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The effect of NH3 content on hydrogen release from LiBH4–NH3 system
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The effect of NH3 content on hydrogen release from LiBH4eNH3 system Xueli Zheng a,*,2, Yongshen Chua b, Zhitao Xiong b, Weidong Chen b, Zhijie Jiang a, Guotao Wu b,*,1, Ping Chen b a
Key Lab of Green Chemistry and Technology, Ministry of Education, The Institute of Homogeneous Catalysis, College of Chemistry, Sichuan University, Chengdu 610064, PR China b Dalian Institute of Chemical Physics, Dalian 116023, PR China
article info
abstract
Article history:
Due to the coordinative nature of NH3 in the LiBH4 ammines, NH3 desorbs predominantly
Received 26 September 2014
at temperatures below 180 C under an open flow mode. Interestingly, the emission of NH3
Received in revised form
can be effectively suppressed in conjunction with improved hydrogen desorption proper-
1 January 2015
ties by introducing a cobalt catalyst (Co-catalyst) and heating the amines in a small closed
Accepted 22 January 2015
vessel. Under such condition, Li(NH3)nBH4, where n ¼ 1, 4/3, 2, releases ca. 15.3 wt%, 17.8 wt
Available online 26 February 2015
%, 14.3 wt% hydrogen at 250 C, respectively. Fairly high H-purity (99.99%) can be achieved in the Co-catalyzed Li(NH3)BH4 sample upon releasing over 15 wt% H2 below 250 C. As for
Keywords:
the Co-catalyzed Li(NH3)2BH4 sample approximately 14.3 wt% of H2 (H-purity: 97.60%) can
Hydrogen storage
be desorbed in a much improved reaction rate.
Lithium borohydride
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Ammonia Cobalt
Introduction Hydrogen is an energy carrier with negligible environmental impact [1]. The importance of developing safe, inexpensive and energy-efficient hydrogen storage materials towards the large scale utilization of hydrogen energy can never be estimated. In the past, tremendous research efforts have been given to high hydrogen content materials with low dehydrogenation temperatures, such as alanates [2e4], borohydrides [5e7], amide-hydrides [8e10], amide-borohydrides [11,12], ammonia borane [13,14] and metal amidoboranes [15e17]. Among these light-weight materials, significant
improvements to enhance the reaction kinetics of boronand nitrogen-based chemical hydrides have been reported [18]. Lithium borohydride (LiBH4) is considered to be promising owing to its high gravimetric (18.4 wt%) and volumetric hydrogen densities, however, it suffers from several drawbacks such as the slow kinetics [6] and unfavorable thermodynamics of dehydrogenation [19], which limit its practical application. Increasing number of efforts has been devoted to the improvement of the dehydrogenation kinetic and thermodynamic properties of LiBH4. Kinetic improvement via the addition of SiO2 [6] or nanosized metal additives, i.e., PdCl2, Pt/Vulcan carbon, NiCl2, CoCl2 have been reported [20,21]. Thermodynamic improvement have also
* Corresponding author. E-mail addresses: [email protected] (X. Zheng), [email protected] (G.T. Wu). 1 Tel.: þ86 411 84379905; fax: þ86 411 84379583. 2 Tel./fax: þ86 28 85412904. http://dx.doi.org/10.1016/j.ijhydene.2015.01.134 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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been achieved via reacting LiBH4 with MgH2 [19], CaH2 [22,23], LiNH2 [24,25] and NH3 [26,27]. Interestingly, introduction of different amount of LiNH2 or NH3 resulted in the formation of Li2BNH6 [24], Li4BN3H10 [25] or Li(NH3)nBH4 [26e28], changing the thermodynamic property of dehydrogenation from endothermic to mild exothermic. It was found that the coexistence of Hdþ in NH2 or NH3 and Hd in BH4 in these systems can effectively facilitate the hydrogen desorption at low temperatures [27,28]. In our previous investigation, we found that decomposition pathway varies with different reaction condition [28]. Under dynamic conditions (open flow mode with argon gas), LiBH4$nNH3 and other X(BH4)2$nNH3 (X ¼ Mg, Ca, Al, n ¼ 1, 2, 4 and 6) release ammonia prior to hydrogen below 200 C [26,29e31]. For Al(BH4)3$6NH3, its hydrogen release is accompanied by high ammonia content (>32 wt%) when heated from RT to 300 C [31]. In the case of Mg(BH4)2$2NH3, hydrogen is released in the temperature range of 150e400 C with 5e7 wt% of NH3 [30]. Under the dynamic system (the majority of NH3 may be evolved into gas phase as flowed by argon and the temperature increased), hydrogen evolution from LiBH4$nNH3 or X(BH4)2$nNH3 might include a gasesolid reaction. On the other hand, in a closed system, Li(NH3)4/3BH4 (17.8 wt%) [28] and Ca(NH2BH3)2$2NH3 (8.0 wt%) [32] could release high amount of H2 under mild conditions. Under the closed condition, the coordinated NH3 molecules in the lattice tend to provide protic H(N) to interact with hydridic H(B) easier than that in dynamic condition (gasesolid reaction). Moreover, the coordination strength between the NH3 molecule and the metal cation (Liþ or Ca2þ) may lead to more favorable dehydrogenation [28,32]. Recently, the dehydrogenation properties of X(BH4)2$nNH3 in dynamic system has been tuned with the addition of NH3BH3 [33], Al2O3 [34] or another borohydride [35,36]. For instances, LiBH4$NH3/ Mg(BH4)2 and Ca(BH4)2$NH3/LiBH4 showed significant improvement in the dehydrogenation kinetic and H-purity as compared to that of the pristine X(BH4)2$nNH3 [35,36]. It is worthy of noting that the additional borohydride is likely to form coordination with NH3, meaning that more NH3 molecule would be ‘tied’ in the solid phase and facilitate the combination of protic H(N) and hydridic H(B) to form H2. From this viewpoint, this strategy is analogous to our closed system dehydrogenation of Li(NH3)4/3BH4, in which the majority of NH3 was ‘tied’ in the lattice owing to the NH3 equilibrium pressure of Li(NH3)4/3BH4. Closed system dehydrogenation not only allows purer hydrogen evolution from lithium borohydride ammoniate, but also avoids the necessity of adding additional ammonia scrubber (MgCl2, ZnCl2 and AlCl3) [27] or other borohydride [35,36] to tie ammonia in the lattice, or introducing highly dispersing supporter Al2O3 for Li(NH3)BH4 [34]. Therefore, this strategy provides an economic and feasible solution for hydrogen storage. It is reported that tuning the coordination number of NH3 would adjust the NH3 evolution [31]. In this study, in order to investigate the effect of NH3 content on hydrogen release from LiBH4eNH3, and seek an optimal composite with better kinetic, higher H-purity with appreciate gravimetric H2 density, we pursued the desorption properties of Li(NH3)nBH4 (n ¼ 1 and 2) in a small reactor. Interestingly, compared to Li(NH3)4/3BH4, Li(NH3)BH4 releases less H2 but of
comparatively higher purity, Li(NH3)2BH4 also evolves less H2 but with better desorption kinetic.
Experimental section Sample preparation and dehydrogenation LiBH4 (95%), anhydrous CoCl2 (97%) and anhydrous NH3 (99.999%) were purchased from Acros, Aldrich and CREDIT, respectively. To prevent air contaminations, all sample loadings and handlings were conducted in a purified argon filled MBRAUN glovebox. A mixture of CoCl2eLiBH4 (molar ratio 0.026/1) was mechanically milled (Retch PM 400) at 200 rpm for 8 h to prepare Co-doped LiBH4. The ball-to-sample mass ratio was about 30/1. At the end of ball milling, the vessel was connected to a pressure gauge to measure the pressure inside the vessel. The gaseous products were then passed through a Mass Spectrometer (MS) for analysis. In all cases, H2 is the only detectable gaseous product. Therefore, by applying the ideal gas equation, the H2 quantity is determined to be 0.07 equiv. (for 1 mol of LiBH4) during ball milling. Two Co-doped Li(NH3)nBH4 samples in different LiBH4/NH3 molar ratios (½ and 1) were prepared in situ and they were dehydrogenated using a home-made closed system (Supplementary Information, Fig. S1). Approximately 100 mg of Codoped LiBH4 powder and pre-weighted amount of ammonia were loaded and tested with time and temperature. The temperature was increased to 250 C at a ramping rate of 0.5 C/min. Gaseous products were analyzed by Mass Spectrometry and hydrogen was the main gas. A Thermo conductivity meter with an accuracy of 0.1 ms/cm was applied to measure the ammonia concentration in the gaseous product.
Characterizations XRD (X-ray diffraction) data was collected at room temperature using a PANalytical X'pert diffractometer equipped with Cu Ka radiation (40 kV, 40 mA) and an in situ cell. FTIR (fourier transform infrared) measurements were conducted on a Varian 3100 FT-IR spectrometer at a resolution of 4 cm1. Raman spectra were recorded on a commercial micro-Raman spectrometer (Renishaw, UK). All the characterizations were conducted at ambient temperature.
Results and discussion Gas evolution from Co-doped Li(NH3)nBH4 (n ¼ 1, 2) NH3 plays a role as ligand owing to the presence of the lone pair electrons on N atom. It coordinates to the metal cation, i.e., Liþ in LiBH4, forming various Li(NH3)nBH4 complexes depending on its content [37]. Due to the relatively weaker coordination between Liþ$$$NH3 as compared to the BeH bond which primarily responsible for the dehydrogenation process, NH3 was desorbed predominantly when Li(NH3)nBH4 (n: 1, 4/3) was heated under an open flow mode at temperatures below 180 C [26,28]. Our previous work on Li(NH3)4/3BH4 successfully manifested that the thermal decomposition properties of
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Li(NH3)4/3BH4 could be altered by conducting the reaction in a closed small vessel, whereby the ammonia emission can be dramatically suppressed through equilibrium-vapor-pressure control (see Supplementary Information) while favored the dissociation of both NeH and BeH bonds to form H2 [28]. Herein, the thermal decomposition of Co-doped Li(NH3)BH4 and Li(NH3)2BH4 samples in a closed vessel were investigated, respectively, and the corresponding results were shown in Fig. 1. The profiles of Co-doped LiBH4 and Li(NH3)4/3BH4 were included as well for comparison. Co was introduced by mechanically milling the mixture of CoCl2eLiBH4 (molar ratio 0.026/1). After the milling process, 0.07 equivalent of H2 per mole of LiBH4 was detached and color change was observed from white to black. As elucidated in the previous investigations, interaction of CoCl2 with LiBH4 is likely to result in the reduction of Co2þ (CoCl2) to metallic Co or CoeB species which may function as an effective catalyst that catalyzed the dehydrogenation of Li(NH3)4/3BH4 [28]. As demonstrated in Fig. 1, the Co-doped LiBH4 liberated only small amount of H2 at 250 C owing to the thermodynamic constraint (curve a) [6]. Similarly, NH3 hardly released H2 under this condition. However, the Co-doped Li(NH3)BH4 released remarkable amount of hydrogen at ca. 140 C (curve b). Finally 15.8 wt% or ca. 3.0 equiv. of H2 was evolved after holding the sample at 250 C for about 11 h. It is noteworthy that most of the NH3 (0.97 equiv. of NH3) was retained in the vicinity of LiBH4 until 140 C (curve g), the calculation of the remaining amount of NH3 at given temperatures was discussed in elsewhere [28]. Upon dehydrogenation to 250 C, the NH3 concentration in the gaseous phase was below 100 ppm (H-purity: 99.99%), in other words, NH3 is completely consumed, which is more superior than LiBH4$NH3/4Al2O3 [34], 2LiBH4$NH3/Mg(BH4)2 [35] and LiBH4$NH3/Ca(BH4)2 [36]. In the case of Co-doped Li(NH3)2BH4, hydrogen was rapidly detached at about 120 C (curve d, Fig. 1A). Approximately 14.0 wt% (or 3.9 equiv.) of H2 can be detached after heating the sample to 250 C. As shown in Fig. 1B (curve e), 1.86 equivalent moles of NH3 were remained in the vicinity of LiBH4 at 120 C, indicating that 0.14 equiv.
Fig. 1 e (A) Volumetric release curves of Co-doped a) LiBH4, b) Li(NH3)BH4, c) Li(NH3)4/3BH4, d) Li(NH3)2BH4 samples. (B) Calculated contents of NH3 retained in the e) Li(NH3)2BH4, f) Li(NH3)1.3BH4 (the average value for Li(NH3)1.2BH4 and Li(NH3)1.4BH4) [28,37], g) Li(NH3)BH4 samples as a function of temperature, respectively.
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NH3 has been detached due to the NH3 equilibrium pressure. It is worth noting that the NH3 equilibrium pressure of the Li(NH3)nBH4 (0 < n 2) ammoniate gradually ascends with the increase of NH3 content [37]. Therefore, NH3 concentration detected in the gaseous phase was ca. 24,000 ppm (H-purity: 97.60%). In comparison, Co-doped Li(NH3)4/3BH4 demonstrated obvious hydrogen evolution at ca. 135 C (curve c, Fig. 1A) and ca. 4.0 equiv. H2 can be detached after holding at 250 C for around 5 h ~1.3 equiv. NH3 was constrained in the vicinity of LiBH4. NH3 concentration in the gaseous phase after dehydrogenation was ca. 320 ppm (H-purity: 99.97%) [28].
Structural analyses As shown in Fig. 1A, the three Co-doped Li(NH3)nBH4 samples released different amount of hydrogen through stepwise processes. In order to clarify their reaction pathways, the samples were collected before and at different stages of dehydrogenation and were subjected to XRD, FTIR, and Raman characterizations. The dehydrogenation process of Co-doped Li(NH3)4/3BH4 has been studied previously [28]. As shown in Fig. 2, absorption of 1 equivalent of NH3 by LiBH4 gave rise to the formation of powdery Li(NH3)BH4 (curve a) [26,37]. Absorption of 2 equivalent of NH3, however, resulted in a liquefied Li(NH3)2BH4 [37], thus, no long range order structure can be detected by XRD. The FTIR measurements (Fig. 3a) showed the presence of the NeH vibrations in the Codoped Li(NH3)BH4 ammoniates at ca. 3291 cm1 and 3375 cm1 [26]. With respect to the Co-doped Li(NH3)2BH4 sample, the NeH vibrations become broader and the resonance at 3291 cm1 red-shifted to 3270 cm1, which indicates that the NeH bonds of Li(NH3)2BH4 are weaker than those in Li(NH3) BH4. During dehydrogenation process of the Co-doped Li(NH3) BH4 sample, the diffraction peaks of Li(NH3)BH4 disappeared; while Li4BN3H10 and LiBH4 with Li2BNH6 minor phase started to build up (See Fig. S2 in the Supplementary Information). FTIR spectra showed the development and consumption of the NeH vibration of Li4BN3H10 or Li2BNH6 successively, the
Fig. 2 e XRD patterns of a) the freshly-made Co-doped Li(NH3)BH4 and b) after dehydrogenation at 250 C, c) the Co-doped Li(NH3)2BH4 after dehydrogenation at 250 C.
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FTIR, which confirms the formation of LiNH2 (3259 cm1 and 3314 cm1) and BN (1373 cm1) (Fig. 4d). Compared to CoeLi(NH3)4/3BH4, Li3BN2 was not observed during the dehydrogenation of CoeLi(NH3)BH4 and CoeLi(NH3)2BH4.
Effect of NH3 on hydrogen release
Fig. 3 e FTIR spectra of the freshly-made Co-doped a) Li(NH3)BH4, b) Li(NH3)2BH4 samples, and the samples after dehydrogenation at 250 C: c) Li(NH3)BH4 d) Li(NH3)2BH4.
gradual decrease of BeH vibration, and the slow development of BeN vibrations of h-BN (1380 cm1: BeN stretching, 800 cm1: BeNeB bending) (See Figs. S3 and S4 in the Supplementary Information) [28,38]. After full dehydrogenation at 250 C, the diffraction peaks of LiH, BN and LiCl were predominantly developed in the dehydrogenated residue (see Fig. 2b). LiCl may be produced by the interaction of CoCl2 and LiBH4, and it was well crystallized after heating to 250 C. Rather than the NeH vibration, two broad vibrations assignable to h-BN was observed via FTIR analysis. Raman spectra match well with the FTIR characterization (Fig. 4). For the CoeLi(NH3)2BH4 sample, however, the intermediates were Li(NH3)BH4, Li4BN3H10, LiBH4 and Li2BNH6 as evidenced by XRD, FTIR and Raman characterizations (See Figs. S5, S6 and S7 in the Supplementary Information). The BeN vibration of BNHx was observed in both CoeLi(NH3)BH4 and CoeLi(NH3)2BH4 samples by FTIR (around 800 cm1 and 1400 cm1) during dehydrogenation. LiNH2 and BN were observed in the complete dehydrogenated residue by XRD characterization (Fig. 2c). FTIR analysis showed the presence of NeH stretches centered at 3310 cm1 and 3256 cm1 (Fig. 3d), which are close to the symmetric and asymmetric NeH stretches of LiNH2. BeH stretches were not observed, while two broad vibration bands around 1380 cm1 and 800 cm1 belonging to h-BN were detected [38]. The corresponding Raman characterization was in accordance with the
The formation of Li(NH3)nBH4 ammoniates depends on the coordination of N in NH3 on Li cation. The generation of H2 in a closed system dehydrogenation, is neither originated from the self-decomposition of NH3 nor from the dehydrogenation of CoeLiBH4, but from the interaction between (N)Hdþ and (B)Hd which coexisted in Li(NH3)nBH4. In previous study, it was reported that Co-catalyst is highly effective in lowering the dehydrogenation temperatures of LiBH4-based hydrogen storage materials [21,28]. Since the NH3 equilibrium pressure will decrease as the temperature decreased, the lowered dehydrogenation temperature will in turn lower the NH3 emission in the gaseous product to certain extent. In addition, under a closed small volume, a significantly lesser amount of NH3 is evolved to achieve its equilibrium pressure as compared to that in a larger volume. Therefore, the overwhelming majority of NH3 (holding Hdþ) is more preferentially to bind with LiBH4 and react with BH4 (Hd) to release hydrogen. The synergic effects of Co-catalyst and small vessel promote direct combination of Hdþ and Hd, and avoid the need of introducing additional additives as ammonia scrubber. In our experiment, we found that when the volumetric release of Co-doped Li(NH3)4/3BH4 was performed in a larger reactor with fourfold volume, 2.0 equivalent moles of gaseous product (instead of 4 equiv.) was evolved upon heated to 250 C for 20 h and with a slower dehydrogenation rate as compared to that performed in the small reactor (6.7 ml). Therefore, we assume that more NH3 coordinated to Liþ is beneficial for hydrogen evolution. Johnson et al. revealed that the presence of NH3 elongated the LieB bond of LiBH4 from ˚ to 2.52e2.57 A ˚ [26]. Chen and Yu also revealed that 2.48e2.54 A metal cations could play vital role in suppressing NH3 emission and destabilizing NeH or BeH bonds [39]. The decreasing melting point of Li(NH3)nBH4 (1 n 2) as n increases could also manifest more NH3 would contribute to the destabilization of Li(NH3)nBH4 [37]. As a matter of fact, the as-prepared Co-doped Li(NH3)BH4 and Li(NH3)2BH4 samples are in solid and liquid form, respectively. As results, the onset temperatures of hydrogen evolution from Co-doped Li(NH3)nBH4 drop gradually as the NH3 content increases (1 n 2) (Fig. 1). Analyzing the tangent slope of the linear parts for initial hydrogen desorption (See Fig. S8 in the Supplementary Information), the rate constant for the sample with n ¼ 2 was estimated to be 0.0012 equiv. H2 min1, about 3.4 and 1.8 times faster than those from n ¼ 1 and 4/3 samples, respectively. As compared to CoeLi(NH3)4/3BH4, the CoeLi(NH3)2BH4 dehydrogenates at lower temperatures with faster rate but at the expense of the H2 capacity. In the case of CoeLi(NH3)BH4, fairly pure H2 with comparative amount of H2 can be obtained.
Dehydrogenation process of Li(NH3)nBH4 (n ¼ 1, 2) Fig. 4 e Raman spectra of the freshly-made Co-doped a) Li(NH3)BH4, b) Li(NH3)2BH4 samples, and the samples after dehydrogenation at 250 C: c) Li(NH3)BH4 d) Li(NH3)2BH4.
The dehydrogenation of Li(NH3)nBH4 may start with the establishment of (H)BeN(H) bond, followed by the
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Table 1 e Dehydrogenation properties of Li(NH3)nBH4. n
Onset T ( C)
Intermediates
Final products
H2 (wt%)
H-purity (%)
140 135 120
Li4BN3H10, LiBH4, Li2BNH6, BNHx Li4BN3H10, Li2BNH6, BNHx Li(NH3)BH4, Li4BN3H10, LiBH4, Li2BNH6, BNHx
LiH, BN Li3BN2, BN LiNH2, BN
15.3 17.8 14.3
99.99 99.97 97.60
1 4/3 2
dissociation of BeH and NeH bonds. Liþ, which initially links to BH4, tends to bind with hydridic H(B) to form LiH. The intermediate LiH will then react with NH3 to form LiNH2 and H2 [40]. Li2BNH6 or Li4BN3H10 is likely to form during the dehydrogenation owing to the presence of LiNH2 and unreacted LiBH4 [41]. Further increasing the temperature will lead to the decomposition of Li4BN3H10, Li2BNH6 and LiBH4 to BN and LiH from Li(NH3)BH4. As for Li(NH3)2BH4, the formation of Li(NH3)BH4, Li2BNH6 or Li4BN3H10 were observed at the initial dehydrogenation stage, evidencing a stepwise consumption of LiBH4 and NH3, and the development of Li2BNH6 and Li4BN3H10. It should be noted that Li2BNH6 decomposes to form Li4BN3H10 and LiBH4 upon heated to ~90 C [24]. The observation of Li2BNH6 in the solid residue is likely due to the recombination of Li4BN3H10 and LiBH4 during the sample cooling process. Summarizing the information obtained from various characterizations, the dehydrogenation of Li(NH3)nBH4 (n ¼ 1, 4/3, 2) systems can be expressed as follow (Table 1): 1 1 1 1 8x H2 LiðNH3 ÞBH4 / Li4 BN3 H10 þ LiBH4 þ Li2 BNH6 þ BNHx þ 8 4 8 2 4 /LiHþBNþ3H2 1 1 2 8x H2 LiðNH3 Þ4=3 BH4 / Li4 BN3 H10 þ Li2 BNH6 þ BNHx þ 6 6 3 3 1 2 / Li3 BN2 þ BNþ4H2 3 3 1 1 1 1 LiðNH3 Þ2 BH4 / Li4 BN3 H10 þ LiBH4 þ Li2 BNH6 þ LiðNH3 ÞBH4 8 8 8 8 1 8x 7 H2 þ NH3 /LiNH2 þBNþ4H2 þ BNHx þ 2 4 8
Conclusion In summary, we have demonstrated a promising hydrogen storage system of Li(NH3)nBH4, which shows significant NH3 suppression and kinetic improvements as compared to neat LiBH4 and NH3 when tested in a closed vessel. The synergic effects of Co catalyst and small vessel promote Li(NH3)BH4 to release 3 equiv. H2 (15.8 wt%) with high purity (99.99%) and yield LiH and BN. Under the same condition, Li(NH3)2BH4 could release 4 equiv. H2 (14.0 wt%) with a purity of 97.60%, and produce LiNH2 and BN.
Acknowledgment The authors would like to acknowledge the financial supports from National Natural Science Foundation of China (51225206, U1232120, 51301161).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.01.134.
references
[1] Cipriani G, Di Dio V, Genduso F, La Cascia D, Liga R, Miceli R, et al. Perspective on hydrogen energy carrier and its automotive applications. Int J Hydrogen Energy 2014;39:8482e94. B, Schwickardi M. Ti-doped alkali metal [2] Bogdanovic aluminium hydrides as potential novel reversible hydrogen storage materials. J Alloys Compd 1997;253:1e9. [3] Chen J, Kuriyama N, Xu Q, Takeshita HT, Sakai T. Reversible hydrogen storage via titanium-catalyzed LiAlH4 and Li3AlH6. J Phys Chem B 2001;105:11214e20. [4] Jensen CM, Gross KJ. Development of catalytically enhanced sodium aluminum hydride as a hydrogen-storage material. Appl Phys A Mater Sci Proc 2001;72:213e9. [5] Amendola SC, Sharp-Goldman SL, Janjua MS, Spencer NC, Kelly MT, Petillo PJ, et al. A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. Int J Hydrogen Energy 2000;25:969e75. [6] Zu¨ttel A, Wenger P, Rentsch S, Sudan P, Mauron P, Emmenegger C. LiBH4 a new hydrogen storage material. J Power Sources 2003;118:1e7. [7] Nakamori Y, Li HW, Miwa K, Towata S, Orimo S. Syntheses and hydrogen desorption properties of metal-borohydrides M(BH4)n (M ¼ Mg, Sc, Zr, Ti, and Zn; n ¼ 2e4) as advanced hydrogen storage materials. Mater Trans 2006;47:1898e901. [8] Leng HY, Ichikawa T, Hino S, Hanada N, Isobe S, Fujii H. New metal-N-H system composed of Mg(NH2)2 and LiH for hydrogen storage. J Phys Chem B 2004;108:8763e5. [9] Lu J, Fang ZZ, Sohn HY. A new LieAleNeH system for reversible hydrogen storage. J Phys Chem B 2006;110:14236e9. [10] Xiong ZT, Wu GT, Hu JJ, Liu YF, Chen P, Luo WF, et al. Reversible hydrogen storage by a LieAleNeH complex. Adv Funct Mater 2007;17:1137e42. [11] Pinkerton FE, Meisner GP, Meyer MS, Balogh MP, Kundrat MD. Hydrogen desorption exceeding ten weight percent from the new quaternary hydride Li3BN2H8. J Phys Chem B 2005;109:6e8. [12] Aoki M, Miwa K, Noritake T, Kitahara G, Nakamori Y, Orimo S, et al. Destabilization of LiBH4 by mixing with LiNH2. Appl Phys A: Mater Sci Proc 2005;80:1409e12. [13] Denney MC, Pons V, Hebden TJ, Heinekey DM, Goldberg KI. Efficient catalysis of ammonia borane dehydrogenation. J Am Chem Soc 2006;128:12048e9. [14] Keaton RJ, Blacquiere JM, Baker RT. Base metal catalyzed dehydrogenation of ammonia-borane for chemical hydrogen storage. J Am Chem Soc 2007;129:1844e5. [15] Diyabalanage HVK, Shrestha RP, Semelsberger TA, Scott BL, Bowden ME, Davis BL, et al. Calcium amidotrihydroborate: a
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[16]
[17]
[18]
[19] [20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
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hydrogen storage material. Angew Chem Int Ed 2007;46:8995e7. Xiong ZT, Yong CK, Wu GT, Chen P, Shaw W, Karkamkar A, et al. High-capacity hydrogen storage in lithium and sodium amidoboranes. Nat Mater 2008;7:138e41. Wu H, Zhou W, Yildirim T. Alkali and alkaline-earth metal amidoboranes: structure, crystal chemistry, and hydrogen storage properties. J Am Chem Soc 2008;130:14834e9. Umegaki T, Yan JM, Zhang XB, Shioyama H, Kuriyama N, Xu Q. Boron- and nitrogen-based chemical hydrogen storage materials. Int J Hydrogen Energy 2009;34:2303e11. Vajo JJ, Skeith SL, Mertens F. Reversible storage of hydrogen in destabilized LiBH4. J Phys Chem B 2005;109:3719e22. Pinkerton FE, Meyer MS, Meisner GP, Balogh MP. Improved hydrogen release from LiB0.33N0.67H2.67 with metal additives: Ni, Fe, and Zn. J Alloys Compd 2007;433:282e91. Tang WS, Wu G, Liu T, Wee ATS, Yong CK, Xiong ZT, et al. Cobalt-catalyzed hydrogen desorption from the LiNH2eLiBH4 system. Dalton Trans 2008:2395e9. Yang J, Sudik A, Wolverton C. Destabilizing LiBH4 with a metal (M ¼ Mg, Al, Ti, V, Cr, or Sc) or metal hydride (MH2, MgH2, TiH2, or CaH2). J Phys Chem C 2007;111:19134e40. Pinkerton FE, Meyer MS. Reversible hydrogen storage in the lithium borohydride-calcium hydride coupled system. J Alloys Compd 2008;464:L1e4. Chater PA, David WIF, Anderson PA. Synthesis and structure of the new complex hydride Li2BH4NH2. Chem Commun 2007:4770e2. Chater PA, Anderson PA, Prendergast JW, Walton A, Mann VSJ, Book D, et al. Synthesis and characterization of amide-borohydrides: new complex light hydrides for potential hydrogen storage. J Alloys Compd 2007;446e447:350e4. Johnson SR, David WIF, Royse DM, Sommariva M, Tang CY, Fabbiani FPA, et al. The monoammoniate of lithium borohydride, Li(NH3)BH4: an effective ammonia storage compound. Chem Asian J 2009;4:849e54. Guo YH, Xia GL, Zhu YH, Gao L, Yu XB. Hydrogen release from amminelithium borohydride, LiBH4$NH3. Chem Commun 2010;46:2599e601. Zheng XL, Wu GT, Li W, Xiong ZT, He T, Guo JP, et al. Releasing 17.8 wt% H2 from lithium borohydride ammoniate. Energy Environ Sci 2011;4:3593e600.
[29] Chen XW, Yuan F, Tan YB, Tang ZW, Yu XB. Improved dehydrogenation properties of Ca(BH4)2$nNH3 (n ¼ 1, 2, and 4) combined with Mg(BH4)2. J Phys Chem C 2012;116:21162e8. [30] Soloveichik G, Her JH, Stephens PW, Gao Y, Rijssenbeek J, Andrus M, et al. Ammine magnesium borohydride complex as a new material for hydrogen storage: structure and properties of Mg(BH4)2$2NH3. Inorg Chem 2008;47:4290e8. [31] Guo YH, Yu XB, Sun WW, Sun DL, Yang WN. The hydrogenenriched AleBeN system as an advanced solid hydrogenstorage candidate. Angew Chem Int Ed 2011;50:1087e91. [32] Chua YS, Wu GT, Xiong ZT, He T, Chen P. Calcium amidoborane ammoniate-synthesis, structure, and hydrogen storage properties. Chem Mater 2009;21:4899e904. [33] Tan YB, Guo YH, Li SF, Sun WW, Zhu YH, Li Q, et al. A liquidbased eutectic system: LiBH4$NH3enNH3BH3 with high dehydrogenation capacity at moderate temperature. J Mater Chem 2011;21:14509e15. [34] Chen XY, Cai WY, Guo YH, Yu XB. Significantly improved dehydrogenation of LiBH4$NH3 assisted by Al2O3 nanoscaffolds. Int J Hydrogen Energy 2012;37:5817e24. [35] Sun WW, Chen XW, Gu QF, Wallwork KS, Tan YB, Tang ZW, et al. A new ammine dual-cation (Li, Mg) borohydride: synthesis, structure, and dehydrogenation enhancement. Chem Eur J 2012;18:6825e34. [36] Tang ZW, Tan YB, Gu QF, Yu XB. A novel aided-cation strategy to advance the dehydrogenation of calcium borohydride monoammoniate. J Mater Chem 2012;22:5312e8. [37] Sullivan EA, Johnson S. The lithium borohydride-ammonia system-pressure-composition-temperature relationships and densities. J Phys Chem 1959;63:233e8. [38] Geick R, Perry CH, Rupprech G. Normal modes in hexagonal boron nitride. Phys Rev 1966;146:543e7. [39] Chen XW, Yu XB. Electronic structure and initial dehydrogenation mechanism of M(BH4)2$2NH3 (M ¼ Mg, Ca, and Zn): a first-principles investigation. J Phys Chem C 2012;116:11900e6. [40] Hu YH, Ruckenstein E. Ultrafast reaction between LiH and NH3 during H2 storage in Li3N. J Phys Chem A 2003;107:9737e9. [41] Meisner GP, Scullin ML, Balogh MP, Pinkerton FE, Meyer MS. Hydrogen release from mixtures of lithium borohydride and lithium amide: a phase diagram study. J Phys Chem B 2006;110:4186e92.
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The effect of NH3 content on hydrogen release from LiBH4eNH3 system Xueli Zheng a,*,2, Yongshen Chua b, Zhitao Xiong b, Weidong Chen b, Zhijie Jiang a, Guotao Wu b,*,1, Ping Chen b a
Key Lab of Green Chemistry and Technology, Ministry of Education, The Institute of Homogeneous Catalysis, College of Chemistry, Sichuan University, Chengdu 610064, PR China b Dalian Institute of Chemical Physics, Dalian 116023, PR China
article info
abstract
Article history:
Due to the coordinative nature of NH3 in the LiBH4 ammines, NH3 desorbs predominantly
Received 26 September 2014
at temperatures below 180 C under an open flow mode. Interestingly, the emission of NH3
Received in revised form
can be effectively suppressed in conjunction with improved hydrogen desorption proper-
1 January 2015
ties by introducing a cobalt catalyst (Co-catalyst) and heating the amines in a small closed
Accepted 22 January 2015
vessel. Under such condition, Li(NH3)nBH4, where n ¼ 1, 4/3, 2, releases ca. 15.3 wt%, 17.8 wt
Available online 26 February 2015
%, 14.3 wt% hydrogen at 250 C, respectively. Fairly high H-purity (99.99%) can be achieved in the Co-catalyzed Li(NH3)BH4 sample upon releasing over 15 wt% H2 below 250 C. As for
Keywords:
the Co-catalyzed Li(NH3)2BH4 sample approximately 14.3 wt% of H2 (H-purity: 97.60%) can
Hydrogen storage
be desorbed in a much improved reaction rate.
Lithium borohydride
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Ammonia Cobalt
Introduction Hydrogen is an energy carrier with negligible environmental impact [1]. The importance of developing safe, inexpensive and energy-efficient hydrogen storage materials towards the large scale utilization of hydrogen energy can never be estimated. In the past, tremendous research efforts have been given to high hydrogen content materials with low dehydrogenation temperatures, such as alanates [2e4], borohydrides [5e7], amide-hydrides [8e10], amide-borohydrides [11,12], ammonia borane [13,14] and metal amidoboranes [15e17]. Among these light-weight materials, significant
improvements to enhance the reaction kinetics of boronand nitrogen-based chemical hydrides have been reported [18]. Lithium borohydride (LiBH4) is considered to be promising owing to its high gravimetric (18.4 wt%) and volumetric hydrogen densities, however, it suffers from several drawbacks such as the slow kinetics [6] and unfavorable thermodynamics of dehydrogenation [19], which limit its practical application. Increasing number of efforts has been devoted to the improvement of the dehydrogenation kinetic and thermodynamic properties of LiBH4. Kinetic improvement via the addition of SiO2 [6] or nanosized metal additives, i.e., PdCl2, Pt/Vulcan carbon, NiCl2, CoCl2 have been reported [20,21]. Thermodynamic improvement have also
* Corresponding author. E-mail addresses: [email protected] (X. Zheng), [email protected] (G.T. Wu). 1 Tel.: þ86 411 84379905; fax: þ86 411 84379583. 2 Tel./fax: þ86 28 85412904. http://dx.doi.org/10.1016/j.ijhydene.2015.01.134 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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been achieved via reacting LiBH4 with MgH2 [19], CaH2 [22,23], LiNH2 [24,25] and NH3 [26,27]. Interestingly, introduction of different amount of LiNH2 or NH3 resulted in the formation of Li2BNH6 [24], Li4BN3H10 [25] or Li(NH3)nBH4 [26e28], changing the thermodynamic property of dehydrogenation from endothermic to mild exothermic. It was found that the coexistence of Hdþ in NH2 or NH3 and Hd in BH4 in these systems can effectively facilitate the hydrogen desorption at low temperatures [27,28]. In our previous investigation, we found that decomposition pathway varies with different reaction condition [28]. Under dynamic conditions (open flow mode with argon gas), LiBH4$nNH3 and other X(BH4)2$nNH3 (X ¼ Mg, Ca, Al, n ¼ 1, 2, 4 and 6) release ammonia prior to hydrogen below 200 C [26,29e31]. For Al(BH4)3$6NH3, its hydrogen release is accompanied by high ammonia content (>32 wt%) when heated from RT to 300 C [31]. In the case of Mg(BH4)2$2NH3, hydrogen is released in the temperature range of 150e400 C with 5e7 wt% of NH3 [30]. Under the dynamic system (the majority of NH3 may be evolved into gas phase as flowed by argon and the temperature increased), hydrogen evolution from LiBH4$nNH3 or X(BH4)2$nNH3 might include a gasesolid reaction. On the other hand, in a closed system, Li(NH3)4/3BH4 (17.8 wt%) [28] and Ca(NH2BH3)2$2NH3 (8.0 wt%) [32] could release high amount of H2 under mild conditions. Under the closed condition, the coordinated NH3 molecules in the lattice tend to provide protic H(N) to interact with hydridic H(B) easier than that in dynamic condition (gasesolid reaction). Moreover, the coordination strength between the NH3 molecule and the metal cation (Liþ or Ca2þ) may lead to more favorable dehydrogenation [28,32]. Recently, the dehydrogenation properties of X(BH4)2$nNH3 in dynamic system has been tuned with the addition of NH3BH3 [33], Al2O3 [34] or another borohydride [35,36]. For instances, LiBH4$NH3/ Mg(BH4)2 and Ca(BH4)2$NH3/LiBH4 showed significant improvement in the dehydrogenation kinetic and H-purity as compared to that of the pristine X(BH4)2$nNH3 [35,36]. It is worthy of noting that the additional borohydride is likely to form coordination with NH3, meaning that more NH3 molecule would be ‘tied’ in the solid phase and facilitate the combination of protic H(N) and hydridic H(B) to form H2. From this viewpoint, this strategy is analogous to our closed system dehydrogenation of Li(NH3)4/3BH4, in which the majority of NH3 was ‘tied’ in the lattice owing to the NH3 equilibrium pressure of Li(NH3)4/3BH4. Closed system dehydrogenation not only allows purer hydrogen evolution from lithium borohydride ammoniate, but also avoids the necessity of adding additional ammonia scrubber (MgCl2, ZnCl2 and AlCl3) [27] or other borohydride [35,36] to tie ammonia in the lattice, or introducing highly dispersing supporter Al2O3 for Li(NH3)BH4 [34]. Therefore, this strategy provides an economic and feasible solution for hydrogen storage. It is reported that tuning the coordination number of NH3 would adjust the NH3 evolution [31]. In this study, in order to investigate the effect of NH3 content on hydrogen release from LiBH4eNH3, and seek an optimal composite with better kinetic, higher H-purity with appreciate gravimetric H2 density, we pursued the desorption properties of Li(NH3)nBH4 (n ¼ 1 and 2) in a small reactor. Interestingly, compared to Li(NH3)4/3BH4, Li(NH3)BH4 releases less H2 but of
comparatively higher purity, Li(NH3)2BH4 also evolves less H2 but with better desorption kinetic.
Experimental section Sample preparation and dehydrogenation LiBH4 (95%), anhydrous CoCl2 (97%) and anhydrous NH3 (99.999%) were purchased from Acros, Aldrich and CREDIT, respectively. To prevent air contaminations, all sample loadings and handlings were conducted in a purified argon filled MBRAUN glovebox. A mixture of CoCl2eLiBH4 (molar ratio 0.026/1) was mechanically milled (Retch PM 400) at 200 rpm for 8 h to prepare Co-doped LiBH4. The ball-to-sample mass ratio was about 30/1. At the end of ball milling, the vessel was connected to a pressure gauge to measure the pressure inside the vessel. The gaseous products were then passed through a Mass Spectrometer (MS) for analysis. In all cases, H2 is the only detectable gaseous product. Therefore, by applying the ideal gas equation, the H2 quantity is determined to be 0.07 equiv. (for 1 mol of LiBH4) during ball milling. Two Co-doped Li(NH3)nBH4 samples in different LiBH4/NH3 molar ratios (½ and 1) were prepared in situ and they were dehydrogenated using a home-made closed system (Supplementary Information, Fig. S1). Approximately 100 mg of Codoped LiBH4 powder and pre-weighted amount of ammonia were loaded and tested with time and temperature. The temperature was increased to 250 C at a ramping rate of 0.5 C/min. Gaseous products were analyzed by Mass Spectrometry and hydrogen was the main gas. A Thermo conductivity meter with an accuracy of 0.1 ms/cm was applied to measure the ammonia concentration in the gaseous product.
Characterizations XRD (X-ray diffraction) data was collected at room temperature using a PANalytical X'pert diffractometer equipped with Cu Ka radiation (40 kV, 40 mA) and an in situ cell. FTIR (fourier transform infrared) measurements were conducted on a Varian 3100 FT-IR spectrometer at a resolution of 4 cm1. Raman spectra were recorded on a commercial micro-Raman spectrometer (Renishaw, UK). All the characterizations were conducted at ambient temperature.
Results and discussion Gas evolution from Co-doped Li(NH3)nBH4 (n ¼ 1, 2) NH3 plays a role as ligand owing to the presence of the lone pair electrons on N atom. It coordinates to the metal cation, i.e., Liþ in LiBH4, forming various Li(NH3)nBH4 complexes depending on its content [37]. Due to the relatively weaker coordination between Liþ$$$NH3 as compared to the BeH bond which primarily responsible for the dehydrogenation process, NH3 was desorbed predominantly when Li(NH3)nBH4 (n: 1, 4/3) was heated under an open flow mode at temperatures below 180 C [26,28]. Our previous work on Li(NH3)4/3BH4 successfully manifested that the thermal decomposition properties of
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Li(NH3)4/3BH4 could be altered by conducting the reaction in a closed small vessel, whereby the ammonia emission can be dramatically suppressed through equilibrium-vapor-pressure control (see Supplementary Information) while favored the dissociation of both NeH and BeH bonds to form H2 [28]. Herein, the thermal decomposition of Co-doped Li(NH3)BH4 and Li(NH3)2BH4 samples in a closed vessel were investigated, respectively, and the corresponding results were shown in Fig. 1. The profiles of Co-doped LiBH4 and Li(NH3)4/3BH4 were included as well for comparison. Co was introduced by mechanically milling the mixture of CoCl2eLiBH4 (molar ratio 0.026/1). After the milling process, 0.07 equivalent of H2 per mole of LiBH4 was detached and color change was observed from white to black. As elucidated in the previous investigations, interaction of CoCl2 with LiBH4 is likely to result in the reduction of Co2þ (CoCl2) to metallic Co or CoeB species which may function as an effective catalyst that catalyzed the dehydrogenation of Li(NH3)4/3BH4 [28]. As demonstrated in Fig. 1, the Co-doped LiBH4 liberated only small amount of H2 at 250 C owing to the thermodynamic constraint (curve a) [6]. Similarly, NH3 hardly released H2 under this condition. However, the Co-doped Li(NH3)BH4 released remarkable amount of hydrogen at ca. 140 C (curve b). Finally 15.8 wt% or ca. 3.0 equiv. of H2 was evolved after holding the sample at 250 C for about 11 h. It is noteworthy that most of the NH3 (0.97 equiv. of NH3) was retained in the vicinity of LiBH4 until 140 C (curve g), the calculation of the remaining amount of NH3 at given temperatures was discussed in elsewhere [28]. Upon dehydrogenation to 250 C, the NH3 concentration in the gaseous phase was below 100 ppm (H-purity: 99.99%), in other words, NH3 is completely consumed, which is more superior than LiBH4$NH3/4Al2O3 [34], 2LiBH4$NH3/Mg(BH4)2 [35] and LiBH4$NH3/Ca(BH4)2 [36]. In the case of Co-doped Li(NH3)2BH4, hydrogen was rapidly detached at about 120 C (curve d, Fig. 1A). Approximately 14.0 wt% (or 3.9 equiv.) of H2 can be detached after heating the sample to 250 C. As shown in Fig. 1B (curve e), 1.86 equivalent moles of NH3 were remained in the vicinity of LiBH4 at 120 C, indicating that 0.14 equiv.
Fig. 1 e (A) Volumetric release curves of Co-doped a) LiBH4, b) Li(NH3)BH4, c) Li(NH3)4/3BH4, d) Li(NH3)2BH4 samples. (B) Calculated contents of NH3 retained in the e) Li(NH3)2BH4, f) Li(NH3)1.3BH4 (the average value for Li(NH3)1.2BH4 and Li(NH3)1.4BH4) [28,37], g) Li(NH3)BH4 samples as a function of temperature, respectively.
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NH3 has been detached due to the NH3 equilibrium pressure. It is worth noting that the NH3 equilibrium pressure of the Li(NH3)nBH4 (0 < n 2) ammoniate gradually ascends with the increase of NH3 content [37]. Therefore, NH3 concentration detected in the gaseous phase was ca. 24,000 ppm (H-purity: 97.60%). In comparison, Co-doped Li(NH3)4/3BH4 demonstrated obvious hydrogen evolution at ca. 135 C (curve c, Fig. 1A) and ca. 4.0 equiv. H2 can be detached after holding at 250 C for around 5 h ~1.3 equiv. NH3 was constrained in the vicinity of LiBH4. NH3 concentration in the gaseous phase after dehydrogenation was ca. 320 ppm (H-purity: 99.97%) [28].
Structural analyses As shown in Fig. 1A, the three Co-doped Li(NH3)nBH4 samples released different amount of hydrogen through stepwise processes. In order to clarify their reaction pathways, the samples were collected before and at different stages of dehydrogenation and were subjected to XRD, FTIR, and Raman characterizations. The dehydrogenation process of Co-doped Li(NH3)4/3BH4 has been studied previously [28]. As shown in Fig. 2, absorption of 1 equivalent of NH3 by LiBH4 gave rise to the formation of powdery Li(NH3)BH4 (curve a) [26,37]. Absorption of 2 equivalent of NH3, however, resulted in a liquefied Li(NH3)2BH4 [37], thus, no long range order structure can be detected by XRD. The FTIR measurements (Fig. 3a) showed the presence of the NeH vibrations in the Codoped Li(NH3)BH4 ammoniates at ca. 3291 cm1 and 3375 cm1 [26]. With respect to the Co-doped Li(NH3)2BH4 sample, the NeH vibrations become broader and the resonance at 3291 cm1 red-shifted to 3270 cm1, which indicates that the NeH bonds of Li(NH3)2BH4 are weaker than those in Li(NH3) BH4. During dehydrogenation process of the Co-doped Li(NH3) BH4 sample, the diffraction peaks of Li(NH3)BH4 disappeared; while Li4BN3H10 and LiBH4 with Li2BNH6 minor phase started to build up (See Fig. S2 in the Supplementary Information). FTIR spectra showed the development and consumption of the NeH vibration of Li4BN3H10 or Li2BNH6 successively, the
Fig. 2 e XRD patterns of a) the freshly-made Co-doped Li(NH3)BH4 and b) after dehydrogenation at 250 C, c) the Co-doped Li(NH3)2BH4 after dehydrogenation at 250 C.
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FTIR, which confirms the formation of LiNH2 (3259 cm1 and 3314 cm1) and BN (1373 cm1) (Fig. 4d). Compared to CoeLi(NH3)4/3BH4, Li3BN2 was not observed during the dehydrogenation of CoeLi(NH3)BH4 and CoeLi(NH3)2BH4.
Effect of NH3 on hydrogen release
Fig. 3 e FTIR spectra of the freshly-made Co-doped a) Li(NH3)BH4, b) Li(NH3)2BH4 samples, and the samples after dehydrogenation at 250 C: c) Li(NH3)BH4 d) Li(NH3)2BH4.
gradual decrease of BeH vibration, and the slow development of BeN vibrations of h-BN (1380 cm1: BeN stretching, 800 cm1: BeNeB bending) (See Figs. S3 and S4 in the Supplementary Information) [28,38]. After full dehydrogenation at 250 C, the diffraction peaks of LiH, BN and LiCl were predominantly developed in the dehydrogenated residue (see Fig. 2b). LiCl may be produced by the interaction of CoCl2 and LiBH4, and it was well crystallized after heating to 250 C. Rather than the NeH vibration, two broad vibrations assignable to h-BN was observed via FTIR analysis. Raman spectra match well with the FTIR characterization (Fig. 4). For the CoeLi(NH3)2BH4 sample, however, the intermediates were Li(NH3)BH4, Li4BN3H10, LiBH4 and Li2BNH6 as evidenced by XRD, FTIR and Raman characterizations (See Figs. S5, S6 and S7 in the Supplementary Information). The BeN vibration of BNHx was observed in both CoeLi(NH3)BH4 and CoeLi(NH3)2BH4 samples by FTIR (around 800 cm1 and 1400 cm1) during dehydrogenation. LiNH2 and BN were observed in the complete dehydrogenated residue by XRD characterization (Fig. 2c). FTIR analysis showed the presence of NeH stretches centered at 3310 cm1 and 3256 cm1 (Fig. 3d), which are close to the symmetric and asymmetric NeH stretches of LiNH2. BeH stretches were not observed, while two broad vibration bands around 1380 cm1 and 800 cm1 belonging to h-BN were detected [38]. The corresponding Raman characterization was in accordance with the
The formation of Li(NH3)nBH4 ammoniates depends on the coordination of N in NH3 on Li cation. The generation of H2 in a closed system dehydrogenation, is neither originated from the self-decomposition of NH3 nor from the dehydrogenation of CoeLiBH4, but from the interaction between (N)Hdþ and (B)Hd which coexisted in Li(NH3)nBH4. In previous study, it was reported that Co-catalyst is highly effective in lowering the dehydrogenation temperatures of LiBH4-based hydrogen storage materials [21,28]. Since the NH3 equilibrium pressure will decrease as the temperature decreased, the lowered dehydrogenation temperature will in turn lower the NH3 emission in the gaseous product to certain extent. In addition, under a closed small volume, a significantly lesser amount of NH3 is evolved to achieve its equilibrium pressure as compared to that in a larger volume. Therefore, the overwhelming majority of NH3 (holding Hdþ) is more preferentially to bind with LiBH4 and react with BH4 (Hd) to release hydrogen. The synergic effects of Co-catalyst and small vessel promote direct combination of Hdþ and Hd, and avoid the need of introducing additional additives as ammonia scrubber. In our experiment, we found that when the volumetric release of Co-doped Li(NH3)4/3BH4 was performed in a larger reactor with fourfold volume, 2.0 equivalent moles of gaseous product (instead of 4 equiv.) was evolved upon heated to 250 C for 20 h and with a slower dehydrogenation rate as compared to that performed in the small reactor (6.7 ml). Therefore, we assume that more NH3 coordinated to Liþ is beneficial for hydrogen evolution. Johnson et al. revealed that the presence of NH3 elongated the LieB bond of LiBH4 from ˚ to 2.52e2.57 A ˚ [26]. Chen and Yu also revealed that 2.48e2.54 A metal cations could play vital role in suppressing NH3 emission and destabilizing NeH or BeH bonds [39]. The decreasing melting point of Li(NH3)nBH4 (1 n 2) as n increases could also manifest more NH3 would contribute to the destabilization of Li(NH3)nBH4 [37]. As a matter of fact, the as-prepared Co-doped Li(NH3)BH4 and Li(NH3)2BH4 samples are in solid and liquid form, respectively. As results, the onset temperatures of hydrogen evolution from Co-doped Li(NH3)nBH4 drop gradually as the NH3 content increases (1 n 2) (Fig. 1). Analyzing the tangent slope of the linear parts for initial hydrogen desorption (See Fig. S8 in the Supplementary Information), the rate constant for the sample with n ¼ 2 was estimated to be 0.0012 equiv. H2 min1, about 3.4 and 1.8 times faster than those from n ¼ 1 and 4/3 samples, respectively. As compared to CoeLi(NH3)4/3BH4, the CoeLi(NH3)2BH4 dehydrogenates at lower temperatures with faster rate but at the expense of the H2 capacity. In the case of CoeLi(NH3)BH4, fairly pure H2 with comparative amount of H2 can be obtained.
Dehydrogenation process of Li(NH3)nBH4 (n ¼ 1, 2) Fig. 4 e Raman spectra of the freshly-made Co-doped a) Li(NH3)BH4, b) Li(NH3)2BH4 samples, and the samples after dehydrogenation at 250 C: c) Li(NH3)BH4 d) Li(NH3)2BH4.
The dehydrogenation of Li(NH3)nBH4 may start with the establishment of (H)BeN(H) bond, followed by the
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Table 1 e Dehydrogenation properties of Li(NH3)nBH4. n
Onset T ( C)
Intermediates
Final products
H2 (wt%)
H-purity (%)
140 135 120
Li4BN3H10, LiBH4, Li2BNH6, BNHx Li4BN3H10, Li2BNH6, BNHx Li(NH3)BH4, Li4BN3H10, LiBH4, Li2BNH6, BNHx
LiH, BN Li3BN2, BN LiNH2, BN
15.3 17.8 14.3
99.99 99.97 97.60
1 4/3 2
dissociation of BeH and NeH bonds. Liþ, which initially links to BH4, tends to bind with hydridic H(B) to form LiH. The intermediate LiH will then react with NH3 to form LiNH2 and H2 [40]. Li2BNH6 or Li4BN3H10 is likely to form during the dehydrogenation owing to the presence of LiNH2 and unreacted LiBH4 [41]. Further increasing the temperature will lead to the decomposition of Li4BN3H10, Li2BNH6 and LiBH4 to BN and LiH from Li(NH3)BH4. As for Li(NH3)2BH4, the formation of Li(NH3)BH4, Li2BNH6 or Li4BN3H10 were observed at the initial dehydrogenation stage, evidencing a stepwise consumption of LiBH4 and NH3, and the development of Li2BNH6 and Li4BN3H10. It should be noted that Li2BNH6 decomposes to form Li4BN3H10 and LiBH4 upon heated to ~90 C [24]. The observation of Li2BNH6 in the solid residue is likely due to the recombination of Li4BN3H10 and LiBH4 during the sample cooling process. Summarizing the information obtained from various characterizations, the dehydrogenation of Li(NH3)nBH4 (n ¼ 1, 4/3, 2) systems can be expressed as follow (Table 1): 1 1 1 1 8x H2 LiðNH3 ÞBH4 / Li4 BN3 H10 þ LiBH4 þ Li2 BNH6 þ BNHx þ 8 4 8 2 4 /LiHþBNþ3H2 1 1 2 8x H2 LiðNH3 Þ4=3 BH4 / Li4 BN3 H10 þ Li2 BNH6 þ BNHx þ 6 6 3 3 1 2 / Li3 BN2 þ BNþ4H2 3 3 1 1 1 1 LiðNH3 Þ2 BH4 / Li4 BN3 H10 þ LiBH4 þ Li2 BNH6 þ LiðNH3 ÞBH4 8 8 8 8 1 8x 7 H2 þ NH3 /LiNH2 þBNþ4H2 þ BNHx þ 2 4 8
Conclusion In summary, we have demonstrated a promising hydrogen storage system of Li(NH3)nBH4, which shows significant NH3 suppression and kinetic improvements as compared to neat LiBH4 and NH3 when tested in a closed vessel. The synergic effects of Co catalyst and small vessel promote Li(NH3)BH4 to release 3 equiv. H2 (15.8 wt%) with high purity (99.99%) and yield LiH and BN. Under the same condition, Li(NH3)2BH4 could release 4 equiv. H2 (14.0 wt%) with a purity of 97.60%, and produce LiNH2 and BN.
Acknowledgment The authors would like to acknowledge the financial supports from National Natural Science Foundation of China (51225206, U1232120, 51301161).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.01.134.
references
[1] Cipriani G, Di Dio V, Genduso F, La Cascia D, Liga R, Miceli R, et al. Perspective on hydrogen energy carrier and its automotive applications. Int J Hydrogen Energy 2014;39:8482e94. B, Schwickardi M. Ti-doped alkali metal [2] Bogdanovic aluminium hydrides as potential novel reversible hydrogen storage materials. J Alloys Compd 1997;253:1e9. [3] Chen J, Kuriyama N, Xu Q, Takeshita HT, Sakai T. Reversible hydrogen storage via titanium-catalyzed LiAlH4 and Li3AlH6. J Phys Chem B 2001;105:11214e20. [4] Jensen CM, Gross KJ. Development of catalytically enhanced sodium aluminum hydride as a hydrogen-storage material. Appl Phys A Mater Sci Proc 2001;72:213e9. [5] Amendola SC, Sharp-Goldman SL, Janjua MS, Spencer NC, Kelly MT, Petillo PJ, et al. A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. Int J Hydrogen Energy 2000;25:969e75. [6] Zu¨ttel A, Wenger P, Rentsch S, Sudan P, Mauron P, Emmenegger C. LiBH4 a new hydrogen storage material. J Power Sources 2003;118:1e7. [7] Nakamori Y, Li HW, Miwa K, Towata S, Orimo S. Syntheses and hydrogen desorption properties of metal-borohydrides M(BH4)n (M ¼ Mg, Sc, Zr, Ti, and Zn; n ¼ 2e4) as advanced hydrogen storage materials. Mater Trans 2006;47:1898e901. [8] Leng HY, Ichikawa T, Hino S, Hanada N, Isobe S, Fujii H. New metal-N-H system composed of Mg(NH2)2 and LiH for hydrogen storage. J Phys Chem B 2004;108:8763e5. [9] Lu J, Fang ZZ, Sohn HY. A new LieAleNeH system for reversible hydrogen storage. J Phys Chem B 2006;110:14236e9. [10] Xiong ZT, Wu GT, Hu JJ, Liu YF, Chen P, Luo WF, et al. Reversible hydrogen storage by a LieAleNeH complex. Adv Funct Mater 2007;17:1137e42. [11] Pinkerton FE, Meisner GP, Meyer MS, Balogh MP, Kundrat MD. Hydrogen desorption exceeding ten weight percent from the new quaternary hydride Li3BN2H8. J Phys Chem B 2005;109:6e8. [12] Aoki M, Miwa K, Noritake T, Kitahara G, Nakamori Y, Orimo S, et al. Destabilization of LiBH4 by mixing with LiNH2. Appl Phys A: Mater Sci Proc 2005;80:1409e12. [13] Denney MC, Pons V, Hebden TJ, Heinekey DM, Goldberg KI. Efficient catalysis of ammonia borane dehydrogenation. J Am Chem Soc 2006;128:12048e9. [14] Keaton RJ, Blacquiere JM, Baker RT. Base metal catalyzed dehydrogenation of ammonia-borane for chemical hydrogen storage. J Am Chem Soc 2007;129:1844e5. [15] Diyabalanage HVK, Shrestha RP, Semelsberger TA, Scott BL, Bowden ME, Davis BL, et al. Calcium amidotrihydroborate: a
4578
[16]
[17]
[18]
[19] [20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
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 4 0 ( 2 0 1 5 ) 4 5 7 3 e4 5 7 8
hydrogen storage material. Angew Chem Int Ed 2007;46:8995e7. Xiong ZT, Yong CK, Wu GT, Chen P, Shaw W, Karkamkar A, et al. High-capacity hydrogen storage in lithium and sodium amidoboranes. Nat Mater 2008;7:138e41. Wu H, Zhou W, Yildirim T. Alkali and alkaline-earth metal amidoboranes: structure, crystal chemistry, and hydrogen storage properties. J Am Chem Soc 2008;130:14834e9. Umegaki T, Yan JM, Zhang XB, Shioyama H, Kuriyama N, Xu Q. Boron- and nitrogen-based chemical hydrogen storage materials. Int J Hydrogen Energy 2009;34:2303e11. Vajo JJ, Skeith SL, Mertens F. Reversible storage of hydrogen in destabilized LiBH4. J Phys Chem B 2005;109:3719e22. Pinkerton FE, Meyer MS, Meisner GP, Balogh MP. Improved hydrogen release from LiB0.33N0.67H2.67 with metal additives: Ni, Fe, and Zn. J Alloys Compd 2007;433:282e91. Tang WS, Wu G, Liu T, Wee ATS, Yong CK, Xiong ZT, et al. Cobalt-catalyzed hydrogen desorption from the LiNH2eLiBH4 system. Dalton Trans 2008:2395e9. Yang J, Sudik A, Wolverton C. Destabilizing LiBH4 with a metal (M ¼ Mg, Al, Ti, V, Cr, or Sc) or metal hydride (MH2, MgH2, TiH2, or CaH2). J Phys Chem C 2007;111:19134e40. Pinkerton FE, Meyer MS. Reversible hydrogen storage in the lithium borohydride-calcium hydride coupled system. J Alloys Compd 2008;464:L1e4. Chater PA, David WIF, Anderson PA. Synthesis and structure of the new complex hydride Li2BH4NH2. Chem Commun 2007:4770e2. Chater PA, Anderson PA, Prendergast JW, Walton A, Mann VSJ, Book D, et al. Synthesis and characterization of amide-borohydrides: new complex light hydrides for potential hydrogen storage. J Alloys Compd 2007;446e447:350e4. Johnson SR, David WIF, Royse DM, Sommariva M, Tang CY, Fabbiani FPA, et al. The monoammoniate of lithium borohydride, Li(NH3)BH4: an effective ammonia storage compound. Chem Asian J 2009;4:849e54. Guo YH, Xia GL, Zhu YH, Gao L, Yu XB. Hydrogen release from amminelithium borohydride, LiBH4$NH3. Chem Commun 2010;46:2599e601. Zheng XL, Wu GT, Li W, Xiong ZT, He T, Guo JP, et al. Releasing 17.8 wt% H2 from lithium borohydride ammoniate. Energy Environ Sci 2011;4:3593e600.
[29] Chen XW, Yuan F, Tan YB, Tang ZW, Yu XB. Improved dehydrogenation properties of Ca(BH4)2$nNH3 (n ¼ 1, 2, and 4) combined with Mg(BH4)2. J Phys Chem C 2012;116:21162e8. [30] Soloveichik G, Her JH, Stephens PW, Gao Y, Rijssenbeek J, Andrus M, et al. Ammine magnesium borohydride complex as a new material for hydrogen storage: structure and properties of Mg(BH4)2$2NH3. Inorg Chem 2008;47:4290e8. [31] Guo YH, Yu XB, Sun WW, Sun DL, Yang WN. The hydrogenenriched AleBeN system as an advanced solid hydrogenstorage candidate. Angew Chem Int Ed 2011;50:1087e91. [32] Chua YS, Wu GT, Xiong ZT, He T, Chen P. Calcium amidoborane ammoniate-synthesis, structure, and hydrogen storage properties. Chem Mater 2009;21:4899e904. [33] Tan YB, Guo YH, Li SF, Sun WW, Zhu YH, Li Q, et al. A liquidbased eutectic system: LiBH4$NH3enNH3BH3 with high dehydrogenation capacity at moderate temperature. J Mater Chem 2011;21:14509e15. [34] Chen XY, Cai WY, Guo YH, Yu XB. Significantly improved dehydrogenation of LiBH4$NH3 assisted by Al2O3 nanoscaffolds. Int J Hydrogen Energy 2012;37:5817e24. [35] Sun WW, Chen XW, Gu QF, Wallwork KS, Tan YB, Tang ZW, et al. A new ammine dual-cation (Li, Mg) borohydride: synthesis, structure, and dehydrogenation enhancement. Chem Eur J 2012;18:6825e34. [36] Tang ZW, Tan YB, Gu QF, Yu XB. A novel aided-cation strategy to advance the dehydrogenation of calcium borohydride monoammoniate. J Mater Chem 2012;22:5312e8. [37] Sullivan EA, Johnson S. The lithium borohydride-ammonia system-pressure-composition-temperature relationships and densities. J Phys Chem 1959;63:233e8. [38] Geick R, Perry CH, Rupprech G. Normal modes in hexagonal boron nitride. Phys Rev 1966;146:543e7. [39] Chen XW, Yu XB. Electronic structure and initial dehydrogenation mechanism of M(BH4)2$2NH3 (M ¼ Mg, Ca, and Zn): a first-principles investigation. J Phys Chem C 2012;116:11900e6. [40] Hu YH, Ruckenstein E. Ultrafast reaction between LiH and NH3 during H2 storage in Li3N. J Phys Chem A 2003;107:9737e9. [41] Meisner GP, Scullin ML, Balogh MP, Pinkerton FE, Meyer MS. Hydrogen release from mixtures of lithium borohydride and lithium amide: a phase diagram study. J Phys Chem B 2006;110:4186e92.