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Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system
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Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system Chao Ping 1, Bao-Qi Feng 1, Jun Ge, Guang-Zhen Li, Wei Zhu, Yun-Lei Teng*, Ya-Ru Zhang, Bao-Xia Dong** School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, PR China
highlights
graphical abstract
Hydrogen desorption properties of the KNH2-doped LieNeH system was examined. Hydrogen desorption kinetics are significantly enhanced by introducing KNH2. The dehydrogenation temperature is lowered drastically. The improved properties induce from the cyclic conversion from KNH2 to KH. KNH2
may
be
formed
dehydrogenation
of
during the
potassium-doped M-N-H system.
article info
abstract
Article history:
Ammonia is a vital intermediate in the hydrogen desorption process of Metal-N-H system.
Received 1 August 2019
KH has strong reactivity with NH3 to form KNH2. We speculate that KNH2 is also an in-
Received in revised form
termediate formed during hydrogen desorption of the potassium-doped M-N-H systems. In
3 September 2019
this research, the dehydrogenation performance of the KNH2-doped LiNH2 and LiH
Accepted 12 September 2019
composition was first studied. Compared with the broad dehydrogenation curve of the
Available online xxx
composite material of LiNH2 and LiH without the catalyst, the dehydrogenation curve of 0.05 mol KNH2-doped composite material was significantly narrowed. The initial and peak
Keywords:
dehydrogenation temperature of the composite to which 0.05 mol of KNH2 was added was
Dehydrogenation
lowered remarkably. Besides, the cyclic dehydrogenation properties of the LiNH2 and LiH
Lithium amides
system was also significantly enhanced by the introduction of KNH2. The cyclic conversion
Lithium hydrides
of KNH2 to KH is the main reason for the enhancement of the hydrogen evolution per-
Potassium amide
formance of the LiNH2eLiH system doped with KNH2. We found the KNH2-doped LieNeH system exhibits similar dehydrogenation property with that of the KH-doped LieNeH
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y.-L. Teng), [email protected] (B.-X. Dong). 1 These authors contributed equally. https://doi.org/10.1016/j.ijhydene.2019.09.109 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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system. This work proves that KNH2 plays a key role in improving the hydrogen desorption performances of the potassium-doped M-N-H systems. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction As the problem of pollution becomes more and more serious, it is more urgent to find a new type of energy that is environmentally friendly. Hydrogen has received wide attention from scientists because of its extremely high energy density, non-polluting properties and its wide distribution in the universe [1e4]. Hydrogen energy is also more probable as a new alternative energy source [5e9]. The hydrogen-energy process chain consists of three major steps: hydrogen production, hydrogen storage, and repowering [10e12]. All three steps are subject to ongoing research [13,14]. In particular, the short board of hydrogen storage technology has further restricted the development of onboard hydrogen. It is well known that hydrogen storage systems composed of light element enable higher mass and bulk densities [15,16]. In 2002, after the first study and publication by Chen et al., the hydrogen storage system containing amide has been extensively studied worldwide. They point out that Li3N can reversibly store more than 10% by mass of hydrogen through the following reaction 1 [17]. Li3N þ 2H2 4 Li2NH þ LiH þ H2 4 LiNH2 þ 2LiH
(1)
Next, the LiNH2 and LiH system has also been proven to be a suitable solid state storage material because it can store 6.5% by mass of hydrogen [18,19]. Li2NH þ H2 4 LiNH2 þ LiH
(2)
Although the system shows a set of thermodynamic parameters that are advantageous, its dehydrogenation temperature is still too high. Works in all aspects were done to improve the kinetics of hydrogen desorption [20e30]. A lot of compounds can effectively enhance the hydrogen storage performance of the LiNH2 and LiH system. Certain potassium compounds play an excellent role in improving the dehydrogenation performance of the Mg(NH2)2e2LiH (LieMgeNeH) system, and KH plays a key role [31e35]. The dehydrogenation properties of the LieNeH systems can be significantly enhanced by doping a potassium compound such as KOH, K2TiF6, KLi3(NH2)4, and K2Mn(NH2)4 [36e40]. These literatures show that some potassium compounds can undergo metathesis reaction with LiH in the process of experiments to convert to KH, thereby significantly enhancing the hydrogen dsorption kinetics of the LieNeH system. We all know that ammonia is a vital intermediate in the hydrogen desorption process of Metal-N-H system. In addition, it has been reported that KH has strong reactivity with NH3 to form potassium amide. Santoru et al. found that a
metal amide-hydride solid solution can be formed by combination of KNH2 and KH [41]. Based on these reports, we speculate that potassium amide may also play a key role in improving the hydrogen desorption performances of the potassium-doped M-N-H systems. Therefore, it is necessary to study the role of potassium amide in enhancing the dehydrogenation performance of the LieNeH system and to make clear its reaction mechanism. It was found that KNH2 exhibits significant influence on enhancing the dehydrogenation performance of the LiNH2 and LiH composition. Compared with the broad dehydrogenation curve of the composite material of LiNH2 and LiH without the catalyst, the dehydrogenation curve of 0.05 mol KNH2-doped composite material was significantly narrowed. The initial and peak dehydrogenation temperature of the composite to which 0.05 mol of KNH2 was added was lowered remarkably. Also, we investigated the role of KNH2 in enhancing the hydrogen desorption performance of the LiNH2 and LiH system and elucidated its reaction mechanism. This work proves that potassium amide may play an important role in improving the hydrogen desorption performances of the potassium compounds-doped M-N-H systems, which helps to further understand the dehydrogenation mechanism of the potassium compounds-doped M-N-H systems.
Experimental procedure Sample preparation The experimental base reagents are lithium hydride (LiH) (98%, J&K Chemical Ltd., China) and lithium amide (LiNH2) (95%, Aldrich). The additives are powdered lithium amide, sodium amide (NaNH2) and potassium amide (KNH2). The corresponding alkali metal hydride is reacted with liquid ammonia for several hours to obtain sodium amide and potassium amide. The additive was uniformly dispersed into the LiNH2 and LiH system by a mechanical ball milling method. The powdered LiNH2 and LiH were mixed with 1e10 mol% additives (300 mg in total) in a steel grinding vessel containing 30 steel balls (6 mm in diameter) for ball milling. The internal volume of steel grinder is about 70 cm3. Then, ball milling was carried out for 2 h at 450 rpm in a 0.6 MPa hydrogen (>99.999%) atmosphere using a planetary ball mill (QM-3SP4). The ball-topowder weight ratio was about 90:1. In order to minimize the temperature rise of the sample during the grinding process, a method of resting for 30 min per hour was used. All samples were processed in a glove box (Mikrouna, China) filled with argon gas (>99.999%) and equipped with a circulation purification system with typical H2O/O2 levels below 0.1 ppm.
Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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Structural characterizations The prepared composites were structurally characterized using an X-ray diffractometer (XRD) (AXS D8 ADVANCE, Bruker, Germany). The polyimide sheets were used to cover the samples to prevent the samples from being oxidized during the measurement. To characterize the NeH stretch mode of the amide, a transmission mode using a Fourier Transform Infrared Spectrometer (FTIR) (TENSOR 27, Bruker, Canada) was used. The samples were mixed with potassium bromide (KBr) powder at a mass ratio of 1:20 and then tableted at room temperature. Each spectrum was created with 16 scans with an average scan resolution of 4 cm1.
Thermal desorption and isothermal dehydrogenation measurements In a Ar atmosphere, using a thermal analyzer (DSC/DTA-TG; Netzsch STA 449 F3 Jupiter®) in combination with a thermal desorption mass spectrometer (DSC/DTA-TG; Netzsch STA 449 F3 Jupiter®, TDMS), the samples were heated at a rate of 10 C min1 and the composition of the evolved gas was analyzed. The dehydrogenation kinetics of the composite was measured using a Hy-Energy PCT Evo commercial volumetric hydrogen storage capacity device (Sieverts apparatus), which is able to accurately measure the amount of hydrogen released by the composite. As usual, about 0.150 g of the sample was placed in a stainless steel autoclave and evacuated. The sample is then quickly heated to the temperature which is required. Under vacuum conditions, dehydrogenation was carried out at 250 and 300 C, respectively. Hydrogenation was carried out at an initial pressure of 2 MPa in a hydrogen atmosphere.
3
Then, we prepared three samples: LiNH2eLiH-xKNH2 (x ¼ 0.01, 0.05 and 0.10). Through the detection of TDMS, we can qualitatively analyze the effect of KNH2 with different amount (Fig. 1). We found that as the KNH2 content increased, the dehydrogenation temperature decreased. For the LiNH2 and LiH system to which only 1 mol% of KNH2 was added, the peak temperature was lowered by 8 C relative to that of the sample without an additive. Increasing the KNH2 content from 1 to 5 mol% further causes the dehydrogenation peak temperature to decrease by another 26 C. Interestingly, the release of by-product (ammonia) is significantly inhibited by the addition of KNH2. The small peak around 100 C in the ammonia gas desorption mass spectra may be due to water since the molar masses of ammonia (17) and water (18) are very close. Further increasing the KNH2 amount to 10 mol% causes the dehydrogenation peak temperature to further decrease by 9 C. For the sample doped with 10 mol% KNH2, a significant ammonia release can be observed at high temperatures. Moreover, as the amount of KNH2 added increases, the hydrogen capacity of the LiNH2 and LiH sample also decreases. Thus, considering the H2 capacity and dehydrogenation temperature, the 5 mol% KNH2-doped LiNH2 and LiH system was selected for further investigation. Compared with the halides, graphite, titaniums, oxides, and hydrides, potassium compounds show better effects on improving the dehydrogenation properties of the LieNeH system [20e28,37e39]. Through the experiments mentioned above, we found the KNH2-doped LieNeH system exhibits similar dehydrogenation property with that of the KH-doped LieNeH system [39]. The investigation on the prepared KNH2-doped LieNeH system may help to understand the dehydrogenation mechanism of potassium compounds-doped LieNeH system. Fig. 2 shows dehydrogenation behaviors of the 5 mol% KNH2-doped LiNH2 and LiH systems at 250 or 300 C. The hydrogen desorption rate of the LiNH2 and LiH sample was
Results and discussion The effects of KNH2 on enhancing dehydrogenation performance of the LiNH2 and LiH system It is reported that the reactivity of alkali metal hydride with NH3 to form alkali metal amide (LiNH2, NaNH2 or KNH2) is KH > NaH > LiH [38]. Alkali metal amide may show different influence on the dehydrogenation properties of the LieNeH system. To understand this, four LiNH2 and LiH systems with no additive, LiNH2, NaNH2 or KNH2 additive were fabricated, and their dehydrogenation behavior was detected by TDMS. In Fig. S1, the LiNH2 and LiH composite containing no additives displayed two broad dehydrogenation curves and an obvious NH3 emission profile [20,21]. The initial dehydrogenation temperature of the composite to which the 5 mol% KNH2 additive was added was lowered by about 40 C relative to the LiNH2eLiH composite, and the peak dehydrogenation temperature was reduced by about 30 C. Also, TDMS tests showed that the addition of KNH2 to the composite material also significantly inhibited the desorption of ammonia at high temperature. Thus, among the three amides (LiNH2, NaNH2, or KNH2), KNH2 exhibit a significant influence on enhancing the dehydrogenation performance of the LiNH2eLiH system.
Fig. 1 e The H2 and NH3 desorption mass spectra for the LiNH2 and LiH systems doped with 1e10 mol% KNH2.
Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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Fig. 4 e The activation energy of the 5 mol% KNH2-doped LiNH2 and LiH system estimated by kissinger's plots. Fig. 2 e Dehydrogenation behaviors of the 5 mol% KNH2doped LiNH2 and LiH systems at 250 (a) or 300 C (b).
significantly enhanced through doping KNH2. At 250 C, the 5 mol% KNH2-doped LiNH2 and LiH sample emitted 2.6 wt% hydrogen in 0.5 h, but the LiNH2eLiH sample desorbed 1.3 wt% hydrogen. When the reaction temperature was changed to 300 C, the LiNH2eLiH-0.05KNH2 sample rapidly released
Fig. 3 e The cyclic dehydrogenation behaviors of the 5 mol % KNH2-doped LiNH2 and LiH system at 300 C.
approximately 4.0 wt% hydrogen within 36 min (the average hydrogen desorption rate reaches 0.11 wt%/min), while the pristine LiNH2 and LiH sample spent 3 h releasing the same amount of hydrogen. In Fig. 3, we show the cyclic dehydrogenation behaviors of the 5 mol% KNH2-doped LiNH2 and LiH system at 300 C. The pristine LiNH2 and LiH system often shows poor hydrogen desorption properties after the second cycle. As mentioned above, many compounds, such as halides, oxides, carbon, hydride, have obvious effects on improving the dehydrogenation kinetics of the LieNeH system, but these compounds have no obvious effects on enhancing the cyclic durability of the LieNeH system [22e27]. Recent literatures reported that potassium compounds not only show obvious effects on improving the dehydrogenation kinetics of the M-N-H system, but also show obvious effects on enhancing the cyclic durability of the M-N-H system [37e39]. As expected, the KNH2doped LiNH2eLiH system shows remarkably enhanced cyclic durability. For the 10th circle, the onset dehydrogenation temperature is increased, but the slope of the dehydrogenation curve and the dehydrogenation amount change little. The dehydrogenation performance of the KNH2-doped LiNH2eLiH system is relatively stable over ten cycles (Fig. 3). The doping of 5 mol% KNH2 significantly enhanced the reversible properties of the LiNH2 and LiH system.
Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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5
Fig. 5 e The XRD profiles for the ball-milled LiNH2 and LiH systems doped with 1e10 mol% KNH2.
The hydrogen desorption mechanism of the KNH2-doped LiNH2 and LiH composition Generally, the activation energy (Ea) can be estimated by the Kissinger formula [42]. Fig. 4 exhibits the Kissinger plot of the sample with KNH2 added. According to previous research data, Ea of the pure LiNH2 and LiH sample is about 110 kJ/mol [24,43]. For the LiNH2eLiH sample to which 5 mol% of KNH2 was added, the Ea value was calculated to be about 78.9 kJ/mol. Compared to the pristine sample, the activation energy of the sample to which 5 mol% of KNH2 was added was reduced by about 28%. These data indicate that KNH2 has a significant impact on reducing energy disorders. This result may be due to the fact that KNH2 changes the energy barrier of the LiNH2 and LiH system during hydrogen desorption. Compared with various compound-doped LieNeH system, the KNH2-doped LieNeH system shows lower activation energy (Table S1). Furthermore, from the integration of the DSC peak, the heat effect of dehydrogenation of the KNH2-doped LieNeH system was estimated to be 47.7 kJ/mol H2. Fig. 5 displays the XRD patterns of the LiNH2 and LiH samples with different amounts of KNH2 before dehydrogenation. In the XRD curve of the sample to which KNH2 was added, we observed the corresponding peaks of potassium hydride but did not find the diffraction peaks of the KNH2 additive. It is noteworthy that the intensity of potassium hydride phase becomes strong with the increase of KNH2 content, indicating that potassium hydride may play a key role in enhancing the dehydrogenation properties of the LiNH2 and LiH system doped with KNH2. As shown in Fig. 6, as the temperature increases from 100 C to 500 C, the intensity of
Fig. 6 e The FTIR and XRD curves for the 5 mol% KNH2doped LiNH2 and LiH system after hydrogen desorption at different temperatures.
potassium hydride phase hardly changes, also indicating that potassium hydride plays a catalytic role during dehydrogenation. The broad diffraction peak between 10 and 20 is because of the polyimide film covered on the samples. Ammonia is considered to be a vital intermediate for the elemental reaction of the LiNH2 and LiH system, as shown in reaction 3 [44,45]. Previous reports indicated the particle size
Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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Fig. 7 e The reaction mechanism of improved hydrogen desorption performance for the 5 mol% KNH2-doped LiNH2 and LiH system.
potassium hydride rapidly interacts with ammonia, producing potassium amide and releasing hydrogen, and the formed potassium amide interacts with lithium hydride to form potassium hydride and lithium amide by a solid-solid reaction during heat treatment, thereby recovering potassium hydride. As shown in Fig. 7, the two conversion reactions between potassium amide and potassium hydride are considered as cyclic interactions that will proceed until lithium hydride is completely consumed. Therefore, the cyclic conversion from potassium amide to potassium hydride, which has good reactivity with ammonia, can enhance the dehydrogenation performance of the KNH2-doped LiNH2 and LiH system. Based on the previous works [38,39] and the experiment in this study, we propose the cyclic conversion between KH and KNH2 will happen if KH or KNH2 exists in the LieNeH system.
Conclusions of LiH, the pressure of ammonia, and catalyst have obvious influence on the kinetics of the reaction of LiH with ammonia [38,46]. In addition, experiments have shown that potassium hydride shows better performance than that of lithium hydride when they react with ammonia [38]. Therefore, when potassium hydride and lithium hydride simultaneously present in ammonia, potassium hydride preferentially reacts with NH3 by the equation (4). LiH þ NH3 4 LiNH2 þ H2
(3)
KH þ LiH þ NH3 / KNH2 þ LiH þ H2
(4)
Reaction (5) represents a reaction in which potassium amide and lithium hydride react to produce lithium amide and potassium hydride. Its enthalpy change DH was calculated to be 17.8 kJ/mol, based on the standard enthalpy change DH0 of the compound is 128.9 kJ/mol (KNH2), 90.5 kJ/ mol (LiH), 57.7 kJ/mol (KH) and 179.5 kJ/mol (LiNH2) [47]. KNH2 þ LiH / KH þ LiNH2
(5)
The dehydrogenation performance of the KNH2-doped LiNH2 and LiH composition was first studied. Compared with the broad dehydrogenation curve of the composite material of LiNH2 and LiH without the catalyst, the dehydrogenation curve of 0.05 mol KNH2-doped composite material was significantly narrowed. The initial and peak dehydrogenation temperature of the composite to which 0.05 mol of KNH2 was added was lowered remarkably. Also, the cyclic dehydrogenation properties of the LiNH2 and LiH system was also significantly enhanced by the introduction of KNH2. The cyclic conversion of KNH2 to KH is the main reason for the enhancement of the hydrogen evolution performance of the LiNH2 and LiH system doped with KNH2. This work suggests that KNH2 is also an intermediate, which plays a key role in improving the hydrogen desorption performances of the potassium-doped M-N-H systems.
Acknowledgments
The value of the DH is negative, indicating the reaction will be easily carried out under an exothermic environment. Previous studies have shown that reaction (5) can be proceeded by ball milling or heat treatment [38,48]. We proposed the dehydrogenation mechanism of the KNH2-doped LiNH2eLiH system, as shown in the following reaction equations.
This study is financially supported by the NNSF of China (Nos. 21573192), the Foundation from the Priority Academic Pro- Q2 gram Development of Jiangsu Higher Education Institutions, and the High-Level Personnel Support Program of Yang-Zhou University.
KNH2 þ LiH / KH þ LiNH2
Appendix A. Supplementary data
2LiNH2/ Li2NH þ NH3
(6)
KH þ NH3 / KNH2 þ H2
(7)
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.109.
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[21] Li Y, Ding X, Wu F, Sun D, Zhang Q, Fang F. Enhancement of hydrogen storage in destabilized LiNH2 with KMgH3 by quick conveyance of N-containing species. J Phys Chem C 2016;120:1415e20. [22] Zhang T, Isobe S, Wang Y, Hashimoto N, Ohnuki S. A homogeneous metal oxide catalyst enhanced solidesolid reaction in the hydrogen desorption of a lithiumehydrogenenitrogen system. ChemCatChem 2014;6:724e7. [23] Nayebossadri S, Aguey-Zinsou KF, Guo ZX. Effect of nitride additives on LiNH hydrogen storage system. Int J Hydrogen Energy 2011;36:7920e6. [24] Varin RA, Jang M. The effects of graphite on the reversible hydrogen storage of nanostructured lithium amide and lithium hydride (LiNH2þ1.2LiH) system. J Alloy Comp 2011;509:7143e51. [25] David WIF, Jones MO, Gregory DH, Jewell CM, Johnson SR, Wallton A, et al. A mechanism for non-stoichiometry in the lithium amide/lithium imide hydrogen storage reaction. J Am Chem Soc 2007;129:1594e601. [26] Leng HY, Wu Z, Duan WY, Xia GL, Li ZL. Effect of MgCl2 addictives on the H-desorption properties of LiNH system. Int J Hydrogen Energy 2012;37:903e7. [27] Amica G, Larochette PA, Gennari FC. Gennari. Hydrogen storage properties of LiNH2 and LiH system with MgH2, CaH2 and TiH2 added. Int J Hydrogen Energy 2015;40:9335e46. [28] Nayebossadri S. Kinetic rate-limiting steps in dehydrogenation of LiNH and LiMgNH systemseffects of elemental Si and Al. Int J Hydrogen Energy 2011;36:8335e43. [29] Yang JZ, Li DC, Fu H, Xin GB, Zheng J, Li XG. In situ hybridization of LiNH2eLiHeMg(BH4)2 nano-composites intermediate and optimized hydrogenation properties. Phys Chem Chem Phys 2012;14:2857e63. [30] Zhang TF, Isobe Y, Wang YM, Oka H, Hashimoto N, Ohnuki S. A metaleoxide catalyst enhanced the desorption properties in complex metal hydrides. J Mater Chem 2014;2:4361e5. [31] Wang JH, Liu T, Wu GT, Li W, Liu YF, Arau´jo CM, et al. Potassium-modified Mg(NH2)2/2LiH system for hydrogen storage. Angew Chem Int Ed 2009;48:5828e32. [32] Luo WF, Stavila V, Klebanoff LE. New insights into the mechanism of activation and hydrogen absorption of (2LiNH2MgH2). Int J Hydrogen Energy 2012;37:6646e52. [33] Liu Y, Yang Y, Zhang X, Li Y, Gao M, Pan H. Insights into the dehydrogenation reaction process of a K-containing Mg (NH2)2e2LiH system. Dalton Trans 2015;44:18012e8. [34] Liang C, Liu YF, Gao MX, Pan HG. Understanding the role of K in the significantly improved hydrogen storage properties of a KOH-doped LiMgNH system. J Mater Chem 2013;1:5031e6. [35] Garroni S, Santoru A, Cao HJ, Dornheim M, Klassen T, Milanese C, et al. Recent progress and new perspectives on metal amide and imide systems for solid-state hydrogen storage. Energies 2018;11:1e28. [36] Zhang YR, Dong BX, Zhao J, Teng YL, Li ZW, Wang L. Synergetic effects of K, Ti and F on the hydrogen storage properties of the Li-N-H system. Int J Hydrogen Energy 2017;42:17149e56. [37] Dong BX, Gao JJ, Teng YL, Tian H, Wang LZ. A novel hydrogen storage system of KLi3(NH2)4-4LiH with superior cycling stability. Int J Hydrogen Energy 2016;41:5371e7. [38] Teng YL, Ichikawa T, Miyaoka H, Kojima Y. Improvement of hydrogen desorption kinetics in the LiHNH3 system by addition of KH. Chem Commun 2011;47:12227e9. [39] Dong BX, Wang L, Ge J, Ping C, TengYL, Li ZW. The effect of KH on enhancing the dehydrogenation properties of the LieNeH system and its catalytic mechanism. Phys Chem Chem Phys 2018;20:11116e22.
Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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[40] Gizer G, Cao HJ, Puszkiel J, Pistidda C, Santoru A, Zhang WJ, et al. Enhancement effect of bimetallic amide K2Mn(NH2)4 and in-situ formed KH and Mn4N on the dehydrogenation/ hydrogenation properties of LieMgeNeH system. Energies 2019;12:2779. [41] Santoru A, Pistidda C, Sørby MH, Chierotti MR, GarroniS Pinatel E, et al. KNH2eKH: a metal amideehydride solid solution. Chem Commun 2016;52:11760e3. [42] Kissinger HE. Reaction kinectics in differential thermal analysis. Anal Chem 1957;29:1702e6. [43] Albanesi LF, Larochette PA, Gennari FC. Destabilization of the LiNH2eLiH hydrogen storage system by aluminum incorporation. Int J Hydrogen Energy 2013;38:12325e34. [44] Ichikawa T, Hanada N, Isobe S, Leng H, Fujii H. Mechanism of novel reaction from LiNH2 and LiH to Li2NH and H2 as a
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Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system Chao Ping 1, Bao-Qi Feng 1, Jun Ge, Guang-Zhen Li, Wei Zhu, Yun-Lei Teng*, Ya-Ru Zhang, Bao-Xia Dong** School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, PR China
highlights
graphical abstract
Hydrogen desorption properties of the KNH2-doped LieNeH system was examined. Hydrogen desorption kinetics are significantly enhanced by introducing KNH2. The dehydrogenation temperature is lowered drastically. The improved properties induce from the cyclic conversion from KNH2 to KH. KNH2
may
be
formed
dehydrogenation
of
during the
potassium-doped M-N-H system.
article info
abstract
Article history:
Ammonia is a vital intermediate in the hydrogen desorption process of Metal-N-H system.
Received 1 August 2019
KH has strong reactivity with NH3 to form KNH2. We speculate that KNH2 is also an in-
Received in revised form
termediate formed during hydrogen desorption of the potassium-doped M-N-H systems. In
3 September 2019
this research, the dehydrogenation performance of the KNH2-doped LiNH2 and LiH
Accepted 12 September 2019
composition was first studied. Compared with the broad dehydrogenation curve of the
Available online xxx
composite material of LiNH2 and LiH without the catalyst, the dehydrogenation curve of 0.05 mol KNH2-doped composite material was significantly narrowed. The initial and peak
Keywords:
dehydrogenation temperature of the composite to which 0.05 mol of KNH2 was added was
Dehydrogenation
lowered remarkably. Besides, the cyclic dehydrogenation properties of the LiNH2 and LiH
Lithium amides
system was also significantly enhanced by the introduction of KNH2. The cyclic conversion
Lithium hydrides
of KNH2 to KH is the main reason for the enhancement of the hydrogen evolution per-
Potassium amide
formance of the LiNH2eLiH system doped with KNH2. We found the KNH2-doped LieNeH system exhibits similar dehydrogenation property with that of the KH-doped LieNeH
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y.-L. Teng), [email protected] (B.-X. Dong). 1 These authors contributed equally. https://doi.org/10.1016/j.ijhydene.2019.09.109 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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system. This work proves that KNH2 plays a key role in improving the hydrogen desorption performances of the potassium-doped M-N-H systems. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction As the problem of pollution becomes more and more serious, it is more urgent to find a new type of energy that is environmentally friendly. Hydrogen has received wide attention from scientists because of its extremely high energy density, non-polluting properties and its wide distribution in the universe [1e4]. Hydrogen energy is also more probable as a new alternative energy source [5e9]. The hydrogen-energy process chain consists of three major steps: hydrogen production, hydrogen storage, and repowering [10e12]. All three steps are subject to ongoing research [13,14]. In particular, the short board of hydrogen storage technology has further restricted the development of onboard hydrogen. It is well known that hydrogen storage systems composed of light element enable higher mass and bulk densities [15,16]. In 2002, after the first study and publication by Chen et al., the hydrogen storage system containing amide has been extensively studied worldwide. They point out that Li3N can reversibly store more than 10% by mass of hydrogen through the following reaction 1 [17]. Li3N þ 2H2 4 Li2NH þ LiH þ H2 4 LiNH2 þ 2LiH
(1)
Next, the LiNH2 and LiH system has also been proven to be a suitable solid state storage material because it can store 6.5% by mass of hydrogen [18,19]. Li2NH þ H2 4 LiNH2 þ LiH
(2)
Although the system shows a set of thermodynamic parameters that are advantageous, its dehydrogenation temperature is still too high. Works in all aspects were done to improve the kinetics of hydrogen desorption [20e30]. A lot of compounds can effectively enhance the hydrogen storage performance of the LiNH2 and LiH system. Certain potassium compounds play an excellent role in improving the dehydrogenation performance of the Mg(NH2)2e2LiH (LieMgeNeH) system, and KH plays a key role [31e35]. The dehydrogenation properties of the LieNeH systems can be significantly enhanced by doping a potassium compound such as KOH, K2TiF6, KLi3(NH2)4, and K2Mn(NH2)4 [36e40]. These literatures show that some potassium compounds can undergo metathesis reaction with LiH in the process of experiments to convert to KH, thereby significantly enhancing the hydrogen dsorption kinetics of the LieNeH system. We all know that ammonia is a vital intermediate in the hydrogen desorption process of Metal-N-H system. In addition, it has been reported that KH has strong reactivity with NH3 to form potassium amide. Santoru et al. found that a
metal amide-hydride solid solution can be formed by combination of KNH2 and KH [41]. Based on these reports, we speculate that potassium amide may also play a key role in improving the hydrogen desorption performances of the potassium-doped M-N-H systems. Therefore, it is necessary to study the role of potassium amide in enhancing the dehydrogenation performance of the LieNeH system and to make clear its reaction mechanism. It was found that KNH2 exhibits significant influence on enhancing the dehydrogenation performance of the LiNH2 and LiH composition. Compared with the broad dehydrogenation curve of the composite material of LiNH2 and LiH without the catalyst, the dehydrogenation curve of 0.05 mol KNH2-doped composite material was significantly narrowed. The initial and peak dehydrogenation temperature of the composite to which 0.05 mol of KNH2 was added was lowered remarkably. Also, we investigated the role of KNH2 in enhancing the hydrogen desorption performance of the LiNH2 and LiH system and elucidated its reaction mechanism. This work proves that potassium amide may play an important role in improving the hydrogen desorption performances of the potassium compounds-doped M-N-H systems, which helps to further understand the dehydrogenation mechanism of the potassium compounds-doped M-N-H systems.
Experimental procedure Sample preparation The experimental base reagents are lithium hydride (LiH) (98%, J&K Chemical Ltd., China) and lithium amide (LiNH2) (95%, Aldrich). The additives are powdered lithium amide, sodium amide (NaNH2) and potassium amide (KNH2). The corresponding alkali metal hydride is reacted with liquid ammonia for several hours to obtain sodium amide and potassium amide. The additive was uniformly dispersed into the LiNH2 and LiH system by a mechanical ball milling method. The powdered LiNH2 and LiH were mixed with 1e10 mol% additives (300 mg in total) in a steel grinding vessel containing 30 steel balls (6 mm in diameter) for ball milling. The internal volume of steel grinder is about 70 cm3. Then, ball milling was carried out for 2 h at 450 rpm in a 0.6 MPa hydrogen (>99.999%) atmosphere using a planetary ball mill (QM-3SP4). The ball-topowder weight ratio was about 90:1. In order to minimize the temperature rise of the sample during the grinding process, a method of resting for 30 min per hour was used. All samples were processed in a glove box (Mikrouna, China) filled with argon gas (>99.999%) and equipped with a circulation purification system with typical H2O/O2 levels below 0.1 ppm.
Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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Structural characterizations The prepared composites were structurally characterized using an X-ray diffractometer (XRD) (AXS D8 ADVANCE, Bruker, Germany). The polyimide sheets were used to cover the samples to prevent the samples from being oxidized during the measurement. To characterize the NeH stretch mode of the amide, a transmission mode using a Fourier Transform Infrared Spectrometer (FTIR) (TENSOR 27, Bruker, Canada) was used. The samples were mixed with potassium bromide (KBr) powder at a mass ratio of 1:20 and then tableted at room temperature. Each spectrum was created with 16 scans with an average scan resolution of 4 cm1.
Thermal desorption and isothermal dehydrogenation measurements In a Ar atmosphere, using a thermal analyzer (DSC/DTA-TG; Netzsch STA 449 F3 Jupiter®) in combination with a thermal desorption mass spectrometer (DSC/DTA-TG; Netzsch STA 449 F3 Jupiter®, TDMS), the samples were heated at a rate of 10 C min1 and the composition of the evolved gas was analyzed. The dehydrogenation kinetics of the composite was measured using a Hy-Energy PCT Evo commercial volumetric hydrogen storage capacity device (Sieverts apparatus), which is able to accurately measure the amount of hydrogen released by the composite. As usual, about 0.150 g of the sample was placed in a stainless steel autoclave and evacuated. The sample is then quickly heated to the temperature which is required. Under vacuum conditions, dehydrogenation was carried out at 250 and 300 C, respectively. Hydrogenation was carried out at an initial pressure of 2 MPa in a hydrogen atmosphere.
3
Then, we prepared three samples: LiNH2eLiH-xKNH2 (x ¼ 0.01, 0.05 and 0.10). Through the detection of TDMS, we can qualitatively analyze the effect of KNH2 with different amount (Fig. 1). We found that as the KNH2 content increased, the dehydrogenation temperature decreased. For the LiNH2 and LiH system to which only 1 mol% of KNH2 was added, the peak temperature was lowered by 8 C relative to that of the sample without an additive. Increasing the KNH2 content from 1 to 5 mol% further causes the dehydrogenation peak temperature to decrease by another 26 C. Interestingly, the release of by-product (ammonia) is significantly inhibited by the addition of KNH2. The small peak around 100 C in the ammonia gas desorption mass spectra may be due to water since the molar masses of ammonia (17) and water (18) are very close. Further increasing the KNH2 amount to 10 mol% causes the dehydrogenation peak temperature to further decrease by 9 C. For the sample doped with 10 mol% KNH2, a significant ammonia release can be observed at high temperatures. Moreover, as the amount of KNH2 added increases, the hydrogen capacity of the LiNH2 and LiH sample also decreases. Thus, considering the H2 capacity and dehydrogenation temperature, the 5 mol% KNH2-doped LiNH2 and LiH system was selected for further investigation. Compared with the halides, graphite, titaniums, oxides, and hydrides, potassium compounds show better effects on improving the dehydrogenation properties of the LieNeH system [20e28,37e39]. Through the experiments mentioned above, we found the KNH2-doped LieNeH system exhibits similar dehydrogenation property with that of the KH-doped LieNeH system [39]. The investigation on the prepared KNH2-doped LieNeH system may help to understand the dehydrogenation mechanism of potassium compounds-doped LieNeH system. Fig. 2 shows dehydrogenation behaviors of the 5 mol% KNH2-doped LiNH2 and LiH systems at 250 or 300 C. The hydrogen desorption rate of the LiNH2 and LiH sample was
Results and discussion The effects of KNH2 on enhancing dehydrogenation performance of the LiNH2 and LiH system It is reported that the reactivity of alkali metal hydride with NH3 to form alkali metal amide (LiNH2, NaNH2 or KNH2) is KH > NaH > LiH [38]. Alkali metal amide may show different influence on the dehydrogenation properties of the LieNeH system. To understand this, four LiNH2 and LiH systems with no additive, LiNH2, NaNH2 or KNH2 additive were fabricated, and their dehydrogenation behavior was detected by TDMS. In Fig. S1, the LiNH2 and LiH composite containing no additives displayed two broad dehydrogenation curves and an obvious NH3 emission profile [20,21]. The initial dehydrogenation temperature of the composite to which the 5 mol% KNH2 additive was added was lowered by about 40 C relative to the LiNH2eLiH composite, and the peak dehydrogenation temperature was reduced by about 30 C. Also, TDMS tests showed that the addition of KNH2 to the composite material also significantly inhibited the desorption of ammonia at high temperature. Thus, among the three amides (LiNH2, NaNH2, or KNH2), KNH2 exhibit a significant influence on enhancing the dehydrogenation performance of the LiNH2eLiH system.
Fig. 1 e The H2 and NH3 desorption mass spectra for the LiNH2 and LiH systems doped with 1e10 mol% KNH2.
Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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Fig. 4 e The activation energy of the 5 mol% KNH2-doped LiNH2 and LiH system estimated by kissinger's plots. Fig. 2 e Dehydrogenation behaviors of the 5 mol% KNH2doped LiNH2 and LiH systems at 250 (a) or 300 C (b).
significantly enhanced through doping KNH2. At 250 C, the 5 mol% KNH2-doped LiNH2 and LiH sample emitted 2.6 wt% hydrogen in 0.5 h, but the LiNH2eLiH sample desorbed 1.3 wt% hydrogen. When the reaction temperature was changed to 300 C, the LiNH2eLiH-0.05KNH2 sample rapidly released
Fig. 3 e The cyclic dehydrogenation behaviors of the 5 mol % KNH2-doped LiNH2 and LiH system at 300 C.
approximately 4.0 wt% hydrogen within 36 min (the average hydrogen desorption rate reaches 0.11 wt%/min), while the pristine LiNH2 and LiH sample spent 3 h releasing the same amount of hydrogen. In Fig. 3, we show the cyclic dehydrogenation behaviors of the 5 mol% KNH2-doped LiNH2 and LiH system at 300 C. The pristine LiNH2 and LiH system often shows poor hydrogen desorption properties after the second cycle. As mentioned above, many compounds, such as halides, oxides, carbon, hydride, have obvious effects on improving the dehydrogenation kinetics of the LieNeH system, but these compounds have no obvious effects on enhancing the cyclic durability of the LieNeH system [22e27]. Recent literatures reported that potassium compounds not only show obvious effects on improving the dehydrogenation kinetics of the M-N-H system, but also show obvious effects on enhancing the cyclic durability of the M-N-H system [37e39]. As expected, the KNH2doped LiNH2eLiH system shows remarkably enhanced cyclic durability. For the 10th circle, the onset dehydrogenation temperature is increased, but the slope of the dehydrogenation curve and the dehydrogenation amount change little. The dehydrogenation performance of the KNH2-doped LiNH2eLiH system is relatively stable over ten cycles (Fig. 3). The doping of 5 mol% KNH2 significantly enhanced the reversible properties of the LiNH2 and LiH system.
Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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5
Fig. 5 e The XRD profiles for the ball-milled LiNH2 and LiH systems doped with 1e10 mol% KNH2.
The hydrogen desorption mechanism of the KNH2-doped LiNH2 and LiH composition Generally, the activation energy (Ea) can be estimated by the Kissinger formula [42]. Fig. 4 exhibits the Kissinger plot of the sample with KNH2 added. According to previous research data, Ea of the pure LiNH2 and LiH sample is about 110 kJ/mol [24,43]. For the LiNH2eLiH sample to which 5 mol% of KNH2 was added, the Ea value was calculated to be about 78.9 kJ/mol. Compared to the pristine sample, the activation energy of the sample to which 5 mol% of KNH2 was added was reduced by about 28%. These data indicate that KNH2 has a significant impact on reducing energy disorders. This result may be due to the fact that KNH2 changes the energy barrier of the LiNH2 and LiH system during hydrogen desorption. Compared with various compound-doped LieNeH system, the KNH2-doped LieNeH system shows lower activation energy (Table S1). Furthermore, from the integration of the DSC peak, the heat effect of dehydrogenation of the KNH2-doped LieNeH system was estimated to be 47.7 kJ/mol H2. Fig. 5 displays the XRD patterns of the LiNH2 and LiH samples with different amounts of KNH2 before dehydrogenation. In the XRD curve of the sample to which KNH2 was added, we observed the corresponding peaks of potassium hydride but did not find the diffraction peaks of the KNH2 additive. It is noteworthy that the intensity of potassium hydride phase becomes strong with the increase of KNH2 content, indicating that potassium hydride may play a key role in enhancing the dehydrogenation properties of the LiNH2 and LiH system doped with KNH2. As shown in Fig. 6, as the temperature increases from 100 C to 500 C, the intensity of
Fig. 6 e The FTIR and XRD curves for the 5 mol% KNH2doped LiNH2 and LiH system after hydrogen desorption at different temperatures.
potassium hydride phase hardly changes, also indicating that potassium hydride plays a catalytic role during dehydrogenation. The broad diffraction peak between 10 and 20 is because of the polyimide film covered on the samples. Ammonia is considered to be a vital intermediate for the elemental reaction of the LiNH2 and LiH system, as shown in reaction 3 [44,45]. Previous reports indicated the particle size
Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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Fig. 7 e The reaction mechanism of improved hydrogen desorption performance for the 5 mol% KNH2-doped LiNH2 and LiH system.
potassium hydride rapidly interacts with ammonia, producing potassium amide and releasing hydrogen, and the formed potassium amide interacts with lithium hydride to form potassium hydride and lithium amide by a solid-solid reaction during heat treatment, thereby recovering potassium hydride. As shown in Fig. 7, the two conversion reactions between potassium amide and potassium hydride are considered as cyclic interactions that will proceed until lithium hydride is completely consumed. Therefore, the cyclic conversion from potassium amide to potassium hydride, which has good reactivity with ammonia, can enhance the dehydrogenation performance of the KNH2-doped LiNH2 and LiH system. Based on the previous works [38,39] and the experiment in this study, we propose the cyclic conversion between KH and KNH2 will happen if KH or KNH2 exists in the LieNeH system.
Conclusions of LiH, the pressure of ammonia, and catalyst have obvious influence on the kinetics of the reaction of LiH with ammonia [38,46]. In addition, experiments have shown that potassium hydride shows better performance than that of lithium hydride when they react with ammonia [38]. Therefore, when potassium hydride and lithium hydride simultaneously present in ammonia, potassium hydride preferentially reacts with NH3 by the equation (4). LiH þ NH3 4 LiNH2 þ H2
(3)
KH þ LiH þ NH3 / KNH2 þ LiH þ H2
(4)
Reaction (5) represents a reaction in which potassium amide and lithium hydride react to produce lithium amide and potassium hydride. Its enthalpy change DH was calculated to be 17.8 kJ/mol, based on the standard enthalpy change DH0 of the compound is 128.9 kJ/mol (KNH2), 90.5 kJ/ mol (LiH), 57.7 kJ/mol (KH) and 179.5 kJ/mol (LiNH2) [47]. KNH2 þ LiH / KH þ LiNH2
(5)
The dehydrogenation performance of the KNH2-doped LiNH2 and LiH composition was first studied. Compared with the broad dehydrogenation curve of the composite material of LiNH2 and LiH without the catalyst, the dehydrogenation curve of 0.05 mol KNH2-doped composite material was significantly narrowed. The initial and peak dehydrogenation temperature of the composite to which 0.05 mol of KNH2 was added was lowered remarkably. Also, the cyclic dehydrogenation properties of the LiNH2 and LiH system was also significantly enhanced by the introduction of KNH2. The cyclic conversion of KNH2 to KH is the main reason for the enhancement of the hydrogen evolution performance of the LiNH2 and LiH system doped with KNH2. This work suggests that KNH2 is also an intermediate, which plays a key role in improving the hydrogen desorption performances of the potassium-doped M-N-H systems.
Acknowledgments
The value of the DH is negative, indicating the reaction will be easily carried out under an exothermic environment. Previous studies have shown that reaction (5) can be proceeded by ball milling or heat treatment [38,48]. We proposed the dehydrogenation mechanism of the KNH2-doped LiNH2eLiH system, as shown in the following reaction equations.
This study is financially supported by the NNSF of China (Nos. 21573192), the Foundation from the Priority Academic Pro- Q2 gram Development of Jiangsu Higher Education Institutions, and the High-Level Personnel Support Program of Yang-Zhou University.
KNH2 þ LiH / KH þ LiNH2
Appendix A. Supplementary data
2LiNH2/ Li2NH þ NH3
(6)
KH þ NH3 / KNH2 þ H2
(7)
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.109.
references KNH2 þ LiH / KH þ LiNH2 In the KNH2-doped LiNH2eLiH system, potassium amide is transformed into potassium hydride during ball milling. Heat-treatment results in the decomposition of lithium amide, forming lithium imide and ammonia. Next,
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Please cite this article as: Ping C et al., Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.109
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