A new strategy to remarkably improve the low-temperature reversible hydrogen desorption performances of LiBH4 by compositing with fluorographene

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A new strategy to remarkably improve the low-temperature reversible hydrogen desorption performances of LiBH4 by compositing with fluorographene Liuting Zhang a,d,1, Jiaguang Zheng a,b,1, Xuezhang Xiao a,b,*, Xuancheng Wang a,b, Xu Huang a,b, Meijia Liu a,b, Qidong Wang a,b, Lixin Chen a,b,c,* a

State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China c Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Hangzhou 310013, PR China d School of Energy and Power, Jiangsu University of Science and Technology, Zhenjiang 212003, PR China b

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abstract

Article history:

LiBH4 is a promising hydrogen storage material for its large capacity. However, high

Received 11 April 2017

desorption temperature, sluggish kinetics and demanding rehydrogenation severely hinder

Received in revised form

its practical use. Surface functional groups of graphene in many cases are treated as

3 May 2017

effective approaches to obtain some kinds of excellent properties of energy storage ma-

Accepted 9 May 2017

terials. In the current work, a new facile and effective strategy to improve the reversible

Available online xxx

hydrogen desorption properties of LiBH4 is proposed by composing with functionalized graphene to form the LiBH4efluorographene composite. The fluorographene (FG) nano-

Keywords:

sheets are successfully exfoliated from fluorographite (FGi) and composed with LiBH4. It is

Hydrogen storage materials

demonstrated that the FG can remarkably improve the hydrogen desorption thermody-

LiBH4

namics, kinetics and reversibility of LiBH4 via reactant destabilization method. An

Graphene

extremely fast hydrogen desorption process with a high capacity of 8.2 wt.% at 148.1
C is

Surface interaction

achieved in the LiBH4e50FG composite. Further research reveals that the enhancement

Modification effect

actually roots in the strengthened interfacial interaction between LiBH4 and exfoliated FG. Moreover, it is confirmed that the LiBH4e40FG composite exhibits a significantly enhanced reversible hydrogen desorption capacity of 7.2 wt.% and LiBH4 is regenerated. Such enhanced reversible hydrogen desorption properties are ascribed to the strengthened interfacial interactions between LiBH4 and FG with large surface, as well as the formation of LiHxF1x phase. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China. Fax: þ86 571 87951152. E-mail addresses: [email protected] (X. Xiao), [email protected] (L. Chen). 1 Liuting Zhang and Jiaguang Zheng contributed equally. http://dx.doi.org/10.1016/j.ijhydene.2017.05.060 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang L, et al., A new strategy to remarkably improve the low-temperature reversible hydrogen desorption performances of LiBH4 by compositing with fluorographene, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.05.060

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Introduction Since the exhaustion and pollution of fossil fuels on earth, exploring a clean and renewable alternative energy is of great importance. Hydrogen, which emits no photochemical smog into the atmosphere, is deemed as one of the most promising sustainable energy carriers [1]. However, reversible and large capacity storage of hydrogen is still challenging the scientists worldwide [2]. Solid-state metal hydrides are most likely to meet the requirement of safe and efficient on-board hydrogen storage [3e6]. Recently, light-metal complex hydrides have drawn extensive interests due to their high volumetric and gravimetric hydrogen capacities [7e12]. LiBH4 is a desirable choice among these light-metal complex hydrides, owing to its large hydrogen storage capacity (18.5 wt.% gravimetrically and 121 kg/m3 volumetrically) [13]. The dehydrogenation process of LiBH4 is divided into two steps, shown as follows:

LiBH4 / LiH þ B þ 1.5H2

(1)

LiH / Li þ 0.5H2

(2)

Unfortunately, its application into practical use is severely hindered due to its high decomposition temperature (above 400
C), sluggish kinetics and demanding rehydrogenation conditions (35 MPa H2 at 600
C) [13]. Several methods have been employed to modify the thermodynamics, kinetics and reversibility of LiBH4, for instance, reactant destabilization [14e20], partial cation/anion substitution [21e24] and catalyst doping [25e27]. Among the additives utilized for LiBH4, metal fluorides have been proved rather effective. Guo et al. [26] found that the onset dehydrogenation temperature of LiBH4 decreased to ca. 100
C after adding TiF3, and such 3LiBH4eTiF3 composite released 5.0 wt.% hydrogen at 250
C. This remarkable improvement in dehydrogenation behavior came from the reaction between LiBH4 and TiF3:

3LiBH4 þ TiF3 / 3LiF þ TiB2 þ B þ 6H2

(3)

This reaction was exothermic because of the formation of LiF, which strongly lowered the dehydrogenation reaction enthalpy of LiBH4. According to another research made by Wang et al. [28], comparative studies of TiF3 and TiCl3 as additive for LiBH4 had been carried out. It was also discovered that TiF3 had better destabilizing effect than TiCl3, owing to the different substitution position of functional halide anions. It was found that Cl tended to replace [BH4] and stabilized the hexagonal structure of LiBH4, while F took the position of H and broke the BeH bond, therefore achieved the destabilization of LiBH4. Theoretically, a study of Yin et al. [29] revealed that the decomposition reaction enthalpy reduced with the increasing number of F anion substituted in LiBH4. Carbon materials have also been applied to improve the hydrogen storage properties of LiBH4 [30e34]. It was reported by Fang et al. [35] that carbon materials such as graphite,

single-walled carbon nanotube and activated carbon could enhance the Hydrogen exchange kinetics and capacity of LiBH4. This effect was mainly attributed to the heterogeneous nucleation and micro-confinement effect of LiBH4. Compared to those most-used carbon materials, 2D-structured graphene was more preferable for the large surface area. Recently, according to Nale et al.'s work [36], the onset decomposition temperature of LiBH4 was lowered for 200
C after milling with reduced graphite oxide (RGO). Deep investigations verified that LiBH4 was well dispersed on the surface of RGO, which caused the enhancement in dehydrogenation behaviors. However, pristine carbon materials had their downsides due to their weak interactions with hydrides. Prospectively thinking, better modification effects might be achieved when the graphene surface was functionalized and able to react with hydrides. However, no research had been reported about using functionalized graphene derivatives for the amelioration of LiBH4. Enlightened by the researches above, here we report a significant improvement in the dehydrogenation thermodynamics and kinetics of LiBH4 modified by fluorographene (FG) in this work. Firstly, FG nanosheets were successfully produced through a liquid phase exfoliation method from fluorographite (FGi), and LiBH4eFG composites with different mass ratios were prepared by mechanical ball-milling. With the increasing amount of FG, the onset dehydrogenation temperature of LiBH4eFG composite is remarkably decreased and the capacity is accordingly increased. The LiBH4e50FG composite started to release hydrogen at a low temperature of 148.1
C with a capacity of 8.2 wt.% H2. Further experiments discovered the better reversibility in the LiBH4eFG system than that of LiBH4eFGi system. Moreover, the modification mechanism of FG has been investigated and discussed in detail.

Experimental details Synthesis of FG nanosheets FG nanosheets could be prepared using a liquid exfoliation method mentioned by Gong et al. [37], which was operated as follows: 1.5 g FGi (Shanghai Carflour Ltd, Grade II) was added into 500 mL NMP (Sinopharm Chemical Reagent Co. Ltd) in the flask. Then the solution was heated to 60
C and refluxed for 2 h. After cooling down to room temperature, the solution was ultrasonically treated for 16 h for liquid exfoliation. After the ultrasonic treatment, the solution was centrifuged under a speed of 8000 rpm, and washed 3 times with deionized water. At last, the precipitate was freeze-dried for 48 h to get FG sample.

Preparation of LiBH4eFG composites LiBH4 (Alfa Aesar, 95%) and reduced graphite oxide (RGO) (Nanjing XFnano Co. Ltd) were used without further treatment. LiBH4eFG composites containing 30 wt.%, 40 wt.%, 45 wt.% and 50 wt.% FG were designated as LiBH4e30FG, LiBH4e40FG, LiBH4e45FG and LiBH4e50FG, respectively. In comparison, composites incorporating 50 wt.% FGi and

Please cite this article in press as: Zhang L, et al., A new strategy to remarkably improve the low-temperature reversible hydrogen desorption performances of LiBH4 by compositing with fluorographene, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.05.060

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50 wt.% RGO were also prepared, denoted as LiBH4e50FGi and LiBH4e50RGO. All the composites were synthesized through mechanical ball-milling (planetary QM-3SP4, Nanjing, China), with the same rotating speed (400 rpm), ball-to-powder ratio (30:1) and milling time (2 h). Pure hydrogen of 2 MPa was charged into the milling jar to protect samples from air and self-decomposition during the milling process. Meanwhile, pristine LiBH4 was also milled under the same parameters, denoted as BM LiBH4.

Temperature programmed desorption (TPD) tests were taken to testify the cycling dehydrogenation properties of LiBH4 modified systems. TPD tests were carried out in a homemade with a Sievert's type apparatus. The operating temperature was heated from 20
C to 500
C at a rate of 5
C/min under the initial hydrogen pressure of 100 Pa.

Characterization

Morphology and structure of as-prepared samples

The composition, morphology and microstructure of the samples were examined by several measurements as follows: X-ray diffraction (XRD) tests were conducted on an X'Pert Pro X-ray diffractometer (PANalytical, Netherlands) using Cu Ka radiation at 40 kV and 40 mA. Fourier Transform Infrared Spectra (FTIR) tests were carried out on a Bruker Tensor 27 unit (transmission mode). Scanning electron microscopy (SEM) observations were taken on a Hitachi SU-70 apparatus. Transmission electron microscopy (TEM, Tecnai G2 F20 working at 200 kV) was also observed. During these measurements and transfer, the sample containers were filled with argon to exclude air and prevent oxidation.

FG nanosheets were prepared through liquid exfoliation from FGi. SEM and TEM observations were taken to look into the structural difference between FGi and FG, as shown in Fig. 1. In Fig. 1(a) and (b) as-received FGi exhibits a flat and smooth surface with thick multi-layered structure. However, TEM photos with different resolutions show clearly that asprepared FG sheets are large, transparent layers, and very few wrinkles are found on FG sheets, indicating a quite highquality surface. Edges of single-layered FG are visible in Fig. 1(d) and (e), which proves the successful preparation of few-layered FG nanosheets. Pictures of commercial graphene were also taken for comparison, shown in Fig. 1(f). It can be

Results and discussion

Fig. 1 e SEM images of (a,b) as-received FGi and TEM images of as-prepared FG (c, d and e) and commercial graphene (f) under different magnifications. Please cite this article in press as: Zhang L, et al., A new strategy to remarkably improve the low-temperature reversible hydrogen desorption performances of LiBH4 by compositing with fluorographene, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.05.060

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discovered from Fig. 1(f) that commercial graphene exhibits much more wrinkles than FG. Interestingly, comparing to graphene surface, FG exhibits numerous small dots on its surface in the TEM images, which may indicate CeF functional groups on FG. To further prove the existence of F on exfoliated FG surface, TEM images of FGi and FG are taken and EDS elemental mappings are made. The results are shown in Figs. S1 and S2. From these results we can clearly see F distributing uniformly on FGi and FG surface. Moreover, from EDS mapping results, FG exhibits a decreased amount of F comparing to FGi, which is due to the exfoliation and washing process. XRD characterizations were carried out to explore the structure of FG more specifically. Fig. 2 shows the XRD patterns of as-prepared FG and as-received FGi. Both FGi and FG exhibit a major peak at 14
, which refers to (002) interlayer reflection in substances highly fluoridized [38]. The sharp diffraction peak at 26
in both samples originates from graphite, indicating the impurity of commercialized FGi. The diffraction peaks at 41
and 77
correspond to the (100) and (110) reflections of CeC in-plane length in the reticular system, respectively [39]. Except for these peaks in common, there is a weak broad peak at 30
in patterns of FG, showing that FG is composed of graphene-like pieces stacking irregularly on different directions [40]. XRD results are in high consistence with TEM observations, demonstrating that functionalized FG nanosheets with fluorine were successfully exfoliated from FGi. LiBH4eFG composites were synthesized through ballmilling LiBH4 and as-prepared FG. LiBH4eFGi and LiBH4eRGO composites were also synthesized for comparison. To explore the effect of FG on the microstructure of LiBH4, XRD and FTIR characterizations were conducted, as shown in Fig. 3. It can be seen clearly that diffraction peaks of LiBH4 can still be detected in all samples, which means that LiBH4 remains stable during the compositing process. It is notable that in LiBH4e50FG composite the XRD diffraction peak at 41
becomes weaker than that of LiBH4e50FGi composite. Given that this peak represents the (100) CeC in-plane length in the

Fig. 2 e XRD patterns of (a) as-received FGi, (b) as-prepared FG.

Fig. 3 e XRD and FTIR profiles of (a) BM LiBH4, as-prepared (b) LiBH4e50FGi, (c) LiBH4e50RGO and (d) LiBH4e50FG composites. reticular system [39], the weakening of this peak (at 41
) indicates that the surface environment of FG changed, demonstrating that some interactions may take place between LiBH4 and FG. FTIR absorption peaks at 2226, 2293 and 2387 cm1 referring to BeH stretching vibration bonds and 1120 cm1 referring to BeH bending vibration bonds [13] further proved the existence of LiBH4 after ball-milling. And the 1215 and 1348 cm1 peaks belong to the bond vibrations of CeF and CeF2 bonds, according to previous researches [38,39]. A special 1190 cm1 peak is shown in the spectra of LiBH4e50FG and LiBH4e50FGi composites. This peak is representing LiBH4 bond vibrations on the basis of reported papers [41,42]. Its appearance after milling with FG and FGi has demonstrated the change of chemical environment in LiBH4. According to the FTIR profiles, after compositing with FG, the BeH stretching vibration peaks are obviously weakened. Meanwhile, the 1215 and 1348 cm1 peaks, which belong to the bond vibrations of CeF and CeF2 bonds, have also been weakened. However, in LiBH4eFGi composite this weakening of FTIR vibration peaks does not occur. Therefore, such conclusions can be drawn that when compositing with FG, LiBH4 particles cover the surface of few-layered FG more thoroughly. This LiBH4 coverage has strengthened the interfacial interactions between LiBH4 and FG surfaces. From the SEM images displayed in Fig. 4, the morphology of as-prepared composites are explored. In Fig. 4(a), it can be found that the pristine LiBH4 particles gathered and stuck together to some big chunks. After ball-milling with FGi, it can be seen from Fig. 4(b) that LiBH4 nanodots adhered to the surface of FGi, which agreed well with our previous report [43]. Since RGO has a lot of wrinkles, a “cabbage” structure of the LiBH4e50RGO composite can be formed after LiBH4 milling with RGO, shown in Fig. 4(c). LiBH4 is well-dispersed on the surface of RGO owing to the wrapping effect. According to our previous findings [43], LiBH4 particles will interact with CeF and CeF2 junctions on FGi and stick to its surface. Whereas FG nanosheets have much larger surface area and all the CeF functional groups on the surfaces can interact with LiBH4. During ball-milling, those LiBH4eFG pieces firstly formed and

Please cite this article in press as: Zhang L, et al., A new strategy to remarkably improve the low-temperature reversible hydrogen desorption performances of LiBH4 by compositing with fluorographene, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.05.060

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Fig. 4 e SEM images of as-received (a) BM LiBH4, (b) LiBH4e50FGi, (c) LiBH4e50RGO and (d) LiBH4e50FG composites.

then become a piled-up structure in Fig. 4(d). This special morphology may benefit the cohesion between LiBH4 and FG, thus may help improving dehydrogenation behavior of LiBH4.

more FG is introduced into the system, and the hydrogen desorption capacity is accordingly increased. In order to explain this trend more intuitively, a bar graph is drawn in Fig. 7. According to previous papers [25,43], the following

Dehydrogenation properties of LiBH4eFG composites Temperature programmed desorption (TPD) measurements were performed on BM LiBH4, LiBH4e50FGi, LiBH4e50FG and LiBH4e50RGO composites to study their dehydrogenation behaviors comparatively. About 150 mg composites were tested each time with a heating rate of 5
C/min from 20
C to 500
C. As the results shown in Fig. 5, BM LiBH4 can barely decompose before 280
C. After the introduction of graphene, the LiBH4e50RGO composite starts to release hydrogen at a lower temperature at about 200
C. Surprisingly, the onset temperature of hydrogen desorption for LiBH4e50FG reduces to 148.1
C, which is even ~150
C lower than that of BM LiBH4. Comparing to LiBH4e50FGi composite which starts to dehydrogenate at 195
C, LiBH4e50FG is also earlier to release hydrogen obviously. Besides, the hydrogen releasing process is so fast that it ends in seconds, indicating the remarkably fast dehydrogenation rate. Moreover, the total dehydrogenation capacity of LiBH4e50FG is 8.2 wt.%, which is larger than 7.2 wt.% of LiBH4e50FGi composite. These results demonstrate a significantly enhanced dehydrogenation behavior both thermodynamically and kinetically. It is proved that FG, with both functional CeF groups and graphene layers, is a great additive for LiBH4 modification. More detailed investigations about LiBH4eFG system were carried out. LiBH4eFG composites with different FG mass ratios were synthesized and investigated to study the effect of FG addition amount on dehydrogenation properties. TPD tests of LiBH4eFG composites below 200
C were carried out and shown in Fig. 6. As displayed in TPD results, the dehydrogenation temperature of LiBH4eFG composite decreases when

Fig. 5 e Dehydrogenation profiles of (a) ball-milled LiBH4, as-prepared (b) LiBH4e50RGO, (c) LiBH4e50FGi and (d) LiBH4e50FG composites from room temperature to 500
C.

Please cite this article in press as: Zhang L, et al., A new strategy to remarkably improve the low-temperature reversible hydrogen desorption performances of LiBH4 by compositing with fluorographene, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.05.060

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hydrogen at 163.8
C, indicating that just partial LiBH4 decomposes at low temperature when FG is insufficient. In addition, LiBH4e40FG has a much larger hydrogen desorption capacity of 7.2 wt.%, along with a lower onset temperature at 154.6
C. This dehydrogenation capacity has already caught up with that of the LiBH4e50FGi composite. When more FG is added, LiBH4e45FG and LiBH4e50FG composites are able to release more hydrogen (7.7 wt.% and 8.2 wt.%) at lower temperatures (149.8
C and 148.1
C), comparing with those of LiBH4e30FG and LiBH4e40FG. These results suggest that stronger thermodynamic modification with larger first dehydrogenation capacities can be achieved when more FG is introduced into LiBH4. However, taking reaction (4) into consideration, LiBH4 may partially dehydrogenate to stable LiF, releasing larger amount of H2 in the first dehydrogenation but sacrificing the reversible hydrogen capacity of LiBH4eFG system. Increased amount of FG may induce more formation of LiF, therefore further discussion about the suitable FG addition amount is presented in Fig. 7. It is shown in Fig. 7 that more than 40 wt.% of FG introduction will result in the measured dehydrogenation capacity higher than its “theoretical dehydrogenation capacity” (LiBH4 / LiH þ B þ 1.5H2), indicating the transition of products from LiH to LiF and partial sacrifice of the system's reversibility. Among all the LiBH4eFG composites, LiBH4e40FG has the measured dehydrogenation capacity mostly close to its theoretical capacity, manifesting a suitable FG addition amount to prevent too much LiF formation and maintain the reversible hydrogen capacity. Therefore, further cycling investigations are focused on LiBH4e40FG composite.

Fig. 6 e Dehydrogenation profiles of (a) LiBH4e30FG, (b) LiBH4e40FG, (c) LiBH4e45FG and (d) LiBH4e50FG composites from room temperature to 200
C.

reaction will take place during LiBH4's dehydrogenation process when F is added: FG

LiBH4 !LiF þ B þ 2H2

(4)

Worth noting, the “theoretical dehydrogenation capacity” in Fig. 7 refers to the hydrogen desorption capacity of the system when LiBH4 is totally decomposed to LiH, B and H2. It can be seen that LiBH4e30FG can only release 3.8 wt.%

Fig. 7 e Dehydrogenation temperatures and hydrogen capacities of (a) LiBH4e30FG, (b) LiBH4e40FG, (c) LiBH4e45FG and (d) LiBH4e50FG.

Cycling dehydrogenation behavior of LiBH4e40FG composite Since our apparatus was difficult to measure hydrogen absorption behaviors precisely under high temperature and pressure, cycling tests were carried out for the second hydrogen desorption measurements. LiBH4e50FGi composite is chosen as the reference composite because it also exhibits 7.2 wt.% first dehydrogenation capacity, which is close to LiBH4e40FG. Both composites were rehydrogenated at 500
C under hydrogen pressure of 10 MPa for 10 h to uptake hydrogen thoroughly. The second dehydrogenation results are displayed in Fig. 8. During the second dehydrogenation process, BM LiBH4 can hardly release hydrogen below 400
C and totally releases no more than 1 wt.% hydrogen. Rehydrogenated LiBH4e50FGi composite starts to release hydrogen at 275
C, and gives off totally 4.4 wt.% hydrogen after 200 min at 500
C. In the case of LiBH4e40FG composite, there is a small amount of hydrogen detected at the temperature lower than 200
C, while the major amount of hydrogen desorption begins at 270
C, and releases 7.1 wt.% hydrogen finally. Both LiBH4e50FGi and LiBH4e40FG composites exhibit much larger reversible hydrogen capacity and lower second-time hydrogen releasing temperature than BM LiBH4. Moreover, it should be noted that the reversible capacity of LiBH4e40FG is greatly improved by 60% comparing to that of LiBH4e50FGi. To further understand the reason why LiBH4e40FG composite exhibits much larger reversible hydrogen capacity, XRD and FTIR characterizations of dehydrogenated and rehydrogenated LiBH4e40FG composites are displayed in Fig. 9 to look

Please cite this article in press as: Zhang L, et al., A new strategy to remarkably improve the low-temperature reversible hydrogen desorption performances of LiBH4 by compositing with fluorographene, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.05.060

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into the hydrogen absorption process of LiBH4e40FG composite. LiBH4, as shown in the XRD results, is completely decomposed since only LiH and LiF (share the similar diffraction peaks) can be detected after the dehydrogenation process. After rehydrogenation, LiBH4 is regenerated but LiF still exists. From the dehydrogenated and rehydrogenated phases the dehydrogenation and rehydrogenation reaction of LiBH4eFG system can be written out as follows: FG

LiBH4 !xLiH þ ð1  xÞLiF þ B þ 2H2

ðx ¼ 0 to 1Þ

LiF

LiH þ B þ 1:5H2 ƒ!LiBH4

Fig. 8 e Second-time dehydrogenation profiles of the rehydrogenated (a) LiBH4e40FG, (b) LiBH4e50FGi and (c) BM LiBH4 composites from room temperature to 500
C.

(5) (6)

The dehydrogenation reaction enthalpy change of reaction (5) can be calculated. According to Zu¨ttel et al.'s work [13] and Barin's thermodynamic data of pure substances [44], the formation enthalpy of LiBH4 at 298.15 K is 194.44 kJ/mol, and the formation enthalpy of LiF and LiH at 298.15 K are given respectively as 616.93 kJ/mol and 90.63 kJ/mol. Considering formation enthalpies of elemental substances under standard conditions are zero, the reaction enthalpy change of reaction S1 can be calculated as (422.49 þ 526.3x) kJ/mol (x ¼ 0e1).

Fig. 9 e XRD and FTIR profile of (a) dehydrogenated and (b) rehydrogenated LiBH4e40FG composites. Please cite this article in press as: Zhang L, et al., A new strategy to remarkably improve the low-temperature reversible hydrogen desorption performances of LiBH4 by compositing with fluorographene, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.05.060

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Fig. 10 e Illustration on the reversible hydrogen storage mechanism of LiBH4eFG systems.

The existence of LiF in the rehydrogenated composite indicates that LiF is too stable to react with B and H2 to re-form LiBH4. But according to a previous report made by GosalawitUtke et al. [45], LiF would help LiH to uptake hydrogen and regenerate LiBH4. This is because LiF can combine with LiH to form a LiHxF1x phase, and this hydridofluoride phase LiHxF1x exhibits significantly improved dehydrogenation and rehydrogenation behaviors [46]. Therefore, it can be easily explained why LiH with LiF can uptake much more hydrogen and regenerate LiBH4. FTIR profiles also demonstrate that bond vibration peaks of [BH4] can be found in rehydrogenated composite [47], further proving the regeneration of LiBH4. Vibration peaks of [B12H12]2 can also be found, showing a small amount formation of Li2B12H12 [48]. Although partial hydrogen recycling capacity is lost due to the

formation of LiF and Li2B12H12, the reversible capacity is still 7.1 wt.%. In fact, the diffraction peak intensities of regenerated LiBH4 in LiBH4e40FG composite are much higher than those of LiBH4e50FGi in our previous report [43], demonstrating that more LiBH4 can be formed in rehydrogenation of LiBH4e40FG composite. According to the experimental results above, the effect of FG on LiBH4 can be described in two competitive aspects. On one hand, FG can destabilize the thermodynamic stability of LiBH4 and the formation of LiF could help LiH to regenerate LiBH4; On the other hand, too much LiF formation would lower the reversible hydrogen capacity of LiBH4. The less FG adds into the LiBH4eFG system, the less LiF would be formed, resulting in the better reversible hydrogen storage performance. So it is obvious that LiBH4e40FG composite has less F addition amount and larger reversible hydrogen capacity than LiBH4e50FGi. Moreover, along with the better reversibility, LiBH4e40FG composite can also exhibit a lower dehydrogenation temperature than that of LiBH4e50FGi. From the perspective of morphology, this superior hydrogen desorption and absorption performance is owing to the large surface area of FG, which enables more LiBH4 nanoparticles to adhere to FG surface, thus strengthens the interfacial interaction between FG and LiBH4 nanoparticles. A schematic diagram is drawn in Fig. 10 to reveal the reversible hydrogen storage mechanism in LiBH4eFG systems. During the first dehydrogenation process, LiBH4 releases hydrogen and turns into LiHxF1x þ B phase, and this phase exhibits large hydrogen absorption capacity. After hydrogen uptake, LiBH4 is regenerated. Among the hydrogen cycling process, LiF seems like an unreactable component, but together with LiH, the LiHxF1x phase can absorb hydrogen to be nearly fully reversible. In this way, LiF can be described as some sort of “catalyst” in LiBH4 dehydrogenation and rehydrogenation. Some comparisons about LiBH4 modified by carbon materials and fluorides are made and listed in Table 1. Among these

Table 1 e Dehydrogenation (DE)/rehydrogenation (RE) properties of LiBH4 modified systems. System LiBH4e30SWNTs LiBH4e30AC LiBH4e20wt.%SiO2e30wt.%TiF3 LiBH4e40FG LiBH4e50FGi LiBH4 þ 0.1ZnF2 LiBH4e30G LiBH4 þ 0.1TiCl3 3LiBH4TiF3 LiBH4 þ HMg3La þ TiCl3 LiBH4@carbon LiBH4 þ 0.5ZnF2 LiBH4 þ C-aerogel (13 nm) Ni/LiBH4/C LiBH4 þ 0.5TiCl3 LiBH4 þ SiO2 (25:75 mass%) LiBH4 þ 0.5TiF3 a

First-time capacitya (wt.%)

Onset DE temp. (
C)

Ending DE temp. (
C)

RE capacitya (wt.%)

Ref.

11 10.2 8.3 7.2 7.2 7 6.8 6 5.9 4.3 4.0 3.7 3.7 3.5 3 2.8 1.5

210 220 70 149.8 180 120 360 100 100 e 200 130 240 180 100 200 100

510 510 450 200 200 500 510 500 290 400 300 500 400 400 500 500 500

3.6 3.9 4 7.1 4.4 4 1.8 3.4 4.0 3.8 e 1 2.6 2.3 Irreversible e 0.2

[35] [35] [25] This work [43] [50] [35] [50] [26] [51] [52] [50] [53] [54] [50] [13] [50]

The hydrogen capacities are measured by the mass of whole system.

Please cite this article in press as: Zhang L, et al., A new strategy to remarkably improve the low-temperature reversible hydrogen desorption performances of LiBH4 by compositing with fluorographene, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.05.060

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 7 ) 1 e1 0

LiBH4ecarbon composites, LiBH4e40FG shows a lower onset dehydrogenation temperature and faster kinetics. LiBH4e30SWNTs and LiBH4e30AC composites cannot release hydrogen below 200
C and are not capable of finishing the dehydrogenation process before 500
C. Besides, the reversible hydrogen capacities of LiBH4e30SWNTs (3.6 wt.%) and LiBH4e30AC (3.9 wt.%) composites are much lower than that of LiBH4e40FG composite (7.1 wt.%). Although the onset dehydrogenation temperature and first-time hydrogen capacity of LiBH4e40FG composite is a little poorer than that of LiBH4e20wt.%SiO2e30wt.%TiF3 system, its ending temperature is far lower t. Comparing with LiBH4 þ 0.5ZnF2, LiBH4 þ 0.5TiCl3 and LiBH4 þ 0.5TiF3, LiBH4e40FG composite is also superior for its fast hydrogen release and large reversible capacity. In fact, LiBH4e40FG composite is able to release 7.2 wt.% hydrogen at 149.8
C along with the better cycling desorption properties, which has already been close to meet the requirement of high-temperature PEM fuel cells [49].

Conclusion In this paper, the surface functional group of activated grapheneefluorographene nanosheets have been successfully synthesized from fluorographite through a liquid exfoliation method, and were introduced to LiBH4 borohydride system. After ball-milling, LiBH4e50FG composite exhibits an extremely fast dehydrogenation at low temperature of 148.1
C, which is 46.9
C lower than that of LiBH4e50FGi composite. Further investigations prove that in LiBH4eFG system, the onset temperature decreases gradually and dehydrogenation capacity improves accordingly with increased FG addition. In addition, the formed LiF during dehydrogenation is discovered to have competitive effects. A LiHxF1x phase is formed by LiH and LiF, helping LiBH4 to regenerate while lowering the reversible hydrogen capacity of the system. The LiBH4e40FG composite has been confirmed to be nearly fully reversible with a second-time dehydrogenation capacity of 7.1 wt.%, 60% larger than that of LiBH4e50FGi. More importantly, these findings have revealed that using FG as additive can strengthen the surficial interaction between LiBH4 nanoparticles and FG, since FG has much larger surface area than FGi. Therefore, promising hydrogen desorption behavior and reversibility can be achieved with less F addition. This work is critically important for applying functionalized graphene derivatives to modify the reversible hydrogen storage thermodynamics and kinetics performances of complex hydride.

Acknowledgments The authors gratefully acknowledge the financial supports for this research from the National Natural Science Foundation of China (51571179 and 51671173), the Program for Innovative Research Team in University of Ministry of Education of China (IRT13037) and the Key Science and Technology Innovation Team of Zhejiang Province (2015C31035).

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Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.05.060.

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Please cite this article in press as: Zhang L, et al., A new strategy to remarkably improve the low-temperature reversible hydrogen desorption performances of LiBH4 by compositing with fluorographene, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.05.060