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Synergetic effects of K, Ti and F on the hydrogen storage properties of the LiNH system
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Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system Ya-Ru Zhang, Bao-Xia Dong**, Juan Zhao, Yun-Lei Teng*, Zong-wei Li, Lu Wang School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, PR China
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abstract
Article history:
In this paper, the hydrogen storage properties of the LiNH2eLiH system doped with K2TiF6
Received 29 March 2017
were investigated and discussed. Interestingly, the hydrogen storage properties are
Received in revised form
significantly enhanced by introducing K2TiF6 into the LiNH2eLiH system. By doping 5 mol%
29 May 2017
K2TiF6 in the LiNH2eLiH system, we obtain the hydrogen desorption peak temperature
Accepted 4 June 2017
(233 C) at a heating rate of 10 C min1, which is approximately 66 C lower than that of the
Available online xxx
pristine LiNH2eLiH system. Moreover, the system begins to desorb H2 at 75 C, which is approximately 124
C lower than in the pristine LiNH2eLiH system. The isothermal
Keywords:
desorption kinetics at 250 C and 300 C clearly reflects the dramatically improved kinetic
Hydrogen storage
properties. Additionally, the reversibility of the LiNH2eLiH system can be drastically
Lithium amide
enhanced by adding K2TiF6. We propose that the dehydrogenation property of the K2TiF6-
Lithium hydride
doped LiNH2eLiH sample is improved by the synergetic effects of K, Ti and F.
Potassium
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Titanium Fluoride
Introduction To transport hydrogen safely and apply hydrogen energy, it is necessary to explore a better hydrogen-carrying system. With the continuous progress in science and technology, light metals are applied to hydrogen storage materials as one of the advanced techniques. According to previous reports, the lightweight metal hydrogen storage material has the advantages of high gravimetric and volumetric hydrogen storage densities at a moderate ab-/desorption temperature [1] compared with the gaseous and liquid hydrogen storage materials. In recent years, the LieNeH system has been considered to be an interesting candidate. The reversibility and relatively
high hydrogen storage capacity of the systems have been investigated since Chen et al. [2] first reported the LieNeH system [3]. Among the LieNeH systems, lithium amide (LiNH2) and its modifications have incessantly drawn the attention of scientists because of their relatively high theoretical hydrogen storage capacities and low cost [4e8]. In addition, the LieNeH system recently receives increased interest because the kinetics and thermodynamics of hydrogen absorption and desorption can be significantly improved by doping a small amount of additives. However, the LieNeH system is still limited in practical applications because a high desorption temperature is required, and the reaction is subjected to slow kinetics. Moreover, in the dehydrogenation
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B.-X. Dong), [email protected] (Y.-L. Teng). http://dx.doi.org/10.1016/j.ijhydene.2017.06.026 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Y-R, et al., Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.026
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process, there is a by-product of NH3 mainly because LiNH2 decomposes at higher temperature [3]. Previous studies report that titanium and titanium compounds can obviously promote the kinetics of the LieNeH hydrogen storage system. Shigehito Isobe et al. demonstrate show a superior catalytic effect that Tinano, TiCl3 and TiOnano 2 on improving the dehydrogenation property of the LieNeH system [9]. In 2014, Dmytro Korablov et al. report that the Mge TieH sample (ternary hydrogen storage material) has a faster absorption-desorption kinetic at 300 C and a lower onset temperature for dehydrogenation (Tonset ¼ 217 C) [10]. In addition, Mao et al. report that the onset temperature of the LiAlH4eLiBH4 system is obviously changed by doping TiF3, where the first and second dehydrogenated steps of this system are decreased by 64 C and 150 C, respectively [11]. It is reported that in addition to the catalysis of TiF3, the reaction of TiF3 with LiAlH4 or LiBH4, may be one of the reasons for the weakening of the BeH and AleH bonding, resulting in the improvement of the dehydrogenation kinetics. Till now, considerable research has been devoted to reducing the operating temperature and improving the dehydrogenation kinetics and thermodynamics of the LieNe H and LieMgeNeH hydrogen storage systems by doping potassium additives [12e22]. The properties and mechanisms of the LieMgeNeH system with potassium halides were studied and analyzed by Liu et al. [19]. Liu et al. report that the 0.08 KFdoped sample had a lower onset dehydrogenation temperature, which is reduced by approximately 80 C, than the pristine sample. The KOH-doped Mg(NH2)2-2 LiH system has an improved cycling stability of de-/hydrogenation according to Liang et al. [20]. The result indicates that the average capacity degradation of the KOH-doped LieNeH system is only 0.002 wt% per cycle within 30 cycles. According to Dong et al., the dehydrogenation onset and peak temperatures of the KOH-doped LiNH2eLiH system are significantly decreased compared to the LiNH2eLiH system without additives. The detailed results reveal that the dehydrogenation onset temperature of the KOH-doped LiNH2eLiH system decreases by 36 C, and the peak temperature decreases by 42 C [21]. The fact indicates that a reaction between the doped KOH and LiH takes place to convert to KH and Li2O during ball milling. KH is responsible for the improvement of the hydrogen storage properties. Recently, the KLi3(NH2)4-4 LiH sample is reported to be a superior hydrogen storage system. The KLi3(NH2)4-4 LiH sample demonstrates a superior cycling property, for which the hydrogen ab-/desorption rates remain unchanged within 30 cycles [22]. According to the relevant literature, the hydrogen storage properties of some hydrogen storage materials can be improved by adding fluoride compounds such as metal fluoride and rare earth fluoride. The FeF3-doped MgH2 powder shows a faster H-sorption kinetics at 300 C. There is a 50 C decrease in the temperature range of hydrogen desorption of the FeF3-doped MgH2 system compared with that of the MgH2 system without additive [23,24]. In addition, the TiF3-doped LiBH4 hydrogen storage system is reported by Guo et al. The results indicate that the desorption begins at approximately 100 C for the TiF3-doped LiBH4, and the hydrogen release capacity significantly increases at 250 C (the hydrogen yield is 5.0 wt%) [25]. In 2011, LiBH4 þ 1/3 (Ce, La) (Cl, F)3 was
investigated to explore its hydrogen storage property by Zhang et al. They reported that the desorption temperatures of the mixtures decreased to 220e300 C, which were much lower than that of pure LiBH4 (approximately 400 C) [26]. In addition, Dong et al. find that KF has a good catalytic effect on improving the hydrogen storage properties of the LieNeH system. They report that the dehydrogenation onset and peak temperatures of the LiNH2eLiH-5-mol% KF composite decrease by approximately 36 C and 38 C, respectively, compared with the LiNH2eLiH sample. Comparing the cyclic hydrogen desorption performance of the LiNH2eLiH-5-mol% KF composite and LiNH2eLiH sample at 300 C, we find that the tenth cycle remains much better than that of the LiNH2e LiH sample in the second cycle [27]. Liu et al. report that K2TiF6 is an excellent additive that can notably improve the dehydrogenation kinetics of NaAlH4 [28]. The results show a synergetic effect of K, Ti and F in enhancing the reversible hydrogen storage properties of NaAlH4. Recently, Nustafa N.S. et al. explored the hydrogen storage system of MgH2 and 4 MgH2eLiAlH4 by doping K2TiF6 [29,30]. In the 10-wt% K2TiF6-doped MgH2 hydrogen storage system, the onset temperature of desorption is 245 C, which is much lower than that of the ball-milled MgH2, and the onset dehydrogenation temperature of the 10-wt% K2TiF6-doped 4 MgH2eLiAlH4 decreases by 50 C compared with the as-milled undoped 4 MgH2eLiAlH4. It is proven that the K2TiF6 additive in MgH2 and 4 MgH2eLiAlH4 samples play a catalytic role in the formation of Tie, Ke, and Al-containing or MgF2 compounds during the ball milling or heating process [29,30]. Therefore, it is reasonable to suppose that K2TiF6 exhibits great potential as a catalyst to enhance the hydrogen storage properties of the LieNeH system. As far as we know, the hydrogen storage properties of the LieNeH system with K2TiF6 additive were never studied. In this work, the hydrogen storage properties of the K2TiF6doped LiNH2eLiH system were investigated and discussed. Interestingly, the hydrogen storage properties were significantly enhanced by introducing 5 mol% K2TiF6 into the LiNH2e LiH system. It is proposed that the dehydrogenation property of the K2TiF6-doped LiNH2eLiH sample is improved by the synergetic effects of K, Ti and F.
Materials and methods Sample preparation The starting materials are lithium hydride (LiH) (98%, J&K Chemical Ltd., China), lithium amide (LiNH2) (95%, SigmaAldrich), potassium titanium fluoride (K2TiF6) (Aldrich, USA), potassium fluoride (KF) (98%, J&K Chemical Ltd., China), titanium (IV) fluoride (TiF4) (Aldrich, USA), which were commercially purchased. We selected 0.01, 0.03 and 0.05 mol K2TiF6, 0.05 mol TiF4 and 0.10 mol KF as additives, and we denote the LieNeH samples with different additives as LiNH2eLiHe xmol% K2TiF6 (x ¼ 1, 3, 5), LiNH2eLiH-5-mol% TiF4 and LiNH2e LiH-10-mol% KF. In total, 300 mg of composites of LiNH2eLiH powders with a 1:1 M ratio and additives were ball-milled under 0.6 MPa hydrogen (>99.999%) atmosphere for 2 h at 450 rpm using a planetary ball-milling apparatus (QM-3SP4).
Please cite this article in press as: Zhang Y-R, et al., Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.026
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Then, 20 steel balls (6 mm in diameter) were put into a steel vessel, whose inner volume is approximately 70 cm3. The ballto-powder weight ratio is approximately 90:1. The milling was interrupted every hour for 30 min to prevent the samples from overheating during the milling. All samples were handled in a glove box filled with high-purity argon atmosphere (>99.999%).
Structural characterizations X-ray diffraction (XRD) (AXS D8 ADVANCE, Bruker, Germany) was used to examine the structural characteristics of reaction products. The samples were covered by polyimide film to prevent oxidation during the measurements.
Thermal desorption and isothermal dehydrogenation measurements Hydrogen and a small amount of ammonia were released from the LieNeH system during the heat treatment. The behaviors of hydrogen and ammonia desorption were measured using a synchronous thermal analyzer (DSC/DTA-TG; Netzsch STA 449 F3 Jupiter®), which was coupled with a thermal desorption mass spectrometer (TDMS; Netzsch QMS 403 D € olos®, Germany). The samples with a weight range of Ae 12e15 mg were heated at the rate of 10 C min1 in an Ar gas flow of 30 ml min1. The absorption and desorption kinetics were also examined by a Hy-Energy PCT apparatus (Sieverts apparatus) using the volumetric method. Approximately 150 mg sample was quickly heated to different objective temperatures (200 C, 250 C and 300 C) for dehydrogenation or hydrogenation. Dehydrogenation was performed in vacuum, and hydrogenation was performed in the initial 2-MPa hydrogen atmosphere. In the cycle performance study, the hydrogen absorption and desorption temperatures were set at 250 C and 300 C, respectively.
Results and discussion Dehydrogenation properties of the K2TiF6-doped LiNH2eLiH systems The composites of LiNH2 and LiH without an additive and with 1 mol%, 3 mol% and 5 mol% K2TiF6 additives were prepared by ball milling at 450 rpm in a hydrogen atmosphere of 0.6 MPa for 2 h. Fig. 1 shows the hydrogen desorption mass spectra, ammonia gas desorption mass spectra, DSC profiles and weight variations during the hydrogen desorption for each composite of LiNH2 and LiH without additive, with 1 mol% K2TiF6, 3 mol% K2TiF6 and 5 mol% K2TiF6. Fig. 1a shows that the hydrogen desorption onset and peak temperatures gradually decrease, and the hydrogen desorption curves gradually become sharp with increasing quantity of the K2TiF6 additive. The hydrogen desorption onset temperatures for each composite of LiNH2 and LiH without additive, with 1 mol% K2TiF6, and with 3 mol% K2TiF6 are 199 C, 92 C and 85 C, respectively (Table 1). The hydrogen desorption peak temperatures for each composite of LiNH2 and LiH without additive, with 1 mol% K2TiF6, and with 3 mol% K2TiF6 are 299 C, 253 C and
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248 C, respectively (Table 1). Notably, the LiNH2eLiH-5-mol% K2TiF6 sample displays a hydrogen desorption onset temperature at 75 C and a peak at 233 C, which decrease by approximately 124 C and 66 C, respectively, compared with those of the pure LiNH2eLiH system. Thus, K2TiF6 has a significant effect on the reduction of the hydrogen desorption peak and onset temperatures of the LiNH2eLiH system. Compared with the LiNH2eLiH composite with 1 mol% AlCl3 or 0.5 mol% LiTi2O4, the onset temperature of the LiNH2eLiH-5mol% K2TiF6 sample decreased by approximately 105 C and 75 C, respectively [31,32]. The peak temperature of the LiNH2eLiH-5-mol% K2TiF6 sample decreased by 92 C compared with the LiNH2eLiH composite with 1 mol% AlCl3. The peak temperature of the LiNH2eLiH-0.5-mol% LiTi2O4 was 227 C [32], which is almost identical to that of the LiNH2eLiH5-mol% K2TiF6 sample (233 C). According to Shigehito Isobe et al., the LiNH2eLiH-5-mol% K2TiF6 sample has a lower peak temperature (approximately 17 C) than the LiNH2eLiH com[9]. posite with TiOmicro 2 It is well known that ammonia gas is produced when the LieNeH system is dehydrogenated at high temperatures [22]. In this study, the ammonia emission during hydrogen desorption is suppressed for the LiNH2eLiH-x-mol% K2TiF6 (x ¼ 1, 3, 5) composites, particularly for the LiNH2eLiH-x-mol% K2TiF6 (x ¼ 3, 5) composites (Fig. 1b). Fig. 1c shows the DSC curves for the LiNH2eLiH-x-mol% K2TiF6 (x ¼ 0, 1, 3, 5) composites at a heating rate of 10 C min1. The pattern indicates that the composite dehydrogenation is an endothermic reaction, and the endothermic onset and peak temperatures of the LiNH2eLiH composites with and without K2TiF6 are consistent with those in Fig. 1a. Fig. 1d shows that the hydrogen desorption rate of the LieNeH system was significantly enhanced by doping K2TiF6 compounds into this system. Within 150 C, the hydrogen desorption amount is 0.651 wt%, 0.680 wt%, 1.478 wt% and 2.496 wt% for the LiNH2eLiH, LiNH2e LiH-1-mol% K2TiF6, LiNH2eLiH-3-mol% K2TiF6 and LiNH2eLiH5-mol% K2TiF6 samples, respectively, which indicates that among the four samples, the LiNH2eLiH-5 mol% K2TiF6 sample has the fastest hydrogen desorption rate. In the hydrogen desorption process from 50 C to 400 C, the pure LiNH2eLiH sample cannot reach a hydrogen desorption plateau, but the LiNH2eLiH-x-mol% K2TiF6 (x ¼ 1, 3, and 5) composites reach the hydrogen desorption plateau at 287 C, 280 C and 265 C, respectively, which further confirms that the K2TiF6 doping can improve the dehydrogenation rate of the LieNeH system. The theoretical hydrogen desorption capacity and experimental weight loss at 400 C for each composite of LiNH2eLiH without additive, with 1 mol% K2TiF6 additive, 3 mol% K2TiF6 additive, or 5 mol% K2TiF6 additive are presented in Table 1. According to the following reaction (1): Li2 NH þ H2 4LiNH2 þ LiH
(1)
the theoretical hydrogen desorption capacities of LiNH2eLiH without additives, with 1 mol% K2TiF6 additive, 3 mol% K2TiF6 additive and 5 mol% K2TiF6 additive are 6.451 wt%, 5.988 wt%, 5.326 wt% and 4.651 wt% (Table 1), respectively. Moreover, the TG curves (Fig. 1d) show that with the increase in temperature to 400 C, the weight loss of LiNH2eLiH without additives with 1 mol%, 3 mol% and 5 mol% K2TiF6 additive is 5.377%, 4.989 wt
Please cite this article in press as: Zhang Y-R, et al., Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.026
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Fig. 1 e Hydrogen desorption mass spectra (a), ammonia gas desorption mass spectra (b), DSC profiles (c) and weight variations during the hydrogen desorption process (d) for each composite of LiNH2 and LiH without additive, with 1 mol% K2TiF6, 3 mol% K2TiF6 and 5 mol% K2TiF6.
Table 1 e Hydrogen desorption on-set temperature, hydrogen desorption peak temperature, theoretical hydrogen desorption capacity and experimental weight loss at 400 C for each composite of LiNH2eLiH without additives, with 1 mol % K2TiF6 additive, 3 mol% K2TiF6 additive, or 5 mol% K2TiF6 additive. Samples
LiNH2eLiH LiNH2eLiH-1 mol% K2TiF6 LiNH2eLiH-3 mol% K2TiF6 LiNH2eLiH-5 mol% K2TiF6
On-set temperature/ C
Peak temperature/ C
Theoretical hydrogen storage/wt %
TG (mass change at 400 C/wt %
199 92 85 75
299 253 248 233
6.451 5.988 5.326 4.651
5.377 4.989 4.025 3.343
%, 4.025 wt% and 3.343 wt% (Table 1), respectively. Although, with increasing K2TiF6 quality, the theoretical and experimental hydrogen desorption capacities decrease, the LiNH2e LiH-5 mol% K2TiF6 sample has the fastest hydrogen desorption properties. Therefore, in this study, the LiNH2eLiH-5-mol % K2TiF6 system was selected for the following series of studies because of its lower dehydrogenation temperature, faster hydrogen desorption rate and less ammonia emission.
In Fig. 2, the isothermal dehydrogenation behaviors of the LiNH2eLiH and LiNH2eLiH-5-mol% K2TiF6 samples at 200 C, 250 C and 300 C were investigated to compare the isothermal desorption properties of the two systems (the K2TiF6-doped sample and the pristine sample). Apparently, the LiNH2eLiH5-mol% K2TiF6 sample released approximately 2.6 wt% hydrogen within 0.6 h at 250 C (Fig. 2b). However, within 0.6 h, only approximately 1.7 wt% hydrogen was released from the
Please cite this article in press as: Zhang Y-R, et al., Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.026
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Fig. 2 e Isothermal kinetic desorption curves of the LiNH2eLiH (a) and LiNH2eLiH-5-mol% K2TiF6 (b) samples at 200 C, 250 C and 300 C.
LiNH2eLiH system at 250 C (Fig. 2a). The dehydrogenation rates of the LiNH2eLiH-5-mol% K2TiF6 and LiNH2eLiH sample within 0.6 h at 250 C are 4.33 and 2.83 wt%/h, respectively. When the dehydrogenation temperature was increased to 300 C, the 5-mol% K2TiF6-doped sample released approximately 2.9 wt% hydrogen and reached a dehydrogenation plateau within 0.5 h, whereas the sample without additive released approximately 2.74 wt% hydrogen within 0.5 h, and it took much longer time to reach a dehydrogenation plateau. According to the above data, it can be obtained at 300 C within 0.5 h the dehydrogenation rates of the LiNH2eLiH-5mol% K2TiF6 and LiNH2eLiH sample are 5.80 and 5.48 wt%/h, respectively. When the operating temperature was increased to 250 C or 300 C, the 5-mol% K2TiF6-doped sample had a faster dehydrogenation rate than the LiNH2eLiH sample. In Fig. 2, the isothermal hydrogen desorption curves of the 5-mol % K2TiF6-doped sample were much steeper than those of the sample without additive. The sample of LiNH2eLiH-5-mol% K2TiF6 obviously has the significantly improved rate of isothermal desorption.
Cycling properties of the K2TiF6-doped LiNH2eLiH systems It is well-known that a good hydrogen storage material should have a good cycling stability. To investigate the cycling
performance of the LiNH2eLiH-5-mol% K2TiF6 sample, the hydrogenation and dehydrogenation behaviors were tested by PCT. The cycling hydrogen desorption curves of the LiNH2e LiH and LiNH2eLiH-5-mol% K2TiF6 samples are exhibited in Fig. 3. The as-milled LiNH2eLiH-5-mol% K2TiF6 sample released approximately 3.1 wt% hydrogen for the first dehydrogenation. In the second, third and fourth hydrogen desorptions, the amount of hydrogen desorption remained at approximately 2.7e2.9 wt%. However, in the fifth cycle, the amount of hydrogen desorption sharply decreased to 1.6 wt%. In the second cycle, the LiNH2eLiH-5-mol% K2TiF6 released 2.9 wt% hydrogen within 1 h (Fig. 3b), but within the identical time length, the LieNeH system without additive only released 0.5 wt% hydrogen (Fig. 3a). In addition, the hydrogen storage capacity of the LiNH2eLiH sample dramatically decreased to 1.2 wt% in the second cycle [31], but the hydrogen storage capacity of the LiNH2eLiH-5-mol% K2TiF6 sample after 5 cycles remained at 1.6 wt%. Based on the experimental results, the 5-mol% K2TiF6-doped LiNH2eLiH displays a faster hydrogen sorption rate and better cyclic stability than the Lie NeH system without additives. Compared with other additives, the K2TiF6 additive moderately improves the cyclic properties of the LiNH2eLiH hydrogen storage system. The LiNH2eLiH-1-mol% TiCl3 can have only 3 cycles of reversible hydrogenation and dehydrogenation [33]. The LiNH2eLiH-1-
Fig. 3 e Reversible cyclic hydrogen sorption (at 250 C)/desorption (at 300 C) curves of the LiNH2eLiH (a) and LiNH2eLiH-5mol% K2TiF6 (b) samples. Please cite this article in press as: Zhang Y-R, et al., Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.026
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mol% AlCl3 sample can perform 7 stable cycles [31]. Interestingly, the KLi3(NH2)4-4 LiH hydrogen storage system has better cycling capability than the LiNH2eLiH-5-mol% K2TiF6 sample. The capacity of KLi3(NH2)4-4 LiH system remained at 2.4 wt% after 30 cycles [22].
Mechanism to improve the hydrogen desorption properties in the K2TiF6-doped LiNH2eLiH sample Fig. 4 shows the XRD patterns of the dehydrogenated LiNH2e LiH-5-mol% K2TiF6 sample at different stages, which demonstrates the structural changes of the sample at different temperatures. The XRD results display no change in the structure of the original reactants after ball milling. After dehydrogenation at 200 C, the XRD curve has a significant change. The diffraction peaks of LiNH2, LiH and K2TiF6 disappeared, but the diffraction peaks of Li2NH, KH and LiF appeared. Particularly, at 27.5 , 32.3 , 53.2 and 64.9 , there are obvious KH diffraction peaks. In addition, strong diffraction peaks appeared at 31.2 , 37.3 and 52.1 because of Li2NH. When the sample was heated to 250 C, no significant change was found in the XRD pattern. With the increase in dehydrogenation temperature to 300 C, the peaks of KH disappeared, but new diffraction peaks because of KF appeared at 27.8 , 33.7 , 43.1 and 57.9 . The Ti intermediates that formed in the dehydrogenation process may be amorphous,
Fig. 4 e XRD patterns for the ball-milled and dehydrogenated LiNH2eLiH-5 mol% K2TiF6 samples at different stages.
which results in the diffraction peaks of Ti compounds in the XRD curves. The broad diffraction peak between 10 and 20 in Fig. 4 is because of the polyimide film covered on the samples. The operating temperature of the hydrogen desorption of hydrogen storage materials is affected by two factors: activation energy barrier and reaction enthalpy change. The Kissinger approach [34] was used to calculate the activation energy (Ea) of the LiNH2eLiH-5-mol% K2TiF6 sample as follows: dln Tb2 E m ¼ a R d T1m where b is the heating rate, Tm is the temperature at which the desorption rate is maximum, and R is the gas constant. Fig. 5 shows the DSC profiles at different heating rates and Kissinger's plot of the LiNH2eLiH-5-mol% K2TiF6 sample. From the DSC curves (Fig. 5a), the hydrogen desorption peak temperatures at various heating rates (8, 9, 10, 11 and 12 K/min) were obtained. The 5-mol% K2TiF6-doped LiNH2eLiH sample exhibits only one endothermic peak regardless of the heating rate. Thus, the dehydrogenation of K2TiF6-doped LiNH2eLiH is a one-step exothermic reaction. With the increase in heating rate, the hydrogen desorption peak temperatures increases. For example, the hydrogen desorption peak temperature is 223 C when the heating rate is 8 K min1 and changes to 233 C when the heating rate is 10 K min1. It is worth noting that the DSC curve of the pure LiNH2eLiH composite shows two endothermic peaks at 10 K min1 [22]: a large peak and a small peak. However, the K2TiF6-doped LiNH2eLiH composites show only one large endothermic peak. By fitting the data, the Kissinger plot of the LiNH2eLiH-5-mol% K2TiF6 sample was obtained, as shown in Fig. 5b. The apparent activation energy of the 5-mol% K2TiF6-doped LiNH2eLiH sample was calculated to be 81.92 kJ mol1, which is much lower than the without additive activation energy of LiNH2eLiH (103.10 kJ mol1) [22]. It is worth mentioning that the apparent activation energy of the MgH2-10 wt% K2TiF6 sample was 132 kJ mol1, which is approximately 32 kJ mol1 lower than that of the as-milled MgH2 (164 kJ mol1) [30]. In addition, the apparent activation energy of the 10 wt% K2TiF6-doped 4 MgH2eLiAlH4 composite was 107 kJ mol1, whereas that of the 4 MgH2eLiAlH4 composite without additive is approximately 127 kJ mol1 [29]. Hence, the experimental results and the literature indicate that the reduction in activation energy is an important factor in decreasing the dehydrogenation temperature of the LiNH2eLiH-5-mol% K2TiF6 sample. To clarify the mechanism of the improved hydrogen desorption properties of the LiNH2eLiH-5-mol% K2TiF6 sample, the hydrogen desorption mass spectra of the LieNeH systems with KF or TiF4 were examined. Fig. 6 compares the hydrogen desorption mass spectra of the LiNH2eLiH-10-mol% KF, LiNH2eLiH-5-mol% K2TiF6 and LiNH2eLiH-5-mol% TiF4 samples. The hydrogen desorption peak temperature for the LiNH2 and LiH composites without additive, with 10 mol% KF, 5 mol% K2TiF6 and 5 mol% TiF4 is 299 C, 250 C, 233 C and 278 C, respectively. The hydrogen desorption peak temperatures of the 5-mol% K2TiF6-doped LieNeH system decreased by approximately 66 C, 17 C and 35 C with respect to that of the LieNeH system without additive, with 10 mol% KF and
Please cite this article in press as: Zhang Y-R, et al., Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.026
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Fig. 5 e DSC profiles of different heating rates (a) and Kissinger's plot (b) of LiNH2eLiH-5-mol% K2TiF6.
with 5 mol% TiF4, respectively. Furthermore, the hydrogen desorption onset temperature is 199 C, 89 C, 75 C and 95 C for the LiNH2 and LiH composite without additive, with 10 mol % KF, 5 mol% K2TiF6 and 5 mol% TiF4 sample, respectively. When only K and F ions were added to the LiNH2eLiH sample, the hydrogen desorption peak and onset temperatures were 250 C and 89 C. When only F and Ti ions were added to the LiNH2eLiH sample, the hydrogen desorption peak and onset temperatures were 278 C and 95 C. The hydrogen desorption peak and onset temperatures were 233 C and 75 C when K, Ti and F ions were added to the LiNH2eLiH sample. The hydrogen desorption peak temperature of the 5-mol% K2TiF6 doped sample decreased by approximately 17 C and 35 C, and the hydrogen desorption onset temperature decreased by approximately 14 C and 20 C compared with those of 10-mol % KF and 5-mol% TiF4-doped samples, respectively. Hence, by comparing the hydrogenation desorption temperatures, we
observe that the hydrogen storage performance of the K2TiF6doped LiNH2eLiH system is improved by the synergetic effects of K, Ti and F. In addition, the 5-mol% K2TiF6-doped LiNH2eLiH sample has much sharper hydrogen desorption curves than the 10-mol% KFe and 5-mol% TiF4-doped LiNH2eLiH samples. This phenomenon is also considered to result from the synergetic effects of K, Ti and F.
Conclusions In this paper, the hydrogen storage properties of the LiNH2e LiH system doped with K2TiF6 were investigated and discussed. Interestingly, the hydrogen storage properties are significantly enhanced by introducing K2TiF6 into the LiNH2e LiH system. By doping 5 mol% K2TiF6 into the LiNH2eLiH system, we obtain the hydrogen desorption peak temperature (233 C) at a heating rate of 10 C min1, which is approximately 66 C lower than that of the pristine LiNH2eLiH system. Moreover, the system begins to desorb H2 at 75 C, which is approximately 124 C lower than that of the pristine LiNH2e LiH system. The isothermal desorption kinetics at 250 C and 300 C clearly reflects the dramatically improved kinetic properties. Additionally, the reversibility of the LiNH2eLiH system can be drastically enhanced by adding K2TiF6. The decrease in activation energy is one of the important causes of the decreased hydrogen desorption operating temperatures of the 5-mol% K2TiF6-doped LiNH2eLiH sample. We propose that the improved dehydrogenation property of the K2TiF6-doped LiNH2eLiH sample is caused by the synergetic effects of K, Ti and F.
Acknowledgments
Fig. 6 e Hydrogen desorption mass spectra for each composite of LiNH2eLiH, LiNH2eLiH-10-mol% KF, LiNH2e LiH-5-mol% K2TiF6 and LiNH2eLiH-5-mol% TiF4.
This work is financially supported by the NNSF of China (Nos. 21573192 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.
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Available online at www.sciencedirect.com
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Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system Ya-Ru Zhang, Bao-Xia Dong**, Juan Zhao, Yun-Lei Teng*, Zong-wei Li, Lu Wang School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, PR China
article info
abstract
Article history:
In this paper, the hydrogen storage properties of the LiNH2eLiH system doped with K2TiF6
Received 29 March 2017
were investigated and discussed. Interestingly, the hydrogen storage properties are
Received in revised form
significantly enhanced by introducing K2TiF6 into the LiNH2eLiH system. By doping 5 mol%
29 May 2017
K2TiF6 in the LiNH2eLiH system, we obtain the hydrogen desorption peak temperature
Accepted 4 June 2017
(233 C) at a heating rate of 10 C min1, which is approximately 66 C lower than that of the
Available online xxx
pristine LiNH2eLiH system. Moreover, the system begins to desorb H2 at 75 C, which is approximately 124
C lower than in the pristine LiNH2eLiH system. The isothermal
Keywords:
desorption kinetics at 250 C and 300 C clearly reflects the dramatically improved kinetic
Hydrogen storage
properties. Additionally, the reversibility of the LiNH2eLiH system can be drastically
Lithium amide
enhanced by adding K2TiF6. We propose that the dehydrogenation property of the K2TiF6-
Lithium hydride
doped LiNH2eLiH sample is improved by the synergetic effects of K, Ti and F.
Potassium
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Titanium Fluoride
Introduction To transport hydrogen safely and apply hydrogen energy, it is necessary to explore a better hydrogen-carrying system. With the continuous progress in science and technology, light metals are applied to hydrogen storage materials as one of the advanced techniques. According to previous reports, the lightweight metal hydrogen storage material has the advantages of high gravimetric and volumetric hydrogen storage densities at a moderate ab-/desorption temperature [1] compared with the gaseous and liquid hydrogen storage materials. In recent years, the LieNeH system has been considered to be an interesting candidate. The reversibility and relatively
high hydrogen storage capacity of the systems have been investigated since Chen et al. [2] first reported the LieNeH system [3]. Among the LieNeH systems, lithium amide (LiNH2) and its modifications have incessantly drawn the attention of scientists because of their relatively high theoretical hydrogen storage capacities and low cost [4e8]. In addition, the LieNeH system recently receives increased interest because the kinetics and thermodynamics of hydrogen absorption and desorption can be significantly improved by doping a small amount of additives. However, the LieNeH system is still limited in practical applications because a high desorption temperature is required, and the reaction is subjected to slow kinetics. Moreover, in the dehydrogenation
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (B.-X. Dong), [email protected] (Y.-L. Teng). http://dx.doi.org/10.1016/j.ijhydene.2017.06.026 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhang Y-R, et al., Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.026
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process, there is a by-product of NH3 mainly because LiNH2 decomposes at higher temperature [3]. Previous studies report that titanium and titanium compounds can obviously promote the kinetics of the LieNeH hydrogen storage system. Shigehito Isobe et al. demonstrate show a superior catalytic effect that Tinano, TiCl3 and TiOnano 2 on improving the dehydrogenation property of the LieNeH system [9]. In 2014, Dmytro Korablov et al. report that the Mge TieH sample (ternary hydrogen storage material) has a faster absorption-desorption kinetic at 300 C and a lower onset temperature for dehydrogenation (Tonset ¼ 217 C) [10]. In addition, Mao et al. report that the onset temperature of the LiAlH4eLiBH4 system is obviously changed by doping TiF3, where the first and second dehydrogenated steps of this system are decreased by 64 C and 150 C, respectively [11]. It is reported that in addition to the catalysis of TiF3, the reaction of TiF3 with LiAlH4 or LiBH4, may be one of the reasons for the weakening of the BeH and AleH bonding, resulting in the improvement of the dehydrogenation kinetics. Till now, considerable research has been devoted to reducing the operating temperature and improving the dehydrogenation kinetics and thermodynamics of the LieNe H and LieMgeNeH hydrogen storage systems by doping potassium additives [12e22]. The properties and mechanisms of the LieMgeNeH system with potassium halides were studied and analyzed by Liu et al. [19]. Liu et al. report that the 0.08 KFdoped sample had a lower onset dehydrogenation temperature, which is reduced by approximately 80 C, than the pristine sample. The KOH-doped Mg(NH2)2-2 LiH system has an improved cycling stability of de-/hydrogenation according to Liang et al. [20]. The result indicates that the average capacity degradation of the KOH-doped LieNeH system is only 0.002 wt% per cycle within 30 cycles. According to Dong et al., the dehydrogenation onset and peak temperatures of the KOH-doped LiNH2eLiH system are significantly decreased compared to the LiNH2eLiH system without additives. The detailed results reveal that the dehydrogenation onset temperature of the KOH-doped LiNH2eLiH system decreases by 36 C, and the peak temperature decreases by 42 C [21]. The fact indicates that a reaction between the doped KOH and LiH takes place to convert to KH and Li2O during ball milling. KH is responsible for the improvement of the hydrogen storage properties. Recently, the KLi3(NH2)4-4 LiH sample is reported to be a superior hydrogen storage system. The KLi3(NH2)4-4 LiH sample demonstrates a superior cycling property, for which the hydrogen ab-/desorption rates remain unchanged within 30 cycles [22]. According to the relevant literature, the hydrogen storage properties of some hydrogen storage materials can be improved by adding fluoride compounds such as metal fluoride and rare earth fluoride. The FeF3-doped MgH2 powder shows a faster H-sorption kinetics at 300 C. There is a 50 C decrease in the temperature range of hydrogen desorption of the FeF3-doped MgH2 system compared with that of the MgH2 system without additive [23,24]. In addition, the TiF3-doped LiBH4 hydrogen storage system is reported by Guo et al. The results indicate that the desorption begins at approximately 100 C for the TiF3-doped LiBH4, and the hydrogen release capacity significantly increases at 250 C (the hydrogen yield is 5.0 wt%) [25]. In 2011, LiBH4 þ 1/3 (Ce, La) (Cl, F)3 was
investigated to explore its hydrogen storage property by Zhang et al. They reported that the desorption temperatures of the mixtures decreased to 220e300 C, which were much lower than that of pure LiBH4 (approximately 400 C) [26]. In addition, Dong et al. find that KF has a good catalytic effect on improving the hydrogen storage properties of the LieNeH system. They report that the dehydrogenation onset and peak temperatures of the LiNH2eLiH-5-mol% KF composite decrease by approximately 36 C and 38 C, respectively, compared with the LiNH2eLiH sample. Comparing the cyclic hydrogen desorption performance of the LiNH2eLiH-5-mol% KF composite and LiNH2eLiH sample at 300 C, we find that the tenth cycle remains much better than that of the LiNH2e LiH sample in the second cycle [27]. Liu et al. report that K2TiF6 is an excellent additive that can notably improve the dehydrogenation kinetics of NaAlH4 [28]. The results show a synergetic effect of K, Ti and F in enhancing the reversible hydrogen storage properties of NaAlH4. Recently, Nustafa N.S. et al. explored the hydrogen storage system of MgH2 and 4 MgH2eLiAlH4 by doping K2TiF6 [29,30]. In the 10-wt% K2TiF6-doped MgH2 hydrogen storage system, the onset temperature of desorption is 245 C, which is much lower than that of the ball-milled MgH2, and the onset dehydrogenation temperature of the 10-wt% K2TiF6-doped 4 MgH2eLiAlH4 decreases by 50 C compared with the as-milled undoped 4 MgH2eLiAlH4. It is proven that the K2TiF6 additive in MgH2 and 4 MgH2eLiAlH4 samples play a catalytic role in the formation of Tie, Ke, and Al-containing or MgF2 compounds during the ball milling or heating process [29,30]. Therefore, it is reasonable to suppose that K2TiF6 exhibits great potential as a catalyst to enhance the hydrogen storage properties of the LieNeH system. As far as we know, the hydrogen storage properties of the LieNeH system with K2TiF6 additive were never studied. In this work, the hydrogen storage properties of the K2TiF6doped LiNH2eLiH system were investigated and discussed. Interestingly, the hydrogen storage properties were significantly enhanced by introducing 5 mol% K2TiF6 into the LiNH2e LiH system. It is proposed that the dehydrogenation property of the K2TiF6-doped LiNH2eLiH sample is improved by the synergetic effects of K, Ti and F.
Materials and methods Sample preparation The starting materials are lithium hydride (LiH) (98%, J&K Chemical Ltd., China), lithium amide (LiNH2) (95%, SigmaAldrich), potassium titanium fluoride (K2TiF6) (Aldrich, USA), potassium fluoride (KF) (98%, J&K Chemical Ltd., China), titanium (IV) fluoride (TiF4) (Aldrich, USA), which were commercially purchased. We selected 0.01, 0.03 and 0.05 mol K2TiF6, 0.05 mol TiF4 and 0.10 mol KF as additives, and we denote the LieNeH samples with different additives as LiNH2eLiHe xmol% K2TiF6 (x ¼ 1, 3, 5), LiNH2eLiH-5-mol% TiF4 and LiNH2e LiH-10-mol% KF. In total, 300 mg of composites of LiNH2eLiH powders with a 1:1 M ratio and additives were ball-milled under 0.6 MPa hydrogen (>99.999%) atmosphere for 2 h at 450 rpm using a planetary ball-milling apparatus (QM-3SP4).
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Then, 20 steel balls (6 mm in diameter) were put into a steel vessel, whose inner volume is approximately 70 cm3. The ballto-powder weight ratio is approximately 90:1. The milling was interrupted every hour for 30 min to prevent the samples from overheating during the milling. All samples were handled in a glove box filled with high-purity argon atmosphere (>99.999%).
Structural characterizations X-ray diffraction (XRD) (AXS D8 ADVANCE, Bruker, Germany) was used to examine the structural characteristics of reaction products. The samples were covered by polyimide film to prevent oxidation during the measurements.
Thermal desorption and isothermal dehydrogenation measurements Hydrogen and a small amount of ammonia were released from the LieNeH system during the heat treatment. The behaviors of hydrogen and ammonia desorption were measured using a synchronous thermal analyzer (DSC/DTA-TG; Netzsch STA 449 F3 Jupiter®), which was coupled with a thermal desorption mass spectrometer (TDMS; Netzsch QMS 403 D € olos®, Germany). The samples with a weight range of Ae 12e15 mg were heated at the rate of 10 C min1 in an Ar gas flow of 30 ml min1. The absorption and desorption kinetics were also examined by a Hy-Energy PCT apparatus (Sieverts apparatus) using the volumetric method. Approximately 150 mg sample was quickly heated to different objective temperatures (200 C, 250 C and 300 C) for dehydrogenation or hydrogenation. Dehydrogenation was performed in vacuum, and hydrogenation was performed in the initial 2-MPa hydrogen atmosphere. In the cycle performance study, the hydrogen absorption and desorption temperatures were set at 250 C and 300 C, respectively.
Results and discussion Dehydrogenation properties of the K2TiF6-doped LiNH2eLiH systems The composites of LiNH2 and LiH without an additive and with 1 mol%, 3 mol% and 5 mol% K2TiF6 additives were prepared by ball milling at 450 rpm in a hydrogen atmosphere of 0.6 MPa for 2 h. Fig. 1 shows the hydrogen desorption mass spectra, ammonia gas desorption mass spectra, DSC profiles and weight variations during the hydrogen desorption for each composite of LiNH2 and LiH without additive, with 1 mol% K2TiF6, 3 mol% K2TiF6 and 5 mol% K2TiF6. Fig. 1a shows that the hydrogen desorption onset and peak temperatures gradually decrease, and the hydrogen desorption curves gradually become sharp with increasing quantity of the K2TiF6 additive. The hydrogen desorption onset temperatures for each composite of LiNH2 and LiH without additive, with 1 mol% K2TiF6, and with 3 mol% K2TiF6 are 199 C, 92 C and 85 C, respectively (Table 1). The hydrogen desorption peak temperatures for each composite of LiNH2 and LiH without additive, with 1 mol% K2TiF6, and with 3 mol% K2TiF6 are 299 C, 253 C and
3
248 C, respectively (Table 1). Notably, the LiNH2eLiH-5-mol% K2TiF6 sample displays a hydrogen desorption onset temperature at 75 C and a peak at 233 C, which decrease by approximately 124 C and 66 C, respectively, compared with those of the pure LiNH2eLiH system. Thus, K2TiF6 has a significant effect on the reduction of the hydrogen desorption peak and onset temperatures of the LiNH2eLiH system. Compared with the LiNH2eLiH composite with 1 mol% AlCl3 or 0.5 mol% LiTi2O4, the onset temperature of the LiNH2eLiH-5mol% K2TiF6 sample decreased by approximately 105 C and 75 C, respectively [31,32]. The peak temperature of the LiNH2eLiH-5-mol% K2TiF6 sample decreased by 92 C compared with the LiNH2eLiH composite with 1 mol% AlCl3. The peak temperature of the LiNH2eLiH-0.5-mol% LiTi2O4 was 227 C [32], which is almost identical to that of the LiNH2eLiH5-mol% K2TiF6 sample (233 C). According to Shigehito Isobe et al., the LiNH2eLiH-5-mol% K2TiF6 sample has a lower peak temperature (approximately 17 C) than the LiNH2eLiH com[9]. posite with TiOmicro 2 It is well known that ammonia gas is produced when the LieNeH system is dehydrogenated at high temperatures [22]. In this study, the ammonia emission during hydrogen desorption is suppressed for the LiNH2eLiH-x-mol% K2TiF6 (x ¼ 1, 3, 5) composites, particularly for the LiNH2eLiH-x-mol% K2TiF6 (x ¼ 3, 5) composites (Fig. 1b). Fig. 1c shows the DSC curves for the LiNH2eLiH-x-mol% K2TiF6 (x ¼ 0, 1, 3, 5) composites at a heating rate of 10 C min1. The pattern indicates that the composite dehydrogenation is an endothermic reaction, and the endothermic onset and peak temperatures of the LiNH2eLiH composites with and without K2TiF6 are consistent with those in Fig. 1a. Fig. 1d shows that the hydrogen desorption rate of the LieNeH system was significantly enhanced by doping K2TiF6 compounds into this system. Within 150 C, the hydrogen desorption amount is 0.651 wt%, 0.680 wt%, 1.478 wt% and 2.496 wt% for the LiNH2eLiH, LiNH2e LiH-1-mol% K2TiF6, LiNH2eLiH-3-mol% K2TiF6 and LiNH2eLiH5-mol% K2TiF6 samples, respectively, which indicates that among the four samples, the LiNH2eLiH-5 mol% K2TiF6 sample has the fastest hydrogen desorption rate. In the hydrogen desorption process from 50 C to 400 C, the pure LiNH2eLiH sample cannot reach a hydrogen desorption plateau, but the LiNH2eLiH-x-mol% K2TiF6 (x ¼ 1, 3, and 5) composites reach the hydrogen desorption plateau at 287 C, 280 C and 265 C, respectively, which further confirms that the K2TiF6 doping can improve the dehydrogenation rate of the LieNeH system. The theoretical hydrogen desorption capacity and experimental weight loss at 400 C for each composite of LiNH2eLiH without additive, with 1 mol% K2TiF6 additive, 3 mol% K2TiF6 additive, or 5 mol% K2TiF6 additive are presented in Table 1. According to the following reaction (1): Li2 NH þ H2 4LiNH2 þ LiH
(1)
the theoretical hydrogen desorption capacities of LiNH2eLiH without additives, with 1 mol% K2TiF6 additive, 3 mol% K2TiF6 additive and 5 mol% K2TiF6 additive are 6.451 wt%, 5.988 wt%, 5.326 wt% and 4.651 wt% (Table 1), respectively. Moreover, the TG curves (Fig. 1d) show that with the increase in temperature to 400 C, the weight loss of LiNH2eLiH without additives with 1 mol%, 3 mol% and 5 mol% K2TiF6 additive is 5.377%, 4.989 wt
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Fig. 1 e Hydrogen desorption mass spectra (a), ammonia gas desorption mass spectra (b), DSC profiles (c) and weight variations during the hydrogen desorption process (d) for each composite of LiNH2 and LiH without additive, with 1 mol% K2TiF6, 3 mol% K2TiF6 and 5 mol% K2TiF6.
Table 1 e Hydrogen desorption on-set temperature, hydrogen desorption peak temperature, theoretical hydrogen desorption capacity and experimental weight loss at 400 C for each composite of LiNH2eLiH without additives, with 1 mol % K2TiF6 additive, 3 mol% K2TiF6 additive, or 5 mol% K2TiF6 additive. Samples
LiNH2eLiH LiNH2eLiH-1 mol% K2TiF6 LiNH2eLiH-3 mol% K2TiF6 LiNH2eLiH-5 mol% K2TiF6
On-set temperature/ C
Peak temperature/ C
Theoretical hydrogen storage/wt %
TG (mass change at 400 C/wt %
199 92 85 75
299 253 248 233
6.451 5.988 5.326 4.651
5.377 4.989 4.025 3.343
%, 4.025 wt% and 3.343 wt% (Table 1), respectively. Although, with increasing K2TiF6 quality, the theoretical and experimental hydrogen desorption capacities decrease, the LiNH2e LiH-5 mol% K2TiF6 sample has the fastest hydrogen desorption properties. Therefore, in this study, the LiNH2eLiH-5-mol % K2TiF6 system was selected for the following series of studies because of its lower dehydrogenation temperature, faster hydrogen desorption rate and less ammonia emission.
In Fig. 2, the isothermal dehydrogenation behaviors of the LiNH2eLiH and LiNH2eLiH-5-mol% K2TiF6 samples at 200 C, 250 C and 300 C were investigated to compare the isothermal desorption properties of the two systems (the K2TiF6-doped sample and the pristine sample). Apparently, the LiNH2eLiH5-mol% K2TiF6 sample released approximately 2.6 wt% hydrogen within 0.6 h at 250 C (Fig. 2b). However, within 0.6 h, only approximately 1.7 wt% hydrogen was released from the
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Fig. 2 e Isothermal kinetic desorption curves of the LiNH2eLiH (a) and LiNH2eLiH-5-mol% K2TiF6 (b) samples at 200 C, 250 C and 300 C.
LiNH2eLiH system at 250 C (Fig. 2a). The dehydrogenation rates of the LiNH2eLiH-5-mol% K2TiF6 and LiNH2eLiH sample within 0.6 h at 250 C are 4.33 and 2.83 wt%/h, respectively. When the dehydrogenation temperature was increased to 300 C, the 5-mol% K2TiF6-doped sample released approximately 2.9 wt% hydrogen and reached a dehydrogenation plateau within 0.5 h, whereas the sample without additive released approximately 2.74 wt% hydrogen within 0.5 h, and it took much longer time to reach a dehydrogenation plateau. According to the above data, it can be obtained at 300 C within 0.5 h the dehydrogenation rates of the LiNH2eLiH-5mol% K2TiF6 and LiNH2eLiH sample are 5.80 and 5.48 wt%/h, respectively. When the operating temperature was increased to 250 C or 300 C, the 5-mol% K2TiF6-doped sample had a faster dehydrogenation rate than the LiNH2eLiH sample. In Fig. 2, the isothermal hydrogen desorption curves of the 5-mol % K2TiF6-doped sample were much steeper than those of the sample without additive. The sample of LiNH2eLiH-5-mol% K2TiF6 obviously has the significantly improved rate of isothermal desorption.
Cycling properties of the K2TiF6-doped LiNH2eLiH systems It is well-known that a good hydrogen storage material should have a good cycling stability. To investigate the cycling
performance of the LiNH2eLiH-5-mol% K2TiF6 sample, the hydrogenation and dehydrogenation behaviors were tested by PCT. The cycling hydrogen desorption curves of the LiNH2e LiH and LiNH2eLiH-5-mol% K2TiF6 samples are exhibited in Fig. 3. The as-milled LiNH2eLiH-5-mol% K2TiF6 sample released approximately 3.1 wt% hydrogen for the first dehydrogenation. In the second, third and fourth hydrogen desorptions, the amount of hydrogen desorption remained at approximately 2.7e2.9 wt%. However, in the fifth cycle, the amount of hydrogen desorption sharply decreased to 1.6 wt%. In the second cycle, the LiNH2eLiH-5-mol% K2TiF6 released 2.9 wt% hydrogen within 1 h (Fig. 3b), but within the identical time length, the LieNeH system without additive only released 0.5 wt% hydrogen (Fig. 3a). In addition, the hydrogen storage capacity of the LiNH2eLiH sample dramatically decreased to 1.2 wt% in the second cycle [31], but the hydrogen storage capacity of the LiNH2eLiH-5-mol% K2TiF6 sample after 5 cycles remained at 1.6 wt%. Based on the experimental results, the 5-mol% K2TiF6-doped LiNH2eLiH displays a faster hydrogen sorption rate and better cyclic stability than the Lie NeH system without additives. Compared with other additives, the K2TiF6 additive moderately improves the cyclic properties of the LiNH2eLiH hydrogen storage system. The LiNH2eLiH-1-mol% TiCl3 can have only 3 cycles of reversible hydrogenation and dehydrogenation [33]. The LiNH2eLiH-1-
Fig. 3 e Reversible cyclic hydrogen sorption (at 250 C)/desorption (at 300 C) curves of the LiNH2eLiH (a) and LiNH2eLiH-5mol% K2TiF6 (b) samples. Please cite this article in press as: Zhang Y-R, et al., Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.026
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mol% AlCl3 sample can perform 7 stable cycles [31]. Interestingly, the KLi3(NH2)4-4 LiH hydrogen storage system has better cycling capability than the LiNH2eLiH-5-mol% K2TiF6 sample. The capacity of KLi3(NH2)4-4 LiH system remained at 2.4 wt% after 30 cycles [22].
Mechanism to improve the hydrogen desorption properties in the K2TiF6-doped LiNH2eLiH sample Fig. 4 shows the XRD patterns of the dehydrogenated LiNH2e LiH-5-mol% K2TiF6 sample at different stages, which demonstrates the structural changes of the sample at different temperatures. The XRD results display no change in the structure of the original reactants after ball milling. After dehydrogenation at 200 C, the XRD curve has a significant change. The diffraction peaks of LiNH2, LiH and K2TiF6 disappeared, but the diffraction peaks of Li2NH, KH and LiF appeared. Particularly, at 27.5 , 32.3 , 53.2 and 64.9 , there are obvious KH diffraction peaks. In addition, strong diffraction peaks appeared at 31.2 , 37.3 and 52.1 because of Li2NH. When the sample was heated to 250 C, no significant change was found in the XRD pattern. With the increase in dehydrogenation temperature to 300 C, the peaks of KH disappeared, but new diffraction peaks because of KF appeared at 27.8 , 33.7 , 43.1 and 57.9 . The Ti intermediates that formed in the dehydrogenation process may be amorphous,
Fig. 4 e XRD patterns for the ball-milled and dehydrogenated LiNH2eLiH-5 mol% K2TiF6 samples at different stages.
which results in the diffraction peaks of Ti compounds in the XRD curves. The broad diffraction peak between 10 and 20 in Fig. 4 is because of the polyimide film covered on the samples. The operating temperature of the hydrogen desorption of hydrogen storage materials is affected by two factors: activation energy barrier and reaction enthalpy change. The Kissinger approach [34] was used to calculate the activation energy (Ea) of the LiNH2eLiH-5-mol% K2TiF6 sample as follows: dln Tb2 E m ¼ a R d T1m where b is the heating rate, Tm is the temperature at which the desorption rate is maximum, and R is the gas constant. Fig. 5 shows the DSC profiles at different heating rates and Kissinger's plot of the LiNH2eLiH-5-mol% K2TiF6 sample. From the DSC curves (Fig. 5a), the hydrogen desorption peak temperatures at various heating rates (8, 9, 10, 11 and 12 K/min) were obtained. The 5-mol% K2TiF6-doped LiNH2eLiH sample exhibits only one endothermic peak regardless of the heating rate. Thus, the dehydrogenation of K2TiF6-doped LiNH2eLiH is a one-step exothermic reaction. With the increase in heating rate, the hydrogen desorption peak temperatures increases. For example, the hydrogen desorption peak temperature is 223 C when the heating rate is 8 K min1 and changes to 233 C when the heating rate is 10 K min1. It is worth noting that the DSC curve of the pure LiNH2eLiH composite shows two endothermic peaks at 10 K min1 [22]: a large peak and a small peak. However, the K2TiF6-doped LiNH2eLiH composites show only one large endothermic peak. By fitting the data, the Kissinger plot of the LiNH2eLiH-5-mol% K2TiF6 sample was obtained, as shown in Fig. 5b. The apparent activation energy of the 5-mol% K2TiF6-doped LiNH2eLiH sample was calculated to be 81.92 kJ mol1, which is much lower than the without additive activation energy of LiNH2eLiH (103.10 kJ mol1) [22]. It is worth mentioning that the apparent activation energy of the MgH2-10 wt% K2TiF6 sample was 132 kJ mol1, which is approximately 32 kJ mol1 lower than that of the as-milled MgH2 (164 kJ mol1) [30]. In addition, the apparent activation energy of the 10 wt% K2TiF6-doped 4 MgH2eLiAlH4 composite was 107 kJ mol1, whereas that of the 4 MgH2eLiAlH4 composite without additive is approximately 127 kJ mol1 [29]. Hence, the experimental results and the literature indicate that the reduction in activation energy is an important factor in decreasing the dehydrogenation temperature of the LiNH2eLiH-5-mol% K2TiF6 sample. To clarify the mechanism of the improved hydrogen desorption properties of the LiNH2eLiH-5-mol% K2TiF6 sample, the hydrogen desorption mass spectra of the LieNeH systems with KF or TiF4 were examined. Fig. 6 compares the hydrogen desorption mass spectra of the LiNH2eLiH-10-mol% KF, LiNH2eLiH-5-mol% K2TiF6 and LiNH2eLiH-5-mol% TiF4 samples. The hydrogen desorption peak temperature for the LiNH2 and LiH composites without additive, with 10 mol% KF, 5 mol% K2TiF6 and 5 mol% TiF4 is 299 C, 250 C, 233 C and 278 C, respectively. The hydrogen desorption peak temperatures of the 5-mol% K2TiF6-doped LieNeH system decreased by approximately 66 C, 17 C and 35 C with respect to that of the LieNeH system without additive, with 10 mol% KF and
Please cite this article in press as: Zhang Y-R, et al., Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.026
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Fig. 5 e DSC profiles of different heating rates (a) and Kissinger's plot (b) of LiNH2eLiH-5-mol% K2TiF6.
with 5 mol% TiF4, respectively. Furthermore, the hydrogen desorption onset temperature is 199 C, 89 C, 75 C and 95 C for the LiNH2 and LiH composite without additive, with 10 mol % KF, 5 mol% K2TiF6 and 5 mol% TiF4 sample, respectively. When only K and F ions were added to the LiNH2eLiH sample, the hydrogen desorption peak and onset temperatures were 250 C and 89 C. When only F and Ti ions were added to the LiNH2eLiH sample, the hydrogen desorption peak and onset temperatures were 278 C and 95 C. The hydrogen desorption peak and onset temperatures were 233 C and 75 C when K, Ti and F ions were added to the LiNH2eLiH sample. The hydrogen desorption peak temperature of the 5-mol% K2TiF6 doped sample decreased by approximately 17 C and 35 C, and the hydrogen desorption onset temperature decreased by approximately 14 C and 20 C compared with those of 10-mol % KF and 5-mol% TiF4-doped samples, respectively. Hence, by comparing the hydrogenation desorption temperatures, we
observe that the hydrogen storage performance of the K2TiF6doped LiNH2eLiH system is improved by the synergetic effects of K, Ti and F. In addition, the 5-mol% K2TiF6-doped LiNH2eLiH sample has much sharper hydrogen desorption curves than the 10-mol% KFe and 5-mol% TiF4-doped LiNH2eLiH samples. This phenomenon is also considered to result from the synergetic effects of K, Ti and F.
Conclusions In this paper, the hydrogen storage properties of the LiNH2e LiH system doped with K2TiF6 were investigated and discussed. Interestingly, the hydrogen storage properties are significantly enhanced by introducing K2TiF6 into the LiNH2e LiH system. By doping 5 mol% K2TiF6 into the LiNH2eLiH system, we obtain the hydrogen desorption peak temperature (233 C) at a heating rate of 10 C min1, which is approximately 66 C lower than that of the pristine LiNH2eLiH system. Moreover, the system begins to desorb H2 at 75 C, which is approximately 124 C lower than that of the pristine LiNH2e LiH system. The isothermal desorption kinetics at 250 C and 300 C clearly reflects the dramatically improved kinetic properties. Additionally, the reversibility of the LiNH2eLiH system can be drastically enhanced by adding K2TiF6. The decrease in activation energy is one of the important causes of the decreased hydrogen desorption operating temperatures of the 5-mol% K2TiF6-doped LiNH2eLiH sample. We propose that the improved dehydrogenation property of the K2TiF6-doped LiNH2eLiH sample is caused by the synergetic effects of K, Ti and F.
Acknowledgments
Fig. 6 e Hydrogen desorption mass spectra for each composite of LiNH2eLiH, LiNH2eLiH-10-mol% KF, LiNH2e LiH-5-mol% K2TiF6 and LiNH2eLiH-5-mol% TiF4.
This work is financially supported by the NNSF of China (Nos. 21573192 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.
Please cite this article in press as: Zhang Y-R, et al., Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.026
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Please cite this article in press as: Zhang Y-R, et al., Synergetic effects of K, Ti and F on the hydrogen storage properties of the LieNeH system, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.06.026