dehydrogenation reaction pathway of the libh mgh composite...

Dehydrogenation Reaction Pathway of the LiBH4−MgH2 Compositeunder Various Pressure ConditionsKee-Bum Kim,†,‡ Jae-Hyeok Shim,*,† So-Hyun Park,† In-Suk Choi,† Kyu Hwan Oh,‡

and Young Whan Cho†

†High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea‡Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea

*S Supporting Information

ABSTRACT: This paper investigates dehydrogenation reaction behavior of theLiBH4−MgH2 composite at 450 °C under various hydrogen and argon back-pressure conditions. While the individual decompositions of LiBH4 and MgH2simultaneously occur under 0.1 MPa H2, the dehydrogenation of MgH2 into Mgfirst takes place and subsequent reaction between LiBH4 and Mg into LiH andMgB2 after an incubation period under 0.5 MPa H2. Under 1 MPa H2, enhanceddehydrogenation kinetics for the same reaction pathway as that under 0.5 MPa H2is obtained without the incubation period. However, the dehydrogenation reactionis significantly suppressed under 2 MPa H2. The formation of Li2B12H12 as anintermediate product during dehydrogenation seems to be responsible for theincubation period. The degradation in hydrogen capacity during hydrogen sorptioncycles is not prevented with the dehydrogenation under 1 MPa H2, whicheffectively suppresses the formation of Li2B12H12. The overall dehydrogenationbehavior under argon pressure conditions is similar to that at hydrogen pressureconditions, except that under 2 MPa Ar.

■ INTRODUCTIONLithium borohydride (LiBH4) has received much attention as apromising solid-state hydrogen storage material owing to itshigh gravimetric (18.5 wt % H2) and volumetric (121 kg H2/m3) hydrogen storage capacities.1 However, its high reactiontemperature and slow kinetics for dehydrogenation should beimproved to be applied for on-board application.2,3 To improvethese drawbacks, extensive efforts have been carried out in thepast decade, and the concept of reactive hydride composite (ordestabilization) was devised as one of the efforts.4,5 In theconcept, the enthalpy change of hydrogen sorption reactions iseffectively reduced compared with pure LiBH4 owing to theformation of thermodynamically stable metal borides instead offree boron during dehydrogenation. Among the reactivehydride composites, the LiBH4−MgH2 composite was firstproposed and has been most investigated so far due to its hightheoretical hydrogen storage capacity and relative low cost ofMgH2.

6 The composite is expected to release 11.4 wt % H2during dehydrogenation through the following reaction:

+ → + +2LiBH MgH 2LiH MgB 4H4 2 2 2 (1)

Upon the dehydrogenation of the LiBH4−MgH2 composite,it has been reported that the application of hydrogen back-pressure plays an important role in the reaction pathway andthe formation of MgB2. Since Vajo et al.

4first found that the

application of 0.5 MPa of hydrogen enhances the formation ofMgB2 during the dehydrogenation of the composite, Nakagawaet al.,7 Bösenberg et al.,8 Pinkerton et al.,9 and Yan et al.10

investigated the dependency of the reaction pathways onhydrogen back-pressures. They found out that the applicationof hydrogen back-pressure effectively suppresses the individualdecomposition of LiBH4 prior to a mutual dehydrogenationreaction between LiBH4 and MgH2, and consequently thesuppression promotes the dehydrogenation through reaction 1.Despite the positive role of hydrogen back-pressures in

dehydrogenation reaction of the LiBH4−MgH2 composite, ithas been found that the hydrogen desorption in the compositeprogresses in two steps under 0.5−2.0 MPa H2.

9,11,12 Duringthe dehydrogenation reaction, MgH2 is first decomposed intoMg releasing ∼3 wt % H2, and then LiBH4 reacts with Mg intoLiH and MgB2 releasing ∼6 wt % H2. Moreover, obviouskinetic retardation has been found between the twodehydrogenation reactions exhibiting an incubation period aslong as 12 h.Bösenberg et al.12 claimed that the kinetic retardation of the

second-step dehydrogenation reaction in the composite isattributed to a lack of heterogeneous nucleation sites for theformation of MgB2. They showed that the incubation for theformation of MgB2 can be effectively reduced by the addition ofMgB2 or transition metal boride, which is believed to provideheterogeneous nucleation sites of MgB2 with a small latticemisfit. Also, there have been attempts to overcome the kinetic

Received: December 12, 2014Revised: April 17, 2015Published: April 17, 2015

Article

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© 2015 American Chemical Society 9714 DOI: 10.1021/jp5123757J. Phys. Chem. C 2015, 119, 9714−9720

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retardation of the composite using various additives.13−19

However, the one-step dehydrogenation according to reaction1 has not been achieved, despite the kinetic enhancement inthese attempts. On the other hand, Yan et al.10 suggested thatthe kinetic retardation is associated with the unexpectedformation of Li2B12H12 as an intermediate phase during thedehydrogenation of the composite, which can block the contactbetween LiBH4 and Mg. They found that the formation ofLi2B12H12 is effectively suppressed under hydrogen back-pressure as high as 2 MPa. However, the kinetic enhancementby the suppression of the Li2B12H12 formation was notconfirmed in their study. In spite of extensive researchconcerning the dehydrogenation reaction pathway of thecomposite, there has been very limited studies reporting thereversibility of the composite without a catalytic additive due toits poor dehydrogenation and rehydrogenation reactionkinetics.19,20

In this study, we address the influence of hydrogen back-pressure on the dehydrogenation reaction pathway of theLiBH4−MgH2 composite with the purpose of confirming thepossibility of the one-step dehydrogenation reaction as well asthe kinetic enhancement under hydrogen back-pressures. Also,the dehydrogenation behavior at various argon back-pressuresis analyzed in comparison with the hydrogen back-pressureeffect. Additionally, the hydrogen sorption cycle performance ofthe composite is investigated up to 20 cycles.

■ EXPERIMENTAL PROCEDURELithium borohydride (LiBH4, Acros, 95% purity) andmagnesium hydride (MgH2, Alfa Aesar, 98% purity) wereused as raw materials without any purification. Three grams ofthe LiBH4−MgH2 mixture with a 2:1 molar ratio was ball-milled using a Retsch PM200 planetary ball mill with 650 rpmfor 12 h. Thirteen 12.7 mm diameter and twenty-four 7.9 mmdiameter Cr-steel balls were employed together with a 140 mLhardened steel bowl, sealed in argon atmosphere with a lidhaving a Viton O-ring. The ball-to-powder weight ratio wasapproximately 50:1.A 0.3 g sample of the LiBH4−MgH2 composite was

dehydrogenated at 450 °C using a Sievert-type volumetricapparatus with a 110 mL reactor. Dehydrogenations werecarried out at various hydrogen or argon back-pressureconditions ranging from static vacuum to 2 MPa. The heatingrate was 30 °C/min in all the cases, and the pressure in thereactor was monitored during dehydrogenation reaction.Hydrogen sorption cycle test was carried out at 450 °C for 3h under 1 and 8 MPa hydrogen for dehydrogenation andhydrogenation, respectively. For cycle test, a compressed pelletsample (5 mm in diameter) was used in order to avoid powderscattering and to enhance heat transfer.21

XRD measurement was performed for dehydrogenated andrehydrogenated samples using a Bruker D8 Advance X-raydiffractometer with Cu Kα radiation. Also, Raman spectroscopywas carried out using a Renishaw inVia Raman microscope witha 532 nm YAG laser. For Raman spectroscopy analysis, thereference peak positions of LiBH4,

8,22−24 Li2B12H12,10,23−25 and

B25−27 were taken from the literature. In the case of MgB2, thespectrum of commercial powder (Sigma-Aldrich, ≥99% purity)was used. To prevent the samples from air exposure during theXRD and Raman spectroscopy measurements, borosilicatecapillary tubes and specially designed alumina holders with acover glass were employed, respectively. To understanddehydrogenation reaction pathway, some samples were

quenched using liquid nitrogen during dehydrogenation beforeXRD and Raman spectroscopy measurements. The wholesample handlings were carried out inside an argon-filledglovebox (mBraun, UniLab), where oxygen and water vaporlevels were kept below 0.1 ppm.

■ RESULTS AND DISCUSSIONFigure 1 shows the dehydrogenation profiles of the LiBH4−MgH2 composite at 450 °C under static vacuum and various

hydrogen pressure conditions. Under static vacuum, thecomposite consistently releases about 8 wt % hydrogen within2 h (Figure 1a). However, additional hydrogen release is rarelyachieved in subsequent 5 h, and the amount of releasedhydrogen for 7 h fails to reach the theoretical hydrogen capacityof the composite (11.4 wt %). Under 0.1 MPa hydrogen, thefinal amount of released hydrogen for 7 h under 0.1 MPahydrogen increases to about 10 wt %, although thedehydrogenation rate is very slightly retarded at the initialstage (Figure 1b). The amount of released hydrogen continuesto increase under 0.3 and 0.5 MPa H2, although the amountunder 0.5 MPa is slightly larger than that under 0.3 MPa(Figure 1c,d). However, the dehydrogenation behavior under0.3 and 0.5 MPa is quite different from that under staticvacuum and 0.1 MPa, exhibiting a distinct two-stage reactionbehavior. In previous study,11 it has been believed that the firstand second reactions are the dehydrogenation of MgH2 intoMg (∼2.8 wt % H2) and the reaction between LiBH4 and Mginto LiH and MgB2 (∼7.2 wt % H2), respectively. Between thetwo reactions, there is an incubation period, which lasts forabout 30 min. Interestingly, the incubation period clarifying thetwo-step dehydrogenation reaction is eliminated with enhanceddehydrogenation kinetics, when 1 MPa of hydrogen is applied(Figure 1e). In addition, the composite releases the mostamount of hydrogen (11.3 wt %) under 1 MPa, which is closeto the theoretical hydrogen capacity. On the other hand, thedehydrogenation reaction is significantly suppressed under 2MPa, releasing only 5.0 wt % hydrogen for 7 h (Figure 1f).XRD patterns of the products dehydrogenated under various

hydrogen pressure conditions are presented in Figure 2. Understatic vacuum and 0.1 MPa H2, Mg and LiH are observed as

Figure 1. Dehydrogenation profiles of the LiBH4−MgH2 compositeunder (a) static vacuum and (b) 0.1, (c) 0.3, (d) 0.5, (e) 1.0, and (f)2.0 MPa of hydrogen. The inset exhibits the reaction rate at the earlystage for 2 h. The symbols on each profile are not real data points buta guide to improve readability.

The Journal of Physical Chemistry C Article

DOI: 10.1021/jp5123757J. Phys. Chem. C 2015, 119, 9714−9720

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main dehydrogenation products together with a small amountof MgB2 (Figure 2a,b). The formation of Mg instead of MgB2implies that the individual decomposition of MgH2 and LiBH4occurs during the dehydrogenation, although the presence of Bis not confirmed in the patterns. On the other hand, MgB2 ismainly found above 0.3 MPa, which is assumed to be formedthrough reaction 1. Under 2 MPa H2, LiBH4 still remainsdistinctly after the dehydrogenation for 7 h, although a smallamount of MgB2 and LiH is observed together with the sharpMg peaks (Figure 2f). The formation of a large amount of Mgand the presence of LiBH4 indicate that the reaction betweenMg and LiBH4 is limited by a low thermodynamic driving forcebecause hydrogen pressure in the reactor approaches theequilibrium pressure of the overall dehydrogenation reaction ofthe composite at 450 °C, which is known to be 2.1 MPa.4 Theresults in Figures 1 and 2 indicate that hydrogen back-pressureabove 0.3 MPa is essential for the dehydrogenation reaction ofthe composite into LiH and MgB2 at 450 °C. Moreover, theamount of formed MgB2 increases with the increase in theamount of dehydrogenated hydrogen, as hydrogen back-pressure increases up to 1 MPa. The dehydrogenation behaviorat 400 °C is similar to that at 450 °C (see Figures S1 and S2).The disappearance of the incubation period under higherhydrogen back-pressure is also observed for the dehydrogen-ation at 400 °C, although the kinetic enhancement is notobvious (Figures S1). Also, the overall dehydrogenationkinetics is significantly suppressed under 0.9 MPa H2,approaching the equilibrium pressure of the composite (∼1.3MPa).4

To analyze the change of reaction pathways of the compositedepending on hydrogen back-pressure, XRD and Ramanspectroscopy measurements of the samples obtained afterbeing quenched during dehydrogenation under 0.1, 0.5, and 1MPa H2 were performed. For the dehydrogenation at 0.1 MPa,small LiH and large Mg peaks are detected as dehydrogenationproducts together with significantly reduced MgH2 peaks and asmall broad MgO peak at point 1 in Figure 3a, which ispresumed to form by oxygen source contained in relatively lowpurity LiBH4 (∼95%). The formation of LiH indicates theindividual decomposition of LiBH4 into LiH and B.Accordingly, the result indicates that both MgH2 and LiBH4are individually decomposed in the early stage of thedehydrogenation. As shown Figure 3b, the result of Raman

spectroscopy shows that Li2B12H12 and B, which are notdetected by the XRD measurement due to their amorphousfeature, rapidly form through the individual decompositionfrom LiBH4 in the early stage, as observed for the LiBH4−YH3composite.28 At point 2 in Figure 3a, it is confirmed thatadditional hydrogen desorption is attributed mainly to theindividual decomposition of LiBH4, considering the reducedpeak of LiBH4 compared with that at the point 1. MgB2, whichis observed at the final stage, seems to form by the reactionbetween Mg and undecomposed LiBH4 during the individualdecompositions. MgB2 might form by a solid state reactionbetween Mg and B, which is decomposed from the individualdecomposition of LiBH4. However, it was reported that thereaction occurs above 600 °C due to strong B−B bonding ofintraicosahedron of α-B.26,29,30 Moreover, there is a lowpossibility that MgB2 forms by a direct reaction between Mgand Li2B12H12, considering that the crystal structure ofLi2B12H12 is similar to that of α-B.

29

Figure 4 elucidates the reaction pathway of the compositedehydrogenated under 0.5 MPa H2. At point 1 in Figure 4a,which is close to the end of the first reaction in the two-stepreaction, large Mg peaks in the XRD pattern without thepresence of LiH and MgB2 shows that the only individualdecomposition of MgH2 occurs. LiBH4 is not likely to

Figure 2. XRD patterns of the reaction products of the LiBH4−MgH2composite dehydrogenated at (a) static vacuum and (b) 0.1, (c) 0.3,(d) 0.5, (e) 1.0, and (f) 2.0 MPa of hydrogen.

Figure 3. (a) XRD patterns and (b) Raman spectrum of the LiBH4−MgH2 composite during dehydrogenation at 450 °C under 0.1 MPa ofhydrogen.



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individually decompose into LiH and B because 0.5 MPa H2 isslightly higher than the equilibrium hydrogen pressure of itsdecomposition reaction at 450 °C (∼0.46 MPa).3 It is foundthat the MgB2 peaks start to form and grow after the incubationperiod (points 2 and 3), indicating that LiBH4 reacts with Mg.The Raman spectra before and after the incubation period(points 1 and 2) during the dehydrogenation are compared inFigure 4b. The main difference between the two spectra is thatthe formation of Li2B12H12 is evident after the incubationperiod. This indicates that a small amount of LiBH4decomposes into Li2B12H12 during the incubation period,which is consistent with the slight increase of released hydrogenduring the incubation period.The dehydrogenation reaction pathway under 1 MPa H2

slightly differs from that under 0.5 MPa. As shown in Figure 5a,the composite releases hydrogen up to 11.3 wt % under 1 MPaH2 without a distinct incubation period. It seems that the fulldecomposition of MgH2 is almost achieved at point 1, and thereaction between LiBH4 and Mg is subsequently under progress

at point 2. However, it is noteworthy that a little peak of MgB2is clearly detected at point 1, which is quite early comparedwith the pattern of point 1 under 0.5 MPa (Figure 4a). Theformation of MgB2 at point 1 indicates that LiBH4 reacts withMg without an incubation period as soon as MgH2 decomposesinto Mg. There is no evidence that LiBH4 and MgH2 directlyreact into LiH and MgB2. As shown in Figure 5b, the Ramanspectrum at point 1 under 1 MPa H2 exhibits a difference inthat the formation of Li2B12H12 is not observed, compared withthat at 0.5 MPa. This implies that the formation of Li2B12H12might be responsible for the incubation period observed duringthe dehydrogenation under 0.3 and 0.5 MPa H2 (Figure 1),which is consistent with the findings observed for the LiBH4−YH3 composite.

28,31 Therefore, the enhancement in dehydro-genation kinetics under 1 MPa H2 observed in Figure 1 seemsto be associated with the suppression of the Li2B12H12formation.Figure 6 presents the dehydrogenation profiles of the

LiBH4−MgH2 composite at various argon back-pressureconditions. The overall dehydrogenation behaviors under Arback-pressure is similar to that under hydrogen back-pressure,

Figure 4. (a) XRD patterns and (b) Raman spectra of the LiBH4−MgH2 composite during dehydrogenation reaction at 450 °C under0.5 MPa of hydrogen.

Figure 5. (a) XRD patterns and (b) Raman spectrum of 2LiBH4−MgH2 composite during dehydrogenation reaction at 450 °C under1.0 MPa of hydrogen.



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exhibiting the two-step reaction feature under 0.3 and 0.5 MPaAr and the enhanced reaction kinetics with the increase in theamount of released hydrogen under 1 MPa Ar, although theamount of released hydrogen under Ar back-pressure is slightlylarger than that under hydrogen back-pressure at the samepressure. One big difference from the dehydrogenation profilesunder hydrogen back-pressure conditions is that the dehydro-genation reaction under 2 MPa Ar is not much suppressedreleasing more than 9 wt % hydrogen compared with thatunder 2 MPa H2, although the kinetics is retarded comparedwith that under 1 MPa Ar. This is presumably because theincrease in Ar back-pressure does not reduce on athermodynamic driving force for the dehydrogenation reaction,in contrast to hydrogen back-pressure. The XRD and Ramanmeasurements of the samples dehydrogenated under 0.1 and 1MPa Ar show similar results to those under hydrogen back-pressure, although they are not presented here.The suppression of the Li2B12H12 formation under high

hydrogen back-pressure can be explained by thermodynamics.According to Ohba et al.,29 the equilibrium hydrogen pressureof the decomposition reaction of LiBH4 into LiH and Li2B12H12at 450 °C is estimated to 0.9 MPa. Therefore, it is expected thatthe application of higher than 1 MPa H2 will suppress theformation of Li2B12H12. However, only with thermodynamics, itcannot be explain why high Ar back-pressure is also effective insuppressing the formation of Li2B12H12. In our previous studyof the LiBH4−YH3 composite,

28,31 we proposed a hypothesisthat gas back-pressure kinetically suppresses the release ofdiborane (B2H6) gas during the dehydrogenation of LiBH4,

32

eventually forming Li2B12H12 by reacting with undecomposedLiBH4.

33 The similar mechanism seems to be true for theLiBH4−MgH2 composite.The amount of dehydrogenated hydrogen in the composite

at 450 °C during 21 cycles is presented in Figure 7. Up to the15th cycle, the hydrogen capacity gradually decreases from 10.8to 6.2 wt %. On the other hand, the capacity tends to bemaintained between 6 and 7 wt % after the 16th cycle. Figure 8exhibits the dehydrogenation behaviors of the composite atdifferent cycles. It is found the dehydrogenation rate graduallydecreases with increasing cycle number. Moreover, thecomposite is not fully dehydrogenated, being interrupted in 3h before the saturation, after the fifth cycle. The dehydrogen-

ation behaviors at the 17th and 21st cycles are similar,indicating that the significant degradation of the dehydrogen-ation kinetics is not in progress after the 17th cycle. Thus, theapplication of relatively high hydrogen back-pressure duringdehydrogenation is unlikely to play a positive role in enhancingthe cycle performance of the composite. There have been acouple of studies on the cycle performance of the composite.Xiao et al.17 and Jepsen et al.21 performed more than 15 cyclesof the composite with 5 mol % NbF5 and TiCl3, respectively, asa catalytic additive. A significant decrease in hydrogen capacitydid not occur during their cycle tests, maintaining more than 8wt %, although a slight degradation was observed in the resultsof Jepsen et al.21 This is in contrast with the significantdegradation tendency observed in the early stage of our cycleresult. Therefore, there seems to be a positive role of a catalyticadditive in improving the cycle property of the composite. Oneof the plausible explanations might be that these catalyticadditives retard microstructural coarsening in the compositeduring the cycles. In addition, the cycle temperature in ourstudy (450 °C) higher than that in the works of Xiao et al.17

and Jepsen et al.21 (350−400 °C) is vulnerable to micro-structural coarsening. Although Shao et al.20 and Mao et al.19

also investigated a few hydrogen sorption cycles of thecomposite at 400 °C without a catalytic additive, however,the number of cycles is too small to understand the overall

Figure 6. Dehydrogenation profiles of the LiBH4−MgH2 compositeunder (a) 0.1, (b) 0.3, (c) 0.5, (d) 1.0, and (e) 2.0 MPa of argon. Theinset exhibits the reaction rate at the early stage for 2 h. The symbolson each profile are not real data points but a guide to improvereadability.

Figure 7. Amount of hydrogen released from the LiBH4−MgH2composite pressed during the cycle test.

Figure 8. Dehydrogenation profiles of the LiBH4−MgH2 compositeduring the cycle test.



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cycle tendency. Hansen et al.34,35 investigated the cycleperformance of the LiBH4−MgH2−Al and LiBH4−Al compo-sites with dehydrogenation pressure of vacuum and 0.5 MPaH2. Although the application of hydrogen back-pressure waseffective in suppressing the formation of Li2B12H12, the initialdegradation was observed during the cycles at both pressureconditions.Figure 9 compares the XRD patterns of the samples after the

21st dehydrogenation and rehydrogenation. After the dehydro-

genation, a significant amount of LiBH4 and Mg is detectedtogether with LiH and MgB2 (Figure 9a), which indicates thereaction between LiBH4 and Mg is uncompleted. As shown inFigure 9b, Mg still remains after the rehydrogenation. Thedegradation in cycle performance of the composite seems toreflect these incomplete dehydrogenation and rehydrogenationreactions. Longer diffusion distance due to gain coarseningduring cycles is presumed to be a main reason for theuncompleted reactions according to microstructural analysis ofthe composite.36 Consequently, the restriction of graincoarsening during cycles seems to be crucial to achieve highcapacity hydrogen storage in the composite.

■ CONCLUSIONSThe dehydrogenation reaction pathway and kinetics of theLiBH4−MgH2 composite without a catalytic additive at 450 °Chave been investigated under various hydrogen and argon back-pressure conditions. LiBH4 and MgH2 individually decomposein the composite under 0.1 MPa H2, releasing about 10 wt %H2 for 7 h. Under 0.5 MPa H2, MgH2 is first dehydrogenatedand subsequently LiBH4 reacts with Mg into LiH and MgB2after an incubation period, which is as long as half an hour.Although the similar reaction pathway seems to be effectiveunder 1 MPa H2, the incubation period disappears withenhanced dehydrogenation kinetics compared with that atlower hydrogen pressure conditions. The composite releasesabout 11 wt % H2 under 0.5 and 1 MPa H2 for 2 h, which isclose to the theoretical hydrogen capacity of the composite.The dehydrogenation reaction is significantly suppressed under2 MPa H2, approaching thermodynamic equilibrium pressure.Raman spectroscopy implies that the formation of Li2B12H12 asan intermediate product during dehydrogenation is responsiblefor the incubation period. Hydrogen sorption cycle of thecomposite has been performed at 450 °C under 1 and 8 MPa

H2 for dehydrogenation and hydrogenation, respectively.Although the dehydrogenation under 1 MPa H2 effectivelysuppresses the formation of Li2B12H12, it fails to prevent thedegradation in hydrogen capacity during hydrogen sorptioncycles. The dehydrogenation behavior at argon pressureconditions is similar to that at hydrogen pressure conditions,except that the dehydrogenation under 2 MPa Ar is not muchsuppressed.

■ ASSOCIATED CONTENT*S Supporting InformationFigure S1 presenting dehydrogenation profiles of the LiBH4−MgH2 composite at 400 °C under 0.3, 0.6, and 0.9 MPa H2;Figure S2 showing XRD patterns of the reaction products ofthe LiBH4−MgH2 composite dehydrogenated at 400 °C under0.3, 0.6, and 0.9 MPa. This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected]; Ph +82-2-958-6760 (J.-H.S.).NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis study has been supported by the KIST InstitutionalProgram (Project No. 2E25322).

■ REFERENCES(1) Züttel, A.; Wenger, P.; Rentsch, S.; Sudan, P.; Mauron, P.;Emmenegger, C. LiBH4 - a New Hydrogen Storage Material. J. PowerSources 2003, 118, 1−7.(2) Orimo, S.; Nakamori, Y.; Kitahara, G.; Miwa, K.; Ohba, N.;Towata, S.; Züttel, A. Dehydriding and Rehydriding Reactions ofLiBH4. J. Alloys Compd. 2005, 404−406, 427−430.(3) Mauron, P.; Buchter, F.; Friedrichs, O.; Remhof, A.; Bielmann,M.; Zwicky, C. N.; Züttel, A. Stability and Reversibility of LiBH4. J.Phys. Chem. B 2008, 112, 906−910.(4) Vajo, J. J.; Skeith, S. L.; Mertens, F. Reversible Storage ofHydrogen in Destabilized LiBH4. J. Phys. Chem. B 2005, 109, 3719−3722.(5) Barkhordarian, G.; Klassen, T.; Dornheim, M.; Bormann, R.Unexpected Kinetic Effect of MgB2 in Reactive Hydride CompositesContaining Complex Borohydrides. J. Alloys Compd. 2007, 440, L18−L21.(6) Bogdanovic,́ B.; Ritter, A.; Spliethoff, B. Active MgH2-MgSystems for Reversible Chemical Energy Storage. Angew. Chem., Int.Ed. Engl. 1990, 29, 223−234.(7) Nakagawa, T.; Ichikawa, T.; Hanada, N.; Kojima, Y.; Fujii, H.Thermal Analysis on the Li−Mg−B−H Systems. J. Alloys Compd.2007, 446−447, 306−309.(8) Bösenberg, U.; Ravnsbaek, D. B.; Hagemann, H.; D’Anna, V.;Minella, C. B.; Pistidda, C.; Beek, W. V.; Jensen, T. R.; Bormann, R.;Dornheim, M. Pressure and Temperature Influence on the DesorptionPathway of the LiBH4-MgH2 Composite System. J. Phys. Chem. C2010, 114, 15212−15217.(9) Pinkerton, F. E.; Meyer, M. S.; Meisner, G. P.; Balogh, M. P.;Vajo, J. J. Phase Boundaries and Reversibility of LiBH4/MgH2Hydrogen Storage Material. J. Phys. Chem. C 2007, 111, 12881−12885.(10) Yan, Y.; Li, H.-W.; Maekawa, H.; Miwa, K.; Towata, S.; Orimo,S. Formation of Intermediate Compound Li2B12H12 during theDehydrogenation Process of the LiBH4−MgH2 System. J. Phys.Chem. C 2011, 115, 19419−19423.(11) Bösenberg, U.; Doppiu, S.; Mosegaard, L.; Barkhordarian, G.;Eigen, N.; Borgschulte, A.; Jensen, T. R.; Cerenius, Y.; Gutfleisch, O.;

Figure 9. XRD patterns of the LiBH4−MgH2 composite after the 21st(a) dehydrogenation and (b) rehydrogenation.



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http://pubs.acs.orgmailto:[email protected]://dx.doi.org/10.1021/jp5123757

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dehydrogenation reaction pathway of the libh mgh composite...

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