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Effect of gas back pressure on hydrogen storage properties and crystal structures of Li2Mg(NH)2
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Effect of gas back pressure on hydrogen storage properties and crystal structures of Li2Mg(NH)2 Chu Liang a,b, Mingxia Gao b, Hongge Pan b,*, Yongfeng Liu b, Mi Yan b a
College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, People's Republic of China b State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province & Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China
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abstract
Article history:
The ternary imide Li2Mg(NH)2 is considered to be one of the most promising on-board
Received 22 May 2014
hydrogen storage materials due to its high reversible hydrogen capacity of 5.86 wt%,
Received in revised form
favorable thermodynamic properties and good cycling stability. In this work, Li2Mg(NH)2
25 August 2014
was synthesized by dynamically dehydrogenating a mixture of Mg(NH2)2e2LiH up to 280 C
Accepted 1 September 2014
under different gas (Ar and H2) and pressures (0e9.0 bar). The crystal structure of
Available online 26 September 2014
Li2Mg(NH)2 was found to depend on the gas back pressure in the dehydrogenation process. The crystal structure of Li2Mg(NH)2 and the dehydrogenation/rehydrogenation properties
Keywords:
of the Mg(NH2)2e2LiH system strongly depend on the gas back pressure in the dehydro-
Hydrogen storage
genation process due to the effect of the pressure on the dehydrogenation kinetics. This
Imide
study provides a new approach for improving the hydrogen storage properties of the amide
Crystal structure
ehydride systems.
Kinetics
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Thermodynamics
Introduction A hydrogen economy based on renewable energy is one possible solution to the problems associated with the use of hydrocarbon fuels, such as the greenhouse gases they emit upon combustion, the ever increasing demand for their utilization, and their inevitable depletion. The feasibility of hydrogen economy mostly depends on the development of safe, efficient and low-cost hydrogen production and storage [1e3]. Hydrogen storage technology has become a bottleneck of the hydrogen economy. Solid-state hydrogen storage is considerably superior in term of safety compared with
gaseous- and liquid-state storage [1,4,5]. Over the past few decades, considerable efforts have been devoted to exploring light-weight metal hydrides, such as LiAlH4, LiBH4, NH3BH3, LiNH2e2LiH, Li2Mg(NH)2, and Li4BN3H10, for solid-state hydrogen storage [6e22]. Among these hydrogen storage materials, the ternary imide Li2Mg(NH)2 has attracted the most attention because of its high reversible capacity, favorable thermodynamics and good cycling stability for hydrogenation/dehydrogenation [23e25]. After absorbing 5.86 wt% of hydrogen, Li2Mg(NH)2 was converted to a Mg(NH2)2e2LiH mixture through the following reaction [26]: Li2 MgðNHÞ2 þ 2H2 4MgðNH2 Þ2 þ 2LiH
* Corresponding author. Tel./fax: þ86 571 87952615. E-mail address: [email protected] (H. Pan). http://dx.doi.org/10.1016/j.ijhydene.2014.09.013 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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The thermodynamic operating temperature for the dehydrogenation of the Mg(NH2)2e2LiH mixture under an equilibrium hydrogen pressure of 1.0 bar was predicted to be ~90 C [25]. Thus, it satisfies the practical requirement of the onboard proton exchange membrane fuel cells [27]. The crystal structure of Li2Mg(NH)2 possesses both cubic and orthorhombic phases, which were observed by several research groups [18,23,28e31]. Xiong et al. [23] detected a cubic Li2Mg(NH)2 by dynamically dehydrogenating a Mg(NH2)2e2LiH mixture from 50 to 300 C at a heating rate of 2 C/min under a flowing gas of Ar. Janot et al. [28] also obtained the cubic Li2Mg(NH)2 by isothermally dehydrogenating a 2LiNH2eMgH2 mixture at 200 C for 2 h under initial vacuum. Orthorhombic Li2Mg(NH)2 was observed by completely dehydrogenating the 2LiNH2eMgH2 [29] and Mg(NH2)2e2LiH [30] mixtures at 180 and 250 C, respectively, in a pressureecomposition (PeC) isotherm process. Our previous work demonstrated that Li2Mg(NH)2 with mixed cubic and orthorhombic phases can be obtained by desorbing NH3 from the Mg(NH2)2e2LiNH2 mixture at 315 C under a N2 atmosphere [18]. Rijssenbeek et al. [31] found that Li2Mg(NH)2 undergoes a transformation from orthorhombic to cubic phase at the temperature over 350 C under initial vacuum. These investigations show that cubic and orthorhombic Li2Mg(NH)2 can be obtained from different starting materials, operating temperatures, and desorption atmospheres. However, the key factors for controlling the crystal structure of Li2Mg(NH)2 remain unclear. Appreciable dehydrogenation rates of the Mg(NH2)2e2LiH mixture were experimentally observed only at the temperatures above 220 C under 1.0 bar hydrogen [32]. A high kinetic barrier from interface reaction and mass transport resulted in the dehydrogenation temperatures of the Mg(NH2)2e2LiH mixture much higher than the thermodynamically predicted equilibrium temperature of ~90 C under 1.0 bar hydrogen [18,32,33]. Considerable efforts have been made to reduce the kinetic barrier of the LieMgeNeH system by adding catalysts and reducing the particle size of the reactants [34e39]. The peak temperature of hydrogen desorption of the Mg(NH2)2e2LiH mixture was lowered from 190 C to less than 154 C by adding 0.5 mol of NaOH [34]. The combined system of Mg(NH2)2e1.9LiHe0.1 KH showed an even lower peak temperature of hydrogen desorption of ca. 132 C [35]. Liu et al. [18] and Xie et al. [36] reported that the hydrogen absorption/desorption kinetic property of the Mg(NH2)2e2LiH system was evidently improved by reducing the particle sizes. However, the dehydrogenation temperatures of the catalyst-added Mg(NH2)2e2LiH system remain higher than those of the thermodynamically predicted one. Further improvements in dehydrogenation/hydrogenation kinetics are highly needed to develop the Mg(NH2)2e2LiH system as an onboard hydrogen storage medium. Previous studies on Li2Mg(NH)2 as a hydrogen storage material mostly focus on kinetic improvements and hydrogen storage mechanisms. To date, the correlation between crystal structures and hydrogen storage properties of Li2Mg(NH)2 remains unclear. In this work, we initially focused on the structure-controllable synthesis of Li2Mg(NH)2 via dehydrogenating the Mg(NH2)2e2LiH mixture under various gas back pressures. The correlations among the crystal structures, gas back pressure, and hydrogen storage properties of Li2Mg(NH)2 as well as the corresponding hydrogen storage mechanisms
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were investigated by first-principles calculations combined with experimental measurements. This study is expected to elucidate the hydrogen storage mechanisms of the LieMgeNeH systems and to serve as a guide to further improve the hydrogen storage properties of Li2Mg(NH)2.
Experimental and computational section Experimental details Lithium hydride LiH (98%, Alfa Aesar) was used as-received without further purification. Magnesium amide Mg(NH2)2 was synthesized in-house by reacting magnesium powder (99%, Sinopharm) with ~7.0 bar ammonia at 300 C. A low-crystalline Li2Mg2(NH)3 sample was prepared by hydrogenating the post36 h milled Li2Mg2(NH)3 at 210 C under 300 bar hydrogen, followed by dehydrogenating the hydrogenated sample from room temperature (RT) to 250 C at a heating rate of 2 C/min under initial vacuum. Mg(NH2)2e2LiH mixture was prepared by ball milling Mg(NH2)2 with LiH in a molar ratio of 1:2 for 36 h. This preparation was carried out on a planetary ball mill (QM-3SP4, Nanjing) with a rotating speed of 500 rpm. The milling jars were filled with the hydrogen pressure of 78 bar to prevent hydrogen desorption during the ball milling. The ball-to-sample weight ratio was about 60:1. All sample handlings were carried out in an MBRAUN glovebox filled with purified argon to prevent contamination from moisture and air (H2O: <1 ppm, O2: <1 ppm). A homemade volumetric Sieverts-type apparatus was used to synthesize Li2Mg(NH)2 and to measure hydrogen desorption/absorption properties. About 200 mg of sample was used in each experiment. Li2Mg(NH)2 was prepared by isothermally and dynamically dehydrogenating the Mg(NH2)2e2LiH mixture at different temperatures under 0e9.0 bar Ar or H2. Three cooling rates of 0.5 C/min, 10e15 C/min, and 150 C/ min for furnace, air, and water cooling were used, respectively, to prepare Li2Mg(NH)2. For the dynamic desorption/ absorption measurements, samples were heated from RT to the preset temperature at a rate of 2 C/min for desorption and to 210 C at a rate of 1 C/min for absorption. The samples for the isothermal desorption and absorption measurements were heated to 150 C at a rate of 15 C/min and dwelled for 750 and 1250 min, respectively. An initial hydrogen pressure of 105 bar was used in the isothermal and dynamic absorption of Li2Mg(NH)2. Initial vacuum and 9.0 bar Ar were applied in the isothermal and dynamic desorption of the Mg(NH2)2e2LiH mixture. The detailed desorption/absorption procedure can be found in our previous work [16]. The experimental enthalpy change of hydrogen desorption was determined by the Van't Hoff method. The homemade Sieverts-type apparatus was used to measure the equilibrium hydrogen pressures at several preset temperatures from 180 to 220 C. A total of 500 mg of the Mg(NH2)2e2LiH mixture was used for these measurements. Equilibrium state was reached when the hydrogen pressure remained unchanged at the preset temperature for 10 h. The Kissinger's method was used to determine the apparent activation energy Ea of dehydrogenation. The temperature for the maximum dehydrogenation rate was obtained from the hydrogen desorption curves measured at heating rates of 5, 6, 7.5, 10 and 15 C/min.
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Crystal structures of samples were characterized by an X'Pert Pro diffractometer with Cu Ka radiation at 40 kV and 40 mA. X-ray diffraction (XRD) data were collected in a 2q range of 10e90 with a step of 0.05 . A homemade container was applied to prevent the powder samples from air and moisture contamination. NeH vibrations of all samples were identified with a Bruker TENSOR 27 Fourier infrared spectrometer (FTIR, Germany) in a transmission mode. Each pellet sample was prepared by cold pressing the mixture of the powder sample and potassium bromide in a weight ratio of about 1:30. FTIR spectra were plotted from 32 scans that were averaged using a resolution of 4 cm1. The morphologies and particle sizes of the as-prepared Li2Mg(NH)2 were observed under a scanning electron microscopy (SEM, Hitachi-S4800).
Computational details The total energies and crystal structures of the related phases in this work were calculated by CASTEP software [40] based on the density functional theory (DFT). The generalized gradient approximation proposed by Perdew et al. (PW91) [41] was employed to include the exchange-correlation energy in the total energy. The ultrasoft pseudopotential was adopted to describe the ioneelectron interactions. Electronic wave functions were expanded on the basis of plane waves with a wellconverged cut-off energy of 490 eV. The Brillouin zone was sampled by a sum over special k-points generated with the MonkhorstePack scheme. The zone was set as 4 4 2 for Mg(NH2)2, 8 8 8 for LiH, 6 6 3 for LiNH2, 2 6 3 for Li2Mg2(NH)3, 6 6 2 for cubic Li2Mg(NH)2, 3 6 6 for orthorhombic Li2Mg(NH)2, and Gamma-point for H2 molecule. A finite basis set correction was used in the evaluation of energy and stress, and the Pulay density-mixing scheme was adopted for self-consistent field calculations. Cubic Li2Mg(NH)2 in the space group P-43m [31] and orthorhombic Li2Mg(NH)2 in the space group Iba2 [31] were employed in our calculations.
All structures calculated were fully relaxed in volume, atomic coordinates, and shape in the structural optimization until the forces <0.03 eV/Å and the stresses <0.05 GPa. Hydrogen was calculated by placing one hydrogen molecule inside a fixed cell of 10 10 10 Å. Finite temperature enthalpy was evaluated within the harmonic approximation [42]. DFT as implanted within the Dmol3 program was used to calculate the harmonic vibrational frequencies. These frequencies were computed by diagonalizing the mass-weighted Hessian matrix of the unit cell. The double numerable plus polarization basis set and the exchange-correlation function of GGA-PW91 were both employed in our calculations. DFT semi-core pseudopots were used to treat the core electrons of the elements of Li, Mg, N, and H. Mixed Li/Mg cation arrangements are present in the crystal lattice of cubic and orthorhombic Li2Mg(NH)2 [31]. In this study, a series of 1 1 3 supercells for cubic Li2Mg(NH)2 and cells for orthorhombic Li2Mg(NH)2 with possible arrangements of Li and Mg cations on the mixed positions were used to optimize geometry and calculate the total energy. The crystal models with the lowest total energy were employed in the thermodynamic calculations of cubic and orthorhombic Li2Mg(NH)2, respectively. The Rietveld refinement results of the XRD data showed that the calculated curves were in good agreement with the experimental curves of Li2Mg(NH)2 [43]. Thus, the crystal models of Li2Mg(NH)2 are reasonable to be used in the calculations.
Results and discussion Effect of gas back pressure on crystal structures of Li2Mg(NH)2 Fig. 1 shows the XRD patterns and FTIR spectra of the Mg(NH2)2e2LiH mixture dehydrogenated at different temperatures under initial vacuum. For the Mg(NH2)2e2LiH mixture
Fig. 1 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dehydrogenated at 190 C for 15 h and from RT to 280 C under initial vacuum.
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Fig. 2 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dehydrogenated at 190 C for 15 h and from RT to 280 C.
both isothermally dehydrogenated at 190 C for 15 h and dynamically dehydrogenated from RT to 280 C, the XRD peaks of the dehydrogenation products corresponded to the cubic Li2Mg(NH)2 phase (Fig. 1a), as reported by Chen et al. [26] and Rijssenbeek et al. [31]. In contrast to the Mg(NH2)2e2LiH mixture dehydrogenated at 190 C, two weak XRD peaks at 21.6 and 33.3 were observed in the XRD pattern of the mixture dehydrogenated to 280 C. This observation indicates that trace Mg3N2 may be present in the dehydrogenation product. FTIR examination (Fig. 1b) presents the absorption band of the NeH vibration of cubic Li2Mg(NH)2 centered at ~3174 cm1 [37] from both dehydrogenation products. In the aforementioned dehydrogenation processes, the hydrogen pressure in the Sieverts-type apparatus was found to increase from the initial vacuum to ca. 0.8 bar, the amount of which equals to the hydrogen released from the Mg(NH2)2e2LiH mixture to Li2Mg(NH)2. Therefore, the cubic Li2Mg(NH)2 can be synthesized by dehydrogenating the Mg(NH2)2e2LiH mixture either at 190 C for 15 h or from RT to 280 C under initial vacuum. Fig. 2 exhibits the XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dehydrogenated at 190 C for 15 h and from RT to 280 C with initial hydrogen pressure of 9.0 bar.
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The product dehydrogenated at 190 C for 15 h was mainly composed of orthorhombic Li2Mg(NH)2 with a little amount of residual LiH and newly formed Li2Mg2(NH)3 and LiNH2. This result implies that an incomplete dehydrogenation of the Mg(NH2)2e2LiH mixture occurred because of the high hydrogen pressure. For the product dehydrogenated from RT to 280 C, the XRD peaks (Fig. 2a) were attributed to those of the orthorhombic Li2Mg(NH)2 reported by Luo et al. [29] and Rijssenbeek et al. [31]. Fig. 2b, shows that the product dehydrogenated to 280 C only had two FTIR absorption peaks of an imide centered at about 3182 and 3161 cm1, which are very close to those of the orthorhombic Li2Mg(NH)2 reported by Markmaitree et al. [44,45]. This result confirms the formation of orthorhombic Li2Mg(NH)2. After the dynamic dehydrogenation to 280 C, the hydrogen pressure in the Sieverts-type apparatus also increased by ~0.8 bar, which is close to the pressure change of the Mg(NH2)2e2LiH mixture dehydrogenated under initial vacuum. This result indicates that almost the same amounts of hydrogen were released in the two dehydrogenation processes despite the formation of cubic or orthorhombic Li2Mg(NH)2. Moreover, orthorhombic Li2Mg(NH)2 was also successfully synthesized by dehydrogenating the Mg(NH2)2e2LiH mixture from RT to 280 C when 9.0 bar Ar pressure was used in the dehydrogenation process (Fig. 2). However, further experiments that only the cubic Li2Mg(NH)2 was formed in the dehydrogenation of the Mg(NH2)2e2LiH mixture from RT to 280 C under Ar with lower pressure of 1.0 and 3.0 bar. Different crystal structures of Li2Mg(NH)2 were formed under initial vacuum and different pressures of Ar or H2 at the same temperature from RT to 280 C. This result indicates that the gas back pressure in the dehydrogenation process of the Mg(NH2)2e2LiH mixture played an important role in determining the crystal structures of the dehydrogenation product of Li2Mg(NH)2. A high H2 or Ar pressure of 9.0 bar favored the formation of orthorhombic Li2Mg(NH)2, whereas a low Ar pressure of 1.0e3.0 bar or initial vacuum favored the formation of cubic Li2Mg(NH)2. The formation of the same crystal structures of Li2Mg(NH)2 at the temperature of 190 C for 15 h and from RT to 280 C indicated that the crystal structure of Li2Mg(NH)2 was not evidently affected by the preparation temperatures (190e280 C). Further temperature-induced phase transition of Li2Mg(NH)2 was reported in our previous paper [43]. In addition, XRD and FTIR analyses (Figs. 3 and 4) of the samples were carried out under initial vacuum and 9.0 bar Ar with different cooling rates of 0.5, 10e15 and 150 C/min. The results of the analyses confirmed that the crystal structure of Li2Mg(NH)2 was not affected by the cooling rates. Consequently, the crystal structure of Li2Mg(NH)2 was strongly determined by the gas back pressure in the dehydrogenation process of the Mg(NH2)2e2LiH system but independent of gas, cooling rates and preparation temperatures in the range of 190e280 C.
Effect of gas back pressure on dehydrogenation/ hydrogenation properties The effect of the gas back pressure on the dehydrogenation behavior of the Mg(NH2)2e2LiH system was evaluated by isothermal and dynamic dehydrogenation. Fig. 5a,b depict
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Fig. 3 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dehydrogenated from RT to 280 C under initial vacuum and then cooled at various rates.
both the dynamic and isothermal dehydrogenation curves, respectively, of the Mg(NH2)2e2LiH mixture dehydrogenated under initial vacuum and 9.0 bar Ar. Apparently, the onset dehydrogenation temperature and the dehydrogenation rates of the Mg(NH2)2e2LiH system were remarkably affected by the gas back pressure. Fig. 5a shows that the operating temperatures of the Mg(NH2)2e2LiH mixture dehydrogenated under initial vacuum were lower than those under 9.0 bar Ar. A higher dehydrogenation rate was also achieved for the initial vacuum. Approximately 4.55 wt% of hydrogen was desorbed from the Mg(NH2)2e2LiH mixture within 700 min at 150 C under initial vacuum, whereas only 2.62 wt% was desorbed
from the 9.0 bar Ar. Analyzing the tangent slop of the linear part of the dehydrogenation curves, an average rate was determined to be ~0.033 wt%/min for the Mg(NH2)2e2LiH mixture dehydrogenated under initial vacuum. This average rate was 3.3 times higher than that obtained under 9.0 bar Ar (0.010 wt%/min). A similar phenomenon of the influence of hydrogen pressure on the dehydrogenation properties was also found in the LiBH4eMetal hydride composite and Ca(BH4)2 systems for hydrogen storage [46e48]. Cubic and orthorhombic Li2Mg(NH)2 were initially prepared by dehydrogenating the Mg(NH2)2e2LiH mixture from RT to 280 C under initial vacuum and 9.0 bar Ar. This
Fig. 4 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dehydrogenated from RT to 280 C under 9.0 bar Ar and then cooled at various rates.
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Fig. 5 e Dehydrogenation curves of the Mg(NH2)2e2LiH mixture dehydrogenated under initial vacuum and 9.0 bar Ar.
Fig. 6 e Dynamic (a) and isothermal (b) hydrogenation curves of the cubic and orthorhombic Li2Mg(NH)2 hydrogenated under 105 bar hydrogen.
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preparation was performed to elucidate the effect of gas back pressure on the hydrogenation properties of the completely dehydrogenated Mg(NH2)2e2LiH mixture. Fig. 6 shows the dynamic and isothermal hydrogenation curves of the cubic and orthorhombic Li2Mg(NH)2 hydrogenated under 105 bar hydrogen. In the dynamic process, both cubic and orthorhombic Li2Mg(NH)2 absorbed 5.74 wt% of hydrogen (Fig. 6a), which is close to the theoretical hydrogen absorption capacity of 5.86 wt% of Li2Mg(NH)2. This result indicates that both the cubic and orthorhombic Li2Mg(NH)2 were completely converted to the Mg(NH2)2e2LiH mixture after hydrogenation at 210 C. However, the cubic and orthorhombic Li2Mg(NH)2 evidently exhibited different hydrogenation behavior. As seen in Fig. 6a, the onset hydrogenation temperature was only ca. 102 C for the cubic Li2Mg(NH)2, whereas ~132 C for the orthorhombic Li2Mg(NH)2. The hydrogenation rate of the cubic Li2Mg(NH)2 was much higher than that of the orthorhombic Li2Mg(NH)2 in the temperature range of 130e170 C. The isothermal hydrogenation rate of both cubic and orthorhombic Li2Mg(NH)2 at 150 C (Fig. 6b) was high within the first 10 min, but become low in the next stage. The hydrogen absorbed for the cubic Li2Mg(NH)2 (1.90 wt%) was still higher than that for the orthorhombic Li2Mg(NH)2 (0.42 wt%). After the initial 10 min, the hydrogenation rates of both the cubic and orthorhombic Li2Mg(NH)2 turned to be a similarly low value (~0.0018 wt%/min). Our previous work [18] revealed that the hydrogenation properties of Li2Mg(NH)2 strongly depend on its particle sizes. Li2Mg(NH)2 with smaller particle sizes exhibit lower hydrogenation temperatures and higher hydrogenation rates. The particle sizes of the as-prepared cubic and orthorhombic Li2Mg(NH)2 were observed under SEM to determine the correlation between its crystal structures and hydrogenation properties. Fig. 7 presents the SEM morphologies of as-prepared Li2Mg(NH)2. The particles of both cubic and orthorhombic Li2Mg(NH)2 were irregular in shape. Cubic Li2Mg(NH)2 generally possessed larger particle sizes than the orthorhombic Li2Mg(NH)2, but still exhibited higher hydrogenation rates and lower hydrogenation temperatures. This result supports that cubic Li2Mg(NH)2 exhibited better hydrogenation properties than the orthorhombic Li2Mg(NH)2.
Fig. 7 e SEM micrographs of the as-prepared cubic (a) and orthorhombic (b) Li2Mg(NH)2.
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Effect of gas back pressure on dehydrogenation reaction pathways The effect of gas back pressure on dehydrogenation reaction pathways was initially investigated to reveal the mechanism of the dehydrogenation/hydrogenation properties of the Mg(NH2)2e2LiH system that is associated with the gas back pressure. The Mg(NH2)2e2LiH mixture dynamically dehydrogenated to different stages under both initial vacuum and 9.0 bar Ar was collected for XRD and FTIR analyses. Fig. 8a,b shows the XRD patterns and FTIR spectra of the Mg(NH2)2e2LiH mixture dynamically dehydrogenated to different temperatures under initial vacuum, respectively. The residual phases of Mg(NH2)2 and LiH were identified in the Mg(NH2)2e2LiH mixture dehydrogenated to 165 C (Fig. 8a). In addition to the typical NeH vibrations of Mg(NH2)2 at ~3326 and 3271 cm1, a new absorption peak of an unknown imide centered at ~3189 cm1 was observed in the FTIR spectrum (Fig. 8b). The increase in dehydrogenation to 180 C resulted in elevated intensity of the absorption peak centered at ~3189 cm1 and the shift of absorption peak to a low wavenumber of ~3183 cm1, which is close to that of the
Fig. 8 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dynamically dehydrogenated to different temperatures under initial vacuum.
low-crystalline Li2Mg2(NH)3 (Fig. 8b). Meanwhile, the FTIR absorption peaks of LiNH2 at ~3312 and 3258 cm1 developed and the absorption peaks of Mg(NH2)2 at ~3326 and 3271 cm1 significantly weakened. Fig. 8a shows that the diffraction peaks of the Mg(NH2)2 and LiH phases weakened whereas a set of diffraction peaks at 30.5, 51.2 and 60.9 significantly intensified with the increase of dehydrogenation temperature to 180 C. Although the XRD peaks of Li2Mg2(NH)3 were mostly overlapped with those of the LiNH2, the exclusively intrinsic diffraction peaks of Li2Mg2(NH)3 at 18.2, 19.4 and 40.2 were still detected, confirming the existence of Li2Mg2(NH)3. The XRD and FTIR results indicated that Mg(NH2)2 and LiH were consumed to convert to LiNH2 and Li2Mg2(NH)3 after the dehydrogenation to 180 C. For the Mg(NH2)2e2LiH mixture dehydrogenated to 200 C, the peaks of cubic Li2Mg(NH)2 dominated the XRD pattern and FTIR spectrum. Fig. 8b shows broad and weak FTIR absorption bands centered at ~3258 cm1, indicating that an incomplete dehydrogenation occurred when the temperature was increased to 200 C. A single phase of cubic Li2Mg(NH)2 was produced after the dehydrogenation temperature was further elevated to 280 C. Fig. 9 shows the XRD patterns and FTIR spectra of the Mg(NH2)2e2LiH mixture dynamically dehydrogenated at different temperatures under 9.0 bar Ar. The residual phases of Mg(NH2)2 and LiH, and the new phase of LiNH2 were detected in the Mg(NH2)2e2LiH mixture dehydrogenated to 190 C. A broad NeH vibration of a new imide centered at ~3184 cm1, which is close to that of the low-crystalline Li2Mg2(NH)3, was also developed as shown in Fig. 9b. As discussed above (Fig. 2), Li2Mg2(NH)3 and LiNH2 were intermediates in the dehydrogenation of the Mg(NH2)2e2LiH mixture dehydrogenated at 190 C for 15 h under 9.0 bar hydrogen (Fig. 2). Therefore, Li2Mg2(NH)3 was possibly formed in the present dehydrogenation process. A previous literature [26] reported that the intermediates of Li2Mg2(NH)3 and LiNH2 were also detected in the Mg(NH2)2e2LiH mixture dehydrogenated at 220 C under an equilibrium hydrogen pressure of ~20 bar. The increase in dehydrogenation temperature to 205 C resulted in the dehydrogenated product showing evident relative intensities of LiNH2 and Li2Mg2(NH)3 in their XRD peaks and FTIR absorption, and almost undetectable Mg(NH2)2 and LiH. A broad diffraction peak in the 2q angle range of 16e20 originated from the overlapped diffraction peaks of LiNH2 and Li2Mg2(NH)3 was also observed. These results indicate that LiH and Mg(NH2)2 were consumed and converted to LiNH2 and Li2Mg2(NH)3, respectively, at 205 C under 9.0 bar Ar. The orthorhombic Li2Mg(NH)2 dominated in the XRD pattern and FTIR spectrum as the dehydrogenated temperature was further elevated to 250 C. Then, a single phase of orthorhombic Li2Mg(NH)2 was produced after the dehydrogenation to 280 C. The information obtained from XRD and FTIR supported that Li2Mg2(NH)3 and LiNH2 are the intermediates in the dehydrogenation process of the Mg(NH2)2e2LiH mixture before the final product of Li2Mg(NH)2 is formed. The dehydrogenation of the Mg(NH2)2e2LiH mixture under both initial vacuum and 9.0 bar Ar can be described by the following reaction:
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operating temperature of the LiNH2eLi2Mg2(NH)3eLiH mixture dehydrogenated under initial vacuum was apparently lower than that of the Mg(NH2)2e3/2LiH mixture, but exhibited a reverse phenomenon for the mixtures dehydrogenated under 9.0 bar Ar. This result may explain the lower operating temperatures and higher dehydrogenation rates of the Mg(NH2)2e2LiH mixture under initial vacuum with respect to that under 9.0 bar Ar (Fig. 5). Fig. 10 shows that the effect of gas back pressure on the dehydrogenation temperature of the LiNH2eLi2Mg2(NH)3eLiH mixture was greater than that on the Mg(NH2)2e3/2LiH mixture. In comparison with the mixtures dehydrogenated under 9.0 bar Ar, the operating temperature was decreased by ca. 35 C for the LiNH2eLi2Mg2(NH)3eLiH mixture dehydrogenated under initial vacuum, whereas it was only ~10 C for the Mg(NH2)2e3/2LiH mixture. The operating temperature of the first-step dehydrogenation reaction of the Mg(NH2)2e2LiH system depended on the gas back pressure but the reaction pathway was independent of the gas back pressure. Thus, the gas back pressure only changed the hydrogen storage kinetics of the first-step reaction. By contrast, the gas back pressure changed both the hydrogen storage thermodynamics and kinetics of the second-step reaction owing to the considerable effect of the gas back pressure on the dehydrogenation properties of the second-step reaction and the different crystal structures of the dehydrogenation product of Li2Mg(NH)2. To verify the effect of the gas back pressure on the thermodynamics and kinetics of the two-step reaction of Eq. (2), the enthalpy changes and apparent activation energies of the and dehydrogenation of Mg(NH2)2e3/2LiH LiNH2eLi2Mg2(NH)3eLiH mixtures under initial vacuum and 9.0 bar Ar were determined by the first-principles calculation and Kissinger's method, respectively. The results are shown in Fig. 11. Our calculations showed that the difference in the thermodynamic enthalpies for the Li2Mg(NH)2 under vacuum and 9.0 bar Ar was less than 0.1 kJ/mol. Thus, the effect of the gas back pressure on the enthalpies was not considered in Fig. 11. For the first-step dehydrogenation reaction, the calculated enthalpy change calculated was 70.8 kJ/mol,
Fig. 9 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dynamically dehydrogenated to different temperatures under 9.0 bar Ar.
1 1 1 MgðNH2 Þ2 þ 2LiH/ Li2 Mg2 ðNHÞ3 þ LiNH2 þ LiH 2 2 2 3 þ H2 /Li2 MgðNHÞ2 þ 2H2 2
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Mechanism for hydrogen storage properties and crystal structures associated with gas back pressure The reaction pathways of the dehydrogenation of the Mg(NH2)2e2LiH mixture were independent of the gas back pressure. By contrast, the dehydrogenation properties of the Mg(NH2)2e2LiH mixture and the crystal structure of the dehydrogenation product of Li2Mg(NH)2 were strongly dependent on the gas back pressure. To examine the effect of the gas back pressure on the first- and second-step dehydrogenation reactions of Eq. (2), a Mg(NH2)2e3/2LiH mixture of the reactant for the first-step reaction and a LiNH2eLi2Mg2(NH)3eLiH mixture of the reactant for the second-step reaction were prepared by ball milling. Fig. 10 shows the dehydrogenation curves of the Mg(NH2)2e3/2LiH and LiNH2eLi2Mg2(NH)3eLiH mixtures dehydrogenated from RT to 280 C under both initial vacuum and 9.0 bar Ar. The
Fig. 10 e Dynamic dehydrogenation curves of the Mg(NH2)2e3/2LiH and LiNH2eLi2Mg2(NH)3eLiH mixtures dehydrogenated under initial vacuum and 9.0 bar Ar.
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Fig. 11 e Schematic enthalpy changes and apparent activation energies of the as-milled Mg(NH2)2e3/2LiH and LiNH2eLi2Mg2(NH)3eLiH mixtures dehydrogenated under both initial vacuum and 9.0 bar Ar.
equivalent to 47.2 kJ/mol-H2, which is close to the experimental value of 42.6 kJ/mol-H2 determined by the Van't Hoff method (Fig. 12). The apparent activation energy of the Mg(NH2)2e3/2LiH mixture dehydrogenated under initial vacuum was 117.8 kJ/mol. A 57.0 kJ/mol increase in kinetic barrier was obtained in the Mg(NH2)2e3/2LiH mixture dehydrogenated under 9.0 bar Ar compared with the mixture dehydrogenated under initial vacuum. This result indicated that the kinetic barrier of the first-step dehydrogenation reaction of Eq. (2) was significantly affected by the gas back pressure. A high pressure of 9.0 bar hydrogen in the dehydrogenation process restrained thermodynamically and kinetically the evolution of hydrogen from the surface of the dehydrogenated products of the Mg(NH2)2e3/2LiH mixture during dehydrogenation. Although the partial pressures of H2 at the equilibrium state of the dehydrogenation under initial vacuum and 9.0 bar Ar upon heating to 280 C were almost the same, the same
Fig. 12 e Van't Hoff plot of the Mg(NH2)2e2LiH mixture milled for 36 h.
amount of H2 was still released under these two conditions (Fig. 5a). The high pressure of 9.0 bar Ar still kinetically restrained the evolution of hydrogen from the surface of the dehydrogenated products of the Mg(NH2)2e3/2LiH mixture compared with the initial vacuum, which contributed to the kinetic barrier affected by the gas back pressure. The enthalpy changes of the second-step dehydrogenation reaction of Eq. (2) were 29.8 kJ/mol (corresponded to 59.5 kJ/ mol-H2) in producing cubic Li2Mg(NH)2 and 24.6 kJ/mol (corresponded to 49.2 kJ/mol-H2) in producing orthorhombic Li2Mg(NH)2. This result indicates that the dehydrogenation from LiNH2eLi2Mg2(NH)3eLiH to orthorhombic Li2Mg(NH)2 possessed much favorable thermodynamics compared with cubic Li2Mg(NH)2. The calculated enthalpy change of 59.5 kJ/ mol-H2 for the dehydrogenation from LiNH2eLi2Mg2(NH)3eLiH to cubic Li2Mg(NH)2 was in good agreement with the experimental value of 59.7 kJ/mol-H2 determined by the Van't Hoff method combined with differential scanning calorimetry [25]. The apparent activation energies were 122.1 and 183.1 kJ/mol for the LiNH2eLi2Mg2(NH)3eLiH mixture dehydrogenated under initial vacuum and 9.0 bar Ar. This result implies that the dehydrogenation from LiNH2eLi2Mg2(NH)3eLiH to cubic Li2Mg(NH)2 possessed much favorable kinetics. Fig. 11 shows that the effect of the gas back pressure on the kinetic barrier of the fist-step reaction of Eq. (2) was less significant than that on the second-step reaction. The second-step reaction of Eq. (2) also showed much favorable thermodynamic properties but low kinetic properties on the first-step reaction under both initial vacuum and 9.0 bar Ar. The thermodynamic and kinetic analyses showed that the kinetic properties in the hydrogen storage process of the Mg(NH2)2e2LiH system could be significantly affected by the gas back pressure. This phenomenon results in strong dependence of the hydrogen storage properties of the Mg(NH2)2e2LiH system and the crystal structure of the dehydrogenated product of Li2Mg(NH)2 on the gas back pressure.
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Conclusions Ternary imides of cubic and orthorhombic Li2Mg(NH)2 were successfully prepared by desorbing the Mg(NH2)2e2LiH mixture from RT to 280 C under initial vacuum and 9.0 bar Ar or H2. The gas back pressure was found to be the key factor for the formation of different crystal structures of Li2Mg(NH)2. A low gas back pressure of 1.0e3.0 bar Ar favored the formation of cubic Li2Mg(NH)2 whereas a high gas back pressure of 9.0 bar H2 or Ar favored the formation of orthorhombic Li2Mg(NH)2. The crystal structure of Li2Mg(NH)2 was insensitive to gases, cooling rates, and the preparation temperatures in the range of 190e280 C. A strong dependence of dehydrogenation/rehydrogenation properties of the Mg(NH2)2e2LiH system on the gas back pressure was found. The dehydrogenation from Mg(NH2)2e2LiH to cubic Li2Mg(NH)2 under initial vacuum showed lower operating temperatures and higher dehydrogenation rates than to orthorhombic Li2Mg(NH)2 under a high gas back pressure of 9.0 bar Ar. Approximately 4.55 wt% of hydrogen was desorbed from the Mg(NH2)2e2LiH mixture within 700 min at 150 C under initial vacuum, whereas only 2.62 wt% of hydrogen was desorbed from the mixture dehydrogenated under 9.0 bar Ar. Cubic Li2Mg(NH)2 exhibited superior hydrogenation properties than orthorhombic Li2Mg(NH)2. The operating temperature for the hydrogenation of cubic Li2Mg(NH)2 lowered by ~30 C compared with orthorhombic Li2Mg(NH)2. Cubic Li2Mg(NH)2 also showed higher hydrogenation rates than orthorhombic Li2Mg(NH)2 in the temperature range of 130e170 C. Structural analyses revealed that the reaction pathways of the dehydrogenation of the Mg(NH2)2e2LiH system were found to be independent of the gas back pressure. LiNH2 and Li2Mg2(NH)3 were the intermediates in the dehydrogenation process of the Mg(NH2)2e2LiH mixture before the final product of Li2Mg(NH)2 was formed. The remarkable effect of the gas back pressure on kinetic properties in the hydrogen storage process of the Mg(NH2)2e2LiH system was mainly responsible for the strong dependence of the hydrogen storage properties and crystal structures of Li2Mg(NH)2 on the gas back pressure.
Acknowledgments We would like to acknowledge the financial supports from Ministry of Science and Technology (No. 2010CB631304), National Nature Science Foundations (Nos. 51025102 and 51201151), Postdoctoral Science Foundation (No. 2012M521168) and Zhejiang Postdoctoral Research Program (No. Bsh1202051) of People's Republic of China.
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Available online at www.sciencedirect.com
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Effect of gas back pressure on hydrogen storage properties and crystal structures of Li2Mg(NH)2 Chu Liang a,b, Mingxia Gao b, Hongge Pan b,*, Yongfeng Liu b, Mi Yan b a
College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, People's Republic of China b State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province & Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China
article info
abstract
Article history:
The ternary imide Li2Mg(NH)2 is considered to be one of the most promising on-board
Received 22 May 2014
hydrogen storage materials due to its high reversible hydrogen capacity of 5.86 wt%,
Received in revised form
favorable thermodynamic properties and good cycling stability. In this work, Li2Mg(NH)2
25 August 2014
was synthesized by dynamically dehydrogenating a mixture of Mg(NH2)2e2LiH up to 280 C
Accepted 1 September 2014
under different gas (Ar and H2) and pressures (0e9.0 bar). The crystal structure of
Available online 26 September 2014
Li2Mg(NH)2 was found to depend on the gas back pressure in the dehydrogenation process. The crystal structure of Li2Mg(NH)2 and the dehydrogenation/rehydrogenation properties
Keywords:
of the Mg(NH2)2e2LiH system strongly depend on the gas back pressure in the dehydro-
Hydrogen storage
genation process due to the effect of the pressure on the dehydrogenation kinetics. This
Imide
study provides a new approach for improving the hydrogen storage properties of the amide
Crystal structure
ehydride systems.
Kinetics
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Thermodynamics
Introduction A hydrogen economy based on renewable energy is one possible solution to the problems associated with the use of hydrocarbon fuels, such as the greenhouse gases they emit upon combustion, the ever increasing demand for their utilization, and their inevitable depletion. The feasibility of hydrogen economy mostly depends on the development of safe, efficient and low-cost hydrogen production and storage [1e3]. Hydrogen storage technology has become a bottleneck of the hydrogen economy. Solid-state hydrogen storage is considerably superior in term of safety compared with
gaseous- and liquid-state storage [1,4,5]. Over the past few decades, considerable efforts have been devoted to exploring light-weight metal hydrides, such as LiAlH4, LiBH4, NH3BH3, LiNH2e2LiH, Li2Mg(NH)2, and Li4BN3H10, for solid-state hydrogen storage [6e22]. Among these hydrogen storage materials, the ternary imide Li2Mg(NH)2 has attracted the most attention because of its high reversible capacity, favorable thermodynamics and good cycling stability for hydrogenation/dehydrogenation [23e25]. After absorbing 5.86 wt% of hydrogen, Li2Mg(NH)2 was converted to a Mg(NH2)2e2LiH mixture through the following reaction [26]: Li2 MgðNHÞ2 þ 2H2 4MgðNH2 Þ2 þ 2LiH
* Corresponding author. Tel./fax: þ86 571 87952615. E-mail address: [email protected] (H. Pan). http://dx.doi.org/10.1016/j.ijhydene.2014.09.013 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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The thermodynamic operating temperature for the dehydrogenation of the Mg(NH2)2e2LiH mixture under an equilibrium hydrogen pressure of 1.0 bar was predicted to be ~90 C [25]. Thus, it satisfies the practical requirement of the onboard proton exchange membrane fuel cells [27]. The crystal structure of Li2Mg(NH)2 possesses both cubic and orthorhombic phases, which were observed by several research groups [18,23,28e31]. Xiong et al. [23] detected a cubic Li2Mg(NH)2 by dynamically dehydrogenating a Mg(NH2)2e2LiH mixture from 50 to 300 C at a heating rate of 2 C/min under a flowing gas of Ar. Janot et al. [28] also obtained the cubic Li2Mg(NH)2 by isothermally dehydrogenating a 2LiNH2eMgH2 mixture at 200 C for 2 h under initial vacuum. Orthorhombic Li2Mg(NH)2 was observed by completely dehydrogenating the 2LiNH2eMgH2 [29] and Mg(NH2)2e2LiH [30] mixtures at 180 and 250 C, respectively, in a pressureecomposition (PeC) isotherm process. Our previous work demonstrated that Li2Mg(NH)2 with mixed cubic and orthorhombic phases can be obtained by desorbing NH3 from the Mg(NH2)2e2LiNH2 mixture at 315 C under a N2 atmosphere [18]. Rijssenbeek et al. [31] found that Li2Mg(NH)2 undergoes a transformation from orthorhombic to cubic phase at the temperature over 350 C under initial vacuum. These investigations show that cubic and orthorhombic Li2Mg(NH)2 can be obtained from different starting materials, operating temperatures, and desorption atmospheres. However, the key factors for controlling the crystal structure of Li2Mg(NH)2 remain unclear. Appreciable dehydrogenation rates of the Mg(NH2)2e2LiH mixture were experimentally observed only at the temperatures above 220 C under 1.0 bar hydrogen [32]. A high kinetic barrier from interface reaction and mass transport resulted in the dehydrogenation temperatures of the Mg(NH2)2e2LiH mixture much higher than the thermodynamically predicted equilibrium temperature of ~90 C under 1.0 bar hydrogen [18,32,33]. Considerable efforts have been made to reduce the kinetic barrier of the LieMgeNeH system by adding catalysts and reducing the particle size of the reactants [34e39]. The peak temperature of hydrogen desorption of the Mg(NH2)2e2LiH mixture was lowered from 190 C to less than 154 C by adding 0.5 mol of NaOH [34]. The combined system of Mg(NH2)2e1.9LiHe0.1 KH showed an even lower peak temperature of hydrogen desorption of ca. 132 C [35]. Liu et al. [18] and Xie et al. [36] reported that the hydrogen absorption/desorption kinetic property of the Mg(NH2)2e2LiH system was evidently improved by reducing the particle sizes. However, the dehydrogenation temperatures of the catalyst-added Mg(NH2)2e2LiH system remain higher than those of the thermodynamically predicted one. Further improvements in dehydrogenation/hydrogenation kinetics are highly needed to develop the Mg(NH2)2e2LiH system as an onboard hydrogen storage medium. Previous studies on Li2Mg(NH)2 as a hydrogen storage material mostly focus on kinetic improvements and hydrogen storage mechanisms. To date, the correlation between crystal structures and hydrogen storage properties of Li2Mg(NH)2 remains unclear. In this work, we initially focused on the structure-controllable synthesis of Li2Mg(NH)2 via dehydrogenating the Mg(NH2)2e2LiH mixture under various gas back pressures. The correlations among the crystal structures, gas back pressure, and hydrogen storage properties of Li2Mg(NH)2 as well as the corresponding hydrogen storage mechanisms
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were investigated by first-principles calculations combined with experimental measurements. This study is expected to elucidate the hydrogen storage mechanisms of the LieMgeNeH systems and to serve as a guide to further improve the hydrogen storage properties of Li2Mg(NH)2.
Experimental and computational section Experimental details Lithium hydride LiH (98%, Alfa Aesar) was used as-received without further purification. Magnesium amide Mg(NH2)2 was synthesized in-house by reacting magnesium powder (99%, Sinopharm) with ~7.0 bar ammonia at 300 C. A low-crystalline Li2Mg2(NH)3 sample was prepared by hydrogenating the post36 h milled Li2Mg2(NH)3 at 210 C under 300 bar hydrogen, followed by dehydrogenating the hydrogenated sample from room temperature (RT) to 250 C at a heating rate of 2 C/min under initial vacuum. Mg(NH2)2e2LiH mixture was prepared by ball milling Mg(NH2)2 with LiH in a molar ratio of 1:2 for 36 h. This preparation was carried out on a planetary ball mill (QM-3SP4, Nanjing) with a rotating speed of 500 rpm. The milling jars were filled with the hydrogen pressure of 78 bar to prevent hydrogen desorption during the ball milling. The ball-to-sample weight ratio was about 60:1. All sample handlings were carried out in an MBRAUN glovebox filled with purified argon to prevent contamination from moisture and air (H2O: <1 ppm, O2: <1 ppm). A homemade volumetric Sieverts-type apparatus was used to synthesize Li2Mg(NH)2 and to measure hydrogen desorption/absorption properties. About 200 mg of sample was used in each experiment. Li2Mg(NH)2 was prepared by isothermally and dynamically dehydrogenating the Mg(NH2)2e2LiH mixture at different temperatures under 0e9.0 bar Ar or H2. Three cooling rates of 0.5 C/min, 10e15 C/min, and 150 C/ min for furnace, air, and water cooling were used, respectively, to prepare Li2Mg(NH)2. For the dynamic desorption/ absorption measurements, samples were heated from RT to the preset temperature at a rate of 2 C/min for desorption and to 210 C at a rate of 1 C/min for absorption. The samples for the isothermal desorption and absorption measurements were heated to 150 C at a rate of 15 C/min and dwelled for 750 and 1250 min, respectively. An initial hydrogen pressure of 105 bar was used in the isothermal and dynamic absorption of Li2Mg(NH)2. Initial vacuum and 9.0 bar Ar were applied in the isothermal and dynamic desorption of the Mg(NH2)2e2LiH mixture. The detailed desorption/absorption procedure can be found in our previous work [16]. The experimental enthalpy change of hydrogen desorption was determined by the Van't Hoff method. The homemade Sieverts-type apparatus was used to measure the equilibrium hydrogen pressures at several preset temperatures from 180 to 220 C. A total of 500 mg of the Mg(NH2)2e2LiH mixture was used for these measurements. Equilibrium state was reached when the hydrogen pressure remained unchanged at the preset temperature for 10 h. The Kissinger's method was used to determine the apparent activation energy Ea of dehydrogenation. The temperature for the maximum dehydrogenation rate was obtained from the hydrogen desorption curves measured at heating rates of 5, 6, 7.5, 10 and 15 C/min.
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Crystal structures of samples were characterized by an X'Pert Pro diffractometer with Cu Ka radiation at 40 kV and 40 mA. X-ray diffraction (XRD) data were collected in a 2q range of 10e90 with a step of 0.05 . A homemade container was applied to prevent the powder samples from air and moisture contamination. NeH vibrations of all samples were identified with a Bruker TENSOR 27 Fourier infrared spectrometer (FTIR, Germany) in a transmission mode. Each pellet sample was prepared by cold pressing the mixture of the powder sample and potassium bromide in a weight ratio of about 1:30. FTIR spectra were plotted from 32 scans that were averaged using a resolution of 4 cm1. The morphologies and particle sizes of the as-prepared Li2Mg(NH)2 were observed under a scanning electron microscopy (SEM, Hitachi-S4800).
Computational details The total energies and crystal structures of the related phases in this work were calculated by CASTEP software [40] based on the density functional theory (DFT). The generalized gradient approximation proposed by Perdew et al. (PW91) [41] was employed to include the exchange-correlation energy in the total energy. The ultrasoft pseudopotential was adopted to describe the ioneelectron interactions. Electronic wave functions were expanded on the basis of plane waves with a wellconverged cut-off energy of 490 eV. The Brillouin zone was sampled by a sum over special k-points generated with the MonkhorstePack scheme. The zone was set as 4 4 2 for Mg(NH2)2, 8 8 8 for LiH, 6 6 3 for LiNH2, 2 6 3 for Li2Mg2(NH)3, 6 6 2 for cubic Li2Mg(NH)2, 3 6 6 for orthorhombic Li2Mg(NH)2, and Gamma-point for H2 molecule. A finite basis set correction was used in the evaluation of energy and stress, and the Pulay density-mixing scheme was adopted for self-consistent field calculations. Cubic Li2Mg(NH)2 in the space group P-43m [31] and orthorhombic Li2Mg(NH)2 in the space group Iba2 [31] were employed in our calculations.
All structures calculated were fully relaxed in volume, atomic coordinates, and shape in the structural optimization until the forces <0.03 eV/Å and the stresses <0.05 GPa. Hydrogen was calculated by placing one hydrogen molecule inside a fixed cell of 10 10 10 Å. Finite temperature enthalpy was evaluated within the harmonic approximation [42]. DFT as implanted within the Dmol3 program was used to calculate the harmonic vibrational frequencies. These frequencies were computed by diagonalizing the mass-weighted Hessian matrix of the unit cell. The double numerable plus polarization basis set and the exchange-correlation function of GGA-PW91 were both employed in our calculations. DFT semi-core pseudopots were used to treat the core electrons of the elements of Li, Mg, N, and H. Mixed Li/Mg cation arrangements are present in the crystal lattice of cubic and orthorhombic Li2Mg(NH)2 [31]. In this study, a series of 1 1 3 supercells for cubic Li2Mg(NH)2 and cells for orthorhombic Li2Mg(NH)2 with possible arrangements of Li and Mg cations on the mixed positions were used to optimize geometry and calculate the total energy. The crystal models with the lowest total energy were employed in the thermodynamic calculations of cubic and orthorhombic Li2Mg(NH)2, respectively. The Rietveld refinement results of the XRD data showed that the calculated curves were in good agreement with the experimental curves of Li2Mg(NH)2 [43]. Thus, the crystal models of Li2Mg(NH)2 are reasonable to be used in the calculations.
Results and discussion Effect of gas back pressure on crystal structures of Li2Mg(NH)2 Fig. 1 shows the XRD patterns and FTIR spectra of the Mg(NH2)2e2LiH mixture dehydrogenated at different temperatures under initial vacuum. For the Mg(NH2)2e2LiH mixture
Fig. 1 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dehydrogenated at 190 C for 15 h and from RT to 280 C under initial vacuum.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 7 7 5 4 e1 7 7 6 4
Fig. 2 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dehydrogenated at 190 C for 15 h and from RT to 280 C.
both isothermally dehydrogenated at 190 C for 15 h and dynamically dehydrogenated from RT to 280 C, the XRD peaks of the dehydrogenation products corresponded to the cubic Li2Mg(NH)2 phase (Fig. 1a), as reported by Chen et al. [26] and Rijssenbeek et al. [31]. In contrast to the Mg(NH2)2e2LiH mixture dehydrogenated at 190 C, two weak XRD peaks at 21.6 and 33.3 were observed in the XRD pattern of the mixture dehydrogenated to 280 C. This observation indicates that trace Mg3N2 may be present in the dehydrogenation product. FTIR examination (Fig. 1b) presents the absorption band of the NeH vibration of cubic Li2Mg(NH)2 centered at ~3174 cm1 [37] from both dehydrogenation products. In the aforementioned dehydrogenation processes, the hydrogen pressure in the Sieverts-type apparatus was found to increase from the initial vacuum to ca. 0.8 bar, the amount of which equals to the hydrogen released from the Mg(NH2)2e2LiH mixture to Li2Mg(NH)2. Therefore, the cubic Li2Mg(NH)2 can be synthesized by dehydrogenating the Mg(NH2)2e2LiH mixture either at 190 C for 15 h or from RT to 280 C under initial vacuum. Fig. 2 exhibits the XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dehydrogenated at 190 C for 15 h and from RT to 280 C with initial hydrogen pressure of 9.0 bar.
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The product dehydrogenated at 190 C for 15 h was mainly composed of orthorhombic Li2Mg(NH)2 with a little amount of residual LiH and newly formed Li2Mg2(NH)3 and LiNH2. This result implies that an incomplete dehydrogenation of the Mg(NH2)2e2LiH mixture occurred because of the high hydrogen pressure. For the product dehydrogenated from RT to 280 C, the XRD peaks (Fig. 2a) were attributed to those of the orthorhombic Li2Mg(NH)2 reported by Luo et al. [29] and Rijssenbeek et al. [31]. Fig. 2b, shows that the product dehydrogenated to 280 C only had two FTIR absorption peaks of an imide centered at about 3182 and 3161 cm1, which are very close to those of the orthorhombic Li2Mg(NH)2 reported by Markmaitree et al. [44,45]. This result confirms the formation of orthorhombic Li2Mg(NH)2. After the dynamic dehydrogenation to 280 C, the hydrogen pressure in the Sieverts-type apparatus also increased by ~0.8 bar, which is close to the pressure change of the Mg(NH2)2e2LiH mixture dehydrogenated under initial vacuum. This result indicates that almost the same amounts of hydrogen were released in the two dehydrogenation processes despite the formation of cubic or orthorhombic Li2Mg(NH)2. Moreover, orthorhombic Li2Mg(NH)2 was also successfully synthesized by dehydrogenating the Mg(NH2)2e2LiH mixture from RT to 280 C when 9.0 bar Ar pressure was used in the dehydrogenation process (Fig. 2). However, further experiments that only the cubic Li2Mg(NH)2 was formed in the dehydrogenation of the Mg(NH2)2e2LiH mixture from RT to 280 C under Ar with lower pressure of 1.0 and 3.0 bar. Different crystal structures of Li2Mg(NH)2 were formed under initial vacuum and different pressures of Ar or H2 at the same temperature from RT to 280 C. This result indicates that the gas back pressure in the dehydrogenation process of the Mg(NH2)2e2LiH mixture played an important role in determining the crystal structures of the dehydrogenation product of Li2Mg(NH)2. A high H2 or Ar pressure of 9.0 bar favored the formation of orthorhombic Li2Mg(NH)2, whereas a low Ar pressure of 1.0e3.0 bar or initial vacuum favored the formation of cubic Li2Mg(NH)2. The formation of the same crystal structures of Li2Mg(NH)2 at the temperature of 190 C for 15 h and from RT to 280 C indicated that the crystal structure of Li2Mg(NH)2 was not evidently affected by the preparation temperatures (190e280 C). Further temperature-induced phase transition of Li2Mg(NH)2 was reported in our previous paper [43]. In addition, XRD and FTIR analyses (Figs. 3 and 4) of the samples were carried out under initial vacuum and 9.0 bar Ar with different cooling rates of 0.5, 10e15 and 150 C/min. The results of the analyses confirmed that the crystal structure of Li2Mg(NH)2 was not affected by the cooling rates. Consequently, the crystal structure of Li2Mg(NH)2 was strongly determined by the gas back pressure in the dehydrogenation process of the Mg(NH2)2e2LiH system but independent of gas, cooling rates and preparation temperatures in the range of 190e280 C.
Effect of gas back pressure on dehydrogenation/ hydrogenation properties The effect of the gas back pressure on the dehydrogenation behavior of the Mg(NH2)2e2LiH system was evaluated by isothermal and dynamic dehydrogenation. Fig. 5a,b depict
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Fig. 3 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dehydrogenated from RT to 280 C under initial vacuum and then cooled at various rates.
both the dynamic and isothermal dehydrogenation curves, respectively, of the Mg(NH2)2e2LiH mixture dehydrogenated under initial vacuum and 9.0 bar Ar. Apparently, the onset dehydrogenation temperature and the dehydrogenation rates of the Mg(NH2)2e2LiH system were remarkably affected by the gas back pressure. Fig. 5a shows that the operating temperatures of the Mg(NH2)2e2LiH mixture dehydrogenated under initial vacuum were lower than those under 9.0 bar Ar. A higher dehydrogenation rate was also achieved for the initial vacuum. Approximately 4.55 wt% of hydrogen was desorbed from the Mg(NH2)2e2LiH mixture within 700 min at 150 C under initial vacuum, whereas only 2.62 wt% was desorbed
from the 9.0 bar Ar. Analyzing the tangent slop of the linear part of the dehydrogenation curves, an average rate was determined to be ~0.033 wt%/min for the Mg(NH2)2e2LiH mixture dehydrogenated under initial vacuum. This average rate was 3.3 times higher than that obtained under 9.0 bar Ar (0.010 wt%/min). A similar phenomenon of the influence of hydrogen pressure on the dehydrogenation properties was also found in the LiBH4eMetal hydride composite and Ca(BH4)2 systems for hydrogen storage [46e48]. Cubic and orthorhombic Li2Mg(NH)2 were initially prepared by dehydrogenating the Mg(NH2)2e2LiH mixture from RT to 280 C under initial vacuum and 9.0 bar Ar. This
Fig. 4 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dehydrogenated from RT to 280 C under 9.0 bar Ar and then cooled at various rates.
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Fig. 5 e Dehydrogenation curves of the Mg(NH2)2e2LiH mixture dehydrogenated under initial vacuum and 9.0 bar Ar.
Fig. 6 e Dynamic (a) and isothermal (b) hydrogenation curves of the cubic and orthorhombic Li2Mg(NH)2 hydrogenated under 105 bar hydrogen.
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preparation was performed to elucidate the effect of gas back pressure on the hydrogenation properties of the completely dehydrogenated Mg(NH2)2e2LiH mixture. Fig. 6 shows the dynamic and isothermal hydrogenation curves of the cubic and orthorhombic Li2Mg(NH)2 hydrogenated under 105 bar hydrogen. In the dynamic process, both cubic and orthorhombic Li2Mg(NH)2 absorbed 5.74 wt% of hydrogen (Fig. 6a), which is close to the theoretical hydrogen absorption capacity of 5.86 wt% of Li2Mg(NH)2. This result indicates that both the cubic and orthorhombic Li2Mg(NH)2 were completely converted to the Mg(NH2)2e2LiH mixture after hydrogenation at 210 C. However, the cubic and orthorhombic Li2Mg(NH)2 evidently exhibited different hydrogenation behavior. As seen in Fig. 6a, the onset hydrogenation temperature was only ca. 102 C for the cubic Li2Mg(NH)2, whereas ~132 C for the orthorhombic Li2Mg(NH)2. The hydrogenation rate of the cubic Li2Mg(NH)2 was much higher than that of the orthorhombic Li2Mg(NH)2 in the temperature range of 130e170 C. The isothermal hydrogenation rate of both cubic and orthorhombic Li2Mg(NH)2 at 150 C (Fig. 6b) was high within the first 10 min, but become low in the next stage. The hydrogen absorbed for the cubic Li2Mg(NH)2 (1.90 wt%) was still higher than that for the orthorhombic Li2Mg(NH)2 (0.42 wt%). After the initial 10 min, the hydrogenation rates of both the cubic and orthorhombic Li2Mg(NH)2 turned to be a similarly low value (~0.0018 wt%/min). Our previous work [18] revealed that the hydrogenation properties of Li2Mg(NH)2 strongly depend on its particle sizes. Li2Mg(NH)2 with smaller particle sizes exhibit lower hydrogenation temperatures and higher hydrogenation rates. The particle sizes of the as-prepared cubic and orthorhombic Li2Mg(NH)2 were observed under SEM to determine the correlation between its crystal structures and hydrogenation properties. Fig. 7 presents the SEM morphologies of as-prepared Li2Mg(NH)2. The particles of both cubic and orthorhombic Li2Mg(NH)2 were irregular in shape. Cubic Li2Mg(NH)2 generally possessed larger particle sizes than the orthorhombic Li2Mg(NH)2, but still exhibited higher hydrogenation rates and lower hydrogenation temperatures. This result supports that cubic Li2Mg(NH)2 exhibited better hydrogenation properties than the orthorhombic Li2Mg(NH)2.
Fig. 7 e SEM micrographs of the as-prepared cubic (a) and orthorhombic (b) Li2Mg(NH)2.
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Effect of gas back pressure on dehydrogenation reaction pathways The effect of gas back pressure on dehydrogenation reaction pathways was initially investigated to reveal the mechanism of the dehydrogenation/hydrogenation properties of the Mg(NH2)2e2LiH system that is associated with the gas back pressure. The Mg(NH2)2e2LiH mixture dynamically dehydrogenated to different stages under both initial vacuum and 9.0 bar Ar was collected for XRD and FTIR analyses. Fig. 8a,b shows the XRD patterns and FTIR spectra of the Mg(NH2)2e2LiH mixture dynamically dehydrogenated to different temperatures under initial vacuum, respectively. The residual phases of Mg(NH2)2 and LiH were identified in the Mg(NH2)2e2LiH mixture dehydrogenated to 165 C (Fig. 8a). In addition to the typical NeH vibrations of Mg(NH2)2 at ~3326 and 3271 cm1, a new absorption peak of an unknown imide centered at ~3189 cm1 was observed in the FTIR spectrum (Fig. 8b). The increase in dehydrogenation to 180 C resulted in elevated intensity of the absorption peak centered at ~3189 cm1 and the shift of absorption peak to a low wavenumber of ~3183 cm1, which is close to that of the
Fig. 8 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dynamically dehydrogenated to different temperatures under initial vacuum.
low-crystalline Li2Mg2(NH)3 (Fig. 8b). Meanwhile, the FTIR absorption peaks of LiNH2 at ~3312 and 3258 cm1 developed and the absorption peaks of Mg(NH2)2 at ~3326 and 3271 cm1 significantly weakened. Fig. 8a shows that the diffraction peaks of the Mg(NH2)2 and LiH phases weakened whereas a set of diffraction peaks at 30.5, 51.2 and 60.9 significantly intensified with the increase of dehydrogenation temperature to 180 C. Although the XRD peaks of Li2Mg2(NH)3 were mostly overlapped with those of the LiNH2, the exclusively intrinsic diffraction peaks of Li2Mg2(NH)3 at 18.2, 19.4 and 40.2 were still detected, confirming the existence of Li2Mg2(NH)3. The XRD and FTIR results indicated that Mg(NH2)2 and LiH were consumed to convert to LiNH2 and Li2Mg2(NH)3 after the dehydrogenation to 180 C. For the Mg(NH2)2e2LiH mixture dehydrogenated to 200 C, the peaks of cubic Li2Mg(NH)2 dominated the XRD pattern and FTIR spectrum. Fig. 8b shows broad and weak FTIR absorption bands centered at ~3258 cm1, indicating that an incomplete dehydrogenation occurred when the temperature was increased to 200 C. A single phase of cubic Li2Mg(NH)2 was produced after the dehydrogenation temperature was further elevated to 280 C. Fig. 9 shows the XRD patterns and FTIR spectra of the Mg(NH2)2e2LiH mixture dynamically dehydrogenated at different temperatures under 9.0 bar Ar. The residual phases of Mg(NH2)2 and LiH, and the new phase of LiNH2 were detected in the Mg(NH2)2e2LiH mixture dehydrogenated to 190 C. A broad NeH vibration of a new imide centered at ~3184 cm1, which is close to that of the low-crystalline Li2Mg2(NH)3, was also developed as shown in Fig. 9b. As discussed above (Fig. 2), Li2Mg2(NH)3 and LiNH2 were intermediates in the dehydrogenation of the Mg(NH2)2e2LiH mixture dehydrogenated at 190 C for 15 h under 9.0 bar hydrogen (Fig. 2). Therefore, Li2Mg2(NH)3 was possibly formed in the present dehydrogenation process. A previous literature [26] reported that the intermediates of Li2Mg2(NH)3 and LiNH2 were also detected in the Mg(NH2)2e2LiH mixture dehydrogenated at 220 C under an equilibrium hydrogen pressure of ~20 bar. The increase in dehydrogenation temperature to 205 C resulted in the dehydrogenated product showing evident relative intensities of LiNH2 and Li2Mg2(NH)3 in their XRD peaks and FTIR absorption, and almost undetectable Mg(NH2)2 and LiH. A broad diffraction peak in the 2q angle range of 16e20 originated from the overlapped diffraction peaks of LiNH2 and Li2Mg2(NH)3 was also observed. These results indicate that LiH and Mg(NH2)2 were consumed and converted to LiNH2 and Li2Mg2(NH)3, respectively, at 205 C under 9.0 bar Ar. The orthorhombic Li2Mg(NH)2 dominated in the XRD pattern and FTIR spectrum as the dehydrogenated temperature was further elevated to 250 C. Then, a single phase of orthorhombic Li2Mg(NH)2 was produced after the dehydrogenation to 280 C. The information obtained from XRD and FTIR supported that Li2Mg2(NH)3 and LiNH2 are the intermediates in the dehydrogenation process of the Mg(NH2)2e2LiH mixture before the final product of Li2Mg(NH)2 is formed. The dehydrogenation of the Mg(NH2)2e2LiH mixture under both initial vacuum and 9.0 bar Ar can be described by the following reaction:
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operating temperature of the LiNH2eLi2Mg2(NH)3eLiH mixture dehydrogenated under initial vacuum was apparently lower than that of the Mg(NH2)2e3/2LiH mixture, but exhibited a reverse phenomenon for the mixtures dehydrogenated under 9.0 bar Ar. This result may explain the lower operating temperatures and higher dehydrogenation rates of the Mg(NH2)2e2LiH mixture under initial vacuum with respect to that under 9.0 bar Ar (Fig. 5). Fig. 10 shows that the effect of gas back pressure on the dehydrogenation temperature of the LiNH2eLi2Mg2(NH)3eLiH mixture was greater than that on the Mg(NH2)2e3/2LiH mixture. In comparison with the mixtures dehydrogenated under 9.0 bar Ar, the operating temperature was decreased by ca. 35 C for the LiNH2eLi2Mg2(NH)3eLiH mixture dehydrogenated under initial vacuum, whereas it was only ~10 C for the Mg(NH2)2e3/2LiH mixture. The operating temperature of the first-step dehydrogenation reaction of the Mg(NH2)2e2LiH system depended on the gas back pressure but the reaction pathway was independent of the gas back pressure. Thus, the gas back pressure only changed the hydrogen storage kinetics of the first-step reaction. By contrast, the gas back pressure changed both the hydrogen storage thermodynamics and kinetics of the second-step reaction owing to the considerable effect of the gas back pressure on the dehydrogenation properties of the second-step reaction and the different crystal structures of the dehydrogenation product of Li2Mg(NH)2. To verify the effect of the gas back pressure on the thermodynamics and kinetics of the two-step reaction of Eq. (2), the enthalpy changes and apparent activation energies of the and dehydrogenation of Mg(NH2)2e3/2LiH LiNH2eLi2Mg2(NH)3eLiH mixtures under initial vacuum and 9.0 bar Ar were determined by the first-principles calculation and Kissinger's method, respectively. The results are shown in Fig. 11. Our calculations showed that the difference in the thermodynamic enthalpies for the Li2Mg(NH)2 under vacuum and 9.0 bar Ar was less than 0.1 kJ/mol. Thus, the effect of the gas back pressure on the enthalpies was not considered in Fig. 11. For the first-step dehydrogenation reaction, the calculated enthalpy change calculated was 70.8 kJ/mol,
Fig. 9 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2e2LiH mixture dynamically dehydrogenated to different temperatures under 9.0 bar Ar.
1 1 1 MgðNH2 Þ2 þ 2LiH/ Li2 Mg2 ðNHÞ3 þ LiNH2 þ LiH 2 2 2 3 þ H2 /Li2 MgðNHÞ2 þ 2H2 2
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(2)
Mechanism for hydrogen storage properties and crystal structures associated with gas back pressure The reaction pathways of the dehydrogenation of the Mg(NH2)2e2LiH mixture were independent of the gas back pressure. By contrast, the dehydrogenation properties of the Mg(NH2)2e2LiH mixture and the crystal structure of the dehydrogenation product of Li2Mg(NH)2 were strongly dependent on the gas back pressure. To examine the effect of the gas back pressure on the first- and second-step dehydrogenation reactions of Eq. (2), a Mg(NH2)2e3/2LiH mixture of the reactant for the first-step reaction and a LiNH2eLi2Mg2(NH)3eLiH mixture of the reactant for the second-step reaction were prepared by ball milling. Fig. 10 shows the dehydrogenation curves of the Mg(NH2)2e3/2LiH and LiNH2eLi2Mg2(NH)3eLiH mixtures dehydrogenated from RT to 280 C under both initial vacuum and 9.0 bar Ar. The
Fig. 10 e Dynamic dehydrogenation curves of the Mg(NH2)2e3/2LiH and LiNH2eLi2Mg2(NH)3eLiH mixtures dehydrogenated under initial vacuum and 9.0 bar Ar.
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Fig. 11 e Schematic enthalpy changes and apparent activation energies of the as-milled Mg(NH2)2e3/2LiH and LiNH2eLi2Mg2(NH)3eLiH mixtures dehydrogenated under both initial vacuum and 9.0 bar Ar.
equivalent to 47.2 kJ/mol-H2, which is close to the experimental value of 42.6 kJ/mol-H2 determined by the Van't Hoff method (Fig. 12). The apparent activation energy of the Mg(NH2)2e3/2LiH mixture dehydrogenated under initial vacuum was 117.8 kJ/mol. A 57.0 kJ/mol increase in kinetic barrier was obtained in the Mg(NH2)2e3/2LiH mixture dehydrogenated under 9.0 bar Ar compared with the mixture dehydrogenated under initial vacuum. This result indicated that the kinetic barrier of the first-step dehydrogenation reaction of Eq. (2) was significantly affected by the gas back pressure. A high pressure of 9.0 bar hydrogen in the dehydrogenation process restrained thermodynamically and kinetically the evolution of hydrogen from the surface of the dehydrogenated products of the Mg(NH2)2e3/2LiH mixture during dehydrogenation. Although the partial pressures of H2 at the equilibrium state of the dehydrogenation under initial vacuum and 9.0 bar Ar upon heating to 280 C were almost the same, the same
Fig. 12 e Van't Hoff plot of the Mg(NH2)2e2LiH mixture milled for 36 h.
amount of H2 was still released under these two conditions (Fig. 5a). The high pressure of 9.0 bar Ar still kinetically restrained the evolution of hydrogen from the surface of the dehydrogenated products of the Mg(NH2)2e3/2LiH mixture compared with the initial vacuum, which contributed to the kinetic barrier affected by the gas back pressure. The enthalpy changes of the second-step dehydrogenation reaction of Eq. (2) were 29.8 kJ/mol (corresponded to 59.5 kJ/ mol-H2) in producing cubic Li2Mg(NH)2 and 24.6 kJ/mol (corresponded to 49.2 kJ/mol-H2) in producing orthorhombic Li2Mg(NH)2. This result indicates that the dehydrogenation from LiNH2eLi2Mg2(NH)3eLiH to orthorhombic Li2Mg(NH)2 possessed much favorable thermodynamics compared with cubic Li2Mg(NH)2. The calculated enthalpy change of 59.5 kJ/ mol-H2 for the dehydrogenation from LiNH2eLi2Mg2(NH)3eLiH to cubic Li2Mg(NH)2 was in good agreement with the experimental value of 59.7 kJ/mol-H2 determined by the Van't Hoff method combined with differential scanning calorimetry [25]. The apparent activation energies were 122.1 and 183.1 kJ/mol for the LiNH2eLi2Mg2(NH)3eLiH mixture dehydrogenated under initial vacuum and 9.0 bar Ar. This result implies that the dehydrogenation from LiNH2eLi2Mg2(NH)3eLiH to cubic Li2Mg(NH)2 possessed much favorable kinetics. Fig. 11 shows that the effect of the gas back pressure on the kinetic barrier of the fist-step reaction of Eq. (2) was less significant than that on the second-step reaction. The second-step reaction of Eq. (2) also showed much favorable thermodynamic properties but low kinetic properties on the first-step reaction under both initial vacuum and 9.0 bar Ar. The thermodynamic and kinetic analyses showed that the kinetic properties in the hydrogen storage process of the Mg(NH2)2e2LiH system could be significantly affected by the gas back pressure. This phenomenon results in strong dependence of the hydrogen storage properties of the Mg(NH2)2e2LiH system and the crystal structure of the dehydrogenated product of Li2Mg(NH)2 on the gas back pressure.
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Conclusions Ternary imides of cubic and orthorhombic Li2Mg(NH)2 were successfully prepared by desorbing the Mg(NH2)2e2LiH mixture from RT to 280 C under initial vacuum and 9.0 bar Ar or H2. The gas back pressure was found to be the key factor for the formation of different crystal structures of Li2Mg(NH)2. A low gas back pressure of 1.0e3.0 bar Ar favored the formation of cubic Li2Mg(NH)2 whereas a high gas back pressure of 9.0 bar H2 or Ar favored the formation of orthorhombic Li2Mg(NH)2. The crystal structure of Li2Mg(NH)2 was insensitive to gases, cooling rates, and the preparation temperatures in the range of 190e280 C. A strong dependence of dehydrogenation/rehydrogenation properties of the Mg(NH2)2e2LiH system on the gas back pressure was found. The dehydrogenation from Mg(NH2)2e2LiH to cubic Li2Mg(NH)2 under initial vacuum showed lower operating temperatures and higher dehydrogenation rates than to orthorhombic Li2Mg(NH)2 under a high gas back pressure of 9.0 bar Ar. Approximately 4.55 wt% of hydrogen was desorbed from the Mg(NH2)2e2LiH mixture within 700 min at 150 C under initial vacuum, whereas only 2.62 wt% of hydrogen was desorbed from the mixture dehydrogenated under 9.0 bar Ar. Cubic Li2Mg(NH)2 exhibited superior hydrogenation properties than orthorhombic Li2Mg(NH)2. The operating temperature for the hydrogenation of cubic Li2Mg(NH)2 lowered by ~30 C compared with orthorhombic Li2Mg(NH)2. Cubic Li2Mg(NH)2 also showed higher hydrogenation rates than orthorhombic Li2Mg(NH)2 in the temperature range of 130e170 C. Structural analyses revealed that the reaction pathways of the dehydrogenation of the Mg(NH2)2e2LiH system were found to be independent of the gas back pressure. LiNH2 and Li2Mg2(NH)3 were the intermediates in the dehydrogenation process of the Mg(NH2)2e2LiH mixture before the final product of Li2Mg(NH)2 was formed. The remarkable effect of the gas back pressure on kinetic properties in the hydrogen storage process of the Mg(NH2)2e2LiH system was mainly responsible for the strong dependence of the hydrogen storage properties and crystal structures of Li2Mg(NH)2 on the gas back pressure.
Acknowledgments We would like to acknowledge the financial supports from Ministry of Science and Technology (No. 2010CB631304), National Nature Science Foundations (Nos. 51025102 and 51201151), Postdoctoral Science Foundation (No. 2012M521168) and Zhejiang Postdoctoral Research Program (No. Bsh1202051) of People's Republic of China.
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