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lithium hydride stoichiometric mixtures with lithium hydride excess
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 5 4 4 8 e5 4 5 3
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
In-situ neutron diffraction study of magnesium amide/lithium hydride stoichiometric mixtures with lithium hydride excess Francesco Dolci a,*, Eveline Weidner a, Markus Hoelzel c, Thomas Hansen d, Pietro Moretto a, Claudio Pistidda b, Michela Brunelli d, Maximilian Fichtner b, Wiebke Lohstroh b a
Institute for Energy, DG Joint Research Centre, European Commission, P.O. Box 2, 1755 ZG Petten, The Netherlands Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Postfach 3640, 76021 Karlsruhe, Germany c Forschungs-Neutronequelle Heinz Maier-Leibnitz, Lichtenbergstrasse 1, 85747, Garching, Germany d Institute Laue-Langevin, Rue Jules Horowitz 6, 38043, Grenoble, France b
article info
abstract
Article history:
The hydrogen sorption of mixtures of magnesium amide (Mg(NH2)2) and lithium hydride
Received 7 January 2010
(LiH) with different molecular ratios have been investigated using in-situ neutron
Received in revised form
diffraction; the experiments were performed at D20/ILL and SPODI/FRMII. The results
2 March 2010
reveal a common reaction pathway for 1:2, 3:8 and 1:4 magnesium amide: lithium hydride
Accepted 7 March 2010
mixtures. Intermediate reaction steps are observed in both ab- and desorption. The ther-
Available online 9 April 2010
modynamic properties of the system at 200 C are not changed by the addition of excess lithium hydride. This finding has important implications for the tailoring the character-
Keywords: Hydrogen storage
istics of this promising hydrogen storage material. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
In-situ neutron diffraction Intermediate reaction Li2Mg2(NH)3
1.
Introduction
Storing hydrogen safely and efficiently is one of the main technological barriers preventing the widespread adoption of hydrogen as an energy carrier and the subsequent transition to a so-called “hydrogen economy”. The solid state storage of hydrogen is considered to be one of the most attractive solutions proposed, thus the search for new materials attracts a significant amount of attention. Complex hydrides are of great interest for solid hydrogen storage as evidenced by the large number of experimental and theoretical works on this topic [1]. Among complex hydrides, the two-component reactive hydride mixture formed from magnesium amide and lithium hydride offers many appealing features. These include reversibility, relatively low operation temperature and
low cost of the material [2,3]. This system, and especially the Mg(NH2)2/2LiH mixture has been thoroughly investigated since the initial reports in 2004 [4e12]. It has been shown that the LiNH2/MgH2 mixtures can be considered equivalent to the Mg(NH2)2/LiH mixtures. A metathesis reaction between MgH2 and LiNH2 occurs after the first absorption/desorption cycle, leading to the formation of Mg(NH2)2 and LiH [13]. In total, 5.58 wt% H2 can be reversibly stored (T ¼ 200 C) and the overall reaction can be given as: Mg(NH2)2 þ 2LiH 4 Li2Mg(NH)2 þ 2H2
(1)
However, the PCI (Pressure Composition Isotherm) of the Mg(NH2)2/2LiH mixture is composed of two regions: one at low pressure (lower than 10 bar at 200 C) and a plateau at higher
* Corresponding author. E-mail address: [email protected] (F. Dolci). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.03.030
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 5 4 4 8 e5 4 5 3
pressure (around 40 bar at 200 C). These regions are associated with the following reactions, respectively [8,14]: 2Li2Mg(NH)2 þ H2 4 LiH þ LiNH2 þ Li2Mg2(NH)3
(2a)
LiNH2 þ Li2Mg2(NH)3 þ 3H2 4 2 Mg(NH2)2 þ 3LiH
(2b)
Mg(NH2)2 þ 8/3LiH 4 1/3Mg3N2 þ 4/3Li2NH þ 8/3H2 w6.9 wt% H2 at 300 C
(3)
Mg(NH2)2 þ 4LiH 4 1/3Mg3N2 þ 4/3Li3N þ 4H2 w9.1 wt% H2 at 500 C
(4)
In principle, increasing the amount of lithium hydride (stoichiometrically) should increase the amount of reversibly stored hydrogen. However, different experimental reports [7,16e18] and theoretical works [19] have pointed out that the dehydrogenated compounds Li3N and Mg2N3 require high hydrogen pressures and temperatures for reversibility and even under extreme conditions complete yield is not always obtained. At operating temperatures around 200 C three reversible reactions with increasing lithium to magnesium stoichiometric ratios have been proposed; n ¼ 2, 8/3, 4 [20,21]: Mg(NH2)2 þ nLiH 4 Li2Mg(NH)2 þ (n 2)LiH þ 2H2
(5)
The PCI curves of all the ratios given above show a twostaged hydrogenation behaviour. This suggests reaction (1) also occurs in samples with excess lithium hydride. In the present study, it will be unambiguously shown that the reversible hydrogen absorption/desorption reaction of each mixture, involving different stoichiometric amounts of Mg(NH2)2 and LiH (reaction (5)), follows the same reaction pathway.
2.
Experimental
2.1.
Materials
in their desorbed state. The 1:4 mixture was used as prepared and cycled at the D20 beamline using the custom built highpressure rig described below.
2.2.
with H2 capacities of 1.46 and 4.43 wt%, respectively. The use of different Mg(NH2)2 to LiH molar stoichiometric ratios has been suggested as an effective way to increase the hydrogen storage capacity of the system. The proposed reactions are [15,16]:
Methods
Neutron diffraction measurements for the 1:2 and 1:4 mixture were performed at the D20 instrument at the Institute Laue Langevin reactor source, Grenoble, France. This instrument provides high flux and a curved linear position sensitive detector (PSD) resulting in rapid data acquisition over the full angular range. Time-resolved diffraction with a resolution of 1 min was possible. Data sets were corrected for detector efficiency against a vanadium standard, using the standard ILL data processing software LAMP. The 3:8 mixture was analysed at the SPODI instrument at the FRMII reactor source, Munich, Germany. This instrument provides better angular resolution compared to D20 at the expenses of a slower data ˚ for acquisition rate. With the selected wavelengths of 2.42 A ˚ for SPODI, a d-spacing range of about 1.4e10 A ˚ D20 and 2.54 A is probed. In order to avoid artefacts due to the slow data collection for SPODI, the diffraction patterns were recorded allowing an equilibration time of 30 min after pressure changes within the sample cell. Low or null-scattering neutron scattering materials such as vanadium or TieZr alloys are not suitable for highpressure measurements with hydrogen. The first cell used for the 2:1 mixture was an Inconel 600 tube, providing the required mechanical properties while minimizing the number of additional Bragg reflections. The resulting two strong reflections (111) and (200) at a d-spacing of 2.05 and ˚ of the main phase of the sample holder were excluded 1.78 A from Rietveld refinement. Similar reflections, but with narrower peak width were observed for the second cell made of stainless steel AISI-316L. This second cell was used for the 3:8 and 1:4 mixtures. Heating was provided via the instrument furnace; the actual sample temperature was determined from refinement of the lattice parameter of the Inconel 600 sample holder for the 1:2 experiment, and from an external thermocouple placed near the walls of the cell in both other cases. Deuterium gas was used in preference to hydrogen, owing to its favourable coherent-to-incoherent scattering ratio. For clarity the chemical formulas are given with hydrogen in the following sections.
2.3. Three different mixtures were produced by ball-milling different stoichiometric amounts of magnesium amide/ lithium hydride. According to the magnesium to lithium molar ratios used in the mixtures they will be labelled as 1:2 (1Mg(NH2)2:2LiH), 3:8 (3Mg(NH2)2:8LiH) and 1:4 (1Mg (NH2)2:4LiH). For each mixture the powder was milled for 12 h under an argon atmosphere in a Fritsch P6 planetary ball mill with silicon nitride vial and balls (powder to-ball ratio of 1:20) at a speed of 600 rpm. For the 3:8 and 1:2 mixtures hydrogen was desorbed for 3 h at 220 C from the resulting powder in a Sieverts’ apparatus, and subsequently deuterium absorption was performed at 95 bar for 3 h. This procedure was repeated two times. Details of the Sieverts equipment used have been published elsewhere [22]. The 1:2 and 3:8 mixtures were used
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Rietveld refinement
Rietveld profile refinement was performed using the MAUD [23,24] software. Published structural models were used: Li2Mg (NH)2 [6], Li2Mg2(NH)3 [14], LiNH2 and Mg(NH2)2 [25]. Profile refinement was significantly complicated because the strongest reflections of three phases are almost overlapping. This is due to the structural similarities of the orthorhombic and tetragonal LieMgeimides as well as of Lieamide with respect to the anion lattice. Because of the overlap and neutron absorption from 6Li, refinements were restricted to few parameters, such as scale factor, lattice and profile parameters. The isotropic DebyeeWaller factor was fixed for all atoms of the phases. With the imposed restrictions reliability factors of weighted agreement factor Rwp of 2.3e4.9% were achieved, for up to 16 refinable
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parameters. The strongest sample holder reflections could be attributed to a main component from the fcc Inconel or steel phase which were excluded from the refinement.
3.
Results and discussion
A previous in-situ neutron diffraction study demonstrated that the 1:2 mixture, when exposed to a deuterium pressure of around 50 bar (T ¼ 200 C), shows a phase evolution from Li2Mg(NH)2 to 1LiNH2/1Li2Mg2(NH)3/1LiH and finally to Mg (NH2)2/2LiH under 70 bar pressure [14]. The results of the in-situ neutron diffraction experiments for the 3:8 and 1:4 ratio samples are shown in Figs. 1 and 2, respectively (in order to compare data obtained with different wavelengths they will be plotted against momentum transfer, hereafter labelled as Q). In Fig. 1, the relevant sections of three diffraction patterns are presented for a 3:8 sample held at 200 C under different deuterium pressures. Owing to the good angular resolution of the SPODI diffractometer the two stage reaction is clearly demonstrated. The starting dehydrogenated material is composed of LiH, the alpha polymorph of Li2Mg(NH)2 and a minor amount of another phase with cubic symmetry which might be either the beta polymorph of Li2Mg(NH)2 or Li2NH. After raising the pressure to 32 bar Li2Mg(NH)2 and the cubic phase completely disappeared. The intensity of LiH is increased and two new phases appear, which can be unambiguously identified as LiNH2 and Li2Mg2(NH)3 (Fig. 1, the calculated reflection intensities are indicated). The Mg(NH2)2 phase only becomes evident as the pressure is raised to 71 bar. Fig. 2 gives an overview of the phase evolution for the 1:4 mixture as obtained in 5 min and 2 min intervals at the high intensity instrument D20. Different time intervals were used to obtain faster data acquisition during the beginning of the absorption process. The temperature was set at 200 C. Starting from the absorbed material a full desorption/re-absorption
Fig. 1 e Comparison of the diffraction pattern for the 3:8 mixture in its desorbed state and under 32 bar and 70 bar of deuterium. An offset has been applied to the grey and blue diffraction profiles to increase clarity. Colour bars show the relative calculated intensities for different phases.
Fig. 2 e Phase evolution for the 1:4 mixture under different deuterium pressures. The arrow indicates a region where the collection time for each diffraction pattern decreased to 2 min instead of five.
cycle was measured. The starting material had not been cycled and the nanocrystallinity of the freshly ball-milled material is evidenced by the very broad Mg(NH2)2 and LiH peaks (Fig. 2 at 0 h). Annealing under deuterium pressures at about 30 bar sharpens the peak profiles (2 h). Lowering the pressure to 10 bar D2 brings about the phase transition outlined in reaction (2b): Mg(NH2)2 disappears, LiNH2 and Li2Mg2(NH)3 are formed and the intensity of LiH decreases (2e8 h). By lowering the pressure to 1 bar D2, Li2Mg(NH)2 is formed at the expense of all the other components, which disappear with the exception of LiH, which decreases in intensity (8e12 h). In the subsequent absorption, the deuterium pressure is set to above 40 and finally above 60 bar D2, which causes the rehydrogenation of the sample to Mg(NH2)2 and LiH. The transition in the first step of the reaction, to LiNH2 and Li2Mg2(NH)3 is very fast at 40 bar pressure and it completes within 4 min (see Fig. 3; the sharp step is clearly noticeable in Fig. 2 and indicated by the arrow in the time axis). Thereby the
Fig. 3 e Diffraction Pattern evolution for the 1:4 mixture after deuterium pressure was raised from 40 to 60 bar. Complete hydrogenation occurs within 4 min.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 5 4 4 8 e5 4 5 3
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Fig. 4 e Comparison of 1:2, 3:8 and 1:4 mixtures at different hydrogenation states. a, c, e represent the low angle regions while b, d and f represent the high angle region. a and b represent the desorbed material. c and d represent the first absorption step. e and f represent the material at during the hydrogenation step.
full reversibility of the intermediate in reactions (2a) and (2b) is demonstrated. Fig. 4 presents a comparison between the three different samples at different stages of hydrogenation. It is evident that all three mixtures present similar diffraction profiles during the hydrogenation steps. No significant deviation from reaction (2a) and (2b), as outlined above, was found. Differences in the line profile are due to the different angular resolution of the D20 and SPODI instruments and to differences in the number of hydrogenation/dehydrogenation cycles the samples have undergone. As explained above, for the 3:8 sample (SPODI measurement) the very good resolution of the high angle region (high Q) clearly allows the identification of all the reaction intermediates involved in the hydrogenation process. The desorbed material consists of a-Li2Mg(NH)2 and LiH (except for the 1:2 mixture). Moreover, as already mentioned, it is possible to observe the presence of a cubic phase in all the three different desorbed mixtures (Fig. 4a and b). This cubic
phase is just a minor fraction of the sample and disappears as soon as the desorbed material is exposed to a hydrogen pressure higher than 1 bar. Clear identification of the cubic phase is difficult. Two hypotheses are most likely and involve a cubic modification of the mixed LieMg imide [6,26] or lithium imide. The intermediate hydrogenation step for pressures between 7 and 40 bar at 200 C is unambiguously identified by the appearance of the peaks associated with LiNH2 and Li2Mg2(NH)3 in the diffraction pattern (Fig. 4c and d). The phase transition summarised in reaction (2a) is especially evident in the high Q (high angle) region of the diffraction patterns (Fig. 1). The molar phase compositions of the sample during the intermediate stage have been obtained from Rietveld refinement (Table 1), the analysis of the 3:8 system is as an example shown in Fig. 5. The angular ranges of the reflections of the steel sample container have been excluded from the refinement. The summary for all three mixtures is presented in Table 1. For each mixture a molar ratio LiNH2:
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Table 1 e Molar phase fraction obtained with Rietveld refinement method for the three different mixtures after reaction (1) is completed. In brackets is the expected theoretical molar fraction value for each component.
1:2 Mixture 3:8 Mixture 1:4 Mixture
LiH mol%
LiNH2 mol%
Li2Mg2(NH)3 mol%
Rw%
32 (33.3) 56 (53.8) 73 (71.4)
36 (33.3) 22 (23.1) 14 (14.3)
32 (33.3) 22 (23.1) 13 (14.3)
3.2 4.9 2.3
Li2Mg2(NH)3 of approximately 1:1 is found, in agreement with reaction (2a). The amount of LiH is obviously different for the three mixtures, and the theoretical phase compositions according to reaction (2a) are also given in Table 1. Considering the errors associated with refinement, the fact that lithium has a high absorption coefficient for neutrons and errors associated with sample purity and weighing procedures during sample preparation, the agreement between the refinement results and the phase fractions expected from reaction (2a) (numbers in brackets) is good. Above 40 bar H2 pressure (T ¼ 200 C) the reaction proceeds to the final state (reaction (2b)) to form Mg(NH2)2 and LiH, albeit with slow kinetics. The diffraction patterns shown in Fig. 4e and f are of samples not yet completely hydrogenated. The constant increase in LiH and the appearance of Mg(NH2)2 reflections, however confirm the expected reaction mechanism. A comparison of the diffraction profiles of the 1:2, 3:8 and 1:4 mixtures (Fig. 2) and Rietveld refinement results (Table 1), supports the assumption that the fundamental reaction path of a magnesium amide/lithium hydride mixture does not change when additional lithium hydride is added. Moreover, the similarities between the PCI curves obtained for different magnesium amide/lithium hydride mixtures, as well as the fact that for each mixture the same reaction intermediates are found in
equal stoichiometric ratios (neglecting the higher amount of LiH) clearly demonstrate that the thermodynamic properties and reaction pathway are independent of the starting amide/ hydride ratio. The lithium hydride excess used in the different mixtures does not take any active part in the hydrogen absorption/desorption process at temperatures around 200 C. However, the addition of LiH can improve system performance, as a decreased ammonia emission has been associated to the increase in the lithium hydride content [27]. For each mixture considered, the nature of intermediates depends only on the partial hydrogen equilibrium pressure and not on the starting lithium hydride to magnesium amide stoichiometry.
4.
Conclusions
It has been unambiguously shown, by means of in-situ neutron diffraction measurements that the hydrogenation/ dehydrogenation reaction pathway at around 200 C of three different magnesium amide/lithium hydride mixtures does not change by increasing the lithium hydride content of the system. The comparison of data collected at the SPODI/FRMII and D20/ILL beam lines offer complementary information on crystallographic identity and phase evolution kinetics. All the observations made for the 1:2, 3:8 and 1:4 mixtures are common and can be summarised as follows: - The desorbed material is composed by a-Li2Mg(NH)2 (and LiH for the 3:8 and 1:4 mixtures) plus an unknown cubic phase. - At 200 C equilibrium pressures below 40 bar and higher than 7 bar the formation of LiNH2 and Li2Mg2(NH)3 is observed and the amount of LiH increases. - At 200 C Mg(NH2)2 appears only after raising the pressure above 40 bar.
Acknowledgments Funding is acknowledged from the European Commission Sixth Framework Program under the Marie Curie Research Training Network COSY (Contract No. MRTN-CT-2006-035366) and the Integrated Project NESSHY.
references
Fig. 5 e Rietveld refinement of the 3:8 mixture under 33 bar of deuterium. The diffraction pattern of the empty sample container is given as a reference. The regions in which the strongest peaks associated to the sample container appear have been excluded from the refinement. For sake of clarity the full spectrum is presented with no deleted areas. The excluded regions (in degrees) are 22.5e27.7, 30.4e42.4, 44.9e49, 59.2e63.9, 69.5e72, 74.4e75.7, and 88.5e90.15.
[1] Orimo SI, Nakamori Y, Eliseo JR, Zuttel A, Jensen CM. Complex hydrides for hydrogen storage. Chem Rev 2007;107: 4111e32. [2] Xiong Z, Wu G, Hu J, Chen P. Ternary imides for hydrogen storage. Adv Mater 2004;16:1522e5. [3] Luo W. (LiNH2eMgH2): a viable hydrogen storage system. J Alloys Compd 2004;381:284e7. [4] Markmaitree T, Osborn W, Shaw LL. Comparisons between MgH2- and LiH-containing systems for hydrogen storage applications. Int J Hydrogen Energy 2008;33:3915e24. [5] Janot R, Eymery JB, Tarascon JM. Investigation of the processes for reversible hydrogen storage in the LieMgeNeH system. J Power Sources 2007;164:496e502.
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[6] Rijssenbeek J, Gao Y, Hanson J, Huang Q, Jones C, Toby B. Crystal structure determination and reaction pathway of amideehydride mixtures. J Alloys Compd 2008;454: 233e44. [7] Leng HY, Ichikawa T, Isobe S, Hino S, Hanada N, Fujii H. Desorption behaviours from metaleNeH systems synthesized by ball milling. J Alloys Compd 2005;404e406: 443e7. [8] Hu J, Liu Y, Wu G, Xiong Z, Chen P. Structural and compositional changes during hydrogenation/ dehydrogenation of the LieMgeNeH system. J Phys Chem C 2007;111:18439e43. [9] Lohstroh W, Fichtner M. Reaction steps in the LieMgeNeH hydrogen storage system. J Alloys Compd 2007;446e447: 332e5. [10] Luo W, Sickafoose S. Thermodynamic and structural characterization of the MgeLieNeH hydrogen storage system. J Alloys Compd 2006;407:274e81. [11] Xiong Z, Hu J, Wu G, Chen P. Hydrogen absorption and desorption in MgeNaeNeH system. J Alloys Compd 2005;395: 209e12. [12] Barison S, Agresti F, Lo Russo S, Maddalena A, Palade P, Principi G, et al. A study of the LiNH2eMgH2 system for solid state hydrogen storage. J Alloys Compd 2008;459:343e7. [13] Luo W, Ronnebro E. Towards a viable hydrogen storage system for transportation application. J Alloys Compd 2005; 404e406:392e5. [14] Weidner E, Dolci F, Hu J, Lohstroh W, Hansen T, Bull DJ, et al. Hydrogenation reaction pathway in Li2Mg(NH)2. J Phys Chem C 2009;113:15772e7. [15] Leng H, Ichikawa T, Fujii H. Hydrogen storage properties of LieMgeNeH systems with different ratios of LiH/Mg(NH2)2. J Phys Chem B 2006;110:12964e8. [16] Nakamori Y, Kitahara G, Miwa K, Towata S, Orimo S. Reversible hydrogen-storage functions for mixtures of Li3N and Mg3N2. Appl Phys A Mater Sci Process 2005;80:1e3.
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[17] Kojima Y, Kawai Y, Ohba N. Hydrogen storage of metal nitrides by a mechanochemical reaction. J Power Sources 2006;159:81e7. [18] Hu J, Wu G, Liu Y, Xiong Z, Chen P, Murata K, et al. Hydrogen release from Mg(NH2)2eMgH2 through mechanochemical reaction. J Phys Chem B 2006;110:14688e92. [19] Akbarzadeh AR, Ozolin V, Wolverton C. First-principles determination of multicomponent hydride phase diagrams: application to the LieMgeNeH system. Adv Mater 2007;19: 3233e9. [20] Aoki M, Noritake T, Nakamori Y, Towata S, Orimo S. Dehydriding and rehydriding properties of Mg(NH2)2eLiH systems. J Alloys Compd 2007;446e447:328e31. [21] Leng HY, Ichikawa T, Hino S, Hanada N, Isobe S, Fujii H. Synthesis and decomposition reactions of metal amides in metaleNeH hydrogen storage system. J Power Sources 2006; 156:166e70. [22] Kircher O, Fichtner M. Hydrogen exchange kinetics in NaAlH4 catalyzed in different decomposition states. J Appl Phys 2004; 95:7748e53. [23] Lutterotti L. The MAUD program, http://www.ing.unitn.it/ wmaud/. [24] Lutterotti L, Scardi P. Simultaneous structure and size-strain refinement by the Rietveld method. J Appl Crystallogr 1990; 23:246e52. [25] Sorby MH, Nakamura Y, Brinks HW, Ichikawa T, Hino S, Fujii H, et al. The crystal structure of LiND2 and Mg(ND2)2. J Alloys Compd 2007;428:297e301. [26] Nakamura Y, Hino S, Ichikawa T, Fujii H, Brinks HW, Hauback BC. Dehydrogenation reaction of LieMgeNeH systems studied by in situ synchrotron powder X-ray diffraction and powder neutron diffraction. J Alloys Compd 2008;457:362e7. [27] Hu J, Fichtner M. Formation and stability of ternary imides in the LieMgeNeH hydrogen storage system. Chem Mater 2009; 21:3485e90.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
In-situ neutron diffraction study of magnesium amide/lithium hydride stoichiometric mixtures with lithium hydride excess Francesco Dolci a,*, Eveline Weidner a, Markus Hoelzel c, Thomas Hansen d, Pietro Moretto a, Claudio Pistidda b, Michela Brunelli d, Maximilian Fichtner b, Wiebke Lohstroh b a
Institute for Energy, DG Joint Research Centre, European Commission, P.O. Box 2, 1755 ZG Petten, The Netherlands Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Postfach 3640, 76021 Karlsruhe, Germany c Forschungs-Neutronequelle Heinz Maier-Leibnitz, Lichtenbergstrasse 1, 85747, Garching, Germany d Institute Laue-Langevin, Rue Jules Horowitz 6, 38043, Grenoble, France b
article info
abstract
Article history:
The hydrogen sorption of mixtures of magnesium amide (Mg(NH2)2) and lithium hydride
Received 7 January 2010
(LiH) with different molecular ratios have been investigated using in-situ neutron
Received in revised form
diffraction; the experiments were performed at D20/ILL and SPODI/FRMII. The results
2 March 2010
reveal a common reaction pathway for 1:2, 3:8 and 1:4 magnesium amide: lithium hydride
Accepted 7 March 2010
mixtures. Intermediate reaction steps are observed in both ab- and desorption. The ther-
Available online 9 April 2010
modynamic properties of the system at 200 C are not changed by the addition of excess lithium hydride. This finding has important implications for the tailoring the character-
Keywords: Hydrogen storage
istics of this promising hydrogen storage material. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
In-situ neutron diffraction Intermediate reaction Li2Mg2(NH)3
1.
Introduction
Storing hydrogen safely and efficiently is one of the main technological barriers preventing the widespread adoption of hydrogen as an energy carrier and the subsequent transition to a so-called “hydrogen economy”. The solid state storage of hydrogen is considered to be one of the most attractive solutions proposed, thus the search for new materials attracts a significant amount of attention. Complex hydrides are of great interest for solid hydrogen storage as evidenced by the large number of experimental and theoretical works on this topic [1]. Among complex hydrides, the two-component reactive hydride mixture formed from magnesium amide and lithium hydride offers many appealing features. These include reversibility, relatively low operation temperature and
low cost of the material [2,3]. This system, and especially the Mg(NH2)2/2LiH mixture has been thoroughly investigated since the initial reports in 2004 [4e12]. It has been shown that the LiNH2/MgH2 mixtures can be considered equivalent to the Mg(NH2)2/LiH mixtures. A metathesis reaction between MgH2 and LiNH2 occurs after the first absorption/desorption cycle, leading to the formation of Mg(NH2)2 and LiH [13]. In total, 5.58 wt% H2 can be reversibly stored (T ¼ 200 C) and the overall reaction can be given as: Mg(NH2)2 þ 2LiH 4 Li2Mg(NH)2 þ 2H2
(1)
However, the PCI (Pressure Composition Isotherm) of the Mg(NH2)2/2LiH mixture is composed of two regions: one at low pressure (lower than 10 bar at 200 C) and a plateau at higher
* Corresponding author. E-mail address: [email protected] (F. Dolci). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.03.030
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 5 4 4 8 e5 4 5 3
pressure (around 40 bar at 200 C). These regions are associated with the following reactions, respectively [8,14]: 2Li2Mg(NH)2 þ H2 4 LiH þ LiNH2 þ Li2Mg2(NH)3
(2a)
LiNH2 þ Li2Mg2(NH)3 þ 3H2 4 2 Mg(NH2)2 þ 3LiH
(2b)
Mg(NH2)2 þ 8/3LiH 4 1/3Mg3N2 þ 4/3Li2NH þ 8/3H2 w6.9 wt% H2 at 300 C
(3)
Mg(NH2)2 þ 4LiH 4 1/3Mg3N2 þ 4/3Li3N þ 4H2 w9.1 wt% H2 at 500 C
(4)
In principle, increasing the amount of lithium hydride (stoichiometrically) should increase the amount of reversibly stored hydrogen. However, different experimental reports [7,16e18] and theoretical works [19] have pointed out that the dehydrogenated compounds Li3N and Mg2N3 require high hydrogen pressures and temperatures for reversibility and even under extreme conditions complete yield is not always obtained. At operating temperatures around 200 C three reversible reactions with increasing lithium to magnesium stoichiometric ratios have been proposed; n ¼ 2, 8/3, 4 [20,21]: Mg(NH2)2 þ nLiH 4 Li2Mg(NH)2 þ (n 2)LiH þ 2H2
(5)
The PCI curves of all the ratios given above show a twostaged hydrogenation behaviour. This suggests reaction (1) also occurs in samples with excess lithium hydride. In the present study, it will be unambiguously shown that the reversible hydrogen absorption/desorption reaction of each mixture, involving different stoichiometric amounts of Mg(NH2)2 and LiH (reaction (5)), follows the same reaction pathway.
2.
Experimental
2.1.
Materials
in their desorbed state. The 1:4 mixture was used as prepared and cycled at the D20 beamline using the custom built highpressure rig described below.
2.2.
with H2 capacities of 1.46 and 4.43 wt%, respectively. The use of different Mg(NH2)2 to LiH molar stoichiometric ratios has been suggested as an effective way to increase the hydrogen storage capacity of the system. The proposed reactions are [15,16]:
Methods
Neutron diffraction measurements for the 1:2 and 1:4 mixture were performed at the D20 instrument at the Institute Laue Langevin reactor source, Grenoble, France. This instrument provides high flux and a curved linear position sensitive detector (PSD) resulting in rapid data acquisition over the full angular range. Time-resolved diffraction with a resolution of 1 min was possible. Data sets were corrected for detector efficiency against a vanadium standard, using the standard ILL data processing software LAMP. The 3:8 mixture was analysed at the SPODI instrument at the FRMII reactor source, Munich, Germany. This instrument provides better angular resolution compared to D20 at the expenses of a slower data ˚ for acquisition rate. With the selected wavelengths of 2.42 A ˚ for SPODI, a d-spacing range of about 1.4e10 A ˚ D20 and 2.54 A is probed. In order to avoid artefacts due to the slow data collection for SPODI, the diffraction patterns were recorded allowing an equilibration time of 30 min after pressure changes within the sample cell. Low or null-scattering neutron scattering materials such as vanadium or TieZr alloys are not suitable for highpressure measurements with hydrogen. The first cell used for the 2:1 mixture was an Inconel 600 tube, providing the required mechanical properties while minimizing the number of additional Bragg reflections. The resulting two strong reflections (111) and (200) at a d-spacing of 2.05 and ˚ of the main phase of the sample holder were excluded 1.78 A from Rietveld refinement. Similar reflections, but with narrower peak width were observed for the second cell made of stainless steel AISI-316L. This second cell was used for the 3:8 and 1:4 mixtures. Heating was provided via the instrument furnace; the actual sample temperature was determined from refinement of the lattice parameter of the Inconel 600 sample holder for the 1:2 experiment, and from an external thermocouple placed near the walls of the cell in both other cases. Deuterium gas was used in preference to hydrogen, owing to its favourable coherent-to-incoherent scattering ratio. For clarity the chemical formulas are given with hydrogen in the following sections.
2.3. Three different mixtures were produced by ball-milling different stoichiometric amounts of magnesium amide/ lithium hydride. According to the magnesium to lithium molar ratios used in the mixtures they will be labelled as 1:2 (1Mg(NH2)2:2LiH), 3:8 (3Mg(NH2)2:8LiH) and 1:4 (1Mg (NH2)2:4LiH). For each mixture the powder was milled for 12 h under an argon atmosphere in a Fritsch P6 planetary ball mill with silicon nitride vial and balls (powder to-ball ratio of 1:20) at a speed of 600 rpm. For the 3:8 and 1:2 mixtures hydrogen was desorbed for 3 h at 220 C from the resulting powder in a Sieverts’ apparatus, and subsequently deuterium absorption was performed at 95 bar for 3 h. This procedure was repeated two times. Details of the Sieverts equipment used have been published elsewhere [22]. The 1:2 and 3:8 mixtures were used
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Rietveld refinement
Rietveld profile refinement was performed using the MAUD [23,24] software. Published structural models were used: Li2Mg (NH)2 [6], Li2Mg2(NH)3 [14], LiNH2 and Mg(NH2)2 [25]. Profile refinement was significantly complicated because the strongest reflections of three phases are almost overlapping. This is due to the structural similarities of the orthorhombic and tetragonal LieMgeimides as well as of Lieamide with respect to the anion lattice. Because of the overlap and neutron absorption from 6Li, refinements were restricted to few parameters, such as scale factor, lattice and profile parameters. The isotropic DebyeeWaller factor was fixed for all atoms of the phases. With the imposed restrictions reliability factors of weighted agreement factor Rwp of 2.3e4.9% were achieved, for up to 16 refinable
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parameters. The strongest sample holder reflections could be attributed to a main component from the fcc Inconel or steel phase which were excluded from the refinement.
3.
Results and discussion
A previous in-situ neutron diffraction study demonstrated that the 1:2 mixture, when exposed to a deuterium pressure of around 50 bar (T ¼ 200 C), shows a phase evolution from Li2Mg(NH)2 to 1LiNH2/1Li2Mg2(NH)3/1LiH and finally to Mg (NH2)2/2LiH under 70 bar pressure [14]. The results of the in-situ neutron diffraction experiments for the 3:8 and 1:4 ratio samples are shown in Figs. 1 and 2, respectively (in order to compare data obtained with different wavelengths they will be plotted against momentum transfer, hereafter labelled as Q). In Fig. 1, the relevant sections of three diffraction patterns are presented for a 3:8 sample held at 200 C under different deuterium pressures. Owing to the good angular resolution of the SPODI diffractometer the two stage reaction is clearly demonstrated. The starting dehydrogenated material is composed of LiH, the alpha polymorph of Li2Mg(NH)2 and a minor amount of another phase with cubic symmetry which might be either the beta polymorph of Li2Mg(NH)2 or Li2NH. After raising the pressure to 32 bar Li2Mg(NH)2 and the cubic phase completely disappeared. The intensity of LiH is increased and two new phases appear, which can be unambiguously identified as LiNH2 and Li2Mg2(NH)3 (Fig. 1, the calculated reflection intensities are indicated). The Mg(NH2)2 phase only becomes evident as the pressure is raised to 71 bar. Fig. 2 gives an overview of the phase evolution for the 1:4 mixture as obtained in 5 min and 2 min intervals at the high intensity instrument D20. Different time intervals were used to obtain faster data acquisition during the beginning of the absorption process. The temperature was set at 200 C. Starting from the absorbed material a full desorption/re-absorption
Fig. 1 e Comparison of the diffraction pattern for the 3:8 mixture in its desorbed state and under 32 bar and 70 bar of deuterium. An offset has been applied to the grey and blue diffraction profiles to increase clarity. Colour bars show the relative calculated intensities for different phases.
Fig. 2 e Phase evolution for the 1:4 mixture under different deuterium pressures. The arrow indicates a region where the collection time for each diffraction pattern decreased to 2 min instead of five.
cycle was measured. The starting material had not been cycled and the nanocrystallinity of the freshly ball-milled material is evidenced by the very broad Mg(NH2)2 and LiH peaks (Fig. 2 at 0 h). Annealing under deuterium pressures at about 30 bar sharpens the peak profiles (2 h). Lowering the pressure to 10 bar D2 brings about the phase transition outlined in reaction (2b): Mg(NH2)2 disappears, LiNH2 and Li2Mg2(NH)3 are formed and the intensity of LiH decreases (2e8 h). By lowering the pressure to 1 bar D2, Li2Mg(NH)2 is formed at the expense of all the other components, which disappear with the exception of LiH, which decreases in intensity (8e12 h). In the subsequent absorption, the deuterium pressure is set to above 40 and finally above 60 bar D2, which causes the rehydrogenation of the sample to Mg(NH2)2 and LiH. The transition in the first step of the reaction, to LiNH2 and Li2Mg2(NH)3 is very fast at 40 bar pressure and it completes within 4 min (see Fig. 3; the sharp step is clearly noticeable in Fig. 2 and indicated by the arrow in the time axis). Thereby the
Fig. 3 e Diffraction Pattern evolution for the 1:4 mixture after deuterium pressure was raised from 40 to 60 bar. Complete hydrogenation occurs within 4 min.
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Fig. 4 e Comparison of 1:2, 3:8 and 1:4 mixtures at different hydrogenation states. a, c, e represent the low angle regions while b, d and f represent the high angle region. a and b represent the desorbed material. c and d represent the first absorption step. e and f represent the material at during the hydrogenation step.
full reversibility of the intermediate in reactions (2a) and (2b) is demonstrated. Fig. 4 presents a comparison between the three different samples at different stages of hydrogenation. It is evident that all three mixtures present similar diffraction profiles during the hydrogenation steps. No significant deviation from reaction (2a) and (2b), as outlined above, was found. Differences in the line profile are due to the different angular resolution of the D20 and SPODI instruments and to differences in the number of hydrogenation/dehydrogenation cycles the samples have undergone. As explained above, for the 3:8 sample (SPODI measurement) the very good resolution of the high angle region (high Q) clearly allows the identification of all the reaction intermediates involved in the hydrogenation process. The desorbed material consists of a-Li2Mg(NH)2 and LiH (except for the 1:2 mixture). Moreover, as already mentioned, it is possible to observe the presence of a cubic phase in all the three different desorbed mixtures (Fig. 4a and b). This cubic
phase is just a minor fraction of the sample and disappears as soon as the desorbed material is exposed to a hydrogen pressure higher than 1 bar. Clear identification of the cubic phase is difficult. Two hypotheses are most likely and involve a cubic modification of the mixed LieMg imide [6,26] or lithium imide. The intermediate hydrogenation step for pressures between 7 and 40 bar at 200 C is unambiguously identified by the appearance of the peaks associated with LiNH2 and Li2Mg2(NH)3 in the diffraction pattern (Fig. 4c and d). The phase transition summarised in reaction (2a) is especially evident in the high Q (high angle) region of the diffraction patterns (Fig. 1). The molar phase compositions of the sample during the intermediate stage have been obtained from Rietveld refinement (Table 1), the analysis of the 3:8 system is as an example shown in Fig. 5. The angular ranges of the reflections of the steel sample container have been excluded from the refinement. The summary for all three mixtures is presented in Table 1. For each mixture a molar ratio LiNH2:
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Table 1 e Molar phase fraction obtained with Rietveld refinement method for the three different mixtures after reaction (1) is completed. In brackets is the expected theoretical molar fraction value for each component.
1:2 Mixture 3:8 Mixture 1:4 Mixture
LiH mol%
LiNH2 mol%
Li2Mg2(NH)3 mol%
Rw%
32 (33.3) 56 (53.8) 73 (71.4)
36 (33.3) 22 (23.1) 14 (14.3)
32 (33.3) 22 (23.1) 13 (14.3)
3.2 4.9 2.3
Li2Mg2(NH)3 of approximately 1:1 is found, in agreement with reaction (2a). The amount of LiH is obviously different for the three mixtures, and the theoretical phase compositions according to reaction (2a) are also given in Table 1. Considering the errors associated with refinement, the fact that lithium has a high absorption coefficient for neutrons and errors associated with sample purity and weighing procedures during sample preparation, the agreement between the refinement results and the phase fractions expected from reaction (2a) (numbers in brackets) is good. Above 40 bar H2 pressure (T ¼ 200 C) the reaction proceeds to the final state (reaction (2b)) to form Mg(NH2)2 and LiH, albeit with slow kinetics. The diffraction patterns shown in Fig. 4e and f are of samples not yet completely hydrogenated. The constant increase in LiH and the appearance of Mg(NH2)2 reflections, however confirm the expected reaction mechanism. A comparison of the diffraction profiles of the 1:2, 3:8 and 1:4 mixtures (Fig. 2) and Rietveld refinement results (Table 1), supports the assumption that the fundamental reaction path of a magnesium amide/lithium hydride mixture does not change when additional lithium hydride is added. Moreover, the similarities between the PCI curves obtained for different magnesium amide/lithium hydride mixtures, as well as the fact that for each mixture the same reaction intermediates are found in
equal stoichiometric ratios (neglecting the higher amount of LiH) clearly demonstrate that the thermodynamic properties and reaction pathway are independent of the starting amide/ hydride ratio. The lithium hydride excess used in the different mixtures does not take any active part in the hydrogen absorption/desorption process at temperatures around 200 C. However, the addition of LiH can improve system performance, as a decreased ammonia emission has been associated to the increase in the lithium hydride content [27]. For each mixture considered, the nature of intermediates depends only on the partial hydrogen equilibrium pressure and not on the starting lithium hydride to magnesium amide stoichiometry.
4.
Conclusions
It has been unambiguously shown, by means of in-situ neutron diffraction measurements that the hydrogenation/ dehydrogenation reaction pathway at around 200 C of three different magnesium amide/lithium hydride mixtures does not change by increasing the lithium hydride content of the system. The comparison of data collected at the SPODI/FRMII and D20/ILL beam lines offer complementary information on crystallographic identity and phase evolution kinetics. All the observations made for the 1:2, 3:8 and 1:4 mixtures are common and can be summarised as follows: - The desorbed material is composed by a-Li2Mg(NH)2 (and LiH for the 3:8 and 1:4 mixtures) plus an unknown cubic phase. - At 200 C equilibrium pressures below 40 bar and higher than 7 bar the formation of LiNH2 and Li2Mg2(NH)3 is observed and the amount of LiH increases. - At 200 C Mg(NH2)2 appears only after raising the pressure above 40 bar.
Acknowledgments Funding is acknowledged from the European Commission Sixth Framework Program under the Marie Curie Research Training Network COSY (Contract No. MRTN-CT-2006-035366) and the Integrated Project NESSHY.
references
Fig. 5 e Rietveld refinement of the 3:8 mixture under 33 bar of deuterium. The diffraction pattern of the empty sample container is given as a reference. The regions in which the strongest peaks associated to the sample container appear have been excluded from the refinement. For sake of clarity the full spectrum is presented with no deleted areas. The excluded regions (in degrees) are 22.5e27.7, 30.4e42.4, 44.9e49, 59.2e63.9, 69.5e72, 74.4e75.7, and 88.5e90.15.
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