Enhanced Hydrogen Storage Properties of Li−Mg−N−H System

May 11, 2009 - Following this mechanistic understanding, incorporation of foreign LiNH2 may provide favorable nucleation/growth sites for the intermed...
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J. Phys. Chem. C 2009, 113, 9944–9949

Enhanced Hydrogen Storage Properties of Li-Mg-N-H System Prepared by Reacting Mg(NH2)2 with Li3N Lai-Peng Ma, Hong-Bin Dai, Zhan-Zhao Fang, Xiang-Dong Kang, Yan Liang, Pei-Jun Wang, Ping Wang,* and Hui-Ming Cheng Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China ReceiVed: NoVember 29, 2008; ReVised Manuscript ReceiVed: April 6, 2009

The Li-Mg-N-H system was prepared by reacting magnesium amide [Mg(NH2)2] with lithium nitride (Li3N) and investigated with regard to the hydrogen storage properties. Our study shows that the present method is superior to the conventional route in enhancing the reversible dehydrogenation properties. Through optimizing the Li3N:Mg(NH2)2 ratio in the starting materials, the reversible capacity of Li-Mg-N-H system increases to 4.9 wt %, 18% higher than that typically obtained from the Mg(NH2)2 + 2LiH mixture at 200 °C. Furthermore, increasing the Li3N:Mg(NH2)2 ratio is effective for mitigating the ammonia release from thus-prepared samples. Combined property/structure investigations indicate that the obtained enhancements should be ascribed to the effects of LiNH2 and LiH that were in situ generated from the excess Li3N. LiNH2 may promote the dehydrogenation reaction via seeding the reaction intermediate. The concurrently generated LiH acts as an effective ammonia trapping agent. These findings highlight the potential of “intermediate seeding” as a strategy to enhance the reversible hydrogen storage properties of metal-N-H systems. 1. Introduction In 2002, Chen et al. reported that lithium nitride (Li3N) reversibly stored over 10 wt % hydrogen via a two-step process involving the reactions as follows:1

Li3N + H2 a Li2NH + LiH

(1)

Li2NH + H2 a LiNH2 + LiH

(2)

This revolutionary discovery immediately stimulated worldwide interest in the metal-N-H systems as potential hydrogen storage media. Several light-metal amide/imide systems were subsequently identified that possess enhanced hydrogen storage properties relative to Li3N.2,3 Furthermore, on the basis of the attractive interaction between Hδ+ and Hδ- from respective amide and hydride, amide-hydride combination provides a novel approach for developing “hybrid” hydrogen storage systems.2-10 Among these newly identified N-containing hydrogen storage materials, Li-Mg-N-H system prepared from Mg(NH2)2 + 2LiH (or 2LiNH2 + MgH2) mixture exhibits the most favorable combination of thermodynamics, reversible capacity, and operating conditions.11-15 The reversible de/rehydrogenation reactions of this system can be described as reaction 3.

Mg(NH2)2 + 2LiH a Li2Mg(NH)2+2H2

(3)

As experimentally determined, Mg(NH2)2 + 2LiH possesses a dehydrogenation reaction enthalpy of ∼44 kJ/mol H2, about 20 kJ/mol H2 lower than that of the “prototype” LiNH2 + LiH system.1,16-18 Thermodynamically, it predicts a dissociation pressure of 0.1 MPa at ∼90 °C. However, such favorable dehydrogenation performance has never been practically achieved as a result of the kinetic limitation.2,19-23 For example, even at an operating temperature of 200 °C, the practical capacity of the Li-Mg-N-H system is typically lower than 4.2 wt %, far * To whom correspondence should be addressed. E-mail: pingwang@ imr.ac.cn. Fax: +86 24 2389 1320.

below the theoretical value of 5.6 wt %.13 Additionally, the problematic ammonia (NH3) release from most metal-N-H systems not only imperils the PEM fuel cell but also causes capacity degradation upon cycling the materials.2,3,24-26 Currently, elimination of ammonia byproduct remains a great challenge in developing metal-N-H hydrogen storage systems. In the present study, we demonstrate an effective method to simultaneously enhance the practical capacity and mitigate the NH3 release of Li-Mg-N-H system. Our study show that reacting magnesium amide (Mg(NH2)2) with lithium nitride (Li3N), instead of lithium hydride (LiH), yields Li-Mg-N-H system with tunable dehydrogenation properties. By optimizing the Li3N:Mg(NH2)2 phase ratio in the starting materials, the practical capacity of thus-prepared material reaches 4.9 wt %, 18% higher than that typically obtained from the Mg(NH2)2 + 2LiH mixture at 200 °C. Furthermore, increasing the Li3N: Mg(NH2)2 ratio is effective for mitigating the ammonia release. Property/structure investigations suggest that the enhancements should be understood from the combined effects of LiNH2 and LiH that were in situ generated from the excess Li3N. We herein report these results. 2. Experimental Section 2.1. Sample Preparation. The starting material Li3N (99.4%) was purchased from Alfa-Aesar Corp. and used as received. Mg(NH2)2 was prepared by milling MgH2 powder (95%) under 0.5 MPa NH3 atmosphere, followed by annealing at 300 °C under 0.5 MPa NH3.27 This process was repeated three times to achieve a purity of ca. 95%, as determined by combined X-ray diffraction and thermogravimetry analyses. The Li-Mg-N-H samples were prepared from the Mg(NH2)2/Li3N mixtures via a two-step process: the Mg(NH2)2/Li3N mixtures in varied molar ratios were first mechanically milled for 5 h and then treated at 240 °C under dynamic vacuum to obtain Li-Mg-N-H samples. For comparison, the Li-Mg-N-H sample was also prepared using the Mg(NH2)2/2LiH mixture as starting materials.

10.1021/jp810494e CCC: $40.75  2009 American Chemical Society Published on Web 05/11/2009

Hydrogen Storage Properties of Li-Mg-N-H System The mechanical milling was performed using a Fritsch P7 planetary mill at 400 rpm under Ar atmosphere. During the milling process, an interval of 5 min was used every 10 min milling to minimize the temperature increment of the sample. All the sample handling was performed in an Ar-filled glovebox equipped with a circulative purification system, in which the typical H2O/O2 levels are below 0.1 ppm. 2.2. Property Measurement. The decomposition behavior of the as-milled Mg(NH2)2/Li3N mixtures was examined using synchronous thermal analyses (thermogravimetry/differential scanning calorimetry/mass spectroscopy, TG/DSC/MS, Netzsch STA 449C/QMS 403C). High purity Ar (99.999%) was used as purge gas, and the ramping rate was 5 °C/min. The hydrogen storage properties of sample, with a typical amount of around 200 mg, were measured using a carefully calibrated Sieverts’ type apparatus. The reference cell with a fixed volume was maintained under room temperature. The slight variation in temperature was recorded to correct the pressure value of the system. Leak tests were performed by verifying the stability of hydrogen pressure before each measurement. Isothermal dehydrogenation kinetics was measured at desired temperatures under an initial hydrogen pressure of 0.1 MPa, with a typical ending pressure of ∼0.14 MPa. The measurement was started from the moment of pushing the reactor into the furnace held at desired temperatures, which resulted in a 30 min delay before reaching equilibrium. Given that no significant dehydrogenation was observed during this stage, only the isothermal curve was presented for clarity. Rehydrogenation treatment of the samples was performed at 200 °C under 5 MPa. To minimize the possible sample sintering, a two-step process was adopted to start the rehydrogenation: hydrogen was first loaded into the sample cell under room temperature, then, the sample cell was heated to 200 °C. The hydrogen supply (99.999%) was further purified by using a hydrogen storage alloy system to minimize the H2O/O2 contamination. Other than specified, the hydrogen capacity was calculated using the total weight of sample to allow evaluation of the practical hydrogen storage property. 2.3. Structural Characterization. The Li-Mg-N-H samples were analyzed using X-ray diffraction (XRD, Rigaku D/MAX2500 diffractometer, Cu KR radiation) and Fourier transform infrared spectrometer (FTIR, Bruker TENSOR 27, MCT detector) equipped with an in situ cell. FTIR spectra were collected in diffuse reflectance infrared Fourier transform (DRIFT) mode at a resolution of 4 cm-1. All the sample preparation was performed in the Ar-filled glovebox. 3. Results 3.1. Preparation of Li-Mg-N-H System from the Mg(NH2)2/Li3N mixture. Upon varying the material composition and operating conditions, Li-Mg-N-H systems may dehydrogenate to different stages, yielding reversible hydrogen capacities ranging from 5.6 to 9.1 wt % (theoretical values).27-32 Considering the operating pressure and temperature, however, the reversible system yielding the ternary imide Li2Mg(NH)2 as the dehydrogenation product is more promising for practical hydrogen storage application. Conventionally, Li2Mg(NH)2 was prepared by milling amide/hydride mixtures [Mg(NH2)2/LiH or LiNH2/MgH2], followed by dehydrogenation at ∼200 °C. Our study shows that Li2Mg(NH)2 can also be prepared using Mg(NH2)2/Li3N mixture as starting materials. Parts a and b of Figure 1 present the XRD patterns of the as-milled 3Mg(NH2)2 + 2Li3N mixture and that after heat-

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Figure 1. XRD patterns of the as-milled 3Mg(NH2)2 + 2Li3N mixture (a), that heat-treated at 240 °C under dynamic vacuum (b), and that after rehydrogenation at 200 °C under 5 MPa hydrogen (c). A small amount of MgO was detected, which should primarily originate from the air contamination during XRD measurements.

Figure 2. TG/DSC profiles (top) of the as-milled 3Mg(NH2)2 + 2Li3N mixture, and synchronous MS signals (bottom) of m/e ) 2 (H2) and m/e ) 17 (NH3). The sample was heated at 5 °C /min under a flow of Ar gas.

treatment at 240 °C under dynamic vacuum. The XRD results indicate that the heat-treatment leads to the conversion of the starting materials into Li2Mg(NH)2 phase, which is also confirmed by the FTIR investigation (results not shown here). TG examination of the milled sample reveals that the sample exhibits a weight loss of ∼12.6 wt % upon heated to 300 °C, as seen in Figure 2. According to the synchronous MS analysis, NH3 is the sole gas species released from the sample. Combined analyses of these results suggest that Li2Mg(NH)2 and NH3 are generated from the 3Mg(NH2)2 + 2Li3N mixture following reaction 4. This reaction yields a theoretical weight loss of 13.7 wt % (after taking into account the impurities), which agrees well with the TG result.

3Mg(NH2)2 + 2Li3N f 3Li2Mg(NH)2 + 2NH3

(4)

3Mg(NH2)2 f 3MgNH + 3NH3

(4.1)

2Li3N + NH3 f 3Li2NH

(4.2)

3MgNH + 3Li2NH f 3Li2Mg(NH)2

(4.3)

Currently, the mechanism underlying reaction 4 is still unclear. Presumably, it proceeds via a three-step process

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Figure 3. Isothermal dehydrogenation curves of the Li-Mg-N-H samples prepared from xLi3N + 3Mg(NH2)2 mixtures (x ) 2, 2.1, 2.3, 2.5, and 3). The insert highlights the evolution of dehydrogenation capacity as a function of starting xLi3N:3Mg(NH2)2 ratio. The dehydrogenation measurements were performed at 200 °C with an initial hydrogen pressure of 0.1 MPa. All the samples were rehydrogenated prior to the dehydrogenation measurements.

following reactions 4.1-4.3. Mg(NH2)2 first decomposes to generate MgNH and NH3; the gaseous NH3 then reacts with Li3N to form Li2NH; finally, the two formed imides, MgNH and Li2NH, react with each other to form Li2Mg(NH)2.33,34 Clearly, capture of NH3 by Li3N is the key in this mechanistic understanding. Similar speculation was also proposed by Hu et al. in their study of Li-N-H system.33 In the present study, strong evidence to support the Li3N + NH3 solid-gas reaction (reaction 4.2) is obtained from the DSC analysis. As seen in Figure 2, an exothermic effect peaked at ∼200 °C superimposes on the wide endothermic peak that agrees with the NH3 release from Mg(NH2)2. Consistently, reaction 4.2 is calculated to be exothermic using the literature data.1,35 In the subsequent rehydrogenation process, thus-prepared Li2Mg(NH)2 then transforms into Mg(NH2)2/LiH mixture (see Figure 1c), consistent with reaction 3. In this regard, the Li-Mg-N-H system prepared from the 3Mg(NH2)2/2Li3N mixture is similar to that obtained using the amide/hydride mixtures. However, further study shows that the present method is superior to the conventional route in readily optimizing the material composition to enhance the practical hydrogen storage properties. 3.2. Property Dependence of Li-Mg-N-H System on the Li3N:Mg(NH2)2 Ratio. Accidentally, we found that slight increase of the starting Li3N:Mg(NH2)2 ratio beyond the stoichiometric value (2:3) resulted in enhanced dehydrogenation properties of resulting Li-Mg-N-H samples. Motivated by this finding, we systematically investigate the property dependence of Li-Mg-N-H system on the Li3N:Mg(NH2)2 ratio. Figure 3 compares the hydrogen desorption curves of the rehydrogenated Li-Mg-N-H samples, which were prepared using the starting materials with varied Li3N:Mg(NH2)2 ratio (ranging from 2/3 to 1). Clearly, corresponding variations in the practical hydrogen capacity are observed. In the present study, the amounts of hydrogen desorbed at 200 °C over 120 min are used to compare the practical capacity, after which only slight increase in capacity is observed for all samples. For example, the sample prepared from the 2Li3N + 3Mg(NH2)2 mixture only desorbs ∼4.2 wt % hydrogen at 200 °C, which agrees well with that typically obtained for the Mg(NH2)2 + 2LiH mixture.13 Upon increasing the Li3N:Mg(NH2)2 ratio to 2.3:3, the hydrogen amount desorbed increases to 4.9 wt % under identical operating conditions. Further increasing the

Ma et al.

Figure 4. Hydrogen cycling properties of the Li-Mg-N-H sample prepared from the 2.3Li3N + 3Mg(NH2)2 mixture. The dehydrogenation measurements were performed at desired temperatures with an initial hydrogen pressure of 0.1 MPa. Rehydrogenation of the samples was performed at 200 °C under 5 MPa.

Figure 5. Comparison of ammonia release from the Li-Mg-N-H samples prepared from the 2Li3N + 3Mg(NH2)2 (open square) and 2.3Li3N + 3Mg(NH2)2 (open circle) mixtures. Both samples were rehydrogenated prior to the dehydrogenation measurements. Ammonia signals (m/e ) 17) were monitored using synchronous MS.

amount of Li3N, however, results in a progressively reduced capacity. When the molar ratio reaches 1:1, the hydrogen capacity falls back to 4.25 wt %, similar to that obtained from the 2Li3N + 3Mg(NH2)2 sample. Further study reveals that the cyclic capacity and kinetics of the sample with the optimal Li3N:Mg(NH2)2 ratio (2.3:3) are highly stable during hydrogen cycling. As shown in Figure 4, its dehydrogenation property at the 10th cycle is almost identical to that at the second cycle. Additionally, comparative MS analyses indicate that less ammonia is detected in this sample. As shown in Figure 5, the ammonia release from the sample prepared using 2.3Li3N + 3Mg(NH2)2 mixture is mitigated relative to the 2Li3N + 3Mg(NH2)2 sample. This finding shows the effect of adding excess Li3N on reducing ammonia byproduct, which will be discussed below in terms of the underlying mechanism. From a practical point of view, suppressing ammonia release is important, not only for stabilizing the cyclic capacity of the material but also for alleviating the technical difficulty in purifying the hydrogen source for feeding the fuel cell. 3.3. Phase and Structure Characterization. In our efforts to understand the property enhancement arising upon adding excess Li3N, we performed combined FTIR and XRD examinations on the series of samples prepared from xLi3N + 3Mg(NH2)2 mixtures (with x ranging from 2 to 3). Here, it

Hydrogen Storage Properties of Li-Mg-N-H System

Figure 6. FTIR spectra of the rehydrogenated (RH) and dehydrogenated (DH) Li-Mg-N-H samples prepared from the xLi3N + 3Mg(NH2)2 mixtures with varied Li3N amount: (a) x ) 2, (b) x ) 2.1, (c) x ) 2.3, (d) x ) 2.5, and (e) x ) 3.

should be noted that all the samples were pretreated by mechanical milling and subsequent heating at 240 °C under dynamic vacuum. Figure 6 presents the FTIR spectra of samples in both rehydrogenated and dehydrogenated states. Overall, the addition of excess Li3N results in no appreciable change on the bonding states of Li-Mg-N-H matrix. For the rehydrogenated samples, the characteristic vibrations from Mg(NH2)2 (3272 and 3326 cm-1) can be clearly observed. For the samples with starting Li3N amount of x > 2, however, the N-H stretches of LiNH2 at 3258 and 3312 cm-1 are also identified.36 Upon increasing the Li3N amount, the N-H modes of LiNH2 gain increased intensity, indicative of increased amount of LiNH2. This is further confirmed by the parallel XRD examinations. As shown in Figure 7 RH, the diffraction peaks from LiNH2 at 2θ ) 30.55° and 35.6° are gradually intensified with the increase of starting Li3N amount. A simultaneous intensity increase of LiH peaks at 2θ ) 38.2° and 44.4° is also observed. These results clearly indicate that the excess Li3N is converted into LiNH2 + LiH in the rehydrogenated samples, which remains stable during hydrogen cycling (results not shown here). Most possibly, the generation of LiNH2 + LiH from excess Li3N proceeds through the Li2NH stage, which was formed during the heat treatment following reaction 4.2. Because of the structural similarity of Li2NH to Li2Mg(NH)2,15 however, it is difficult to obtain convincing evidence to clarify this point. In the subsequent dehydrogenation process, Mg(NH2)2 reacts with LiH to generate Li2Mg(NH)2 and H2 following reaction 3. Meanwhile, LiNH2 and partial LiH remain stable upon dehydrogenation, as indicated by the FTIR (Figure 6 DH) and XRD (Figure 7 DH) results. The stability of the in situ generated

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Figure 7. XRD patterns of the rehydrogenated (RH) and dehydrogenated (DH) Li-Mg-N-H samples prepared from the xLi3N + 3Mg(NH2)2 mixtures with varied Li3N amount: (a) x ) 2, (b) x ) 2.1, (c) x ) 2.3, (d) x ) 2.5, and (e) x ) 3. The squares highlight the increase in the intensity of LiNH2 and LiH phases upon increasing the Li3N addition. A small amount of MgO was detected, which should primarily originate from the air contamination during XRD measurements.

LiNH2/LiH is quite understandable from the thermodynamic limitation of the dehydrogenation reaction (see reaction 2) by the operating condition. In this work, the dehydrogenation measurements were carried out at 200 °C under an initial hydrogen pressure of 0.1 MPa, which is higher than the dissociation pressure of LiNH2 + LiH system at this temperature.1,16 3.4. Mechanistic Understanding. Reacting Li3N with Mg(NH2)2 provides an alternative route to the preparation of Li-Mg-N-H system for reversible hydrogen storage. The advantage of the present method over the conventional route is that the practical dehydrogenation capacity can be readily enhanced by optimizing the Li3N:Mg(NH2)2 ratio. For example, the Li-Mg-N-H system prepared from 2.3Li3N + 3Mg(NH2)2 mixture exhibits a reversible capacity of 4.9 wt %, 18% higher than that typically obtained from the Mg(NH2)2 + 2LiH mixture. As discussed above, the 0.3 mol excess Li3N does not directly participate in the reversible hydrogen storage. Additionally, the presence of excess LiH (originating from excess Li3N) results in no change of reaction pathway. In this regard, the present system is substantially different from the Mg(NH2)2 + 8/3LiH or Mg(NH2)2 + 4LiH. The latter two systems exhibit higher capacity by proceeding the dehydrogenation reaction beyond the Li2Mg(NH)2 stage to Mg3N2/Li2NH or Mg3N2/Li3N. Such “deeper” reactions invariably require the application of low hydrogen partial pressure and/or high operating temperature.27-32 In the present study, however, the operating conditions (i.e.,

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Figure 8. Evolution of FTIR spectra for the Li-Mg-N-H sample prepared from the 2Li3N + 3Mg(NH2)2 mixture. The FTIR spectra were collected at selected points of the dehydrogenation process (under an initial pressure of 0.1 MPa H2 at 200 °C): (a) rehydrogenated, (b, c, and d) partially dehydrogenated. The signal intensities of dehydrogenated samples were all normalized to that of the rehydrogenated sample.

0.1 MPa hydrogen back pressure and 200 °C) restrain the proceeding of further dehydrogenation reactions,32 as confirmed by the FTIR and XRD results. The precluded compositional modification suggests that kinetic enhancement may be responsible for the improved capacity arising upon adding excess Li3N. In this regard, careful examination of the dehydrogenation reaction by using FTIR provides a valuable clue. Here, we select the Li-Mg-N-H sample prepared from the stoichiometric 2Li3N + 3Mg(NH2)2 mixture and examine the evolution of FTIR spectra at selected points of the dehydrogenation process. As shown in Figure 8, with proceeding the dehydrogenation reaction, the N-H bands of Mg(NH2)2 gradually weaken and finally disappear; meanwhile, the N-H stretch of Li2Mg(NH)2 appears and gains increasing intensity. Of particular interest, the N-H stretches of LiNH2 are also identified during dehydrogenation, which shows an initial intensity increase followed by intensity decrease. The structural similarity between LiNH2 and Li2Mg(NH)2 suggests that the identified LiNH2 should act as an important reaction intermediate of reaction 3. Similar speculation was also proposed by Hu et al. in their study of Li-Mg-N-H system.37 Following this mechanistic understanding, incorporation of foreign LiNH2 may provide favorable nucleation/growth sites for the intermediate LiNH2, which is similar to the promoting effect of product seeding.21 As a result of the lowered kinetic barriers associated with the intermediate nucleation, the dehydrogenation reaction of Li-Mg-N-H system can be promoted, thus leading to improved dehydrogenation capacity. This speculation is supported by our designed experiments. Starting from the Mg(NH2)2 + 2LiH mixture, we directly added 0.15 equiv of LiNH2 additive to obtain Mg(NH2)2 + 2LiH + 0.15LiNH2 sample. As shown in Figure 9, direct LiNH2 addition results in a property enhancement comparable to that obtained from the 2.3Li3N + 3Mg(NH2)2 mixture. A better understanding of the promoting effect of incorporated LiNH2 is obtained by establishing a quantitative relation between the LiNH2 addition and kinetic barrier. We applied the Arrhenius treatment to the dehydrogenation kinetics data to determine the apparent activation energies (Ea) and further compared the Ea values of Li-Mg-N-H samples prepared from the 2Li3N + 3Mg(NH2)2 and 2.3Li3N + 3Mg(NH2)2 mixtures. As shown in Figure 10, the Ea for dehydrogenation reaction of the sample prepared from 2Li3N + 3Mg(NH2)2 was calculated to be 92.2 kJ/mol. For the

Ma et al.

Figure 9. Comparison of dehydrogenation curves for the Li-Mg-N-H samples prepared from the 2.3Li3N + 3Mg(NH2)2 (open circle) and Mg(NH2)2 + 2LiH + 0.15LiNH2 (solid line) mixtures. The dehydrogenation measurements were performed at desired temperatures with an initial hydrogen pressure of 0.1 MPa. Both samples were rehydrogenated prior to the dehydrogenation measurements.

Figure 10. Arrhenius profiles of the dehydrogenation kinetics of the Li-Mg-N-H samples prepared from the 2Li3N + 3Mg(NH2)2 (black circle) and 2.3Li3N + 3Mg(NH2)2 (red square) mixtures for determination of apparent activation energy (Ea). Straight lines show that data are exponential with reciprocal temperature and comply with the Arrhenius equation: rate (wt %/h) ) ko exp(-Ea/RT).

latter reaction, in which 0.15 equiv of LiNH2 seed was in situ incorporated, the Ea was reduced to 88.1 kJ/mol. Clearly, the above results suggest that incorporation of intermediate seed is effective for lowering energy barriers associated with intermediate nucleation, which favors the proceeding of dehydrogenation reaction. In fact, the dehydrogenation reaction of the latter system has been driven toward completion while comparing its practical capacity 4.9 wt % with the theoretical value 5 wt % (after taking into account the amounts of excess Li3N and impurities). Consistent with our speculation, these findings suggest that the capacity enhancement arising upon adding excess Li3N originates from the enhanced dehydrogenation kinetics by the “intermediate seeding” of incorporated LiNH2. As LiNH2 does not participate in the dehydrogenation reaction under the applied conditions, its seeding effect is well maintained upon de/ rehydrogenation cycling. This may account for the cyclic stability of the obtained enhancement. Additionally, the property dependence on Li3N:Mg(NH2)2 ratio may be readily understood from that LiNH2 seeding can enhance the practical capacity via promoting dehydrogenation kinetics but overloading “inert” LiNH2 (together with LiH) inherently lowers the theoretical capacity of the system. Besides the enhanced hydrogen capacity,

Hydrogen Storage Properties of Li-Mg-N-H System addition of excess Li3N in the starting materials also effectively mitigates the ammonia release from thus-prepared Li-Mg-N-H system. This effect should be primarily attributed to the presence of excess LiH, which has been well established to act as an effective ammonia trapping agent.38,39 For the first time, we demonstrate that Li-Mg-N-H system can be prepared by reacting Mg(NH2)2 with Li3N. This new route allows us to readily enhance the reversible dehydrogenation properties of Li-Mg-N-H system by in situ incorporating LiNH2/LiH additives from excess Li3N. LiNH2 may promote the dehydrogenation reaction via seeding the reaction intermediate. The concurrently generated LiH is effectively for trapping the ammonia byproduct. 4. Conclusions Li-Mg-N-H system was successfully prepared from the Mg(NH2)2/Li3N mixture. The advantage of this method over the conventional route is that optimizing the Li3N:Mg(NH2)2 ratio in the starting materials can effectively enhance the practical hydrogen storage properties. Our screening study show that increasing the Li3N:Mg(NH2)2 ratio from 2:3 to 2.3:3 results in a capacity increase from 4.2 to 4.9 wt %. Such capacity enhancement is highly stable during hydrogen cycling. Additionally, the ammonia release from this sample is significantly mitigated. Systematic FTIR and XRD examinations indicate that the excess Li3N transforms to LiNH2 and LiH phases in the first rehydrogenation, which remain stable in the subsequent hydrogen cycling. Combined structure/property investigations suggest that the in situ generated LiNH2 promotes the dehydrogenation reaction via seeding the reaction intermediate. The concurrently generated LiH can effectively suppress the ammonia release. Our findings highlight the guidance of mechanistic understanding in exploring effective means for property enhancement and may be of significance for designing related metal-N-H systems for high-performance hydrogen storage. Acknowledgment. The financial supports for this research from the Hundred Talents Project of Chinese Academy of Sciences, the National Natural Science Foundation of China (grant nos. 50571099, 50671107, and 50771094) and National High-Tech R&D Program of China (863 Program, grant no. 2006AA05Z104) are gratefully acknowledged. References and Notes (1) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. Nature (London) 2002, 420, 302–304. (2) Chen, P.; Xiong, Z.; Wu, G.; Liu, Y.; Hu, J.; Luo, W. Scripta Mater. 2007, 56, 817–822. (3) Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Zu¨ttel, A.; Jensen, C. M. Chem. ReV. 2007, 107, 4111–4132. (4) Nakamori, Y.; Orimo, S. J. Alloys Compd. 2004, 370, 271–275. (5) Nakamori, Y.; Ninomiya, A.; Kitahara, G.; Aoki, M.; Noritake, T.; Miwa, K.; Kojima, Y.; Orimo, S. J. Power Sources 2006, 155, 447– 455. (6) Pinkerton, F. E.; Meisner, G. P.; Meyer, M. S.; Balogh, M. P.; Kundrat, M. D. J. Phys. Chem. B 2005, 109, 6–8.

J. Phys. Chem. C, Vol. 113, No. 22, 2009 9949 (7) Kojima, Y.; Matsumoto, M.; Kawai, Y.; Haga, T.; Ohba, N.; Miwa, K.; Towata, S. I.; Nakamori, Y.; Orimo, S. J. Phys. Chem. B 2006, 110, 9632–9636. (8) Xiong, Z. T.; Wu, G. T.; Hu, J. J.; Liu, Y. F.; Chen, P.; Luo, W. F.; Wang, J. AdV. Funct. Mater. 2007, 17, 1137–1142. (9) Janot, R.; Eymery, J. B.; Tarascon, J. M. J. Phys. Chem. C 2007, 111, 2335–2340. (10) Lu, J.; Fang, Z. Z.; Sohn, H. Y.; Bowman, R. C.; Hwang, S. J. J. Phys. Chem. C 2007, 111, 16686–16692. (11) Xiong, Z.; Wu, G.; Hu, J.; Chen, P. AdV. Mater. 2004, 16, 1522– 1525. (12) Luo, W. J. Alloys Compd. 2004, 381, 284–287. (13) Chen, Y.; Wu, C. Z.; Wang, P.; Cheng, H. M. Int. J. Hydrogen Energy 2006, 31, 1236–1240. (14) Chen, P.; Xiong, Z.; Yang, L.; Wu, G.; Luo, W. J. Phys. Chem. B 2006, 110, 14221–14225. (15) Rijssenbeek, J.; Gao, Y.; Hanson, J.; Huang, Q.; Jones, C.; Toby, B. J. Alloys Compd. 2008, 454, 233–244. (16) Kojima, Y.; Kawai, Y. J. Alloys Compd. 2005, 395, 236–239. (17) Xiong, Z. T.; Hu, J. J.; Wu, G. T.; Chen, P.; Luo, W. F.; Gross, K.; Wang, J. J. Alloys Compd. 2005, 398, 235–239. (18) Yang, J.; Sudik, A; Wolverton, C. J. Alloys Compd. 2007, 430, 334–338. (19) Liu, Y.; Hu, J.; Xiong, Z.; Wu, G. J. Mater. Res. 2007, 22, 1339– 1345. (20) Luo, S.; Flanagan, T. B.; Luo, W. J. Alloys Compd. 2007, 440, L13-L17. (21) Sudik, A.; Yang, J.; Halliday, D.; Wolverton, C. J. Phys. Chem. C 2007, 111, 6568–6573. (22) Yang, J.; Sudik, A.; Siegel, D. J.; Halliday, D.; Drews, A.; Carter, R. O.; Wolverton, C.; Lewis, G. J.; Sachtler, J. W. A.; Low, J. J.; Faheem, S. A.; Lesch, D. A.; Ozolins, V. Angew. Chem., Int. Ed. 2008, 47, 882– 887. (23) Hu, J. J.; Liu, Y. F.; Wu, G. T.; Xiong, Z. T.; Chua, Y. S.; Chen, P. Chem. Mater. 2008, 20, 4398–4402. (24) Luo, W. F.; Stewart, K. J. Alloys Compd. 2007, 440, 357–361. (25) Rajalakshmi, N.; Jayanth, T.; Dhathathreyan, K. Fuel Cells 2003, 3, 177–180. (26) Uribe, F. A.; Gottesfeld, S.; Zawodzinski, T. A., Jr. J. Electrochem. Soc. 2002, 149, A293–A296. (27) Leng, H.; Ichikawa, T.; Hino, S.; Hanada, N.; Isobe, S.; Fujii, H. J. Phys. Chem. B 2004, 108, 8763–8765. (28) Orimo, S.; Nakamori, Y.; Kitahara, G.; Miwa, K.; Ohba, N.; Noritake, T.; Towata, S. Appl. Phys. A: Mater. Sci. Process. 2004, 79, 1765– 1767. (29) Nakamori, Y.; Kitahara, G.; Miwa, K.; Towata, S.; Orimo, S. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 1–3. (30) Nakamori, Y.; Kitahara, G.; Ninomiya, A.; Aoki, M.; Noritake, T.; Towata, S.; Orimo, S. Mater. Trans. 2005, 46, 2093–2097. (31) Leng, H.; Ichikawa, T.; Fujii, H. J. Phys. Chem. B 2006, 110, 12964–12968. (32) Nakamura, Y.; Hino, S.; Ichikawa, T.; Fujii, H.; Brinks, H. W.; Hauback, B. C. J. Alloys Compd. 2008, 457, 362–367. (33) Hu, Y. H.; Ruckenstein, E. Ind. Eng. Chem. Res. 2006, 45, 4993– 4998. (34) Markmaitree, T.; Osbon, W.; Shaw, L. L. Int. J. Hydrogen Energy 2008, 33, 3915–3924. (35) Barin, I. Thermochemical Data of Pure Substances, 3rd ed.; WileyVCH Verlag GmbH: Weinheim, Germany, 1995; pp 1006-1008. (36) Bohger, J. P. O.; Essmann, R. R.; Jacobs, H. J. Mol. Struct. 1995, 348, 325–328. (37) Hu, J.; Liu, Y.; Wu, G.; Xiong, Z.; Chen, P. J. Phys. Chem. C 2007, 111, 18439–18443. (38) Hu, Y.; Ruckenstein, E. J. Phys. Chem. A 2003, 107, 9737–9739. (39) Ichikawa, T.; Hanada, N.; Isobe, S.; Leng, H. Y.; Fujii, H. J. Phys. Chem. B 2004, 108, 7887–7892.

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