Pressure and Temperature Influence on the Desorption Pathway of the

Aug 13, 2010 - Pressure and Temperature Influence on the Desorption Pathway of the LiBH4−MgH2 Composite System ..... Formation of Intermediate Compo...
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Pressure and Temperature Influence on the Desorption Pathway of the LiBH4-MgH2 Composite System Ulrike Bo¨senberg,*,† Dorthe B. Ravnsbæk,‡ Hans Hagemann,§ Vincenza D’Anna,§ Christian Bonatto Minella,† Claudio Pistidda,† Wouter van Beek,| Torben R. Jensen,‡ Ru¨diger Bormann,† and Martin Dornheim† Institute of Materials Research, Materials Technology, GKSS-Research Centre Geesthacht, D-21502 Geesthacht, Germany, Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, UniVersity of Aarhus, DK-8000, Denmark, De´partement de Chimie Physique, UniVersite´ de Gene`Ve, 30, quai E. Ansermet, CH1211 GeneVa 4, Switzerland, and Swiss-Norwegian Beamlines at ESRF, BP-220, 38043 Grenoble, France ReceiVed: May 26, 2010; ReVised Manuscript ReceiVed: July 26, 2010

The decomposition pathway in LiBH4-MgH2 reactive hydride composites was investigated systematically as a function of pressure and temperature. Individual decomposition of MgH2 and LiBH4 is observed at higher temperatures and low pressures (T g 450 °C and p(H2) e 3 bar), whereas simultaneous desorption of H2 from LiBH4 and formation of MgB2 was observed at 400 °C and a hydrogen backpressure of p(H2) ) 5 bar. The simultaneous desorption of H2 from LiBH4 and MgH2 without intermediate formation of metallic Mg could not be observed. In situ X-ray diffraction (XRD) and infrared (IR) spectroscopy reveal the present crystalline and amorphous phases. Introduction For a future society using hydrogen as an energy carrier, safe and efficient storage of hydrogen is one of the key issues. One very promising option regarding safety and volumetric capacity is the storage in metal hydrides. However, no single metal hydride or complex hydride fulfills all the requirements concerning gravimetric capacity, reaction enthalpy, and reaction kinetics for efficient hydrogen storage for mobile applications at present. Against this background, high capacity reactive hydride composites (RHCs) were developed in 2004.1-6 In these systems, two hydrides are combined with each other to react exothermally during the overall endothermic desorption reaction to form a new compound, thereby lowering the overall reaction enthalpy. The concept is similar to the early approaches of Libowitz et al.7 and Reilly and Wiswall,8,9 but the high capacity needed for mobile applications is preserved by the combination of two or more metal hydrides. One of the most prominent and best studied systems of this kind is the LiBH4-MgH2 composite, reacting to form LiH-MgB2 composites and H2 during desorption according to LiBH4 + MgH2 T LiH + MgB2 + 4H2. Reversibility and feasibility of the overall reaction was proven previously. The observed reaction kinetics are slow, although great improvement was achieved by the addition of transition metal additives.10 Despite or because of the extensive studies, contradictory observations, and statements on the reaction pathway, a number of reaction steps and intermediate phases exist. As first proposed by Vajo et al.1 and confirmed by experiments in refs 10 and 11, a hydrogen backpressure facilitates the formation of MgB2. Shim et al.12 systematically investigated the influence of hydrogen backpressures in LiBH4-based reactive hydride com* To whom correspondence should be addressed. E-mail: boesenberg@ gkss.de. Telephone: +49 (0) 4152 87 2565. Fax: +49 (0) 87 2625. † GKSS-Research Centre Geesthacht. ‡ University of Aarhus. § Universite´ de Gene`ve. | Swiss-Norwegian Beamlines at ESRF.

posites forming borides and made very similar observations for all systems. Nakagawa et al.11 investigated the reaction in the LiBH4-MgH2 system under hydrogen and inert gas pressure of several bar at temperatures up to 450 °C and confirmed only in the case of hydrogen atmosphere the formation of MgB2. The formation of MgB2 is considered to be crucial for the reversible reaction and thus the formation of LiBH4.2,11,13 Yang et al.14 showed the partial formation of MgB2 under a backpressure of 1 bar hydrogen at 400 °C and the fraction of MgB2 increases with increasing hydrogen backpressure. Pinkerton et al.13 have drawn phase boundary conditions for thermodynamically and kinetically stable regions for a reversible reaction in TiCl3 catalyzed composites. For example, a minimum of 3 bar hydrogen at temperatures up to 450 °C suppresses the individual decomposition of LiBH4. For the first time, this work clarifies the simultaneous desorption of LiBH4 and formation of MgB2 by in situ X-ray diffraction with simultaneous measurement of the pressure change. Walker and co-workers15-18 on the other hand found formation of a Li-Mg alloy during desorption up to temperatures of 550 °C and no formation of MgB2 under dynamic vacuum. However, they were able to show slow partial reabsorption from the Li-Mg alloy to LiBD4 and MgD2 without observing the formation of MgB2 in the desorbed state. Investigations on the effect of stoichiometry have only shown a change in the fraction of reaction products, not in the basic mechanism. Recent theoretical and experimental investigations19-23 revealed the formation of Li2B12H12 as an intermediate step or as a side reaction during the decomposition of LiBH4, pure or in the composite, according to LiBH4 f 1/12Li2B12H12 + 5/6LiH + 13/12H2 f LiH + B + 3/2H2, lowering the reaction enthalpy by approximately 20 kJ/mol H2 for the first reaction step.20 The possible intermediate formation of Li2B12H12 and the related change in reaction enthalpy has to be considered also for the LiBH4-MgH2 composites. Nonetheless, all experiments evaluated so far took place at elevated temperatures around 400 °C; however, for practical

10.1021/jp104814u  2010 American Chemical Society Published on Web 08/13/2010

Desorption Pathway of the LiBH4-MgH2 Composite System

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TABLE 1: Desorption Conditions for the Prepared Samples A-D and the Conditions for the in Situ XRD Measurements at the Synchrotron with the Expected Reaction Pathway, that is, the Intermediate Step sample

expected reaction pathway, intermediate state + + + + + +

MgH2 MgH2 MgH2 MgH2 MgH2 MgH2

2LiH + Mg + B + 4H2 1 /6Li2B12H12 + 5/3LiH + Mg + 19/6H2 2LiBH4 + Mg + H2 2LiH + MgB2 + 4H2 2LiH + Mg + B + 4H2 2LiBH4 + Mg + H2

A B C D in situ XRD, Figure 2 in situ XRD, Figure 3

2LiBH4 2LiBH4 2LiBH4 2LiBH4 2LiBH4 2LiBH4

in situ XRD, Figure 5

2LiBH4 + MgH2 T 2LiH + MgB2 + 4H2

T T T T T T

application, lower temperatures in combination with desorption pressures around 5 bar H2 are required. In this temperature/ pressure region of T < 360 °C and p ≈ 5 bar, a direct formation of MgB2 and LiH from LiBH4 and MgH2 without intermediate formation of Mg is expected. A first study in this direction was performed by Wan et al.24 showing a hydrogen release of 2 wt % at 265 °C into vacuum, however, with the intermediate formation of metallic Mg. In this work, direct desorption at 350 °C and 5-6 bar hydrogen is attempted. Clearly, the decomposition of LiBH4-MgH2 composites may exhibit different reaction pathways depending on the physical conditions. For practical applications and future tank integration, exact knowledge of the reaction pathway is essential because the obtained reaction products are crucial for possible rehydrogenation. The decomposition pathway in dependence of pressure and temperature and its correlation to the individual compounds and the composite is demonstrated in this Article. Because of slow reaction kinetics, equilibrium is unlikely to be found in dynamic measurements with a variation of pressure and temperature. In this work, the decomposition pathway is followed by in situ X-ray diffraction (XRD) under isothermal and isobaric conditions. Partially desorbed samples are prepared in a Sievert type instrument and analyzed by XRD and infrared (IR) spectroscopy. Thus, changes in the reaction mechanism with possible amorphous intermediate states can be observed. Experimental Section The initial powders were purchased from Alfa Aesar in the highest available purity. The composites were produced by highenergy ball milling in a Spex 8000 M Mixer mill using stainless steel vials and balls. MgH2 was premilled for 5 h before LiBH4,

desorption conditions 440 °C, 3 bar H2 420 °C, 4 bar H2 400 °C, 5 bar H2 350 °C, 5.5 bar H2 heating rate 5 K/min, final T ) 450 °C, 3 bar H2 heating rate 10 K/min between steps, final T ) 415 °C, 5 bar H2 heating rate 5 K/min, final T ) 320 °C, 5 bar H2

and if appropriate the additive was added for further 5 h of milling. The final ball-to-powder ratio was 10:1. All handling and milling was performed under continuously purified argon atmosphere. In situ XRD measurements were performed at the European Synchrotron Research Facility (ESRF) in Grenoble, France at the Swiss-Norwegian Beamline (SNBL), B station. The selected wavelength for these experiments was λ ) 0.5005 Å. For comparison, XRD data are referring to q ) 4π sin(θ/λ), relating the reflections to the normalized reciprocal lattice vectors. The diffracted intensities were collected via a high resolution six arm diffractometer. The samples were prepared in single crystal sapphire capillaries under inert atmosphere and set under pressure at the experimental station. Reaction of the composite with the capilaries was not observed. During the reaction, the absolute pressure in the sample cell was monitored and the changes were analyzed qualitatively. The sample temperature was not monitored directly; therefore, temperature inaccuracies of (25 K cannot be excluded. Further in situ XRD experiments were performed at I711 at MAX-II, MAX-lab, Sweden.25 The samples were mounted in single crystal sapphire tubes under inert conditions in an argonfilled glovebox. The diffracted intensity was measured using a MAR 165 CCD plate detector. The selected wavelength was λ ) 1.072 Å. The sample is heated by a tungsten wire below the capillary, and the temperature is controlled by an inserted thermocouple and an external PID regulator. Details of the experimental setup are given in ref 26. The detailed conditions regarding pressure and temperature for all in situ XRD experiments are listed in Table 1. For the preparation of the ex situ samples for XRD and IR spectroscopy, a Sievert type apparatus designed by HERA Hydrogen Systems, Quebec, Canada was used. Heating to the final temperature was performed under 50 bar hydrogen, and the desired pressure was then set without intermediate evacuation. The samples were cooled to room temperature under the selected hydrogen pressure. The preparation conditions and the expected reaction pathway for each sample are listed in Table 1. The expected final state for all samples is a LiH-MgB2 composite because of the thermodynamic stability. Because of the sluggish reaction kinetics, the intermediate state of the reaction can be observed by cooling the samples rapidly to room temperature under the selected hydrogen pressure. Further characterization with X-rays was achieved with a Siemens D5000 diffractometer using Cu-KR radiation.

Figure 1. van’t Hoff diagram of the experimental and theoretical determined ∆H and ∆S for the composite as well as the individual compounds.1,19,22,27,28

IR spectroscopy of the partially dehydrogenated samples was performed using a Biorad Excalibur FT-IR instrument equipped with a Specac Golden Gate ATR setup. All samples were loaded in a glovebox into the ATR cell. The nominal resolution was 1 or 2 cm-1.

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Figure 2. In situ XRD and pressure record of the first desorption reaction of pure LiBH4-MgH2 composite under 3 bar hydrogen with 5 K/min to a final temperature of 450 °C, measured at SNBL, ESRF.

Results and Discussion The selected experimental conditions and their relation to the thermodynamic properties of the individual compounds, reaction pathway, and composite are illustrated in a van’t Hoff plot in Figure 1. According to measurements of the thermodynamic properties of LiBH427 and MgH2,28 neither compound is stable under the conditions for sample A (T ) 440 °C, p(H2) ) 3 bar), and therefore, an independent decomposition of MgH2 and LiBH4 is expected according to 2LiBH4 + MgH2 T 2LiH + Mg + B + 4H2 T 2LiH + MgB2 + 4H2. Formation of MgB2 only depends on the reactivity between B and metallic Mg. However, under conditions for sample B (T ) 420 °C, p(H2) ) 4 bar), MgH2 decomposes into metallic Mg but LiBH4 can decompose only into Li2B12H1219 and not into LiH and B. Thus, an intermediate formation of Li2B12H12 according to 2LiBH4 + MgH2 T 1/6Li2B12H12 + 5/3LiH + Mg + 19/6H2 T 2LiH + MgB2 + 4H2 depends on the reaction kinetics and is expected in the composites. At lower temperatures such as condition C (T ) 400 °C, p(H2) ) 5 bar), individual decomposition of MgH2 into metallic Mg is possible and expected; however, LiBH4 itself is stable under these conditions according to refs 19 and 27. Hydrogen release from LiBH4 under these conditions can only occur by simultaneous formation of MgB2 and LiH, by effective lowering of the reaction enthalpy. It is in good agreement with the measured thermodynamic properties of LiBH4-MgH2 composites by Vajo et al.1 The expected reaction pathway is 2LiBH4 + MgH2 T 2LiBH4 + Mg + H2 T 2LiH + MgB2 + 4H2. A further decrease in reaction temperature such as that for sample D (T ) 350 °C, p(H2) ) 5.5 bar) is expected to lead to the simultaneous release of hydrogen from MgH2 and LiBH4 coupled with the formation of LiH and MgB2, that is, 2LiBH4 + MgH2 T 2LiH + MgB2 + 4H2. Under these conditions, the reaction enthalpy is effectively lowered regarding LiBH4 as well as MgH2. Sample A is prepared under high temperatures and low pressures. According to the van’t Hoff diagrams shown in Figure 1, the individual decomposition of LiBH4 and MgH2 is expected, which is indeed confirmed by in situ XRD, measured at SNBL, ESRF (see Figure 2). As-milled and uncatalyzed LiBH4-MgH2 composites are heated under 3 bar hydrogen to a final temperature of approximately 450 °C with a heating rate of 5 K/min. During the

Bo¨senberg et al.

Figure 3. In situ XRD and pressure record of the first desorption reaction of pure LiBH4-MgH2 composite under 5 bar hydrogen to a final temperature of 415 °C, measured at SNBL, ESRF. Heating rate between the temperature steps was 10 K/min.

heating period, four distinct events can be observed. At approximately 110 °C, the phase transformation of LiBH4 from orthorhombic to hexagonal occurs. At 275 °C, the LiBH4 diffraction peaks fade out because of melting of this phase. At temperatures around 330 °C, desorption of hydrogen from MgH2 occurs, and this reaction is coupled to the formation of metallic Mg and a increase in pressure. Subsequently, a further increase in pressure occurs without any changes in the diffraction pattern, very likely due to the decomposition of LiBH4 and the corresponding release of hydrogen gas. Further heating the sample to temperatures of 450 °C and keeping isothermal leads to the formation of MgB2 without any further evolution of gas. Therefore, desorption of LiBH4 and subsequent formation of MgB2 are proposed for these experimental conditions. However, heating the same material at slightly higher pressure (p(H2) ) 5 bar) to lower temperatures (415 °C), such as condition C, leads to a different reaction mechanism which is shown in Figure 3. The heating rate between the temperature steps was 10 K/min. Again, the phase transformation and melting of LiBH4 as well as desorption of MgH2 into Mg is observed upon heating. Then, an incubation period similar to that observed in the Sievert type apparatus for the first desorption of pure composites can be observed.10 After several hours, a further increase in pressure, and thus hydrogen release, takes place simultaneous to the upcoming of peaks in the XRD pattern corresponding to MgB2. Therefore, the simultaneous desorption of LiBH4 and formation of MgB2 is concluded for these experimental conditions. Both observations are in good agreement with the thermodynamic properties of the individual compounds and the composite; see Figure 1. To obtain more information on possible amorphous intermediate phases, partially desorbed samples A-D were produced in a Sievert type apparatus, where approximately 5 wt % H2 was desorbed in samples A-C. The results from XRD and IR spectroscopy are shown in Figure 4a and b, respectively. In all samples, residual LiBH4 is observed, by XRD as well as IR spectroscopy. IR spectroscopy clearly shows the corresponding signals for the bending modes of the [BH4]- tetrahedra in the region 1000-1400 cm-1 as well as the stretching modes from 2100 to 2300 cm-1; see Table 2 for a more specific assignment of the peaks to the corresponding IR modes. As expected for the samples A-C, metallic Mg is observed from the XRD patterns and only in sample C partial formation of

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Figure 4. XRD patterns (a) and IR spectra (b) of the as-milled LiBH4-MgH2 composites and samples A-D. Reference spectra of LiBH4 and LiB12H12 · xH2O and as-milled sample are shown in the bottom part.

TABLE 2: Mode Assignment for Experimental IR Spectra frequency (cm-1)

mode assignment

715 broad band around 850 1075 1090 1150 1230 1310 1395 1635 2180 2270 2300 2465 2485 2505

[B12H12]2MgH2 [B12H12]2LiBH4, bending unknown, possible B-H bending LiBH4, bending LiBH4, bending unknown, possible B-H bending H2O bending from contamination LiBH4, stretching LiBH4, stretching LiBH4, stretching [B12H12]2[B12H12]2[B12H12]2-

MgB2 is found. According to the calculations by Ozolins et al.,22 Li2B12H12 is expected to be present in all samples, and according to Orimo et al.20 it should be traced in sample B. However, in none of the samples is Li2B12H12 clearly detected (see Figure 4b). (The weak signal around 2500 cm-1 seen in sample A may possibly correspond to trace amounts of Li2B12H12). For sample D, only the initial materials LiBH4 and MgH2 can be observed; however, this sample did not show any hydrogen release under the present conditions of 350 °C and 5.5 bar H2. Nonetheless, the IR spectra show interesting peaks at 1150 and 1395 cm-1 which cannot be found in the reference spectra of as-received LiBH4. Both peaks are in the region of the B-H bending modes, and therefore, such an interaction is likely. The active B-H mode at 1150 cm-1 forms already during high-energy ball milling and is further present in the samples heated up to 400 °C. After heating to higher temperatures, this mode has disappeared. However, for samples A and B, heated to at least 420 °C, a new active B-H mode is observed at 1395 cm-1. This is very similar to the bending mode of BH-4 tetrahedra in Mg(BH4)2 produced by metathesis reaction of LiBH4 and MgCl2;29,30 therefore, a similar vibration and network of the [BH4]- ions as in Mg(BH4)2 are possibly present in the composites at higher temperatures. A similar active mode was observed in LiBH4-MgH2 composites in refs 14, 24, and 31 after cycling the material, but was not identified in detail. Note that pure Mg(BH4)2 exhibits a very strong band at 1258 cm-1 which is not observed in our samples.32 Starting from LiH-MgB2

Figure 5. In situ XRD of the second desorption reaction of LiBH4-MgH2 composite with 5 mol % NbF5 as additive under 5 bar hydrogen heated with 5 K/min to a final temperature of 320 °C, measured at I711, MAX-lab. Temperature profile and quantitative analysis of the peak area of the MgH2 (110) reflection are shown on the right.

composites, they were not observed after absorption (results not shown) in our samples performing the absorption reaction at 350 °C. It should be noted that B-O bonds exhibit stretching vibrations in a broad spectral range of 1300-1600 cm-1.33 The samples were always handled carefully under inert conditions; oxidation is very unlikely but cannot be excluded completely. Of highest interest is the low temperature region where MgH2 and LiBH4 are proposed to desorb hydrogen and form MgB2 and LiH simultaneously. This was investigated in more detail by in situ XRD at approximately T ) 350 °C and p(H2) ) 5 bar hydrogen at I711, MAX-lab; see Figure 5. For this experiment, a previously cycled sample of NbF5 doped composite was used to avoid possible nucleation barriers for the formation of MgB2.34,35 Previous experiments (results not shown) had shown very high reaction rates for NbF5 doped composites, indicating NbF5 to be a very effective catalytic additive. In the initial composites, LiBH4 as well as MgH2 is present. Upon heating, the phase transformation of LiBH4 occurs and at approximately 275 °C the melting. Further heating is stopped before the decomposition of MgH2 can be traced, and the sample

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is held isothermally for 5.5 h. The diffracted intensity is corrected for decreasing intensity of the beam with time and allows thus for quantitative analysis; the herein applied procedure is described in more detail in ref 35. The development of the MgH2 peak area with time is shown on the right-hand side in Figure 5. No significant change in the peak area of the MgH2 (110) reflection can be observed during the whole experiment. Small changes might be due to the movement of MgH2 crystals in the melt of LiBH4. A peak around q ≈ 2.55 Å-1 appears after approximately 1 h isothermal; however, it could not be related to any known Li-B, Li-B-H, Li-Mg, or Mg-B-H phase. At present, the simultaneous decomposition reaction of MgH2 and LiBH4 with the formation of LiH and MgB2 has not been observed yet. The chosen conditions are already close to the conditions of the thermodynamic equilibrium, and the driving force for the reaction can therefore be assumed to be small. This is especially important because the thermodynamic properties were derived from absorption reaction measurements.1 The conclusion is supported by a recent study of nanoconfined composites in carbon aerogels with significantly lower reaction temperatures.36 Conclusions Detailed studies of LiBH4-MgH2 reactive hydride composites have shown a strong dependency of the reaction mechanism on pressure and temperature. In situ XRD combined with pressure observation proves to be a powerful tool for such investigations. At high temperatures and low pressures, independent decomposition of all hydrides and subsequent formation of MgB2 are observed, whereas at slightly lower temperatures and slightly higher pressures first decomposition of MgH2 and subsequently simultaneous decomposition of LiBH4 and formation of MgB2 can be observed. Simultaneous decomposition of MgH2 and LiBH4 along with the formation of MgB2 and LiH could not be observed. IR spectroscopic measurements could not confirm the proposed formation of Li2B12H12 for any of the applied conditions. Acknowledgment. The authors wish to thank Dr. Olga Sofanova of SNBL at ESRF and Dr. Yngve Cerenius I711, MAX-lab for their support and help. Partial funding by the Helmholtz Initiative “Functional Materials for Mobile Hydrogen Storage” is gratefully acknowledged by the authors. This work was supported in part by the Swiss National Science Foundation. References and Notes (1) Vajo, J. J.; Skeith, S. L.; Mertens, F. Reversible Storage of Hydrogen in Destabilized LiBH4. J. Phys. Chem. B 2005, 109, 3719–3722. (2) Barkhordarian, G.; Klassen, T.; Dornheim, M.; Bormann, R. Unexpected Kinetic Effect of MgB2 in Reactive Hydride Composites Containing Complex Borohydrides. J. Alloys Compd. 2007, 440, L18–L21. (3) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Interaction of hydrogen with metal nitrides and imides. Nature 2002, 420, 302–304. (4) Cho, Y. W.; Shim, J.-H.; Lee, B. J. Thermal destabilization of binary and complex metal hydrides by chemical reaction: A thermodynamic analysis. CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 2006, 30, 65–69. (5) Luo, W. (LiNH2-MgH2): A viable hydrogen storage system. J. Alloys Compd. 2004, 381 (1-2), 284–287. (6) Dornheim. M. Handbook of Hydrogen Storage; Hirscher, M., Ed.;Wiley-VCH: 2010; Chapter Tailoring Reaction Enthalpies of Hydrides. (7) Libowitz, G. G.; Hayes, H. F.; Gibb Jr, T. R. G. The System Zirkonium-Nickel and Hydrogen. J. Phys. Chem. 1958, 62, 76. (8) Reilly, J. J.; Wiswall, R. H. The Reaction of Hydrogen with Alloys of Magnesium and Copper. Inorg. Chem. 1967, 6 (12), 2220–2223. (9) Reilly, J. J.; Wiswall, R. H. Reaction of hydrogen with alloys of magnesium and nickel and the formation of Mg2NiH4. Inorg. Chem. 1968, 7 (11), 2254–2256.

Bo¨senberg et al. (10) Bo¨senberg, U.; Doppiu, S.; Mosegaard, L.; Barkhordarian, G.; Eigen, N.; Borgschulte, A.; Jensen, T. R.; Cerenius, Y.; Gutfleisch, O.; Klassen, T.; Dornheim, M.; Bormann, R. Hydrogen sorption properties of 2LiBH4/ MgH2 composites. Acta Mater. 2007, 55, 3951–3958. (11) Nakagawa, T.; Ichikawa, T.; Hanada, N.; Kojima, Y.; Fujii, H. Thermal analysis on the Li-Mg-B-H systems. J. Alloys Compd. 2007, 446447, 306–309. (12) Shim, J.-H.; Lim, J.-H.; Rather, S.-u.; Lee, Y.-S.; Reed, D.; Kim, Y.; Book, D.; Cho, Y. W. Effect of Hydrogen Back Pressure on Dehydrogenation Behavior of LiBH4-Based Reactive Hydride Composites. J. Phys. Chem. Lett. 2010, 1, 59–63. (13) Pinkerton, F. E.; Meyer, M. S.; Meisner, G. P.; Balogh, M. P.; Vajo, J. J. Phase Boundaries and Reversibility of LiBH4/MgH2 Hydrogen Storage Material. J. Phys. Chem. C 2007, 111 (35), 12881–12885. (14) Yang, J.; Sudik, A.; Wolverton, C. 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