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Enhanced dehydrogenation and rehydrogenation properties of LiBH4 catalyzed by graphene
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 3 8 ( 2 0 1 3 ) 2 7 9 6 e2 8 0 3
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Enhanced dehydrogenation and rehydrogenation properties of LiBH4 catalyzed by graphene Juan Xu a,b,c, Rongrong Meng a, Jianyu Cao a, Xiaofang Gu a, Zhongqing Qi a,b, Wenchang Wang a, Zhidong Chen a,b,c,* a
School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China Key Laboratory for Solar Cell Materials and Technology of Jiangsu Province, Changzhou 213164, China c Changzhou University and Qualtec Co. Ltd., Changzhou 213164, China b
article info
abstract
Article history:
The investigation of thermally induced dehydrogenation of LiBH4 reveals that LiBH4 doped
Received 15 September 2012
with the graphene catalysts shows superior dehydrogenation and rehydrogenation perfor-
Received in revised form
mance to that of Vulcan XC-72, carbon nanotube and BP2000 doped LiBH4. For doping with
5 December 2012
20 wt.% graphene, thermal dehydrogenation of LiBH4 is found to start at ca. 230 C and a total
Accepted 7 December 2012
weight loss of 11.4 wt.% can be obtained below 700 C. With increased loading of graphene
Available online 5 January 2013
within a LiBH4 sample, the onset dehydrogenation temperature and the two main desorption peaks from LiBH4 are found to decrease while the hydrogen release amount is found to
Keywords:
increase. Moreover, variation of the equilibrium pressure obtained from isotherms
Hydrogen storage
measured at 350e450 C indicate the dehydrogenation enthalpy is reduced from 74 kJ mol1
Lithium borohydride
H2 for pure LiBH4 to ca. 40 kJ mol1 H2 for 20 wt.% graphene doped LiBH4. Importantly, the
Graphene
reversible dehydrogenation/rehydrogenation process was achieved under 3 MPa H2 at 400 C
Mechanically ball milling
for 10 h, with a capacity of ca. 4.0 wt.% in the tenth cycle. Especially, LiBH4 is reformed and new species, Li2B10H10, is detected after the rehydrogenation process. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen-based energy economy has been limited by some insurmountable difficulties such as safe and high on-board hydrogen storage as well as hazards associated with emerging transportation applications [1]. Traditionally, hydrogen is available mainly as compressed gas in highly pressurized cylinders (up to 800 bar), as liquid hydrogen in cryogenic tanks (at 252 C), and as adsorbed hydrogen in solid metal, alloy and carbon materials [2]. The safety of pressurized cylinders and the harsh requirement of low critical temperature for cryogenic storage are present challenging
barriers to the widespread commercialization of hydrogenpowered vehicles. On the other hand, the amount of adsorbed hydrogen in various metal/alloy hydrides and nanostructure materials is currently too low to fully meet US Department of Energy (DOE) 2015 system target for mobile hydrogen storage. As a result, extensive efforts have been directed at developing new and improved solid hydrogen storage materials with high hydrogen density, low dehydrogenation temperature, and excellent reversibility [3e5]. In particular, lithium borohydride (LiBH4) with a high theoretical storage capacity of 18.5 wt.% of H2, has been extensively considered as a leading hydrogen storage
* Corresponding author. Changzhou University and Qualtec Co. Ltd., No. 1 Gehu Road, Changzhou 213164, China. Tel./fax: þ86 0519 86330239. E-mail address: [email protected] (Z. Chen). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.12.046
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 3 8 ( 2 0 1 3 ) 2 7 9 6 e2 8 0 3
candidate for on-board applications that are dependent on total weight [6]. However, extraction of hydrogen from LiBH4 at mild temperature has been hampered by its strong covalent and ionic bonds [7,8]. Thus, LiBH4 starts to decompose and release hydrogen at ca. 410 C and only half of the hydrogen is released below 600 C [9]. Moreover, the rehydrogenation reaction has remained an enormous challenge due to extremely rigorous reaction conditions, requiring high hydrogen pressure (35 MPa) and elevated temperature (600 C) [7]. Consequently, several novel approaches have been developed to lower the H2 evolution temperature, increase the hydrogen desorption capacity and promote the rehydrogenation reaction under mild condition [10]. Some conventional strategies such as the destabilization with various effective catalysts [11,12], confinement in nanoporous materials [13], and partial cation substitution [14] have been tried to decrease the dehydrogenation temperature and improve the reversibility of LiBH4. Of particular interest is the potential to utilize nanostructured and porous carbon materials as effective catalysts and nanoscale frameworks for LiBH4, which help to realize high hydrogen release amount at markedly reduced temperature and easy rehydrogenation reaction under moderate condition through combining large surface area and tunable pore size [15e23]. In the case that LiBH4 was ballmilled with multi-walled carbon nanotubes (MWNTs), with a mass ratio of 2:1, the LiBH4/MWNTs mixtures showed excellent dehydrogenation properties, hydrogen desorption starting at 250 C, and ca. 3.8 wt.% hydrogen can be released after rehydrogenated at 10 MPa hydrogen pressure and 400 C [15]. When LiBH4 is doped with single-walled carbon nanotubes (SWCNTs), with a mass ratio of 7:3, over 6.0 wt % hydrogen can be recharged at 400 C under an initial hydrogen pressure of 10 MPa [16]. Agresti et al. also reported that the dehydrogenation temperature of LiBH4 can be decreased by more than 60 C when LiBH4 was dispersed on multi-walled carbon nanotubes (MWCNTs) by solvent infiltration method [17]. Addition of purified SWNTs into LiBH4/MgH2, with a mass ratio of 9:1, shows that the initial hydrogen desorption temperature is lowered to 320 C, and nearly 10 wt.% of hydrogen can be released within 20 min at 450 C, which is about two times faster than that of the neat LiBH4/MgH2 sample [18]. Experimental measurements of Majzoub et al. indicate that the onset dehydrogenation temperature of highly ordered hexagonal nanoporous hard carbon (NPC) supported premelted LiBH4, with the average size of 2 nm, was dramatically reduced from 460 to 220 C [19]. Vajo et al. infiltrated LiBH4 into carbon aerogel hosts with average pore sizes of 13 and 25 nm, and demonstrated enhanced desorption kinetics and reversibility [20]. Also, it has been qualitatively confirmed that LiBH4 doped with disordered mesoporous carbon, to a mass ratio of 1:1, gives rise to a large amount of hydrogen at 332 C [21]. Furthermore, our recent findings prove that Vulcan XC-72 carbon is highly effective for improving the dehydrogenation properties of LiBH4. The first dehydrogenation temperature for the Vulcan XC-72 carbon doped LiBH4 sample starts at ca. 320 C and nearly 13.2 wt. % H2 can be desorbed below 700 C [22,23]. Similar catalytic effects of carbon addition on the dehydrogenation kinetics and cyclic reversibility have also been observed for the LieBeMgeH system [18].
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Quite recently, graphene, the first two-dimensional atomic crystal, emerges as a conceptually new class of carbon material and its unique planar nanostructure and unusual properties promise the potential applications in a wide range of areas such as electronics, sensors, and energy storage [24]. Furthermore, graphene with large specific area and tunable pore structure have received continuous interest as potential hydrogen storage media by physical adsorption [25,26]. In this regard, we report a doping strategy to improve the dehydrogenating properties of LiBH4 that involves the addition of various quantities of graphene as a host for LiBH4 in this paper.
2.
Experimental section
2.1.
Materials
Commercially acquired LiBH4 (95% purity, J&K Chemical Ltd, Sweden), natural graphite (99.9995 wt.% purity, Alfa Aesar Ltd, UK), NaNO3 (99% purity, Sinopharm Chemical Reagent Co. Ltd, China), H2SO4 (96% purity, Sinopharm Chemical Reagent Co. Ltd, China), KMnO4 (99% purity, Sinopharm Chemical Reagent Co. Ltd, China), H2O2 (30 wt.% aqueous solution, Sinopharm Chemical Reagent Co. Ltd, China), Vulcan XC-72 carbon (with a specific surface area of 254 m2 g1, Cobat. Inc., USA), BP2000 carbon (with a specific surface area of 1450 m2 g1, Cobat. Inc., USA), and Carbon nanotube (>95% purity, with a specific surface area of 150 m2 g1, Shenzhen Nanotech Port Ltd. Co., China) were used as received.
2.2.
Preparation of graphene
As described in literatures [27], graphene used in present study was made by complete oxidation followed by microwave and thermal reduction method. In brief, 1 g natural graphite powders and 1 g NaNO3 were placed in a reaction vessel that was preliminarily immersed in an ice bath, followed by slowly adding 35 ml H2SO4 under violent stirring. After 5 g KMnO4 was gradually added over about 1 h, the mixture was gently stirred at room temperature and allowed to go on for 120 h to fully oxidize graphite powder to graphite oxide (GO). Subsequently, the GO obtained was added to 100 ml of 5 wt.% H2SO4 aqueous solutions over about 2 h with gentle stirring, and then 100 ml of 30 wt.% H2O2 was added. After centrifugation and washing procedure with a mixed aqueous solution of 3 wt.% H2SO4 and 0.5 wt.% H2O2, GO powders obtained were irradiated in a domestic microwave oven (1100 W for 1 min) and then filtrated, washed and dried at 65 C for 24 h. Afterwards, the above chemical activated GO powders were put in a tube furnace under flowing argon and heated at 800 C for 1 h. Finally, the final products were then centrifuged, washed, and vacuum-dried.
2.3.
Doping LiBH4 with graphene
The mixture of LiBH4 and graphene was prepared through a ball milling method: 1 g of LiBH4 and graphene mixture with
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various mass ratios was mechanically milled for 1.5 h (Planetary QM-1SP2) under argon atmosphere at room temperature, respectively. The ball-to-power weight ratio was 30:1 at 580 rpm using stainless steel balls of 10 mm diameter. The mass ratios of graphene and LiBH4 are 20:80, 40:60, 70:30 and 85:15, respectively.
2.4.
Hydrogen releasing property measurements
A Netzsch STA449C TG-DSC thermoanalyzer coupled with a Balzers Thermostar Quadrupole Mass Spectrometer was used to investigate the hydrogen releasing properties. The heating rate was 10 C min1, with argon flowing at a purging rate of 20 cm3 min1. Typical sample quantity used was ca. 5e10 mg. For comparison with the neat system, the hydrogen capacity is calculated based only on the mass of LiBH4. Dehydrogenation/rehydrogenation performances of the samples were examined by using a carefully calibrated Sievert’s type apparatus. A typical cyclic experiment entailed dehydrogenation at 550 C, and rehydrogenation at 400 C with an initial 3 MPa hydrogen pressure for 10 h. As it is difficult to accurately characterize the rehydrogenation process under high hydrogen pressure, the restored hydrogen amounts were precisely determined in the subsequent dehydrogenation half-cycle.
2.5.
Structure and morphology characterization
The morphology of the graphene as prepared was analyzed by transmission electron microscopy (TEM) using a Technai G2 20s-Twin Microscope (FEP Inc., USA). TEM samples were prepared by dispersing dry graphene powders in ethanol to form a homogeneous suspension, and then, the suspension was dropped on 200-mesh copper grid for observation. X-ray diffraction (XRD) measurements were carried out on a Rigaku D/MAX-2500 diffractometer using Cu Ka radiation. Diffraction patterns were collected at a scanning rate of 2 min1 and with a step of 0.02 . Surface area and porosity of the graphene sample was obtained from N2 adsorptionedesorption isotherm and BarreteJoynereHalenda (BJH) plots (Micromeritics porosimeter ASAP 2010).
3.
Results and discussion
3.1.
Microstructure characterization of graphene
The morphology of the graphene as prepared was observed by TEM analysis, as presented in Fig. 1. Large, disordered and over-lapped multilayer graphene nanosheets resemble rippled silk veil waves, entangled with each other to form an interconnected network. The area of the above graphene sheets reaches a few hundred square nanometers and its thickness approximates to 1.9 nm. Compared with the reported thicknesses of different layer graphemes [28], the graphene sheets obtained by complete oxidation followed by microwave and thermal reduction method in nitrogen atmosphere possess approximately three stacked individual graphene layers. The nitrogen adsorptionedesorption isotherm of graphene sheets and pore size distribution plot are shown in Fig. 2. Specific surface area and pore-size characterization of the graphene were performed by coupling high-resolution nitrogen at 77.4 K. The BrunauereEmmetteTeller (BET) specific surface area of obtained graphene sheets is about 2340 m2 g1 calculated from the desorption line, less than the theoretical specific surface area of single-layer graphene sheets (2620 m2 g1). Furthermore, it should be pointed out that a hysteresis loop in the nitrogen desorption branch was also observed, proving that the as-prepared graphene sheets are porous system [27]. At the same time, the pore size distribution plot reveals that the pores in graphene have a narrow micromesopores size distribution centered at ca. 3.6 nm, with a pore volume of 1.9 cm3 g1 (see the inset in Fig. 2).
3.2. LiBH4
Dehydrogenation properties of graphene doped
The thermal dehydrogenation curves as a function of temperature present the hydrogen release properties of LiBH4 doped with and without graphene. Fig. 3 shows temperature programmed hydrogen release profiles (A) and corresponding thermogravimetric (TG) curves (B) of ball-milled pure LiBH4 and as-prepared graphene doped LiBH4 with different mass ratios; while the detailed data are listed in Table 1. Four hydrogen desorption peaks of ball-milled pure LiBH4 respectively corresponds to the structural transition (80e110 C),
Fig. 1 e TEM images of graphene prepared by microwave and thermal reduction method.
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Quantity Adsorbed (cm 3 g-1 )
1800 1500
Adsorption Desorption Pore Volume (cm3 g-1 )
1200 900 600 300 0
0.0
0.2
2.0 1.5 1.0 0.5 0.0 0
0.4
2
4
6
8
10 12 14 16
Pore Diameter (nm)
0.6
0.8
1.0
Relative Pressure (P/Po) Fig. 2 e N2 adsorption/desorption isotherm curves of graphene (w2340 m2 gL1) at 77.4 K. The inset shows the pore size distribution plot for the as-prepared graphene.
(A) o
300 C o
102 C 65 C o
o
195 C
(e) o
300 C
Relative Intensity (a.u.)
o
90 C
o
200 C
(d) o
300 C
o
88 C
o
465 C
o
215 C
(c) o
310 C o
o
515 C
o
82 C
230 C
(b) o
485 C o 420 C
o
98 C
100
200
300
o
610 C
(a)
400
500
600
700
600
700
o
Temperature ( C)
(B) 100
(a)
Weight (%)
95
(b) 90
(c) (d)
85
(e)
100
200
300
400
500
Temperature ( oC) Fig. 3 e Temperature programmed hydrogen release profiles (A) and TG curves (B) for ball-milled pure LiBH4 (a) and the mixture of graphene and LiBH4 with various mass ratio (bee). The mass ratios of graphene and LiBH4 are 20:80 (b), 40:60 (c), 70:30 (d) and 85:15 (e), respectively.
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melting reaction (280e310 C) and dehydrogenation reactions (420e510 and 560e650 C) (curve a) [7,29]. The onset temperature for main hydrogen desorption is ca. 420 C, while the two main desorption peaks are located at 485 and 610 C, and the weight loss corresponds to 10.7 wt.%. However, by adding 20 wt.% graphene catalyst to the LiBH4 compound, the dehydrogenation temperature of LiBH4 is significantly reduced (curve b). The onset hydrogen desorption temperature of the 20 wt.% graphene doped LiBH4 is located at 230 C, resulting in a release of ca. 5.9 wt.% hydrogen from LiBH4, which is quite different from pure LiBH4, where only a minor dehydrogenation of 0.6 wt.% occurs below 360 C. Also, in comparison to pure LiBH4, the other main dehydrogenation peaks of the 20 wt.% graphene doped LiBH4 shift to lower temperatures, with an increased total weight loss of 11.4 wt.% below 700 C. With increase in graphene content in the mixtures of graphene and LiBH4 (up to 85 wt. %), the onset dehydrogenation temperature and the two main desorption peaks from LiBH4 are found to gradually decrease while the total hydrogen release amount is found to increase. When LiBH4 is doped with 85 wt.% graphene, the onset dehydrogenation temperature approach 195 C, and the major desorption peaks occur at 310 C (as shown in Part A of Fig. 3, top curve). Importantly, the dehydrogenation capacity of LiBH4 doped with 85 wt.% catalyst come to 17.9 wt.% based on LiBH4, which is very close to the theoretical value of ca. 18.5 wt.%. Hence, these results clearly demonstrate that dehydrogenation properties of LiBH4 is significantly improved by doping various amounts of graphene into LiBH4, which may due to the synergistic effect of remarkable increase in the contact area between graphene and LiBH4 and confinement in nanoporous graphene, which help to uniformly disperse LiBH4 and effectively avoid its agglomeration. In order to compare the catalytic effect of different carbon on the dehydrogenation and rehydrogenation properties of LiBH4, hydrogen desorption mass spectra (MS) and TG profiles of various carbon doped LiBH4 composites and that of pure LiBH4 were presented in Fig. 4; while the detailed data are listed in Table 2. It was observed that the onset dehydrogenation temperature of LiBH4 was respectively reduced from 420 C for pure LiBH4 to 320, 280, 280, and 230 C for 20 wt.% vulcan C, carbon nanotube, BP2000, and graphene doped LiBH4. Furthermore, the released hydrogen amount was greatly increased after adding various carbon additives, in comparison with neat LiBH4, demonstrating that by mechanically milling with different carbon additives, the dehydrogenation performances of LiBH4 sample can be considerably improved. The difference of onset dehydrogenation temperature may come from their distinct specific surface area and pore distribution. Noticeably, among these carbon additives, graphene exhibits the best catalytic effect on the decomposition of LiBH4, with a dehydrogenation capacity of 9.8 wt.% below 550 C, which may derive from its enormous specific surface area and suitable pore size. After the hydrogen desorption performance of graphene doped LiBH4 has been ascertained, its dehydrogenation pressureecomposition isotherms were collected to further investigate its thermodynamic properties and dehydrogenation behavior. Pressureecompositionetemperature (PCT) measurements on the ball-milled pure LiBH4 and 20 wt.% graphene doped LiBH4 were collected in the range of 350e450 C, as shown
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Table 1 e Hydrogen release properties of the mixtures of graphene and LiBH4 with various mass ratios. Sample
LiBH4 Graphene Graphene Graphene Graphene
doped doped doped doped
Graphene doped amount (wt.%)
Onset dehydrogenation temperature ( C)
Main dehydrogenation temperature ( C)
Total dehydrogenation capacity (wt.%)
0 20 40 70 85
420 230 215 200 195
485; 610 310; 515 300; 465 300 300
10.7 11.4 12.3 13.3 17.9
LiBH4 LiBH4 LiBH4 LiBH4
in Fig. 5. No plateaus can be observed for the pure LiBH4 sample measured at 400 C (curve a), while the dehydrogenation isotherms of graphene doped LiBH4 show obvious plateaus (curve bed). As for the dehydrogenation of 20 wt.% graphene
(A)
o
310 C o
o
82 C
(e)
230 C o
Relative Intensity (a.u.)
460 C o
o
280 C
90 C
(d) o
490 C o
(c)
o
90 C
280 C o
485 C o
(b)
o
80 C
320 C o
o
o
98 C
485 C
420 C
100
200
300
(a)
400
500
o
Temperature ( C)
(B) 100
(c)
98
Weight (%)
3.3. LiBH4
(a)
(b)
(d) (e)
96 94 92 90
100
200
300
400
doped LiBH4 at 450 C (curve d), the dehydrogenation plateau was kept from 2.2 to 7.8 wt.%, with total hydrogen capacities of approximately 9.2 wt.%. The Van’t Hoff plot for the dehydrogenation reaction of the 20 wt.% graphene doped LiBH4 system (logarithm of the equilibrium pressure versus the inverse of the absolute temperature), using the medium equilibrium pressures, is shown in the inset of Fig. 5. According to Van’t Hoff equation and the determined thermodynamic parameters, the resultant Van’t Hoff equation of 20 wt.% graphene doped LiBH4 sample can be expressed numerically as ln ( peq/p0) ¼ 4853/ T þ 9.19, and the dehydrogenation reaction enthalpy changes (DH ) are calculated as ca. 40 kJ mol1 H2, which is far less than that of pure LiBH4 (ca. 74 kJ mol1 H2) [11,17], demonstrating that the dehydrogenation thermodynamics of LiBH4 is largely improved through the doping of graphene. Fig. 6 presents the first dehydrogenation profiles of pure LiBH4 and as-prepared 20 wt.% graphene doped LiBH4 sample at 450 C. Within 90 min, pure LiBH4 released only ca. 4.3 wt.% hydrogen. In contrast, ca. 9.2 wt.% hydrogen could be released from the graphene doped LiBH4 sample. Evidently, graphene possess a promoting effect on the dehydrogenation amount and kinetics of LiBH4.
500
Temperature ( oC ) Fig. 4 e Temperature programmed hydrogen release profiles (A) and TG curves (B) for pure LiBH4 (a), and Vulcan C (b), carbon nanotube (c), BP2000 (d), graphene (e) doped LiBH4 that were mechanically milled under Ar atmosphere for 1 h. The doping amount of various carbon is 20 wt.% and applied heating rate is 10 C minL1.
Rehydrogenation properties of graphene doped
The theoretical reversible hydrogen storage of pure LiBH4 is governed by the equilibrium, LiBH4 # LiH þ B þ 1.5H2, which accounts for a reversible capacity of 13.9 wt %. However, the rehydrogenation condition is rather demanding due to slow kinetics, which leads to serious cyclic capacity degradation [30]. In order to develop a reversible H2 storage and release system and recover LiBH4 at mild condition, which is important for practical applications, 20 wt.% graphene doped LiBH4 sample that had been dehydrogenated under a vacuum up to 550 C was rehydrogenated at 400 C for 10 h under 3 MPa hydrogen pressure. Fig. 7 shows a comparison on cycling dehydrogenation properties between neat LiBH4 and 20 wt.% graphene doped LiBH4 after recharging with H2. Clearly, for the pure LiBH4 sample, only 2.1 wt.% hydrogen could be restored within 10 h at 400 C with an initial hydrogen pressure of 3 MPa after the first dehydrogenation (Fig. 7(A)). In contrast, the total H2 released amounts of the 20 wt.% graphene doped LiBH4 sample decreased from the initial 9.8 wt.% to the 6.9 wt.% in the second cycle, and further to 4.9 wt.% in the fifth cycle and to 4.0 wt.% in the tenth cycle (Fig. 7(B)),
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Table 2 e Hydrogen release properties of various carbon doped LiBH4 with a doping amount of 20 wt.% below 550 C. Sample
BET area of carbon (m2 g1)
Onset dehydrogenation temperature ( C)
Main dehydrogenation temperature ( C)
Total dehydrogenation capacity (wt.%)
254 150 1450 2340
320 280 280 230
485 490 460 310
9.5 8.8 9.6 9.8
Vulcan C doped LiBH4 Carbon nanotube doped LiBH4 BP2000 doped LiBH4 Graphene doped LiBH4
25
ln(p eq /p 0 )=-4853/T+9.19
100
ln(peq/p0)
20
2.0
1.5
15
98
D H =40.35K Jmol -1 H 2 1.4
1.5
-1
1000/T(K )
1.6
Weight (%)
Pressure(atm)
(A)
2.5
10
5
(d) 0
(c)
-8
-6
-4
96 94 92
(a)
(b)
2nd
-2
90
0
1st 50
100
2
H 2 amount desorped (wt.%)
10
(b)
98 96
10th
94
5th 2nd
92
6
90
0 80
200
300
400 o
500
600
Temperature ( C)
2
60
1st 100
(a)
4
40
600
100
8
20
500
(B)
Weight (%)
Fig. 5 e Desorption PCT curves for pure LiBH4 at 400 C (a), 20 wt.% graphene doped LiBH4 at 350 (b), 400 (c), and 450 C (d), respectively. The inset shows the Van’t Hoff plot for the dehydrogenated graphene-doped LiBH4 sample.
0
400
Temperature ( oC)
H (weight%)
100
Fig. 7 e Comparison on the dehydrogenation cycle profiles between pure LBH4 (A) and 20 wt.% graphene doped LBH4 (B). The graphene doped LBH4 sample can be dehydrogenated (at 550 C for 5 h) and rehydrogenated (at 3 MPa H2, 400 C for 10 h) repeatedly and measured gravimetrically.
Time (min) Fig. 6 e Comparison on the first dehydrogenation curves between the pure LiBH4 (a) and 20 wt.% graphene doped LiBH4 (b) at 450 C.
indicative of higher cyclic capacity and better capacity retention than pure LiBH4. On the other hand, although the graphene doped LiBH4 sample still suffers from serious cyclic degradation, its capacity retention rate is obviously higher
than that of carbon nanotube, carbon aerogel, and mesoporous carbon doped LiBH4 sample [15,16,18,20,21]. Actually, similar degradation behavior, particularly the nearly or even more than 50 wt.% capacity loss in the first two cycles, was repeatedly observed in the studies of various carbon doped LiBH4 samples. Furthermore, as compared with transition metal (fluoride and hydroxides) doped LiBH4 samples reported in literatures [7,22,23,31,32], the rehydrogenation temperature and pressure conditions applied in the present study are
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$ $
&
B B $
*
B
$
B
Relative intensity (a.u.)
& # $ & # #
$ #
B
$
*
(d) & Li2B10H10
B B
*
# LiH * Li2C2
#
B
##
#
*
$ LiBH4
(c)
B $ $
&
B
*
$
B $
B
$
#
&& #
# $ $
$
*
(b)
$
(a) 10
15
20
25
30
35
40
45
50
55
60
65
70
o
2q ( ) Fig. 8 e X-ray diffraction patterns of as-prepared graphene (a), 20 wt.% graphene doped LiBH4 before (b) and after (c) the first dehydrogenation at 550 C for 5 h, and after the tenth rehydrogenation (d) at 400 C for 10 h with an initial hydrogen pressure of 3 MPa.
significantly lower, indicating that the graphene additive is highly effective for promoting the rehydrogenation reaction and rendering the system partially reversible under markedly reduced temperature and pressure conditions, which may come from the existence of some irreversible dehydrogenation products. A discussion of the possible mechanism underlying this characteristic phenomenon will be given in the following section.
3.4. Dehydrogenation/rehydrogenation reaction mechanism In our efforts to understand the kinetic enhancement arising upon adding graphene, explore any possible structural change, and infer possible reaction mechanism during the dehydrogenation/rehydrogenation process, XRD patterns of 20 wt.% graphene doped LiBH4 before and after the initial dehydrogenation, and the sample after the tenth dehydrogenation at 550 C for 5 h and then rehydrogenation at 400 C for 10 h were acquired, as shown in Fig. 8. For graphene sheets, broad XRD out-of-plane graphitic reflections can be observed (curve a) [33], proving their stacks structure [27], in accordance with their TEM results. LiBH4, LiH, boron and Li2C2 diffraction peaks are observed for the mixture before the first dehydrogenation (curve b) except for broad graphene reflections, indicating new species appeared during the ball milling process of graphene and LiBH4; the appearance of LiH and boron peaks may result from slight decomposition of LiBH4 [22]. As for pure LiBH4 system, only LiH, B and H2 can be formed after the decomposition of LiBH4 [9]. However, after the first dehydrogenation at 550 C for 5 h, the LiBH4 phase disappears, and Li2C2, LiH and B phases are identified (curve c), indicating that the possible dehydrogenation reaction of LiBH4, catalyzed by graphene, may be described as LiBH4 / LiH þ B þ 1.5H2 and 2LiBH4 þ 2C / Li2C2 þ 2B þ 4H2, just like carbon nanotube and mesoporous carbon doped sample [15,21]. In the case of the
rehydrogenated sample (curve d), LiBH4 is reformed and new species, Li2B10H10, is detected, confirming the reversible dehydrogenation/rehydrogenation reactions of the graphene doped LiBH4 sample. Nevertheless, Li2C2, LiH and B phases still exist, showing incomplete rehydrogenation reaction, which may be the reason for the cyclic degradation of the graphene doped LiBH4 sample. Thus, we deduce that the reversible hydrogen uptake/release of graphene doped LiBH4 may be represented as follows: LiH þ B þ 1.5H2 # LiBH4 and 2LiH þ 10B þ 4H2 # Li2B10H10.
4.
Conclusions
Graphene prepared by microwave and thermal reduction method exhibited significantly improved catalytic effect on the reversible dehydrogenation and rehydrogenation properties of LiBH4. The mixture of graphene and LiBH4 prepared by ball milling method could reduce the onset decomposition temperature from 420 to 195 C, two major desorption temperature of LiBH4 decrease from 485 and 610 to 300 C, and the dehydrogenation enthalpy is reduced by ca. 34 kJ mol1 H2, which may be ascribed to the synergistic effect of remarkable increase in the contact area between graphene and LiBH4 and confinement in nanoporous graphene. Furthermore, graphene doped LiBH4 has better hydrogenation/dehydrogenation reversibility than other nanoporous carbon materials, with a capacity of approximately 4.0 wt.% in the tenth cycle under mild conditions. Thus, destabilizing LiBH4 using graphene is a new and promising way for the catalytic dehydrogenation and rehydrogenation of LiBH4.
Acknowledgment The authors gratefully acknowledge financial support from the National Science Foundation of China (21003015 and 21103014), the Science Foundation of Jiangsu Province (BK2012591, 12KJA150003, BE201113 and 2011Z0062), the Science Foundation of Changzhou (CJ20115020), the Foundation of Jiangsu Key Laboratory for Solar Cell Materials and Technology (201106), and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions for support of this work.
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[5] Leng HY, Ichikawa T, Hino S, Hanada N, Isobe S, Fujii H. New metaleNeH system composed of Mg(NH2)2 and LiH for hydrogen storage. J Phys Chem B 2004;108:8763e5. [6] Orimo S, Nakamori Y, Eliseo JR, Zu¨ttel A, Jensen CM. Complex hydrides for hydrogen storage. Chem Rev 2007;107:4111e2. [7] Orimo S, Nakamori Y, Kitahara G, Miwa K, Ohba N, Towata S, et al. Dehydriding and rehydriding reactions of LiBH4. J Alloys Compd 2005;404e406:427e30. [8] Wang PJ, Fang ZZ, Ma LP, Kang XD, Wang P. Effect of SWNTs on the reversible hydrogen storage properties of LiBH4eMgH2 composite. Int J Hydrogen Energy 2008;33:5611e6. [9] Zu¨ttel A, Wenger P, Rentsch S, Sudan P, Mauron P, Emmenegger C. LiBH4 a new hydrogen storage material. J Power Sources 2003;118:1e7. [10] Li C, Peng P, Zhou DW, Wan L. Research progress in LiBH4 for hydrogen storage: a review. Int J Hydrogen Energy 2011;36: 14512e26. [11] Gennari FC. Destabilization of LiBH4 by MH2 (M ¼ Ce, La) for hydrogen storage: nanostructural effects on the hydrogen sorption kinetics. Int J Hydrogen Energy 2011;36:15231e8. [12] Zhou Y, Liu Y, Wu W, Zhang Y, Gao M, Pan H. Improved hydrogen storage properties of LiBH4 destabilized by in situ formation of MgH2 and LaH3. J Phys Chem C 2012;116:1588e95. [13] Nielsen TK, Bo¨senberg U, Gosalawit R, Dornheim M, Cerenius Y, Besenbacher F, et al. A reversible nanoconfined chemical reaction. ACS Nano 2010;4:3903e8. [14] Bardajı´ EG, Karger ZZ, Boucharat N, Nale A, Setten MJ, Lohstroh W, et al. LiBH4eMg(BH4)2: a physical mixture of metal borohydrides as hydrogen storage material. J Phys Chem C 2011;115:6095e101. [15] Yu XB, Wu Z, Chen QR, Li ZL, Weng BC, Huang TS. Improved hydrogen storage properties of LiBH4 destabilized by carbon. Appl Phys Lett 2007;90:034106-1e034106-3. [16] Fang ZZ, Kang XD, Wang P, Cheng HM. Improved reversible dehydrogenation of lithium borohydride by milling with asprepared single-walled carbon nanotubes. J Phys Chem C 2008;112:17023e9. [17] Agresti F, Khandelwal A, Capurso G, Russo SL, Maddalena A, Principi G. Improvement of dehydrogenation kinetics of LiBH4 dispersed on modified multi-walled carbon nanotubes. Nanotechnology 2010;21:065707-1e065707-5. [18] Wang PJ, Fang ZZ, Ma LP, Kang XD, Wang P. Effect of carbon addition on hydrogen storage behaviors of LieMgeBeH system. Int J Hydrogen Energy 2010;35:3072e5. [19] Liu X, Peaslee D, Jost CZ, Majzoub EH. Controlling the decomposition pathway of LiBH4 via confinement in highly ordered nanoporous carbon. J Phys Chem C 2010;114:14036e41.
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Enhanced dehydrogenation and rehydrogenation properties of LiBH4 catalyzed by graphene Juan Xu a,b,c, Rongrong Meng a, Jianyu Cao a, Xiaofang Gu a, Zhongqing Qi a,b, Wenchang Wang a, Zhidong Chen a,b,c,* a
School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China Key Laboratory for Solar Cell Materials and Technology of Jiangsu Province, Changzhou 213164, China c Changzhou University and Qualtec Co. Ltd., Changzhou 213164, China b
article info
abstract
Article history:
The investigation of thermally induced dehydrogenation of LiBH4 reveals that LiBH4 doped
Received 15 September 2012
with the graphene catalysts shows superior dehydrogenation and rehydrogenation perfor-
Received in revised form
mance to that of Vulcan XC-72, carbon nanotube and BP2000 doped LiBH4. For doping with
5 December 2012
20 wt.% graphene, thermal dehydrogenation of LiBH4 is found to start at ca. 230 C and a total
Accepted 7 December 2012
weight loss of 11.4 wt.% can be obtained below 700 C. With increased loading of graphene
Available online 5 January 2013
within a LiBH4 sample, the onset dehydrogenation temperature and the two main desorption peaks from LiBH4 are found to decrease while the hydrogen release amount is found to
Keywords:
increase. Moreover, variation of the equilibrium pressure obtained from isotherms
Hydrogen storage
measured at 350e450 C indicate the dehydrogenation enthalpy is reduced from 74 kJ mol1
Lithium borohydride
H2 for pure LiBH4 to ca. 40 kJ mol1 H2 for 20 wt.% graphene doped LiBH4. Importantly, the
Graphene
reversible dehydrogenation/rehydrogenation process was achieved under 3 MPa H2 at 400 C
Mechanically ball milling
for 10 h, with a capacity of ca. 4.0 wt.% in the tenth cycle. Especially, LiBH4 is reformed and new species, Li2B10H10, is detected after the rehydrogenation process. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen-based energy economy has been limited by some insurmountable difficulties such as safe and high on-board hydrogen storage as well as hazards associated with emerging transportation applications [1]. Traditionally, hydrogen is available mainly as compressed gas in highly pressurized cylinders (up to 800 bar), as liquid hydrogen in cryogenic tanks (at 252 C), and as adsorbed hydrogen in solid metal, alloy and carbon materials [2]. The safety of pressurized cylinders and the harsh requirement of low critical temperature for cryogenic storage are present challenging
barriers to the widespread commercialization of hydrogenpowered vehicles. On the other hand, the amount of adsorbed hydrogen in various metal/alloy hydrides and nanostructure materials is currently too low to fully meet US Department of Energy (DOE) 2015 system target for mobile hydrogen storage. As a result, extensive efforts have been directed at developing new and improved solid hydrogen storage materials with high hydrogen density, low dehydrogenation temperature, and excellent reversibility [3e5]. In particular, lithium borohydride (LiBH4) with a high theoretical storage capacity of 18.5 wt.% of H2, has been extensively considered as a leading hydrogen storage
* Corresponding author. Changzhou University and Qualtec Co. Ltd., No. 1 Gehu Road, Changzhou 213164, China. Tel./fax: þ86 0519 86330239. E-mail address: [email protected] (Z. Chen). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.12.046
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candidate for on-board applications that are dependent on total weight [6]. However, extraction of hydrogen from LiBH4 at mild temperature has been hampered by its strong covalent and ionic bonds [7,8]. Thus, LiBH4 starts to decompose and release hydrogen at ca. 410 C and only half of the hydrogen is released below 600 C [9]. Moreover, the rehydrogenation reaction has remained an enormous challenge due to extremely rigorous reaction conditions, requiring high hydrogen pressure (35 MPa) and elevated temperature (600 C) [7]. Consequently, several novel approaches have been developed to lower the H2 evolution temperature, increase the hydrogen desorption capacity and promote the rehydrogenation reaction under mild condition [10]. Some conventional strategies such as the destabilization with various effective catalysts [11,12], confinement in nanoporous materials [13], and partial cation substitution [14] have been tried to decrease the dehydrogenation temperature and improve the reversibility of LiBH4. Of particular interest is the potential to utilize nanostructured and porous carbon materials as effective catalysts and nanoscale frameworks for LiBH4, which help to realize high hydrogen release amount at markedly reduced temperature and easy rehydrogenation reaction under moderate condition through combining large surface area and tunable pore size [15e23]. In the case that LiBH4 was ballmilled with multi-walled carbon nanotubes (MWNTs), with a mass ratio of 2:1, the LiBH4/MWNTs mixtures showed excellent dehydrogenation properties, hydrogen desorption starting at 250 C, and ca. 3.8 wt.% hydrogen can be released after rehydrogenated at 10 MPa hydrogen pressure and 400 C [15]. When LiBH4 is doped with single-walled carbon nanotubes (SWCNTs), with a mass ratio of 7:3, over 6.0 wt % hydrogen can be recharged at 400 C under an initial hydrogen pressure of 10 MPa [16]. Agresti et al. also reported that the dehydrogenation temperature of LiBH4 can be decreased by more than 60 C when LiBH4 was dispersed on multi-walled carbon nanotubes (MWCNTs) by solvent infiltration method [17]. Addition of purified SWNTs into LiBH4/MgH2, with a mass ratio of 9:1, shows that the initial hydrogen desorption temperature is lowered to 320 C, and nearly 10 wt.% of hydrogen can be released within 20 min at 450 C, which is about two times faster than that of the neat LiBH4/MgH2 sample [18]. Experimental measurements of Majzoub et al. indicate that the onset dehydrogenation temperature of highly ordered hexagonal nanoporous hard carbon (NPC) supported premelted LiBH4, with the average size of 2 nm, was dramatically reduced from 460 to 220 C [19]. Vajo et al. infiltrated LiBH4 into carbon aerogel hosts with average pore sizes of 13 and 25 nm, and demonstrated enhanced desorption kinetics and reversibility [20]. Also, it has been qualitatively confirmed that LiBH4 doped with disordered mesoporous carbon, to a mass ratio of 1:1, gives rise to a large amount of hydrogen at 332 C [21]. Furthermore, our recent findings prove that Vulcan XC-72 carbon is highly effective for improving the dehydrogenation properties of LiBH4. The first dehydrogenation temperature for the Vulcan XC-72 carbon doped LiBH4 sample starts at ca. 320 C and nearly 13.2 wt. % H2 can be desorbed below 700 C [22,23]. Similar catalytic effects of carbon addition on the dehydrogenation kinetics and cyclic reversibility have also been observed for the LieBeMgeH system [18].
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Quite recently, graphene, the first two-dimensional atomic crystal, emerges as a conceptually new class of carbon material and its unique planar nanostructure and unusual properties promise the potential applications in a wide range of areas such as electronics, sensors, and energy storage [24]. Furthermore, graphene with large specific area and tunable pore structure have received continuous interest as potential hydrogen storage media by physical adsorption [25,26]. In this regard, we report a doping strategy to improve the dehydrogenating properties of LiBH4 that involves the addition of various quantities of graphene as a host for LiBH4 in this paper.
2.
Experimental section
2.1.
Materials
Commercially acquired LiBH4 (95% purity, J&K Chemical Ltd, Sweden), natural graphite (99.9995 wt.% purity, Alfa Aesar Ltd, UK), NaNO3 (99% purity, Sinopharm Chemical Reagent Co. Ltd, China), H2SO4 (96% purity, Sinopharm Chemical Reagent Co. Ltd, China), KMnO4 (99% purity, Sinopharm Chemical Reagent Co. Ltd, China), H2O2 (30 wt.% aqueous solution, Sinopharm Chemical Reagent Co. Ltd, China), Vulcan XC-72 carbon (with a specific surface area of 254 m2 g1, Cobat. Inc., USA), BP2000 carbon (with a specific surface area of 1450 m2 g1, Cobat. Inc., USA), and Carbon nanotube (>95% purity, with a specific surface area of 150 m2 g1, Shenzhen Nanotech Port Ltd. Co., China) were used as received.
2.2.
Preparation of graphene
As described in literatures [27], graphene used in present study was made by complete oxidation followed by microwave and thermal reduction method. In brief, 1 g natural graphite powders and 1 g NaNO3 were placed in a reaction vessel that was preliminarily immersed in an ice bath, followed by slowly adding 35 ml H2SO4 under violent stirring. After 5 g KMnO4 was gradually added over about 1 h, the mixture was gently stirred at room temperature and allowed to go on for 120 h to fully oxidize graphite powder to graphite oxide (GO). Subsequently, the GO obtained was added to 100 ml of 5 wt.% H2SO4 aqueous solutions over about 2 h with gentle stirring, and then 100 ml of 30 wt.% H2O2 was added. After centrifugation and washing procedure with a mixed aqueous solution of 3 wt.% H2SO4 and 0.5 wt.% H2O2, GO powders obtained were irradiated in a domestic microwave oven (1100 W for 1 min) and then filtrated, washed and dried at 65 C for 24 h. Afterwards, the above chemical activated GO powders were put in a tube furnace under flowing argon and heated at 800 C for 1 h. Finally, the final products were then centrifuged, washed, and vacuum-dried.
2.3.
Doping LiBH4 with graphene
The mixture of LiBH4 and graphene was prepared through a ball milling method: 1 g of LiBH4 and graphene mixture with
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various mass ratios was mechanically milled for 1.5 h (Planetary QM-1SP2) under argon atmosphere at room temperature, respectively. The ball-to-power weight ratio was 30:1 at 580 rpm using stainless steel balls of 10 mm diameter. The mass ratios of graphene and LiBH4 are 20:80, 40:60, 70:30 and 85:15, respectively.
2.4.
Hydrogen releasing property measurements
A Netzsch STA449C TG-DSC thermoanalyzer coupled with a Balzers Thermostar Quadrupole Mass Spectrometer was used to investigate the hydrogen releasing properties. The heating rate was 10 C min1, with argon flowing at a purging rate of 20 cm3 min1. Typical sample quantity used was ca. 5e10 mg. For comparison with the neat system, the hydrogen capacity is calculated based only on the mass of LiBH4. Dehydrogenation/rehydrogenation performances of the samples were examined by using a carefully calibrated Sievert’s type apparatus. A typical cyclic experiment entailed dehydrogenation at 550 C, and rehydrogenation at 400 C with an initial 3 MPa hydrogen pressure for 10 h. As it is difficult to accurately characterize the rehydrogenation process under high hydrogen pressure, the restored hydrogen amounts were precisely determined in the subsequent dehydrogenation half-cycle.
2.5.
Structure and morphology characterization
The morphology of the graphene as prepared was analyzed by transmission electron microscopy (TEM) using a Technai G2 20s-Twin Microscope (FEP Inc., USA). TEM samples were prepared by dispersing dry graphene powders in ethanol to form a homogeneous suspension, and then, the suspension was dropped on 200-mesh copper grid for observation. X-ray diffraction (XRD) measurements were carried out on a Rigaku D/MAX-2500 diffractometer using Cu Ka radiation. Diffraction patterns were collected at a scanning rate of 2 min1 and with a step of 0.02 . Surface area and porosity of the graphene sample was obtained from N2 adsorptionedesorption isotherm and BarreteJoynereHalenda (BJH) plots (Micromeritics porosimeter ASAP 2010).
3.
Results and discussion
3.1.
Microstructure characterization of graphene
The morphology of the graphene as prepared was observed by TEM analysis, as presented in Fig. 1. Large, disordered and over-lapped multilayer graphene nanosheets resemble rippled silk veil waves, entangled with each other to form an interconnected network. The area of the above graphene sheets reaches a few hundred square nanometers and its thickness approximates to 1.9 nm. Compared with the reported thicknesses of different layer graphemes [28], the graphene sheets obtained by complete oxidation followed by microwave and thermal reduction method in nitrogen atmosphere possess approximately three stacked individual graphene layers. The nitrogen adsorptionedesorption isotherm of graphene sheets and pore size distribution plot are shown in Fig. 2. Specific surface area and pore-size characterization of the graphene were performed by coupling high-resolution nitrogen at 77.4 K. The BrunauereEmmetteTeller (BET) specific surface area of obtained graphene sheets is about 2340 m2 g1 calculated from the desorption line, less than the theoretical specific surface area of single-layer graphene sheets (2620 m2 g1). Furthermore, it should be pointed out that a hysteresis loop in the nitrogen desorption branch was also observed, proving that the as-prepared graphene sheets are porous system [27]. At the same time, the pore size distribution plot reveals that the pores in graphene have a narrow micromesopores size distribution centered at ca. 3.6 nm, with a pore volume of 1.9 cm3 g1 (see the inset in Fig. 2).
3.2. LiBH4
Dehydrogenation properties of graphene doped
The thermal dehydrogenation curves as a function of temperature present the hydrogen release properties of LiBH4 doped with and without graphene. Fig. 3 shows temperature programmed hydrogen release profiles (A) and corresponding thermogravimetric (TG) curves (B) of ball-milled pure LiBH4 and as-prepared graphene doped LiBH4 with different mass ratios; while the detailed data are listed in Table 1. Four hydrogen desorption peaks of ball-milled pure LiBH4 respectively corresponds to the structural transition (80e110 C),
Fig. 1 e TEM images of graphene prepared by microwave and thermal reduction method.
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Quantity Adsorbed (cm 3 g-1 )
1800 1500
Adsorption Desorption Pore Volume (cm3 g-1 )
1200 900 600 300 0
0.0
0.2
2.0 1.5 1.0 0.5 0.0 0
0.4
2
4
6
8
10 12 14 16
Pore Diameter (nm)
0.6
0.8
1.0
Relative Pressure (P/Po) Fig. 2 e N2 adsorption/desorption isotherm curves of graphene (w2340 m2 gL1) at 77.4 K. The inset shows the pore size distribution plot for the as-prepared graphene.
(A) o
300 C o
102 C 65 C o
o
195 C
(e) o
300 C
Relative Intensity (a.u.)
o
90 C
o
200 C
(d) o
300 C
o
88 C
o
465 C
o
215 C
(c) o
310 C o
o
515 C
o
82 C
230 C
(b) o
485 C o 420 C
o
98 C
100
200
300
o
610 C
(a)
400
500
600
700
600
700
o
Temperature ( C)
(B) 100
(a)
Weight (%)
95
(b) 90
(c) (d)
85
(e)
100
200
300
400
500
Temperature ( oC) Fig. 3 e Temperature programmed hydrogen release profiles (A) and TG curves (B) for ball-milled pure LiBH4 (a) and the mixture of graphene and LiBH4 with various mass ratio (bee). The mass ratios of graphene and LiBH4 are 20:80 (b), 40:60 (c), 70:30 (d) and 85:15 (e), respectively.
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melting reaction (280e310 C) and dehydrogenation reactions (420e510 and 560e650 C) (curve a) [7,29]. The onset temperature for main hydrogen desorption is ca. 420 C, while the two main desorption peaks are located at 485 and 610 C, and the weight loss corresponds to 10.7 wt.%. However, by adding 20 wt.% graphene catalyst to the LiBH4 compound, the dehydrogenation temperature of LiBH4 is significantly reduced (curve b). The onset hydrogen desorption temperature of the 20 wt.% graphene doped LiBH4 is located at 230 C, resulting in a release of ca. 5.9 wt.% hydrogen from LiBH4, which is quite different from pure LiBH4, where only a minor dehydrogenation of 0.6 wt.% occurs below 360 C. Also, in comparison to pure LiBH4, the other main dehydrogenation peaks of the 20 wt.% graphene doped LiBH4 shift to lower temperatures, with an increased total weight loss of 11.4 wt.% below 700 C. With increase in graphene content in the mixtures of graphene and LiBH4 (up to 85 wt. %), the onset dehydrogenation temperature and the two main desorption peaks from LiBH4 are found to gradually decrease while the total hydrogen release amount is found to increase. When LiBH4 is doped with 85 wt.% graphene, the onset dehydrogenation temperature approach 195 C, and the major desorption peaks occur at 310 C (as shown in Part A of Fig. 3, top curve). Importantly, the dehydrogenation capacity of LiBH4 doped with 85 wt.% catalyst come to 17.9 wt.% based on LiBH4, which is very close to the theoretical value of ca. 18.5 wt.%. Hence, these results clearly demonstrate that dehydrogenation properties of LiBH4 is significantly improved by doping various amounts of graphene into LiBH4, which may due to the synergistic effect of remarkable increase in the contact area between graphene and LiBH4 and confinement in nanoporous graphene, which help to uniformly disperse LiBH4 and effectively avoid its agglomeration. In order to compare the catalytic effect of different carbon on the dehydrogenation and rehydrogenation properties of LiBH4, hydrogen desorption mass spectra (MS) and TG profiles of various carbon doped LiBH4 composites and that of pure LiBH4 were presented in Fig. 4; while the detailed data are listed in Table 2. It was observed that the onset dehydrogenation temperature of LiBH4 was respectively reduced from 420 C for pure LiBH4 to 320, 280, 280, and 230 C for 20 wt.% vulcan C, carbon nanotube, BP2000, and graphene doped LiBH4. Furthermore, the released hydrogen amount was greatly increased after adding various carbon additives, in comparison with neat LiBH4, demonstrating that by mechanically milling with different carbon additives, the dehydrogenation performances of LiBH4 sample can be considerably improved. The difference of onset dehydrogenation temperature may come from their distinct specific surface area and pore distribution. Noticeably, among these carbon additives, graphene exhibits the best catalytic effect on the decomposition of LiBH4, with a dehydrogenation capacity of 9.8 wt.% below 550 C, which may derive from its enormous specific surface area and suitable pore size. After the hydrogen desorption performance of graphene doped LiBH4 has been ascertained, its dehydrogenation pressureecomposition isotherms were collected to further investigate its thermodynamic properties and dehydrogenation behavior. Pressureecompositionetemperature (PCT) measurements on the ball-milled pure LiBH4 and 20 wt.% graphene doped LiBH4 were collected in the range of 350e450 C, as shown
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Table 1 e Hydrogen release properties of the mixtures of graphene and LiBH4 with various mass ratios. Sample
LiBH4 Graphene Graphene Graphene Graphene
doped doped doped doped
Graphene doped amount (wt.%)
Onset dehydrogenation temperature ( C)
Main dehydrogenation temperature ( C)
Total dehydrogenation capacity (wt.%)
0 20 40 70 85
420 230 215 200 195
485; 610 310; 515 300; 465 300 300
10.7 11.4 12.3 13.3 17.9
LiBH4 LiBH4 LiBH4 LiBH4
in Fig. 5. No plateaus can be observed for the pure LiBH4 sample measured at 400 C (curve a), while the dehydrogenation isotherms of graphene doped LiBH4 show obvious plateaus (curve bed). As for the dehydrogenation of 20 wt.% graphene
(A)
o
310 C o
o
82 C
(e)
230 C o
Relative Intensity (a.u.)
460 C o
o
280 C
90 C
(d) o
490 C o
(c)
o
90 C
280 C o
485 C o
(b)
o
80 C
320 C o
o
o
98 C
485 C
420 C
100
200
300
(a)
400
500
o
Temperature ( C)
(B) 100
(c)
98
Weight (%)
3.3. LiBH4
(a)
(b)
(d) (e)
96 94 92 90
100
200
300
400
doped LiBH4 at 450 C (curve d), the dehydrogenation plateau was kept from 2.2 to 7.8 wt.%, with total hydrogen capacities of approximately 9.2 wt.%. The Van’t Hoff plot for the dehydrogenation reaction of the 20 wt.% graphene doped LiBH4 system (logarithm of the equilibrium pressure versus the inverse of the absolute temperature), using the medium equilibrium pressures, is shown in the inset of Fig. 5. According to Van’t Hoff equation and the determined thermodynamic parameters, the resultant Van’t Hoff equation of 20 wt.% graphene doped LiBH4 sample can be expressed numerically as ln ( peq/p0) ¼ 4853/ T þ 9.19, and the dehydrogenation reaction enthalpy changes (DH ) are calculated as ca. 40 kJ mol1 H2, which is far less than that of pure LiBH4 (ca. 74 kJ mol1 H2) [11,17], demonstrating that the dehydrogenation thermodynamics of LiBH4 is largely improved through the doping of graphene. Fig. 6 presents the first dehydrogenation profiles of pure LiBH4 and as-prepared 20 wt.% graphene doped LiBH4 sample at 450 C. Within 90 min, pure LiBH4 released only ca. 4.3 wt.% hydrogen. In contrast, ca. 9.2 wt.% hydrogen could be released from the graphene doped LiBH4 sample. Evidently, graphene possess a promoting effect on the dehydrogenation amount and kinetics of LiBH4.
500
Temperature ( oC ) Fig. 4 e Temperature programmed hydrogen release profiles (A) and TG curves (B) for pure LiBH4 (a), and Vulcan C (b), carbon nanotube (c), BP2000 (d), graphene (e) doped LiBH4 that were mechanically milled under Ar atmosphere for 1 h. The doping amount of various carbon is 20 wt.% and applied heating rate is 10 C minL1.
Rehydrogenation properties of graphene doped
The theoretical reversible hydrogen storage of pure LiBH4 is governed by the equilibrium, LiBH4 # LiH þ B þ 1.5H2, which accounts for a reversible capacity of 13.9 wt %. However, the rehydrogenation condition is rather demanding due to slow kinetics, which leads to serious cyclic capacity degradation [30]. In order to develop a reversible H2 storage and release system and recover LiBH4 at mild condition, which is important for practical applications, 20 wt.% graphene doped LiBH4 sample that had been dehydrogenated under a vacuum up to 550 C was rehydrogenated at 400 C for 10 h under 3 MPa hydrogen pressure. Fig. 7 shows a comparison on cycling dehydrogenation properties between neat LiBH4 and 20 wt.% graphene doped LiBH4 after recharging with H2. Clearly, for the pure LiBH4 sample, only 2.1 wt.% hydrogen could be restored within 10 h at 400 C with an initial hydrogen pressure of 3 MPa after the first dehydrogenation (Fig. 7(A)). In contrast, the total H2 released amounts of the 20 wt.% graphene doped LiBH4 sample decreased from the initial 9.8 wt.% to the 6.9 wt.% in the second cycle, and further to 4.9 wt.% in the fifth cycle and to 4.0 wt.% in the tenth cycle (Fig. 7(B)),
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Table 2 e Hydrogen release properties of various carbon doped LiBH4 with a doping amount of 20 wt.% below 550 C. Sample
BET area of carbon (m2 g1)
Onset dehydrogenation temperature ( C)
Main dehydrogenation temperature ( C)
Total dehydrogenation capacity (wt.%)
254 150 1450 2340
320 280 280 230
485 490 460 310
9.5 8.8 9.6 9.8
Vulcan C doped LiBH4 Carbon nanotube doped LiBH4 BP2000 doped LiBH4 Graphene doped LiBH4
25
ln(p eq /p 0 )=-4853/T+9.19
100
ln(peq/p0)
20
2.0
1.5
15
98
D H =40.35K Jmol -1 H 2 1.4
1.5
-1
1000/T(K )
1.6
Weight (%)
Pressure(atm)
(A)
2.5
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-2
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H 2 amount desorped (wt.%)
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Weight (%)
Fig. 5 e Desorption PCT curves for pure LiBH4 at 400 C (a), 20 wt.% graphene doped LiBH4 at 350 (b), 400 (c), and 450 C (d), respectively. The inset shows the Van’t Hoff plot for the dehydrogenated graphene-doped LiBH4 sample.
0
400
Temperature ( oC)
H (weight%)
100
Fig. 7 e Comparison on the dehydrogenation cycle profiles between pure LBH4 (A) and 20 wt.% graphene doped LBH4 (B). The graphene doped LBH4 sample can be dehydrogenated (at 550 C for 5 h) and rehydrogenated (at 3 MPa H2, 400 C for 10 h) repeatedly and measured gravimetrically.
Time (min) Fig. 6 e Comparison on the first dehydrogenation curves between the pure LiBH4 (a) and 20 wt.% graphene doped LiBH4 (b) at 450 C.
indicative of higher cyclic capacity and better capacity retention than pure LiBH4. On the other hand, although the graphene doped LiBH4 sample still suffers from serious cyclic degradation, its capacity retention rate is obviously higher
than that of carbon nanotube, carbon aerogel, and mesoporous carbon doped LiBH4 sample [15,16,18,20,21]. Actually, similar degradation behavior, particularly the nearly or even more than 50 wt.% capacity loss in the first two cycles, was repeatedly observed in the studies of various carbon doped LiBH4 samples. Furthermore, as compared with transition metal (fluoride and hydroxides) doped LiBH4 samples reported in literatures [7,22,23,31,32], the rehydrogenation temperature and pressure conditions applied in the present study are
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 2 7 9 6 e2 8 0 3
$ $
&
B B $
*
B
$
B
Relative intensity (a.u.)
& # $ & # #
$ #
B
$
*
(d) & Li2B10H10
B B
*
# LiH * Li2C2
#
B
##
#
*
$ LiBH4
(c)
B $ $
&
B
*
$
B $
B
$
#
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# $ $
$
*
(b)
$
(a) 10
15
20
25
30
35
40
45
50
55
60
65
70
o
2q ( ) Fig. 8 e X-ray diffraction patterns of as-prepared graphene (a), 20 wt.% graphene doped LiBH4 before (b) and after (c) the first dehydrogenation at 550 C for 5 h, and after the tenth rehydrogenation (d) at 400 C for 10 h with an initial hydrogen pressure of 3 MPa.
significantly lower, indicating that the graphene additive is highly effective for promoting the rehydrogenation reaction and rendering the system partially reversible under markedly reduced temperature and pressure conditions, which may come from the existence of some irreversible dehydrogenation products. A discussion of the possible mechanism underlying this characteristic phenomenon will be given in the following section.
3.4. Dehydrogenation/rehydrogenation reaction mechanism In our efforts to understand the kinetic enhancement arising upon adding graphene, explore any possible structural change, and infer possible reaction mechanism during the dehydrogenation/rehydrogenation process, XRD patterns of 20 wt.% graphene doped LiBH4 before and after the initial dehydrogenation, and the sample after the tenth dehydrogenation at 550 C for 5 h and then rehydrogenation at 400 C for 10 h were acquired, as shown in Fig. 8. For graphene sheets, broad XRD out-of-plane graphitic reflections can be observed (curve a) [33], proving their stacks structure [27], in accordance with their TEM results. LiBH4, LiH, boron and Li2C2 diffraction peaks are observed for the mixture before the first dehydrogenation (curve b) except for broad graphene reflections, indicating new species appeared during the ball milling process of graphene and LiBH4; the appearance of LiH and boron peaks may result from slight decomposition of LiBH4 [22]. As for pure LiBH4 system, only LiH, B and H2 can be formed after the decomposition of LiBH4 [9]. However, after the first dehydrogenation at 550 C for 5 h, the LiBH4 phase disappears, and Li2C2, LiH and B phases are identified (curve c), indicating that the possible dehydrogenation reaction of LiBH4, catalyzed by graphene, may be described as LiBH4 / LiH þ B þ 1.5H2 and 2LiBH4 þ 2C / Li2C2 þ 2B þ 4H2, just like carbon nanotube and mesoporous carbon doped sample [15,21]. In the case of the
rehydrogenated sample (curve d), LiBH4 is reformed and new species, Li2B10H10, is detected, confirming the reversible dehydrogenation/rehydrogenation reactions of the graphene doped LiBH4 sample. Nevertheless, Li2C2, LiH and B phases still exist, showing incomplete rehydrogenation reaction, which may be the reason for the cyclic degradation of the graphene doped LiBH4 sample. Thus, we deduce that the reversible hydrogen uptake/release of graphene doped LiBH4 may be represented as follows: LiH þ B þ 1.5H2 # LiBH4 and 2LiH þ 10B þ 4H2 # Li2B10H10.
4.
Conclusions
Graphene prepared by microwave and thermal reduction method exhibited significantly improved catalytic effect on the reversible dehydrogenation and rehydrogenation properties of LiBH4. The mixture of graphene and LiBH4 prepared by ball milling method could reduce the onset decomposition temperature from 420 to 195 C, two major desorption temperature of LiBH4 decrease from 485 and 610 to 300 C, and the dehydrogenation enthalpy is reduced by ca. 34 kJ mol1 H2, which may be ascribed to the synergistic effect of remarkable increase in the contact area between graphene and LiBH4 and confinement in nanoporous graphene. Furthermore, graphene doped LiBH4 has better hydrogenation/dehydrogenation reversibility than other nanoporous carbon materials, with a capacity of approximately 4.0 wt.% in the tenth cycle under mild conditions. Thus, destabilizing LiBH4 using graphene is a new and promising way for the catalytic dehydrogenation and rehydrogenation of LiBH4.
Acknowledgment The authors gratefully acknowledge financial support from the National Science Foundation of China (21003015 and 21103014), the Science Foundation of Jiangsu Province (BK2012591, 12KJA150003, BE201113 and 2011Z0062), the Science Foundation of Changzhou (CJ20115020), the Foundation of Jiangsu Key Laboratory for Solar Cell Materials and Technology (201106), and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions for support of this work.
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