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MgH2 mixture
international journal of hydrogen energy 33 (2008) 6188–6194
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effects of mechanical milling on desorption kinetics and phase transformation of LiNH2/MgH2 mixture Rohit R. Shahi, T.P. Yadav, M.A. Shaz, O.N. Srivastava* Department of Physics, Banaras Hindu University, VNS 221005, India
article info
abstract
Article history:
We have investigated the ball-milling (5–45 h) followed by dehydrogenation kinetics of
Received 14 December 2007
LiNH2–MgH2 mixture. A new result of ball-milling reveals the formation of Mg(NH2)2 for
Received in revised form
mixtures ball-milled for more than 15 h. The desorption kinetics has been found to
31 March 2008
improve appreciably by increasing the duration of milling from 5 to 45 h. This enhance-
Accepted 10 July 2008
ment is attributed to formation of Mg(NH2)2 during this process. It was found that the
Available online 2 October 2008
Mg(NH2)2 form through the reaction 2LiNH2 þ MgH2 / Mg(NH2)2 þ 2LiH. The phase and microstructural characterizations have been done through X-ray diffraction, IR-spec-
Keywords:
trometry, transmission and scanning electron microscopy. Correlation between particle
Hydrogen storage materials
size and hydrogenation behavior has also been investigated.
Ball-milling
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
Dehydrogenation kinetics
reserved.
LiNH2 Mg(NH2)2
1.
Introduction
It is generally agreed now that for harnessing hydrogen, particularly for vehicular transport, storage forms the crucial issue. The R&D effort in intermetallic hydrides, which corresponded to first storage system for hydrogen in solids, for more than two decades, has resulted in obtaining storage capacity of only w2 wt.% [1,2]. The required storage capacity for vehicular transport is >9 wt.% [3]. The first result on high (6 wt.%) hydrogen storage capacity in carbon nanotubes/ nanofibers [4], does not seem to have survived the test of time. There are several controversial results in hydrogen storage capacities of carbon nanotubes/nanofibers. Reproducible high hydrogen storage capacities in these materials have not been obtained [4]. However, researchers at the University of Virginia have claimed to have formed carbon-based materials with 8 wt.% storage capacity. The full details of this material are still not available [5]. Therefore, there is an ever ongoing
research for finding new hydrogen storage materials with adequate (w6 wt.%) storage capacities. In the past several years, new storage systems like light weight complex hydrides such as alanates typified by sodium alanate NaAlH4 (storage capacity of 7.4 wt.%) have drawn considerable attention [6–8]. Another compound namely Li–N–H system also possesses significant hydrogen-storage capacity [9]. Chen et al were the first to report a novel light-weight storage system Li–N–H that can absorb 10.6 wt.% hydrogen reversibly. However, this system possesses unfavorable thermodynamics [10,11]. It needs to be modified in order to make it a viable storage material [12]. Several studies have recently been conducted, which reveal that the material tailoring through replacement of part of Li by Mg gives rise to a system that is close to a viable material. It is difficult to synthesize such material because of its highly oxidizing nature and since these materials are prepared at high temperature they are prone to get oxidized. One feasible way of synthesizing the
* Corresponding author. Tel.: þ91 542 2368468; fax: þ91 542 2369889. E-mail address: [email protected] (O.N. Srivastava). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.07.029
international journal of hydrogen energy 33 (2008) 6188–6194
Li–Mg–N–H system is to ball-mill mixture of LiNH2 and MgH2. This process allows reaction to take place at much lower temperature [12]. In this article, we report the effect of ball-milling duration of LiNH2 and MgH2 mixtures on dehydrogenation behavior particularly on dehydrogenation kinetics of Li–Mg–N–H storage system. The investigation carried out in present study reveals the new result on the formation of Mg(NH2)2 phase. This phase has been formed as a result of ball-milling alone of the LiNH2–MgH2 mixture. The formation of this phase leads to the enhancement of desorption kinetics (up to 49%) for the 45 h ball-milled sample.
2.
Experimental details
The lithium-amide (Aldrich, 95% pure) and magnesium hydride (Alfa aeser, 98% pure) mixture was prepared with a molar ratio of 2:1.1. The 10% excess of MgH2 was added to minimize the loss of NH3 during dehydriding process [12]. High-energy ball-milling conducted using attriter ball-mill (Szegvari attritor) with modification in milling vial. The modified vial (volume of modified vial is 10 times smaller than the original vial) of attriter ball-mill and balls (each having diameter w6.5 mm) were both made of stainless steel. The loading of balls and powder to the vial was performed in the glove box. Ball-milling was conducted under high-purity inert gas (argon) atmosphere. The ball to powder weight ratio was 50:1, milling speed was 400 rpm and milling temperature was maintained at room temperature achieved by water cooling arrangement. The entire sample before and after ball-milling was handled in glove box. To minimize the contamination of the sample through H2O/O2 during sample transportation and measurements, parafilm (Pechinery plastic packaging) was used to cover the surface of sample. We have studied the effect of milling duration on kinetics and phase transformation during dehydrogenation reaction. Hydrogen desorption behavior of materials, each having weight 1 g, were examined by using a carefully calibrated Sievert’s type apparatus [7]. The desorption rate was measured in weight percent per hour. The desorption back pressure for 5 and 45 h ball-milled sample has been found to be 5.61 and 5.96 atm, respectively. Ball-milling time was varied from 5 to 45 h. The powder mixture with and without high-energy ball-milling were analyzed by using powder X-ray diffraction (XRD) (‘X’ Pert PRO PANalytical), IR-spectrometry (NICOLET 57000, FT-IR Thermo Electron Corporation), transmission (TEM) (FEI, TECNAI 20G2) and scanning (SEM) (Philips XL-20) electron microscopy. In addition to identification of phase transformations, X-ray diffraction patterns were also used to estimate crystallite sizes of initial LiNH2, MgH2 and ball-milled LiNH2/MgH2 mixture. The use of the Voigt function for the analysis of the integral breadths of broadened X-ray diffraction peak forms the basis of a rapid and powerful single-peak method of crystallite size and strain determination [13]. In this case, the constituent Couchy and Gaussian components can be obtained from the ratio of full width at half maximum intensity (2u) and integral breadth (b). In a single-peak analysis, the apparent crystallite size ‘D’ and strain ‘e’ can be related to Couchy (bc) and Gaussian (bG)
6189
widths of the diffraction peak at the Bragg’s angle q; D¼
l bC cos q
and e ¼
bG 4 tan q
The constituent Couchy and Gaussian components can be given as ! 1=2 2 bC ¼ a0 þ a1 j þ a2 j2 b; bG ¼ b0 þ b1=2 j þb1 j þ b2 j2 b p where a0, a1 and a2 are Couchy constants; b0, b1/2, b1 and b2 are Gaussian constants and j ¼ 2u/b where b is the integral breadth obtained from XRD peak. The values of Couchy and Gaussian constant have been taken from the Langford table [14]. These are a0 ¼ 2.0207, a1 ¼ 0.4803, a2 ¼ 1.7756; b0 ¼ 0.6420, b1/2 ¼ 1.4187, b1 ¼ 2.2043, b2 ¼ 1.8706. Using Voigt function technique, we have calculated the crystallite size D and the lattice strain ‘‘e’’ for the initial LiNH2, MgH2, ball-milled LiNH2/MgH2 mixture, and Mg(NH2)2/LiH phases.
3.
Results and discussion
LiNH2/MgH2 mixture exhibited improvement in desorption kinetics with respect to ball-milling duration. The XRD pattern evaluation revealed that the reaction involved in the first dehydrogenation. Fig. 1 gives representative XRD patterns of initial LiNH2, MgH2, the ball-milled mixture and those obtained after first de/rehydrogenation cycle. The sample was dehydrogenated at 200 C. As a result of this a mixture of lithium and magnesium imide was formed. The MgNH phase found to have tetragonal unit cell with lattice parameter ˚ , c ¼ 3.677 A ˚ and Li2NH have cubic unit cell with a ¼ 11.58 A ˚ . The sample was rehydrogenated lattice parameter a ¼ 5.053 A at 220 C and 110 atm. of hydrogen pressure, the imide
Fig. 1 – X-ray diffraction patterns of (a) initial LiNH2, (b) initial MgH2, (c) 5 h ball-milled sample of 2:1.1 molar mixture of LiNH2 and MgH2, (d) dehydrogenated (at 200 8C) sample and (e) rehydrogenated (at 220 8C and 120 atm of H2) sample.
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international journal of hydrogen energy 33 (2008) 6188–6194
mixture was converted to Mg(NH2)2 and LiH rather than the initial LiNH2 and MgH2, which is recently reported by Lohstroh and Fichtner [15]. As the final product of both reactions a mixed phase of composition Li2Mg(NH)2 has also been proposed [12,16,17]. However, this phase is still under discussion. In addition to X-ray diffraction patterns the phase changes have also been identified by FTIR spectroscopy as shown in Figs. 2 and 5. The spectra shown in Fig. 2 display amide/imide conversion of 5 h ball-milled mixture at different stages of reaction. Fig. 2a depicts the characteristic IR absorption of 5 h ball-milled mixture of LiNH2 and MgH2. The IR absorption peaks are located at 3256 and 3310 cm1 for LiNH2, symmetric and asymmetric stretching [18], respectively. Fig. 2b shows the IR absorption spectra of dehydrogenated sample, it shows two absorption peaks at 3192 and 3162 cm1 that corresponds to MgNH [19] and Li2NH [20], respectively. The IR absorption for rehydrogenated state shown in Fig 2c displays two bands at 3282 and 3330 cm1 for Mg(NH2)2 [19]. These results are supported by X-ray diffraction data shown in Fig. 1. Both X-ray diffraction and IR spectroscopy show that the dehydrogenation products in present investigation are Li2NH and MgNH. The de/rehydrogenation process of material prepared in the present investigation by milling of 2:1.1 molar ratio of LiNH2/MgH2 may be described by the following reactions. 2LiNH2 þ MgH2 / Li2 NH þ MgNH þ 2H2 4 MgðNH2 Þ2 þ 2LiH (1) The structural change resulting upon the first dehydrogenation cycle was further evidenced by transmission electron microscopy. The microstructure and selected area electron diffraction pattern (SAED) of milled and dehydrogenated samples are shown in Fig. 3a and b, respectively. For the milled sample, the SAED pattern exhibits diffraction rings as well as spots, which were identified to correspond to crystallites of MgH2 and LiNH2, respectively, the microstructure shows that the MgH2 gets homogeneously mixed with LiNH2. For dehydrogenated sample, SAED pattern also exhibits Fig. 3 – TEM images of (a) 5 h ball-milled and (b) dehydrogenated sample of 2:1.1 molar LiNH2 and MgH2 mixture.
Fig. 2 – Infrared spectra of 2:1.1 LiNH2–MgH2: (a) 5 h ballmilled mixture, (b) dehydrogenated sample at 200 8C and (c) rehydrogenated under 120 atm at 220 8C.
diffraction rings as well as spots that have been identified to be from MgNH and Li2NH, respectively. The representative XRD patterns of ball-milled samples with different milling durations are shown in Fig. 4. After ballmilling for 5 h, it is possible to recognize the two main phases namely LiNH2 and MgH2. As can be seen XRD peak revealing the presence of Mg(NH2)2 phase starts appearing after 15 h of ball-milling. This phase increases with increasing ball-milling duration along with LiH phase. This shows that prolonged ball-milling produces reaction-based phase transformation. Further, these phases are part of reversible reaction. However, the appearance of MgO peak after prolonged ball-milling (30 h) is the inhibiting factor. MgO seems to arise due to oxidation of the mixture with smaller Mg particles. A feasible phase transformation scheme taking place due to prolonged ball-milling is outlined in the following. Further, the product, Mg(NH2)2 þ 2LiH undergoes reversible
international journal of hydrogen energy 33 (2008) 6188–6194
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Fig. 4 – X-ray diffraction patterns of (a) 5 h, (b) 15 h, (c) 30 h, (d) 35 h and (e) 45 h ball-milled samples of 2:1.1 molar mixture of LiNH2 and MgH2, (f) dehydrogenated (at 200 8C) sample of 45 h ball-milled mixture, (g) rehydrogenated (at 220 8C and 120 atm of H2) sample of 45 h ball-milled mixture, (h) enlarged view of XRD peak at 2q [ 30.1268 Mg(NH2)2 and 2q [ 30.698 LiNH2.
dehydrogenation and rehydrogenation. This has also been shown in the following. 2LiNH2 þ MgH2 / MgðNH2 Þ2 þ 2LiH 4 Li2 NH þ MgNH þ 2H2 (2) As can be seen from Fig. 4 the characteristic XRD peak for Mg(NH2)2 phase at 2q ¼ 23.265 , which was indexed as the most intense peak of this phase starts developing after 15 h of ball-milling and become pronounced for the milling duration of 45 h. It is interesting to note that in addition to X-ray ˚ ) other diffraction peak from Mg(NH2)2 at 2q ¼ 23.26 (d w 3.82A
Fig. 5 – Infrared spectra of (a) initial, (b) 5 h, (c) 15 h, (d) 35 h and (e) 45 h, ball-milled mixture of 2:1.1 LiNH2–MgH2 (bold line for LiNH2 and dotted line for Mg(NH2)2 bands).
Fig. 6 – Kinetics of (a) 5 h, (b) 45 h ball-milled and (c) comparison of kinetics of different hours ball-milled samples of 2:1.1 molar mixtures of LiNH2 and MgH2 at 200 8C.
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international journal of hydrogen energy 33 (2008) 6188–6194
Table 1 – Variation in kinetics of 2:1.1 molar mixtures of LiNH2/MgH2 Sl. No. 1. 2. 3. 4. 5.
Ball-milling time (h)
Desorption temperature ( C)
Storage capacity (wt.%)
Desorption time (min)
Desorption kinetics (wt.%/h)
Increment in kinetics (%/h)
5 15 30 35 45
200 200 200 200 200
4.0 4.05 4.15 4.21 4.28
175 160 145 140 125
1.37 1.52 1.72 1.82 2.05
Starting material 10.94 25.54 33.84 49.63
diffraction peaks for Mg(NH2)2 phase also appear. However, the d values for several of these are very close to the d values for LiNH2. Therefore, the X-ray diffraction peaks from Mg(NH2)2 phase merge with that from LiNH2. This is exem˚ plified for Mg(NH2)2:(224) diffraction peak with d w 2.964A (2q ¼ 30.126 ) together with LiNH2:(112) diffraction peak with ˚ (2q ¼ 30.69 ) in Fig. 4h. The two d values are so close d w 2.91A that the two peaks merge together. Fig. 4h brings out the magnified version of the observed XRD peak for different ballmilling duration. As can be seen, the smooth peak for 5 h ball-milling, developed shoulder (marked by arrows) for ballmilling durations of 15, 30, 35, and 45 h. The appearance of shoulder is indicative of development of another phase that is presumably Mg(NH2)2. Similar is the case for diffraction peaks where the presence of both LiNH2 and Mg(NH2)2 have been indicated. These feature coupled with invariable presence of diffraction peak at 2q ¼ 23.265 that is the most intense XRD peak of Mg(NH2)2 confirm that this phase indeed gets formed due to ball-milling. In order to further establish the formation of Mg(NH2)2 through ball-milling, we have characterized the samples through FTIR at different stages of ball-milling duration. Fig. 5 shows that the IR spectrums of initial LiNH2 and different hours ball-milled LiNH2/MgH2 mixture. The LiNH2 spectrum exhibits characteristic bands at 3256 and 3310 cm1 [18] as shown in Fig. 5a. The LiNH2 is not decomposed upon 5 h of ball-milling (Fig. 5b). With intense ball-milling (15 h) the characteristic bands at 3278 and 3325 cm1 appear, these values are close to the bands for Mg(NH2)2 [19] as shown in Fig. 5c. The peaks corresponding to these wave numbers grow with increasing ball-milling duration. At the prolonged ballmilling greater than 15 h, the characteristic band intensity of LiNH2 decreases along with increasing Mg(NH2)2 band intensity (Fig. 5d and e). These results support the XRD data shown in Fig. 5. Both XRD and IR-spectroscopy show that the prolonged ball-milling (15 h) leads to phase transformations. In particular it reveals the formation of Mg(NH2)2. The effect of milling duration on desorption kinetics of 2:1.1 molar mixture of LiNH2/MgH2 is shown in Fig. 6. It has been found that the desorption kinetics of milled sample increases with increasing milling duration. Fig. 6a represents typical dehydrogenation profile of 2:1.1 molar LiNH2/MgH2 mixtures under inert atmosphere for six cycles at 200 C. It has been found that the hydrogen uptake capacity of 5 h milled materials w4 wt.% within 3 h. The hydrogen uptake capacity of 45 h ball-milled sample is found to be w4.3 wt.% within w2 h as shown in Fig. 6b. The desorption kinetics improved with degree of mechanical milling as shown in Fig. 6c. It has been found that the desorption kinetics of ball-milled sample increases up to 49% with ball-milling duration ranging from 5 to 45 h at
200 C (Table 1). As regards possible release of ammonia, we did not find presence of ammonia in the desorbed hydrogen. It may be pointed out that even if NH3 is released, it will recombine with LiH that is a known ammonia getter [21]. The diffraction peaks at 2q ¼ 30.4 and 2q ¼ 27.5 in Fig. 7 could be indexed as (112) peak of LiNH2 and (110) peak of MgH2, respectively. Substantial peak broadening was found to result with prolonged milling duration. This possibly arises due to continuous deformation and breaking of material particles. There will also be disorder produced due to strain stored in the particles due to mechanical milling. This strain has been calculated employing Voigt function technique. The variation of calculated crystallite size and lattice strain with ball-milling duration is shown in Table 2. For the mixture, the calculated crystallite size of LiNH2 and MgH2 decreased for longer ballmilling duration. It has been found that after 5 h milling the crystallite sizes of LiNH2 and MgH2 are 127 and 54 nm, respectively. The increase in ball-milling time beyond 5 h resulted in further decrease in crystallite size. However, the reduction rate in crystallite size is dramatically reduced. It has been found that after ball-milling for 45 h, the crystallite sizes of LiNH2 and MgH2 are 11 and 12 nm, respectively. As can be seen from Table 2, the crystallite size of the formed Mg(NH2)2 and LiH also decreases with increasing ball-milling duration ranging from 15 to 45 h. The X-ray diffraction peaks started becoming excessively broadened approaching halo-like feature for ball-milling durations exceeding 45 h. Therefore, we could not separate particle size reduction effect on the XRD peak broadening after
Fig. 7 – Peak broadening with ball milling durations of (110) and (112) diffraction peaks of MgH2 and LiNH2, respectively.
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international journal of hydrogen energy 33 (2008) 6188–6194
Table 2 – The variation of crystallite size and lattice strain with milling duration Sl. Ball-milling No. time (h)
MgH2
LiNH2
Compounds formed to reaction Mg(NH2)2
1. 2. 3. 4. 5. 6.
0 5 15 30 35 45
Crystalline size (D) 0.5 (nm)
Lattice strain (e)
Crystalline size (D) 0.5 (nm)
Lattice strain (e)
378 127 42 18 13 11
Initial strain 0.23 0.26 0.33 0.37 0.39
150 54 29 20 17 12
Initial strain 0.21 0.26 0.34 0.44 0.52
LiH
Crystalline size Lattice Crystalline size Lattice (D) 0.5 (nm) strain (e) (D) 0.5 (nm) strain (e) – – 48 16 15 14
– – 0.002 0.040 0.045 0.070
– – 60 33 25 16
– – 0.018 0.020 0.028 0.039
Fig. 8 – SEM images of 2:1.1 molar mixture of LiNH2 and MgH2 (a) initial powder, (b) 5 h, (c) 15 h, (d) 30 h, (e) 35 h, and (f) 45 h.
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international journal of hydrogen energy 33 (2008) 6188–6194
45 h ball-milling. Also ball-milling beyond 45 h led to increase of MgO content. Therefore, ball-milling durations greater from 45 h were not employed. Fig. 8 shows that the SEM images of the LiNH2 and MgH2 mixture before and after ball milling. The initial LiNH2 and MgH2 have faceted grains and bimodal particle-size distribution with small number of particles around 4 mm and large particles of size around 30 mm being present. Ball-milling for 5 h has led substantial reduction in average particle size w2.5 mm and some large particles of particle size w16 mm are still present. Further milling beyond 5 h led to decrease in particle size of large particle but not of small ones. The 45 h milled sample possesses particle size of w2.36 and w6.07 mm. This result combined with the variation of crystallite size suggested that ball-milling beyond 5 h leads to breaking down of large particles but not of small ones. Further breaking of small particles is very limited for ball-milling beyond 5 h. Chen et al. [16], reported that the reversibility of this system is related to local interaction between Mg(NH2)2 and LiH as is clear from Fig. 8c to f. The sample ball-milled for longer durations (>5 h) are characterized by lower crystallite size and comparatively more homogeneous dispersion of the constituent particles. The small particle size that starts becoming discernible after 5 h ball-milling will result in mechanical milling-induced diffusion. This LiNH2/MgH2 will leads to reaction-based transformation yielding Mg(NH2)2 together with LiH. Since Mg(NH2)2 and LiH are known [22] to have suitable DH and DS for low desorption temperature (90 C) and pressure (1 bar). Therefore, formation of Mg(NH2)2 will result in improved desorption kinetics. This is what has actually been assured. The desorption kinetics has been found to increase progressively as the ball-milling increases from 15 to 45 h resulting in increase in formation of Mg(NH2)2 phase. As outlined earlier, ball-milling duration greater than 45 h leads to significant increase in undesirable phase MgO hence ball-milling duration greater than 45 h were not tried.
4.
Conclusions
In conclusion, it can be said that the major and new result of our investigation is the appearance of Mg(NH2)2 phase directly by ball-milling of LiNH2 and MgH2 mixture. This procedure skips a step in reaction leading to the formation of Mg(NH2)2 through dehydrogenation/rehydrogenation of Li–Mg–N–H system as found by other workers [12,15,16]. A related new result corresponds to appreciable enhancement of desorption kinetics up to 49% for the 45 h ball-milled mixtures consisting Mg(NH2)2 and LiH.
Acknowledgments The authors acknowledge Prof. A.R. Verma (Delhi, India), Prof. T.N. Veziroglu (President, IAHE, Florida, USA), Prof. R.S. Tiwari for their encouragement and helpful discussion. They also thank Prof. A.S.K.Sinha, Chemical Engineering IIT, BHU for his help in carrying out the FTIR Spectroscopy investigation. Financial assistance from Ministry of New and Renewable
Energy (MNRE) and DST (UNANST), New Delhi (India) is gratefully acknowledged.
references
[1] Schlapbach L, Zu¨ttel A. Hydrogen-storage materials for mobile applications. Nature 2001;414:353–8. [2] Pandey SK, Srivastava A, Srivastava ON. Improvement in hydrogen storage capacity in LaNi5 through substitution of Ni by Fe. Int J Hydrogen Energy 2007;32:2461–5. [3] Shi S, Hwang J. Research frontier on new materials and concepts for hydrogen storage. Int J Hydrogen Energy 2007;32:224–8. [4] Strobel R, Garche J, Maseley PT, Jaorissen L, Walf G. Hydrogen storage by carbon materials. J Power sources 2006;159:781–801. [5] Phillips AB, Shivaram. Record-breaking hydrogen storage materials for use in fuel cells developed University of Virginia, (2007 November 13). Available at: http://www. sciencedaily.com/releases/2007/11/071112133751.htm. [6] Boganovic B, Schwickardi M. Ti-doped alkali metal aluminium hydrides as potential novel reversible hydrogen storage materials. J Alloys Compd 1997;253–254:1–9. [7] Sandrock G, Gross K, Thomas G. Effect of Ti-catalyst content on the reversible hydrogen storage properties of the sodium alanates. J Alloys Compd 2002;339:299–308. [8] Sterlin Leo HM, Pukazhselvan D, Irena SG, Srivastava ON. Studies on synthesis and dehydrogenation behavior of magnesium alanate and magnesium–sodium alanate mixture. Int J Hydrogen Energy 2007;32:4933–8. [9] Chen P, Xiong Z, Luo J, Tan J, Lin KL. Interaction of hydrogen with metal nitrides and imides. Nature 2002;420:302–4. [10] Chen P, Xiong Z, Luo J, Tan J, Lin KL. Interaction between lithium amide and lithium hydride. J Phys Chem B 2003;107: 10967–70. [11] Ichikawa T, Hanada N, Isobe S, Len H. Mechanism of novel reaction from LiNH2 and LiH to Li2NH and H2 as a promising hydrogen storage system. J Phys Chem B 2004;108:7887–92. [12] Weifang L. (LiNH2–MgH2): a viable hydrogen storage system. J Alloys Compd 2004;381:284–7. [13] Yadava TP, Mukhopadhyay NK, Tiwari RS, Srivastava ON. Studies on the formation and stability of nano-crystalline Al50Cu28Fe22 alloy synthesized through high-energy ball milling. Mater Sci Eng A 2005;393:366–73. [14] Keljserde Th H, Langford JI, Mittemeijer EJ, Vogels ABP. Use of the Voigt function in a single-line method for the analysis of X-ray diffraction line broadening. J Appl Cryst 1982;15:308. [15] Lohstroh W, Fichtner M. Reaction steps in the Li–Mg–N–H hydrogen storage system. J Alloys Compd 2007;446–447:332–5. [16] Chen Y, Wu CZ, Wang P, Chang HM. Structure and hydrogen storage property of ball-milled LiNH2/MgH2 mixture. Int J Hydrogen Energy 2006;31:1236–40. [17] Luo W, Sickafoose S. Thermodynamic and structural characterization of the Mg–Li–N–H hydrogen storage system. J Alloys Compd 2006;407:274. [18] Bohger JPO, Ebmannb R, Jacobsa H. Infrared and Raman studies on the internal modes of lithium amide. J Molecular Struct 1995;348:325–8. [19] Linde G, Juza R, Anorg Z. IR spectra of amides and imides of divalent and trivalent metals. Allg Chem 1974;409:199. [20] Y Kojima, Y Kawai. IR characterizations of lithium imide and amide. J Alloys Compd 2005;395:236–9. [21] Janot R, Eymery J-B, Tarascon J-M. Investigation of the processes for reversible hydrogen storage in the Li–Mg–N–H system. J Power Sources 2007;164:496–502. [22] J Yang, A Sudik, Wolverton C. Activation of hydrogen storage materials in the Li–Mg–N–H system: effect on storage properties. J Alloys Compd 2006;430:334–6.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effects of mechanical milling on desorption kinetics and phase transformation of LiNH2/MgH2 mixture Rohit R. Shahi, T.P. Yadav, M.A. Shaz, O.N. Srivastava* Department of Physics, Banaras Hindu University, VNS 221005, India
article info
abstract
Article history:
We have investigated the ball-milling (5–45 h) followed by dehydrogenation kinetics of
Received 14 December 2007
LiNH2–MgH2 mixture. A new result of ball-milling reveals the formation of Mg(NH2)2 for
Received in revised form
mixtures ball-milled for more than 15 h. The desorption kinetics has been found to
31 March 2008
improve appreciably by increasing the duration of milling from 5 to 45 h. This enhance-
Accepted 10 July 2008
ment is attributed to formation of Mg(NH2)2 during this process. It was found that the
Available online 2 October 2008
Mg(NH2)2 form through the reaction 2LiNH2 þ MgH2 / Mg(NH2)2 þ 2LiH. The phase and microstructural characterizations have been done through X-ray diffraction, IR-spec-
Keywords:
trometry, transmission and scanning electron microscopy. Correlation between particle
Hydrogen storage materials
size and hydrogenation behavior has also been investigated.
Ball-milling
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
Dehydrogenation kinetics
reserved.
LiNH2 Mg(NH2)2
1.
Introduction
It is generally agreed now that for harnessing hydrogen, particularly for vehicular transport, storage forms the crucial issue. The R&D effort in intermetallic hydrides, which corresponded to first storage system for hydrogen in solids, for more than two decades, has resulted in obtaining storage capacity of only w2 wt.% [1,2]. The required storage capacity for vehicular transport is >9 wt.% [3]. The first result on high (6 wt.%) hydrogen storage capacity in carbon nanotubes/ nanofibers [4], does not seem to have survived the test of time. There are several controversial results in hydrogen storage capacities of carbon nanotubes/nanofibers. Reproducible high hydrogen storage capacities in these materials have not been obtained [4]. However, researchers at the University of Virginia have claimed to have formed carbon-based materials with 8 wt.% storage capacity. The full details of this material are still not available [5]. Therefore, there is an ever ongoing
research for finding new hydrogen storage materials with adequate (w6 wt.%) storage capacities. In the past several years, new storage systems like light weight complex hydrides such as alanates typified by sodium alanate NaAlH4 (storage capacity of 7.4 wt.%) have drawn considerable attention [6–8]. Another compound namely Li–N–H system also possesses significant hydrogen-storage capacity [9]. Chen et al were the first to report a novel light-weight storage system Li–N–H that can absorb 10.6 wt.% hydrogen reversibly. However, this system possesses unfavorable thermodynamics [10,11]. It needs to be modified in order to make it a viable storage material [12]. Several studies have recently been conducted, which reveal that the material tailoring through replacement of part of Li by Mg gives rise to a system that is close to a viable material. It is difficult to synthesize such material because of its highly oxidizing nature and since these materials are prepared at high temperature they are prone to get oxidized. One feasible way of synthesizing the
* Corresponding author. Tel.: þ91 542 2368468; fax: þ91 542 2369889. E-mail address: [email protected] (O.N. Srivastava). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.07.029
international journal of hydrogen energy 33 (2008) 6188–6194
Li–Mg–N–H system is to ball-mill mixture of LiNH2 and MgH2. This process allows reaction to take place at much lower temperature [12]. In this article, we report the effect of ball-milling duration of LiNH2 and MgH2 mixtures on dehydrogenation behavior particularly on dehydrogenation kinetics of Li–Mg–N–H storage system. The investigation carried out in present study reveals the new result on the formation of Mg(NH2)2 phase. This phase has been formed as a result of ball-milling alone of the LiNH2–MgH2 mixture. The formation of this phase leads to the enhancement of desorption kinetics (up to 49%) for the 45 h ball-milled sample.
2.
Experimental details
The lithium-amide (Aldrich, 95% pure) and magnesium hydride (Alfa aeser, 98% pure) mixture was prepared with a molar ratio of 2:1.1. The 10% excess of MgH2 was added to minimize the loss of NH3 during dehydriding process [12]. High-energy ball-milling conducted using attriter ball-mill (Szegvari attritor) with modification in milling vial. The modified vial (volume of modified vial is 10 times smaller than the original vial) of attriter ball-mill and balls (each having diameter w6.5 mm) were both made of stainless steel. The loading of balls and powder to the vial was performed in the glove box. Ball-milling was conducted under high-purity inert gas (argon) atmosphere. The ball to powder weight ratio was 50:1, milling speed was 400 rpm and milling temperature was maintained at room temperature achieved by water cooling arrangement. The entire sample before and after ball-milling was handled in glove box. To minimize the contamination of the sample through H2O/O2 during sample transportation and measurements, parafilm (Pechinery plastic packaging) was used to cover the surface of sample. We have studied the effect of milling duration on kinetics and phase transformation during dehydrogenation reaction. Hydrogen desorption behavior of materials, each having weight 1 g, were examined by using a carefully calibrated Sievert’s type apparatus [7]. The desorption rate was measured in weight percent per hour. The desorption back pressure for 5 and 45 h ball-milled sample has been found to be 5.61 and 5.96 atm, respectively. Ball-milling time was varied from 5 to 45 h. The powder mixture with and without high-energy ball-milling were analyzed by using powder X-ray diffraction (XRD) (‘X’ Pert PRO PANalytical), IR-spectrometry (NICOLET 57000, FT-IR Thermo Electron Corporation), transmission (TEM) (FEI, TECNAI 20G2) and scanning (SEM) (Philips XL-20) electron microscopy. In addition to identification of phase transformations, X-ray diffraction patterns were also used to estimate crystallite sizes of initial LiNH2, MgH2 and ball-milled LiNH2/MgH2 mixture. The use of the Voigt function for the analysis of the integral breadths of broadened X-ray diffraction peak forms the basis of a rapid and powerful single-peak method of crystallite size and strain determination [13]. In this case, the constituent Couchy and Gaussian components can be obtained from the ratio of full width at half maximum intensity (2u) and integral breadth (b). In a single-peak analysis, the apparent crystallite size ‘D’ and strain ‘e’ can be related to Couchy (bc) and Gaussian (bG)
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widths of the diffraction peak at the Bragg’s angle q; D¼
l bC cos q
and e ¼
bG 4 tan q
The constituent Couchy and Gaussian components can be given as ! 1=2 2 bC ¼ a0 þ a1 j þ a2 j2 b; bG ¼ b0 þ b1=2 j þb1 j þ b2 j2 b p where a0, a1 and a2 are Couchy constants; b0, b1/2, b1 and b2 are Gaussian constants and j ¼ 2u/b where b is the integral breadth obtained from XRD peak. The values of Couchy and Gaussian constant have been taken from the Langford table [14]. These are a0 ¼ 2.0207, a1 ¼ 0.4803, a2 ¼ 1.7756; b0 ¼ 0.6420, b1/2 ¼ 1.4187, b1 ¼ 2.2043, b2 ¼ 1.8706. Using Voigt function technique, we have calculated the crystallite size D and the lattice strain ‘‘e’’ for the initial LiNH2, MgH2, ball-milled LiNH2/MgH2 mixture, and Mg(NH2)2/LiH phases.
3.
Results and discussion
LiNH2/MgH2 mixture exhibited improvement in desorption kinetics with respect to ball-milling duration. The XRD pattern evaluation revealed that the reaction involved in the first dehydrogenation. Fig. 1 gives representative XRD patterns of initial LiNH2, MgH2, the ball-milled mixture and those obtained after first de/rehydrogenation cycle. The sample was dehydrogenated at 200 C. As a result of this a mixture of lithium and magnesium imide was formed. The MgNH phase found to have tetragonal unit cell with lattice parameter ˚ , c ¼ 3.677 A ˚ and Li2NH have cubic unit cell with a ¼ 11.58 A ˚ . The sample was rehydrogenated lattice parameter a ¼ 5.053 A at 220 C and 110 atm. of hydrogen pressure, the imide
Fig. 1 – X-ray diffraction patterns of (a) initial LiNH2, (b) initial MgH2, (c) 5 h ball-milled sample of 2:1.1 molar mixture of LiNH2 and MgH2, (d) dehydrogenated (at 200 8C) sample and (e) rehydrogenated (at 220 8C and 120 atm of H2) sample.
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mixture was converted to Mg(NH2)2 and LiH rather than the initial LiNH2 and MgH2, which is recently reported by Lohstroh and Fichtner [15]. As the final product of both reactions a mixed phase of composition Li2Mg(NH)2 has also been proposed [12,16,17]. However, this phase is still under discussion. In addition to X-ray diffraction patterns the phase changes have also been identified by FTIR spectroscopy as shown in Figs. 2 and 5. The spectra shown in Fig. 2 display amide/imide conversion of 5 h ball-milled mixture at different stages of reaction. Fig. 2a depicts the characteristic IR absorption of 5 h ball-milled mixture of LiNH2 and MgH2. The IR absorption peaks are located at 3256 and 3310 cm1 for LiNH2, symmetric and asymmetric stretching [18], respectively. Fig. 2b shows the IR absorption spectra of dehydrogenated sample, it shows two absorption peaks at 3192 and 3162 cm1 that corresponds to MgNH [19] and Li2NH [20], respectively. The IR absorption for rehydrogenated state shown in Fig 2c displays two bands at 3282 and 3330 cm1 for Mg(NH2)2 [19]. These results are supported by X-ray diffraction data shown in Fig. 1. Both X-ray diffraction and IR spectroscopy show that the dehydrogenation products in present investigation are Li2NH and MgNH. The de/rehydrogenation process of material prepared in the present investigation by milling of 2:1.1 molar ratio of LiNH2/MgH2 may be described by the following reactions. 2LiNH2 þ MgH2 / Li2 NH þ MgNH þ 2H2 4 MgðNH2 Þ2 þ 2LiH (1) The structural change resulting upon the first dehydrogenation cycle was further evidenced by transmission electron microscopy. The microstructure and selected area electron diffraction pattern (SAED) of milled and dehydrogenated samples are shown in Fig. 3a and b, respectively. For the milled sample, the SAED pattern exhibits diffraction rings as well as spots, which were identified to correspond to crystallites of MgH2 and LiNH2, respectively, the microstructure shows that the MgH2 gets homogeneously mixed with LiNH2. For dehydrogenated sample, SAED pattern also exhibits Fig. 3 – TEM images of (a) 5 h ball-milled and (b) dehydrogenated sample of 2:1.1 molar LiNH2 and MgH2 mixture.
Fig. 2 – Infrared spectra of 2:1.1 LiNH2–MgH2: (a) 5 h ballmilled mixture, (b) dehydrogenated sample at 200 8C and (c) rehydrogenated under 120 atm at 220 8C.
diffraction rings as well as spots that have been identified to be from MgNH and Li2NH, respectively. The representative XRD patterns of ball-milled samples with different milling durations are shown in Fig. 4. After ballmilling for 5 h, it is possible to recognize the two main phases namely LiNH2 and MgH2. As can be seen XRD peak revealing the presence of Mg(NH2)2 phase starts appearing after 15 h of ball-milling. This phase increases with increasing ball-milling duration along with LiH phase. This shows that prolonged ball-milling produces reaction-based phase transformation. Further, these phases are part of reversible reaction. However, the appearance of MgO peak after prolonged ball-milling (30 h) is the inhibiting factor. MgO seems to arise due to oxidation of the mixture with smaller Mg particles. A feasible phase transformation scheme taking place due to prolonged ball-milling is outlined in the following. Further, the product, Mg(NH2)2 þ 2LiH undergoes reversible
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Fig. 4 – X-ray diffraction patterns of (a) 5 h, (b) 15 h, (c) 30 h, (d) 35 h and (e) 45 h ball-milled samples of 2:1.1 molar mixture of LiNH2 and MgH2, (f) dehydrogenated (at 200 8C) sample of 45 h ball-milled mixture, (g) rehydrogenated (at 220 8C and 120 atm of H2) sample of 45 h ball-milled mixture, (h) enlarged view of XRD peak at 2q [ 30.1268 Mg(NH2)2 and 2q [ 30.698 LiNH2.
dehydrogenation and rehydrogenation. This has also been shown in the following. 2LiNH2 þ MgH2 / MgðNH2 Þ2 þ 2LiH 4 Li2 NH þ MgNH þ 2H2 (2) As can be seen from Fig. 4 the characteristic XRD peak for Mg(NH2)2 phase at 2q ¼ 23.265 , which was indexed as the most intense peak of this phase starts developing after 15 h of ball-milling and become pronounced for the milling duration of 45 h. It is interesting to note that in addition to X-ray ˚ ) other diffraction peak from Mg(NH2)2 at 2q ¼ 23.26 (d w 3.82A
Fig. 5 – Infrared spectra of (a) initial, (b) 5 h, (c) 15 h, (d) 35 h and (e) 45 h, ball-milled mixture of 2:1.1 LiNH2–MgH2 (bold line for LiNH2 and dotted line for Mg(NH2)2 bands).
Fig. 6 – Kinetics of (a) 5 h, (b) 45 h ball-milled and (c) comparison of kinetics of different hours ball-milled samples of 2:1.1 molar mixtures of LiNH2 and MgH2 at 200 8C.
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Table 1 – Variation in kinetics of 2:1.1 molar mixtures of LiNH2/MgH2 Sl. No. 1. 2. 3. 4. 5.
Ball-milling time (h)
Desorption temperature ( C)
Storage capacity (wt.%)
Desorption time (min)
Desorption kinetics (wt.%/h)
Increment in kinetics (%/h)
5 15 30 35 45
200 200 200 200 200
4.0 4.05 4.15 4.21 4.28
175 160 145 140 125
1.37 1.52 1.72 1.82 2.05
Starting material 10.94 25.54 33.84 49.63
diffraction peaks for Mg(NH2)2 phase also appear. However, the d values for several of these are very close to the d values for LiNH2. Therefore, the X-ray diffraction peaks from Mg(NH2)2 phase merge with that from LiNH2. This is exem˚ plified for Mg(NH2)2:(224) diffraction peak with d w 2.964A (2q ¼ 30.126 ) together with LiNH2:(112) diffraction peak with ˚ (2q ¼ 30.69 ) in Fig. 4h. The two d values are so close d w 2.91A that the two peaks merge together. Fig. 4h brings out the magnified version of the observed XRD peak for different ballmilling duration. As can be seen, the smooth peak for 5 h ball-milling, developed shoulder (marked by arrows) for ballmilling durations of 15, 30, 35, and 45 h. The appearance of shoulder is indicative of development of another phase that is presumably Mg(NH2)2. Similar is the case for diffraction peaks where the presence of both LiNH2 and Mg(NH2)2 have been indicated. These feature coupled with invariable presence of diffraction peak at 2q ¼ 23.265 that is the most intense XRD peak of Mg(NH2)2 confirm that this phase indeed gets formed due to ball-milling. In order to further establish the formation of Mg(NH2)2 through ball-milling, we have characterized the samples through FTIR at different stages of ball-milling duration. Fig. 5 shows that the IR spectrums of initial LiNH2 and different hours ball-milled LiNH2/MgH2 mixture. The LiNH2 spectrum exhibits characteristic bands at 3256 and 3310 cm1 [18] as shown in Fig. 5a. The LiNH2 is not decomposed upon 5 h of ball-milling (Fig. 5b). With intense ball-milling (15 h) the characteristic bands at 3278 and 3325 cm1 appear, these values are close to the bands for Mg(NH2)2 [19] as shown in Fig. 5c. The peaks corresponding to these wave numbers grow with increasing ball-milling duration. At the prolonged ballmilling greater than 15 h, the characteristic band intensity of LiNH2 decreases along with increasing Mg(NH2)2 band intensity (Fig. 5d and e). These results support the XRD data shown in Fig. 5. Both XRD and IR-spectroscopy show that the prolonged ball-milling (15 h) leads to phase transformations. In particular it reveals the formation of Mg(NH2)2. The effect of milling duration on desorption kinetics of 2:1.1 molar mixture of LiNH2/MgH2 is shown in Fig. 6. It has been found that the desorption kinetics of milled sample increases with increasing milling duration. Fig. 6a represents typical dehydrogenation profile of 2:1.1 molar LiNH2/MgH2 mixtures under inert atmosphere for six cycles at 200 C. It has been found that the hydrogen uptake capacity of 5 h milled materials w4 wt.% within 3 h. The hydrogen uptake capacity of 45 h ball-milled sample is found to be w4.3 wt.% within w2 h as shown in Fig. 6b. The desorption kinetics improved with degree of mechanical milling as shown in Fig. 6c. It has been found that the desorption kinetics of ball-milled sample increases up to 49% with ball-milling duration ranging from 5 to 45 h at
200 C (Table 1). As regards possible release of ammonia, we did not find presence of ammonia in the desorbed hydrogen. It may be pointed out that even if NH3 is released, it will recombine with LiH that is a known ammonia getter [21]. The diffraction peaks at 2q ¼ 30.4 and 2q ¼ 27.5 in Fig. 7 could be indexed as (112) peak of LiNH2 and (110) peak of MgH2, respectively. Substantial peak broadening was found to result with prolonged milling duration. This possibly arises due to continuous deformation and breaking of material particles. There will also be disorder produced due to strain stored in the particles due to mechanical milling. This strain has been calculated employing Voigt function technique. The variation of calculated crystallite size and lattice strain with ball-milling duration is shown in Table 2. For the mixture, the calculated crystallite size of LiNH2 and MgH2 decreased for longer ballmilling duration. It has been found that after 5 h milling the crystallite sizes of LiNH2 and MgH2 are 127 and 54 nm, respectively. The increase in ball-milling time beyond 5 h resulted in further decrease in crystallite size. However, the reduction rate in crystallite size is dramatically reduced. It has been found that after ball-milling for 45 h, the crystallite sizes of LiNH2 and MgH2 are 11 and 12 nm, respectively. As can be seen from Table 2, the crystallite size of the formed Mg(NH2)2 and LiH also decreases with increasing ball-milling duration ranging from 15 to 45 h. The X-ray diffraction peaks started becoming excessively broadened approaching halo-like feature for ball-milling durations exceeding 45 h. Therefore, we could not separate particle size reduction effect on the XRD peak broadening after
Fig. 7 – Peak broadening with ball milling durations of (110) and (112) diffraction peaks of MgH2 and LiNH2, respectively.
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Table 2 – The variation of crystallite size and lattice strain with milling duration Sl. Ball-milling No. time (h)
MgH2
LiNH2
Compounds formed to reaction Mg(NH2)2
1. 2. 3. 4. 5. 6.
0 5 15 30 35 45
Crystalline size (D) 0.5 (nm)
Lattice strain (e)
Crystalline size (D) 0.5 (nm)
Lattice strain (e)
378 127 42 18 13 11
Initial strain 0.23 0.26 0.33 0.37 0.39
150 54 29 20 17 12
Initial strain 0.21 0.26 0.34 0.44 0.52
LiH
Crystalline size Lattice Crystalline size Lattice (D) 0.5 (nm) strain (e) (D) 0.5 (nm) strain (e) – – 48 16 15 14
– – 0.002 0.040 0.045 0.070
– – 60 33 25 16
– – 0.018 0.020 0.028 0.039
Fig. 8 – SEM images of 2:1.1 molar mixture of LiNH2 and MgH2 (a) initial powder, (b) 5 h, (c) 15 h, (d) 30 h, (e) 35 h, and (f) 45 h.
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45 h ball-milling. Also ball-milling beyond 45 h led to increase of MgO content. Therefore, ball-milling durations greater from 45 h were not employed. Fig. 8 shows that the SEM images of the LiNH2 and MgH2 mixture before and after ball milling. The initial LiNH2 and MgH2 have faceted grains and bimodal particle-size distribution with small number of particles around 4 mm and large particles of size around 30 mm being present. Ball-milling for 5 h has led substantial reduction in average particle size w2.5 mm and some large particles of particle size w16 mm are still present. Further milling beyond 5 h led to decrease in particle size of large particle but not of small ones. The 45 h milled sample possesses particle size of w2.36 and w6.07 mm. This result combined with the variation of crystallite size suggested that ball-milling beyond 5 h leads to breaking down of large particles but not of small ones. Further breaking of small particles is very limited for ball-milling beyond 5 h. Chen et al. [16], reported that the reversibility of this system is related to local interaction between Mg(NH2)2 and LiH as is clear from Fig. 8c to f. The sample ball-milled for longer durations (>5 h) are characterized by lower crystallite size and comparatively more homogeneous dispersion of the constituent particles. The small particle size that starts becoming discernible after 5 h ball-milling will result in mechanical milling-induced diffusion. This LiNH2/MgH2 will leads to reaction-based transformation yielding Mg(NH2)2 together with LiH. Since Mg(NH2)2 and LiH are known [22] to have suitable DH and DS for low desorption temperature (90 C) and pressure (1 bar). Therefore, formation of Mg(NH2)2 will result in improved desorption kinetics. This is what has actually been assured. The desorption kinetics has been found to increase progressively as the ball-milling increases from 15 to 45 h resulting in increase in formation of Mg(NH2)2 phase. As outlined earlier, ball-milling duration greater than 45 h leads to significant increase in undesirable phase MgO hence ball-milling duration greater than 45 h were not tried.
4.
Conclusions
In conclusion, it can be said that the major and new result of our investigation is the appearance of Mg(NH2)2 phase directly by ball-milling of LiNH2 and MgH2 mixture. This procedure skips a step in reaction leading to the formation of Mg(NH2)2 through dehydrogenation/rehydrogenation of Li–Mg–N–H system as found by other workers [12,15,16]. A related new result corresponds to appreciable enhancement of desorption kinetics up to 49% for the 45 h ball-milled mixtures consisting Mg(NH2)2 and LiH.
Acknowledgments The authors acknowledge Prof. A.R. Verma (Delhi, India), Prof. T.N. Veziroglu (President, IAHE, Florida, USA), Prof. R.S. Tiwari for their encouragement and helpful discussion. They also thank Prof. A.S.K.Sinha, Chemical Engineering IIT, BHU for his help in carrying out the FTIR Spectroscopy investigation. Financial assistance from Ministry of New and Renewable
Energy (MNRE) and DST (UNANST), New Delhi (India) is gratefully acknowledged.
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