Structural differences in LiBH₄ before and after the melting reaction at approximately 550 K were investigated to clarify the experimental method for the confirmation of reversible dehydriding and rehydriding reactions. Since the long-range order of LiBH₄ begins to disappear after the melting reaction was achieved, investigation of the atomistic vibrations of the [BH₄]-anion in LiBH₄ was found to be effective for the confirmation of the reversibility. In the present study, LiBH₄ was successively dehydrided (decomposed) into LiH and B under 1 MPa of hydrogen at 873 K, and then rehydrided (recombined) into LiBH₄ under 35 MPa of hydrogen at the same temperature (873 K). The temperatures at the beginning and ending of the dehydriding reaction are lowered, by approximately 30 K, for LiBH₄ substituted (or mixed) with Mg (atomic ratio of Li:Mg=9:1) as compared to those for LiBH₄ alone. This is similar to the tendency exhibited by LiNH₂

Kitahara, G.

Miwa, K.

Nakamori, Y.

Ohba, N.

Orimo, S.

Towata, S.

Züttel, Andreas

RERO DOC Digital Library

http://doc.rero.chDehydriding and rehydriding reactions of LiBH4S. Orimoa,∗, Y. Nakamoria, G. Kitaharaa, K. Miwab, N. Ohbab, S. Towatab, A. Zu¨ttelca Institute for Materials Research, Tohoku University, Sendai 980-8577, Japanb Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japanc University of Fribourg, Physics Institute, Perolles 1700, SwitzerlandAbstractStructural differences in LiBH4 before and after the melting reaction at approximately 550 K were investigated to clarify the experimentalmethod for the confirmation of reversible dehydriding and rehydriding reactions. Since the long-range order of LiBH4 begins to disappearafter the melting reaction was achieved, investigation of the atomistic vibrations of the [BH4]-anion in LiBH4 was found to be effective forthe confirmation of the reversibility. In the present study, LiBH4 was successively dehydrided (decomposed) into LiH and B under 1 MPa ofhydrogen at 873 K, and then rehydrided (recombined) into LiBH4 under 35 MPa of hydrogen at the same temperature (873 K). The temperaturesat the beginning and ending of the dehydriding reaction are lowered, by approximately 30 K, for LiBH4 substituted (or mixed) with Mg (atomicratio of Li:Mg = 9:1) as compared to those for LiBH4 alone. This is similar to the tendency exhibited by LiNH2.1. IntroductionLiBH4 is a potential candidate for hydrogen storage ma-terials because of its high gravimetric and volumetric hydro-gen densities: 18.5 mass% and 121 kg H2/m3, respectively[1]. After the melting reaction at approximately 550 K, thedehydriding reaction accompanied by the phase decomposi-tion proceeds mainly above 573 K according to the followingequation [2]:LiBH4 ⇒ LiH + B + (3/2)H2 (1)During the dehydriding (decomposition) reaction,13.8 mass% of hydrogen (“3/2 mol of hydrogen” per “1 molof LiBH4”) can be released from LiBH4.The dehydriding temperatures of LiBH4 have been re-ported to decrease to approximately 473 K when SiO2 isadded as a catalyst [3]. Moreover, experimental studies on thephysical properties of MBH4, where M = Li, Na, K [4,5],∗ Corresponding author. Tel.: +81 22 215 2093; fax: +81 22 215 2091.E-mail address: orimo@imr.tohoku.ac.jp (S. Orimo).and the first-principles calculations of LiBH4 [6] have indi-cated that an effective way to thermodynamically lower thedehydriding temperature of LiBH4 is to suppress the chargetransfer from the Li-cation to the [BH4]-anion by the par-tial substitution of Li; a similar technique has already beenapplied for Li(-Mg)NH2 [7–9].In addition, the rehydriding (recombination) reaction,which proceeds as follows:LiH + B + (3/2)H2 ⇒ LiBH4 (2)should be improved upon to functionalize LiBH4 as a re-versible hydrogen storage medium. However, until recently,it has been reported that direct synthesis of elemental Li and Byield only LiH [10], and LiBH4 cannot be synthesized underelevated conditions: 15 MPa of hydrogen at 923 K [2,3].Therefore, in this study, we have attempted to rehydrideLiBH4 in order to confirm the reversible reactions of Eqs.(1) and (2). Prior to that, the structural differences in LiBH4before and after the melting reaction at approximately 550 Kwere investigated to clarify the experimental method for theconfirmation of the reversibility.0925-8388/$ – see front matter 2005 Elsevier B.V. All rights reserved.1Published in "Journal of Alloys and Compounds 404-406: 427-430, 2005"which should be cited to refer to this work. http://doc.rero.ch2. ExperimentalLiBH4 of 95% purity was purchased from Aldrich Co.Ltd. The melting and dehydriding temperatures of LiBH4were measured using differential scanning calorimetry (DSC)(Bruker-AXS, PDSC 3100S, under 0.1 MPa of hydrogen at aheating rate of 10 K/min). Based on the results, LiBH4 wasdehydrided according to Eq. (1) as follows: 100 mg of LiBH4was placed in a Mo crucible in a glove box filled with purifiedargon (dew point below 180 K). The crucible containing theLiBH4 was then sealed into a reaction-tube equipped with aconnection valve for the evacuation and introduction of hy-drogen. After carrying out the evacuation for 12 h, 1 MPa ofhydrogen (99.99999%) was introduced into the reaction-tubeand heated up to 873 K for 5 min. Subsequently, the dehy-drided LiBH4, i.e., the mixture of LiH and B, was rehydridedaccording to Eq. (2) in a similar method as the dehydriding re-action explained above: 35 MPa of hydrogen was introducedinto the reaction-tube using the high-pressure gas reactionunit, and heated up to 873 K for 12 h.Crystal and atomistic structures were examined at roomtemperature by powder X-ray diffraction measurement(PANalytical, X’Pert, Cu K) and in situ Raman spec-troscopy (Nicolet, Almega-HD with color-CCD, 532 nmlaser with back scattering geometry), respectively. The de-hydriding reaction was investigated using thermal desorptionspectroscopy detected by gas chromatography (GL Science,GC323, at a heating rate of 10 K/min). The samples werealways handled in the glove box filled with purified argonwithout exposure to air before and during measurements.3. Results and discussion3.1. Structural differences in LiBH4 before and aftermelting reactionThe DSC profile of LiBH4 is shown in Fig. 1. Two sharpand one broad endothermic reactions were observed, whichcorrespond to the structural transition (Ts = 380 K) [11–13],melting reaction (Tm = 550 K), and dehydriding reaction(Td = 600–700 K), respectively.Fig. 2 shows the powder X-ray diffraction profiles ofLiBH4 at room temperature (a) before the melting reaction(as purchased) and (b) after the melting reaction under 1 MPaof hydrogen at 573 K. After the melting reaction (and thencooled down to room temperature), the diffraction peaks forLiBH4 became smaller and broader than those before themelting reaction. These observations suggest that the long-range order of LiBH4 began to disappear after the meltingreaction was achieved.However, the B–H atomistic vibrations of the [BH4]-anionwere clearly detected using in situ Raman spectroscopy, bothbefore and after the melting reaction, as shown in Fig. 3. Thus,we emphasize that the investigation of the atomistic vibra-tions (directly affected by the short-range order or bonding)Fig. 1. DSC profile of LiBH4. The sample was heated under 0.1 MPa ofhydrogen at a rate of 10 K/min.of the [BH4]-anion in LiBH4 is effective for the confirmationof the reversibility according to Eqs. (1) and (2), especiallyin the case of the rehydriding reaction above the melting tem-perature.3.2. Rehydriding and dehydriding reactions of LiBHThe mixture of LiH and B was prepared by the dehydridingreaction of LiBH4 under 1 MPa of hydrogen at 873 K for5 min. As shown in Fig. 4(b), only LiH peaks were observedafter the dehydriding reaction. No diffraction peak of boronwas observed, indicating that boron is in an amorphous-likephase. The dehydrided LiBH4, i.e., the mixture of LiH and B,was subsequently rehydrided under 35 MPa of hydrogen at873 K. After the rehydriding reaction, only small diffractionpeaks of LiBH4 were observed, as shown in Fig. 4(c). This isprobably due to the disappearance of the long-range order at873 K, which is a higher temperature than that for the meltingreaction mentioned above.Fig. 2. Powder X-ray diffraction profiles (at room temperature) of LiBH4 (a)before the melting reaction (as purchased) and (b) after the melting reactionunder 1 MPa of hydrogen at 573 K.2http://doc.rero.chFig. 3. Raman spectra (at room temperature) of (a) before the melting re-action (as purchased) and (b) after the melting reaction under 1 MPa ofhydrogen at 573 K.Therefore, the B–H atomistic vibrations in LiBH4 wereexamined, as shown in Fig. 5. The vibration modes wereclearly observed in the sample after the rehydriding reaction;this was expected from the result described in Section 3.1.From these results, LiBH4 was confirmed to rehydride under35 MPa of hydrogen at 873 K.The thermal desorption profile of the rehydrided LiBH4 isshown in Fig. 6. The dehydriding reaction of LiBH4 beginsat approximately 700 K, and a sharp peak appears at approx-imately 850 K. A preliminary result of the rehydrided LiBH4substituted (or mixed) with Mg (atomic ratio of Li:Mg = 9:1)is also shown in Fig. 6. The temperatures at the beginning andending of the dehydriding reaction are lowered, by approx-imately 30 K, for LiBH4 substituted (or mixed) with Mg ascompared to those for LiBH4 alone. A similar tendency hasbeen reported in the case of LiNH2 [5,8]. Detailed investi-Fig. 4. Powder X-ray diffraction profiles (at room temperature) of (a) beforethe dehydriding reaction (as purchased), (b) dehydrided LiBH4 under 1 MPaof hydrogen at 873 K, and (c) rehydrided LiBH4 under 35 MPa of hydrogenat 873 K.Fig. 5. Raman spectra (at room temperature) of LiBH4 (a) before the dehy-driding reaction (as purchased, same to Fig. 3(a)) and (b) after rehydridingreaction.Fig. 6. Thermal desorption profiles of rehydrided (a) LiBH4 and (b) sub-stituted (or mixed) with Mg (atomic ratio of Li:Mg = 9:1). The sample washeated under 0.1 MPa of argon at a rate of 10 K/min.gations of the effects of Mg substitution (or mixing) on thedehydriding and rehydriding reactions of LiBH4 are under-way.4. ConclusionsLiBH4, a potential candidate for hydrogen storage materi-als, was successively dehydrided (decomposed) into LiH andB under 1 MPa of hydrogen at 873 K, and then rehydrided (re-combined) into LiBH4 under 35 MPa of hydrogen at the sametemperature (873 K). An effective way for the confirmationof the reversibility is to investigate the atomistic vibrationsof the [BH4]-anion in LiBH4, because the long-range orderof LiBH4 began to disappear after the melting reaction wasachieved. The Mg substitution (or mixing) effects for thedehydriding and rehydriding reactions of LiBH4 have beenpreliminary investigated.3http://doc.rero.chAcknowledgementsThis work was partially supported, by the Ministryof Education, Culture, Sports, Science and Technology,“Grant-in-Aid for Encouragement of Young Scientists (A),#15686027” and “Grant-in-Aid for Exploratory Research#15656001”, by the New Energy and Industrial TechnologyDevelopment Organization (NEDO), “Development of SafeUtilization Technology and an Infrastructure for HydrogenUse (2003-2004), #03001387”, by the Thermal and ElectricEnergy Technology Foundation, by Tohoku Intelligent Cos-mos Research Foundation, and by the Collaborative Researchin Center for Interdisciplinary Research, Tohoku University.References[1] L. Schlapbach, A. Zu¨ttel, Nature 414 (2001) 353.[2] A. Zu¨ttel, P. Wenger, S. Rentsch, P. Sudan, Ph. Mauron, Ch. Emmeneg-ger, J. Power Sources 5194 (2003) 1.[3] A. Zu¨ttel, S. Rentsch, P. Fisher, P. Wenger, P. Sudan, Ph. Mauron, Ch.Emmenegger, J. Alloys Compd. 356/357 (2003) 515.[4] S. Orimo, Y. Nakamori, A. Zu¨ttel, Mater. Sci. Eng. B 108 (2004)51.[5] Y. Nakamori, S. Orimo, J. Alloys Compd. 370 (2004) 271.[6] K. Miwa, N. Ohba, S. Towata, Y. Nakamori, S. Orimo, Phys. Rev. B69 (2004) 245120.[7] Y. Nakamori, S. Orimo, Mater. Sci. Eng. B 108 (2004) 48.[8] S. Orimo, Y. Nakamori, G. Kitahara, K. Miwa, N. Ohba, T. Noritake,S. Towata, Appl. Phys. A (Rapid Commun.) 79 (2004) 1765.[9] M.E. Arroyo y de Dompablo, G. Ceder, J. Alloys Compd. 364 (2004)6.[10] H.I. Schlesinger, H.C. Brown, H.R. Hoekstra, L.R. Rapp, J. Am. Chem.Soc. 69 (1949) 1231.[11] S. Gomes, H. Hagemann, K. Yvon, J. Alloys Compd. 346 (2002) 206.[12] J.-Ph. Soulie´, G. Renaudin, R. Cerny´, K. Yvon, J. Alloys Compd. 346(2002) 200.[13] H. Hagemann, S. Gomes, G. Renaudin, K. Yvon, J. Alloys Compd.363 (2004) 129.4

Dehydriding and rehydriding reactions of LiBH₄

Abstract

Similar works

Full text

Available Versions

RERO DOC Digital Library