Hydrogen storage performance of lithium borohydride decorated activated hexagonal boron nitride nanocomposite for fuel cell applications

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 1 5 5 8 6 e1 5 5 9 6

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Hydrogen storage performance of lithium borohydride decorated activated hexagonal boron nitride nanocomposite for fuel cell applications R. Naresh Muthu a, S. Rajashabala a,*, R. Kannan b a b

School of Physics, Madurai Kamaraj University, Madurai 625021, Tamil Nadu, India Department of Physics, University College of Engineering, Anna University, Dindigul 624622, Tamil Nadu, India

article info

abstract

Article history:

A safe and cost effective material for hydrogen storage is indispensable for developing

Received 12 February 2017

hydrogen fuel cell technology to reach its greater heights. The present work deals with

Received in revised form

hydrogen storage performance of lithium borohydride decorated activated hexagonal

21 April 2017

boron nitride (LiBH4@Ah-BN) nanocomposite. where a facile chemical impregnation

Accepted 22 April 2017

method was adopted for the preparation of LiBH4@Ah-BN nanocomposite. The prepared

Available online 23 May 2017

nanocomposite was subjected to various characterization techniques such as X-ray Diffraction (XRD), Micro-Raman Spectroscopy, Fourier Transform Infrared Spectroscopy

Keywords:

(FTIR), Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy

Activated hexagonal boron nitride

(EDX), BrunauereEmmetteTeller (BET) Studies, CHNS-Elemental Analysis and Thermo

(Ah-BN)

Gravimetric Analysis (TGA). From BET studies, it is confirmed that, there is an enhance-

Lithium borohydride (LiBH4)

ment in the specific surface area of LiBH4@Ah-BN nanocomposite (122 m2/g) compared to

LiBH4@Ah-BN nanocomposite

Ah-BN (70 m2/g). The hydrogen storage ability was examined using a Sieverts-like hy-

Hydrogen storage

drogenation setup. An excellent hydrogen storage capacity of 2.3 wt% at 100
C was noticed for LiBH4@Ah-BN nanocomposite. The TGA study indicates the dehydrogenation profile of stored hydrogen in the range of 110e150
C. The binding energy of stored hydrogen (0.31 eV) lies in recommended range of US-DOE 2020 targets for fuel cell applications. The present investigation demonstrates the preparation of LiBH4@Ah-BN nanocomposite based hydrogen storage medium which has remarkable cycling stability and hydrogen storage capacity. Hence these desirable traits make LiBH4@Ah-BN nanocomposite as a potential hydrogen storage candidate for fuel cell applications in near future. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The world is facing ever growing appetite for renewable energy owing to shrinkage of fossil fuels and global warming. Being light weight, nontoxic and most abundant element in

the universe, Hydrogen is trusted as a clean and renewable energy carrier that guarantees zero-emission. However the main bottleneck of hydrogen, as energy of future, is the development of safe, compact and cost effective hydrogen storage medium. The way to accomplish is still remains as a steep uphill climbing [1e5].

* Corresponding author. E-mail address: [email protected] (S. Rajashabala). http://dx.doi.org/10.1016/j.ijhydene.2017.04.240 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Boron nitride (BN) is considered as a boon in the nanotechnology era. Having astonishing properties, h-BN material poses as a praiseworthy candidate in hydrogen storage, exclusion of dye/toxic metals, toxic gas sensor (pollutant adsorption in harsh environment), drug delivery, and bio sensor applications [6e8]. In recent years, h-BN is fascinated among scientific community towards hydrogen storage owing to its dipolar nature of BeN bonds and high surface-to-volume ratio. Moreover the B and N atoms in h-BN may attract more hydrogens at all possible adsorption sites and enhance the storage capacity [9e17]. Weng et al. [9] reported 2.3 wt% of hydrogen for BN porous microbelts at 77 K and 1 MPa. The hBN nanoparticles decorated multi-walled carbon nanotubes could store 2.3 wt% of hydrogen at 100
C [10]. Wang et al. [11] achieved a 2.6 wt% of storage capacity at 1 MPa in the case of h-BN powders. The multiwalled BNNT and bamboo-like BNNTs have a hydrogen uptake of 1.8 and 2.6 wt%, respectively at room temperature and 10 MPa [12]. Hydrogen storage capacity of sulfonated poly (ether-ether-ketone) - h-BN polymer nanocomposite was examined by Muthu et al. [13] and noticed 2.98 wt% of hydrogen at 150
C. The morphology of BN nano material plays a crucial role for the hydrogen storage. The bamboo-type BNNTs, straight-walled BNNTs and flower type BN were able to store 3.0, 2.7 and 2.5 wt% at around 100 bar, respectively [14]. The h-BN decorated acid treated halloysite clay nanotubes (A-HNT-h-BN) could store 2.88 wt% at 50
C [15]. The reduced graphene oxide/acid treated halloysite nanotubes/hexagonal boron nitride (RGO/A-HNT/hBN) hybrid nanocomposite had the ability to store 3.3 wt% of hydrogen at 50
C [16]. A highest hydrogen storage capacity of 5.7 wt% was reported in the case of oxygen doped BN nanosheets at room temperature and 5 MPa [17]. High-pressure and low temperature hydrogen storage systems are unrealistic for onboard applications due to safety concerns [18e20]. Complex hydrides such as alanates (AlH4), amides (NH2) and borohydrides (BH4) are successfully employed as lightweight and safe solid state hydrogen storage media [21,22]. Among these complex hydrides, lithium borohydride (LiBH4) has received significant attention over the other traditional storage materials due to its high gravimetric (18.5 wt%) and volumetric (121 kg/m3) hydrogen densities [23e25]. However LiBH4 is projected as a suitable candidate for onboard applications, it suffers from sensitivity air, high thermal stability (decomposes at about 400
C) and energetically expensive to reverse the reaction (reversibility requires high pressure) [26e28]. But there are ways to resolve the high dehydrogenation temperature, slow sorption kinetics and reversibility of LiBH4 [29e33]. Ren et al. [34] theoretically found that the highly porous natured LiBH4 (porosity of 50%) has 9.36 wt% of storage capacity at 77 K and 100 bar. Recently the destabilization behavior of LiBH4 was investigated by Gennari [35] by the addition of metal hydrides MH2 (M ¼ La, Ce) and found that the LiBH4eLaH2 composite exhibits improved hydrogen desorption performance than LiBH4eCeH2 composite. Mao et al. [36] reported that 6.7 wt % of hydrogen for LiBH4/Mg composite and less than 1 wt % of hydrogen for pure Mg could be stored at 250
C. The reduction of dehydrogenation temperature of LiBH4 can be achieved via by incorporating metals, metal halides, oxides, sulfides and hydrides. Wang et al. [37] reported the addition of Ce and S could reduce

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the dehydrogenation temperature of LiBH4-20 wt%Ce2S3 composite to 250
C, which is 80
C lower than that of the pristine LiBH4 system. Johnson et al. [38] studied the hydrogen desorption kinetics and storage performance of MgH2, ball milled MgH2 and MgH2/LiBH4. where the hydrogen desorption of MgH2/LiBH4 composite is faster than ball milled MgH2 and MgH2. A storage capacity of 6.01 wt%, 5.86 wt% and 4.86 wt% were noticed at 300
C and 10 bar in the case of MgH2/LiBH4, ball milled MgH2 and MgH2 respectively. Fan et al. [39] shown that the presence of Nb2O5 catalyst in LiBH4eMgH2 composite yields a destabilized and reversible hydrogen storage material with a capacity of approximately 6 - 8 wt% hydrogen releasing below 400
C. The reaction mechanisms of LiBH4 with hydrogenated Mg11CeNi alloy were also investigated [40]. In which LiBH4/Mg11CeNi hydride system undergoes a two step dehydrogenation reactions one at around 280
C and other in the temperature range of 350 - 425
C. Xia et al. [41] demonstrated the effect of destabilizer (MoCl3) in LiBH4eMgH2 composite and observed destabilized and reversible hydrogen storage medium with a capacity of around 7 wt% at 300
C. In the case of NaAlH4eMgH2eLiBH4eTiF3 composite, a 4.5 wt% of hydrogen storage capacity is noticed at 350
C [42]. Commercialization of hydrogen refueling stations to worldwide are not yet realized due to the hindrance of several factors such as high operating temperature/pressure, reversibility and slow kinetics of hydrogen storage materials. Moreover none of the prepared materials achieve the targets set by United States Department of Energy (US-DOE). At present the challenges ahead the scientific community is to develop a safe, cost effective and efficient hydrogen storage medium for fuel cell applications. The present work is aimed at the synthesis and characterization of LiBH4 decorated activated h-BN nanocomposite for fuel cell applications. The proposed non-carbon and light weight elements (B, Li and N) based hydrogen storage medium may offer large surface area, more adsorption sites, excellent storage capacity, good reversibility and fast kinetics [43e45]. During hydrogenation, the hydrogen can be absorbed at boron sites, nitrogen sites, BN bridge sites and Li sites. The presence of electron deficient character of boron atoms, electron rich character of nitrogen atoms and strong affinity of Liþ ions, enable to attract more hydrogen. Moreover the acid treated h-BN may create more defect sites (voids/pores) which in turn enhance the surface area of Ah-BN. Hence the hydrogen rich LiBH4, dipolar character of BeN atoms, Liþ ions and defect sites are believed to be the key factors for the enhancement of hydrogen storage capacity of proposed LiBH4 decorated Ah-BN nanocomposite [46e48].

Experimental methods Materials Lithium borohydride (LiBH4), hexagonal boron nitride (h-BN) nanoparticles were supplied by Sigma Aldrich and Sisco Research Laboratories, respectively. Anhydrous tetrahydrofuran (THF), nitric acid (HNO3), sulfuric acid (H2SO4) were received from Merck. All received chemicals were used without any further purification and double distilled water was used throughout this study.

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Preparation of activated h-BN (Ah-BN) Initially, the Ah-BN is prepared by adding activating reagent (1: 1 ratio of 1 M H2SO4 aqueous solution and 8 M HNO3 aqueous solution) with 1 g of h-BN nanoparticles [48]. Both were mixed well, followed by ultrasonication for 30 min at room temperature. The obtained slurry was stirred at 150
C in water bath for 2 h. The mixture was washed with distilled water for several times until the pH attains 7. Then the precipitation was dried in vacuum for overnight at 100
C. The resultant product was labeled as Ah-BN.

Preparation of LiBH4@Ah-BN nanocomposite The Ah-BN nanoparticles were dispersed in THF (5 mg/1 ml) and 10% LiBH4 was dissolved in THF (50 mg/1 ml) under magnetic stirring [22]. Then the LiBH4 solution was slowly added into Ah-BN solution and stirred for 12 h at room temperature. Further it was dried at 80
C for 1 h under vacuum in order to remove the traces of THF. The obtained product was grounded well using agate mortar and the resultant product was labeled as LiBH4@Ah-BN nanocomposite.

Hydrogenation The prepared LiBH4@Ah-BN nanocomposite based hydrogen storage medium was subjected to hydrogenation studies in order to verify its storage capacity for fuel cell applications. The hydrogenation study was carried out using Sieverts-like hydrogenation setup and the hydrogenation process is described as follows. Initially, the nanocomposite is loaded in the hydrogenation setup. The nanocomposite is maintained in a vacuum at 50
C and 100
C for 1 h, following which the hydrogen is allowed to pass at constant flow rate to the chamber for certain time. where the chamber was maintained at equilibrium pressure of 1 kg/cm2. After that, the hydrogenated LiBH4@Ah-BN nanocomposite was removed from the chamber and allowed to attain room temperature. Then the amount of hydrogenation in the hydrogenated LiBH4@Ah-BN nanocomposite is estimated using CHNS elemental analysis and TGA studies.

Results and discussion Structural and morphological analysis Fig. 1 depicts the XRD patterns of h-BN, Ah-BN and LiBH4@AhBN nanocomposite. Both h-BN and Ah-BN [see Fig. 1(a and b)] exhibit similar diffraction peaks at 2q ¼ 26.6, 41.6, 43.8, 50.1, 55.1 and 75.8
which are indexed as (002), (100), (101), (102), (004) and (110) planes of hexagonal phase of boron nitride (JCPDS card no. 14-0033) [49,50]. The crystal structure of Ah-BN remains same even after the surface modification and it is evident from the absence of any new peaks in Ah-BN. In the case of LiBH4@Ah-BN nanocomposite, the presence of new characteristic peaks at 2q ¼ 22.2, 25.5, 30.6, 31.5, 34.8, 36.4 and 39.0
for LiBH4 [51,52] besides the peaks of Ah-BN confirms that LiBH4 is incorporated at the surface of Ah-BN. The crystalline sizes (using Scherrer's formula) of h-BN, Ah-BN and LiBH4@Ah-BN nanocomposite were found to be 5.32, 5.68 and 6.84 nm, respectively. The presence of vibrational modes of h-BN, Ah-BN and LiBH4@Ah-BN nanocomposite were analyzed using FTIR spectroscopy (see Fig. 2). The h-BN has two strong modes located at 1385 and 810 cm1 which are assigned to in plane BeN stretching vibrations and out-of-plane BeNeB bending vibrations, respectively. Even though the FTIR spectra of AhBN and pristine h-BN [see Fig. 2(a and b)] are appears to be quite similar, three new peaks were noticed at 3450 cm1 (stretching modes of OeH groups), 3090 cm1 (BeOH) and 930 cm1 (BeNeO) for Ah-BN [6,48,53,54]. In the case of LiBH4@Ah-BN nanocomposite (Fig. 2c), new modes such as BeH stretching (2385, 2289 and 2225 cm1), BeH bending (1257 and 1215 cm1), OeH (3502, 3424, 3309 and 3262 cm1), LieH

Characterization The structural and morphological properties of the synthesized nanocomposite were examined using X-ray Diffraction (XRD, X'Pert PAN analytical X-ray diffractometer) with Cu Ka radiation and Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDX, JEOL model JSM 6390LV). Fourier Transform Infrared (FTIR) spectra were recorded in the range of 400e4000 cm1 using a shimadzu FTIR-8400 spectrophotometer. Micro-Raman studies (Labram HR800) and CHNS-elemental analysis (Elementar Vario EL III model analyzer) were also performed. The specific surface area of prepared samples were estimated from BrunauereEmmetteTeller (BET) studies using Micromeritics ASAP 2010. The Thermo Gravimetric Analysis (TGA) was carried out using a Perkin Elmer-Diamond thermal analyzer in the temperature range of 50e600
C at a heating rate of 10
C/min.

Fig. 1 e XRD patterns of (a) h-BN (b) Ah-BN and (c) LiBH4@Ah-BN nanocomposite.

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Fig. 2 e FTIR spectra of (a) h-BN (b) Ah-BN and (c) LiBH4@AhBN nanocomposite.

(1010 cm1) and LieO (487 and 442 cm1) vibrations were noticed for LiBH4 in addition to the modes of Ah-BN [24,26,55]. The absence of LieB/LieN vibrations suggests that there may be a weak (van der walls attraction) interaction between LiBH4 and Ah-BN during the synthesis of LiBH4@Ah-BN nanocomposite via chemical impregnation method. But the presence of LiBH4 in LiBH4@Ah-BN nanocomposite is confirmed from XRD studies.

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The prepared h-BN, Ah-BN and LiBH4@Ah-BN nanocomposite were further analyzed using micro-Raman. Fig. 3(a and b) corresponds to the micro-Raman spectra of h-BN and Ah-BN. Both the spectra exhibit a sharp peak at 1365 cm1 which corresponds to in-plane vibrational Raman active E2g mode (B and N atoms are moving against each other within a plane). This result confirms that the prepared Ah-BN is in crystalline nature and free from impurities [14,56]. wherein LiBH4@Ah-BN nanocomposite, the new mode at 1285 cm1 corresponds to BeH vibration of LiBH4, indicates that LiBH4 is decorated on Ah-BN [57,58]. Micro-Raman studies also confirm the presence of LiBH4 in LiBH4@Ah-BN nanocomposite. The synthesized LiBH4@Ah-BN nanocomposite is subjected to EDX analysis in order to analyze the chemical compositions present in the samples. The EDX spectra of hBN, Ah-BN and LiBH4@Ah-BN nanocomposite and their corresponding atomic % of elements are shown in Fig. 4. It is evident that boron (B) and nitrogen (N) elements are alone observed in the pristine h-BN and Ah-BN. In addition to B and N elements, oxygen (O) elements (due to the interaction of LiBH4 with atmosphere) have also been detected in the case of LiBH4@Ah-BN nanocomposite. Hence, it is clear that the prepared nanocomposite is free from other impurities. However, this technique is not suitable to detect the presence of LiBH4 in the LiBH4@Ah-BN nanocomposite due to the weak scattering of electrons by the light elements such as Li and H elements [59], the presence of Li element could indirectly be detected via micro-Raman, FTIR and XRD analysis. The surface morphology of h-BN, Ah-BN and LiBH4@AhBN nanocomposite are shown in Fig. 5. The SEM image of AhBN (see Fig. 5b) depicts the uniform distribution of h-BN nanoparticles with an average particle size of 40e50 nm which are highlighted in red circule. Moreover, less agglomeration of nanoparticles is noticed in the case of AhBN than pristine h-BN (see Fig. 5a). From Fig. 5c, it is clear that the h-BN nanoparticles are homogeneously dispersed and strongly confirms that the LiBH4 are anchored at the surface of Ah-BN. The nitrogen adsorptionedesorption isotherms of the synthesized Ah-BN and LiBH4@Ah-BN nanocomposite were analyzed (see Fig. 6). Based on the IUPAC classification, the Ah-BN and LiBH4@Ah-BN nanocomposite could exhibit type II isotherms and type H3 hysteresis loops with more mesopored and micropored structures [60,61]. Table 1 represents the BET specific surface area and pore volume of prepared Ah-BN and LiBH4@Ah-BN nanocomposite. It is observed that the prepared LiBH4@Ah-BN nanocomposite has excellent external surface area and BET specific surface area than Ah-BN, which is mainly attributed to the incorporation of LiBH4 on the surface of Ah-BN.

Hydrogen adsorption analysis

Fig. 3 e Raman spectra of (a) h-BN (b) Ah-BN and (c) LiBH4@Ah-BN nanocomposite.

The amount of stored hydrogen in the prepared nanocomposite is estimated from CHNS-elemental analysis. Fig. 7 represents the amount of hydrogen stored in h-BN, Ah-BN and LiBH4@Ah-BN nanocomposite at different hydrogenation temperatures, hydrogen flow rates and flow durations.

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Fig. 4 e EDX spectra of (a) h-BN (b) Ah-BN and (c) LiBH4@Ah-BN nanocomposite and the insert shows the corresponding atomic % of elements.

Fig. 8 shows the comparison of storage capacity of hydrogen in h-BN, Ah-BN and LiBH4@Ah-BN nanocomposite. The maximum hydrogen uptake of h-BN and Ah-BN are found to be 0.19 and 0.39 wt%, respectively, whereas LiBH4@Ah-BN nanocomposite reaches 2.3 wt% which is more than the total storage capacity of h-BN and Ah-BN. The high storage capacity of LiBH4@Ah-BN nanocomposite could be resulted from the following factors. The more agglomeration nature of bare h-BN nanoparticles led to less hydrogen adsorption sites and resulted in less storage capacity. But in the case of Ah-BN, the homogenous dispersion of h-BN nanoparticles offers more adsorption sites to absorb more hydrogens and more LiBH4 and in turn increased the storage capacity. In LiBH4@Ah-BN nanocomposite, the presence of more non-agglomerated h-BN nanoparticles and their dipolar nature and high affinity of Liþ ions towards hydrogen are the authentic factors for the enhancement of storage capacity. In addition the high specific surface area of LiBH4@Ah-BN nanocomposite offers more adsorption sites for the hydrogen to be stored. No significant rise in the hydrogen storage capacity (2.31 wt%) of LiBH4@Ah-BN
nanocomposite even at 150 C was observed.

The obtained results are compared with earlier reports of BN based hydrogen storage materials and are given in Table 2. However, most of the hydrogenation experiments reported in Table 2 are conducted either at low temperature or at high pressure conditions. Moreover, the storage of hydrogen at low temperature and high pressure conditions are not realistic for automobile applications. In the present case a maximum of 2.3 wt% of hydrogen storage capacity for LiBH4@Ah-BN nanocomposite was achieved at 100
C which may suitable for onboard applications. Fig. 9 presents the comparison of FTIR spectra of hydrogenated h-BN, Ah-BN and LiBH4@Ah-BN nanocomposite with as-synthesized h-BN, Ah-BN and LiBH4@Ah-BN nanocomposite. It is concluded that, there is a suppression of intensity for the prominent modes of hydrogenated h-BN, AhBN nanocomposite compared to as-synthesized h-BN, Ah-BN nanocomposite. It may due to the absorbance of more hydrogen during hydrogenation process. But the intensities of vibration bands (1190 cm1 and 2470 cm1) of hydrogenated LiBH4@Ah-BN nanocomposite are augmented compared to assynthesized LiBH4@Ah-BN nanocomposite (highlighted by dotted rectangular box).

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Fig. 5 e SEM photographs of (a) h-BN (b) Ah-BN and (c) LiBH4@Ah-BN nanocomposite.

Hydrogen desorption analysis The TGA spectra of hydrogenated h-BN, Ah-BN and LiBH4@Ah-BN nanocomposite are shown in Fig. 10. The hydrogenated Ah-BN exhibits desorption temperature of stored

hydrogen in the temperature range of 135e290
C. whereas the hydrogenated LiBH4@Ah-BN nanocomposite shows an initial weight loss of 2.3 wt% in the temperature range of 110e150
C that corresponds to desorption temperature of stored hydrogen. A second weight loss at 170
C indicates the disintegration of LiBH4. Usually the decomposition of LiBH4 occurs at 380
C. From our results it is concluded that the presence of Ah-BN nanoparticles greatly reduce the desorption temperature of hydrogenated LiBH4@Ah-BN (110e150
C). In the LiBH4@Ah-BN nanocomposite, the lithium borohydrides are weakly bonded with h-BN nanoparticles and resulted in the destabilization of LiBH4. Our results are well agreed with earlier reports [24,59,64]. The activation energy of desorption (Ed) can be calculated from the desorption temperature using the following equation [10,13,15].  2 T Ed ln m ¼ b RTm

Fig. 6 e Nitrogen adsorptionedesorption isotherms of AhBN and LiBH4@Ah-BN nanocomposite (The open and solid symbols represent absorption and desorption isotherms, respectively).

where, Tm is the desorption temperature, b is the heating rate (10
C/min) and R is the universal gas constant. The binding energy (EB) of stored hydrogen is calculated using van't Hoff equation [16,65]. The Ed and EB values at the desorption temperatures are found to be 19.91 kJ/mol, 22.69 kJ/mol, 0.29 eV and 0.33 eV respectively in the case of LiBH4@Ah-BN nanocomposite. The desorption temperature, activation energy of desorption and the binding energy of hydrogenated h-

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Table 1 e Textural properties of Ah-BN and LiBH4@Ah-BN nanocomposite based on the nitrogen adsorptionedesorption isotherms. Nanocomposite BET surface Micropore surface Micropore volume Mesopore volume External surface Total pore volume area (m2/g) area (m2/g) (cm3/g) (cm3/g) area (m2/g) (cm3/g) Ah-BN LiBH4@Ah-BN

70 122

63 84

0.04 0.06

0.26 0.48

42 106

0.30 0.54

Table 2 e Hydrogen storage capacity of BN based materials. Samples

Fig. 7 e Hydrogen adsorption of h-BN, Ah-BN and LiBH4@Ah-BN nanocomposite at various operating conditions.

BN, Ah-BN and LiBH4@Ah-BN nanocomposite are displayed in Table 3. The hydrogen storage capacity of hydrogenated LiBH4@Ah-BN nanocomposite and the desorption temperature of stored hydrogen can be measured from CHNS-elemental analysis and TGA studies. Furthermore in the prepared

BN nanopowder h-BN nanoparticles Ah-BN Multi-walled BNNT BN porous microbelts MWCNT/h-BN nanocomposite Flower type BN h-BN powders Bamboo-like BNNTs Straight-walled BNNTs A-HNT-h-BN nanocomposite SPEEK-h-BN nanocomposite Bamboo type BNNTs LiBH4@Ah-BN nanocomposite BN hollow spheres BN nanotubes collapsed walls BN whiskers Oxygen doped BN nanosheets

H2 (wt%)

Conditions

Reference

0.2 0.19 0.39 1.8 2.3 2.3

RT and 10 MPa 100
C 100
C RT and 10 MPa 77 K and 1 MPa 100
C

[12] Present work Present work [12] [9] [10]

2.5 2.6 2.6 2.7 2.88

100 bar 1 MPa RT and 10 MPa 100 bar 50
C

[14] [11] [12] [14] [15]

2.98

150
C

[13]

3.0 2.3

100 bar 100
C

[14] Present work

4.07 4.2

298 K and 10 MPa [62] 10 MPa [63]

5.6 5.7

RT and 3 MPa RT and 5 MPa

[7] [16]

nanocomposite, the amount of hydrogen stored is equal to the amount of hydrogen desorbed and ensures 100% desorption for LiBH4@Ah-BN nanocomposite. As per US-DOE targets, for any ideal hydrogen storage material, the binding energy of stored hydrogen should be in the range of 0.2e0.4 eV for fuel cell applications [66,67]. The calculated binding energy of hydrogenated LiBH4@Ah-BN nanocomposite lies in the recommended range (see Table 3) and the stored hydrogens are weakly chemisorped. Hence, the present hydrogen storage medium may serve as a better solid hydrogen storage medium in the realm of hydrogen fuel cell applications.

Reusability test

Fig. 8 e Comparison of hydrogen adsorptions of h-BN, AhBN and LiBH4@Ah-BN nanocomposite at a constant flow rate of 1 L/min for 30 min.

The performance of prepared LiBH4@Ah-BN nanocomposite is further investigated for its reusability. The reusability profile of prepared LiBH4@Ah-BN nanocomposite is depicted in Fig. 11. Initially the hydrogenated LiBH4@Ah-BN nanocomposite was annealed at 200
C for 30 min. After that, the LiBH4@Ah-BN nanocomposite was again hydrogenated at 100
C at a constant hydrogen flow rate of 1 L/min for 30 min using Sieverts like hydrogenation setup. Then the amount of

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Table 3 e Hydrogen desorption parameters. Nanocomposites h-BN Ah-BN LiBH4@Ah-BN

H2 (wt%)

Tm (
C)

Ed (kJ/mol)

EB (eV)

0.19 0.39 2.3

275e370 135e290 110e150

31.75e38.97 21.64e32.87 19.91e22.69

0.43e0.50 0.32e0.44 0.29e0.33

Fig. 9 e FTIR spectra of hydrogenated (a) h-BN (b) Ah-BN and (c) LiBH4@Ah-BN nanocomposite at 100
C and a flow rate of 1 L/min for 30 min.

Fig. 11 e Reusability test of LiBH4@Ah-BN nanocomposite. stored hydrogen was estimated from CHNS-elemental analysis. This procedure was repeated for at least four times under same conditions. The obtained results are shown as Fig. 11. It is understood that the prepared LiBH4@Ah-BN nanocomposite maintains 85.7% of hydrogen storage capacity even after four cycles and shows the sign of better cycling performance.

To confirm the storage capacity estimated from CHNS e elemental analysis, TGA studies were also carried out for the hydrogenated LiBH4@Ah-BN nanocomposite (see Fig. 12). From CHNS and TGA studies, it is observed that the amount of

Fig. 10 e TGA spectra of (a) h-BN (b) Ah-BN and (c) LiBH4@Ah-BN nanocomposite hydrogenated at 100
C with a flow rate of 1 L/min for 30 min.

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this work under UGC-MRP. The authors acknowledge UGC-UPE for micro-Raman and USIC-MKU for FTIR characterizations.

references

Fig. 12 e TGA spectra of hydrogenated LiBH4@Ah-BN nanocomposite for four cycles.

stored hydrogen (CHNS studies) in LiBH4@Ah-BN nanocomposite and the amount of desorbed hydrogen (TGA studies) is one and the same for each cycle.

Conclusion In summary, a non-carbon and light weight elements based LiBH4@Ah-BN nanocomposite hydrogen storage medium was prepared and its storage capacity was tested. where chemical impregnation method was adopted for the synthesis of LiBH4@Ah-BN nanocomposite. The effects of hydrogen flow rate, flow duration and hydrogenation temperature on the storage capacity of LiBH4@Ah-BN nanocomposite were examined. The LiBH4@Ah-BN nanocomposite based adsorbent has hydrogen storage capacity of 2.3 wt% at 100
C and at a flow rate of 1 L/ min. The hydrogen storage capacity increases with increase of hydrogen flow rate and operating temperatures. Based on the hydrogen storage capacity the LiBH4@Ah-BN nanocomposite performs well than Ah-BN and h-BN. The dehydrogenation temperature of hydrogenated nanocomposite lies in the temperature range of 110e150
C. From our results it is concluded that the presence of Ah-BN nanoparticles greatly reduces the desorption temperature of hydrogenated LiBH4@Ah-BN (110e150
C) and could serve as a catalyst to improve the kinetics of hydrogen uptake and release. The reusability test of LiBH4@Ah-BN nanocomposite confirms its better storage performance during hydrogenation and dehydrogenation. Having excellent storage capacity, low operating temperature (100
C) and 100% desorption, the prepared LiBH4@Ah-BN nanocomposite may serve as a promising hydrogen storage material for fuel cell applications.

Acknowledgment One of the authors Dr. S. Rajashabala thanks University Grants Commission of India for providing grant to carry out

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