Synthesis of CsH and its effect on the hydrogen storage properties of the Mg(NH2)2-2LiH system

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Synthesis of CsH and its effect on the hydrogen storage properties of the Mg(NH2)2-2LiH system Jiaxun Zhang a, Yongfeng Liu a,*, Xin Zhang a, Yaxiong Yang a, Qihang Zhang a, Ting Jin a, Yuxuan Wang a, Mingxia Gao a, Lixian Sun b, Hongge Pan a a

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China b Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, Guangxi Key Laboratory of Information Material, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China

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abstract

Article history:

Cesium hydride (CsH) was successfully synthesized by ball milling Cs under a hydrogen

Received 21 February 2016

atmosphere of 50 bar at 50
C. The effect of the prepared CsH on the hydrogen storage

Received in revised form

properties of the Mg(NH2)2-2LiH system was systematically investigated. The Mg(NH2)2-

22 March 2016

2LiH-0.08CsH composite exhibited optimal hydrogen storage properties because it revers-

Accepted 9 April 2016

ibly stores approximately 4.62 wt% hydrogen with a dehydrogenation onset temperature of

Available online 28 April 2016

70
C via a two-stage reaction. At 150
C, approximately 80% of the reversible capacity was

Keywords:

dehydrogenated CsH-containing sample began to absorb hydrogen at 55
C and took up

Hydrogen storage

approximately 4.58 wt% hydrogen at 130
C. A cycling analysis indicated that the CsH-

quickly released from the Mg(NH2)2-2LiH-0.08CsH composite within 100 min. The fully

Amides

containing Mg(NH2)2-2LiH system exhibited good reversible hydrogen storage abilities.

Hydrides

Detailed mechanistic studies revealed that during the initial heating process, CsH gradually

Additives

reacted with Mg(NH2)2 to afford CsMg(NH)(NH2), and CsH acted as a catalyst to reduce the

Hydrogen storage properties

activation energy barrier of the first dehydrogenation step. As the operating temperature increased, CsMg(NH)(NH2) as a reactant participated in a second dehydrogenation step to decrease the desorption enthalpy change. This behavior reasonably explains the significantly improved hydrogen storage properties of the CsH-containing Mg(NH2)2-2LiH system. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Hydrogen is an ideal synthetic fuel for future energy systems due to its lightweight, abundance, lack of greenhouse gas

emissions, and potentially renewable nature [1]. A key technological challenge for the large-scale use of hydrogen involves its safe, efficient and economical storage [1e4]. Recently, metaleNeH systems have attracted increasing attention since the reversible storage of 11.5 wt% hydrogen

* Corresponding author. Tel./fax: þ86 571 87952615. E-mail address: [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.ijhydene.2016.04.057 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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was reported for Li3N [5]. A variety of metal amide-hydride combinations including LieNeH, MgeNeH, LieMgeNeH and LieCaeNeH have been investigated and developed [5e9]. Among these combinations, the Mg(NH2)2-2LiH combination is considered to be one of the most promising hydrogen storage media due to its relatively high hydrogen capacity, good reversibility, and favorable thermodynamics [7e9]. Approximately 5.6 wt% of hydrogen was reversibly stored in the Mg(NH2)2-2LiH system through the following reaction [8]:

Mg(NH2)2 þ 2LiH 4 Li2MgN2H2 þ 2H2

(1)

However, a large kinetic barrier (Ea: ~102e120 kJ/mol) results in high operating temperatures for hydrogen storage in this reaction (i.e., typically higher than 200
C) [7,10]. Considerable efforts have been focused on lowering the operating temperature and enhancing the dehydrogenation kinetics of the LieMgeNeH system, and these efforts have included adjusting the composition, reducing the particle size, and additive doping [11e29]. Studies of the Mg(NH2)2xLiH mixtures (x ¼ 1.5e2.7) confirmed that the optimal molar ratio for combining Mg(NH2)2 and LiH was 1:2 [11]. The addition of a small amount of TiN, VCl3, graphite-supported Ru, Li3AlH6, LiBH4, NaBH4, Mg(BH4)2, Ca(BH4)2, CaH2, CaCl2 or LiCl can be effective for enhancing the de-/hydrogenation kinetics of the Mg(NH2)2-2LiH system [12e22]. The Mg(NH2)22LiH-0.1Ca(BH4)2 sample exhibited an approximately 4 wt% reversible hydrogen storage capacity below 140
C due to the concerted effects of the in situ generated CaH2 and LiBH4 during ball milling [19]. More importantly, the presence of Na, K or Rb in the LieMgeNeH system significantly improved its hydrogen storage properties. The majority of hydrogen desorption in the Mg(NH2)2-2LiH-0.5NaOH system was completed below 175
C, which is 36
C less than that of the pristine sample [23]. The peak dehydrogenation temperature of the Mg(NH2)2-1.9LiH-0.1KH sample was reduced from 186 to 132
C, and reversible hydrogen storage was achieved at temperatures as low as 107
C in a pressure-compositiontemperature (PCT) model [24]. Further investigations of KF and RbF also confirmed their positive effects for reducing the de-/hydrogenation temperatures of the LieMgeNeH systems [25,26]. In particular, the K and Rb codoped Mg(NH2)2-2LiH composite was able to reversibly store 5.2 wt% H2 while operating at 130
C, and more than 93% of the hydrogen storage capacity remained after 50 cycles [27]. Moreover, it is important to note that the Rb-containing sample exhibited much better hydrogenation kinetics than that of the K-doped sample [26]. Therefore, the K and Rb-based additives offer superior catalytic function, which reduces the operating temperatures and enhances the reaction kinetics for hydrogen storage in the Mg(NH2)2-2LiH system. Based on physical and chemical similarities, the Cs-based compounds (i.e., CsH) are also expected to exhibit the ability to improve hydrogen storage properties of the Mg(NH2)2-2LiH system, especially for the hydrogenation kinetics. However, systematic studies on the effects of CsH on the hydrogen storage behavior of the Mg(NH2)2-2LiH system have not been previously reported, and the role played by CsH remains unclear.

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In this study, CsH was successfully synthesized by ball milling molten Cs under a H2 atmosphere at 50
C. The effects of the prepared CsH on the hydrogen storage behavior of the Mg(NH2)2-2LiH system were investigated in detail. The results indicated that the Mg(NH2)2-2LiH-0.08CsH composite reversibly stored approximately 4.62 wt% hydrogen starting at 70
C. At 150
C, approximately 80% of the reversible capacity was quickly released from the Mg(NH2)2-2LiH-0.08CsH composite within 100 min. The fully dehydrogenated CsH-containing sample started absorbing hydrogen at 55
C, and approximately 4.58 wt% of hydrogen was taken up at 130
C. Structural analyses revealed the role of CsH based on analysis of its chemical state at different dehydrogenation stages.

Experimental section Commercial LiH (purity 98%) was purchased from Alfa Aesar and used as received. Mg(NH2)2 was produced in-house by directly reacting Mg powder (99%, Sinopharm) with 7 bar of NH3 at 300
C. CsH was synthesized in our laboratory by ball milling metallic Cs (99.8%, Alfa Aesar) under a H2 pressure of 50 bar at 500 rpm on a planetary ball mill (QM-3SP4, Nanjing) for 12 h. Prior to ball-milling, the milling vessel was heated to 50
C using a water bath and then wrapped with gauze for heat protection. The samples consisting of Mg(NH2)2-2LiH-xCsH (x ¼ 0, 0.01, 0.03, 0.05, 0.08, 0.10, 0.15) were prepared by ball milling the corresponding chemicals at 500 rpm for 36 h. The milling vessels were filled with a hydrogen pressure of 50 bar to prevent hydrogen release during ball milling. All of the samples were handled in a MBRAUN glove box filled with pure argon to prevent contamination by air and moisture (O2: <1 ppm, H2O: <1 ppm). The dehydrogenation/hydrogenation quantities were identified using a volumetric method on a homemade Sieverttype apparatus. Both isothermal and nonisothermal approaches were employed, and approximately 120 mg samples were used for each experiment. In the non-isothermal experiments, the temperature was gradually increased from ambient temperature at a rate of 2
C/min for dehydrogenation and 1
C/min for hydrogenation. For the isothermal experiments, the samples were quickly heated to and maintained at a given temperature during the entire measurement. The sample temperature was measured and controlled with an automatic temperature controller and a thermocouple that was inserted into the interior of the sample. The reactor was evacuated prior to making the measurements. For dehydrogenation, the gas was desorbed into a calibrated volume initially at 1  103 Torr, and the pressure increase was ~0.5 bar after completion of hydrogen desorption. For hydrogenation, the initial hydrogen pressure was 100 bar, and the pressure above the sample was maintained at more than 100 bar during the experiment due to the temperature effects. The dehydrogenation/hydrogenation quantities were computed according to the pressure changes in the calibration volume using the equation of state. The experiments that were used to determine the temperature dependence of hydrogen desorption were performed on a homemade temperature-programmed desorption (TPD) system coupled with a QIC-20 mass spectrometer (Hiden,

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England), which simultaneously recorded hydrogen (m/z: 2) and ammonia (m/z: 17). Approximately 50 mg of the sample was loaded and tested each time. Pure Ar was passed through the sample as the carrier gas in the testing process, and the sample was gradually heated to the preset temperature at a ramping rate of 2
C/min. Differential scanning calorimetry (DSC) measurements were conducted on a Netzsch DSC 200 F3 unit. Approximately 3 mg of the sample was gradually heated from room temperature to 300
C at a rate of 2
C/min under a flow of pure Ar. The phase structures were characterized using an X'Pert PRO (MiniFlex 600) X-ray diffractometer with Cu Ka radiation at 40 kV and 15 mA. The XRD data were collected over a 2q range of 10e90
with step increments of 0.02
. A homemade container was utilized to prevent the samples from coming into contact with moisture in the air during sample transfer and testing. The infrared measurements were performed using a Bruker Tensor 27 Fourier transform infrared spectrometer (FTIR, Germany) in transmission mode. The powder samples and potassium bromide (KBr) powder in a 1:30 weight ratio were cold-pressed in a glove box to form pellets, and then, the pellets were quickly transferred to the FTIR apparatus for testing. Each spectrum was created using an average of 16 scans at a resolution of 4 cm1. The morphology of the samples was observed using scanning electron microscopy (SEM, Hitachi S-4800). The distribution of elemental Cs in the samples was analyzed using an energy-dispersive X-ray spectrometer (EDS) attached to a Hitachi S-4800 scanning electron microscope. The samples were transferred to the SEM facility in a sealed box filled with pure Ar from the glove box. The SEM sample holder was installed within 10 s under flowing nitrogen.

Results and discussion Cesium hydride (CsH) was first synthesized in our laboratory by ball milling metallic Cs under 50 bar of H2 at 500 rpm for 12 h. Prior to ball milling, the milling vessel was heated to 50
C for 30 min using a water bath and then wrapped with gauze for heat protection. The ball milling process was carried out on a planetary ball mill. After ball milling for 12 h, a white powder sample was obtained as the product, which was collected and subjected to XRD and SEM characterization. The results are shown in Fig. 1. The XRD pattern contained typical reflections for CsH, and no additional peaks were observed, indicating that the ball-milled product was composed of a single CsH phase. Further SEM observation revealed that the as-prepared CsH sample consisted of particles in irregular shapes and sizes, and the size of most of the particles was 50 mm in diameter, as shown in the inset of Fig. 1. The as-synthesized CsH was introduced into the Mg(NH2)22LiH system by ball milling the corresponding chemicals under 50 bar of H2 to evaluate its catalytic effects. Seven samples consisting of Mg(NH2)2-2LiH-xCsH (x ¼ 0, 0.01, 0.03, 0.05, 0.08, 0.10, 0.15) were prepared. Fig. 2 shows the XRD patterns and FTIR spectra of the as-milled samples. As observed in Fig. 2a, the samples with x ¼ 0e0.05 exhibited very similar reflections in the XRD profile because only two constituent phases (i.e., LiH and Mg(NH2)2) were present even though their diffraction

Intensity (a.u.)

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50μm

CsH PDF#65-9024

10

20

30

40

50

60

70

80

90

2θ (°) Fig. 1 e XRD pattern of the as-synthesized CsH sample. The inset shows a SEM image of the as-synthesized CsH sample.

peak intensities weakened gradually. Further FTIR analysis (Fig. 2b) indicated that the characteristic doublet NeH vibration of the amide ions in Mg(NH2)2 at 3272 and 3326 cm1 was discernable [28], confirming the presence of Mg(NH2)2. As x increased from 0.08 to 1.5, the peak intensities of LiH and Mg(NH2)2 further decreased along with the appearance of CsH at 2q ¼ 23.9, 27.8 and 39.7
, especially for the sample with x ¼ 0.15. The very weak reflections of CsH in the Mg(NH2)22LiH-xCsH sample were possibly due to its poor crystallization after ball milling and/or the chemical reaction between CsH and Mg(NH2)2-2LiH. To understand the reason, the as-milled Mg(NH2)2-2LiH-0.08CsH sample was recrystallized at 120
C under 100 bar of H2 for 12 h. Based on the results, significantly intensified and narrowed reflections were observed for CsH after recrystallization (Fig. 3a). In particular, the dehydrogenation measurement revealed nearly identical dehydrogenation behavior for the samples before and after recrystallization (Fig. 3b), indicating that no additional hydrogen was absorbed during recrystallization. Therefore, the as-milled Mg(NH2)22LiH-xCsH samples were primarily composed of Mg(NH2)2, LiH and CsH, and the crystallinities of these components were poor due to strong collusion during ball milling since no chemical reaction took place. Further SEM observation indicated the relatively homogenous and smaller particles for the CsH-containing sample (Fig. 4b) compared to that of the pristine sample (Fig. 4a). This morphological change should be beneficial for improving the hydrogen storage properties. To understand how the addition of CsH affects the hydrogen storage behavior, the as-milled Mg(NH2)2-2LiH-xCsH samples were subjected to thermal dehydrogenation. Fig. 5a shows the TPD curves of the Mg(NH2)2-2LiH-xCsH samples. As previously reported [29], the TPD curve of the pristine Mg(NH2)2-2LiH sample exhibits a distinct hydrogen desorption peak that starts at 132
C and reaches a maximum at approximately 189
C. In the presence of CsH, the operating temperature for hydrogen release was significantly lower, and the dehydrogenation peak became broader. For x > 0.05, the dehydrogenation curve divided into two peaks, suggesting a

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Fig. 2 e XRD patterns (a) and FTIR spectra (b) of the Mg(NH2)2-2LiH-xCsH samples.

Fig. 3 e XRD patterns (a) and volumetric release curves (b) of the Mg(NH2)2-2LiH-0.08CsH samples before and after recrystallization.

change in the dehydrogenation process. In addition, it is important to note that the onset temperature for hydrogen desorption monotonously decreased from 132 to 70
C when x increased from 0 to 0.08, which results in a decrease of 62
C. However, no additional reduction in the dehydrogenation temperature was observed when the CsH content increased from x ¼ 0.08 to 0.15. Simultaneous detection of the NH3 signal

revealed that the addition of CsH dramatically decreased the evolution of the NH3 by-product (Fig. 5b), especially for the x > 0.03 sample, because nearly no NH3 release was detected by MS. The reduced NH3 evolution increased the purity of the hydrogen released from the LieMgeNeH system, which is advantageous for onboard practical applications as a hydrogen storage medium.

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Fig. 4 e SEM images of the Mg(NH2)2-2LiH (a, c) and Mg(NH2)2-2LiH-0.08CsH (b, d) samples before and after cycling.

Intensity (a.u.)

(a) 1

Mg(NH2)2-2LiH

2 3 4 5 6 7

+0.01CsH +0.03CsH +0.05CsH +0.08CsH +0.10CsH +0.15CsH

H2 signal 1 2 3 4 7

(b)

5

6

NH3 signal

1 2

3

4-7

50

100

150

200

250

Temperature (°C) Fig. 5 e H2 (a) and NH3 (b) TPD-MS signals of the Mg(NH2)22LiH-xCsH samples as a function of temperature.

Fig. 6a shows the quantitative hydrogen release curves of Mg(NH2)2-2LiH-xCsH samples as a function of temperature obtained using the volumetric method. As expected, the operating temperatures, especially for the onset temperatures, substantially decreased as the CsH content increased, which is consistent with the TPD results shown in Fig. 5a. The midpoint temperature of the hydrogen release reaction was reduced from 187
C for x ¼ 0e137
C for x ¼ 0.15. At the same

time, the available dehydrogenation amount also gradually decreased with the increasing CsH amount because the molecular weight of Cs (132.9) is much larger than those of Li (6.9) and Mg (24.3). Upon heating, the total amount of hydrogen released from the pristine Mg(NH2)2-2LiH sample was approximately 5.4 wt%. For the sample with 0.15 mol CsH, the total amount of hydrogen released decreased to 4.3 wt%, which is a 20% reduction relative to that of the pristine sample. A comprehensive comparison revealed that in the current study, the x ¼ 0.08 sample exhibited the optimal hydrogen storage properties. This sample released approximately 4.62 wt% H2 with an onset temperature of 70
C and a midpoint dehydrogenation temperature of 155
C. Here, it should be mentioned that such a hydrogen storage capacity is slightly lower than those of RbH or KH-containing samples as reported previously [27]. To further evaluate the effects of CsH on the dehydrogenation kinetics, the isothermal dehydrogenation behaviors of Mg(NH2)2-2LiH and Mg(NH2)2-2LiH-0.08CsH at 110e150
C were compared, as shown in Fig. 6b and c. The addition of CsH significantly improved the dehydrogenation kinetics of the Mg(NH2)2-2LiH system, especially at lower operating temperatures. For example, the CsH-containing sample released 40% of its hydrogen capacity within 300 min at 110
C, and no hydrogen desorption was detected for the pristine sample under identical conditions. When the temperature was increased to 150
C, approximately 80% of the hydrogen was released from the CsH-containing sample within 100 min but the pristine sample required 600 min. By analyzing the

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Fig. 6 e Nonisothermal (a) and isothermal (b, c) dehydrogenation curves of the Mg(NH2)2-2LiH-xCsH samples.

tangential slope of the linear portions, the rate constant for the Mg(NH2)2-2LiH-0.08CsH sample was estimated to be 2.78%/min at 150
C. This rate is 8 times faster than that of the pristine sample (0.34%/min). The apparent activation energies (Ea) for the first and second dehydrogenation steps were determined to be 86.5 ± 3.6 and 104.5 ± 2.4 kJ/mol, respectively, for the Mg(NH2)2-2LiH-0.08CsH sample using Kissinger's method (Fig. 7b) [30]. A 20% decrease in the Ea value for the first dehydrogenation step was observed compared to that of the pristine sample (107.2 ± 2.8 kJ/mol) (Fig. 7a), and this decrease is responsible for the significantly decreased operating temperatures for the CsH-containing sample. The DSC measurement (Fig. 7c) revealed that the heat flow curve of the Mg(NH2)2-2LiH-0.08CsH sample contained two adjacent peaks below 250
C along with an appreciable low-temperature shift with respect to the Mg(NH2)2-2LiH sample. This result is in good agreement with the TPD result shown in Fig. 5a, confirming the occurrence of two dehydrogenation stages with different thermodynamics and/or kinetics. By fitting the DSC curves, the reaction enthalpy changes were calculated to be approximately 38.2 ± 1.1 and 32.5 ± 0.9 kJ/mol of H2 for the first and second dehydrogenation steps, respectively. Apparently, the enthalpy change of the first step was very similar to that of the pristine sample but an 18% decrease was observed for the

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Fig. 7 e Kissinger's plots (a, b) and DSC curves (c) of the Mg(NH2)2-2LiH and Mg(NH2)2-2LiH-0.08CsH samples.

second step, which is the most likely reason for the lower temperature of the second step. In addition, it is important to note that an additional exothermic event was observed for the Mg(NH2)2-2LiH-0.08CsH sample at 250e280
C, which may be due to the phase transformation of Li2MgN2H2 that has been previously reported [25]. After dehydrogenation, the Mg(NH2)2-2LiH and Mg(NH2)22LiH-0.08CsH samples were subjected to hydrogenation under a hydrogen pressure of 100 bar to evaluate the hydrogenation behavior. Fig. 8a shows the hydrogenation curves of the dehydrogenated Mg(NH2)2-2LiH and Mg(NH2)2-2LiH-0.08CsH samples as a function of temperature. The onset hydrogenation temperature of the Mg(NH2)2-2LiH-0.08CsH sample was only approximately 55
C, which is 40
C lower than that of the pristine sample (ca. 95
C). Approximately 4.58 wt% hydrogen was recharged into the dehydrogenated CsH-containing sample even at temperatures as low as 130
C, which indicates good hydrogen storage reversibility. However, the dehydrogenated pristine sample was only completely hydrogenated above 200
C. An additional isothermal hydrogenation experiment (Fig. 8b) indicated that more than 3 wt% hydrogen was rapidly charged into the dehydrogenated CsHcontaining sample within 100 min at 100
C and 100 bar of H2. However, at the same temperature and hydrogen pressure, the dehydrogenated pristine sample could only absorb 1.2 wt

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Fig. 8 e Nonisothermal (a) and isothermal (b) hydrogenation curves of the dehydrogenated Mg(NH2)2-2LiH and Mg(NH2)22LiH-0.08CsH samples.

% hydrogen even when the reaction time was extended to 400 min. At 120
C, the dehydrogenated CsH-containing sample took up more than 4 wt% hydrogen within 20 min at 120
C. These hydrogenation kinetics are superior to the Kand Rb-containing samples [25e27,31]. According to these results, the presence of CsH significantly decreased the operating temperature and improved the reaction kinetics for hydrogen storage in the Mg(NH2)2-2LiH system. To understand the role played by CsH, the dehydrogenated Mg(NH2)2-2LiH-0.08CsH samples that were collected at different stages during the variable temperature experiment were subjected to XRD and FTIR analyses. The results are shown in Fig. 9. The XRD results (Fig. 9a) indicated that the sample collected at 75
C primarily consisted of Mg(NH2)2, LiH, and CsH. In the FTIR spectrum (Fig. 9b), the typical doublet NeH vibration for Mg(NH2)2 at 3272/3328 cm1 was observed. After dehydrogenation at 120
C, a new broad absorbance that was centered at 3187 cm1 appeared in the FTIR spectrum even though the XRD pattern was similar to that of the sample dehydrogenated at 75
C. This absorbance was due to the absorbance of a ternary imide (Li2Mg2N3H3), as previously reported [25]. When the sample was heated to 140
C, three major diffraction peaks for Li2Mg2N3H3 were observed at 30.3, 51.0, and 60.7
(2q) in the XRD profile. Correspondingly, the FTIR absorbance of Li2Mg2N3H3 at 3187 cm1 intensified as the absorbance for Mg(NH2)2 weakened. A further increase in the operating temperature to 170
C resulted in an increase in the intensities of the diffraction peaks located at 30.3, 51.0, and 60.7
, which correspond to Li2Mg2N3H3. In addition, the signals for LiH were absent in the XRD profile. The FTIR spectra demonstrated that the absorbance at 3187 cm1 that originated from Li2Mg2N3H3 shifted to 3178 cm1, and the signature doublet vibration of Mg(NH2)2 was not visible. Moreover, the

NeH vibration doublet of LiNH2 at 3257/3312 cm1 was observed. Therefore, the dehydrogenation product was most likely composed of Li2Mg2N3H3 and LiNH2 when the sample was heated from room temperature to 170
C, which can be catalyzed by CsH as discussed above.

2Mg(NH2)2 þ 3LiH / LiNH2 þ Li2Mg2N3H3 þ 3H2

(2)

After dehydrogenation at 200
C, the diffraction peaks for LiH and CsH disappeared completely, and the signals located at 17.3, 30.7, 50.9, and 61.0
dominated the XRD pattern. Simultaneously, the FTIR spectrum exhibited a strong symmetric absorbance peak at 3174 cm1, which corresponds to the cubic Li2MgN2H2 imide according to a previous report [25]. No specific XRD data were observed for the Cs-containing species at 170e200
C, which may be due to the poor crystallization of the Cs-containing product or its structure being similar to the ternary imide of Li2MgN2H2. After full dehydrogenation at 280
C, the reflections of orthorhombic Li2MgN2H2 were identified with substantial intensities in the XRD profile along with the disappearance of cubic Li2MgN2H2. The FTIR spectrum contained the NeH vibration of orthorhombic Li2MgN2H2 at 3160/3183 cm1. Therefore, a polymorphic transformation from a cubic structure to an orthorhombic structure occurred in Li2MgN2H2. This result explains the exothermic peak in the DSC curve (Fig. 7c) because the orthorhombic structure is the ground-state configuration for the Li2MgN2H2 phase [32]. Moreover, a series of new peaks at 18.9, 23.4, 26.0, and 29.4
appeared at 280
C, which may originate from a Cs-based compound (e.g., CsLi(NH2)2 or CsMg(NH)2) or unknown amide-imide complexes (i.e., CsMg(NH)x(NH2)y).

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Fig. 9 e XRD patterns (a, c) and FTIR spectra (b, d) of the dehydrogenated Mg(NH2)2-2LiH-0.08CsH and Mg(NH2)2-xCsH samples.

To identify the Cs-based intermediates that were produced during the thermal dehydrogenation process, three samples were designed and prepared by ball milling Mg(NH2)2 and CsH in molar ratios of 1:0.5, 1:1, 1:2 at 500 rpm for 36 h followed by dehydrogenation. Fig. 9c shows the XRD patterns of the dehydrogenated Mg(NH2)2-2LiH-0.08CsH and Mg(NH2)2-xCsH samples. Careful examination indicated that the newly developed peaks of the Mg(NH2)2-2LiH-0.08CsH sample dehydrogenated at 280
C are consistent with those of the

dehydrogenated Mg(NH2)2-CsH (molar ratio: 1:1) sample. In addition, three NeH vibrations at 3302, 3258 and 3247 cm1 were observed in the FTIR spectra of both the dehydrogenated Mg(NH2)2-2LiH-0.08CsH sample and the dehydrogenated Mg(NH2)2-CsH sample, as shown in Fig. 9d. Further quantitative dehydrogenation measurement indicated that approximately 1.03 wt% was released from the Mg(NH2)2-CsH mixture at 70e250
C, which is equivalent to 0.98 mol of H2. Thus, the dehydrogenation product of the Mg(NH2)2-CsH mixture can be

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expressed as CsMg(NH)(NH2). As a result, the following reaction was proposed for hydrogen desorption from a mixture of Mg(NH2)2 and CsH upon heating:

2Mg(NH2)2 þ CsH / CsMg(NH)(NH2) þ H2

(3)

A similar phenomenon was also observed for the Mg(NH2)2-KH and Mg(NH2)2-RbH system [26,33]. Therefore, CsH reacts with Mg(NH2)2 to afford CsMg(NH)(NH2) during the heating process, which changes the reaction process of the Mg(NH2)2-2LiH-0.08CsH sample. As a result, the second step of dehydrogenation can be expressed as follows:

0.08Mg(NH2)2 þ 0.08CsH þ 0.62LiH þ 0.46LiNH2 þ 0.46Li2Mg2N3H3 / 0.08CsMg(NH)(NH2) þ 0.16LiH þ 0.92Li2MgN2H2 þ 0.46H2

(4)

By summarizing reactions (2) and (4), the overall reaction process of the Mg(NH2)2-2LiH-0.08CsH sample upon heating can be described by the following reaction:

Mg(NH2)2 þ 2LiH þ 0.08CsH / 0.92Li2MgN2H2 þ 0.08CsMg(NH)(NH2) þ 0.16LiH þ 1.92H2

(5)

The theoretical dehydrogenation capacity of this reaction was calculated to be 4.63 wt%, which agrees well with the experimental value of 4.62 wt%. Based on our thermodynamic, kinetic and reaction mechanistic analyses, the added CsH persisted in the first step of dehydrogenation of the Mg(NH2)22LiH system and primarily acted as a catalyst to reduce the operating temperature. In the second dehydrogenation step, CsH directly participated in the dehydrogenation reaction as a reactant and was converted to CsMg(NH)(NH2), which improves the dehydrogenation reaction thermodynamics. These two factors are responsible for the change in the reaction

5

Mg(NH2)2-2LiH

250 °C-des 210 °C-abs

(a)

4 3

H/M (wt%)

2 1 0 0

1000

5

2000

3000

180 °C-des 140 °C-abs

4000

5000

Mg(NH2)2-2LiH-0.08CsH

(b)

4 3 2 1 0 0

1000

2000

Time (min)

3000

4000

Fig. 10 e Hydrogen desorption/absorption cycle curves of the Mg(NH2)2-2LiH (a) and Mg(NH2)2-2LiH-0.08CsH (b) samples.

process and the reduced dehydrogenation temperatures of the CsH-containing Mg(NH2)2-2LiH system. Fig. 10a and b shows the cycling dehydrogenation/hydrogenation curves of the pristine Mg(NH2)2-2LiH sample and Mg(NH2)2-2LiH-0.08CsH sample, respectively. The available hydrogen capacity of the pristine sample rapidly decreased from 5.29 to 2.83 wt% after 10 cycles with operating conditions of 250
C and 180 min for dehydrogenation and 210
C, 100 bar of H2 and 60 min for hydrogenation. The capacity retention was determined to be 53%. For the Mg(NH2)2-2LiH-0.08CsH sample, the hydrogen capacity remained at 4.41 wt% after 10 cycles with operating conditions of 180
C and 180 min for dehydrogenation and 140
C, 100 bar of H2 and 60 min for hydrogenation, which corresponds to a 98% hydrogenation capacity retention. Obviously, the cycling stability of the Mg(NH2)2-2LiH sample was significantly improved by the addition of CsH, which is further confirmed by XRD and FTIR analyses as the hydrogenated CsH-containing sample after 10 cycles was still composed of Mg(NH2)2, LiH and CsH without additional species (Fig. 11). The significantly improved cycling stability is most likely due to the reduced evolution of the NH3 by-product and the limited growth of the particle size for the dehydrogenation product. As shown in Fig. 4b, the evolution of the NH3 by-product was undetectable upon heating of the Mg(NH2)2-2LiH-0.08CsH sample. Moreover, SEM observation indicated that the particles of the pristine sample were distinctly aggregated, and the particle sized increased (~3e5 mm) after 10 cycles (Fig. 4c). In contrast, the particles of the cycled CsH-containing sample remained separate, and the size of most of the particles was less than 1 mm (Fig. 4d). EDS examination revealed that the distribution of Cs was relatively homogenous without apparent agglomeration in the Mg(NH2)2-2LiH-0.08CsH sample after cycling (Fig. 12). These results reasonably explain the significantly improved cycling stability of the CsH-containing Mg(NH2)2-2LiH sample.

Conclusions CsH was synthesized by ball milling Cs under a hydrogen atmosphere of 50 bar at 500 rpm for 12 h and then introduced into the Mg(NH2)2-2LiH system to improve its hydrogen storage properties. The results indicated that the addition of a small amount of CsH significantly reduced the operating temperature and enhanced the de-/hydrogenation kinetics of the Mg(NH2)2-2LiH system. The optimized composite was determined to be Mg(NH2)2-2LiH-0.08CsH, which could reversibly store 4.6 wt% hydrogen via a two-step reaction. The onset dehydrogenation temperature of the Mg(NH2)2-2LiH0.08CsH sample was only 70
C, which is a 62
C decrease relative to the pristine sample. More than 80% of the hydrogen was rapidly released within 100 min at 150
C, which is an 8fold enhancement in the rate constant. Moreover, the fully dehydrogenated CsH-containing sample at 130
C took up 4 wt % hydrogen at 120
C and 100 bar of H2 within 20 min. Therefore, this sample exhibited superior hydrogenation kinetics. For the Mg(NH2)2-2LiH-0.08CsH sample, the apparent activation energy and reaction enthalpy change values were calculated to be 86.5 ± 3.6 kJ/mol and 38.2 ± 1.1 kJ/mol-H2, respectively, as well as 104.5 ± 2.4 kJ/mol and 32.5 ± 0.9 kJ/mol-

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 4 1 ( 2 0 1 6 ) 1 1 2 6 4 e1 1 2 7 4

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Fig. 11 e XRD pattern (a) and FTIR spectrum (b) of the Mg(NH2)2-2LiH-0.08CsH sample after 10 dehydrogenation/ hydrogenation cycles.

Fig. 12 e SEM images (a, c) and EDS element maps (b, d) of the Mg(NH2)2-2LiH-0.08CsH sample before and after cycling.

H2, respectively, for the first and second dehydrogenation steps, respectively. After 10 cycles, the hydrogen capacity of the Mg(NH2)2-2LiH-0.08CsH sample remained at 4.41 wt%, corresponding to 98% capacity retention under operating conditions of 180
C and 180 min for dehydrogenation and 140
C, 100 bar of H2 and 60 min for hydrogenation.

Mechanistic studies revealed that CsH acted as a catalyst to reduce the activation energy barrier of the first dehydrogenation step during the initial heating process. As the reaction temperature increased, CsH reacted gradually with Mg(NH2)2 to form CsMg(NH)(NH2) in the second dehydrogenation step, which changes the reaction pathway and results in the

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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 4 1 ( 2 0 1 6 ) 1 1 2 6 4 e1 1 2 7 4

formation of CsMg(NH)(NH2). These factors are responsible for the improved thermodynamics and kinetics of hydrogen storage in the Mg(NH2)2-2LiH system.

Acknowledgments We gratefully acknowledge the financial support received from the Zhejiang Provincial Natural Science Foundation of China (LR16E010002), the National Natural Science Foundation of China (51222101), the Research Fund for the Doctoral Program of Higher Education of China (20130101130007), and the National Youth Top-notch Talent Support Program.

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