat AB and LiBH4$NH3 were also measured for comparison.
attribute to the combination of Hdþ and Hd, leading to the formation of BeN bonds [36]. Upon full dehydrogenation, the BeH stretching modes in the dehydrogenation products appear at the frequencies of 2218, 2286, and 2380 cm1, and the BH2 deformation bands are assigned to the absorptions at 1120 cm1, which is well consistent with the BH 4 anion [37]. The same results were also observed in LiHeAB/LiBH4$NH3 and 2LiHe2AB/LiBH4$NH3, indicating that the BH 4 group in LiBH4$NH3 keeps unchanged during the reaction (Fig. 4b). However, the XRD pattern (Fig. S2) of 3LiHe3AB/LiBH4$NH3 sample after dehydrogenation indicate the formation of an amorphous phase, suggesting that the LiBH4 may form with poor crystal structure in decomposition process. The 11B NMR spectrum of 3LiHe3AB/LiBH4$NH3 decomposed under Ar presents a boron species with a chemical shift of 39.5 ppm (Fig. 6a), which is assigned to the boron nucleus in the tetrahedral BH 4 units, consistent with the results of FTIR. Moreover, the original BNH3 resonance has converted completely into the new resonance features (at 21.5, 12.1 and 1.6 ppm), which corresponds to the characteristics of B in trigonal planar BN3 and/or HBN2 environments, similar to the chemical shift of B in the 3AB/LiBH4$NH3 (Fig. 6b) and Li(NH3) NH2BH3 decomposed [31,35]. This strongly suggests that the final product is a borazine-like or polyborazin-like compound [38]. According to the above analysis, it is concluded that the dehydrogenation of x (LiHeAB)/LiBH4$NH3 composites is also based on the combination mechanism of Hdþ and Hd through the interaction between LiHeAB and NH3 group in LiBH4$NH3. Therefore, the balanced BeH and NeH for this combination reaction can be achieved by adjusting the ratio of LiHeAB and LiBH4$NH3. Moreover, as LiAB can be formed through the reaction of AB and LiH [7], the hydrogen release in the x (LiHeAB)/LiBH4$NH3 composites may start with the formation of LiAB, and then LiAB further react with the NH3 group in the LiBH4$NH3 to release H2 and form LiNBNH
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Fig. 6 e Low-field 11B NMR (400 MHz) spectra for 3LiHe3AB/ LiBH4$NH3 (a) and 3AB/LiBH4$NH3 (b) after dehydrogenation at 250 C.
(borazine-like or polyborazin-like compound) and LiBH4. The TPD measurement for 3LiAB/LiBH4$NH3 in Fig. S3 shows similar dehydrogenation properties to the 3(LiHeAB)/ LiBH4$NH3, except a capacity decrease of 1 equivalent H2. In addition, the IR results also confirm the residual BH1 4 after the 3LiAB/LiBH4$NH3 dehydrogenated to 250 C as did in the decomposed 3(LiHeAB)/LiBH4$NH3 (Fig. S4). These results supported the above assumption. Therefore, the decomposition reaction of 3(LiHeAB)/LiBH4$NH3 can be speculated as Eqs. (3) and (4). Compared with the reported amminelithium amidoborane (Li(NH3)NH2BH3), which releases a large amount of ammonia (0.53 mol) during the dehydrogenation, owing to the weak N:/Liþ coordination bond, only pure hydrogen evolution was observed from the x (LiHeAB)/ LiBH4$NH3 composites, indicating the crucial role of LiBH4, derived form the LiBH4$NH3, in stabilizing the ammonia and promoting its combination with the formed LiAB [35]. Kim et al. and Xia et al. suggested a dehydrogenation mechanism for LiNH2BH3 and Li(NH3)NH2BH3 based on the dimer (LiNH2BH3)2 and (Li(NH3)NH2BH3)2 as a model system [35,39]. Here we can also use the dimer decomposition mechanism to explain the LiHeAB/LiBH4$NH3 system. In LiHeAB/LiBH4$NH3, the dehydrogenation process occurred through the transfer of H from B to the midpoint between two Li cations forming the triangular moiety (denoted d “Lidþ 2 :H ”), and then the redox reaction of the dihydrogen bond in LiHdþ.dHN to form H2, which is the same as the dehydrogenation of LiNH2BH3 and Li(NH3)NH2BH3 [35,39]. In a similar way to the dehydrogenation of the solid materials in the cases of LiNH2 and LiNH2BH3, the transfer of a Li cation and a hydride is a principal step for hydrogen release in LiHeAB/LiBH4$NH3. In addition, a strong Lewis base H in LiH acts as a hydride source for the dehydrogenation and the Li cation plays an important role in decreasing the energy barrier for intermolecular NeB bond formation in the hydrogen
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release, which would result in the formation of NeB oligomers/polymers as products. LiH þ NH3 BH3 /LiNH2 BH3 þ H2
(3)
LiNH2 BH3 þ LiBH4 $NH3 /LiNBNH2 þ 3H2 þ LiBH4
(4)
4.
Conclusion
In this article, our results have shown that x (LiHeAB)/ LiBH4$NH3 composites exhibit much higher dehydrogenation capacity with enhanced hydrogen purity, and faster kinetics than the previously reported AB/LiBH4$NH3 composites. For example, in the case of the 3LiHe3AB/LiBH4$NH3 composite, about 10 wt. % high-pure hydrogen can be released below 100 C, which exceeds even the thermolysis of LiAB. The hydrogen release in the x (LiHeAB)/LiBH4$NH3 composites was found to start with the formation of LiAB, and then LiAB further reacted with the NH3 group in LiBH4$NH3 to release H2 and form LiNBNH polymer and LiBH4. The controllable protic hydrogen source from the stabilized NH3 group in LiBH4$NH3 played a crucial role in providing optimal stoichiometric ratio of Hdþ and Hd, which contributed to the pure hydrogen release without undesired volatile by-products. Our work may provide an effective method to promote the dehydrogenation and suppress volatile by-products evolution of AB, as well as further information on understanding the decomposition mechanism of the ammonia complexes of AB.
Acknowledgements This work was partially supported by the Ministry of Science and Technology of China (2010CB631302), the National Natural Science Foundation of China (Grant No. 51071047), the PhD Programs Foundation of Ministry of Education of China (20110071110009), Science and Technology Commission of Shanghai Municipality (11JC1400700, 11520701100) and Shanghai Rising-Star Program (11QH1400900).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2012.09.069.
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NH3BH3/LiBH4$NH3 modified by metal hydrides for advanced dehydrogenation Yingbin Tan a,b, Ziwei Tang a, Shaofeng Li a, Qian Li b,**, Xuebin Yu a,* a b
Department of Materials Science, Fudan University, Shanghai 200433, China Shanghai Key Laboratory of Modern Metallurgy & Materials Processing, Shanghai University, Shanghai 200072, China
article info
abstract
Article history:
The significantly enhanced dehydrogenation performance of binary complex system,
Received 18 June 2012
NH3BH3/LiBH4$NH3, were achieved through a chemical modification of LiH to form ternary
Received in revised form
composites of x (LiHeNH3BH3)/LiBH4$NH3. Among the studied composites, 3LiHe3NH3BH3/
5 September 2012
LiBH4$NH3 released ca. 10 wt. % high-pure hydrogen (>99.9 mol%) below 100 C with fast
Accepted 10 September 2012
kinetics, while less than 8 wt. % hydrogen, accompanied with a fair number of volatile
Available online 10 October 2012
byproducts, were released from 3NH3BH3/LiBH4$NH3 at the same conditions. Further
Keywords:
composites is based on the combination mechanism of Hdþ and Hd through the interac-
investigations revealed that the hydrogen emission from x (LiHeNH3BH3)/LiBH4$NH3 Dehydrogenation properties
tion between LiHeNH3BH3 and NH3 group in LiBH4$NH3, in which the controllable protic
Binary complex system
hydrogen source from the stabilized NH3 group played a crucial role in providing optimal
Kinetics
stoichiometric ratio of Hdþ and Hd, and thus leading to the significant improvement of
Protic hydrogen
dehydrogenation capacity and purity. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The development of a safe and efficient storage medium for hydrogen is vital to its use as an alternative energy carrier [1]. Decades of extensive efforts on metal/alloy hydrides [2,3], nanostructured carbon materials [4], and complex hydrides [5] have been conducted to solve this problem. However, the multiple target criteria accepted as necessary for the successful application of such stores have not yet been achieved by any single material [6e8]. Recently, ammonia borane, NH3BH3 (AB), owning a high hydrogen-storage capacity (19.6 wt. %) and relatively favorable thermal stability, has been regarded as one of the most promising candidates for chemical hydrogen storage [9e15]. However, the practical
application of AB as on-board hydrogen source has been hindered due to the following several problems: (1) the hydrogen release kinetics from solid AB is comparably slow with a long induction period [16,17]; (2) the release of volatile by-products such as borazine, diborane, ammonia and monomeric aminoborane [18,19], which are undesired in combination with a fuel cell; (3) serious foaming during the decomposition process of AB that complicates the apparatus design [20]. To solve these problems, several approaches, including adoption of transition metals catalysts [21e23], nanoscaffolds [24e26], ionic liquids [27e29] and additives [30] etc., have been developed to improve the dehydrogenation properties of AB in terms of the improved dehydrogenation kinetics, eliminated induction period and suppressed volatile
* Corresponding author. Tel.: þ86 21 5566 4581. ** Corresponding author. Tel./fax: þ86 21 56338065. E-mail addresses: [email protected] (Q. Li), [email protected] (X. Yu). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.09.069
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by-products release. However, from a practical point of view, these approaches are still far from ideal for hydrogen storage applications, owing to the reduction of hydrogen capacity with these non-hydrogen foreign materials added. Therefore, development of new strategies to achieve a generation of high-purity hydrogen from AB without significant decrease on the system’s hydrogen capacity still remains a huge challenge. We have previously reported that the hydrogen-enriched AB/LiBH4$NH3 eutectic system presented improved decomposition properties via an interaction of AB and the NH3 group in the LiBH4$NH3, in which LiBH4 worked as a carrier of ammonia and played a crucial role in promoting the interaction between the NH3 group and AB [31]. However, along with the hydrogen evolution, release of a large amount of volatile by-products (ammonia, diborane and borazine) has also occurred [32], which promotes us to pursue various modifications on this complex system so as to further improve its dehydrogenation properties. In this paper, we report our latest results on the study of the AB/LiBH4$NH3 modified via metal hydrides, in which the LiH was found to be the most effective additive in enhancing the dehydrogenation properties of AB/LiBH4$NH3. Our results showed that 3LiHe3AB/LiBH4$NH3 could release ca. 10 wt % high-pure hydrogen at 100 C within 3 h, suggesting the potential for this complex system as a promising candidate for chemical hydrogen storage.
2.
Experimental
2.1.
Reagents and synthesis
H2 ðCH2 Þ and NH3 ðCNH3 Þ can be calculated from the follow two equations, CH2 þ CNH3 ¼ 1 CH2 2:02 þ CNH3 17:03 Mp ¼ Wp
2.2.
(2)
Decomposition behavior of the sample was also studied by the powder X-ray diffraction (XRD, Rigaku D/max 2400, Cu Ka radiation, 16 kW) to confirm the phase structure. Amorphous polymer tape was used to cover the surface of the powder to avoid sample oxidation during the XRD measurement. Solid-state infrared spectra of the samples (as KBr pellets) were recorded with Nicolet Nexus 470 in the range of 500e4000 cm1. During the IR measurement (KBr pellets), samples were loaded into one closed tube with CaF2 windows. Raman spectra were collected at room temperature in backscattering geometry using the 632.8 nm line of an Arþ-ion gas laser with a power of 100 mW, and the samples were mounted with a thin cover glass to isolate them from air. Solid-state MAS NMR spectra were measured using a Bruker Avance 400 MHz spectrometer, using a Doty CP-MAS probe with no probe background. The powder samples collected after decomposition reaction was spun at 5 kHz, using 4 mm ZrO2 rotors filled up in purified argon atmosphere glove boxes. A 0.55 ms single-pulse excitation was employed, with repetition times of 1.5 s.
3. The starting materials, LiBH4 (95%), LiH (95%) and NH3BH3 (97%), were purchased from SigmaeAldrich and used in asreceived form without further purification. NH3 was purified by soda lime before using. LiBH4$NH3 was synthesized by exposing LiBH4 to NH3 atmosphere at room-temperature, LiHeAB/LiBH4$NH3, 2LiHe2AB/LiBH4$NH3 and 3LiHe3AB/ LiBH4$NH3 were prepared by grinding LiH-AB and LiBH4$NH3 for 10 min at mole ratios of 1:1, 2:1 and 3:1, respectively. All samples were handled in an argon-filled glove box. Heat treatments were carried out in a close test tube under an argon atmosphere, and hydrogen was released into a carrier stream of argon through a T-joint with a thin connection tube to maintain the argon atmosphere over the samples.
(1)
Results and discussion
The initial investigation of metal hydrides addition on the dehydrogenation of 2AB/LiBH4$NH3 was conducted [31]. The temperature-programmed desorption (TPD) results for the MHe2AB/LiBH4$NH3 (MH ¼ LiH, NaH, MgH2, and CaH2, the mole ratio of MH to AB and LiBH4$NH3 is 1:2:1) composites are shown in Fig. 1. It shows that, except the MgH2, the addition of LiH, NaH and CaH2 significantly lowered the decomposition
Instrumentation and analyses
Hydrogen release property measurements were performed by eliminating ammonia temperature programmed desorption (EATPD) and temperature programmed desorption (TPD) performed on a Sieverts-type apparatus, connected with a reactor filled with sample (w0.1 g) under argon atmosphere (1 bar) with a heating rate of 5 C min1. EATPD instrument with molecular sieves was used to eliminate the ammonia during thermal decomposition. The contents of H2 and NH3 in the emission gas were determined using gravimetric and volumetric results. Firstly, the mass percent (Wp) and mole per gram (Mp) of gas released from the samples were calculated from the weights of the samples and volumetric results, then the mole proportion of
Fig. 1 e TPD results for 2LiHe2AB/LiBH4$NH3, 2NaHe2AB/ LiBH4$NH3, CaH2e2AB/LiBH4$NH3, MgH2e2AB/LiBH4$NH3 and 2AB/LiBH4$NH3. The heating rate is 5 C minL1.
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Fig. 2 e EATPD results for (a) x (LiHeAB)/LiBH4$NH3 (x [ 1, 2, 3) samples. The x AB/LiBH4$NH3 (x [ 1, 2, 3) samples were also measured for comparison. (b) A comparison of TPD results for 3LiHe3AB/LiBH4$NH3 with LiH/LiBH4$NH3, LiH/AB and 3AB/LiBH4$NH3. The heating rate is 5 C minL1.
temperature of 2AB/LiBH4$NH3 sample, to which the onset temperature was decreased from 100 C to 50, 50 and 75 C, respectively. Particularly, for the LiH addition, which has the most reactive and negative charged hydrogen among these metal hydrides, a significant improvement on the dehydrogenation temperature, while without decrease on the overall gravimetric hydrogen yield of the mixture by 200 C, was observed. Given that LiH presented the most effective activity on the dehydrogenation improvement of the composites above, further investigations on the x (LiHeAB)/LiBH4$NH3 samples with various molar ratios were conducted. Fig. 2a shows the TPD results for the dehydrogenation of x (LiHeAB)/LiBH4$NH3 (x ¼ 1, 2, 3) samples with a heating rate of 5 C min1. Clearly, compared to the AB/LiBH4$NH3 system, addition of LiH leads to significant improvement in terms of both hydrogen release temperature and capacity. During decomposition, the AB/ LiBH4$NH3 system released hydrogen in a two-step decomposition feature with the onset temperature at ca. 100 C (Fig. 2a), while the hydrogen release temperature of LiHeAB/ LiBH4$NH3 system was lowered to ca. 50 C, and the majority of hydrogen was released at temperature below 100 C. Changing the LiH content did not have significant effect on the initial dehydrogenation properties, while the total dehydrogenation capacity by 250 C was increased with the increasing LiH content. In addition, an apparent second-step dehydrogenation was observed for the composites with a high ratio of LiH. Notably, the dehydrogenation properties of x (LiHeAB)/ LiBH4$NH3 are also better than other two binary systems, i.e. LiH/AB and LiH/LiBH4$NH3 (Fig. 2b), indicating the dehydrogenation advantages of the multi-component composites [33]. More importantly, the addition of LiH was found to eliminate the detrimental gas impurities significantly. A summary of the gravimetric and volumetric measurement resulted from the decomposition of AB/LiBH4$NH3 and x (LiHeAB)/LiBH4$NH3 composites after heating to 250 C are listed in Table 1. It can be seen that the volatile by-products concentration was reduced from 19.12 mol % of AB/LiBH4$NH3 to 1.2 mol % for LiHeAB/LiBH4$NH3. Furthermore, the by-products concentration was decreased to 0.88 mol % and 0.11 mol % for 2LiHe2AB/ LiBH4$NH3 and 3LiHe3AB/LiBH4$NH3, respectively, suggesting that the increased LiHeAB content, which results in the
decrease of LiBH4$NH3 ratio in the composites, may further contribute to the volatile by-products suppression. The dehydrogenation behavior for the 3LiHe3AB/ LiBH4$NH3 sample was further investigated by using isothermal volumetric EATPD measurements [31] at various temperatures as shown in Fig. 3. In agreement with the literature [34], the neat AB produced negligible hydrogen below 80 C, and only approximately 2.1 wt % and 4.2 wt % of H2 was released at 90 and 100 C within 60 min, respectively. Meanwhile, the 3AB/LiBH4$NH3 started to release hydrogen at above 80 C with sluggish kinetics, releasing 3.4, 5.6 and 7.3 wt % hydrogen at 80, 90 and 100 C within 1 h, respectively. As for the LiH added sample, a significantly increased H2-release rate was present even at 70 C, releasing more than 6 wt% hydrogen with 30 min. At elevated temperatures (>90 C), the majority of hydrogen release was completed in a few minutes. Particularly, at 90 and 100 C, 7.8 wt % and 9.5 wt% H2 were generated within 10 min, respectively. Note that in all the temperature range, the dehydrogenation properties of 3LiHe3AB/LiBH4$NH3 is advanced compared with the LiAB, a recently reported appearing hydrogen storage material [18]. Clearly, the fast dehydrogenation rate and high hydrogen capacity and purity displayed by the 3LiHe3AB/LiBH4$NH3 in the temperature below 100 C enable this material to be a promising hydrogen storage candidate.
Table 1 e Summary of gas evolution for the x AB/ LiBH4$NH3 and x (LiHeAB)/AB/LiBH4$NH3 mixtures. Samples
AB/LiBH4$NH3 2AB/LiBH4$NH3 3AB/LiBH4$NH3 LiHeAB/LiBH4$NH3 2LiHe2AB/LiBH4$NH3 3LiHe3AB/LiBH4$NH3
Total weight loss (wt. %)
Gas desorption (mol/g)
H-purity (mol %)
23.96 25.98 28.39 10.34 12.29 13.56
0.049 0.059 0.066 0.0469 0.0574 0.065
80.88 84.14 84.80 98.8 99.12 99.89
Assumed that the released gases consisted of hydrogen and ammonia.
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Fig. 3 e Time dependences of hydrogen release from 3LiHe3AB/LiBH4$NH3 at 100 C, 90 C, 80 C and 70 C. The neat AB, LiAB and LiBH4$NH3/3AB at 90 C were also measured for comparison.
In order to understand the thermal decomposition mechanism of the x (LiHeAB)/LiBH4$NH3 composites, X-ray diffraction (XRD), 11B solid state nuclear magnetic resonance (11B NMR), Fourier transform infrared (FTIR) and Raman spectrometry measurements were conducted for the 3LiHe3AB/LiBH4$NH3 sample. Fig. 4 shows the FTIR data of the sample before and after dehydrogenation. Notably, the position of the LieH vibration is observed in the as-prepared 3LiHe3AB/LiBH4$NH3 (Fig. 4 and Fig. S1), suggesting that there is no chemical reaction occurred before heat treatment. After dehydrogenation at 150 C, FTIR analysis reveals that the intensity of BeH bands at 1169 and 1073 cm1 is dramatic decay accompanied by the distinct reduction of the NeH stretch (3200e3400 cm1), resulting in the formation of BeN
bending vibration peak at 799 cm1. Meanwhile, disappearance of the LieH bond was also observed, indicating the occurrence of the interaction among the LiH, AB and LiBH4$NH3. The Raman spectra (Fig. 5) show that the NeH vibration attributed to AB (at 3246 cm1) is clearly identified, while the NeH vibration ascribed to LiBH4$NH3 (at 3293 cm1) is disappeared, accompanied by the disappearance of the BeH bending (at 1575 cm1), which suggests that the protic NeH bonds in LiBH4$NH3 is more active than that in AB to combine with the hydridic BeH bonds [35]. Moreover, it was observed that the NeH bending ascribed to AB (at 3246 cm1) is also disappeared, and a new vibration BeN centered at 1326 cm1 emerged. The results above illustrate that the hydrogen released from 3LiHe3AB/LiBH4$NH3 at below 150 C may
Fig. 4 e FTIR spectra of (a) 3LiHe3AB/LiBH4$NH3 at 25 C and its products after heating to 150 C and 250 C. The neat AB and LiBH4$NH3 were also measured for comparison. (b) LiHeAB/LiBH4$NH3, 2LiHe2AB/LiBH4$NH3 and 3LiHe3AB/ LiBH4$NH3$dehydrogenated to 250 C. FTIR spectra of LiBH4 after heating to 250 C were also measured for comparison.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 8 1 0 1 e1 8 1 0 7
Fig. 5 e Raman spectra of 3LiHe3AB/LiBH4$NH3 at 25 C and its products after heating to 150 C and 250 C. The neat AB and LiBH4$NH3 were also measured for comparison.
attribute to the combination of Hdþ and Hd, leading to the formation of BeN bonds [36]. Upon full dehydrogenation, the BeH stretching modes in the dehydrogenation products appear at the frequencies of 2218, 2286, and 2380 cm1, and the BH2 deformation bands are assigned to the absorptions at 1120 cm1, which is well consistent with the BH 4 anion [37]. The same results were also observed in LiHeAB/LiBH4$NH3 and 2LiHe2AB/LiBH4$NH3, indicating that the BH 4 group in LiBH4$NH3 keeps unchanged during the reaction (Fig. 4b). However, the XRD pattern (Fig. S2) of 3LiHe3AB/LiBH4$NH3 sample after dehydrogenation indicate the formation of an amorphous phase, suggesting that the LiBH4 may form with poor crystal structure in decomposition process. The 11B NMR spectrum of 3LiHe3AB/LiBH4$NH3 decomposed under Ar presents a boron species with a chemical shift of 39.5 ppm (Fig. 6a), which is assigned to the boron nucleus in the tetrahedral BH 4 units, consistent with the results of FTIR. Moreover, the original BNH3 resonance has converted completely into the new resonance features (at 21.5, 12.1 and 1.6 ppm), which corresponds to the characteristics of B in trigonal planar BN3 and/or HBN2 environments, similar to the chemical shift of B in the 3AB/LiBH4$NH3 (Fig. 6b) and Li(NH3) NH2BH3 decomposed [31,35]. This strongly suggests that the final product is a borazine-like or polyborazin-like compound [38]. According to the above analysis, it is concluded that the dehydrogenation of x (LiHeAB)/LiBH4$NH3 composites is also based on the combination mechanism of Hdþ and Hd through the interaction between LiHeAB and NH3 group in LiBH4$NH3. Therefore, the balanced BeH and NeH for this combination reaction can be achieved by adjusting the ratio of LiHeAB and LiBH4$NH3. Moreover, as LiAB can be formed through the reaction of AB and LiH [7], the hydrogen release in the x (LiHeAB)/LiBH4$NH3 composites may start with the formation of LiAB, and then LiAB further react with the NH3 group in the LiBH4$NH3 to release H2 and form LiNBNH
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Fig. 6 e Low-field 11B NMR (400 MHz) spectra for 3LiHe3AB/ LiBH4$NH3 (a) and 3AB/LiBH4$NH3 (b) after dehydrogenation at 250 C.
(borazine-like or polyborazin-like compound) and LiBH4. The TPD measurement for 3LiAB/LiBH4$NH3 in Fig. S3 shows similar dehydrogenation properties to the 3(LiHeAB)/ LiBH4$NH3, except a capacity decrease of 1 equivalent H2. In addition, the IR results also confirm the residual BH1 4 after the 3LiAB/LiBH4$NH3 dehydrogenated to 250 C as did in the decomposed 3(LiHeAB)/LiBH4$NH3 (Fig. S4). These results supported the above assumption. Therefore, the decomposition reaction of 3(LiHeAB)/LiBH4$NH3 can be speculated as Eqs. (3) and (4). Compared with the reported amminelithium amidoborane (Li(NH3)NH2BH3), which releases a large amount of ammonia (0.53 mol) during the dehydrogenation, owing to the weak N:/Liþ coordination bond, only pure hydrogen evolution was observed from the x (LiHeAB)/ LiBH4$NH3 composites, indicating the crucial role of LiBH4, derived form the LiBH4$NH3, in stabilizing the ammonia and promoting its combination with the formed LiAB [35]. Kim et al. and Xia et al. suggested a dehydrogenation mechanism for LiNH2BH3 and Li(NH3)NH2BH3 based on the dimer (LiNH2BH3)2 and (Li(NH3)NH2BH3)2 as a model system [35,39]. Here we can also use the dimer decomposition mechanism to explain the LiHeAB/LiBH4$NH3 system. In LiHeAB/LiBH4$NH3, the dehydrogenation process occurred through the transfer of H from B to the midpoint between two Li cations forming the triangular moiety (denoted d “Lidþ 2 :H ”), and then the redox reaction of the dihydrogen bond in LiHdþ.dHN to form H2, which is the same as the dehydrogenation of LiNH2BH3 and Li(NH3)NH2BH3 [35,39]. In a similar way to the dehydrogenation of the solid materials in the cases of LiNH2 and LiNH2BH3, the transfer of a Li cation and a hydride is a principal step for hydrogen release in LiHeAB/LiBH4$NH3. In addition, a strong Lewis base H in LiH acts as a hydride source for the dehydrogenation and the Li cation plays an important role in decreasing the energy barrier for intermolecular NeB bond formation in the hydrogen
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release, which would result in the formation of NeB oligomers/polymers as products. LiH þ NH3 BH3 /LiNH2 BH3 þ H2
(3)
LiNH2 BH3 þ LiBH4 $NH3 /LiNBNH2 þ 3H2 þ LiBH4
(4)
4.
Conclusion
In this article, our results have shown that x (LiHeAB)/ LiBH4$NH3 composites exhibit much higher dehydrogenation capacity with enhanced hydrogen purity, and faster kinetics than the previously reported AB/LiBH4$NH3 composites. For example, in the case of the 3LiHe3AB/LiBH4$NH3 composite, about 10 wt. % high-pure hydrogen can be released below 100 C, which exceeds even the thermolysis of LiAB. The hydrogen release in the x (LiHeAB)/LiBH4$NH3 composites was found to start with the formation of LiAB, and then LiAB further reacted with the NH3 group in LiBH4$NH3 to release H2 and form LiNBNH polymer and LiBH4. The controllable protic hydrogen source from the stabilized NH3 group in LiBH4$NH3 played a crucial role in providing optimal stoichiometric ratio of Hdþ and Hd, which contributed to the pure hydrogen release without undesired volatile by-products. Our work may provide an effective method to promote the dehydrogenation and suppress volatile by-products evolution of AB, as well as further information on understanding the decomposition mechanism of the ammonia complexes of AB.
Acknowledgements This work was partially supported by the Ministry of Science and Technology of China (2010CB631302), the National Natural Science Foundation of China (Grant No. 51071047), the PhD Programs Foundation of Ministry of Education of China (20110071110009), Science and Technology Commission of Shanghai Municipality (11JC1400700, 11520701100) and Shanghai Rising-Star Program (11QH1400900).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2012.09.069.
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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 8 1 0 1 e1 8 1 0 7
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