Magnesium borohydride (Mg(BH₄)₂) is a promising material for hydrogen storage because of its high gravimetric storage density (15.0 mass%). We intended to synthesize Mg(BH₄)₂ by decomposition reaction of LiBH₄ with MgCl₂ by heat treatment without using a solvent, where the product consists of LiCl and a compound of magnesium, boron and hydrogen. Hydrogen desorption temperature of the product is approximately 100 K lower than that of LiBH₄ and the decomposition consists of a two-step reaction. The products of the 1st and 2nd decomposition reactions are MgH₂ and Mg, respectively. This result indicates the following two-step reaction (1st reaction: Mg(BH₄)₂→MgH₂+2B+3H₂, 2nd reaction: MgH₂→Mg+H₂). The first decomposition peak is dominant and is around 563 K. The 2nd decomposition occurs at the temperature greater than 590 K

Buchter, Florian

Matsunaga, Tomoya

Miwa, K.

Orimo, S.

Towata, S.

Züttel, Andreas

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http://doc.rero.chMagnesium borohydride: A new hydrogen storage materialT. Matsunagaa,, F. Buchtera, K. Miwac, S. Towatac, S. Orimod, A. Zu¨ttela,baPhysics Department, University of Fribourg, Pe´rolles, 1700 Fribourg, SwitzerlandbEMPA, Department Environment, Energy and Mobility, Abt. 138 ‘‘Hydrogen & Energy’’, U¨berlandstrasse 129, 8600 Du¨bendorf, SwitzerlandcToyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, JapandInstitute for Materials Research, Tohoku University, Sendai 980-8577, JapanAbstractMagnesium borohydride (Mg(BH4)2) is a promising material for hydrogen storage because of its high gravimetric storage density(15.0mass%). We intended to synthesize Mg(BH4)2 by decomposition reaction of LiBH4 with MgCl2 by heat treatment without using asolvent, where the product consists of LiCl and a compound of magnesium, boron and hydrogen. Hydrogen desorption temperature ofthe product is approximately 100K lower than that of LiBH4 and the decomposition consists of a two-step reaction. The products of the1st and 2nd decomposition reactions are MgH2 and Mg, respectively. This result indicates the following two-step reaction (1st reaction:Mg(BH4)2-MgH2+2B+3H2, 2nd reaction: MgH2-Mg+H2). The ﬁrst decomposition peak is dominant and is around 563K. The2nd decomposition occurs at the temperature greater than 590K.Keywords: Hydrogen absorbing materials; Thermodynamic properties; Thermal analysis1. IntroductionHigh-density hydrogen storage is one of the mostimportant issues to introduce fuel cell vehicles. Complexhydrides, consisting of light elements, are very promisingmaterials for hydrogen storage because of their highgravimetric and volumetric hydrogen density [1]. SinceBogdanovic and Schwickardi reported the reversibility ofthe catalyzed hydrogen sorption reaction of NaAlH4 [2],many efforts have been made to investigate complexhydrides as hydrogen storage materials [3–5]. Besides thebasic physical properties, the challenges are to tailor thestability and to investigate the reversibility of the hydrogensorption reaction of complex hydrides.The hydrogen in the complex hydrides is often located inthe corners of a tetrahedron with B or Al in the center. Thenegative charge of the anion, [BH4] and [AlH4], iscompensated by a cation, e.g. Li or Na. Therefore, thebonding character and the properties of the complexhydrides are largely determined by the difference inelectronegativity between the cation and the boron oraluminum. For example, the docomposition temperaturesof complex hydrides consist of alkaline metal and [BH4]or [AlH4] anion have a good correlation with electro-negativity of cation as is shown in Fig. 1. Based on thiscorrelation, experimental studies have been preformed tolower the stability of complex hydrides [6–8]. Also,theoretical calculations indicate that this tendency can beapplied not only to alkaline metal borohydride, but also toother borohydrides with alkaline-earth metals or some oftransition metals [9,10].Magnesium borohydride (Mg(BH4)2) is one of thepromising materials for hydrogen storage because of itshigh gravimetric storage density (15.0mass%). The Paulingelectronegativity of magnesium is 1.31, which is greaterthan that of lithium (0.98). This implies that Mg(BH4)2 isless stable than LiBH4. Until now, physical properties ofMg(BH4)2 as a hydrogen storage material are not knownbecause it is difﬁcult to synthesize the material. The processto synthesize Mg(BH4)2 has been mainly examined by twodifferent approaches. One is the reaction of diborane(B2H6) with magnesium or its compounds [11]. The other isthe double decomposition of magnesium halides withCorresponding author.E-mail address: tmatu@mvj.biglobe.ne.jp (T. Matsunaga).1Published in "Renewable Energy 33(2): 193-196, 2007"which should be cited to refer to this work. http://doc.rero.chalkaline metal borohydrides in an organic solvent, e.g.diethyl ether or tetrahydrofuran [11,12]. The secondapproach has the advantage that it does not requirediborane, which is a toxic compound. However, theproduct which is synthesized by double decomposition ina solvent is a solvate consisting of Mg(BH4)2 and solvent(e.g. Mg(BH4)2  3THF) and the extraction of Mg(BH4)2from the solvate is difﬁcult because of its afﬁnity with thesolvent [13].In this study, we report about a new method intended tosynthesize Mg(BH4)2 by dry process without using asolvent. Lithium borohydride and magnesium chlorideare used as starting materials. The synthesis products aswell as the hydrogen desorption products were investigatedby X-ray diffraction and hydrogen desorption measure-ment of the product synthesized by the new method havebeen performed.2. ExperimentalThe samples were purchased from Aldrich Co. Ltd.: thepurities are 495% for LiBH4, 499.9% for MgCl2,respectively. The samples were always handled in an argonglove box to avoid any possible reaction with moisture orair. The reaction between LiBH4 and MgCl2 was investi-gated by differential scanning calorimetry (DSC) (MettlerToledo Inc. HP DSC827e). Five milligrams of the sample(for 2:1 mole ratio) was mixed in an argon glove box andﬁlled in a DSC sample cell made of aluminum. The samplecell was sealed up in argon atmosphere. Therefore, it hasnever been exposed to any gases except pure argon duringthe measurement. DSC measurement of LiBH4 and MgCl2mixture was carried out in the temperature range from 313to 513K at the heating (or cooling) rate of 5K/min in thesealed sample cell for three heating and cooling cycles.Measurements with pure LiBH4 and MgCl2 were alsoperformed at the same heating condition.Heat treatments of LiBH4 and MgCl2 mixture werecarried out in a stainless steel cylinder o10MPa ofhydrogen at 453, 523 or 593K. In each heat treatment,approximately 600mg of LiBH4 and MgCl2 mixture (for2:1 mole ratio) was pressed in order to make a pellet andﬁlled into a cylinder in an argon glove box. After evacu-ating by rotary vacuum pump for 1 h at room temperature,10MPa of hydrogen was introduced into the cylinder andheated up. It was kept at the ﬁnal temperature mentionedabove for 3 h and then slowly cooled to room temperature.Crystal structures of the samples were investigated atroom temperature by powder X-ray diffraction measure-ment (SIEMENS, D-500, Cu Ka). To avoid exposure toair, each sample was ﬁlled into a sample holder in an argonglove box and covered with plastic wrap ﬁlm during X-raydiffraction measurement.Temperature-programmed desorption (TPD) measure-ment was carried out in vacuum continuously after heattreatment at 593K in the same stainless steel cylinder. Thedesorbed gas volume was measured by a mass ﬂowcontroller (Brooks instruments, 5850E, max. ﬂow 5standard cm3/min), with a maximum full-scale error of 1%.3. Results and discussionThe DSC proﬁles of LiBH4 and MgCl2 mixture areshown in Fig. 2. Proﬁles of pure LiBH4 and pure MgCl2were also investigated as references. For LiBH4, there is anendothermic peak (at T ¼ 386K, DQ ¼ 206.0 J/g) duringheating from 313 to 513K and an exothermic peak duringcooling both of them are corresponding to the phasetransition of LiBH4, whereas no peak was observed forMgCl2 in this temperature range. For the LiBH4 andMgCl2 mixture sample, there is an endothermic peak(at T ¼ 385K, DQ ¼ 56.3 J/g) during the 1st heatingprocedure, corresponding to the phase transition of LiBH4.However, this peak disappears after the cooling of 1stcycle. This result indicates that there is no pure LiBH4remains after the 1st heating. In addition, for the LiBH4and MgCl2 mixture, other exothermic and endothermicpeaks are observed after the 1st cooling at T ¼ 440K(DQ ¼ 23.6 J/g). These peaks are not observed for pureLiBH4 or pure MgCl2. This result implies that somereaction took place during the 1st heating. The endother-mic peak at 440K observed after the 1st cooling is inagreement with the DSC measurement result whichStasinevich and Egorenko has reported for Mg(BH4)2[14]. Therefore, the following reaction during the 1stheating procedure is assumed:2LiBH4 þMgCl2 !MgðBH4Þ2 þ 2LiCl:XRD measurement results after heat treatment alsoindicate that LiBH4 react with MgCl2 during heattreatment. Fig. 3 shows the XRD measurement resultsafter heat treatment at various temperatures. Simplymixing LiBH4 and MgCl2 in an inert atmosphere (Ar)does not lead to any reaction. However, after heattreatment, the peaks corresponding to LiBH4 and MgCl2disappear. New peaks were observed which correspond toLiCl. Although there is still a small amount of LiBH4and MgCl2 after heat treatment at 453K, neither LiBH4nor MgCl2 was observed after heat treatment at theDecomposition temperature [K]1020040060080010001200-2000200400600800KBH4NaBH4LiBH4LiAlH4NaAlH4KAlH4Pauling electronegativity of the cation0.6 0.7 0.8 0.9 1.1 1.2 1.3 1.4 1.5Decomposition temperature [°C]Fig. 1. Decomposition temperature as a function of Pauling electro-negativity of the cation.2http://doc.rero.chtemperature 4523K. The reason for the absence of adiffraction pattern for Mg(BH4)2 may be that thecompound synthesized by this method does not crystallizedsufﬁciently. Orimo et al. [15] have reported that long-rangeorder of LiBH4 disappears when LiBH4 melts and solidiﬁesagain. The same phenomena may happen in the sampleafter heat treatment.TPD measurement of the LiBH4 and MgCl2 mixture wascarried out after heat treatment at 593K for 3 h o10MPaof hydrogen. After heat treatment, the sample was cooleddown to room temperature o10MPa of hydrogen.Subsequently, hydrogen gas was extracted from thecylinder to vacuum at room temperature, where no gasevolution was observed from the sample during thisprocedure. After evacuation, the sample was heated witha heating rate of 0.2K/min. The thermograms of the TPDmeasurement are shown in Fig. 4. The hydrogen deso-rption temperature of the sample after the heat treatment isapproximately 100K lower as compared to that of the pureLiBH4 sample. The sample after heat treatment has twodesorption peaks. It starts to decompose at 500K and the1st desorption peak appears around 563K. This peak isdominant and more than half of hydrogen is evolved. Thesecond desorption peak starts at 600K and it is very sharpas compared to the 1st desorption peak. The shape of twoindependent desorption peaks implies that decompositionreaction consists of two steps.350 400 450 50050 100 150 200350 400 450 50050 100 150 200(1st cycle)(2nd cycle)(3rd cycle)LiBH4MgCl2LiBH4+MgCl2 mixture (2:1 mole ratio)10 [mW]30 [mW]Temperature [K]Temperature [°C]Fig. 2. Differential scanning calorimetry proﬁles of LiBH4 and MgCl2mixture with a sealed sample cell. The heating rate is 5K/min.Measurements with pure LiBH4 and MgCl2 were also performed at thesame heating condition. Solid lines and dotted lines show heating andcooling, respectively. Five milligrams of the sample was used for eachmeasurement.10 20 30 40 50 60 70 80 902000 [counts]LiClMgCl2Plastic wrap filmLiBH4(a)(b)(c)(d)2 [°]Fig. 3. Powder X-ray diffraction proﬁles of LiBH4 and MgCl2 mixture:(a) before heat treatment; (b) after heat treatment at 453K; (c) after heattreatment at 523K; and (d) after heat treatment at 593K, respectively.Each heat treatment was carried out o10MPa of hydrogen.300 400 500 600 700024681012100 200 300 400 500300 400 500 600 700100 200 300 400 500(b)H2 desorption [mass %]Temperature [K](a)(b)(a)Temperature [°C]Fig. 4. Temperature-programmed desorption spectra of: (a) LiBH4 andMgCl2 mixture after heat treatment at 593K o10MPa of hydrogen; and(b) pure LiBH4. The samples were heated after evacuation at roomtemperature with a heating rate of 0.2K/min.3http://doc.rero.chFig. 5 shows the XRD measurement results of the LiBH4and MgCl2 mixture (a) before heat treatment, (b) after heattreatment at 593K for 3 h, (c) after TPD measurement upto 593K and (d) after TPD measurement up to 773K. As itis shown in Fig. 5(c), after the desorption measurement upto 593K, the peaks which correspond to MgH2 areobserved, whereas the diffraction pattern of metallic Mgis not observed. On the other hand, as is shown in Fig. 5(d),after heated up to 773K, the peaks corresponding to Mgare clearly present, where the peaks from MgH2 are nolonger observed. This result supports the following two-step reaction:MgðBH4Þ2 !MgH2 þ 2Bþ 3H2 ð1st stepÞ;MgH2 !MgþH2 ð2nd stepÞ:4. ConclusionsWe intended to synthesize Mg(BH4)2 by decompositionreaction of LiBH4 with MgCl2 by heat treatment withoutusing a solvent, where the product consists of LiCl and acompound of magnesium, boron and hydrogen. Hydrogendesorption property of the product was examined by TPDmeasurement. It was found that the decompositiontemperature of the product is approximately 100K lowerthan that of LiBH4 and the decomposition consists of atwo-step reaction. The 1st decomposition peak is dominantand is around 563K. The 2nd decomposition reactionoccurs at the temperature4590K. The products of the 1stand 2nd decomposition reaction are MgH2 and Mg,respectively. Therefore, this is an evidence for Mg(BH4)2as the possible reaction product, although no diffractionpattern was found.AcknowledgmentsThis study was partially supported by the New Energyand Industrial Technology Development Organization(NEDO), International Joint Research under the ‘‘Devel-opment for Safe Utilization and Infrastructure of hydro-gen’’ Project (2005–2006) and the Swiss Federal Ofﬁce ofEnergy (BFE) project ‘‘Hydrogen storage in metal- andcomplex hydrides’’, No. 100324 (2003–2006).References[1] Zu¨ttel A. Materialstoday 2003:24.[2] Bogdanovic BM, Schwickardi J. J Alloys Compds 1997;253–254:1.[3] Zu¨ttel A, Rentsch S, Fischer P, Wenger P, Sudan P, Mauron Ph, et al.J Alloys Compds 2003;356:515.[4] Chen P, Xiong Z, Luo J, Lin J, Tan KL. Nature 2002;420:302.[5] Vajo JJ, Skeith SL, Mertens F. J Phys Chem B 2005;109(9):3719.[6] Orimo S, Nakamori Y, Zu¨ttel A. Mater Sci Eng B 2004;108:51.[7] Nakamori Y, Orimo S. J Alloys Compds 2004;370:271.[8] Nakamori Y, Miwa K, Ninomiya A, Li H, Ohba N, Towata S, ZuttelA, Orimo S. Communicated.[9] Miwa K, Ohba N, Towata S, Nakamori Y, Orimo S. J AlloysCompds 2005;404–406:140.[10] Yoshino M, Komiya K, Takahashi Y, Shinzato Y, Yukawa H,Morinaga M. J Alloys Compds 2005;404–406:185.[11] Plesek J, Hermanek S. Collect Czech Chem Commun 1966;31:3845.[12] Konoplev VN. Russ J Inorg Chem 1980;25(7):964.[13] Konoplev VN, Silina TA. Russ J Inorg Chem 1974;19(9):1383.[14] Stasinevich DS, Egorenko GA. Russ J Inorg Chem 1968;13(3):341.[15] Orimo S, Nakamori Y, Kitahara G, Miwa K, Ohba N, Towata S, etal. J Alloys Compds 2005;404–406:427–30.10 20 30 40 50 60 70 80 90LiClMgMgCl2Plastic wrap filmLiBH4(a)(b)(c)(d)MgH22000 [counts]2 [°]Fig. 5. Powder X-ray diffraction proﬁles of LiBH4 and MgCl2 mixture:(a) before heat treatment; (b) after heat treatment at 593K o10MPa ofhydrogen; (c) after desorption up to 593K; and (d) after desorption up to773K in vacuum, respectively.4

Magnesium borohydride: A new hydrogen storage material

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