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MgH2 composites improved by the catalysis of CoNiB nanoparticles
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Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles Yanping Zhao b, Liangzhong Ding a,*, Tongsheng Zhong a, Huatang Yuan b, Lifang Jiao b,* a
College of Chemistry and Environment Engineering, Hunan City University, Yiyang, Hunan 413000, PR China Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Key Laboratory of Advanced Energy Materials Chemistry (MOE), Tianjin Key Lab of Metal and Moleculebased Material Chemistry, Nankai University, Tianjin 300071, PR China
b
article info
abstract
Article history:
2LiBH4/MgH2 system is a representative and promising reactive hydride composite for
Received 14 March 2014
hydrogen storage. However, the high desorption temperature and sluggish desorption ki-
Received in revised form
netics hamper its practical application. In our present report, we successfully introduce
30 April 2014
CoNiB nanoparticles as catalysts to improve the dehydrogenation performances of the
Accepted 11 May 2014
2LiBH4/MgH2 composite. The sample with CoNiB additives shows a significant desorption
Available online xxx
property. Temperature programmed desorption (TPD) measurement demonstrates that the peak decomposition temperatures of MgH2 and LiBH4 are lowered to be 315 C and 417 C
Keywords:
for the CoNiB-doped 2LiBH4/MgH2. Isothermal dehydrogenation analysis demonstrates
LiBH4
that approximately 10.2 wt% hydrogen can be released within 360 min at 400 C. In addi-
Hydrogen storage
tion, this study gives a preliminary evidence for understanding the CoNiB catalytic
Catalysis
mechanism of 2LiBH4/MgH2
CoNiB
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the rising demands for clear and more environmental friendly energy, hydrogen has attracted a lot of attention because its combustion product is water, a zero pollutant and its energy density is 142 MJ kg1 (three times higher than that of petroleum) [1,2]. However, achieving a safe, convenient hydrogen storage system is generally regarded as a critical prerequisite to realize the widespread use of hydrogen as a fuel [3,4]. Recently, intensive interest has been focused on
MgH2 and borohydrides due to their high hydrogen content and potential use for hydrogen storage. Especially, LiBH4 appears more intriguing for its high gravity (18.5 wt%) and volumetric hydrogen density (121 kg/m3) [5e8]. However, the strong chemical bonding in LiBH4 and chemical inertness of the product elemental boron (B) result in a high desorption temperature, sluggish dehydrogenation kinetic and poor reversibility. To address these problems, effective strategies, such as nanoconfinement [4,9e11], catalyst addition [12e14],
* Corresponding authors. Tel.: þ86 22 23504527; fax: þ86 22 23502604. E-mail addresses: [email protected] (L. Ding), [email protected] (L. Jiao). http://dx.doi.org/10.1016/j.ijhydene.2014.05.063 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhao Y, et al., Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.063
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destabilization [15,16] and partial anion/cation substitution [17,18] have been explored. Particularly, destabilization by various reactive additives, that is, the reactive hydride composites (RHCs) has opened up the possibility to develop higheperformance hydrogen storage materials. As destabilized agent, metal hydrides are of great interest and the destabilization concept was successfully exemplified by LiBH4/MH2 (M ¼ Nd, Ce, Mg, Ca, Sr, Y) system [19e27]. For example, LiBH4 þ NdH2 þ x liberated 6.0 wt% H2 at 370 C within 1.5 h [19]. In respect to 6LiBH4 þ CeH2 sample, a reversible hydrogen capacity was up to 6.0 wt%, near to the theoretical value [20]. In the case of LiBH4/MH2 system, a 2:1 mole ratio of LiBH4/MgH2 has been one of the most promising composites. Vajo et al.[28] reported that the reaction enthalpy of 2LiBH4/MgH2 was reduced by 25 kJ/mol H2 according to the following reaction 1: 2LiBH4 þ MgH2 42LiH þ MgB2 þ 4H2
(1)
In addition, the theoretical capacity of reaction 1 is 11.5 wt %. And the generation of MgB2 is prerequisite for the reverse reaction [29,30]. Unfortunately, it is difficult to form the MgB2 from Mg and LiBH4, which results in a long incubation period before the second dehydrogenation step and greatly degrades the desorption kinetics. The addition of suitable additives, such as chlorides [29,31,32], oxides [33,34], metal [35] to 2LiBH4/MgH2 can shorten or even eliminate the incubation period. However, these dopants will cause a decrease in the hydrogen capacity or generate some byproducts during continuous cycling. Interestingly, the dehydrogenation process of various transition metal salts shares the same mechanism, that is, the product transition metal borides actually act as active sites [31,36e39]. Motivated by these considerations, we have centered on catalyst screening, which is satisfied to not only lower desorption temperature of 2LiBH4/MgH2 but also promote the hydrogen release rate. CoNiB nanoparticles exhibit much superior activity [40e42]. For example, J.C. Ingersoll et al.[41] adopted CoNiB to catalyze the hydrolysis of NaBH4, finding that the hydrogen generation rate was 11 times faster than the reported rates. And in our previous work [43], CoNiB nanoparticles can enhance the hydrogen storage of LiBH4. Therefore, in our present work, we focus on the improvement of dehydrogenation properties of 2LiBH4/MgH2 by CoNiB additives and determine the corresponding dehydrogenation mechanism.
Experimental section Preparation of CoNiB nanoparticles The CoNiB nanoparticles were prepared by reducing NiCl2$6H2O, CoCl2$6H2O with KBH4. Firstly, 0.005 mol metal salts (CoCl2$6H2O and NiCl2$6H2O) were dissolved in 50 ml anhydrous ethanol, and the Co/(Co þ Ni) molar ratio (cCo) was 0.85. Secondly, 50 ml of 0.02 mol KBH4 aqueous solution containing 0.002 mol NaOH was added dropwise into the above solution and with vigorous stirring for 25 min. An ice bath was used to control the reaction temperature. The black reaction products were collected to concentrate, wash thoroughly with
distilled water and anhydrous ethanol until pH ¼ 7. Then, the sample was dried at 60 C under vacuum for 24 h, then annealed at 450 C for 3 h, and 400 C for 1 h under an argon atmosphere, respectively.
Synthesis of 2LiBH4/MgH2-10 wt% CoNiB The mixtures of 2LiBH4/MgH2 (mole ratio 2:1) and CoNiB in a weight ratio of 9:1 were milled for 4 h via a planetary ball milling device at a rate of 450 rpm under a hydrogen pressure of 10 bar. The ball-to-powder weight ratio was around 40:1. As a control sample, 2LiBH4/MgH2 was prepared by the same procedure.
Characterization The phase structure and texture/morphology of the asprepared samples were studied by powder X-ray diffraction (XRD, Rigaku D/Max PC2500, Cu Ka radiation), Scanning electron microscopy (SEM, HITACHI S-4800). Temperature programmed desorption (TPD) was conducted at a heating rate of 2 C min1 to measure the desorption properties of the composites. About 70 mg of sample was loaded in the reactor, then heated in the temperature range of 30 Ce600 C, at a ramping rate of 2 C/min in an Ar flow of 35 ml min1. Differential scanning calorimetry (DSC) was performed via a Q20P, TA Instruments. The hydrogen desorption/adsorption kinetics were evaluated by using a Sieverts type apparatus. Unless stated otherwise, the hydrogen capacity is calculated excluding the weight of the catalyst.
Results and discussion Materials characterization The XRD patterns and structures of CoNiB and 2LiBH4/MgH210wt% CoNiB were demonstrated in Fig. 1. As we can see in Fig. 1(a), there are no typical diffraction crystalline peaks appearing for CoNiB. One broad bump with low intensity at 45 is discernible, which indicates CoNiB is amorphous. And the TEM image (Fig. 1(b)) shows that with particle size of 10 nm, the as-synthesized CoNiB particles are uniformly distributed. An overall SEM morphology of 2LiBH4/MgH2-10wt % CoNiB is given in Fig. 1(c). It is shown that the product is comprised of particles in several microns. The appearances of Co, Ni signals given from the EDS elemental mapping in Fig. 2 assure the presences of CoNiB. Meanwhile, CoNiB nanoparticles are well dispersed in the sample 2LiBH4/MgH2-10wt% CoNiB, which plays an important role in strengthening its catalytic activity.
Hydrogen desorption properties The hydrogen desorption performances of as-prepared samples 2LiBH4/MgH2 and 2LiBH4/MgH2-10wt% CoNiB were examined via a TPD apparatus. As is shown in Fig. 3(a), there are two major hydrogen desorption peaks and a minor shoulder peak observed for the two samples. According to the neat 2LiBH4/MgH2, the first peak desorption temperature is at
Please cite this article in press as: Zhao Y, et al., Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.063
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Fig. 1 e (a and b) XRD pattern and TEM image of CoNiB, (c) SEM image of 2LiBH4/MgH2-10wt% CoNiB.
370 C, which is attributed to the decomposition of mainly MgH2. With further heating, a second decomposition occurs at 442 C. Since LiBH4 is known to release hydrogen in several steps, the minor shoulder peak taken place at 301 C may be the fusion of LiBH4. After adding a small amount of CoNiB (10 wt%), the sample 2LiBH4/MgH2-10 wt% CoNiB exhibits a remarkable reduction in temperature of two main dehydrogenation steps. The maximal dehydrogenation temperatures are at 315 C and 417 C, which are 55 C and 26 C lower than those of 2LiBH4/MgH2 composite, respectively. Moreover, it starts to release hydrogen at 180 C. The quantification of the total amount of hydrogen release evaluated is given in Fig. 3(b). A total hydrogen release capacity of 10.8 wt% was obtained for 2LiBH4/MgH2-10 wt% CoNiB below 500 C, while 2LiBH4/MgH2 only emitted 9.4 wt% H2. From the above results, it is clearly seen that by doping only 10 wt% CoNiB in the 2LiBH4/MgH2 system, pronounced property improvements with lower desorption temperatures and higher hydrogen storage capacity are achieved. The thermal decomposition behavior of the as-milled 2LiBH4/MgH2 and 2LiBH4/MgH2-10 wt% CoNiB without hydrogen back pressure was conducted by DSC in Fig. 4. As we can see, there are four distinct endothermic peaks, which are assigned to the orthorhombic-hexagonal phase transition of LiBH4, the melting of LiBH4, the decomposition of MgH2, and the dehydrogenation of LiBH4. Interestingly, the four peaks of 2LiBH4/MgH2-10 wt% CoNiB are shifted to lower temperatures. Especially, the decomposition temperature of LiBH4, MgH2 in 2LiBH4/MgH2-10 wt% CoNiB are reduced from 380 C,
427 C to 353 C, 388 C, respectively. The desorption temperatures of LiBH4, MgH2 for the two samples obtained by TPD, DSC are different. The distinct dehydrogenation measurement conditions may account for this phenomenon, which has been reported previously [37]. The TPD measurement was operated in a 35 ml Ar flow with a heating rate of 2 C/min, while DSC measurement was run under 1 bar Ar with a 4 C/ min heating rate. Generally, the faster the heating rate, the higher the decomposition temperature. In order to further clarify the dehydrogenation properties of the 2LiBH4/MgH2-10 wt% CoNiB system, the isothermal experiment was conducted via a Sievert-type apparatus. As shown in Fig. 5, when the heating temperature is at 350 C, the neat 2LiBH4/MgH2 sample has a poor kinetic, with a long incubation period for the second dehydrogenation step. Interestingly, the 2LiBH4/MgH2-10 wt% CoNiB composite has a similar desorption step with that of pure 2LiBH4/MgH2 at 350 C, but a much faster desorption kinetics occurs. With the temperature increases, an incubation period gradually disappears. At 400 C, the problematic incubation completely disappears for 2LiBH4/MgH2-10 wt% CoNiB. It can escape about 10.2 wt% hydrogen within 360 min. These results demonstrate that the mixing with CoNiB exerts a remarkable acceleration on the desorption rate of 2LiBH4/MgH2 without hydrogen back pressure. To gain insight into the catalytic effect of CoNiB on the desorption kinetics, the activation energy Ea for 2LiBH4/MgH210 wt% CoNiB was determined by using the non-isothermal Kissinger method as described below:
Fig. 2 e (a) SEM image of 2LiBH4/MgH2-10 wt% CoNiB and the corresponding EDS elemental mapping images of, (b) Co, (c) Ni. Please cite this article in press as: Zhao Y, et al., Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.063
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Fig. 3 e (a) Thermally programmed H2 desorption curves and, (b) the corresponding thermally programmed H2 desorption capacity curves of 2LiBH4/MgH2, 2LiBH4/MgH2-10 wt% CoNiB.
ln
b T2m
!
Ea ¼ RT
where b is the heating rate, Tm is the absolute temperature for the maximum hydrogen release, R is the gas constant. Herein, Tm was extracted from TPD curves with heating rates of 5, 8, 10 C/min (not shown here). Fig. 6 presents the Kissinger plot for the 2LiBH4/MgH2-10 wt% CoNiB system. The intrinsic linearity of all the curves demonstrates that the desorption rate of 2LiBH4/MgH2-10 wt% CoNiB system is well characterized by the non-isothermal Kissinger method. The calculated apparent activation energies are 137, 116 kJ/mol, which are lower than those of pure MgH2 (~168 kJ/mol) [7,44,45], LiBH4 (~146 kJ/mol) [46]. The above decreases of activation energy suggest the kinetic barrier of 2LiBH4/MgH2 is effectively reduced by CoNiB nanoparticles under no hydrogen back pressure conditions. In order to understand the catalytic mechanism of CoNiB, the phase evolution of 2LiBH4/MgH2-10 wt% CoNiB before and after dehydrogenation without hydrogen back pressure was carefully examined by XRD. With respect to 2LiBH4/MgH210wt% CoNiB, no new phases show up except for LiBH4, MgH2, indicating that in the milling process no solid-phase reaction
Fig. 4 e DSC profiles of the as-milled 2LiBH4/MgH2 and 2LiBH4/MgH2-10 wt% CoNiB at a heating ramp of 4 C/min.
Fig. 5 e Isothermal dehydrogenation curves of the neat 2LiBH4/MgH2 and 2LiBH4/MgH2-10 wt% CoNiB at different temperatures.
Fig. 6 e Kissinger plot of 2LiBH4/MgH2-10 wt% CoNiB composites: (a) the dehydrogenation of MgH2, (b) the dehydrogenation of LiBH4.
Please cite this article in press as: Zhao Y, et al., Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.063
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(51171083), MOE (IRT-13R30 and IRT13022), the Natural Science Fund of Hunan (2012FJ3023) and 111 Project (B12015).
references
Fig. 7 e XRD patterns of 2LiBH4/MgH2-10wt% CoNiB before, (I) and after, (II) dehydrogenation at 400 C.
occurs. As displayed in Fig. 7(II), except the peaks of parafilm, the major diffraction peaks are corresponding to Mg. And a trace mount of MgO, LiH can be found in the desorption products. There are no signals for the formation of element B, which may be attributed to the fact that the boron is amorphous. Also, the peaks of Co, Ni-containing phases can be detected in XRD patterns, making us believe that boride nanoparticles indeed act as active sites. The above major dehydrogenation product obtained under the condition of no hydrogen back pressure is different from that of the condition with hydrogen back pressure, that is, the decomposition mechanism is related to the physical conditions [29,32,47e50]. Therefore, later study is supposed on the effect of hydrogen back pressure on the 2LiBH4/MgH2-10 wt% CoNiB.
Conclusion In this work, the catalytic effect of CoNiB on the hydrogen storage performances of 2LiBH4/MgH2 system was investigated. CoNiB exhibits a good activity. For example, compared with the pristine 2LiBH4/MgH2, two lower dehydrogenation temperatures declined by 55 C, 26 C via a TPD analysis were observed for 2LiBH4/MgH2-10 wt% CoNiB. Moreover, the apparent activation energies for the first and second decomposition step are significantly decreased to 137, 116 kJ/mol after adding 10 wt% CoNiB. Therefore, directly introducing CoNiB truly enhances the dehydrogenation properties of 2LiBH4/MgH2 system. And the excellent catalysis of CoNiB may provide other transition metal borides a valuable hint to improve the dehydrogenation properties of other complex RHC composites such as LiBH4eMg(BH4)2, LiBH4eAl etc.
Acknowledgments This work was financially supported by 973 program (2010CB631303), National Natural Science Foundation of China
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Please cite this article in press as: Zhao Y, et al., Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.063
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ScienceDirect journal homepage: www.elsevier.com/locate/he
Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles Yanping Zhao b, Liangzhong Ding a,*, Tongsheng Zhong a, Huatang Yuan b, Lifang Jiao b,* a
College of Chemistry and Environment Engineering, Hunan City University, Yiyang, Hunan 413000, PR China Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Key Laboratory of Advanced Energy Materials Chemistry (MOE), Tianjin Key Lab of Metal and Moleculebased Material Chemistry, Nankai University, Tianjin 300071, PR China
b
article info
abstract
Article history:
2LiBH4/MgH2 system is a representative and promising reactive hydride composite for
Received 14 March 2014
hydrogen storage. However, the high desorption temperature and sluggish desorption ki-
Received in revised form
netics hamper its practical application. In our present report, we successfully introduce
30 April 2014
CoNiB nanoparticles as catalysts to improve the dehydrogenation performances of the
Accepted 11 May 2014
2LiBH4/MgH2 composite. The sample with CoNiB additives shows a significant desorption
Available online xxx
property. Temperature programmed desorption (TPD) measurement demonstrates that the peak decomposition temperatures of MgH2 and LiBH4 are lowered to be 315 C and 417 C
Keywords:
for the CoNiB-doped 2LiBH4/MgH2. Isothermal dehydrogenation analysis demonstrates
LiBH4
that approximately 10.2 wt% hydrogen can be released within 360 min at 400 C. In addi-
Hydrogen storage
tion, this study gives a preliminary evidence for understanding the CoNiB catalytic
Catalysis
mechanism of 2LiBH4/MgH2
CoNiB
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the rising demands for clear and more environmental friendly energy, hydrogen has attracted a lot of attention because its combustion product is water, a zero pollutant and its energy density is 142 MJ kg1 (three times higher than that of petroleum) [1,2]. However, achieving a safe, convenient hydrogen storage system is generally regarded as a critical prerequisite to realize the widespread use of hydrogen as a fuel [3,4]. Recently, intensive interest has been focused on
MgH2 and borohydrides due to their high hydrogen content and potential use for hydrogen storage. Especially, LiBH4 appears more intriguing for its high gravity (18.5 wt%) and volumetric hydrogen density (121 kg/m3) [5e8]. However, the strong chemical bonding in LiBH4 and chemical inertness of the product elemental boron (B) result in a high desorption temperature, sluggish dehydrogenation kinetic and poor reversibility. To address these problems, effective strategies, such as nanoconfinement [4,9e11], catalyst addition [12e14],
* Corresponding authors. Tel.: þ86 22 23504527; fax: þ86 22 23502604. E-mail addresses: [email protected] (L. Ding), [email protected] (L. Jiao). http://dx.doi.org/10.1016/j.ijhydene.2014.05.063 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhao Y, et al., Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.063
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destabilization [15,16] and partial anion/cation substitution [17,18] have been explored. Particularly, destabilization by various reactive additives, that is, the reactive hydride composites (RHCs) has opened up the possibility to develop higheperformance hydrogen storage materials. As destabilized agent, metal hydrides are of great interest and the destabilization concept was successfully exemplified by LiBH4/MH2 (M ¼ Nd, Ce, Mg, Ca, Sr, Y) system [19e27]. For example, LiBH4 þ NdH2 þ x liberated 6.0 wt% H2 at 370 C within 1.5 h [19]. In respect to 6LiBH4 þ CeH2 sample, a reversible hydrogen capacity was up to 6.0 wt%, near to the theoretical value [20]. In the case of LiBH4/MH2 system, a 2:1 mole ratio of LiBH4/MgH2 has been one of the most promising composites. Vajo et al.[28] reported that the reaction enthalpy of 2LiBH4/MgH2 was reduced by 25 kJ/mol H2 according to the following reaction 1: 2LiBH4 þ MgH2 42LiH þ MgB2 þ 4H2
(1)
In addition, the theoretical capacity of reaction 1 is 11.5 wt %. And the generation of MgB2 is prerequisite for the reverse reaction [29,30]. Unfortunately, it is difficult to form the MgB2 from Mg and LiBH4, which results in a long incubation period before the second dehydrogenation step and greatly degrades the desorption kinetics. The addition of suitable additives, such as chlorides [29,31,32], oxides [33,34], metal [35] to 2LiBH4/MgH2 can shorten or even eliminate the incubation period. However, these dopants will cause a decrease in the hydrogen capacity or generate some byproducts during continuous cycling. Interestingly, the dehydrogenation process of various transition metal salts shares the same mechanism, that is, the product transition metal borides actually act as active sites [31,36e39]. Motivated by these considerations, we have centered on catalyst screening, which is satisfied to not only lower desorption temperature of 2LiBH4/MgH2 but also promote the hydrogen release rate. CoNiB nanoparticles exhibit much superior activity [40e42]. For example, J.C. Ingersoll et al.[41] adopted CoNiB to catalyze the hydrolysis of NaBH4, finding that the hydrogen generation rate was 11 times faster than the reported rates. And in our previous work [43], CoNiB nanoparticles can enhance the hydrogen storage of LiBH4. Therefore, in our present work, we focus on the improvement of dehydrogenation properties of 2LiBH4/MgH2 by CoNiB additives and determine the corresponding dehydrogenation mechanism.
Experimental section Preparation of CoNiB nanoparticles The CoNiB nanoparticles were prepared by reducing NiCl2$6H2O, CoCl2$6H2O with KBH4. Firstly, 0.005 mol metal salts (CoCl2$6H2O and NiCl2$6H2O) were dissolved in 50 ml anhydrous ethanol, and the Co/(Co þ Ni) molar ratio (cCo) was 0.85. Secondly, 50 ml of 0.02 mol KBH4 aqueous solution containing 0.002 mol NaOH was added dropwise into the above solution and with vigorous stirring for 25 min. An ice bath was used to control the reaction temperature. The black reaction products were collected to concentrate, wash thoroughly with
distilled water and anhydrous ethanol until pH ¼ 7. Then, the sample was dried at 60 C under vacuum for 24 h, then annealed at 450 C for 3 h, and 400 C for 1 h under an argon atmosphere, respectively.
Synthesis of 2LiBH4/MgH2-10 wt% CoNiB The mixtures of 2LiBH4/MgH2 (mole ratio 2:1) and CoNiB in a weight ratio of 9:1 were milled for 4 h via a planetary ball milling device at a rate of 450 rpm under a hydrogen pressure of 10 bar. The ball-to-powder weight ratio was around 40:1. As a control sample, 2LiBH4/MgH2 was prepared by the same procedure.
Characterization The phase structure and texture/morphology of the asprepared samples were studied by powder X-ray diffraction (XRD, Rigaku D/Max PC2500, Cu Ka radiation), Scanning electron microscopy (SEM, HITACHI S-4800). Temperature programmed desorption (TPD) was conducted at a heating rate of 2 C min1 to measure the desorption properties of the composites. About 70 mg of sample was loaded in the reactor, then heated in the temperature range of 30 Ce600 C, at a ramping rate of 2 C/min in an Ar flow of 35 ml min1. Differential scanning calorimetry (DSC) was performed via a Q20P, TA Instruments. The hydrogen desorption/adsorption kinetics were evaluated by using a Sieverts type apparatus. Unless stated otherwise, the hydrogen capacity is calculated excluding the weight of the catalyst.
Results and discussion Materials characterization The XRD patterns and structures of CoNiB and 2LiBH4/MgH210wt% CoNiB were demonstrated in Fig. 1. As we can see in Fig. 1(a), there are no typical diffraction crystalline peaks appearing for CoNiB. One broad bump with low intensity at 45 is discernible, which indicates CoNiB is amorphous. And the TEM image (Fig. 1(b)) shows that with particle size of 10 nm, the as-synthesized CoNiB particles are uniformly distributed. An overall SEM morphology of 2LiBH4/MgH2-10wt % CoNiB is given in Fig. 1(c). It is shown that the product is comprised of particles in several microns. The appearances of Co, Ni signals given from the EDS elemental mapping in Fig. 2 assure the presences of CoNiB. Meanwhile, CoNiB nanoparticles are well dispersed in the sample 2LiBH4/MgH2-10wt% CoNiB, which plays an important role in strengthening its catalytic activity.
Hydrogen desorption properties The hydrogen desorption performances of as-prepared samples 2LiBH4/MgH2 and 2LiBH4/MgH2-10wt% CoNiB were examined via a TPD apparatus. As is shown in Fig. 3(a), there are two major hydrogen desorption peaks and a minor shoulder peak observed for the two samples. According to the neat 2LiBH4/MgH2, the first peak desorption temperature is at
Please cite this article in press as: Zhao Y, et al., Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.063
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Fig. 1 e (a and b) XRD pattern and TEM image of CoNiB, (c) SEM image of 2LiBH4/MgH2-10wt% CoNiB.
370 C, which is attributed to the decomposition of mainly MgH2. With further heating, a second decomposition occurs at 442 C. Since LiBH4 is known to release hydrogen in several steps, the minor shoulder peak taken place at 301 C may be the fusion of LiBH4. After adding a small amount of CoNiB (10 wt%), the sample 2LiBH4/MgH2-10 wt% CoNiB exhibits a remarkable reduction in temperature of two main dehydrogenation steps. The maximal dehydrogenation temperatures are at 315 C and 417 C, which are 55 C and 26 C lower than those of 2LiBH4/MgH2 composite, respectively. Moreover, it starts to release hydrogen at 180 C. The quantification of the total amount of hydrogen release evaluated is given in Fig. 3(b). A total hydrogen release capacity of 10.8 wt% was obtained for 2LiBH4/MgH2-10 wt% CoNiB below 500 C, while 2LiBH4/MgH2 only emitted 9.4 wt% H2. From the above results, it is clearly seen that by doping only 10 wt% CoNiB in the 2LiBH4/MgH2 system, pronounced property improvements with lower desorption temperatures and higher hydrogen storage capacity are achieved. The thermal decomposition behavior of the as-milled 2LiBH4/MgH2 and 2LiBH4/MgH2-10 wt% CoNiB without hydrogen back pressure was conducted by DSC in Fig. 4. As we can see, there are four distinct endothermic peaks, which are assigned to the orthorhombic-hexagonal phase transition of LiBH4, the melting of LiBH4, the decomposition of MgH2, and the dehydrogenation of LiBH4. Interestingly, the four peaks of 2LiBH4/MgH2-10 wt% CoNiB are shifted to lower temperatures. Especially, the decomposition temperature of LiBH4, MgH2 in 2LiBH4/MgH2-10 wt% CoNiB are reduced from 380 C,
427 C to 353 C, 388 C, respectively. The desorption temperatures of LiBH4, MgH2 for the two samples obtained by TPD, DSC are different. The distinct dehydrogenation measurement conditions may account for this phenomenon, which has been reported previously [37]. The TPD measurement was operated in a 35 ml Ar flow with a heating rate of 2 C/min, while DSC measurement was run under 1 bar Ar with a 4 C/ min heating rate. Generally, the faster the heating rate, the higher the decomposition temperature. In order to further clarify the dehydrogenation properties of the 2LiBH4/MgH2-10 wt% CoNiB system, the isothermal experiment was conducted via a Sievert-type apparatus. As shown in Fig. 5, when the heating temperature is at 350 C, the neat 2LiBH4/MgH2 sample has a poor kinetic, with a long incubation period for the second dehydrogenation step. Interestingly, the 2LiBH4/MgH2-10 wt% CoNiB composite has a similar desorption step with that of pure 2LiBH4/MgH2 at 350 C, but a much faster desorption kinetics occurs. With the temperature increases, an incubation period gradually disappears. At 400 C, the problematic incubation completely disappears for 2LiBH4/MgH2-10 wt% CoNiB. It can escape about 10.2 wt% hydrogen within 360 min. These results demonstrate that the mixing with CoNiB exerts a remarkable acceleration on the desorption rate of 2LiBH4/MgH2 without hydrogen back pressure. To gain insight into the catalytic effect of CoNiB on the desorption kinetics, the activation energy Ea for 2LiBH4/MgH210 wt% CoNiB was determined by using the non-isothermal Kissinger method as described below:
Fig. 2 e (a) SEM image of 2LiBH4/MgH2-10 wt% CoNiB and the corresponding EDS elemental mapping images of, (b) Co, (c) Ni. Please cite this article in press as: Zhao Y, et al., Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.063
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Fig. 3 e (a) Thermally programmed H2 desorption curves and, (b) the corresponding thermally programmed H2 desorption capacity curves of 2LiBH4/MgH2, 2LiBH4/MgH2-10 wt% CoNiB.
ln
b T2m
!
Ea ¼ RT
where b is the heating rate, Tm is the absolute temperature for the maximum hydrogen release, R is the gas constant. Herein, Tm was extracted from TPD curves with heating rates of 5, 8, 10 C/min (not shown here). Fig. 6 presents the Kissinger plot for the 2LiBH4/MgH2-10 wt% CoNiB system. The intrinsic linearity of all the curves demonstrates that the desorption rate of 2LiBH4/MgH2-10 wt% CoNiB system is well characterized by the non-isothermal Kissinger method. The calculated apparent activation energies are 137, 116 kJ/mol, which are lower than those of pure MgH2 (~168 kJ/mol) [7,44,45], LiBH4 (~146 kJ/mol) [46]. The above decreases of activation energy suggest the kinetic barrier of 2LiBH4/MgH2 is effectively reduced by CoNiB nanoparticles under no hydrogen back pressure conditions. In order to understand the catalytic mechanism of CoNiB, the phase evolution of 2LiBH4/MgH2-10 wt% CoNiB before and after dehydrogenation without hydrogen back pressure was carefully examined by XRD. With respect to 2LiBH4/MgH210wt% CoNiB, no new phases show up except for LiBH4, MgH2, indicating that in the milling process no solid-phase reaction
Fig. 4 e DSC profiles of the as-milled 2LiBH4/MgH2 and 2LiBH4/MgH2-10 wt% CoNiB at a heating ramp of 4 C/min.
Fig. 5 e Isothermal dehydrogenation curves of the neat 2LiBH4/MgH2 and 2LiBH4/MgH2-10 wt% CoNiB at different temperatures.
Fig. 6 e Kissinger plot of 2LiBH4/MgH2-10 wt% CoNiB composites: (a) the dehydrogenation of MgH2, (b) the dehydrogenation of LiBH4.
Please cite this article in press as: Zhao Y, et al., Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.063
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(51171083), MOE (IRT-13R30 and IRT13022), the Natural Science Fund of Hunan (2012FJ3023) and 111 Project (B12015).
references
Fig. 7 e XRD patterns of 2LiBH4/MgH2-10wt% CoNiB before, (I) and after, (II) dehydrogenation at 400 C.
occurs. As displayed in Fig. 7(II), except the peaks of parafilm, the major diffraction peaks are corresponding to Mg. And a trace mount of MgO, LiH can be found in the desorption products. There are no signals for the formation of element B, which may be attributed to the fact that the boron is amorphous. Also, the peaks of Co, Ni-containing phases can be detected in XRD patterns, making us believe that boride nanoparticles indeed act as active sites. The above major dehydrogenation product obtained under the condition of no hydrogen back pressure is different from that of the condition with hydrogen back pressure, that is, the decomposition mechanism is related to the physical conditions [29,32,47e50]. Therefore, later study is supposed on the effect of hydrogen back pressure on the 2LiBH4/MgH2-10 wt% CoNiB.
Conclusion In this work, the catalytic effect of CoNiB on the hydrogen storage performances of 2LiBH4/MgH2 system was investigated. CoNiB exhibits a good activity. For example, compared with the pristine 2LiBH4/MgH2, two lower dehydrogenation temperatures declined by 55 C, 26 C via a TPD analysis were observed for 2LiBH4/MgH2-10 wt% CoNiB. Moreover, the apparent activation energies for the first and second decomposition step are significantly decreased to 137, 116 kJ/mol after adding 10 wt% CoNiB. Therefore, directly introducing CoNiB truly enhances the dehydrogenation properties of 2LiBH4/MgH2 system. And the excellent catalysis of CoNiB may provide other transition metal borides a valuable hint to improve the dehydrogenation properties of other complex RHC composites such as LiBH4eMg(BH4)2, LiBH4eAl etc.
Acknowledgments This work was financially supported by 973 program (2010CB631303), National Natural Science Foundation of China
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Please cite this article in press as: Zhao Y, et al., Hydrogen storage behavior of 2LiBH4/MgH2 composites improved by the catalysis of CoNiB nanoparticles, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.05.063