Enhancement of Hydrogen Storage in Destabilized LiNH2 with

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Enhancement of Hydrogen Storage in Destabilized LiNH with KMgH by Quick Conveyance of N-containing Species 3

Yongtao Li, Xiaoli Ding, Feilong Wu, Dalin Sun, Qingan Zhang, and Fang Fang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11388 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 15, 2016

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The Journal of Physical Chemistry

Enhancement of Hydrogen Storage in Destabilized LiNH2 with KMgH3 by Quick Conveyance of N-containing Species Yongtao Li,a,b Xiaoli Ding,a Feilong Wu,b Dalin Sun,b Qingan Zhanga and Fang Fang∗,b a

School of Materials Science and Engineering, Anhui University of Technology, Maanshan, 243002,

China. b

Department of Materials Science, Fudan University, Shanghai, 200433, China.

Abstract: The advantage of potassium–magnesium bimetallic hydride as novel destabilization agent for LiNH2 was demonstrated by a comparative study among MgH2–, NaMgH3– and KMgH3– combined systems. The LiNH2 combined with KMgH3 not only results in a substantial decreased desorption temperature by >60 ºC and suppression of NH3 release, but also enhances sorption kinetics 2–5 times in comparison to the single MgH2 or NaMgH3 does. A high reversible capacity of more than 65% is also achieved even recharging under milder conditions. Through detailed structural analysis on the desorption products at various stages of these combined systems, we proposed a “self-concerted” reaction mechanism that the in situ formed active MgH2 and KH by KMgH3 involve in LiNH2 decompositon. In addition, the remarkable promotion in conveyance of N–containing species by highly actived KH than NaH is believed to be responsible for the superior destabilizing effect of KMgH3 over NaMgH3 in improving hydrogen sorption kinetics of LiNH2. Keywords: Hydrogen storage; Amides; Reactive hydrides composite; Bimetallic hydrides; Potassium;

∗

Corresponding author. Tel. & Fax: +86-21-6564 2873; E-mail: [email protected]

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1. Introduction Complex hydrides, particularly lithium amide (LiNH2), have attracted more attentions as one of promising hydrogen storage candidates due to its high capacity of ~8.8 wt.%.1‒5 However, on one hand, the decomposition temperature of amides to release hydrogen is above 300 ºC, which is still too high for practical application.6 On the other hand, the amides decomposition usually involves an intermediate by-product ammonia (NH3) emission. This NH3 emission is extremely detrimental when amides are used as hydrogen sources for fuel cell by reason that the NH3 by-product not only decreases the yield of H2 but also poisons nafion membrane and noble metal catalysts for fuel cell.7‒10 Thus, substantial effort has been devoted to altering thermodynamics and suppressing undesirable NH3.11‒14 One effective approach to improve the decomposition properties of amides is forming LiNH2–MH (M = metal) reactive hydrides composite systems (RHCs) based on the coulombic interaction between Hδ- in MH and Hδ+ in amides.15,16 Many LiNH2–hydrides RHCs have been successfully developed.17‒19 For instance, combining LiH with LiNH2 to form LiNH2–LiH system resulted in a decrease of the desorption temperature to ~230 °C, as well as partially inhibition of NH3 emission in compared to pure LiNH2.5 In a subsequent investigation, Lu et al.20 reported that the desorption temperature can be further reduced to around 160 °C and only a trace of NH3 emission was observed in LiNH2–NaH system, but unfortunately this composite system was hardly reloaded under milder conditions. Excitingly, Chen and Luo et al.21,22 further found that LiNH2–MgH2 system started to desorb hydrogen at 150 ºC and showed better reversibility, but this combined system released amounts of ammonia (NH3) gas. Mechanistic studies demonstrate that introducing alkali metal hydride into LiNH2 can suppress NH3 emission and alter kinetics due to their strong affinity with N–H units, while the added alkaline-earth metal hydride would favour reversibility due to the formation of a thermodynamically favourable desorbed product with a vacancy-containing structure.12, 23‒25 Following these observations, our previous work showed that a facile desorption starting at 45 °C without detectable NH3 emission 2 ACS Paragon Plus Environment

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was achieved in LiNH2–NaMgH3 system, which was further believed to be ascribed to two reasons as following: (i) the highly reactive between NH3 and in situ formed NaH by NaMgH3 decomposition; and (ii) rapid conversion of N–H units by NaH along with MgH2.26 This clearly suggested that enhancement of N–H units conversion by alkali metal hydride is crucial for obtaining superior properties. Recently, Kojima et al.27‒30 reported that the reactive kinetics of KH with N–H units (such as liquid NH3) is three times faster than NaH. Therefore, it is reasonable to speculate that jointly in situ introducing highly active KH and MgH2 via KMgH3 would further improve the hydrogen storage properties of LiNH2–MH systems. In this study, our results are presented to reveal the advantages of bimetallic hydrides as a novel destabilization agent for LiNH2 by a comparative investigation among MgH2–, NaMgH3– and KMgH3–added systems. These reactive hydrides systems involved in situ forming highly active MgH2 and MH from MMgH3 in LiNH2 decomposition through a concerted reaction pathway. Their de-/rehydriding behaviors showed that as compared to MgH2 and NaMgH3, KMgH3 exhibits a better destabilizing effect for LiNH2 including inhibiting NH3 emission, lowering thermodynamic thresholds as well as concomitantly speeding up kinetic and reversible events.

2. Experimental section Bimetallic hydrides MMgH3 (M = Na, K) precursors were synthesized by a solid-phase reaction method using NaH or KH (95% purity, Sigma-Aldrich) and MgH2 (98% purity, Sigma-Aldrich) as starting materials. The preparation procedure of MMgH3 involves mechanical milling of the MH and MgH2 mixture in a 1:1 molar ratio at ambient temperature under an argon atmosphere for 10 h, followed by isothermal treatment at 350 °C under 5 MPa hydrogen pressure for 24 h. And then, the LiNH2–MMgH3 composites were prepared by mechanically milling of LiNH2 powder (95% purity, Sigma-Aldrich) with MMgH3 in a molar ratio of 2:1 for 2 h under an argon atmosphere using a 3 ACS Paragon Plus Environment


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planetary mill at 42 rad/s with a 60:1 ball-to-powder mass ratio. Post-milled composites were denoted as 2LiNH2–NaMgH3 and 2LiNH2–KMgH3 for brief. For comparison, the same procedure was employed to prepare the 2LiNH2–MgH2 composites. All sample procedures were carried out in an Ar-filled glovebox equipped with circulative purification system. The sample thus prepared were subjected to powder X-ray diffraction measurements (XRD, Bruker D8 Advance Diffractometer with Cu Kα), Fourier transform infrared spectra (FTIR, Perkin –Elmer Spectrum 100 Spectrometer) and thermogravimetry/mass spectroscopy (TG/MS, Netzsch STA 409 PC) under a flowing Ar (or Ar/NH3 mixture) atmosphere. The reversibility with de-/rehydrogenation cycles were evaluated by Sieverts-type measurements. A sample of about 0.5 g sealed in the autoclave was heated to 300 °C at a rate of 5 °C/min; this temperature was maintained for 90 min for hydrogen release. After completion, the sample was cooled to room temperature and pressurized with 5 MPa H2 and then heated up to 180 °C for re-hydrogenation.

3. Results and disscussion 3.1 Altered thermal desorption behaviors The effect of bimetallic hydrides MMgH3 on LiNH2 decomposition was first investigated by MS. Fig. 1 shows the temperature–dependent H2 and NH3 released from the 2LiNH2–MgH2, 2LiNH2– NaMgH3 and 2LiNH2–KMgH3, from which two points can be obtained: (i) the H2 desorption temperatures including both onset and peak values for the 2LiNH2–KMgH3 are reduced significantly as shown in Fig. 1A. For the 2LiNH2–MgH2, it starts to release hydrogen at about 215 °C and exhibits two peaks located at 298 and 389 °C. After introducing NaMgH3, the onset temperature is reduced to about 45 °C, and the main peak at about 262 ºC with two shoulders at 133 and 304 ºC were observed for the 2LiNH2–NaMgH3. For the 2LiNH2–KMgH3, a similar onset temperature at ~42 ºC is observed, but its main dehydrogenation peak is dramatically shifted to around 195 ºC with two shoulders at 90 and 275 ºC and completed below 307 ºC, much lower than the temperatures for the 2LiNH2–MgH2 and 4 ACS Paragon Plus Environment

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2LiNH2–NaMgH3 by ~103 and 67 ºC, respectively; (ii) the emission of NH3 is effectively suppressed as shown in Fig. 1B. For the 2LiNH2–MgH2, considerable amounts of NH3 are released with starting at about 52 ºC and peaking at 82 and 305 ºC. For both 2LiNH2–NaMgH3 and 2LiNH2–KMgH3, excitingly, no detectable NH3 are observed within the temperature range of the main dehydrogenation peaks. Unexpectedly, at the temperature higher than 328 °C, some evolved NH3 is detected for the 2LiNH2–KMgH3 while no NH3 is found for the 2LiNH2–NaMgH3. This unexpected NH3 emission suggests different destabilizing mechanism or reaction pathway between 2LiNH2–NaMgH3 and 2LiNH2–KMgH3, which will be discussed in section 3.3. These reduced dehydriding temperatures and suppressed NH3 emission from the 2LiNH2–MMgH3 clearly indicate that as compared to single MgH2, the involvement of bimetallic hydrides, especially for KMgH3, can remarkably promote the LiNH2 desorption. Fig. 2 presents the DSC curves for the 2LiNH2–MgH2, 2LiNH2–NaMgH3 and 2LiNH2– KMgH3. The 2LiNH2–MgH2 exhibits three distinct endothermic processes: the first process in the range of 60–80 °C (step I, as marked by grey shadows) contributes to the decomposition of LiNH2 to release NH3 as suggested by Fig. 1B (a); the second (step II) and third (step III) processes both occur over a wide temperature range with a peak at around 300 and 390 °C, respectively, being well corresponding to its two hydrogen release processes as shown in MS results (see Fig. 1A (a)). For the 2LiNH2–NaMgH3, similar three endothermic peaks can be found, but the peak for steps II and III shift to 260 °C and 295 °C, respectively, and the peak intensity for step III is obviously reduced. For 2LiNH2–KMgH3 composite, a much more dramatic shift towards lower temperature is observed. Except for a similar step I in the rage of 60–80 °C, the endothermic peak for step II shifts to 195 °C, which is lower than the peaks of 2LiNH2–MgH2 and 2LiNH2–NaMgH3 by 105 and 65 °C, respectively. No obvious peak for step III can be found for the 2LiNH2–KMgH3 but the shoulder peak at 275 °C in the MS curve 5 ACS Paragon Plus Environment


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in Fig. 1A(c) suggests that a wide peak with very low intensity for step III still presents but merges into step II at the temperature range of 220–300 °C. Besides these endothermic peaks for step I, II and III, a new endothermic process at the temperature higher than 330 °C is found (marked by step IV), which may be corresponding to the unusual NH3 emission in Fig. 1B(c). Compared to the 2LiNH2–MgH2, the remarkable shift towards lower temperature for both MS and DSC peaks of the 2LiNH2–NaMgH3 and 2LiNH2–KMgH3 provide direct evidence that the LiNH2 is thermodynamically destabilized by alkali metal–magnesium bimetallic hydrides where the KMgH3 shows better destabilizing effect than NaMgH3.

3.2 Structural evolutions upon dehydrogenation To understood how alkali metal–magnesium bimetallic hydrides significantly promote hydrogen release from LiNH2 and why KMgH3 shows better destabilizing effect than NaMgH3, the phase evolution of the 2LiNH2–NaMgH3 and 2LiNH2–KMgH3 during heating process was studied in detail. Fig. 3 shows the ex-situ XRD patterns for the 2LiNH2–NaMgH3 (a–d) and 2LiNH2–KMgH3 (e–h) before and after heating. It can be observed that at the initial stage, no diffraction peaks except for MMgH3 and LiNH2 are observed for both samples after ball milling (Figs. 3(a, e)), but their diffraction intensity are slightly weakened after heating to 120 °C (Figs. 3(b, f)). This indicates that the reaction between MMgH3 and LiNH2 is initiated, even though the peaks for newly formed phase are not visible. Further close examination of the XRD patterns reveals that compared to the milled sample, the intensity ratio between LiNH2 and MMgH3 peaks increases in the partially dehydrogenated samples at 120 °C. For instance, the intensity ratio between the LiNH2 peak at 30.3° and the KMgH3 peak at 31.4° rises from ~0.25 after ball-milling to ~0.60 after heating to 120 °C. This evolution of intensity ratio are ascribed to both the decomposition of little MMgH3 and the newly formed Li2NH phase whose diffraction peaks almost overlap all peaks of LiNH2 phase.5,25

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With a further temperature rise to 200 °C, distinct features are obtained for the 2LiNH2–NaMgH3 and 2LiNH2–KMgH3 (Figs. 3(c, g)). The appearance of new peaks is obtained for both samples that can be assigned to the hydride phases such as NaH, LiH or KH; but the difference is also obtained that the element Mg from NaMgH3 is suggested to be captured by amides and/or imides to generate amorphous products involving Mg–N–H complexes while the element Mg from KMgH3 reacts with amides and/or imides to form crystalline K2Mg(NH)2. The corresponding FTIR results also display different features (see Fig. 4). Except for the starting N–H stretches at about 3311 and 3259 cm-1, an additional stretch at 3178 cm-1 is observed for Mg–N–H complexes in the 2LiNH2–NaMgH3.31,32 But the 2LiNH2–KMgH3 exhibits more stretches appeared at ύ = 3375, 3314, 3297, 3256 and 3171 cm-1 that can be ascribed to the K2Mg(NH)2, LiNH2 and Li2NH phases.14 When the temperature increased to 320 °C, NaH and Li2Mg(NH)2 phases are observed for the 2LiNH2–NaMgH3, but LiH phase plus two imides including K2Mg(NH)2 and Li2Mg(NH)2 are simultaneously found for the 2LiNH2–KMgH3. Based on the above phase evolution with the temperature rise, it is therefore concluded that (i) the introduction of bimetallic hydrides into LiNH2 results in altered reaction pathways and different desorbed products; (ii) the involvement of KMgH3 in LiNH2 decomposition is much faster than NaMgH3 one as indicated by Figs. 3(c, g), which agrees well with the improved dehydrogenation for NaMgH3– and KMgH3–added composites obtained in Fig. 1A.

3.3 Proposed pathway for dehydrogenation In terms of the above structural evolution upon heating and the point of view of thermodynamic energetics, a “self-synergistic” reaction route involved in dehydrogenation process of the bimetallic hydrides combined system is proposed as shown in Fig. 5. LiNH2 first decomposes to release NH3 and converts into Li2NH due to the negative △G value of about –2 kJ/mol (reaction (1)). Meanwhile, the MMgH3 decomposes to MH and MgH2 upon heating (reaction (2)), and then the newly generated MH species reacts preferentially with NH3 to form MNH2 and H2 that proceeds in an exothermic manner 7 ACS Paragon Plus Environment


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(reaction (3)).28,29 Furthermore, the newborn MNH2 can be immediately captured by the in-situ formed MgH2 to generate the amorphous Mg(NH2)2 and crystalline MH (reaction (4));31,33 Upon further heating, the metastable Mg(NH2)2 rapidly decomposes into MgNH and NH3 (reaction (5)). The released NH3 by reaction (5) again participates in reacting with MH that is generated from reactions (2) and/or (4) until it is exhausted. Obviously, combining with reactions (3), (4) and (5) demonstrates that the ‘N’ is quickly transferred from NH3 to Mg-containing imides by MH, which is exactly the reason why no NH3 emission is observed for the 2LiNH2–MMgH3 under low temperature (marked by light green shade). When the temperature further increases to 320 °C, the resulting MgNH combines with Li2NH to form Li2Mg(NH)2 according to reaction (6) in the 2LiNH2–MMgH3. Interestingly, for the 2LiNH2–KMgH3, an additional reaction is obtained (i.e. reaction (7), marked by blue-grew shade in Fig. 5) that the formed Li2Mg(NH)2 most likely react with the active KH to generate K2Mg(NH)2 and LiH as obtained in Fig. 3(g). Until the KH is exhausted (i.e., being completely transformed into K2Mg(NH)2), the excessive Li2Mg(NH)2 would remain in the end products. Note that, similar reactions between Mg(NH2)2 and NaH do not occur for the 2LiNH2–NaMgH3 composite,32,34 thus only Li2Mg(NH)2 is obtained for the 2LiNH2–NaMgH3 (see Figs. 3(d, h)). Furthermore, according to the features reported by Wang et al.,14 the obtained K2Mg(NH)2 would decompose into KMgN with release of NH3 gas when the temperature was elevated over ~330 °C, while the Li2Mg(NH)2 can be kept without decomposition during the heating period.8 This may reasonably explain why some NH3 gas is released only from the 2LiNH2–KMgH3 after heating upon 328 °C in Fig. 1B. The desorption mechanism proposed above emphasizes that both hydrogen evolution and inhibition of NH3 emission mainly depend on the combination of NH3 and hydrides (i.e., reaction (3) in Fig. 5). To support the superior combination of hydrides with NH3, the weight changes of MgH2, NaMgH3 and KMgH3 under NH3 atmosphere are further examined. Fig. 6 compares the representative isothermal TG curves for three hydrides at 200 ºC. It was found that within 60 min only 8% increase in weight 8 ACS Paragon Plus Environment

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was obtained for the MgH2, while about 20% and 58% increase for NaMgH3 and KMgH3 ones, respectively. Namely, the reaction yield of MgH2 and NaMgH3 with NH3 was about 7% and 22%, respectively, while the KMgH3-NH3 system reaches up to 85% relative to their theoretical values of completely conversion into imides. Obviously, the reaction activity to combine with NH3 increases in an order from MgH2, NaMgH3 and KMgH3. Further combining with the phase evolution upon heating (see Fig. 4), it can be reasonably concluded that MH that is formed by in-situ decomposition of MMgH3 greatly promotes the H2 emission and the ‘N’ transfer from NH3 to amides and/or imides. Namely, for the alkali metal–magnesium bimetallic hydrides combined systems, the in-situ formed MgH2 and MH from MMgH3 successively participate in LiNH2 decomposition via concerted pathways, thus leading to improved hydrogen storage properties.

3.4 Enhanced kinetics and reversibility Fig. 7 compares the moderate temperature kinetics for desorption between 2LiNH2–NaMgH3 and 2LiNH2–KMgH3. Obviously, 2LiNH2–NaMgH3 exhibits a slow and multistage desorption without completion until 60 min, while only 30 min is needed for completed dehydrogenation of the 2LiNH2–KMgH3. These features clearly indicate that KMgH3 is superior to NaMgH3 for promoting the desorption kinetics of LiNH2. Further increasing temperature to 300 °C, a cyclic dehydrogenation was carried out to compare both kinetics and reversibility of the LiNH2–MgH2, 2LiNH2–NaMgH3 and 2LiNH2– KMgH3. For the first dehydrogenation (Fig. 8A), a distinct incubation period of about 30, 8 and 2 min is obtained for 2LiNH2–MgH2, 2LiNH2–NaMgH3 and 2LiNH2–KMgH3, respectively. This is most likely to be associated with the temperature-dependent nucleation process of these N-containing species, which can be promoted by introducing of the alkali metals hydrides. Moreover, the 2LiNH2–MgH2 needs about 80 min for completion, while the values for the 2LiNH2–NaMgH3 and 2LiNH2–KMgH3 only are less 30 min and 10 min, respectively. For the 9 ACS Paragon Plus Environment


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second dehydrogenation (Fig. 8B), in comparison to 28% for the 2LiNH2–MgH2, much higher capacity retention is achieved by 85% for the 2LiNH2–NaMgH3 and 65% for the 2LiNH2– KMgH3. These indicate that both kinetics and reversibility of LiNH2 can be improved significantly by adding bimetallic metal hydrides. Note that, the slight low capacity retention of 2LiNH2–KMgH3 as compared to 2LiNH2–NaMgH3 is most likely to be related to the possible loss of N–containing species due to the NH3 release (Fig. 1B) and the formation of K2Mg(NH)2 except for Li2Mg(NH)2 in the resulting products of the 2LiNH2–KMgH3 (see Fig. 3). In this regard, further investigation into the chemical state of Na or K with designed experiments that employ in situ photoelectron spectroscopic technique is currently underway to define the mechanism during rehydriding process.

4. Conclusion In summary, a dramatic destabilization in LiNH2 was achieved by combining potassium– magnesium bimetallic hydrides through self-concerted reaction pathway. As compared to 2LiNH2–MgH2 and 2LiNH2–NaMgH3, the 2LiNH2–KMgH3 shows lower temperature, less NH3 emission and faster kinetics for desorption. An enhanced reversibility is also accomplished by a high retention of more than 65% as well as rehydriding under a milder condition of 180 ºC and 5 MPa H2 pressure. The concept of constructing bimetallic hydrides destabilized multicomponent systems with concerted reaction pathways exhibits great potential for improving the hydrogen storage properties of amides and related complex hydrides.

This work was financially supported by the National Natural Science Foundation of China (Nos. 51271002, 51301002, 51471052, U1201241) and the Science and Technology Commission of Shanghai Municipality (No. 11XD1400600).


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(21) Xiong, Z. T.; Wu, G. T.; Hu, J.; Chen, P. Ternary Imides for Hydrogen Storage. Adv. Mater., 2004, 16, 1522−1525. (22) Luo, W. (LiNH2–MgH2): a Viable Hydrogen Storage System. J. Alloys Compd., 2004, 381, 284−287. (23) Wu, H. Structure of Ternary Imide Li2Ca(NH)2 and Hydrogen Storage Mechanisms in Amide-Hydride System. J. Am. Chem. Soc., 2008, 130, 6515−6522. (24) Liu, D. M.; Liu, Q. Q.; Si, T. Z.; Zhang, Q. A. Synthesis and Crystal Structure of a Novel Nitride Hydride Sr2LiNH2. J. Alloys Compd., 2010, 495, 272−274. (25) Hayes, J.; Goudy, A. Thermodynamics, Kinetics and Modeling Studies of KH- RbH- and CsH-doped 2LiNH2/MgH2 Hydrogen Storage Systems. Int. J. Hydrogen Energy, 2015, 40 (36), 12336–12342. (26) Li, Y.; Fang, F.; Sun, D.; Zheng, S.; Bendersky, L. A.; Zhang, Q.; Ouyang, L.; Zhu, M. Hydrogen Storage of a Novel Combined System of LiNH2-NaMgH3: Synergistic Effects of In Situ Formed Alkali and Alkaline-earth Metal Hydrides. Dalton Trans., 2013, 42, 1810− 1819. (27) Teng, Y.; Ichikawa, T.; Miyaoka, H.; Kojima, Y. Improvement of Desorption Kinetics in the LiH–NH3 System by Addition of KH. Chem. Commun., 2011, 47, 12227− 12229. (28) Kojima, Y.; Tange, K.; Hino, S.; Nakamura, K.; Miyaoka, H.; Yamamoto, H.; Ichikawa, T. Molecular Hydrogen Carrier with Activated Nanohydride and Ammonia J. Mater. Res., 2009, 24, 2185−2190. (29) Yamamoto, H.; Miyaoka, H.; Hino, S.; Nakanishi, H.; Ichikawa, T.; Kojima, Y.; Recyclable Hydrogen Storage System Composed of Ammonia and Alkali Metal Hydride. Int. J. Hydrogen Energy, 2009, 34(24): 9760–9764.



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(30) Miyaoka, H.; Nakajima, K.; Yamaguchi, S.; Aoki, T.; Yamamoto, H.; Okuda, T.; Goshome, K.; Ichikawa, T.; Kojima, Y. Catalysis of Lithium Chloride and Alkali Metal Borohydrides on Hydrogen Generation of Ammonia and Lithium Hydride System. J. Phys. Chem. C, 2015, 119(34), 19922–19927. (31) Sheppard, D. A.; Paskevicious, M. Buckley, C. E. Hydrogen Desorption from the NaNH2MgH2 System. J. Phys. Chem. C, 2011, 115, 8407−8413. (32) Wang, J.; Chen, P.; Pan, H.; Xiong, Z.; Gao, M.; Wu, G.; Liang, C.; Wang, J. Solid-Solid Heterogeneous Catalysis: the Role of Potassium in Promoting the Dehydrogenation of the Mg(NH2)2/2LiH Composite. ChemSusChem, 2013, 6(11), 2181−2189. (33) Singh, N. K.; Kobayashi, T.; Dolotko, O.; Wiehch, J.; Pruskia, M.; Pecharsky, V. K. Mechanochemical Transformations in NaNH2-MgH2 Mixtures. J. Alloys Compd., 2012, 513, 324−327. (34) Dong, B.; Song, L.; Ge, J.; Teng, Y.; Zhang, S. The Ternary Amide KLi3(NH2)4: An Important Intermediate in the Potassium Compound-added Li–N–H Systems. RSC Adv., 2014, 4, 10702−10707.


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Caption of Figures Figure 1. Temperature–dependent hydrogen (H2, m/e = 2, (A)) and ammonia (NH3, m/e = 17, (B)) released from (a) 2LiNH2–MgH2, (b) 2LiNH2–NaMgH3 and (c) 2LiNH2–KMgH3. Figure 2. DSC curves for the (a) 2LiNH2–MgH2, (b) 2LiNH2–NaMgH3 and (c) 2LiNH2–KMgH3. Figure 3. Ex-situ XRD patterns for the 2LiNH2–NaMgH3 (lower) and 2LiNH2–KMgH3 (upper) before (a, e) and after desorption at 120 °C (b, f), 200 °C (c, g) and 320 °C (d, h). Figure 4. FTIR spectra for the (a) 2LiNH2–NaMgH3 and (b) 2LiNH2–KMgH3 after desorption at 200 °C. Figure 5. A proposed pathway for 2LiNH2–MMgH3 (M = Na, K) systems upon dehydrogenation. Figure 6. Isothermal TG curves for MgH2 (a), NaMgH3 (b) and KMgH3 (c) at 200 ºC under a flow of NH3/Ar (70/30) gas. Figure 7. Desorption kinetics for the (a) 2LiNH2–NaMgH3 and (b) 2LiNH2–KMgH3 at 200 °C. Figure 8. Kinetic curves of first (A) and second (B) dehydrogenation that was normalized to the initial desorbed capacity for the (a) 2LiNH2–MgH2, (b) 2LiNH2–NaMgH3 and (c) 2LiNH2– KMgH3 at 300 ºC.



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TOC


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Figure 1. Temperature–dependent hydrogen (H2, m/e = 2, (A)) and ammonia (NH3, m/e = 17, (B)) released from (a) 2LiNH2–MgH2, (b) 2LiNH2–NaMgH3 and (c) 2LiNH2–KMgH3. 80x87mm (300 x 300 DPI)



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Figure 2. DSC curves for the (a) 2LiNH2–MgH2, (b) 2LiNH2–NaMgH3 and (c) 2LiNH2–KMgH3. 80x62mm (300 x 300 DPI)


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Figure 3. Ex-situ XRD patterns for the 2LiNH2–NaMgH3 (lower) and 2LiNH2–KMgH3 (upper) before (a, e) and after desorption at 120 °C (b, f), 200 °C (c, g) and 320 °C (d, h). 80x137mm (300 x 300 DPI)



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Figure 4. FTIR spectra for the (a) 2LiNH2–NaMgH3 and (b) 2LiNH2–KMgH3 after desorption at 200 °C. 80x60mm (300 x 300 DPI)


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Figure 5. A proposed pathway for 2LiNH2–MMgH3 (M = Na, K) systems upon dehydrogenation. 140x54mm (300 x 300 DPI)



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Figure 6. Isothermal TG curves for MgH2 (a), NaMgH3 (b) and KMgH3 (c) at 200 ºC under a flow of NH3/Ar (70/30) gas. 80x61mm (300 x 300 DPI)


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Figure 7. Desorption kinetics for the (a) 2LiNH2–NaMgH3 and (b) 2LiNH2–KMgH3 at 200 °C. 80x54mm (300 x 300 DPI)



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Figure 8. Kinetic curves of first (A) and second (B) dehydrogenation that was normalized to the initial desorbed capacity for the (a) 2LiNH2–MgH2, (b) 2LiNH2–NaMgH3 and (c) 2LiNH2–KMgH3 at 300 ºC. 80x102mm (300 x 300 DPI)


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Enhancement of Hydrogen Storage in Destabilized LiNH2 with

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