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Improved dehydrogenation performance of LiBH4 by 3D hierarchical flower-like MoS2 spheres additives
Journal of Power Sources 300 (2015) 358e364
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Improved dehydrogenation performance of LiBH4 by 3D hierarchical flower-like MoS2 spheres additives Yan Zhao b, Yongchang Liu b, Huiqiao Liu b, Hongyan Kang b, Kangzhe Cao b, Qinghong Wang a, **, Chunling Zhang b, Yijing Wang b, Huatang Yuan b, Lifang Jiao b, * a School of Chemistry and Chemical Engineering, Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou, Jiangsu 221116, PR China b Key Laboratory of Advanced Energy Materials Chemistry (MOE), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3D hierarchical flower-like MoS2 spheres have been fabricated. The obtained product was consisted of few-layered MoS2 nanosheets. The obtained MoS2 was featured with high density of basal edges and active sites. LiBH4eMoS2 (as-prepared) mixture exhibited favorable dehydrogenation properties.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 March 2015 Received in revised form 20 September 2015 Accepted 23 September 2015 Available online 1 October 2015
In this work, 3D hierarchical flower-like MoS2 spheres are successfully fabricated via a hydrothermal method followed by a heat treatment. The obtained product is composed of few-layered MoS2 nanosheets with enlarged interlayer distance (ca. 0.66 nm) of the (002) plane. Meanwhile, the hydrogen storage properties of the as-prepared MoS2 ball milled with LiBH4 are systematically investigated. The results of temperature programmed desorption (TPD) and isothermal measurement suggest that the LiBH4eMoS2 (as-prepared) mixture exhibits favorable dehydrogenation properties in both lowering the hydrogen release temperature and improving kinetics of hydrogen release rate. LiBH4eMoS2 (as-prepared) sample (the preparation mass ratio is 1:1) starts to release hydrogen at 171 C, and roughly 5.6 wt % hydrogen is released within 1 h when isothermally heated to 320 C, which presents superior dehydrogenation performance compared to that of the bulk LiBH4. The excellent dehydrogenation performance of the LiBH4eMoS2 (as-prepared) mixture may be attributed to the high active site density and enlarged interlayer distance of the MoS2 nanosheets, 3D architectures and hierarchical structures. © 2015 Elsevier B.V. All rights reserved.
Keywords: LiBH4 MoS2 spheres Few-layered MoS2 nanosheets Hydrogen release
1. Introduction * Corresponding author. Tel.: þ86 22 23498089; fax: þ86 22 23502604. ** Corresponding author. Tel.: þ86 51683500063. E-mail addresses: [email protected] (Q. Wang), [email protected] (L. Jiao). http://dx.doi.org/10.1016/j.jpowsour.2015.09.088 0378-7753/© 2015 Elsevier B.V. All rights reserved.
To develop highly efficient hydrogen storage materials with favorable volumetric and gravimetric hydrogen densities is
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necessary for automobile fueling applications. Tremendous efforts have been devoted to the research in the past few decades [1e3]. Recently, complex metal hydrides of the light elements such as alanates ðAlH4 Þ [4e7], amides ðNH2 Þ [8,9], and borohydrides ðBH4 Þ [10e14] have been developed and investigated intensively as potential hydrogen storage materials. Among them, lithium borohydride (LiBH4) with a high theoretical storage capacity (18.5 wt%, 121 kg m3), has been widely accepted as a leading candidate for onboard applications. However, the practical applications are restricted due to its harsh thermodynamics, kinetics and reversibility. The pure LiBH4 starts to release hydrogen at approximately 400 C, and the re-hydrogenation reaction only takes place under high temperature and pressure conditions (600 C and 350 atm H2) [11,12]. On this account, various attempts have been carried out to improve the dehydrogenation and re-hydrogenation properties of LiBH4. Doping additives, such as transition metals [15e17], metal oxides [16,18], metal hydrides [19,20] and graphite [21], has been proposed to improve the thermodynamic and kinetic characteristics of LiBH4. Meanwhile, mechanical milling has been used by many researchers for reducing particle size and enlarging the concentration of reaction interfaces. Kyle Crosby et al. reported that the onset dehydrogenation temperature of LiBH4 was reduced to below 300 C after ball milling with MgH2 [17]. And according to Si and co-workers [22], 11 wt% of hydrogen was liberated at 260 C from the LiBH4/M-TNT (M ¼Ni, Fe) composite prepared by ball-milling. Recently, transition metal sulfides were investigated to improve the dehydrogenation and hydrogenation properties of LiBH4. In particular, molybdenum disulfide (MoS2) has been well studied as the catalyst due to its layered structure, high chemical stability, and excellent electrocatalytic properties [23]. Experimental measurements of Yan suggest that MoS2 nanoplate with high density of basal edges and abundant unsaturated active S atoms has a superior performance as catalyst in hydrogen evolution reaction (HER) [24]. According to Han's study [25], the onset dehydrogenation temperature of sample was reduced to 230 C after ball milling with commercial MoS2, which had a decrease of 80 C in contrast with that of bulk LiBH4. In addition, Wang et al. added commercial MoS2 to 2LiBH4eMgH2 system, finding that the onset dehydrogenation temperature was reduced to 206 C and the total dehydrogenation amount increased from 9.26 wt% to 10.47 wt% [26]. However, the performance of commercial MoS2 is still unsatisfactory when used as the additives to improve the dehydrogenation performance of LiBH4, suffering from the thicker sheets that tightly stacked together, and with few active sites on the surface. What's more, most of the reported MoS2 which used as the additives for hydrogen storage materials were still based on two-dimensional (2D) planar structures. Considering that the reaction activity of MoS2 may be closely linked with their morphology and size, the structure modification has been attracting great attention and need to be further studied [27,28]. Motivated by the above considerations, in this paper, 3D hierarchical flower-like MoS2 spheres were fabricated by a simple hydrothermal method and used as a specific additive for LiBH4. The as-prepared MoS2 was composed of ultrathin nanoplates with enlarged interlayer spacing of the (002) plane, and featured with certain amounts of active site and catalytic action. Furthermore, LiBH4 mixed with the as-prepared MoS2 by short-time ball milling exhibits an excellent dehydrogenation performance. The onset dehydrogenation temperature is about 171 C, with 5.6 wt% hydrogen released within 1 h when isothermally heated to 320 C. In addition, the possible dehydrogenation mechanism is proposed and discussed.
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2. Experiment details 2.1. Materials The source materials, namely, LiBH4 (95%) was purchased from Acros, (NH4)6Mo7O24$4H2O, CS(NH2)2 and bulk MoS2 were purchased from Feng Chuan technology company of Tianjin. 2.2. Synthesis of 3D hierarchical flower-like MoS2 spheres The MoS2 composite was synthesized via a hydrothermal method according to the previously published literature [29,30]. In a typical synthesis, 0.61 g (NH4)6Mo7O24$4H2O, 0.89 g and CS(NH2)2 were dissolved into 80 ml deionized water. After stirring for 30 min, the obtained clear solution was transferred into a 100 ml Teflon-lined stainless steel autoclave, then tightly sealed and maintained at 200 C for 24 h. After cooled to room temperature naturally, the resulting black precipitates were centrifuged, washed with deionized water and ethanol for several times, and then dried in a vacuum over at 60 C for 12 h, followed by further thermal treatment at 800 C in an Ar atmosphere for 2 h, the products were achieved. 2.3. Preparation of LiBH4-xMoS2/C mixtures The LiBH4-xMoS2 (x ¼ 0, 0.5, 1, 2, 3) mixtures were prepared by mechanically milled for 4 h (Planetary QM-1SP2) under argon atmosphere. The ball-to-power weight ratio is 40:1 and rotating rate is 450 rpm. For comparison, mixtures of LiBH4eMoS2 with a mass ratio of 1:1 were prepared by mechanically milled for different time (1 h, 2 h and 6 h) and hand mixing. In addition, the other composites: LiBH4eMoS2 (bulk) and LiBH4eMoS2 (as-prepared) with a mass ratio of 1:1 was also ball milled for 4 h, respectively. All samples were handled in an Ar-filled glove box (H2O < 1 ppm; O2 < 1 ppm). 2.4. Characterization The phase structure and morphology of the samples were characterized by X-ray diffraction (XRD, Rigaku D/Max-2500, Cu K, radiation) and scanning electron microscopy (SEM, Hitachi X-650). X-ray diffraction (XRD) patterns were collected at a scanning rate of 4 C min1. During the XRD measurements, all the composite samples were smeared on a glass slide in an Ar-filled glove box and then covered with plastic film to avoid any moisture or oxygen contact. Raman spectrum (Renishaw inVia, excitation 514.5 nm) were used to characterize the synthesized materials. Infrared spectra were collected via a FT-IR-650 spectrometer (Tianjin Gangdong) at a resolution of 4 cm1. Actually, the sample was air exposed for the FTIR and Raman spectra. X-ray photoelectron spectrometer (XPS, PHI 5000 Versaprobe, ULVAC PHI) was also used to characterize the dehydrogenated materials. The dehydrogenation behaviors of the samples were examined by using a homemade temperature programmed desorption (TPD) system at a ramping rate of 2 C min1 with the temperature range from 30 C to 600 C. Kinetics performances were further detected via a Sieverts-type isothermal measurement at different temperatures. 3. Results and discussions 3.1. Structural and morphology characterization The crystallographic structure of as-prepared MoS2 and bulk MoS2 composite were characterized by X-ray diffraction (XRD). As can be seen from Fig. 1a, the pattern of the as prepared MoS2 is in
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Fig. 1. (a) XRD patterns and (b) Raman spectra of the as-prepared MoS2 and bulk MoS2.
good agreement with a hexagonal structure (JCPDS No. 37e1492). The peaks located at (002), (004), (100), (103), (110) planes suggest that a pure phase of MoS2 has been obtained by a solvothermal method (Experimental details see ESIy). However, the (002) diffraction peak of the as prepared MoS2 is significantly weaker than that in the bulk sample, indicating the stack of the MoS2 single layers has been significantly inhibited [27e29]. Additionally, comparing with the sharp peaks of bulk MoS2, the as-prepared sample shows weak and broaden peaks, and the d-spacing corresponding to the peak (002) is 0.66 nm calculated according to the diffraction angles using the Bragg equation, implying the interplanar spacing in as-prepared MoS2 exhibits a slight increase to some degree [30]. Raman spectroscopy is quite sensitive to the thickness of layered material systems, and our results are consistent with the reported values [28,31]. Fig. 1b shows the comparative Raman spectrum of bulk and as-prepared MoS2. The two dominant peaks of bulk MoS2 at 380 and 406 cm1 correspond to the E12g and A1g modes of the hexagonal MoS2 crystal, respectively. The E12g mode involves the in-layer displacement of Mo and S atoms, whereas the A1g mode involves the out-of-layer symmetric displacements of S atoms along the c axis [32]. In comparison, for as-prepared MoS2, the slight decrease of the difference between Raman peak frequencies of E12g and A1g is particular significant for the decreasing number of MoS2 layers [33]. Additionally, comparing with the Raman spectrum of the bulk sample, the relatively larger peak width and weaker intensity of E12g peak of the as-prepared MoS2 indicate that the crystal structure of the MoS2 is not perfect and in-layer disorder or defects exist between the Mo and S atoms, and the lower intensity of E12g peak compared with A1g peak further reveals the basal-edge-rich feature of the few-layered MoS2 nanoplates. Accordingly, we also suspect that certain amount of “defect sites’’ may exist on the edge of the nanoplates, which in turn provide more active site to favor the dehydrogenation of LiBH4. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) provide insights into the morphology and detailed structure of the as-prepared MoS2 and bulk MoS2. As is shown in Fig. 2a, we can see that the building blocks of bulk MoS2 are large micrometer-sized scaled sheets, which are tightly stacked together. Differently, from the image in Fig. 2b, large numbers of uniform hierarchical 3D flower-like spheres are clearly observed. Meanwhile, the nanoflowers have a diameter in the range of 500e800 nm, and many of them are independent of each other. Furthermore, the hierarchical nanoflower consists of ultrathin
nanoplates with a thickness less than 10 nm. As previous reports [30,34], the formation mechanism of 3D hierarchical flower-like MoS2 spheres could be concluded like that: the ultrathin MoS2 nanosheets were firstly prepared by hydrothermal reaction between (NH4)6Mo7O24$4H2O and CS(NH2)2, then attached to each other by van der Waals interaction and finally self-assembled into the three-dimensional nanostructures (illustrated in Scheme 1). Furthermore, the TEM image in Fig. 2d shows that the asprepared MoS2 has a good flower-like spheres microstructure which exhibits rippled sheets morphology due to its ultrathin features, and also suggest the basal-edge-rich feature of the ultrathin MoS2 nanoplates, in consistent with SEM observation. In comparison, the bulk MoS2 (Fig. 2c) exhibits obvious thicker nanosheet structure tightly stacked together. Fig. 2e and f display the representative HRTEM images of the bulk MoS2 and asprepared MoS2. It can be clearly noted that the thickness of asprepared MoS2 is composed of fewer layers (3e6) with an interlayer separation of 0.65e0.66 nm (Fig. S1, ESIy), which is larger than the interlayer distance (0.62 nm) for the (002) plane of the bulk MoS2, matching well with the results of the XRD analysis. Thus, we speculate that ultrathin MoS2 nanoplates with basal-edge-rich feature may provide more active site to favor the dehydrogenation of LiBH4. 3.2. Hydrogen release performance The effects of the as-prepared MoS2 on the dehydrogenation performance of LiBH4 were systematically investigated. The TPD curves of pure LiBH4, LiBH4eMoS2 (bulk) and LiBH4eMoS2 (asprepared) are shown in Fig. 3a. It is clearly observed that both the onset dehydrogenation temperatures (Ton-set) and maximal dehydrogenation temperatures (T-max) are reduced obviously with the addition of the bulk or as-prepared MoS2. In the case of pure LiBH4, multiple desorption peaks are presented when heated up to 600 C. Meanwhile, the onset dehydrogenation temperature is around 290 C, close to its melting point, and the decomposition occurs at around 445 C. However, when compared with the multiple desorption peaks of pure LiBH4, other composites tend to be a unimodal pattern during dehydrogenation, particularly for LiBH4eMoS2 (as-prepared). Obviously, compared to the pure LiBH4, the maximal dehydrogenation temperatures of the LiBH4eMoS2 (bulk) and LiBH4eMoS2 (as-prepared) samples were reduced to 335 C and 287 C, respectively. Additionally, it is noteworthy that the LiBH4eMoS2 (as-prepared) sample showed the lowest onset
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Fig. 2. SEM images of (a) bulk MoS2, (b) as-prepared MoS2; TEM and HRTEM images of (c, e) bulk MoS2, (d, f) as-prepared MoS2.
Scheme 1. Schematic formation process of 3D hierarchical flower-like MoS2 spheres.
dehydrogenation temperature of about 171 C, which was 119 C and 88 C lower than that of the pure LiBH4 and LiBH4eMoS2 (bulk) samples, showing an effective effect on improving the dehydrogenation of LiBH4. Detailed information about the onset dehydrogenation temperature is revealed in Fig. 3b. Fig. 3c illustrates the total amount of hydrogen evaluated and dehydrogenation performance of the above samples. Compared to the pure LiBH4, the dehydrogenation capacity shows a gradual increase with those different additives added before 400 C. As to the LiBH4eMoS2 (as-prepared) sample, the hydrogen desorption capacity is 7.8 wt% when heated up to 600 C, and shows a superior
dehydrogenation performances from 200 C to 350 C. For the pure LiBH4 and LiBH4eMoS2 (bulk), the dehydrogenation capacities are 13.7 wt% and 6.2 wt%, respectively. As reported by Liu et al. [33,35], the decreased hydrogen capacity should be due to the doped MoS2 additives and the dehydrogenation product in samples. In order to further study the effect of the two different additives on the dehydrogenation properties of LiBH4, the isothermal hydrogen desorption measurements were conducted. Fig. 3d presents the kinetic behaviors of LiBH4eMoS2 (as-prepared), LiBH4eMoS2 (bulk) and pure LiBH4 samples at about 320 C, respectively. The kinetics of LiBH4 is greatly enhanced after doping
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Fig. 3. (a) TPD dehydrogenation curves, (b) onset dehydrogenation temperature and (c) the corresponding hydrogen weight loss of LiBH4eMoS2 (as-prepared), LiBH4eMoS2 (bulk) and pure LiBH4 samples, respectively, the milling time is 4 h and the mass ratio is 1:1. (d) Hydrogen release from the above samples at 320 C.
the two different additives, especially for the addition of asprepared MoS2. Dehydrogenation was completed within nearly 1 h, with 5.6 wt% hydrogen released. By contrast, the dehydrogenation capacities of LiBH4eMoS2 (bulk) and pure LiBH4 respectively are 2.57 wt% and 0.3 wt% when at 320 C in an hour. Thus, considering lower dehydrogenation temperature, higher hydrogen capacity and better kinetic behavior, the LiBH4eMoS2 (as-prepared) sample is chosen for the later research. In these regards, to make a confirmation of the best performance of LiBH4eMoS2 (as-prepared) sample, we research the different mass ratios of LiBH4 and MoS2 (as-prepared), and investigate the different ball-milling time on the mixture (Fig. S2 and S3, ESIy). The best results can be obtained from the LiBH4eMoS2 (asprepared) sample with the mass ratio of (1:1) and with ball-milling time at 4 h. The dehydrogenation performance also matches with the details in Fig. 3. To further analyze the dehydrogenation kinetic behaviors of the best sample, experiments of LiBH4eMoS2 (as-prepared) (1:1)-4 h dehydrogenated at 250 C, 300 C, 350 C and 400 C were employed respectively. As is shown in Fig. 4, only about 3.1 wt% of hydrogen is released in an hour when at about 250 C. With the dehydrogenation temperature increasing from 250 C to 400 C, the dehydrogenation rate becomes faster and the dehydrogenation capacity becomes higher. Approximately 6.6 wt% of hydrogen is released within 1 h at 400 C, and a dehydrogenation plateau is also obtained. The above results reveal that the as-prepared MoS2 demonstrates an effect on improving the dehydrogenation properties of LiBH4. In order to have a better understand of the reaction mechanism,
Fig. 4. Isothermal hydrogen desorption profiles of LiBH4eMoS2 (as-prepared)-(1:1)4 h at different temperatures.
the phase compositions of MoS2 (as-prepared) on the dehydrogenation process of LiBH4 at different temperatures were investigated by XRD patterns and FT-IR. As is shown in Fig. 5a, the phase products of the sample at different temperatures are clearly incarnated. When heating at 250 C, MoB2 and Li2S phases are detected as the main products, suggesting that LiBH4 has been
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Fig. 5. a: XRD patterns of dehydrogenated of LiBH4eMoS2(Prepared)/C(1:1) 4 h sample at 250 C, 300 C, 350 C, 400 C, and 600 C; b: FT-IR spectra of (a) pure LiBH4, and the dehydrogenated products of LiBH4eMoS2(Prepared)/C(1:1)-4 h composite at (b) 250 C, (c) 300 C, (d) 350 C, (e) 400 C.
decomposed and reacted with MoS2 (as-prepared) in some degree. As temperature raising, nearly no new phases are observed and the ultimate products are still contains MoB2 and Li2S. It should be noted that the similar reactions may be caused at different temperatures. According to the previous reports [22,25], the coexistence of MoB2 and Li2S could decreases the energy for the breaking
of ionic bond between B and H, thus catalyzing the dehydrogenation reaction and decreasing the dehydrogenation temperature. To our best knowledge, the elemental B should have an existence in the dehydrogenated products. However, the fact was that no boronelemental phases were detected by XRD, indicating that the element may be in an amorphous state, as previously reported
Fig. 6. XPS of dehydrogenated products of LiBH4eMoS2 (as-prepared)/C (1:1)-4 h sample at 250 C, 300 C, 350 C and 400 C.
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[23,26,35,37,38]. Meanwhile, the FT-IR results of the dehydrogenated LiBH4eMoS2 (as-prepared) at different temperatures are performed and shown in Fig. 5b, and the pure LiBH4 is also included for comparison. As is shown, typical BeH vibrations in LiBH4 exist at around 2319 and 1114 cm1 [36,39,41], indicating the emergence of LiBH4 in the dehydrogenated products. However, the absorbance intensities of characteristic peaks detected gradually become weak with increasing the dehydrogenation temperature, which may attribute to the decomposition of the LiBH4eMoS2 (as-prepared) mixture. XPS was used to gain insight into the chemical states of boron for further investigation. As is shown in Fig. 6a, two peaks are appeared corresponding to the B-1s level at the binding energies (BE) of 187.4 eV and 191.4 eV, indicating that B element in the samples exists in both elemental(B) and restored states (MoeB). It is calculated that the restored boron in MoeB may be attributed to the formation of MoB2 during the redox reaction occurs in LiBH4eMoS2 (as-prepared) mixture. Meanwhile, it is worth noting that the similar peaks with according binding energies are observed as temperature raising (Fig. 6bed), which is in greatly agreement with the results in Fig. 5 [40e43]. From above results, it can be inferred that during the hydrogen desorption process, following reactions may occur:
LiBH4 þ MoS2/MoB2 þ LiS2 þ H2 þ B
(1)
According to the previous reports [25,44], commercial MoS2 can noticeably improve the reversibility of LiBH4 and reduce its initial dehydrogenation temperature. Additionally, the preferable performance of LiBH4 doped with MoS2 is attributed to the formation of MoB2 and Li2S. Both MoB2 and Li2S act together to improve the reversibility and catalyze the decomposition of LiBH4. Our MoS2 contains more defects relative to commercial MoS2 and expected to improve the reversibility of LiBH4. 4. Conclusions In summary, we have successfully synthesized the 3D hierarchical flower-like MoS2 spheres via a hydrothermal method, and then prepared LiBH4eMoS2 (as-prepared) by ball-milling. Experimental results show that the dehydrogenation properties of LiBH4 are significantly improved after doping with the as-prepared MoS2. The onset dehydrogenation temperature for the LiBH4eMoS2 (asprepared) (1:1)-4 h sample is reduced to 171 C, and the maximal desorption peak occurs at about 287 C, much lower than that of pure LiBH4. Simultaneously, the mixture also exhibits superior hydrogen desorption kinetics, with 5.6 wt% of hydrogen liberated from LiBH4 at 320 C within 1 h. Furthermore, the desorption mechanism investigation of LiBH4eMoS2 (as-prepared) indicates that a redox reaction has taken place during the dehydrogenation process. MoB2 and Li2S phases as the main products always exist in the dehydrogenated products. The above results confirm that MoS2 (as-prepared) as additives milling with LiBH4 possesses an improved hydrogen desorption performance in decreasing dehydrogenation temperature and enhancing hydrogen release kinetics. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (51171083, 51571124), the Ministry of Education of China (IRT-13R30), the 111 Project (B12015) and Tianjin Research Program of Application Foundation and Advanced Technology (12JCYBJC10900).
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Improved dehydrogenation performance of LiBH4 by 3D hierarchical flower-like MoS2 spheres additives Yan Zhao b, Yongchang Liu b, Huiqiao Liu b, Hongyan Kang b, Kangzhe Cao b, Qinghong Wang a, **, Chunling Zhang b, Yijing Wang b, Huatang Yuan b, Lifang Jiao b, * a School of Chemistry and Chemical Engineering, Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou, Jiangsu 221116, PR China b Key Laboratory of Advanced Energy Materials Chemistry (MOE), Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3D hierarchical flower-like MoS2 spheres have been fabricated. The obtained product was consisted of few-layered MoS2 nanosheets. The obtained MoS2 was featured with high density of basal edges and active sites. LiBH4eMoS2 (as-prepared) mixture exhibited favorable dehydrogenation properties.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 March 2015 Received in revised form 20 September 2015 Accepted 23 September 2015 Available online 1 October 2015
In this work, 3D hierarchical flower-like MoS2 spheres are successfully fabricated via a hydrothermal method followed by a heat treatment. The obtained product is composed of few-layered MoS2 nanosheets with enlarged interlayer distance (ca. 0.66 nm) of the (002) plane. Meanwhile, the hydrogen storage properties of the as-prepared MoS2 ball milled with LiBH4 are systematically investigated. The results of temperature programmed desorption (TPD) and isothermal measurement suggest that the LiBH4eMoS2 (as-prepared) mixture exhibits favorable dehydrogenation properties in both lowering the hydrogen release temperature and improving kinetics of hydrogen release rate. LiBH4eMoS2 (as-prepared) sample (the preparation mass ratio is 1:1) starts to release hydrogen at 171 C, and roughly 5.6 wt % hydrogen is released within 1 h when isothermally heated to 320 C, which presents superior dehydrogenation performance compared to that of the bulk LiBH4. The excellent dehydrogenation performance of the LiBH4eMoS2 (as-prepared) mixture may be attributed to the high active site density and enlarged interlayer distance of the MoS2 nanosheets, 3D architectures and hierarchical structures. © 2015 Elsevier B.V. All rights reserved.
Keywords: LiBH4 MoS2 spheres Few-layered MoS2 nanosheets Hydrogen release
1. Introduction * Corresponding author. Tel.: þ86 22 23498089; fax: þ86 22 23502604. ** Corresponding author. Tel.: þ86 51683500063. E-mail addresses: [email protected] (Q. Wang), [email protected] (L. Jiao). http://dx.doi.org/10.1016/j.jpowsour.2015.09.088 0378-7753/© 2015 Elsevier B.V. All rights reserved.
To develop highly efficient hydrogen storage materials with favorable volumetric and gravimetric hydrogen densities is
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necessary for automobile fueling applications. Tremendous efforts have been devoted to the research in the past few decades [1e3]. Recently, complex metal hydrides of the light elements such as alanates ðAlH4 Þ [4e7], amides ðNH2 Þ [8,9], and borohydrides ðBH4 Þ [10e14] have been developed and investigated intensively as potential hydrogen storage materials. Among them, lithium borohydride (LiBH4) with a high theoretical storage capacity (18.5 wt%, 121 kg m3), has been widely accepted as a leading candidate for onboard applications. However, the practical applications are restricted due to its harsh thermodynamics, kinetics and reversibility. The pure LiBH4 starts to release hydrogen at approximately 400 C, and the re-hydrogenation reaction only takes place under high temperature and pressure conditions (600 C and 350 atm H2) [11,12]. On this account, various attempts have been carried out to improve the dehydrogenation and re-hydrogenation properties of LiBH4. Doping additives, such as transition metals [15e17], metal oxides [16,18], metal hydrides [19,20] and graphite [21], has been proposed to improve the thermodynamic and kinetic characteristics of LiBH4. Meanwhile, mechanical milling has been used by many researchers for reducing particle size and enlarging the concentration of reaction interfaces. Kyle Crosby et al. reported that the onset dehydrogenation temperature of LiBH4 was reduced to below 300 C after ball milling with MgH2 [17]. And according to Si and co-workers [22], 11 wt% of hydrogen was liberated at 260 C from the LiBH4/M-TNT (M ¼Ni, Fe) composite prepared by ball-milling. Recently, transition metal sulfides were investigated to improve the dehydrogenation and hydrogenation properties of LiBH4. In particular, molybdenum disulfide (MoS2) has been well studied as the catalyst due to its layered structure, high chemical stability, and excellent electrocatalytic properties [23]. Experimental measurements of Yan suggest that MoS2 nanoplate with high density of basal edges and abundant unsaturated active S atoms has a superior performance as catalyst in hydrogen evolution reaction (HER) [24]. According to Han's study [25], the onset dehydrogenation temperature of sample was reduced to 230 C after ball milling with commercial MoS2, which had a decrease of 80 C in contrast with that of bulk LiBH4. In addition, Wang et al. added commercial MoS2 to 2LiBH4eMgH2 system, finding that the onset dehydrogenation temperature was reduced to 206 C and the total dehydrogenation amount increased from 9.26 wt% to 10.47 wt% [26]. However, the performance of commercial MoS2 is still unsatisfactory when used as the additives to improve the dehydrogenation performance of LiBH4, suffering from the thicker sheets that tightly stacked together, and with few active sites on the surface. What's more, most of the reported MoS2 which used as the additives for hydrogen storage materials were still based on two-dimensional (2D) planar structures. Considering that the reaction activity of MoS2 may be closely linked with their morphology and size, the structure modification has been attracting great attention and need to be further studied [27,28]. Motivated by the above considerations, in this paper, 3D hierarchical flower-like MoS2 spheres were fabricated by a simple hydrothermal method and used as a specific additive for LiBH4. The as-prepared MoS2 was composed of ultrathin nanoplates with enlarged interlayer spacing of the (002) plane, and featured with certain amounts of active site and catalytic action. Furthermore, LiBH4 mixed with the as-prepared MoS2 by short-time ball milling exhibits an excellent dehydrogenation performance. The onset dehydrogenation temperature is about 171 C, with 5.6 wt% hydrogen released within 1 h when isothermally heated to 320 C. In addition, the possible dehydrogenation mechanism is proposed and discussed.
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2. Experiment details 2.1. Materials The source materials, namely, LiBH4 (95%) was purchased from Acros, (NH4)6Mo7O24$4H2O, CS(NH2)2 and bulk MoS2 were purchased from Feng Chuan technology company of Tianjin. 2.2. Synthesis of 3D hierarchical flower-like MoS2 spheres The MoS2 composite was synthesized via a hydrothermal method according to the previously published literature [29,30]. In a typical synthesis, 0.61 g (NH4)6Mo7O24$4H2O, 0.89 g and CS(NH2)2 were dissolved into 80 ml deionized water. After stirring for 30 min, the obtained clear solution was transferred into a 100 ml Teflon-lined stainless steel autoclave, then tightly sealed and maintained at 200 C for 24 h. After cooled to room temperature naturally, the resulting black precipitates were centrifuged, washed with deionized water and ethanol for several times, and then dried in a vacuum over at 60 C for 12 h, followed by further thermal treatment at 800 C in an Ar atmosphere for 2 h, the products were achieved. 2.3. Preparation of LiBH4-xMoS2/C mixtures The LiBH4-xMoS2 (x ¼ 0, 0.5, 1, 2, 3) mixtures were prepared by mechanically milled for 4 h (Planetary QM-1SP2) under argon atmosphere. The ball-to-power weight ratio is 40:1 and rotating rate is 450 rpm. For comparison, mixtures of LiBH4eMoS2 with a mass ratio of 1:1 were prepared by mechanically milled for different time (1 h, 2 h and 6 h) and hand mixing. In addition, the other composites: LiBH4eMoS2 (bulk) and LiBH4eMoS2 (as-prepared) with a mass ratio of 1:1 was also ball milled for 4 h, respectively. All samples were handled in an Ar-filled glove box (H2O < 1 ppm; O2 < 1 ppm). 2.4. Characterization The phase structure and morphology of the samples were characterized by X-ray diffraction (XRD, Rigaku D/Max-2500, Cu K, radiation) and scanning electron microscopy (SEM, Hitachi X-650). X-ray diffraction (XRD) patterns were collected at a scanning rate of 4 C min1. During the XRD measurements, all the composite samples were smeared on a glass slide in an Ar-filled glove box and then covered with plastic film to avoid any moisture or oxygen contact. Raman spectrum (Renishaw inVia, excitation 514.5 nm) were used to characterize the synthesized materials. Infrared spectra were collected via a FT-IR-650 spectrometer (Tianjin Gangdong) at a resolution of 4 cm1. Actually, the sample was air exposed for the FTIR and Raman spectra. X-ray photoelectron spectrometer (XPS, PHI 5000 Versaprobe, ULVAC PHI) was also used to characterize the dehydrogenated materials. The dehydrogenation behaviors of the samples were examined by using a homemade temperature programmed desorption (TPD) system at a ramping rate of 2 C min1 with the temperature range from 30 C to 600 C. Kinetics performances were further detected via a Sieverts-type isothermal measurement at different temperatures. 3. Results and discussions 3.1. Structural and morphology characterization The crystallographic structure of as-prepared MoS2 and bulk MoS2 composite were characterized by X-ray diffraction (XRD). As can be seen from Fig. 1a, the pattern of the as prepared MoS2 is in
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Fig. 1. (a) XRD patterns and (b) Raman spectra of the as-prepared MoS2 and bulk MoS2.
good agreement with a hexagonal structure (JCPDS No. 37e1492). The peaks located at (002), (004), (100), (103), (110) planes suggest that a pure phase of MoS2 has been obtained by a solvothermal method (Experimental details see ESIy). However, the (002) diffraction peak of the as prepared MoS2 is significantly weaker than that in the bulk sample, indicating the stack of the MoS2 single layers has been significantly inhibited [27e29]. Additionally, comparing with the sharp peaks of bulk MoS2, the as-prepared sample shows weak and broaden peaks, and the d-spacing corresponding to the peak (002) is 0.66 nm calculated according to the diffraction angles using the Bragg equation, implying the interplanar spacing in as-prepared MoS2 exhibits a slight increase to some degree [30]. Raman spectroscopy is quite sensitive to the thickness of layered material systems, and our results are consistent with the reported values [28,31]. Fig. 1b shows the comparative Raman spectrum of bulk and as-prepared MoS2. The two dominant peaks of bulk MoS2 at 380 and 406 cm1 correspond to the E12g and A1g modes of the hexagonal MoS2 crystal, respectively. The E12g mode involves the in-layer displacement of Mo and S atoms, whereas the A1g mode involves the out-of-layer symmetric displacements of S atoms along the c axis [32]. In comparison, for as-prepared MoS2, the slight decrease of the difference between Raman peak frequencies of E12g and A1g is particular significant for the decreasing number of MoS2 layers [33]. Additionally, comparing with the Raman spectrum of the bulk sample, the relatively larger peak width and weaker intensity of E12g peak of the as-prepared MoS2 indicate that the crystal structure of the MoS2 is not perfect and in-layer disorder or defects exist between the Mo and S atoms, and the lower intensity of E12g peak compared with A1g peak further reveals the basal-edge-rich feature of the few-layered MoS2 nanoplates. Accordingly, we also suspect that certain amount of “defect sites’’ may exist on the edge of the nanoplates, which in turn provide more active site to favor the dehydrogenation of LiBH4. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) provide insights into the morphology and detailed structure of the as-prepared MoS2 and bulk MoS2. As is shown in Fig. 2a, we can see that the building blocks of bulk MoS2 are large micrometer-sized scaled sheets, which are tightly stacked together. Differently, from the image in Fig. 2b, large numbers of uniform hierarchical 3D flower-like spheres are clearly observed. Meanwhile, the nanoflowers have a diameter in the range of 500e800 nm, and many of them are independent of each other. Furthermore, the hierarchical nanoflower consists of ultrathin
nanoplates with a thickness less than 10 nm. As previous reports [30,34], the formation mechanism of 3D hierarchical flower-like MoS2 spheres could be concluded like that: the ultrathin MoS2 nanosheets were firstly prepared by hydrothermal reaction between (NH4)6Mo7O24$4H2O and CS(NH2)2, then attached to each other by van der Waals interaction and finally self-assembled into the three-dimensional nanostructures (illustrated in Scheme 1). Furthermore, the TEM image in Fig. 2d shows that the asprepared MoS2 has a good flower-like spheres microstructure which exhibits rippled sheets morphology due to its ultrathin features, and also suggest the basal-edge-rich feature of the ultrathin MoS2 nanoplates, in consistent with SEM observation. In comparison, the bulk MoS2 (Fig. 2c) exhibits obvious thicker nanosheet structure tightly stacked together. Fig. 2e and f display the representative HRTEM images of the bulk MoS2 and asprepared MoS2. It can be clearly noted that the thickness of asprepared MoS2 is composed of fewer layers (3e6) with an interlayer separation of 0.65e0.66 nm (Fig. S1, ESIy), which is larger than the interlayer distance (0.62 nm) for the (002) plane of the bulk MoS2, matching well with the results of the XRD analysis. Thus, we speculate that ultrathin MoS2 nanoplates with basal-edge-rich feature may provide more active site to favor the dehydrogenation of LiBH4. 3.2. Hydrogen release performance The effects of the as-prepared MoS2 on the dehydrogenation performance of LiBH4 were systematically investigated. The TPD curves of pure LiBH4, LiBH4eMoS2 (bulk) and LiBH4eMoS2 (asprepared) are shown in Fig. 3a. It is clearly observed that both the onset dehydrogenation temperatures (Ton-set) and maximal dehydrogenation temperatures (T-max) are reduced obviously with the addition of the bulk or as-prepared MoS2. In the case of pure LiBH4, multiple desorption peaks are presented when heated up to 600 C. Meanwhile, the onset dehydrogenation temperature is around 290 C, close to its melting point, and the decomposition occurs at around 445 C. However, when compared with the multiple desorption peaks of pure LiBH4, other composites tend to be a unimodal pattern during dehydrogenation, particularly for LiBH4eMoS2 (as-prepared). Obviously, compared to the pure LiBH4, the maximal dehydrogenation temperatures of the LiBH4eMoS2 (bulk) and LiBH4eMoS2 (as-prepared) samples were reduced to 335 C and 287 C, respectively. Additionally, it is noteworthy that the LiBH4eMoS2 (as-prepared) sample showed the lowest onset
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Fig. 2. SEM images of (a) bulk MoS2, (b) as-prepared MoS2; TEM and HRTEM images of (c, e) bulk MoS2, (d, f) as-prepared MoS2.
Scheme 1. Schematic formation process of 3D hierarchical flower-like MoS2 spheres.
dehydrogenation temperature of about 171 C, which was 119 C and 88 C lower than that of the pure LiBH4 and LiBH4eMoS2 (bulk) samples, showing an effective effect on improving the dehydrogenation of LiBH4. Detailed information about the onset dehydrogenation temperature is revealed in Fig. 3b. Fig. 3c illustrates the total amount of hydrogen evaluated and dehydrogenation performance of the above samples. Compared to the pure LiBH4, the dehydrogenation capacity shows a gradual increase with those different additives added before 400 C. As to the LiBH4eMoS2 (as-prepared) sample, the hydrogen desorption capacity is 7.8 wt% when heated up to 600 C, and shows a superior
dehydrogenation performances from 200 C to 350 C. For the pure LiBH4 and LiBH4eMoS2 (bulk), the dehydrogenation capacities are 13.7 wt% and 6.2 wt%, respectively. As reported by Liu et al. [33,35], the decreased hydrogen capacity should be due to the doped MoS2 additives and the dehydrogenation product in samples. In order to further study the effect of the two different additives on the dehydrogenation properties of LiBH4, the isothermal hydrogen desorption measurements were conducted. Fig. 3d presents the kinetic behaviors of LiBH4eMoS2 (as-prepared), LiBH4eMoS2 (bulk) and pure LiBH4 samples at about 320 C, respectively. The kinetics of LiBH4 is greatly enhanced after doping
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Fig. 3. (a) TPD dehydrogenation curves, (b) onset dehydrogenation temperature and (c) the corresponding hydrogen weight loss of LiBH4eMoS2 (as-prepared), LiBH4eMoS2 (bulk) and pure LiBH4 samples, respectively, the milling time is 4 h and the mass ratio is 1:1. (d) Hydrogen release from the above samples at 320 C.
the two different additives, especially for the addition of asprepared MoS2. Dehydrogenation was completed within nearly 1 h, with 5.6 wt% hydrogen released. By contrast, the dehydrogenation capacities of LiBH4eMoS2 (bulk) and pure LiBH4 respectively are 2.57 wt% and 0.3 wt% when at 320 C in an hour. Thus, considering lower dehydrogenation temperature, higher hydrogen capacity and better kinetic behavior, the LiBH4eMoS2 (as-prepared) sample is chosen for the later research. In these regards, to make a confirmation of the best performance of LiBH4eMoS2 (as-prepared) sample, we research the different mass ratios of LiBH4 and MoS2 (as-prepared), and investigate the different ball-milling time on the mixture (Fig. S2 and S3, ESIy). The best results can be obtained from the LiBH4eMoS2 (asprepared) sample with the mass ratio of (1:1) and with ball-milling time at 4 h. The dehydrogenation performance also matches with the details in Fig. 3. To further analyze the dehydrogenation kinetic behaviors of the best sample, experiments of LiBH4eMoS2 (as-prepared) (1:1)-4 h dehydrogenated at 250 C, 300 C, 350 C and 400 C were employed respectively. As is shown in Fig. 4, only about 3.1 wt% of hydrogen is released in an hour when at about 250 C. With the dehydrogenation temperature increasing from 250 C to 400 C, the dehydrogenation rate becomes faster and the dehydrogenation capacity becomes higher. Approximately 6.6 wt% of hydrogen is released within 1 h at 400 C, and a dehydrogenation plateau is also obtained. The above results reveal that the as-prepared MoS2 demonstrates an effect on improving the dehydrogenation properties of LiBH4. In order to have a better understand of the reaction mechanism,
Fig. 4. Isothermal hydrogen desorption profiles of LiBH4eMoS2 (as-prepared)-(1:1)4 h at different temperatures.
the phase compositions of MoS2 (as-prepared) on the dehydrogenation process of LiBH4 at different temperatures were investigated by XRD patterns and FT-IR. As is shown in Fig. 5a, the phase products of the sample at different temperatures are clearly incarnated. When heating at 250 C, MoB2 and Li2S phases are detected as the main products, suggesting that LiBH4 has been
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Fig. 5. a: XRD patterns of dehydrogenated of LiBH4eMoS2(Prepared)/C(1:1) 4 h sample at 250 C, 300 C, 350 C, 400 C, and 600 C; b: FT-IR spectra of (a) pure LiBH4, and the dehydrogenated products of LiBH4eMoS2(Prepared)/C(1:1)-4 h composite at (b) 250 C, (c) 300 C, (d) 350 C, (e) 400 C.
decomposed and reacted with MoS2 (as-prepared) in some degree. As temperature raising, nearly no new phases are observed and the ultimate products are still contains MoB2 and Li2S. It should be noted that the similar reactions may be caused at different temperatures. According to the previous reports [22,25], the coexistence of MoB2 and Li2S could decreases the energy for the breaking
of ionic bond between B and H, thus catalyzing the dehydrogenation reaction and decreasing the dehydrogenation temperature. To our best knowledge, the elemental B should have an existence in the dehydrogenated products. However, the fact was that no boronelemental phases were detected by XRD, indicating that the element may be in an amorphous state, as previously reported
Fig. 6. XPS of dehydrogenated products of LiBH4eMoS2 (as-prepared)/C (1:1)-4 h sample at 250 C, 300 C, 350 C and 400 C.
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[23,26,35,37,38]. Meanwhile, the FT-IR results of the dehydrogenated LiBH4eMoS2 (as-prepared) at different temperatures are performed and shown in Fig. 5b, and the pure LiBH4 is also included for comparison. As is shown, typical BeH vibrations in LiBH4 exist at around 2319 and 1114 cm1 [36,39,41], indicating the emergence of LiBH4 in the dehydrogenated products. However, the absorbance intensities of characteristic peaks detected gradually become weak with increasing the dehydrogenation temperature, which may attribute to the decomposition of the LiBH4eMoS2 (as-prepared) mixture. XPS was used to gain insight into the chemical states of boron for further investigation. As is shown in Fig. 6a, two peaks are appeared corresponding to the B-1s level at the binding energies (BE) of 187.4 eV and 191.4 eV, indicating that B element in the samples exists in both elemental(B) and restored states (MoeB). It is calculated that the restored boron in MoeB may be attributed to the formation of MoB2 during the redox reaction occurs in LiBH4eMoS2 (as-prepared) mixture. Meanwhile, it is worth noting that the similar peaks with according binding energies are observed as temperature raising (Fig. 6bed), which is in greatly agreement with the results in Fig. 5 [40e43]. From above results, it can be inferred that during the hydrogen desorption process, following reactions may occur:
LiBH4 þ MoS2/MoB2 þ LiS2 þ H2 þ B
(1)
According to the previous reports [25,44], commercial MoS2 can noticeably improve the reversibility of LiBH4 and reduce its initial dehydrogenation temperature. Additionally, the preferable performance of LiBH4 doped with MoS2 is attributed to the formation of MoB2 and Li2S. Both MoB2 and Li2S act together to improve the reversibility and catalyze the decomposition of LiBH4. Our MoS2 contains more defects relative to commercial MoS2 and expected to improve the reversibility of LiBH4. 4. Conclusions In summary, we have successfully synthesized the 3D hierarchical flower-like MoS2 spheres via a hydrothermal method, and then prepared LiBH4eMoS2 (as-prepared) by ball-milling. Experimental results show that the dehydrogenation properties of LiBH4 are significantly improved after doping with the as-prepared MoS2. The onset dehydrogenation temperature for the LiBH4eMoS2 (asprepared) (1:1)-4 h sample is reduced to 171 C, and the maximal desorption peak occurs at about 287 C, much lower than that of pure LiBH4. Simultaneously, the mixture also exhibits superior hydrogen desorption kinetics, with 5.6 wt% of hydrogen liberated from LiBH4 at 320 C within 1 h. Furthermore, the desorption mechanism investigation of LiBH4eMoS2 (as-prepared) indicates that a redox reaction has taken place during the dehydrogenation process. MoB2 and Li2S phases as the main products always exist in the dehydrogenated products. The above results confirm that MoS2 (as-prepared) as additives milling with LiBH4 possesses an improved hydrogen desorption performance in decreasing dehydrogenation temperature and enhancing hydrogen release kinetics. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (51171083, 51571124), the Ministry of Education of China (IRT-13R30), the 111 Project (B12015) and Tianjin Research Program of Application Foundation and Advanced Technology (12JCYBJC10900).
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