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Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system
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Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system Bao-Xia Dong, Ya-Ru Zhang, Lin-Ting Chen, Yun-Lei Teng*, Juan Zhao College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, PR China
article info
abstract
Article history:
In this paper, the influence of multi-walled carbon nanotube (MWCNT)-doping on the
Received 8 July 2016
hydrogen storage properties of the Li3N system was systematically investigated. Compared
Received in revised form
with the pure Li3N sample, the MWCNT-doped Li3N sample shows faster hydrogen ab-
1 September 2016
sorption and desorption kinetics and a drastically improved cycling stability. Along with
Accepted 14 September 2016
increasing MWCNT content, the hydrogen storage improvement becomes more apparent.
Available online xxx
When the MWCNT doping level reaches 10 mol%, the enhancement effect is significant. The improved hydrogen storage properties of the Li3N system by MWCNT-doping can be
Keywords:
reduced to the physical effect of ball milling, the increased specific surface area and large
Lithium nitride
pores as well as the good thermal conductivity of MWCNTs.
MWCNTs
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Hydrogen storage Cycling stability Kinetics
Introduction Hydrogen is regarded as a source of clean energy, as it is a zero-pollution and highly efficient fuel [1,2]. However, the problem of hydrogen storage must be solved to fully realize the extensive utilization of hydrogen. Traditional hydrogen storage based on physical methods cannot meet practical application requirements because of its high risk. In the ongoing process of exploring hydrogen storage media, solid chemical hydrogen storage is receiving strong scientific interest, and has the potential to become a safe and effective hydrogen storage method. Li3N has received much attention as a hydrogen storage material because of its high theoretical hydrogen storage capacity (10.4 wt%) and reversibility since Ping Chen first reported it in 2002 [3]. Many studies have demonstrated that the hydrogen absorption and release processes of the Li3N system
can be divided into two steps (reactions 1 and 2) as follows [4e10]:
Li3N þ H2 4 Li2NH þ LiH
(1)
Li2NH þ H2 4 LiNH2þLiH
(2)
However, it was found that there were several problems with thermodynamics, kinetics and reversibility that limit the practical application of Li3N as a hydrogen storage material. First, at moderate temperatures, its theoretical reversible hydrogen storage capacity is only approximately 6.5 wt% because hydrogen cannot be completely desorbed. The enthalpy changes in the first and second steps of the dehydrogenation reaction are 45 kJ/mol and 116 kJ/mol, respectively. The high enthalpy change in the second step of
* Corresponding author. Fax: þ86 514 87979201. E-mail address: [email protected] (Y.-L. Teng). http://dx.doi.org/10.1016/j.ijhydene.2016.09.102 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Dong B-X, et al., Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.102
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the dehydrogenation reaction will result in a high hydrogen desorption temperature (higher than 320 C) [4e7]. Second, it is believed that the long solid-state diffusion paths of macroscopic phase segregation upon dehydrogenation/hydrogenation are responsible for the slow kinetics. Incorporating other materials into the Li3N system was reported as an effective approach toward improving its dehydrogenation/ hydrogenation kinetics [8e12]. Third, during the process of hydrogen absorption, the rapidly released heat due to the fast hydrogen absorption reaction will cause the Li3N sample to sinter, which will seriously degrade the hydrogen absorption/ desorption performance of the Li3N sample. Experiments show that at the initial 180 C, as the hydrogenation reaction progresses, the temperature of the Li3N sample will be higher than 400 C. Therefore, large temperature changes cause the Li3N sintering, which will result in difficulty for further hydrogen absorption/desorption [13,14]. To solve the problems mentioned above, scientists have attempted to improve the hydrogen storage properties of Li3N by various methods [4e12]. Hu et al. studied the effect of doping with Li2O and LiNH2 on the hydrogen storage properties of Li3N by partially oxidizing the Li3N sample in air and doing a hydrogenation/dehydrogenation pretreatment, respectively [11,12]. At 180 C, the hydrogenation level of the Li2O-doped Li3N system is 5 wt% within 3 min. Furthermore, the Li2O-doped Li3N material shows good cycling stability; after 6 cycles the hydrogen storage capacity and hydrogen absorption and desorption rates are not weakened [11]. It was found that, in combination with hydrogen storage capacity and XRD characterization, the Li2O doping can partially suppress the sintering of Li3N. Compared with the Li2O-doped Li3N system, doping with LiNH2 can clearly improve the hydrogen storage capacity of Li3N with the effect depending on the doping level of LiNH2. When the LiNH2 content is within the range of 28e50 mol%, the maximum attainable amount of hydrogen storage is 6.8 wt% [12]. These reports proved that doping additives into the Li3N system is an effective method to improve the hydrogenation/dehydrogenation reaction rate and cycling stability. Liu et al. found that Li3N in the Li3N/Li2C2 composite showed a significantly enhanced sequential hydrogen absorption capacity and a lowered hydrogenation/ dehydrogenation temperature [8]. Recently, Demir-Cakan et al. reported an innovative route to synthesize Li3N@carbon composites that have desirable hydrogen storage properties with fast hydrogen absorption/desorption kinetics at 200 C as well as a completely reversible hydrogen storage process [4]. Xia et al. successfully developed a simple one-pot method to synthesize carbon-coated Li3N fibers by an electrospinning technique. These fibers exhibit significantly enhanced hydrogen storage properties, with a stable reversibility close to the theoretical value (10.4 wt%) over ten cycles at 250 C [5]. These reports demonstrated that carbon materials have clear effects on enhancing the hydrogen storage properties of the Li3N system. Recently, carbon nanotubes have been added into hydrogen storage systems including magnesium hydride, sodium aluminum hydride, LieNeH and LieMgeNeH to improve the hydrogen storage properties of these materials [15e29]. It was found that carbon nanotubes possess excellent heat transfer performance and can promote dispersion of
hydrogen storage materials, which, to a certain extent, can improve hydrogen storage performance. However, the effect of carbon nanotubes on the hydrogen storage behaviors of the Li3N system has not been studied to date. In this paper, the influence of MWCNT-doping on the hydrogen storage properties of Li3N was systematically investigated. The hydrogen storage kinetics as well as the cycling stability of the Li3N system were improved by MWCNT-doping. The causes for the improvement of the hydrogen storage properties of the Li3N system by MWCNTdoping are proposed on the basis of experimental results.
Experimental details Sample preparation Lithium nitride (Li3N) (99.5%, Aldrich) and multi-walled carbon nanotubes (MWCNTs) (>95%, Shenzhen Nanotech Port Ltd. Co, China) were used for the following experiments. The MWCNTs were dispersed into the Li3N sample by the following mechanical ball-milling method. Li3N and MWCNTs (300 mg total), together with 20 steel balls (6 mm in diameter), were placed into a milling vessel made of steel with an inner volume of ~70 cm3. The ball milling was performed under a 0.6 MPa hydrogen (>99.999%) atmosphere for 2 h at 450 rpm using a planetary ball milling apparatus (QM-3SP4). The ballto-powder weight ratio was approximately 90:1. To minimize sample heating, the milling process was paused for 30 min in every hour of milling. We selected four Li3N samples to study: without additive, with 5 mol%, 10 mol% or 20 mol% MWCNTs additive, which are labeled Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs, respectively. All of the samples were handled in an Ar-filled (>99.999%) glove box (Mikrouna, China) equipped with a circulative purification system, in which the typical H2O/O2 levels were below 0.1 ppm.
Structural and morphological characterizations The structures and morphologies of the produced composites were examined by X-ray diffraction (XRD) (AXS D8 ADVANCE, Bruker, German) and Scanning Electron Microscopy (SEM) (S4800II, Hitachi, Japan), respectively. The NeH vibrations in all samples were identified using a Fourier Transform IR spectrometer (FTIR) (TENSOR 27, Bruker, Canada). Transmission mode was used, with a scan resolution of 4 cm1, and 16 scans were performed and accumulated. BET specific surface area and pore size distribution measurements were performed on a Micromeritics ASAP 2020 HD88 surface area and pore size analyzer.
Thermal desorption and isothermal hydrogenation/ dehydrogenation measurements The thermal desorption behavior, the evolved gas composition, and the thermogravimetry-differential scanning calorimetry were studied using synchronous thermal analysis (DSC/DTA-TG; Netzsch STA 449 F3 Jupiter®) combined with thermal desorption mass spectroscopy (TDMS; Netzsch QMS
Please cite this article in press as: Dong B-X, et al., Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.102
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€olos®, Germany) with a heating rate of 10 C/min 403 D Ae under 30 ml/min Ar gas flow. The isothermal hydrogen desorption and absorption kinetics were measured using a Hy-Energy PCT evo commercial volumetric hydrogen storage capacity apparatus (Sievert apparatus), which facilitates the accurate volumetric determination of the amount of evolved hydrogen. Typically, approximately 0.15 g of sample was loaded into a stainless steel autoclave and evacuated. Then, the sample was rapidly heated to desired temperatures. Dehydrogenation at 300 C was performed under a vacuum. Full hydrogenation was carried out at 250 C under a 2 MPa hydrogen atmosphere.
Results and discussion The structures and morphologies of the pure and MWCNTdoped Li3N samples after mechanical ball milling To study the influence of the MWCNT doping on Li3N during the ball milling process, the morphology and particle size of the pure and MWCNT-doped Li3N samples after mechanical ball milling were characterized using SEM, as shown in Fig. 1. The original morphology and structure of MWCNTs were damaged after mechanical ball milling at 450 rpm for 2 h, presenting a granular image. For the ball-milled pure Li3N sample, the particle size is very uneven (0.5e5 mm). However, the particle size and morphology of the MWCNT-doped Li3N samples after ball milling changed. Compared with that of the ball-milled pure Li3N sample, the particle size of the ballmilled MWCNT-doped Li3N samples is reduced (0.1e2 mm)
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and becomes more even. The experimental results demonstrate that, in the process of ball milling, the doped MWCNTs may have a physical ball milling effect [30], which can further decrease the particle size of Li3N and result in better sample dispersion. In other words, the formation of small crystalline size and clean surface are further induced due to the addition of MWCNTs in the process of ball milling. To determine whether MWCNTs react with Li3N during the process of ball milling, the MWCNT-doped Li3N samples were analyzed after ball milling by XRD (Fig. 2). For all four samples there are clear diffraction peaks due to the two crystal types of Li3N (a-Li3N and b-Li3N). However, compared with the ballmilled pure Li3N sample, the three MWCNT-doped samples show a weak unknown diffraction peak at 30 , which may be due to the interaction of MWCNTs with Li3N. In addition, the diffraction peaks due to MWCNTs are not seen in Fig. 2, indicating the doped samples become amorphous after ball milling. As shown in Table 1, BET specific surface areas of the pure Li3N sample and the Li3N samples with 5 mol%, 10 mol%, and 20 mol% of MWCNTs after ball milling is 10.8631, 15.4728, 16.4178, and 18.8292 m2/g, respectively. In comparison with the ball-milled pure Li3N sample, the ball-milled MWCNTdoped Li3N samples show larger specific surface areas. Along with the increase of MWCNT content, specific surface areas of the MWCNT-doped Li3N samples display an increasing trend. Compared with the specific surface area of pure MWCNTs (139.8623 m2/g), the specific surface area of the four samples is quite small, which may explain why, with the increasing MWCNT content, the specific surface areas of the MWCNTdoped samples increase.
Fig. 1 e SEM images of the Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs samples after ball milling. Please cite this article in press as: Dong B-X, et al., Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.102
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Fig. 2 e XRD curves of the Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs samples after ball milling.
Fig. 3 shows the pore size distribution of the Li3N samples without additive, and with 5 mol%, 10 mol%, and 20 mol% of MWCNTs after ball milling. The pore size of the four samples is mainly distributed in the following two ranges: 10e50 A and 50e300 A. Compared with the ball-milled pure Li3N sample, the MWCNT-doped samples have larger pores, especially the pores in the range of 50e300 A. The quantity of large pores increases with increasing MWCNT content. In 2005, Hu et al. studied the influence of high temperature dehydrogenation on the hydrogen storage performance of Li3N and analyzed the changes of the specific surface area and pore size under different hydrogenation/dehydrogenation conditions [13]. They found that after high temperature dehydrogenation, the quantity of large pores of the Li3N sample was significantly reduced, which may be an important factor that results in the difficulty of further dehydrogenation. On the basis of this result and the above experimental data, we predict that MWCNT-doping may improve the hydrogen desorption and absorption properties of Li3N.
Fig. 3 e Pore size distribution of the Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs samples after ball milling.
without additive and with 5 mol%, 10 mol% or 20 mol% MWCNT additive, of which the hydrogen storage capacities are 10.4, 10.1, 9.9 and 9.7 wt%, respectively. Fig. 4 compares the first hydrogen absorption behaviors of these samples. For the four samples, the onset hydrogen absorption temperature was approximately 120 C and the plateau of hydrogen uptake at 250 C is approximately 8.3 wt% (when the hydrogen absorption amount is calculated, the MWCNT weight is excluded, as described in the literature [4,5]). From Fig. 4, it can be observed that all samples exhibit a fast hydrogenation reaction rate within 30 min at the beginning of the hydrogen absorption. However, as the reaction progresses, the hydrogen absorption rate gradually slows. In comparison with the pure Li3N sample, the hydrogen absorption rate of the MWCNTdoped Li3N sample increases, and the rate increases with the MWCNT quantity increasing. However, the hydrogen absorption rate of the Li3N-10 mol% MWCNTs sample is near that of the Li3N-20 mol% MWCNTs sample. Within 30 min, the
The influence of the doped MWCNTs on the hydrogen absorption and desorption kinetics of Li3N To study the influence of the doped MWCNTs on the hydrogen absorption properties of Li3N, we selected Li3N samples
Table 1 e BET specific surface area of the Li3N, Li3N-5 mol % MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs samples after ball milling. Samples BET specific surface area (m2/g)
Li3N 10.8631
Li3N-5 mol Li3N-10 mol Li3N-20 mol % MWCNTs % MWCNTs % MWCNTs 15.4728
16.4178
18.8292
Fig. 4 e The first isothermal hydrogenation curves at 250 C of the ball-milled Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol % MWCNTs and Li3N-20 mol% MWCNTs samples.
Please cite this article in press as: Dong B-X, et al., Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.102
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average hydrogen absorption rate of the Li3N-10 mol% MWCNTs sample is approximately 0.232 wt/min, which is 1.3 times faster than that of the pure Li3N sample. Within the same time period, the hydrogen absorption of Li3N samples doped with MWCNTs is larger than that of the pure Li3N sample. In addition, for the Li3N samples doped with 10 mol% MWCNTs and 20 mol% MWCNTs, the hydrogen absorption within 4 h is close to the plateau of hydrogen uptake, but the hydrogen absorption amount of the Li3N sample without additive fails to reach the plateau within such a short time. Therefore, the doped MWCNTs accelerate the hydrogen absorption rate of Li3N and thereby enhance the hydrogen absorption kinetics. Possible reasons for the improved kinetics were analyzed as follows. First, the MWCNT-doping better disperses the Li3N, and the interaction between Li3N particles and MWCNTs will increase the phase interface, which benefits the effective contact of hydrogen with Li3N [23,31]. Secondly, the MWCNT-doped Li3N samples have larger specific surface areas and more 50e300 A large pores, which will provide further opportunities for the contact of hydrogen with Li3N. To study the influence of the MWCNTs on the hydrogen desorption properties of the hydrogenated Li3N, the hydrogen desorption behaviors of the four samples after the first hydrogen absorption were analyzed by TDMS. As shown in Fig. 5, for all samples, there is only one hydrogen desorption peak from 50 to 500 C, indicating that only the reaction of LiH with LiNH2 generating Li2NH and hydrogen proceeds (reaction 2). The MWCNT-doping does not change the onset hydrogen desorption temperature of the Li3N after the first hydrogen absorption. However, the hydrogen desorption peak temperature is decreased by MWCNT-doping and along with the increase of MWCNT content, the hydrogen desorption peak temperature gradually lowers. Compared with the pure Li3N sample after the first hydrogen absorption, the hydrogen desorption peak temperatures of the 5 mol%, 10 mol% and 20 mol% MWCNT-doped Li3N samples after the first hydrogen absorption are decreased by 1.5, 4.4 and 7.4 C, respectively. Moreover, the NH3 mass spectrum shows that for all the samples there is almost no ammonia emission during the process of hydrogen desorption. Therefore, it is apparent that the MWCNT-doping can improve the hydrogen desorption kinetics to some extent. The hydrogenated 20 mol% MWCNTsdoped sample shows the best hydrogen desorption kinetics. The reason for the improvement of the hydrogen desorption kinetics may partially be because the MWCNTs can suppress Li3N sintering during hydrogenation. It has been reported that MWCNTs have good thermal conductivity and can disperse hydrogen absorption reaction heat rapidly [23,32], which may weaken the compaction of Li3N. The volumetric method was also adopted to examine the hydrogen desorption kinetics of the pure and MWCNT-doped Li3N samples under vacuum at 300 C. As shown in Fig. 6, the four samples have almost the same onset hydrogen desorption temperature (at approximately 215 C), indicating that the MWCNT-doping has little influence on the onset hydrogen desorption temperature of the hydrogenated Li3N sample. All four samples also exhibit a fast dehydrogenation reaction rate within 20e30 min from the beginning of the hydrogen desorption. However, as the reaction progresses, the hydrogen
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Fig. 5 e The first dehydrogenation TDMS curves of the Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N20 mol% MWCNTs samples after the first hydrogen absorption.
desorption rate gradually slows. Within 30 min, the average hydrogen desorption rate of the Li3N-10 mol% MWCNTs sample is approximately 0.117 wt/min, which is 1.2 times faster than that of the pure Li3N sample. In comparison with the pure Li3N sample, the hydrogen desorption rate of the MWCNT-doped Li3N sample increases, and the rate increases with the MWCNTs quantity increasing. The hydrogen
Fig. 6 e Isothermal dehydrogenation curves at 300 C of the Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs samples after the first hydrogen absorption.
Please cite this article in press as: Dong B-X, et al., Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.102
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desorption rate of the Li3N sample doped with 10 mol% MWCNTs is near that of the Li3N sample doped with 20 mol% MWCNTs. In addition, the MWCNT-doped Li3N samples can reach the plateau of hydrogen desorption (3.8 wt%) within 2 h, but it takes at least 4 h for the pure Li3N sample to reach its hydrogen desorption plateau. Therefore, the isothermal dehydrogenation tests using the volumetric method further confirm that the MWCNT-doping can improve the hydrogen desorption kinetics of the hydrogenated Li3N.
The influence of the MWCNTs on the hydrogen absorption/ desorption reversibility of Li3N An ideal hydrogen storage material with superior performance not only has fast hydrogen absorption/desorption kinetics but also has good cycling stability. To study the influence of MWCNTs on the cycling properties of Li3N, ten hydrogen absorption/desorption cycles for the Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs samples were performed. Fig. 7 shows the second, sixth, eighth and tenth hydrogen absorption curves of the four samples to compare their hydrogen absorption cycling properties. Fig. 7(a) shows that, for the pure Li3N sample, the onset hydrogen absorption temperature clearly increases and the hydrogen absorption rate gradually slows as more cycles pass. The pure Li3N sample can absorb 3.8 wt% hydrogen within 1.5 h for the second hydrogen absorption, but the amount of
the tenth hydrogen absorption within 5 h is only 1 wt%. These experimental results show that the cycling stability of the pure Li3N sample is poor. For the 5 mol% MWCNTs-doped Li3N sample, as more cycles pass, the onset absorption temperature and the hydrogen absorption rate change little within six cycles, indicating that the cycling stability of Li3N is improved by doping with 5 mol% MWCNTs (Fig. 7(b)). However, from the eighth hydrogen absorption, the hydrogen absorption curve slope clearly decreases, indicating that the hydrogen absorption rate notably reduces. As shown in Fig. 7(c and d), when the MWCNT level reaches 10 mol%, the sample exhibits good cycling performance. For the 10 mol% and 20 mol% MWCNTdoped Li3N samples, the onset absorption temperature and the hydrogen absorption rate change little within ten cycles. When examining the change of the hydrogen absorption with cycle number, the capacity of the Li3N samples with 10 mol% and 20 mol% MWCNTs remain ca. 3.8 wt% during ten cycles, which is 2.3 times as many as that of the pure Li3N sample. Therefore, MWCNT-doping can improve the hydrogen absorption cycling stability of Li3N, and when the doping level of MWCNTs reaches 10 mol%, the enhancement effect is significant. Fig. 8 shows the first, sixth, eighth and tenth hydrogen desorption curves of the hydrogenated Li3N samples to compare their hydrogen desorption cycling properties. Fig. 8(a) shows that, for the pure Li3N sample, the onset hydrogen desorption temperature obviously increases and the
Fig. 7 e Reversible full hydrogen absorption curves for the Li3N (a), Li3N-5 mol% MWCNTs (b), Li3N-10 mol% MWCNTs (c) and Li3N-20 mol% MWCNTs (d) samples. Please cite this article in press as: Dong B-X, et al., Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.102
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hydrogen desorption rate gradually slows as the cycles pass. The pure Li3N sample can desorb 3.8 wt% hydrogen within 1.8 h for the first hydrogen desorption, but the amount of the tenth hydrogen desorption within 4 h is far less than 1 wt%. These experimental results show that the cycling desorption property of the pure Li3N sample is poor. Fig. 8(b) demonstrates that, for the Li3N sample with 5 wt% MWCNTs, the onset desorption temperature and the hydrogen desorption rate change little within six cycles. However, from the eighth hydrogen desorption, the slope of hydrogen desorption curve clearly decreases, indicating that the hydrogen desorption rate significantly reduces. As shown in Fig. 8(c and d), when the MWCNTs reach 10 mol%, the sample exhibits good cycling performance. For the Li3N samples with 10 mol% and 20 mol% MWCNTs, the onset desorption temperatures change little and the hydrogen desorption rates also change little within ten cycles. By examining the change in the hydrogen desorption with the cycle number, it was determined that the maximum hydrogen desorption of the 10 mol% and 20 mol% MWCNTs-doped Li3N samples remains ca. 3.8 wt% within ten cycles while the maximum hydrogen desorption of the pure Li3N sample decreases drastically, from 3.8 wt% for the first desorption to 0.7 wt% for the tenth desorption. For the tenth cycle, the Li3N samples with 10 mol% and 20 mol% MWCNTs can desorb approximately 3.5 wt% hydrogen within 2 h, which is 7 times as many as that of the pure Li3N sample. Therefore, MWCNT-doping can improve the hydrogen desorption cycling
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stability of Li3N, and when the doping level of MWCNTs reaches 10 mol%, the enhancement effect is significant.
The morphologies and composition of the pure and MWCNTdoped Li3N samples after the tenth hydrogen desorption Fig. S1 shows the morphologies of Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs after the tenth hydrogen desorption. Although all the samples become cylinders formed of powder after the tenth hydrogen desorption, the compactness of the cylinders differs. The pure Li3N and Li3N-5 mol% MWCNTs samples form very compact cylinders after the tenth hydrogen absorption, indicating that the cyclic hydrogen absorption and desorption notably sinters the two samples. However, the sintering degree of the Li3N-10 mol% MWCNTs and Li3N20 mol% MWCNTs samples after the tenth hydrogen desorption is low, as the formed cylinders are less compact and their uneven cross sections can be observed with the naked eye. This demonstrates that MWCNT-doping can weaken the degree of Li3N sintering. Compared with the pure Li3N sample, MWCNT-doped Li3N samples present good cycling stability. To more deeply understand the causes of the improved cycling performance, we characterized the products of the pure and MWCNT-doped Li3N samples after the tenth hydrogen desorption by XRD and FTIR. As shown in Fig. 9(a), the characteristic diffraction of
Fig. 8 e Reversible full hydrogen desorption curves for the Li3N (a), Li3N-5 mol% MWCNTs (b), Li3N-10 mol% MWCNTs (c) and Li3N-20 mol% MWCNTs (d) samples. Please cite this article in press as: Dong B-X, et al., Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.102
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infrared spectra, showing that LiNH2 has been completely consumed during hydrogen desorption reactions for these samples (Fig. 9). Therefore, the XRD and FTIR data can further explain why doping with MWCNTs can improve the hydrogen storage cycling performance of Li3N. LiNH2 can be completely consumed in the hydrogen desorption reaction of MWCNTdoped Li3N samples after several cycles because doping with MWCNTs weakens the sintering degree of Li3N.
Conclusions In this paper, the influence of MWCNT-doping on the hydrogen storage properties of Li3N was systematically investigated. Compared with the pure Li3N sample, the MWCNT-doped Li3N samples show faster hydrogen absorption and desorption kinetics and drastically improved cycling stability. The effect becomes more apparent with increasing MWCNT content. When the MWCNT content reaches 10 mol %, the enhancement effect is significant. The improvement of the hydrogen storage properties of the Li3N system by MWCNT-doping comes down to the following causes: (1) In the process of ball milling, the MWCNTs add to the physical effect of ball milling, which can further decrease the particle size of Li3N, better dispersing the samples; (2) MWCNT-doping increases the specific surface area and the amount of large pores, which is conducive to the absorption and release of hydrogen; (3) the good thermal conductivity of MWCNTs quickly disperses hydrogen absorption reaction heat and weakens the sintering degree of the Li3N.
Acknowledgments
Fig. 9 e The XRD profiles and FTIR spectra of the Li3N, Li3N5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol % MWCNTs samples after the tenth hydrogen desorption.
LiNH2 remains in the XRD curve of pure Li3N after the tenth hydrogen desorption. Two obvious NeH stretching vibration peaks from LiNH2 can also be observed in the infrared spectra of pure Li3N after the tenth hydrogen desorption. Moreover, only very weak NeH stretching vibration peaks due to Li2NH can be observed in the infrared spectra of the pure Li3N after the tenth hydrogen desorption. Coupled with the lower hydrogen desorption maximum, this suggests that the hydrogen desorption solidesolid reaction between LiNH2 and LiH has great difficulty proceeding during the tenth hydrogen desorption. Of course, the appearance of this experimental phenomenon during hydrogen desorption cycles may be caused by the sample sintering, which hinders the migrations of Liþ and Hþ and therefore influences the efficiency of the reaction [33]. For the MWCNT-doped Li3N after the tenth hydrogen desorption, only the diffraction peaks due to Li2NH and LiH are observed in the XRD curves and there are only obvious NeH stretching vibration peaks due to Li2NH in the
This work is financially supported by the NNSF of China (Nos. 21573192, 21301152 and 21671169), SRF of SEM for ROCS, RFDP (No. 20133250120008), Six Talent Peaks Project in Jiangsu Province (No. 2015-XYN-011), and the Foundation from the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.09.102.
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Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system Bao-Xia Dong, Ya-Ru Zhang, Lin-Ting Chen, Yun-Lei Teng*, Juan Zhao College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, PR China
article info
abstract
Article history:
In this paper, the influence of multi-walled carbon nanotube (MWCNT)-doping on the
Received 8 July 2016
hydrogen storage properties of the Li3N system was systematically investigated. Compared
Received in revised form
with the pure Li3N sample, the MWCNT-doped Li3N sample shows faster hydrogen ab-
1 September 2016
sorption and desorption kinetics and a drastically improved cycling stability. Along with
Accepted 14 September 2016
increasing MWCNT content, the hydrogen storage improvement becomes more apparent.
Available online xxx
When the MWCNT doping level reaches 10 mol%, the enhancement effect is significant. The improved hydrogen storage properties of the Li3N system by MWCNT-doping can be
Keywords:
reduced to the physical effect of ball milling, the increased specific surface area and large
Lithium nitride
pores as well as the good thermal conductivity of MWCNTs.
MWCNTs
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Hydrogen storage Cycling stability Kinetics
Introduction Hydrogen is regarded as a source of clean energy, as it is a zero-pollution and highly efficient fuel [1,2]. However, the problem of hydrogen storage must be solved to fully realize the extensive utilization of hydrogen. Traditional hydrogen storage based on physical methods cannot meet practical application requirements because of its high risk. In the ongoing process of exploring hydrogen storage media, solid chemical hydrogen storage is receiving strong scientific interest, and has the potential to become a safe and effective hydrogen storage method. Li3N has received much attention as a hydrogen storage material because of its high theoretical hydrogen storage capacity (10.4 wt%) and reversibility since Ping Chen first reported it in 2002 [3]. Many studies have demonstrated that the hydrogen absorption and release processes of the Li3N system
can be divided into two steps (reactions 1 and 2) as follows [4e10]:
Li3N þ H2 4 Li2NH þ LiH
(1)
Li2NH þ H2 4 LiNH2þLiH
(2)
However, it was found that there were several problems with thermodynamics, kinetics and reversibility that limit the practical application of Li3N as a hydrogen storage material. First, at moderate temperatures, its theoretical reversible hydrogen storage capacity is only approximately 6.5 wt% because hydrogen cannot be completely desorbed. The enthalpy changes in the first and second steps of the dehydrogenation reaction are 45 kJ/mol and 116 kJ/mol, respectively. The high enthalpy change in the second step of
* Corresponding author. Fax: þ86 514 87979201. E-mail address: [email protected] (Y.-L. Teng). http://dx.doi.org/10.1016/j.ijhydene.2016.09.102 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Dong B-X, et al., Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.102
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the dehydrogenation reaction will result in a high hydrogen desorption temperature (higher than 320 C) [4e7]. Second, it is believed that the long solid-state diffusion paths of macroscopic phase segregation upon dehydrogenation/hydrogenation are responsible for the slow kinetics. Incorporating other materials into the Li3N system was reported as an effective approach toward improving its dehydrogenation/ hydrogenation kinetics [8e12]. Third, during the process of hydrogen absorption, the rapidly released heat due to the fast hydrogen absorption reaction will cause the Li3N sample to sinter, which will seriously degrade the hydrogen absorption/ desorption performance of the Li3N sample. Experiments show that at the initial 180 C, as the hydrogenation reaction progresses, the temperature of the Li3N sample will be higher than 400 C. Therefore, large temperature changes cause the Li3N sintering, which will result in difficulty for further hydrogen absorption/desorption [13,14]. To solve the problems mentioned above, scientists have attempted to improve the hydrogen storage properties of Li3N by various methods [4e12]. Hu et al. studied the effect of doping with Li2O and LiNH2 on the hydrogen storage properties of Li3N by partially oxidizing the Li3N sample in air and doing a hydrogenation/dehydrogenation pretreatment, respectively [11,12]. At 180 C, the hydrogenation level of the Li2O-doped Li3N system is 5 wt% within 3 min. Furthermore, the Li2O-doped Li3N material shows good cycling stability; after 6 cycles the hydrogen storage capacity and hydrogen absorption and desorption rates are not weakened [11]. It was found that, in combination with hydrogen storage capacity and XRD characterization, the Li2O doping can partially suppress the sintering of Li3N. Compared with the Li2O-doped Li3N system, doping with LiNH2 can clearly improve the hydrogen storage capacity of Li3N with the effect depending on the doping level of LiNH2. When the LiNH2 content is within the range of 28e50 mol%, the maximum attainable amount of hydrogen storage is 6.8 wt% [12]. These reports proved that doping additives into the Li3N system is an effective method to improve the hydrogenation/dehydrogenation reaction rate and cycling stability. Liu et al. found that Li3N in the Li3N/Li2C2 composite showed a significantly enhanced sequential hydrogen absorption capacity and a lowered hydrogenation/ dehydrogenation temperature [8]. Recently, Demir-Cakan et al. reported an innovative route to synthesize Li3N@carbon composites that have desirable hydrogen storage properties with fast hydrogen absorption/desorption kinetics at 200 C as well as a completely reversible hydrogen storage process [4]. Xia et al. successfully developed a simple one-pot method to synthesize carbon-coated Li3N fibers by an electrospinning technique. These fibers exhibit significantly enhanced hydrogen storage properties, with a stable reversibility close to the theoretical value (10.4 wt%) over ten cycles at 250 C [5]. These reports demonstrated that carbon materials have clear effects on enhancing the hydrogen storage properties of the Li3N system. Recently, carbon nanotubes have been added into hydrogen storage systems including magnesium hydride, sodium aluminum hydride, LieNeH and LieMgeNeH to improve the hydrogen storage properties of these materials [15e29]. It was found that carbon nanotubes possess excellent heat transfer performance and can promote dispersion of
hydrogen storage materials, which, to a certain extent, can improve hydrogen storage performance. However, the effect of carbon nanotubes on the hydrogen storage behaviors of the Li3N system has not been studied to date. In this paper, the influence of MWCNT-doping on the hydrogen storage properties of Li3N was systematically investigated. The hydrogen storage kinetics as well as the cycling stability of the Li3N system were improved by MWCNT-doping. The causes for the improvement of the hydrogen storage properties of the Li3N system by MWCNTdoping are proposed on the basis of experimental results.
Experimental details Sample preparation Lithium nitride (Li3N) (99.5%, Aldrich) and multi-walled carbon nanotubes (MWCNTs) (>95%, Shenzhen Nanotech Port Ltd. Co, China) were used for the following experiments. The MWCNTs were dispersed into the Li3N sample by the following mechanical ball-milling method. Li3N and MWCNTs (300 mg total), together with 20 steel balls (6 mm in diameter), were placed into a milling vessel made of steel with an inner volume of ~70 cm3. The ball milling was performed under a 0.6 MPa hydrogen (>99.999%) atmosphere for 2 h at 450 rpm using a planetary ball milling apparatus (QM-3SP4). The ballto-powder weight ratio was approximately 90:1. To minimize sample heating, the milling process was paused for 30 min in every hour of milling. We selected four Li3N samples to study: without additive, with 5 mol%, 10 mol% or 20 mol% MWCNTs additive, which are labeled Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs, respectively. All of the samples were handled in an Ar-filled (>99.999%) glove box (Mikrouna, China) equipped with a circulative purification system, in which the typical H2O/O2 levels were below 0.1 ppm.
Structural and morphological characterizations The structures and morphologies of the produced composites were examined by X-ray diffraction (XRD) (AXS D8 ADVANCE, Bruker, German) and Scanning Electron Microscopy (SEM) (S4800II, Hitachi, Japan), respectively. The NeH vibrations in all samples were identified using a Fourier Transform IR spectrometer (FTIR) (TENSOR 27, Bruker, Canada). Transmission mode was used, with a scan resolution of 4 cm1, and 16 scans were performed and accumulated. BET specific surface area and pore size distribution measurements were performed on a Micromeritics ASAP 2020 HD88 surface area and pore size analyzer.
Thermal desorption and isothermal hydrogenation/ dehydrogenation measurements The thermal desorption behavior, the evolved gas composition, and the thermogravimetry-differential scanning calorimetry were studied using synchronous thermal analysis (DSC/DTA-TG; Netzsch STA 449 F3 Jupiter®) combined with thermal desorption mass spectroscopy (TDMS; Netzsch QMS
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€olos®, Germany) with a heating rate of 10 C/min 403 D Ae under 30 ml/min Ar gas flow. The isothermal hydrogen desorption and absorption kinetics were measured using a Hy-Energy PCT evo commercial volumetric hydrogen storage capacity apparatus (Sievert apparatus), which facilitates the accurate volumetric determination of the amount of evolved hydrogen. Typically, approximately 0.15 g of sample was loaded into a stainless steel autoclave and evacuated. Then, the sample was rapidly heated to desired temperatures. Dehydrogenation at 300 C was performed under a vacuum. Full hydrogenation was carried out at 250 C under a 2 MPa hydrogen atmosphere.
Results and discussion The structures and morphologies of the pure and MWCNTdoped Li3N samples after mechanical ball milling To study the influence of the MWCNT doping on Li3N during the ball milling process, the morphology and particle size of the pure and MWCNT-doped Li3N samples after mechanical ball milling were characterized using SEM, as shown in Fig. 1. The original morphology and structure of MWCNTs were damaged after mechanical ball milling at 450 rpm for 2 h, presenting a granular image. For the ball-milled pure Li3N sample, the particle size is very uneven (0.5e5 mm). However, the particle size and morphology of the MWCNT-doped Li3N samples after ball milling changed. Compared with that of the ball-milled pure Li3N sample, the particle size of the ballmilled MWCNT-doped Li3N samples is reduced (0.1e2 mm)
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and becomes more even. The experimental results demonstrate that, in the process of ball milling, the doped MWCNTs may have a physical ball milling effect [30], which can further decrease the particle size of Li3N and result in better sample dispersion. In other words, the formation of small crystalline size and clean surface are further induced due to the addition of MWCNTs in the process of ball milling. To determine whether MWCNTs react with Li3N during the process of ball milling, the MWCNT-doped Li3N samples were analyzed after ball milling by XRD (Fig. 2). For all four samples there are clear diffraction peaks due to the two crystal types of Li3N (a-Li3N and b-Li3N). However, compared with the ballmilled pure Li3N sample, the three MWCNT-doped samples show a weak unknown diffraction peak at 30 , which may be due to the interaction of MWCNTs with Li3N. In addition, the diffraction peaks due to MWCNTs are not seen in Fig. 2, indicating the doped samples become amorphous after ball milling. As shown in Table 1, BET specific surface areas of the pure Li3N sample and the Li3N samples with 5 mol%, 10 mol%, and 20 mol% of MWCNTs after ball milling is 10.8631, 15.4728, 16.4178, and 18.8292 m2/g, respectively. In comparison with the ball-milled pure Li3N sample, the ball-milled MWCNTdoped Li3N samples show larger specific surface areas. Along with the increase of MWCNT content, specific surface areas of the MWCNT-doped Li3N samples display an increasing trend. Compared with the specific surface area of pure MWCNTs (139.8623 m2/g), the specific surface area of the four samples is quite small, which may explain why, with the increasing MWCNT content, the specific surface areas of the MWCNTdoped samples increase.
Fig. 1 e SEM images of the Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs samples after ball milling. Please cite this article in press as: Dong B-X, et al., Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.102
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Fig. 2 e XRD curves of the Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs samples after ball milling.
Fig. 3 shows the pore size distribution of the Li3N samples without additive, and with 5 mol%, 10 mol%, and 20 mol% of MWCNTs after ball milling. The pore size of the four samples is mainly distributed in the following two ranges: 10e50 A and 50e300 A. Compared with the ball-milled pure Li3N sample, the MWCNT-doped samples have larger pores, especially the pores in the range of 50e300 A. The quantity of large pores increases with increasing MWCNT content. In 2005, Hu et al. studied the influence of high temperature dehydrogenation on the hydrogen storage performance of Li3N and analyzed the changes of the specific surface area and pore size under different hydrogenation/dehydrogenation conditions [13]. They found that after high temperature dehydrogenation, the quantity of large pores of the Li3N sample was significantly reduced, which may be an important factor that results in the difficulty of further dehydrogenation. On the basis of this result and the above experimental data, we predict that MWCNT-doping may improve the hydrogen desorption and absorption properties of Li3N.
Fig. 3 e Pore size distribution of the Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs samples after ball milling.
without additive and with 5 mol%, 10 mol% or 20 mol% MWCNT additive, of which the hydrogen storage capacities are 10.4, 10.1, 9.9 and 9.7 wt%, respectively. Fig. 4 compares the first hydrogen absorption behaviors of these samples. For the four samples, the onset hydrogen absorption temperature was approximately 120 C and the plateau of hydrogen uptake at 250 C is approximately 8.3 wt% (when the hydrogen absorption amount is calculated, the MWCNT weight is excluded, as described in the literature [4,5]). From Fig. 4, it can be observed that all samples exhibit a fast hydrogenation reaction rate within 30 min at the beginning of the hydrogen absorption. However, as the reaction progresses, the hydrogen absorption rate gradually slows. In comparison with the pure Li3N sample, the hydrogen absorption rate of the MWCNTdoped Li3N sample increases, and the rate increases with the MWCNT quantity increasing. However, the hydrogen absorption rate of the Li3N-10 mol% MWCNTs sample is near that of the Li3N-20 mol% MWCNTs sample. Within 30 min, the
The influence of the doped MWCNTs on the hydrogen absorption and desorption kinetics of Li3N To study the influence of the doped MWCNTs on the hydrogen absorption properties of Li3N, we selected Li3N samples
Table 1 e BET specific surface area of the Li3N, Li3N-5 mol % MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs samples after ball milling. Samples BET specific surface area (m2/g)
Li3N 10.8631
Li3N-5 mol Li3N-10 mol Li3N-20 mol % MWCNTs % MWCNTs % MWCNTs 15.4728
16.4178
18.8292
Fig. 4 e The first isothermal hydrogenation curves at 250 C of the ball-milled Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol % MWCNTs and Li3N-20 mol% MWCNTs samples.
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average hydrogen absorption rate of the Li3N-10 mol% MWCNTs sample is approximately 0.232 wt/min, which is 1.3 times faster than that of the pure Li3N sample. Within the same time period, the hydrogen absorption of Li3N samples doped with MWCNTs is larger than that of the pure Li3N sample. In addition, for the Li3N samples doped with 10 mol% MWCNTs and 20 mol% MWCNTs, the hydrogen absorption within 4 h is close to the plateau of hydrogen uptake, but the hydrogen absorption amount of the Li3N sample without additive fails to reach the plateau within such a short time. Therefore, the doped MWCNTs accelerate the hydrogen absorption rate of Li3N and thereby enhance the hydrogen absorption kinetics. Possible reasons for the improved kinetics were analyzed as follows. First, the MWCNT-doping better disperses the Li3N, and the interaction between Li3N particles and MWCNTs will increase the phase interface, which benefits the effective contact of hydrogen with Li3N [23,31]. Secondly, the MWCNT-doped Li3N samples have larger specific surface areas and more 50e300 A large pores, which will provide further opportunities for the contact of hydrogen with Li3N. To study the influence of the MWCNTs on the hydrogen desorption properties of the hydrogenated Li3N, the hydrogen desorption behaviors of the four samples after the first hydrogen absorption were analyzed by TDMS. As shown in Fig. 5, for all samples, there is only one hydrogen desorption peak from 50 to 500 C, indicating that only the reaction of LiH with LiNH2 generating Li2NH and hydrogen proceeds (reaction 2). The MWCNT-doping does not change the onset hydrogen desorption temperature of the Li3N after the first hydrogen absorption. However, the hydrogen desorption peak temperature is decreased by MWCNT-doping and along with the increase of MWCNT content, the hydrogen desorption peak temperature gradually lowers. Compared with the pure Li3N sample after the first hydrogen absorption, the hydrogen desorption peak temperatures of the 5 mol%, 10 mol% and 20 mol% MWCNT-doped Li3N samples after the first hydrogen absorption are decreased by 1.5, 4.4 and 7.4 C, respectively. Moreover, the NH3 mass spectrum shows that for all the samples there is almost no ammonia emission during the process of hydrogen desorption. Therefore, it is apparent that the MWCNT-doping can improve the hydrogen desorption kinetics to some extent. The hydrogenated 20 mol% MWCNTsdoped sample shows the best hydrogen desorption kinetics. The reason for the improvement of the hydrogen desorption kinetics may partially be because the MWCNTs can suppress Li3N sintering during hydrogenation. It has been reported that MWCNTs have good thermal conductivity and can disperse hydrogen absorption reaction heat rapidly [23,32], which may weaken the compaction of Li3N. The volumetric method was also adopted to examine the hydrogen desorption kinetics of the pure and MWCNT-doped Li3N samples under vacuum at 300 C. As shown in Fig. 6, the four samples have almost the same onset hydrogen desorption temperature (at approximately 215 C), indicating that the MWCNT-doping has little influence on the onset hydrogen desorption temperature of the hydrogenated Li3N sample. All four samples also exhibit a fast dehydrogenation reaction rate within 20e30 min from the beginning of the hydrogen desorption. However, as the reaction progresses, the hydrogen
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Fig. 5 e The first dehydrogenation TDMS curves of the Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N20 mol% MWCNTs samples after the first hydrogen absorption.
desorption rate gradually slows. Within 30 min, the average hydrogen desorption rate of the Li3N-10 mol% MWCNTs sample is approximately 0.117 wt/min, which is 1.2 times faster than that of the pure Li3N sample. In comparison with the pure Li3N sample, the hydrogen desorption rate of the MWCNT-doped Li3N sample increases, and the rate increases with the MWCNTs quantity increasing. The hydrogen
Fig. 6 e Isothermal dehydrogenation curves at 300 C of the Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs samples after the first hydrogen absorption.
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desorption rate of the Li3N sample doped with 10 mol% MWCNTs is near that of the Li3N sample doped with 20 mol% MWCNTs. In addition, the MWCNT-doped Li3N samples can reach the plateau of hydrogen desorption (3.8 wt%) within 2 h, but it takes at least 4 h for the pure Li3N sample to reach its hydrogen desorption plateau. Therefore, the isothermal dehydrogenation tests using the volumetric method further confirm that the MWCNT-doping can improve the hydrogen desorption kinetics of the hydrogenated Li3N.
The influence of the MWCNTs on the hydrogen absorption/ desorption reversibility of Li3N An ideal hydrogen storage material with superior performance not only has fast hydrogen absorption/desorption kinetics but also has good cycling stability. To study the influence of MWCNTs on the cycling properties of Li3N, ten hydrogen absorption/desorption cycles for the Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs samples were performed. Fig. 7 shows the second, sixth, eighth and tenth hydrogen absorption curves of the four samples to compare their hydrogen absorption cycling properties. Fig. 7(a) shows that, for the pure Li3N sample, the onset hydrogen absorption temperature clearly increases and the hydrogen absorption rate gradually slows as more cycles pass. The pure Li3N sample can absorb 3.8 wt% hydrogen within 1.5 h for the second hydrogen absorption, but the amount of
the tenth hydrogen absorption within 5 h is only 1 wt%. These experimental results show that the cycling stability of the pure Li3N sample is poor. For the 5 mol% MWCNTs-doped Li3N sample, as more cycles pass, the onset absorption temperature and the hydrogen absorption rate change little within six cycles, indicating that the cycling stability of Li3N is improved by doping with 5 mol% MWCNTs (Fig. 7(b)). However, from the eighth hydrogen absorption, the hydrogen absorption curve slope clearly decreases, indicating that the hydrogen absorption rate notably reduces. As shown in Fig. 7(c and d), when the MWCNT level reaches 10 mol%, the sample exhibits good cycling performance. For the 10 mol% and 20 mol% MWCNTdoped Li3N samples, the onset absorption temperature and the hydrogen absorption rate change little within ten cycles. When examining the change of the hydrogen absorption with cycle number, the capacity of the Li3N samples with 10 mol% and 20 mol% MWCNTs remain ca. 3.8 wt% during ten cycles, which is 2.3 times as many as that of the pure Li3N sample. Therefore, MWCNT-doping can improve the hydrogen absorption cycling stability of Li3N, and when the doping level of MWCNTs reaches 10 mol%, the enhancement effect is significant. Fig. 8 shows the first, sixth, eighth and tenth hydrogen desorption curves of the hydrogenated Li3N samples to compare their hydrogen desorption cycling properties. Fig. 8(a) shows that, for the pure Li3N sample, the onset hydrogen desorption temperature obviously increases and the
Fig. 7 e Reversible full hydrogen absorption curves for the Li3N (a), Li3N-5 mol% MWCNTs (b), Li3N-10 mol% MWCNTs (c) and Li3N-20 mol% MWCNTs (d) samples. Please cite this article in press as: Dong B-X, et al., Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.102
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hydrogen desorption rate gradually slows as the cycles pass. The pure Li3N sample can desorb 3.8 wt% hydrogen within 1.8 h for the first hydrogen desorption, but the amount of the tenth hydrogen desorption within 4 h is far less than 1 wt%. These experimental results show that the cycling desorption property of the pure Li3N sample is poor. Fig. 8(b) demonstrates that, for the Li3N sample with 5 wt% MWCNTs, the onset desorption temperature and the hydrogen desorption rate change little within six cycles. However, from the eighth hydrogen desorption, the slope of hydrogen desorption curve clearly decreases, indicating that the hydrogen desorption rate significantly reduces. As shown in Fig. 8(c and d), when the MWCNTs reach 10 mol%, the sample exhibits good cycling performance. For the Li3N samples with 10 mol% and 20 mol% MWCNTs, the onset desorption temperatures change little and the hydrogen desorption rates also change little within ten cycles. By examining the change in the hydrogen desorption with the cycle number, it was determined that the maximum hydrogen desorption of the 10 mol% and 20 mol% MWCNTs-doped Li3N samples remains ca. 3.8 wt% within ten cycles while the maximum hydrogen desorption of the pure Li3N sample decreases drastically, from 3.8 wt% for the first desorption to 0.7 wt% for the tenth desorption. For the tenth cycle, the Li3N samples with 10 mol% and 20 mol% MWCNTs can desorb approximately 3.5 wt% hydrogen within 2 h, which is 7 times as many as that of the pure Li3N sample. Therefore, MWCNT-doping can improve the hydrogen desorption cycling
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stability of Li3N, and when the doping level of MWCNTs reaches 10 mol%, the enhancement effect is significant.
The morphologies and composition of the pure and MWCNTdoped Li3N samples after the tenth hydrogen desorption Fig. S1 shows the morphologies of Li3N, Li3N-5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol% MWCNTs after the tenth hydrogen desorption. Although all the samples become cylinders formed of powder after the tenth hydrogen desorption, the compactness of the cylinders differs. The pure Li3N and Li3N-5 mol% MWCNTs samples form very compact cylinders after the tenth hydrogen absorption, indicating that the cyclic hydrogen absorption and desorption notably sinters the two samples. However, the sintering degree of the Li3N-10 mol% MWCNTs and Li3N20 mol% MWCNTs samples after the tenth hydrogen desorption is low, as the formed cylinders are less compact and their uneven cross sections can be observed with the naked eye. This demonstrates that MWCNT-doping can weaken the degree of Li3N sintering. Compared with the pure Li3N sample, MWCNT-doped Li3N samples present good cycling stability. To more deeply understand the causes of the improved cycling performance, we characterized the products of the pure and MWCNT-doped Li3N samples after the tenth hydrogen desorption by XRD and FTIR. As shown in Fig. 9(a), the characteristic diffraction of
Fig. 8 e Reversible full hydrogen desorption curves for the Li3N (a), Li3N-5 mol% MWCNTs (b), Li3N-10 mol% MWCNTs (c) and Li3N-20 mol% MWCNTs (d) samples. Please cite this article in press as: Dong B-X, et al., Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.102
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infrared spectra, showing that LiNH2 has been completely consumed during hydrogen desorption reactions for these samples (Fig. 9). Therefore, the XRD and FTIR data can further explain why doping with MWCNTs can improve the hydrogen storage cycling performance of Li3N. LiNH2 can be completely consumed in the hydrogen desorption reaction of MWCNTdoped Li3N samples after several cycles because doping with MWCNTs weakens the sintering degree of Li3N.
Conclusions In this paper, the influence of MWCNT-doping on the hydrogen storage properties of Li3N was systematically investigated. Compared with the pure Li3N sample, the MWCNT-doped Li3N samples show faster hydrogen absorption and desorption kinetics and drastically improved cycling stability. The effect becomes more apparent with increasing MWCNT content. When the MWCNT content reaches 10 mol %, the enhancement effect is significant. The improvement of the hydrogen storage properties of the Li3N system by MWCNT-doping comes down to the following causes: (1) In the process of ball milling, the MWCNTs add to the physical effect of ball milling, which can further decrease the particle size of Li3N, better dispersing the samples; (2) MWCNT-doping increases the specific surface area and the amount of large pores, which is conducive to the absorption and release of hydrogen; (3) the good thermal conductivity of MWCNTs quickly disperses hydrogen absorption reaction heat and weakens the sintering degree of the Li3N.
Acknowledgments
Fig. 9 e The XRD profiles and FTIR spectra of the Li3N, Li3N5 mol% MWCNTs, Li3N-10 mol% MWCNTs and Li3N-20 mol % MWCNTs samples after the tenth hydrogen desorption.
LiNH2 remains in the XRD curve of pure Li3N after the tenth hydrogen desorption. Two obvious NeH stretching vibration peaks from LiNH2 can also be observed in the infrared spectra of pure Li3N after the tenth hydrogen desorption. Moreover, only very weak NeH stretching vibration peaks due to Li2NH can be observed in the infrared spectra of the pure Li3N after the tenth hydrogen desorption. Coupled with the lower hydrogen desorption maximum, this suggests that the hydrogen desorption solidesolid reaction between LiNH2 and LiH has great difficulty proceeding during the tenth hydrogen desorption. Of course, the appearance of this experimental phenomenon during hydrogen desorption cycles may be caused by the sample sintering, which hinders the migrations of Liþ and Hþ and therefore influences the efficiency of the reaction [33]. For the MWCNT-doped Li3N after the tenth hydrogen desorption, only the diffraction peaks due to Li2NH and LiH are observed in the XRD curves and there are only obvious NeH stretching vibration peaks due to Li2NH in the
This work is financially supported by the NNSF of China (Nos. 21573192, 21301152 and 21671169), SRF of SEM for ROCS, RFDP (No. 20133250120008), Six Talent Peaks Project in Jiangsu Province (No. 2015-XYN-011), and the Foundation from the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.09.102.
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Please cite this article in press as: Dong B-X, et al., Effects of MWCNTs on improving the hydrogen storage performance of the Li3N system, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.102