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Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4
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Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4 Liuting Zhang, Jiaguang Zheng, Lixin Chen, Xuezhang Xiao*, Teng Qin, Yiqun Jiang, Shouquan Li, Hongwei Ge, Qidong Wang State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
article info
abstract
Article history:
Mg(BH4)2 is considered as one of the most promising light metal complex hydrides because
Received 30 June 2015
of its high volumetric and gravimetric hydrogen capacities and world-wide abundance.
Received in revised form
However, its higher major desorption temperatures (above 300 C) and poor reaction ki-
23 August 2015
netics have to be improved for the practical application. Herein, Mg(BH4)2 was successfully
Accepted 24 August 2015
synthesized via wet-chemical technique and a significant enhancement in dehydrogena-
Available online xxx
tion performance of Mg(BH4)2 is achieved by the synergetic effect of fluorographite (FGi) and LiBH4. Under the effect of FGi, the hydrogen desorption of 6 Mg(BH4)2-4FGi composite
C in seconds. However, the hydrogen released from
Keywords:
could be completed below 170
Hydrogen storage
6 Mg(BH4)2-4FGi suffers from impurities of B2H6 and HF. More importantly, it is demon-
Mg(BH4)2
strated that almost all the B2H6 and HF impurities can be suppressed by synergetic
LiBH4
modifying Mg(BH4)2 with FGi and LiBH4. The 3 Mg(BH4)2e3LiBH4-4FGi composite exhibits a
Fluorographite
capacity over 8.0 wt% H2 and starts to release hydrogen at 125.7 C, which is 143.8 C and
Synergetic effect
254.3 C lower than that of pure Mg(BH4)2 and LiBH4, respectively. These significant improvements could be attributed to both the novel morphology that numerous nano-scale borohydride spots formed on the surface of FGi, and the formations of stable fluorides (MgF2 and LiF) from the interaction between borohydrides and FGi. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen, which has high energy density, almost zero emission and promising renewable nature, is a potential energy carrier especially in proton exchange membrane fuel cells (PEMFCs). Nevertheless, reversible hydrogen storage with high capacity still hinds the practical application of
hydrogen energy. Because of the high gravimetric, volumetric hydrogen capacities and abundant resouses, solidstate hydrogen storage materials especially light-metal complex hydrides have received significant attention in the past ten years [1e8]. Among these materials, LiBH4 is competitive and widely investigated due to its large theoretical hydrogen capacity (13.8 wt%) and volumetric
* Corresponding author. Tel./fax: þ86 571 8795 1876. E-mail address: [email protected] (X. Xiao). http://dx.doi.org/10.1016/j.ijhydene.2015.08.090 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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hydrogen density (121 kg/m3) [1,9,10]. However, the main evolution of hydrogen starts at 380 C, and the reaction products (LiH and B) can only be rehydrogenated at high hydrogen pressure of 35 MPa and high temperature of 600 C hinds the practical application of LiBH4 [11]. Compared with LiBH4, Mg(BH4)2 has larger practical hydrogen capacity (14.8 wt%) and comparatively low onset dehydrogenation temperature [12]. Mg(BH4)2 was reported to start releasing hydrogen at 270 C and to reach its hydrogen desorption peak temperature at about 350 C [13e15]. Unfortunately, the dehydrogenation of Mg(BH4)2 still needs to be greatly improved for practical hydrogen storage applications due to its high desorption temperatures and poor kinetics. In order to improve the dehydrogenation property of Mg(BH4)2, ball milling Mg(BH4)2 with additives has been demonstrated to be an effect way [16e19]. For example, Li et al. found that Mg(BH4)2 ball milled with Ti-based additives including TiO2, TiH2 and Ti can start to release hydrogen at 227 C [16]. Transition metal halides were well known to be effective in destablizing borohydrides and by adding Nb or Ti chlorides, the onset temperature of Mg(BH4)2 can be lowered by more than 100 C [17]. Moreover, Mg(BH4)2 with NbF5 begins to release hydrogen at 75 C and the maximum hydrogen yield obtained from Mg(BH4)2 in the presence of 15 wt% NbF5, is 3.7, 7.4, 10.0, 11.4 wt% for 150, 250, 300 and 350 C, respectively [18]. It was also reported that the hydrogen storage properties of Mg(BH4)2 can be effectively improved by combining with other complex hydrides to form a reactive composite [20e22]. Yu et al. found that the onset temperature of dehydrogenation of the Mg(BH4)2eLiNH2 composite decreased from 270 C to 160 C but NH3 (poisonous for PEMFCs) formed in the decomposition process [20]. Recently, Juahir et al. [22] found that the TiF3 doped NaAlH4eMg(BH4)2 (2:1) composite could start to release hydrogen at about 75 C, 60 C lower than that of undoped one. So far, such a hydrogen desorption temperature is the best achievement by modifying Mg(BH4)2 with complex hydrides, but the terminal dehydrogenation temperature (>400 C) is still too high for practical applications. Nanoconfinement is another widely used way to improve the hydrogen storage properties of Mg(BH4)2. Recently, Fichtner et al. achieved a very significant decrease in hydrogen desorption temperature (the onset temperature was about 170 C) by confining Mg(BH4)2 into disordered activated carbon [23]. Later, Wahab et al. studied the dehydrogenation performance of Mg(BH4)2 confined in highly ordered CMK3 and the hydrogen desorption of the Mg(BH4)2-CMK3 started at 155 C [24]. Moreover, the hydrogen desorption was further decreased by incorporating Ni nanoparticles into the Mg(BH4)2-CMK3 system. However, the actual amount of Mg(BH4)2 confined to the templete was so limited, leading to a large decrease in practical hydrogen storage capacity. Our previous study shows that FGi has a remarkable effect on both the dehydrogenation temperature and kinetics of LiBH4 and NaBH4 [4,25]. So far as we know, the effect of FGi on the hydrogen desorption property of Mg(BH4)2 is still unreported. In addition, Nale et al. found that LiBH4eMg(BH4)2 eutectic mixture was shown to release hydrogen at 220 C, which is lower compared to the hydrogen desorption temperature of both LiBH4 and Mg(BH4)2 [26]. Inspired by above investigations, we hypothesize that a further significant
decrease of desorption temperature and reasonable high gravimetric hydrogen capacity can be realized if Mg(BH4)2 can be synergistically modified by means of FGi combining with LiBH4. In this study, we first report the hydrogen desorption performance of as-synthesized Mg(BH4)2-FGi composite. Because previous studies confirm that the desorption performance of Mg(BH4)2 can be improved by adding LiBH4 to form Mg(BH4)2eLiBH4 eutectic mixture [26e31], we also introduced LiBH4 into the Mg(BH4)2-FGi system, aiming to synergistically improve the dehydrogenation properties of Mg(BH4)2. In the Mg(BH4)2eLiBH4-FGi system, hydrogen starts to be released at a surprisingly lower temperature of 125.7 C with a hydrogen capacity over 8.0 wt%, which means the synergistic effects of FGi and LiBH4 can significantly enhance the hydrogen desorption performance of Mg(BH4)2, and makes it possible to use for mobile hydrogen storage applications.
Experimental details The starting materials were all stored and handled in an argon-filled glove box to prevent H2O and O2 contamination. LiBH4 (Alfa Aesar, 95%) and FGi (CarFluor Ltd, Grade II) were used as received. The fluorine content in FGi is 62 wt% and the chemical formula of fluorographite is written as CF [32,33]. The properties and crystal structure diagram of FGi has already been presented in our published paper [4]. Mg(BH4)2 was synthesized from MgCl2 (Aladdin, 99%) and NaBH4 (Aladdin, 98%) in anhydrous diethyl ether (AR, Sinopharm chemical Reagent Co. Ltd) as described in the previous paper [7]. The composites contained 30 wt%, 40 wt%, 50 wt% FGi were prepared by ball milling the mixture of Mg(BH4)2 and FGi, denoted as 7 Mg(BH4)2-3FGi, 6 Mg(BH4)2-4FGi and 5 Mg(BH4)25FGi, respectively. In addition, half amount of Mg(BH4)2 was replaced by LiBH4 in the 6 Mg(BH4)2-4FGi sample to prepare 3 Mg(BH4)2e3LiBH4-4FGi composite. The composites were all ball milled for 2 h (planetary QM-3SP4, Nanjing, China) under 0.1 MPa Argon pressure at a speed of 450 r/min. The ball-topower ratio was 40:1. A Sievert's type apparatus was applied to determine the dehydrogenation properties of the composites quantitatively using a volumetric method. In brief, about 150 mg of sample was loaded in a home-made reactor for each temperature programmed desorption (TPD) measurement. The heating rate was 5 C/min (from room temperature to 500 C) and the whole system was pumped to static vacuum before the measurement. To better compare the dehydrogenation properties, all the dehydrogenation capacity of measured composites are presented as weight percent (wt%) of the whole composite (i.e. borohydride plus FGi). In addition, Mass Spectrometry (MS) measurement was also carried out on a QIC-20 (HIDEN ANALYTICAL LTD) gas analysis system to detect the gas released from the composites (from room temperature to 500 C at a heating rate of 5 C/min under proactive Argon flow). X-ray diffraction analysis (XRD) was obtained from an X'Pert Pro X-ray diffractometer (PANalytical, Netherlands) using Cu Ka radiation at 40 kV and 40 mA. Fourier Transform Infrared Spectra (FTIR) was carried out on a Bruker Tensor 27
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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unit (transmission mode). Scanning Electron Microscope (SEM, Hitachi SU-70) was adopted to analyze the morphology of the composites with 3.0 kV e-beam energy. X-ray Photoelectron Spectroscopy (XPS) was applied to analyse the binding energy (BE) values, which were referenced to the C1s peak at 284.6 eV with an uncertainty of ±0.2 eV, on a VGESCALAB MARK II system with Mg Ka radiation (1253.6 eV) at a base pressure of 1 108 Torr. During the XPS, XRD, FTIR and SEM measurements, special caution was taken to reduce the time of samples exposed to air.
Results and discussion The microstructures of as-received FGi, as-synthesized Mg(BH4)2, 5 Mg(BH4)2-5FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)23FGi composites were characterised by XRD and FTIR, as shown in Fig. 1. It can be found in the XRD part of Fig. 1, the asreceived FGi displays two broaden peaks at 2q ¼ 14 corresponding to (001) reflection in compounds with a very high level of fluorine and 41 corresponding to (100) reflection of CeC in-plane length in the reticular system [34,35]. Also, a sharp peak at 2q ¼ 26 is attributed to graphite, indicating the incomplete fluorination of FGi. In Fig. 1(b), the XRD pattern of as-synthesized Mg(BH4)2 matches perfectively with bMg(BH4)2 reported by Chłopek et al., indicating the Mg(BH4)2 is successfully synthesized by wet chemical reaction [7]. After ball milling, the 5 Mg(BH4)2-5FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)2-3FGi composites still exhibit the obvious diffration
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peaks corresponding to Mg(BH4)2 and FGi. Moreover, the signal of FGi becomes weaker with decreasing the amount of FGi from 50 to 30 wt%, while Mg(BH4)2 tends to be amorphous and/ or fine crystallite state by the effect of ball milling. To further explore the vibration of bands in these composites, FTIR measurement was carried out. It can be seen from the FTIR part in Fig. 1(a) that, the characteristic absorption of CeF covalent band appears at 1215 cm1. At the same time, the stretching vibration of CeF2 moieties located in the edges of segments at 1348 cm1 are present in the FGi [36]. Typical features of [BH4]1 group can be observed in the spectra of assynthesized Mg(BH4)2, the BeH absorption band is split into three contributions at 2386 cm1, 2291 cm1, and 2223 cm1. While the BeH bending vibration is split into two contributions at 1125 cm1 and 1267 cm1 [7]. For the as-prepared 5 Mg(BH4)2-5FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)2-3FGi composites, it can be seen from the FTIR part of Fig. 1(c), (d) and (e) that the intensity of CeF functional group in FGi also decreases with the decreasing amount of FGi gradually. Interestingly, the intensity of BeH absorption bands (2386 cm1 and 2223 cm1) in Mg(BH4)2 are distinctly disappeared, indicating that the Mg(BH4)2 may have some interaction with the surfaces of FGi and result in the change of the chemical atmosphere of BeH bands in Mg(BH4)2, which is similar with our previous investigations [25]. SEM examinations were applied to explore the microstructure feature of FGi, as-synthesized Mg(BH4)2, 5 Mg(BH4)25FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)2-3FGi composites, as shown in Fig. 2. It is obviously found in Fig. 2(a) that FGi is
Fig. 1 e XRD and FTIR profiles of (a) FGi, (b) Mg(BH4)2, (c) 5 Mg(BH4)2-5FGi, (d) 6 Mg(BH4)2-4FGi and (e) 7 Mg(BH4)2-3FGi composites. Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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Fig. 2 e SEM images of (a) FGi, (b) Mg(BH4)2, (c) 5 Mg(BH4)2-5FGi, (d) 6 Mg(BH4)2-4FGi and (e) 7 Mg(BH4)2-3FGi composites.
layered sheet structures. The particle size of as-synthesized Mg(BH4)2 is ranging from 0.1 to 1 mm in Fig. 2(b). After ball milling with FGi, it seems that the Mg(BH4)2 becomes flaky and covers the surfaces of FGi, while the layered sheet structure of FGi has been destroyed, as shown in Fig. 2(c), (d) and (e). According to XRD and FTIR analyses in Fig. 1, it can be deduced that the Mg(BH4)2 has some physical interactions with CeF2 and CeF functional groups, and then cover the surface of FGi as a result. Fig. 3 presents the TPD profiles of the as-synthesized Mg(BH4)2, 5 Mg(BH4)2-5FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)23FGi composites. It is noteworthy that FGi can significantly improve the dehydrogenation performance of Mg(BH4)2. For the as-synthesized Mg(BH4)2, no gas is released below 250 C. While for the 5 Mg(BH4)2-5FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)23FGi composites, 7.0 wt%, 8.6 wt% and 9.3 wt% gas can be released at 161.0 C, 169.8 C and 172.6 C, respectively.
Interestingly, the majority of the hydrogen is released in seconds and a sudden temperature rise is occurred at the same time. This phenomenon has never been reported in the hydrogen desorption of Mg(BH4)2. XRD and FTIR profiles of the dehydrogenated assynthesized Mg(BH4)2, 5 Mg(BH4)2-5FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)2-3FGi composites are also presented in Fig. 4. There is no signal of BeH band detected in the FTIR spectrum, indicating that all the composites are fully decomposed after dehydrogenation process. XRD results suggest that Mg(BH4)2 decomposed into Mg, MgB2 and amorphous B (can't be detected by XRD). Fig. 4(b), (c) and (d) show that MgF2 is the main phase in all three dehydrogenated Mg(BH4)2-FGi composites. Further more, it is reasonable to observe that the XRD patterns of Mg and MgB2 show up while that of C decreases with the increasing amount of Mg(BH4)2.
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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In order to shed light on the gas components released from the sample, MS measurement was carried out for the 6 Mg(BH4)2-4FGi composite, as shown in Fig. 5. It is clearly seen that the hydrogen desorption of the 6 Mg(BH4)2-4FGi composite can be classified as three stages corresponding to approximately 200, 310 and 360 C. Moreover, the impurities of B2H6 and HF are also released from the composite starting at the first stage. Therefore, it can be concluded that total mass loss is attributed to the releasing of impurities of B2H6 and HF together with H2. Combining the gas amount from MS together with the XRD and FTIR results, take 6 Mg(BH4)2-4FGi composite as an example, the dehydrogenation equation (1) can be written as follows:
22 Mg(BH4)2 þ 37 CF / x MgF2 þ y MgB2 þ z B2H6 þ (22 x y) Mg þ (44 2y 2z) B þ 37 C þ (37 2x) HF þ (69.5 þ x 3z) H2 (x ¼ 8~9, y ¼ 12.5~13.1 z ¼ 0.9~1.1) (1) Though the dehydrogenation temperature of Mg(BH4)2 could be largely decreased by modifying with FGi, the current dehydrogennation temperature is still to high for PEMFCs. Moreover, the B2H6 and HF impurities are also released along
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with H2, which not only reduce the actual dehydrogenation capacity but also poison the PEMFCs. Therefore, an effective strategy that can not only decrease the hydrogen desorption temperature of Mg(BH4)2 but also suppress B2H6 impurity is of great significance and highly desirable. Recently, it has been proposed that mixed-metal borohydrides could have acceptable thermodynamics and some examples such as LiBH4eMg(BH4)2 mixture have already been reported [26e31]. By forming eutectic composition, the material starts to decompose in the liquid state just after melting, resulting in a relative low dehydrogenation temperature (220 C) compared to those of both LiBH4 and Mg(BH4)2 [26,27]. Thus, the 3 Mg(BH4)2e3LiBH4-4FGi composite was further prepared by ball milling 30 wt% Mg(BH4)2, 30 wt% LiBH4 and 40 wt% FGi for 2 h under 0.1 MPa Ar. For comparison, 5 Mg(BH4)2e5LiBH4 composite was also prepared under the same condition. Fig. 6 is the XRD and FTIR profiles of as-prepared 5 Mg(BH4)2e5LiBH4 and 3 Mg(BH4)2e3LiBH4-4FGi composites. XRD result shows that as-prepared 50 Mg(BH4)2e50LiBH4 is just a mixture of Mg(BH4)2 and LiBH4, agreeing well with previous studies [27]. While in the case of 3 Mg(BH4)2e3LiBH4-4FGi composite, both the diffraction intensities of Mg(BH4)2 and LiBH4 become weaker after ball milling with FGi. In addition, the FTIR profiles of 5 Mg(BH4)2e5LiBH4 and 3 Mg(BH4)2e3LiBH4-4FGi composites still show the typical features of [BH4]1 group, while only the CeF characteristic absorption band presents in the 3 Mg(BH4)2e3LiBH4-4FGi composites. This results indicate that some interaction may occur between borohydrides and FGi, resulting in the decreasing signal of Mg(BH4)2 and LiBH4 in XRD patterns and the disappearing signal of CeF2 vibration band in FTIR profiles. To further explore the morphology of 3 Mg(BH4)2e3LiBH44FGi composite, SEM measurements were also employed. It is clearly shown in Fig. 7 that the 3 Mg(BH4)2e3LiBH4-4FGi composite has a rough surface. Meanwhile, numerous nano-scale spots formed on the surface of FGi. According to FTIR analysis above and the unique morphology of 3 Mg(BH4)2e3LiBH4-4FGi, it can be concluded that the CeF2 and CeF functional groups in the surface of FGi are beneficial to catch Mg(BH4)2/LiBH4 mixture and reduce their grain sizes to nanoscale. A similar phenomenon was also found in our previous work [25], this unique surface morphology may improve the hydrogen storage properties of Mg(BH4)2, as far as we know. To testify the synergetic effect of FGi and LiBH4 for enhancing dehydrogenation properties of Mg(BH4)2, TPD and MS measurements were further carried out. Fig. 8 presents the TPD profiles and corresponding temperature variation of the as-synthesized Mg(BH4)2, 5 Mg(BH4)2e5LiBH4, 6 Mg(BH4)2-4FGi and 3 Mg(BH4)2e3LiBH4-4FGi composites. Fig. 8 (a) and (b) demonstrate that the onset dehydrogenation temperature of can be reduced by forming eutectic Mg(BH4)2 5 Mg(BH4)2e5LiBH4 composite [26e31]. As mentioned above, the dehydrogenation temperature of Mg(BH4)2 can be significantly decreased by modifying with FGi (Fig. 8(c)). In addition, it is noteworthy that the dehydrogenation performance of Mg(BH4)2 can be further improved by synergetic modifying with LiBH4 and FGi. The 3 Mg(BH4)2e3LiBH4-4FGi can obtain 8.1 wt% hydrogen at 125.7 C, which is 44.1, 122.2 and 143.8 C lower than that of 6 Mg(BH4)2-4FGi, 5 Mg(BH4)2e5LiBH4 and pure Mg(BH4)2, respectively. The obvious decrease of onset
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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Fig. 4 e XRD and IR profiles of (a) Mg(BH4)2, (b) 5 Mg(BH4)2-5FGi, (c) 6 Mg(BH4)2-4FGi and (d) 7 Mg(BH4)2-3FGi composites dehydrogenated at 500 C.
hydrogen desorption temperature is clearly shown in the inset picture of Fig. 8, which is a substantial improvement. Moreover, the gas components released from the 3 Mg(BH4)2e3LiBH4-4FGi were also confirmed by MS technique, as shown in Fig. 9. It is of interest that the dehydrogenation of 3 Mg(BH4)2e3LiBH4-4FGi becomes one step compared with three steps of 6 Mg(BH4)2-4FGi, indicating a
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change in dehydrogenation path when Mg(BH4)2 synergetic modified with FGi and LiBH4. Notably, almost all the HF and H2B6 impurities are suppressed during the dehydrogenation process. TPD and MS measurements show that the 3 Mg(BH4)2e3LiBH4-4FGi composite has achieved a relatively low onset dehydrogenation temperature, for comparison, the dehydrogenation properties of state-of-the-art Mg(BH4)2 modified systems are displayed in Table 1. It can be clearly seen in the table that the on-set dehydrogenation temperature 3 Mg(BH4)2e3LiBH4-4FGi composite is inferior to that of NaAlH4eMg(BH4)2 þ 5wt%TiF3, Mg(BH4)2 þ 15wt%NbF5, Mg(BH4)2e2LiAlH4 and Mg(BH4)2 þ 0.05TiCl3 þ 0.05NbCl5 systems, however, its terminal dehydrogenation temperature is superior to all the Mg(BH4)2 modified systems presented in Table 1. Moreover, the 3 Mg(BH4)2e3LiBH4-4FGi composite possesses the hydrogen desorption capacities of 8.1 wt% at the lower temperature of 160 C in seconds, which is the best achievement among all the modified Mg(BH4)2 systems, as far as we know. This result indicates that our strategy in term of not only further decreasing the desorption temperature of Mg(BH4)2 but also maintaining a reasonable hydrogen purity achieves great success. For understanding the dehydrogenation process of such 3 Mg(BH4)2e3LiBH4-4FGi composite, the solid residues of the dehydrogenated 3Mg(BH4)2e3LiBH4-4FGi were further collected for XRD, FTIR and XPS examinations (see Fig. 10). XRD and FTIR profiles suggest that the 3Mg(BH4)2e3LiBH44FGi composite is fully decomposed under the experimental conditions and the decomposition products are identified as
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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Fig. 6 e XRD and IR profiles of (a) 5 Mg(BH4)2e5LiBH4 and (b) 3 Mg(BH4)2e3LiBH4-4FGi composites.
MgF2, MgB2, LiH, LiF and C. In addition, XPS profiles shows that the B1S peak can be fairly fitted by two different peaks. The boron alloying state at 187.7 ev should be assigned to MgB2, which is identical to Talapatraa's work [37]. The peak at lower energy of 187.1 ev belongs to elemental B [38], indicating the existence of amorphous B in the dehydrogenated products. Based on above experimental results, the dehydrogenation equation of the 3 Mg(BH4)2e3LiBH4-4FGi composite can be drawn as follows:
22 Mg(BH4)2 þ 79 LiBH4 þ 74 CF / x MgF2 þ (22 x) MgB2 þ (74 2x) LiF þ (2x þ 5) LiH þ 74 C þ (79 þ 2x) B þ (243.5 x) H2 (x ¼ 20~21)
(2)
Based on the above analyses, we try to figure out how LiBH4 and FGi worked together to synergetic modifying Mg(BH4)2. Fig. 2 has already revealed that after ball milling with FGi, the Mg(BH4)2 becomes flaky and covers the surfaces of FGi, the greatly reduced dehydrogenation was totally originated from
Fig. 7 e SEM images of (a) 3 Mg(BH4)2e3LiBH4-4FGi (£20 K), (b) 3 Mg(BH4)2e3LiBH4-4FGi (£50 K).
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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60
70
Time (min)
Fig. 8 e TPD and corresponding Temperature profiles of (a) Mg(BH4)2, (b) 5 Mg(BH4)2e5LiBH4, (c) 6 Mg(BH4)2-4FGi and (d) 3 Mg(BH4)2e3LiBH4-4FGi from room temperature to 500 C at heating rate of 5 C/min.
the reaction between FGi and Mg(BH4)2. While LiBH4 was also ball milled with Mg(BH4)2 and FGi, numerous nano-scale spots formed on the surface of FGi, which surely do favor to destabilize the whole system, thus reduce the dehydrogenation temperature. In addition, as the reaction is exothermic, the system releases heat when the reaction begins. Once the nano spots start to release gas at a low temperature, they can serve as lighters to kindle the remaining reaction just like a chain reaction and help to complete the whole dehydrogenation process. Furthermore, the formation of stable fluorides (MgF2 and LiF) during dehydrogenation process also contributes to the drastically enhanced dehydrogenation kinetics performance of 3 Mg(BH4)2e3LiBH4-4FGi composite. In order to explore reversible hydrogen storage performance of the 3 Mg(BH4)2e3LiBH4-4FGi composite, the rehydrogenation experiment of the dehydrided 3 Mg(BH4)2e3LiBH4-4FGi sample was carried out under 12 MPa of H2 at 400 C for 12 h. It is found that the 3 Mg(BH4)2e3LiBH44FGi exhibits partial reversibility, which rehydrogenated sample is composed of typical [BH4] and [B12H12] groups after rehydrogenation, as shown in Fig. S1. The rehydrogenated 3 Mg(BH4)2e3LiBH4-4FGi could release 4.3 wt% H2 in the second cycle, which is much high than that of 6 Mg(BH4)2-4FGi with about 1.2 wt% hydrogen H2, shown in Fig. S2. Although the reversible hydrogen storage of the as-prepared Mg(BH4)2eLiBH4-FGi composite is poor, the hydrogen desorption temperature is as low as ~125 C and a capacity over 8.0 wt% H2 is released from the composite at the first cycle. In a word, a synergetic thermodynamic and kinetic improvement can be achieved for initial hydrogen desorption in Mg(BH4)2 complex hydride.
Conclusions
1600
H2
Ion Current/nA (normalized)
1400
B2H6
1200
HF
1000 800 600 400 200 0 50
100
150
200
250
300
350
400
450
500
Temperature (°C)
Fig. 9 e MS profiles of 3 Mg(BH4)2e3LiBH4-4FGi composite from room temperature to 500 C at heating rate of 5 C/ min.
In summary, it has been successfully demonstrated that the hydrogen desorption properties of Mg(BH4)2 can be significantly improved by synergetic modifying with FGi and LiBH4. XRD, FTIR, TPD and MS analyses indicate that the 6 Mg(BH4)24FGi composite starts to desorb hydrogen below 170 C, while the B2H6 and HF impurities also release along with H2. Notably, under the synergetic effect of FGi and LiBH4, almost all B2H6 and HF impurities could be suppressed during dehydrogenation in 3 Mg(BH4)2e3LiBH4-4FGi composite. More importantly, the 3 Mg(BH4)2e3LiBH4-4FGi begins to release hydrogen at 125.7 C with a capacity over 8.0 wt% H2. It is believed that the drastically improved dehydrogenation kinetics is ascribed to the greatly size reduced nanoscale borohydrides in 3 Mg(BH4)2e3LiBH4-4FGi system, as well as the formation of stable MgF2 and LiF fluorides during dehydrogenation process. In addition, the partial reversibility for hydrogen storage has also been proved in 3 Mg(BH4)2e3LiBH4-4FGi composite, but reversibility of the composite still needs to be improved due to the formation of incomplete B12H12 intermediate hydrides. Thus strategy of synergetic modifying with FGi and LiBH4 opens up a new way for hydrogen desorption kinetics and capacity improvement in Mg(BH4)2 system.
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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Table 1 e Dehydrogenation properties of Mg(BH4)2 modified systems. Initial dehydrogenation capacity (wt%)a
On-set dehydrogenation temperature ( C)
Terminal dehydrogenation temperature ( C)
Ref.
Mg(BH4)2 þ 15wt%NbF5 NaAlH4eMg(BH4)2 þ 5wt%TiF3 Mg(BH4)2e2LiAlH4 Mg(BH4)2 þ 0.05TiCl3 þ 0.05NbCl5 3 Mg(BH4)2e3LiBH4-4FGi
8.5 8.0 11.0 ~ 8.1
75 75 120 125 125.7
370 500 370 360 160
Mg(BH4)2eLiNH2 LMBH/IRH33
7.2 2.2
160 220
300 380
[18] [22] [21] [17] Current work [20] [28]
System
a
The hydrogen storage capacity is given as weight percent (wt%) of the whole composite system.
Fig. 10 e (a) XRD, (b) IR and (c) XPS profiles of 3 Mg(BH4)2e3LiBH4-4FGi composite dehydrogenated at 500 C.
Acknowledgements The authors gratefully acknowledge the financial supports for this research from the National Natural Science Foundation of China (51571179, 51471151, 51171173), the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037), the Zhejiang Provincial Science & Technology Program of China (2015C31035, 2014C31134), and the Fundamental Research Funds for the Central Universities (2015QNA4010).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.08.090.
references
[1] Schlapbach L, Zu¨ttel A. Hydrogen-storage materials for mobile applications. Nature 2001;414:353e8.
[2] Bogdanovic B, Schwickardi M. Ti-doped alkali metal aluminium hydrides as potential novel reversible hydrogen storage materials. J Alloys Compd 1997;253:1e9. [3] Fan XL, Xiao XZ, Chen LX, Yu KR, Wu Z, Li SQ, et al. Active species of CeAl4 in the CeCl3-doped sodium aluminium hydride and its enhancement on reversible hydrogen storage performance. Chem Commun 2009;44:6857e9. [4] Zhang L, Xiao X, Fan X, Li S, Ge H, Wang Q, et al. Fast hydrogen release from NaBH4 destabilized by fluorographite under moderate conditions. RSC Adv 2014;4:2550e6. [5] Chen P, Xiong ZT, Luo JZ, Lin JY, Tan KL. Interaction of hydrogen with metal nitrides and imides. Nature 2002;420:302e4. [6] Mao JF, Yu XB, Guo ZP, Poh CK, Liu HK, Wu Z, et al. Improvement of the LiAlH4-NaBH4 system for reversible hydrogen storage. J Phys Chem C 2009;113:10813e8. on A, Zabara O, Fichtner M. [7] Chłopek K, Frommen C, Le Synthesis and properties of magnesium tetrahydroborate, Mg(BH4)2. J Mater Chem 2007;17:3496e503. [8] Chong L, Zou J, Zengab X, Dingab W. Reversible hydrogen sorption in NaBH4 at lower temperatures. J Mater Chem A 2013;1:13510e23. [9] Vajo JJ, Skeith SL, Mertens F. Reversible storage of hydrogen in destabilized LiBH4. J Phys Chem B 2005;109:3719e22. [10] Yu X, Grant D, Walker G. Low-temperature dehydrogenation of LiBH4 through destabilization with TiO2. J Phys Chem C 2008;112:11059e62. [11] Orimo S, Nakamoriy Y, Kitahara G, Miwa K, Ohba N, Towata S, et al. Dehydriding and rehydriding reactions of LiBH4. J Alloys Compd 2005;404:427e30. [12] Konoplev V, Bakulina V. Some properties of magnesium borohydride. Russ Chem Bull 1971;20:136e8. [13] Nakamori Y, Miwa K, Ninomiya A, Li H, Ohba N, Towata S, et al. Correlation between thermodynamical stabilities of metal borohydrides and cation electronegativites: firstprinciples calculations and experiments. Phys Rev B 2006;74:045126. [14] Matsunaga T, Buchter F, Mauron P, Bielman M, Nakamori Y, Orimo S, et al. Hydrogen storage properties of Mg[BH4]2. J Alloys Compd 2008;459:583e8. [15] Soloveichik GL, Gao Y, Rijssenbeek J, Andrus M, Kniajanski S, Bowman RC, et al. Magnesium borohydride as a hydrogen storage material: properties and dehydrogenation pathway of unsolvated Mg(BH4)2. Int J Hydrogen Energy 2009;34:916e28. [16] Li H-W, Kikuchi K, Nakamori Y, Miwa K, Towata S, Orimo S. Effects of ball milling and additives on dehydriding behaviors of well-crystallized Mg(BH4)2. Scr Mater 2007;57:679e82. [17] Bardajı´ EG, Hanada N, Zabara O, Fichtner M. Effect of several metal chlorides on the thermal decomposition behaviour of a-Mg(BH4)2. Int J Hydrogen Energy 2011;36:12313e8.
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
10
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 x x x ( 2 0 1 5 ) 1 e1 0
[18] Al-Kukhun A, Hwang HT, Varma A. NbF5 additive improves hydrogen release from magnesium borohydride. Int J Hydrogen Energy 2012;37:17671e7. [19] Shi B, Song Y. Influence of transition metals Fe, Ni, and Nb on dehydrogenation characteristics of Mg(BH4)2: electronic structure mechanisms. Int J Hydrogen Energy 2013;38:6417e24. [20] Yu X, Guo Y, Sun D, Yang Z, Ranjbar A, Guo Z, et al. A combined hydrogen storage system of Mg(BH4)2-LiNH2 with favorable dehydrogenation. J Phys Chem C 2010;114:4733e7. [21] Yang Y, Gao M, Liu Y, Wang J, Gu J, Pan H, et al. Multi-hydride systems with enhanced hydrogen storage properties derived from Mg(BH4)2 and LiAlH4. Int J Hydrogen Energy 2012;37:10733e42. [22] Juahir N, Mustafa NS, Yap FAH, Ismail M. Study on the hydrogen storage properties and reaction mechanism of NaAlH4-Mg(BH4)2 (2:1) with and without TiF3 additive. Int J Hydrogen Energy 2015;40:7628e35. [23] Fichtner M, Zhao-Karger Z, Hu J, Roth A, Weidler P. The kinetic properties of Mg(BH4)2 infiltrated in activated carbon. Nanotechnology 2009;20:204029. [24] Wahab MA, Jia YA, Yang D, Zhao H, Yao X. Enhanced hydrogen desorption from Mg(BH4)2 by combining nanoconfinement and a Ni catalyst. J Mater Chem A 2013;1:3471e8. [25] Zhang L, Chen L, Xiao X, Chen Z, Wang S, Fan X, et al. Superior dehydrogenation performance of nanoscale lithium borohydride modified with fluorographite. Int J Hydrogen Energy 2014;39:896e904. [26] Nale A, Catti M, Bardajı´ EG, Fichtner M. On the decomposition of the 0.6LiBH4-0.4Mg(BH4)2 eutectic mixture for hydrogen storage. Int J Hydrogen Energy 2011;36:13676e82. [27] Bardajı´ EG, Zhao-Karger Z, Boucharat N, Nale A, van Setten MJ, Lohstroh W, et al. LiBH4-Mg(BH4)2: a physical mixture of metal borohydrides as hydrogen storage material. J Phys Chem C 2011;115:6095e101.
[28] Chen J, Zhang Y, Xiong Z, Wu G, Chu H, He T, et al. Enhanced hydrogen desorption from the Co-catalyzed LiBH4-Mg(BH4)2 eutectic composite. Int J Hydrogen Energy 2012;37:12425e31. [29] Zhao-Karger Z, Witter R, Bardajı´ EG, Wang D, Cossementc D, Fichtner M. Altered reaction pathways of eutectic LiBH4Mg(BH4)2 by nanoconfinement. J Mater Chem A 2013;1:3379e86. [30] Liu X, Peaslee D, Sheehan TP, Majzoub EH. Decomposition behavior of eutectic LiBH4-Mg(BH4)2 and its confinement effects in ordered nanoporous carbon. J Phys Chem C 2014;118:27265e71. [31] Javadian P, Jensen TR. Enhanced hydrogen reversibility of nanoconfined LiBH4-Mg(BH4)2. Int J Hydrogen Energy 2014;39:9871e6. [32] Panich A, Yudanov N. Phase transition, structure, and molecular dynamics in fluorographite C2F intercalated with acetonitrile. J Struct Chem 1991;32:220e3. [33] Kamarchik Jr P, Margrave JL. Poly (carbon monofluoride): a solid, layered fluorocarbon. Acc Chem Res 1978;11:296e300. [34] Hamwi A. Fluorine reactivity with graphite and fullerenes. Fluoride derivatives and some practical electrochemical applications. J Phys Chem Solids 1996;57:677e88. [35] Mouras S, Hamm A, Djurado D, Cousseins JC. Synthesis of first stage graphite intercalation compounds with fluorides. rale 1987;24:572e82. Rev Chim Mine [36] Lee YS, Cho TH, Lee BK, Rho JS, An KH, Lee YH. Surface properties of fluorinated single-walled carbon nanotubes. J Fluor Chem 2003;120:99e104. [37] Talapatraa A, Bandyopadhyaya SK, Sena P, Barata P, Mukherjeeb S, Mukherjeeb M. X-ray photoelectron spectroscopy studies of MgB2 for valence state of Mg. Phys C 2005;419:141e7. [38] Moulder JF, Stickle WF, Sool PE, Bomben KD. Handbook of Xray photoelectron spectroscopy. Eden Prairie: Perkin-Elmer; 1992.
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Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4 Liuting Zhang, Jiaguang Zheng, Lixin Chen, Xuezhang Xiao*, Teng Qin, Yiqun Jiang, Shouquan Li, Hongwei Ge, Qidong Wang State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
article info
abstract
Article history:
Mg(BH4)2 is considered as one of the most promising light metal complex hydrides because
Received 30 June 2015
of its high volumetric and gravimetric hydrogen capacities and world-wide abundance.
Received in revised form
However, its higher major desorption temperatures (above 300 C) and poor reaction ki-
23 August 2015
netics have to be improved for the practical application. Herein, Mg(BH4)2 was successfully
Accepted 24 August 2015
synthesized via wet-chemical technique and a significant enhancement in dehydrogena-
Available online xxx
tion performance of Mg(BH4)2 is achieved by the synergetic effect of fluorographite (FGi) and LiBH4. Under the effect of FGi, the hydrogen desorption of 6 Mg(BH4)2-4FGi composite
C in seconds. However, the hydrogen released from
Keywords:
could be completed below 170
Hydrogen storage
6 Mg(BH4)2-4FGi suffers from impurities of B2H6 and HF. More importantly, it is demon-
Mg(BH4)2
strated that almost all the B2H6 and HF impurities can be suppressed by synergetic
LiBH4
modifying Mg(BH4)2 with FGi and LiBH4. The 3 Mg(BH4)2e3LiBH4-4FGi composite exhibits a
Fluorographite
capacity over 8.0 wt% H2 and starts to release hydrogen at 125.7 C, which is 143.8 C and
Synergetic effect
254.3 C lower than that of pure Mg(BH4)2 and LiBH4, respectively. These significant improvements could be attributed to both the novel morphology that numerous nano-scale borohydride spots formed on the surface of FGi, and the formations of stable fluorides (MgF2 and LiF) from the interaction between borohydrides and FGi. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen, which has high energy density, almost zero emission and promising renewable nature, is a potential energy carrier especially in proton exchange membrane fuel cells (PEMFCs). Nevertheless, reversible hydrogen storage with high capacity still hinds the practical application of
hydrogen energy. Because of the high gravimetric, volumetric hydrogen capacities and abundant resouses, solidstate hydrogen storage materials especially light-metal complex hydrides have received significant attention in the past ten years [1e8]. Among these materials, LiBH4 is competitive and widely investigated due to its large theoretical hydrogen capacity (13.8 wt%) and volumetric
* Corresponding author. Tel./fax: þ86 571 8795 1876. E-mail address: [email protected] (X. Xiao). http://dx.doi.org/10.1016/j.ijhydene.2015.08.090 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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hydrogen density (121 kg/m3) [1,9,10]. However, the main evolution of hydrogen starts at 380 C, and the reaction products (LiH and B) can only be rehydrogenated at high hydrogen pressure of 35 MPa and high temperature of 600 C hinds the practical application of LiBH4 [11]. Compared with LiBH4, Mg(BH4)2 has larger practical hydrogen capacity (14.8 wt%) and comparatively low onset dehydrogenation temperature [12]. Mg(BH4)2 was reported to start releasing hydrogen at 270 C and to reach its hydrogen desorption peak temperature at about 350 C [13e15]. Unfortunately, the dehydrogenation of Mg(BH4)2 still needs to be greatly improved for practical hydrogen storage applications due to its high desorption temperatures and poor kinetics. In order to improve the dehydrogenation property of Mg(BH4)2, ball milling Mg(BH4)2 with additives has been demonstrated to be an effect way [16e19]. For example, Li et al. found that Mg(BH4)2 ball milled with Ti-based additives including TiO2, TiH2 and Ti can start to release hydrogen at 227 C [16]. Transition metal halides were well known to be effective in destablizing borohydrides and by adding Nb or Ti chlorides, the onset temperature of Mg(BH4)2 can be lowered by more than 100 C [17]. Moreover, Mg(BH4)2 with NbF5 begins to release hydrogen at 75 C and the maximum hydrogen yield obtained from Mg(BH4)2 in the presence of 15 wt% NbF5, is 3.7, 7.4, 10.0, 11.4 wt% for 150, 250, 300 and 350 C, respectively [18]. It was also reported that the hydrogen storage properties of Mg(BH4)2 can be effectively improved by combining with other complex hydrides to form a reactive composite [20e22]. Yu et al. found that the onset temperature of dehydrogenation of the Mg(BH4)2eLiNH2 composite decreased from 270 C to 160 C but NH3 (poisonous for PEMFCs) formed in the decomposition process [20]. Recently, Juahir et al. [22] found that the TiF3 doped NaAlH4eMg(BH4)2 (2:1) composite could start to release hydrogen at about 75 C, 60 C lower than that of undoped one. So far, such a hydrogen desorption temperature is the best achievement by modifying Mg(BH4)2 with complex hydrides, but the terminal dehydrogenation temperature (>400 C) is still too high for practical applications. Nanoconfinement is another widely used way to improve the hydrogen storage properties of Mg(BH4)2. Recently, Fichtner et al. achieved a very significant decrease in hydrogen desorption temperature (the onset temperature was about 170 C) by confining Mg(BH4)2 into disordered activated carbon [23]. Later, Wahab et al. studied the dehydrogenation performance of Mg(BH4)2 confined in highly ordered CMK3 and the hydrogen desorption of the Mg(BH4)2-CMK3 started at 155 C [24]. Moreover, the hydrogen desorption was further decreased by incorporating Ni nanoparticles into the Mg(BH4)2-CMK3 system. However, the actual amount of Mg(BH4)2 confined to the templete was so limited, leading to a large decrease in practical hydrogen storage capacity. Our previous study shows that FGi has a remarkable effect on both the dehydrogenation temperature and kinetics of LiBH4 and NaBH4 [4,25]. So far as we know, the effect of FGi on the hydrogen desorption property of Mg(BH4)2 is still unreported. In addition, Nale et al. found that LiBH4eMg(BH4)2 eutectic mixture was shown to release hydrogen at 220 C, which is lower compared to the hydrogen desorption temperature of both LiBH4 and Mg(BH4)2 [26]. Inspired by above investigations, we hypothesize that a further significant
decrease of desorption temperature and reasonable high gravimetric hydrogen capacity can be realized if Mg(BH4)2 can be synergistically modified by means of FGi combining with LiBH4. In this study, we first report the hydrogen desorption performance of as-synthesized Mg(BH4)2-FGi composite. Because previous studies confirm that the desorption performance of Mg(BH4)2 can be improved by adding LiBH4 to form Mg(BH4)2eLiBH4 eutectic mixture [26e31], we also introduced LiBH4 into the Mg(BH4)2-FGi system, aiming to synergistically improve the dehydrogenation properties of Mg(BH4)2. In the Mg(BH4)2eLiBH4-FGi system, hydrogen starts to be released at a surprisingly lower temperature of 125.7 C with a hydrogen capacity over 8.0 wt%, which means the synergistic effects of FGi and LiBH4 can significantly enhance the hydrogen desorption performance of Mg(BH4)2, and makes it possible to use for mobile hydrogen storage applications.
Experimental details The starting materials were all stored and handled in an argon-filled glove box to prevent H2O and O2 contamination. LiBH4 (Alfa Aesar, 95%) and FGi (CarFluor Ltd, Grade II) were used as received. The fluorine content in FGi is 62 wt% and the chemical formula of fluorographite is written as CF [32,33]. The properties and crystal structure diagram of FGi has already been presented in our published paper [4]. Mg(BH4)2 was synthesized from MgCl2 (Aladdin, 99%) and NaBH4 (Aladdin, 98%) in anhydrous diethyl ether (AR, Sinopharm chemical Reagent Co. Ltd) as described in the previous paper [7]. The composites contained 30 wt%, 40 wt%, 50 wt% FGi were prepared by ball milling the mixture of Mg(BH4)2 and FGi, denoted as 7 Mg(BH4)2-3FGi, 6 Mg(BH4)2-4FGi and 5 Mg(BH4)25FGi, respectively. In addition, half amount of Mg(BH4)2 was replaced by LiBH4 in the 6 Mg(BH4)2-4FGi sample to prepare 3 Mg(BH4)2e3LiBH4-4FGi composite. The composites were all ball milled for 2 h (planetary QM-3SP4, Nanjing, China) under 0.1 MPa Argon pressure at a speed of 450 r/min. The ball-topower ratio was 40:1. A Sievert's type apparatus was applied to determine the dehydrogenation properties of the composites quantitatively using a volumetric method. In brief, about 150 mg of sample was loaded in a home-made reactor for each temperature programmed desorption (TPD) measurement. The heating rate was 5 C/min (from room temperature to 500 C) and the whole system was pumped to static vacuum before the measurement. To better compare the dehydrogenation properties, all the dehydrogenation capacity of measured composites are presented as weight percent (wt%) of the whole composite (i.e. borohydride plus FGi). In addition, Mass Spectrometry (MS) measurement was also carried out on a QIC-20 (HIDEN ANALYTICAL LTD) gas analysis system to detect the gas released from the composites (from room temperature to 500 C at a heating rate of 5 C/min under proactive Argon flow). X-ray diffraction analysis (XRD) was obtained from an X'Pert Pro X-ray diffractometer (PANalytical, Netherlands) using Cu Ka radiation at 40 kV and 40 mA. Fourier Transform Infrared Spectra (FTIR) was carried out on a Bruker Tensor 27
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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unit (transmission mode). Scanning Electron Microscope (SEM, Hitachi SU-70) was adopted to analyze the morphology of the composites with 3.0 kV e-beam energy. X-ray Photoelectron Spectroscopy (XPS) was applied to analyse the binding energy (BE) values, which were referenced to the C1s peak at 284.6 eV with an uncertainty of ±0.2 eV, on a VGESCALAB MARK II system with Mg Ka radiation (1253.6 eV) at a base pressure of 1 108 Torr. During the XPS, XRD, FTIR and SEM measurements, special caution was taken to reduce the time of samples exposed to air.
Results and discussion The microstructures of as-received FGi, as-synthesized Mg(BH4)2, 5 Mg(BH4)2-5FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)23FGi composites were characterised by XRD and FTIR, as shown in Fig. 1. It can be found in the XRD part of Fig. 1, the asreceived FGi displays two broaden peaks at 2q ¼ 14 corresponding to (001) reflection in compounds with a very high level of fluorine and 41 corresponding to (100) reflection of CeC in-plane length in the reticular system [34,35]. Also, a sharp peak at 2q ¼ 26 is attributed to graphite, indicating the incomplete fluorination of FGi. In Fig. 1(b), the XRD pattern of as-synthesized Mg(BH4)2 matches perfectively with bMg(BH4)2 reported by Chłopek et al., indicating the Mg(BH4)2 is successfully synthesized by wet chemical reaction [7]. After ball milling, the 5 Mg(BH4)2-5FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)2-3FGi composites still exhibit the obvious diffration
3
peaks corresponding to Mg(BH4)2 and FGi. Moreover, the signal of FGi becomes weaker with decreasing the amount of FGi from 50 to 30 wt%, while Mg(BH4)2 tends to be amorphous and/ or fine crystallite state by the effect of ball milling. To further explore the vibration of bands in these composites, FTIR measurement was carried out. It can be seen from the FTIR part in Fig. 1(a) that, the characteristic absorption of CeF covalent band appears at 1215 cm1. At the same time, the stretching vibration of CeF2 moieties located in the edges of segments at 1348 cm1 are present in the FGi [36]. Typical features of [BH4]1 group can be observed in the spectra of assynthesized Mg(BH4)2, the BeH absorption band is split into three contributions at 2386 cm1, 2291 cm1, and 2223 cm1. While the BeH bending vibration is split into two contributions at 1125 cm1 and 1267 cm1 [7]. For the as-prepared 5 Mg(BH4)2-5FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)2-3FGi composites, it can be seen from the FTIR part of Fig. 1(c), (d) and (e) that the intensity of CeF functional group in FGi also decreases with the decreasing amount of FGi gradually. Interestingly, the intensity of BeH absorption bands (2386 cm1 and 2223 cm1) in Mg(BH4)2 are distinctly disappeared, indicating that the Mg(BH4)2 may have some interaction with the surfaces of FGi and result in the change of the chemical atmosphere of BeH bands in Mg(BH4)2, which is similar with our previous investigations [25]. SEM examinations were applied to explore the microstructure feature of FGi, as-synthesized Mg(BH4)2, 5 Mg(BH4)25FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)2-3FGi composites, as shown in Fig. 2. It is obviously found in Fig. 2(a) that FGi is
Fig. 1 e XRD and FTIR profiles of (a) FGi, (b) Mg(BH4)2, (c) 5 Mg(BH4)2-5FGi, (d) 6 Mg(BH4)2-4FGi and (e) 7 Mg(BH4)2-3FGi composites. Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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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 x x x ( 2 0 1 5 ) 1 e1 0
Fig. 2 e SEM images of (a) FGi, (b) Mg(BH4)2, (c) 5 Mg(BH4)2-5FGi, (d) 6 Mg(BH4)2-4FGi and (e) 7 Mg(BH4)2-3FGi composites.
layered sheet structures. The particle size of as-synthesized Mg(BH4)2 is ranging from 0.1 to 1 mm in Fig. 2(b). After ball milling with FGi, it seems that the Mg(BH4)2 becomes flaky and covers the surfaces of FGi, while the layered sheet structure of FGi has been destroyed, as shown in Fig. 2(c), (d) and (e). According to XRD and FTIR analyses in Fig. 1, it can be deduced that the Mg(BH4)2 has some physical interactions with CeF2 and CeF functional groups, and then cover the surface of FGi as a result. Fig. 3 presents the TPD profiles of the as-synthesized Mg(BH4)2, 5 Mg(BH4)2-5FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)23FGi composites. It is noteworthy that FGi can significantly improve the dehydrogenation performance of Mg(BH4)2. For the as-synthesized Mg(BH4)2, no gas is released below 250 C. While for the 5 Mg(BH4)2-5FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)23FGi composites, 7.0 wt%, 8.6 wt% and 9.3 wt% gas can be released at 161.0 C, 169.8 C and 172.6 C, respectively.
Interestingly, the majority of the hydrogen is released in seconds and a sudden temperature rise is occurred at the same time. This phenomenon has never been reported in the hydrogen desorption of Mg(BH4)2. XRD and FTIR profiles of the dehydrogenated assynthesized Mg(BH4)2, 5 Mg(BH4)2-5FGi, 6 Mg(BH4)2-4FGi and 7 Mg(BH4)2-3FGi composites are also presented in Fig. 4. There is no signal of BeH band detected in the FTIR spectrum, indicating that all the composites are fully decomposed after dehydrogenation process. XRD results suggest that Mg(BH4)2 decomposed into Mg, MgB2 and amorphous B (can't be detected by XRD). Fig. 4(b), (c) and (d) show that MgF2 is the main phase in all three dehydrogenated Mg(BH4)2-FGi composites. Further more, it is reasonable to observe that the XRD patterns of Mg and MgB2 show up while that of C decreases with the increasing amount of Mg(BH4)2.
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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 x x x ( 2 0 1 5 ) 1 e1 0
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(a) (b) (c) (d)
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10 9 8 7 6 5
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Fig. 3 e TPD and corresponding Temperature profiles of (a) Mg(BH4)2, (b) 5 Mg(BH4)2-5FGi, (c) 6 Mg(BH4)2-4FGi and (d) 7 Mg(BH4)2-3FGi composites from room temperature to 500 C at heating rate of 5 C/min.
In order to shed light on the gas components released from the sample, MS measurement was carried out for the 6 Mg(BH4)2-4FGi composite, as shown in Fig. 5. It is clearly seen that the hydrogen desorption of the 6 Mg(BH4)2-4FGi composite can be classified as three stages corresponding to approximately 200, 310 and 360 C. Moreover, the impurities of B2H6 and HF are also released from the composite starting at the first stage. Therefore, it can be concluded that total mass loss is attributed to the releasing of impurities of B2H6 and HF together with H2. Combining the gas amount from MS together with the XRD and FTIR results, take 6 Mg(BH4)2-4FGi composite as an example, the dehydrogenation equation (1) can be written as follows:
22 Mg(BH4)2 þ 37 CF / x MgF2 þ y MgB2 þ z B2H6 þ (22 x y) Mg þ (44 2y 2z) B þ 37 C þ (37 2x) HF þ (69.5 þ x 3z) H2 (x ¼ 8~9, y ¼ 12.5~13.1 z ¼ 0.9~1.1) (1) Though the dehydrogenation temperature of Mg(BH4)2 could be largely decreased by modifying with FGi, the current dehydrogennation temperature is still to high for PEMFCs. Moreover, the B2H6 and HF impurities are also released along
5
with H2, which not only reduce the actual dehydrogenation capacity but also poison the PEMFCs. Therefore, an effective strategy that can not only decrease the hydrogen desorption temperature of Mg(BH4)2 but also suppress B2H6 impurity is of great significance and highly desirable. Recently, it has been proposed that mixed-metal borohydrides could have acceptable thermodynamics and some examples such as LiBH4eMg(BH4)2 mixture have already been reported [26e31]. By forming eutectic composition, the material starts to decompose in the liquid state just after melting, resulting in a relative low dehydrogenation temperature (220 C) compared to those of both LiBH4 and Mg(BH4)2 [26,27]. Thus, the 3 Mg(BH4)2e3LiBH4-4FGi composite was further prepared by ball milling 30 wt% Mg(BH4)2, 30 wt% LiBH4 and 40 wt% FGi for 2 h under 0.1 MPa Ar. For comparison, 5 Mg(BH4)2e5LiBH4 composite was also prepared under the same condition. Fig. 6 is the XRD and FTIR profiles of as-prepared 5 Mg(BH4)2e5LiBH4 and 3 Mg(BH4)2e3LiBH4-4FGi composites. XRD result shows that as-prepared 50 Mg(BH4)2e50LiBH4 is just a mixture of Mg(BH4)2 and LiBH4, agreeing well with previous studies [27]. While in the case of 3 Mg(BH4)2e3LiBH4-4FGi composite, both the diffraction intensities of Mg(BH4)2 and LiBH4 become weaker after ball milling with FGi. In addition, the FTIR profiles of 5 Mg(BH4)2e5LiBH4 and 3 Mg(BH4)2e3LiBH4-4FGi composites still show the typical features of [BH4]1 group, while only the CeF characteristic absorption band presents in the 3 Mg(BH4)2e3LiBH4-4FGi composites. This results indicate that some interaction may occur between borohydrides and FGi, resulting in the decreasing signal of Mg(BH4)2 and LiBH4 in XRD patterns and the disappearing signal of CeF2 vibration band in FTIR profiles. To further explore the morphology of 3 Mg(BH4)2e3LiBH44FGi composite, SEM measurements were also employed. It is clearly shown in Fig. 7 that the 3 Mg(BH4)2e3LiBH4-4FGi composite has a rough surface. Meanwhile, numerous nano-scale spots formed on the surface of FGi. According to FTIR analysis above and the unique morphology of 3 Mg(BH4)2e3LiBH4-4FGi, it can be concluded that the CeF2 and CeF functional groups in the surface of FGi are beneficial to catch Mg(BH4)2/LiBH4 mixture and reduce their grain sizes to nanoscale. A similar phenomenon was also found in our previous work [25], this unique surface morphology may improve the hydrogen storage properties of Mg(BH4)2, as far as we know. To testify the synergetic effect of FGi and LiBH4 for enhancing dehydrogenation properties of Mg(BH4)2, TPD and MS measurements were further carried out. Fig. 8 presents the TPD profiles and corresponding temperature variation of the as-synthesized Mg(BH4)2, 5 Mg(BH4)2e5LiBH4, 6 Mg(BH4)2-4FGi and 3 Mg(BH4)2e3LiBH4-4FGi composites. Fig. 8 (a) and (b) demonstrate that the onset dehydrogenation temperature of can be reduced by forming eutectic Mg(BH4)2 5 Mg(BH4)2e5LiBH4 composite [26e31]. As mentioned above, the dehydrogenation temperature of Mg(BH4)2 can be significantly decreased by modifying with FGi (Fig. 8(c)). In addition, it is noteworthy that the dehydrogenation performance of Mg(BH4)2 can be further improved by synergetic modifying with LiBH4 and FGi. The 3 Mg(BH4)2e3LiBH4-4FGi can obtain 8.1 wt% hydrogen at 125.7 C, which is 44.1, 122.2 and 143.8 C lower than that of 6 Mg(BH4)2-4FGi, 5 Mg(BH4)2e5LiBH4 and pure Mg(BH4)2, respectively. The obvious decrease of onset
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
6
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 x x x ( 2 0 1 5 ) 1 e1 0
Fig. 4 e XRD and IR profiles of (a) Mg(BH4)2, (b) 5 Mg(BH4)2-5FGi, (c) 6 Mg(BH4)2-4FGi and (d) 7 Mg(BH4)2-3FGi composites dehydrogenated at 500 C.
hydrogen desorption temperature is clearly shown in the inset picture of Fig. 8, which is a substantial improvement. Moreover, the gas components released from the 3 Mg(BH4)2e3LiBH4-4FGi were also confirmed by MS technique, as shown in Fig. 9. It is of interest that the dehydrogenation of 3 Mg(BH4)2e3LiBH4-4FGi becomes one step compared with three steps of 6 Mg(BH4)2-4FGi, indicating a
200
Ion Current/nA (normalized)
H2 B 2H 6 150
HF
100
50
0 50
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Temperature ( °C) Fig. 5 e MS profiles of 6 Mg(BH4)2-4FGi composite from room temperature to 500 C with a heating rate of 5 C/min.
change in dehydrogenation path when Mg(BH4)2 synergetic modified with FGi and LiBH4. Notably, almost all the HF and H2B6 impurities are suppressed during the dehydrogenation process. TPD and MS measurements show that the 3 Mg(BH4)2e3LiBH4-4FGi composite has achieved a relatively low onset dehydrogenation temperature, for comparison, the dehydrogenation properties of state-of-the-art Mg(BH4)2 modified systems are displayed in Table 1. It can be clearly seen in the table that the on-set dehydrogenation temperature 3 Mg(BH4)2e3LiBH4-4FGi composite is inferior to that of NaAlH4eMg(BH4)2 þ 5wt%TiF3, Mg(BH4)2 þ 15wt%NbF5, Mg(BH4)2e2LiAlH4 and Mg(BH4)2 þ 0.05TiCl3 þ 0.05NbCl5 systems, however, its terminal dehydrogenation temperature is superior to all the Mg(BH4)2 modified systems presented in Table 1. Moreover, the 3 Mg(BH4)2e3LiBH4-4FGi composite possesses the hydrogen desorption capacities of 8.1 wt% at the lower temperature of 160 C in seconds, which is the best achievement among all the modified Mg(BH4)2 systems, as far as we know. This result indicates that our strategy in term of not only further decreasing the desorption temperature of Mg(BH4)2 but also maintaining a reasonable hydrogen purity achieves great success. For understanding the dehydrogenation process of such 3 Mg(BH4)2e3LiBH4-4FGi composite, the solid residues of the dehydrogenated 3Mg(BH4)2e3LiBH4-4FGi were further collected for XRD, FTIR and XPS examinations (see Fig. 10). XRD and FTIR profiles suggest that the 3Mg(BH4)2e3LiBH44FGi composite is fully decomposed under the experimental conditions and the decomposition products are identified as
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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 x x x ( 2 0 1 5 ) 1 e1 0
7
Fig. 6 e XRD and IR profiles of (a) 5 Mg(BH4)2e5LiBH4 and (b) 3 Mg(BH4)2e3LiBH4-4FGi composites.
MgF2, MgB2, LiH, LiF and C. In addition, XPS profiles shows that the B1S peak can be fairly fitted by two different peaks. The boron alloying state at 187.7 ev should be assigned to MgB2, which is identical to Talapatraa's work [37]. The peak at lower energy of 187.1 ev belongs to elemental B [38], indicating the existence of amorphous B in the dehydrogenated products. Based on above experimental results, the dehydrogenation equation of the 3 Mg(BH4)2e3LiBH4-4FGi composite can be drawn as follows:
22 Mg(BH4)2 þ 79 LiBH4 þ 74 CF / x MgF2 þ (22 x) MgB2 þ (74 2x) LiF þ (2x þ 5) LiH þ 74 C þ (79 þ 2x) B þ (243.5 x) H2 (x ¼ 20~21)
(2)
Based on the above analyses, we try to figure out how LiBH4 and FGi worked together to synergetic modifying Mg(BH4)2. Fig. 2 has already revealed that after ball milling with FGi, the Mg(BH4)2 becomes flaky and covers the surfaces of FGi, the greatly reduced dehydrogenation was totally originated from
Fig. 7 e SEM images of (a) 3 Mg(BH4)2e3LiBH4-4FGi (£20 K), (b) 3 Mg(BH4)2e3LiBH4-4FGi (£50 K).
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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Fig. 8 e TPD and corresponding Temperature profiles of (a) Mg(BH4)2, (b) 5 Mg(BH4)2e5LiBH4, (c) 6 Mg(BH4)2-4FGi and (d) 3 Mg(BH4)2e3LiBH4-4FGi from room temperature to 500 C at heating rate of 5 C/min.
the reaction between FGi and Mg(BH4)2. While LiBH4 was also ball milled with Mg(BH4)2 and FGi, numerous nano-scale spots formed on the surface of FGi, which surely do favor to destabilize the whole system, thus reduce the dehydrogenation temperature. In addition, as the reaction is exothermic, the system releases heat when the reaction begins. Once the nano spots start to release gas at a low temperature, they can serve as lighters to kindle the remaining reaction just like a chain reaction and help to complete the whole dehydrogenation process. Furthermore, the formation of stable fluorides (MgF2 and LiF) during dehydrogenation process also contributes to the drastically enhanced dehydrogenation kinetics performance of 3 Mg(BH4)2e3LiBH4-4FGi composite. In order to explore reversible hydrogen storage performance of the 3 Mg(BH4)2e3LiBH4-4FGi composite, the rehydrogenation experiment of the dehydrided 3 Mg(BH4)2e3LiBH4-4FGi sample was carried out under 12 MPa of H2 at 400 C for 12 h. It is found that the 3 Mg(BH4)2e3LiBH44FGi exhibits partial reversibility, which rehydrogenated sample is composed of typical [BH4] and [B12H12] groups after rehydrogenation, as shown in Fig. S1. The rehydrogenated 3 Mg(BH4)2e3LiBH4-4FGi could release 4.3 wt% H2 in the second cycle, which is much high than that of 6 Mg(BH4)2-4FGi with about 1.2 wt% hydrogen H2, shown in Fig. S2. Although the reversible hydrogen storage of the as-prepared Mg(BH4)2eLiBH4-FGi composite is poor, the hydrogen desorption temperature is as low as ~125 C and a capacity over 8.0 wt% H2 is released from the composite at the first cycle. In a word, a synergetic thermodynamic and kinetic improvement can be achieved for initial hydrogen desorption in Mg(BH4)2 complex hydride.
Conclusions
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Fig. 9 e MS profiles of 3 Mg(BH4)2e3LiBH4-4FGi composite from room temperature to 500 C at heating rate of 5 C/ min.
In summary, it has been successfully demonstrated that the hydrogen desorption properties of Mg(BH4)2 can be significantly improved by synergetic modifying with FGi and LiBH4. XRD, FTIR, TPD and MS analyses indicate that the 6 Mg(BH4)24FGi composite starts to desorb hydrogen below 170 C, while the B2H6 and HF impurities also release along with H2. Notably, under the synergetic effect of FGi and LiBH4, almost all B2H6 and HF impurities could be suppressed during dehydrogenation in 3 Mg(BH4)2e3LiBH4-4FGi composite. More importantly, the 3 Mg(BH4)2e3LiBH4-4FGi begins to release hydrogen at 125.7 C with a capacity over 8.0 wt% H2. It is believed that the drastically improved dehydrogenation kinetics is ascribed to the greatly size reduced nanoscale borohydrides in 3 Mg(BH4)2e3LiBH4-4FGi system, as well as the formation of stable MgF2 and LiF fluorides during dehydrogenation process. In addition, the partial reversibility for hydrogen storage has also been proved in 3 Mg(BH4)2e3LiBH4-4FGi composite, but reversibility of the composite still needs to be improved due to the formation of incomplete B12H12 intermediate hydrides. Thus strategy of synergetic modifying with FGi and LiBH4 opens up a new way for hydrogen desorption kinetics and capacity improvement in Mg(BH4)2 system.
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
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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 x x x ( 2 0 1 5 ) 1 e1 0
Table 1 e Dehydrogenation properties of Mg(BH4)2 modified systems. Initial dehydrogenation capacity (wt%)a
On-set dehydrogenation temperature ( C)
Terminal dehydrogenation temperature ( C)
Ref.
Mg(BH4)2 þ 15wt%NbF5 NaAlH4eMg(BH4)2 þ 5wt%TiF3 Mg(BH4)2e2LiAlH4 Mg(BH4)2 þ 0.05TiCl3 þ 0.05NbCl5 3 Mg(BH4)2e3LiBH4-4FGi
8.5 8.0 11.0 ~ 8.1
75 75 120 125 125.7
370 500 370 360 160
Mg(BH4)2eLiNH2 LMBH/IRH33
7.2 2.2
160 220
300 380
[18] [22] [21] [17] Current work [20] [28]
System
a
The hydrogen storage capacity is given as weight percent (wt%) of the whole composite system.
Fig. 10 e (a) XRD, (b) IR and (c) XPS profiles of 3 Mg(BH4)2e3LiBH4-4FGi composite dehydrogenated at 500 C.
Acknowledgements The authors gratefully acknowledge the financial supports for this research from the National Natural Science Foundation of China (51571179, 51471151, 51171173), the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037), the Zhejiang Provincial Science & Technology Program of China (2015C31035, 2014C31134), and the Fundamental Research Funds for the Central Universities (2015QNA4010).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.08.090.
references
[1] Schlapbach L, Zu¨ttel A. Hydrogen-storage materials for mobile applications. Nature 2001;414:353e8.
[2] Bogdanovic B, Schwickardi M. Ti-doped alkali metal aluminium hydrides as potential novel reversible hydrogen storage materials. J Alloys Compd 1997;253:1e9. [3] Fan XL, Xiao XZ, Chen LX, Yu KR, Wu Z, Li SQ, et al. Active species of CeAl4 in the CeCl3-doped sodium aluminium hydride and its enhancement on reversible hydrogen storage performance. Chem Commun 2009;44:6857e9. [4] Zhang L, Xiao X, Fan X, Li S, Ge H, Wang Q, et al. Fast hydrogen release from NaBH4 destabilized by fluorographite under moderate conditions. RSC Adv 2014;4:2550e6. [5] Chen P, Xiong ZT, Luo JZ, Lin JY, Tan KL. Interaction of hydrogen with metal nitrides and imides. Nature 2002;420:302e4. [6] Mao JF, Yu XB, Guo ZP, Poh CK, Liu HK, Wu Z, et al. Improvement of the LiAlH4-NaBH4 system for reversible hydrogen storage. J Phys Chem C 2009;113:10813e8. on A, Zabara O, Fichtner M. [7] Chłopek K, Frommen C, Le Synthesis and properties of magnesium tetrahydroborate, Mg(BH4)2. J Mater Chem 2007;17:3496e503. [8] Chong L, Zou J, Zengab X, Dingab W. Reversible hydrogen sorption in NaBH4 at lower temperatures. J Mater Chem A 2013;1:13510e23. [9] Vajo JJ, Skeith SL, Mertens F. Reversible storage of hydrogen in destabilized LiBH4. J Phys Chem B 2005;109:3719e22. [10] Yu X, Grant D, Walker G. Low-temperature dehydrogenation of LiBH4 through destabilization with TiO2. J Phys Chem C 2008;112:11059e62. [11] Orimo S, Nakamoriy Y, Kitahara G, Miwa K, Ohba N, Towata S, et al. Dehydriding and rehydriding reactions of LiBH4. J Alloys Compd 2005;404:427e30. [12] Konoplev V, Bakulina V. Some properties of magnesium borohydride. Russ Chem Bull 1971;20:136e8. [13] Nakamori Y, Miwa K, Ninomiya A, Li H, Ohba N, Towata S, et al. Correlation between thermodynamical stabilities of metal borohydrides and cation electronegativites: firstprinciples calculations and experiments. Phys Rev B 2006;74:045126. [14] Matsunaga T, Buchter F, Mauron P, Bielman M, Nakamori Y, Orimo S, et al. Hydrogen storage properties of Mg[BH4]2. J Alloys Compd 2008;459:583e8. [15] Soloveichik GL, Gao Y, Rijssenbeek J, Andrus M, Kniajanski S, Bowman RC, et al. Magnesium borohydride as a hydrogen storage material: properties and dehydrogenation pathway of unsolvated Mg(BH4)2. Int J Hydrogen Energy 2009;34:916e28. [16] Li H-W, Kikuchi K, Nakamori Y, Miwa K, Towata S, Orimo S. Effects of ball milling and additives on dehydriding behaviors of well-crystallized Mg(BH4)2. Scr Mater 2007;57:679e82. [17] Bardajı´ EG, Hanada N, Zabara O, Fichtner M. Effect of several metal chlorides on the thermal decomposition behaviour of a-Mg(BH4)2. Int J Hydrogen Energy 2011;36:12313e8.
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090
10
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 x x x ( 2 0 1 5 ) 1 e1 0
[18] Al-Kukhun A, Hwang HT, Varma A. NbF5 additive improves hydrogen release from magnesium borohydride. Int J Hydrogen Energy 2012;37:17671e7. [19] Shi B, Song Y. Influence of transition metals Fe, Ni, and Nb on dehydrogenation characteristics of Mg(BH4)2: electronic structure mechanisms. Int J Hydrogen Energy 2013;38:6417e24. [20] Yu X, Guo Y, Sun D, Yang Z, Ranjbar A, Guo Z, et al. A combined hydrogen storage system of Mg(BH4)2-LiNH2 with favorable dehydrogenation. J Phys Chem C 2010;114:4733e7. [21] Yang Y, Gao M, Liu Y, Wang J, Gu J, Pan H, et al. Multi-hydride systems with enhanced hydrogen storage properties derived from Mg(BH4)2 and LiAlH4. Int J Hydrogen Energy 2012;37:10733e42. [22] Juahir N, Mustafa NS, Yap FAH, Ismail M. Study on the hydrogen storage properties and reaction mechanism of NaAlH4-Mg(BH4)2 (2:1) with and without TiF3 additive. Int J Hydrogen Energy 2015;40:7628e35. [23] Fichtner M, Zhao-Karger Z, Hu J, Roth A, Weidler P. The kinetic properties of Mg(BH4)2 infiltrated in activated carbon. Nanotechnology 2009;20:204029. [24] Wahab MA, Jia YA, Yang D, Zhao H, Yao X. Enhanced hydrogen desorption from Mg(BH4)2 by combining nanoconfinement and a Ni catalyst. J Mater Chem A 2013;1:3471e8. [25] Zhang L, Chen L, Xiao X, Chen Z, Wang S, Fan X, et al. Superior dehydrogenation performance of nanoscale lithium borohydride modified with fluorographite. Int J Hydrogen Energy 2014;39:896e904. [26] Nale A, Catti M, Bardajı´ EG, Fichtner M. On the decomposition of the 0.6LiBH4-0.4Mg(BH4)2 eutectic mixture for hydrogen storage. Int J Hydrogen Energy 2011;36:13676e82. [27] Bardajı´ EG, Zhao-Karger Z, Boucharat N, Nale A, van Setten MJ, Lohstroh W, et al. LiBH4-Mg(BH4)2: a physical mixture of metal borohydrides as hydrogen storage material. J Phys Chem C 2011;115:6095e101.
[28] Chen J, Zhang Y, Xiong Z, Wu G, Chu H, He T, et al. Enhanced hydrogen desorption from the Co-catalyzed LiBH4-Mg(BH4)2 eutectic composite. Int J Hydrogen Energy 2012;37:12425e31. [29] Zhao-Karger Z, Witter R, Bardajı´ EG, Wang D, Cossementc D, Fichtner M. Altered reaction pathways of eutectic LiBH4Mg(BH4)2 by nanoconfinement. J Mater Chem A 2013;1:3379e86. [30] Liu X, Peaslee D, Sheehan TP, Majzoub EH. Decomposition behavior of eutectic LiBH4-Mg(BH4)2 and its confinement effects in ordered nanoporous carbon. J Phys Chem C 2014;118:27265e71. [31] Javadian P, Jensen TR. Enhanced hydrogen reversibility of nanoconfined LiBH4-Mg(BH4)2. Int J Hydrogen Energy 2014;39:9871e6. [32] Panich A, Yudanov N. Phase transition, structure, and molecular dynamics in fluorographite C2F intercalated with acetonitrile. J Struct Chem 1991;32:220e3. [33] Kamarchik Jr P, Margrave JL. Poly (carbon monofluoride): a solid, layered fluorocarbon. Acc Chem Res 1978;11:296e300. [34] Hamwi A. Fluorine reactivity with graphite and fullerenes. Fluoride derivatives and some practical electrochemical applications. J Phys Chem Solids 1996;57:677e88. [35] Mouras S, Hamm A, Djurado D, Cousseins JC. Synthesis of first stage graphite intercalation compounds with fluorides. rale 1987;24:572e82. Rev Chim Mine [36] Lee YS, Cho TH, Lee BK, Rho JS, An KH, Lee YH. Surface properties of fluorinated single-walled carbon nanotubes. J Fluor Chem 2003;120:99e104. [37] Talapatraa A, Bandyopadhyaya SK, Sena P, Barata P, Mukherjeeb S, Mukherjeeb M. X-ray photoelectron spectroscopy studies of MgB2 for valence state of Mg. Phys C 2005;419:141e7. [38] Moulder JF, Stickle WF, Sool PE, Bomben KD. Handbook of Xray photoelectron spectroscopy. Eden Prairie: Perkin-Elmer; 1992.
Please cite this article in press as: Zhang L, et al., Remarkable enhancement in dehydrogenation properties of Mg(BH4)2 modified by the synergetic effect of fluorographite and LiBH4, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.090