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Acta Materialia 55 (2007) 3951–3958

Hydrogen sorption properties of MgH2–LiBH4 composites

Ulrike Bosenberg a,*, Stefania Doppiu b, Lene Mosegaard c, Gagik Barkhordarian a,Nico Eigen a, Andreas Borgschulte a,1, Torben R. Jensen c, Yngve Cerenius d,

Oliver Gutfleisch b, Thomas Klassen e, Martin Dornheim a, Rudiger Bormann a

a Institute for Materials Research, GKSS Research Centre Geesthacht, D-21502 Geesthacht, Germanyb IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany

c Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, University of Aarhus, DK-8000, Denmarkd MAX-lab, Lund University, S-22100 Lund, Sweden

e Institute of Materials Technology, Helmut-Schmidt-University, D-22043 Hamburg, Germany

Received 21 December 2006; received in revised form 5 March 2007; accepted 10 March 2007Available online 4 May 2007

Abstract

A detailed analysis of the reaction mechanism of the reactive hydride composite (RHC) MgH2 + 2LiBH4 M MgB2 + 2LiH + 4H2 wasperformed using high-pressure differential scanning calorimetry (HP-DSC) measurements and in situ synchrotron powder X-ray diffrac-tion (XRD) measurements along with kinetic investigations using a Sievert-type apparatus. For the desorption the following two-stepreaction has been observed: MgH2 + 2LiBH4 M Mg + 2LiBH4 + H2 M MgB2 + 2LiH + 4H2. However, this reaction is kineticallyrestricted and proceeds only at elevated temperatures. In contrast to the desorption reaction, LiBH4 and MgH2 are found to form simul-taneously under fairly moderate conditions of 50 bar hydrogen pressure in the temperature range of 250–300�C. As found in pure lightmetal hydrides, significant improvement of sorption kinetics is possible if suitable additives are used.� 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Synchrotron radiation; Phase transformation; Hydrogen storage; Differential scanning calorimetry (DSC)

1. Introduction

Light metal hydrides are the favoured materials for solidhydrogen storage in mobile applications. The complexhydride LiBH4 is of special interest as it is among the mate-rials with the highest theoretical gravimetric hydrogen stor-age density, amounting to 18.5 wt.%. LiBH4 is known todecompose only at temperatures above 400 �C to LiH andB, releasing 13.5 wt.% hydrogen via this reaction. The cor-responding heat of decomposition is 68.6 kJ mol�1 H2 [1].However, despite the high driving force, absorption is notpossible under moderate conditions, i.e. hydrogen pressures

1359-6454/$30.00 � 2007 Acta Materialia Inc. Published by Elsevier Ltd. All

doi:10.1016/j.actamat.2007.03.010

* Corresponding author. Tel.: +49 4152 87 2565; fax: +49 4152 87-2625.E-mail address: ulrike.boesenberg@gkss.de (U. Bosenberg).

1 Present address: EMPA-Materials Science & Technology, CH-8600Dubendorf, Switzerland.

below 100 bar and temperatures below 350 �C. Thus, thereversibility of hydrogenation of LiBH4 is rather limited.

The thermodynamic and kinetic properties of this com-plex hydride have been subject to multiple experimentaland theoretical investigations to understand the strongbonding properties in the complex [2–7]. By use of suitableadditives such as SiO2, the onset temperature of the desorp-tion reaction can be lowered to about 200 �C [4]; however,from a technical point of view, the reaction is still notreversible as no rehydrogenation is possible under moder-ate conditions due to the formation of lithium silicates[8]. Orimo et al. [5] were able to reduce the desorption tem-perature by approximately 30 K by substitution (or mix-ing) of LiBH4 with Mg. In that study, Orimo et al. firstsucceeded in hydriding the mixture of LiH and B under35 MPa and 600 �C for 12 h. However, the effect of Mgon the absorption reaction was not investigated.

rights reserved.

Fig. 1. Schematic diagram for the in situ XRD setup.

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Independently, Vajo et al. [9] and Barkhordarian et al.[10] found a new reaction mechanism for composites con-sisting of MgH2 and borohydrides. The reaction principleof the so-called Reactive Hydride Composites (RHCs)[10] is explained for the system of MgH2 + 2LiBH4 as fol-lows. During the endothermic dehydrogenation reactionthe exothermic formation of MgB2 proceeds and the totalreaction enthalpy is thereby lowered. Absorption pro-ceeds again from MgB2 and LiH. Experimental investiga-tions proving the reaction principle have been performedby Vajo et al. [9] and Barkhordarian et al. [10]. A simplecalculation based on the published thermodynamic data[11], yields a reaction enthalpy of �46 kJ mol�1 H2 forthe system. This gives an estimate for the equilibriumtemperature of 169 �C at 1 bar hydrogen pressure. PCTmeasurements performed by Vajo et al. gave a reactionenthalpy of dehydrogenation of �40.5 kJ mol�1 H2,which is in reasonably good agreement with the calcula-tion above. First-principles calculations for this systemwere performed by Alapati et al. [12]. However, theirresults show considerable quantitative discrepancies whencompared with tabulated [11] or experimental data [9]. Inspite of the lowered value of the reaction enthalpy in theMgH2–LiBH4 RHC, rehydrogenation is possible at pres-sures of 50 bar hydrogen and temperatures below300 �C. These conditions are substantially more favour-able than those of the single-phase LiBH4 system andcan be considered as a breakthrough in utilizing borohy-drides for reversible hydrogen storage.

In this study, the reaction mechanisms during absorp-tion and desorption in the MgH2–LiBH4 system and theinfluence of different additives on the reaction kinetics havebeen experimentally investigated by using high-pressuredifferential scanning calorimetry (HP-DSC), volumetricmeasurements in a Sievert-type apparatus and in situ syn-chrotron X-ray diffraction (XRD). The combination ofthese different techniques gives a first insight into the com-plex reaction mechanism of the system.

2. Experimental

The initial MgH2 powder (95% purity; residue magne-sium) was purchased from Goldschmidt GmbH. LiBH4

(95% purity), LiH (99.4% purity), MgB2 and the additivestitanium isopropoxide (99.995% purity) and VCl3 (99%purity) were produced by Alfa Aesar. The added SiO2

(99% purity) was purchased from Sigma Aldrich.For the investigations of the desorption reaction mech-

anism, MgH2 was premilled in a Spex 8000 M Mixer Millfor 5 h. Afterwards, LiBH4 and the dopants were addedand milled for additional 5 h. This ensured identical start-ing conditions for all materials. For the observation of theabsorption reaction, unless investigated in a cycling exper-iment, MgB2–LiH composites were milled for 5 h. The ballto powder ratio was in both cases 4:1. One batch wasmilled with a ball to powder ratio of 10:1 for 20 h in aFritsch 5 planetary ball mill (PBM). All handling and

milling was performed under a continuously purified argonatmosphere. Average crystallite sizes of 10–15 nm weredetermined from XRD measurements by applying theScherrer formula.

Thermodynamic properties were investigated by HP-DSC (Netzsch DSC 204 HP Phoenix, principle pressurerange 1–150 bar) under a hydrogen atmosphere with theentire apparatus placed in a dedicated glovebox under anargon atmosphere to enable the handling of the oxida-tion-sensitive materials. The HP-DSC apparatus works indynamic mode, i.e. keeping the hydrogen pressure constantfor the entire temperature range. Absorption and desorp-tion experiments were performed at 50 and 3 bar of hydro-gen under a continuous hydrogen flow of 200 and20 ml min�1, respectively. The heating rate was 5 K min�1

in both cases.For all desorption reactions a residual hydrogen pres-

sure of 1–5 bar was applied as this facilitates the formationof MgB2 [9].

In addition, DSC and thermogravimetric (TG) mea-surements were performed simultaneously in a NetzschSTA 409 C in an argon atmosphere with the entire appa-ratus placed in a dedicated glovebox under an argonatmosphere. The heating rate was 5 K min�1 and a con-tinuous argon flow of 200 ml min�1 was applied. The con-centration of H2 and Ar in the exhaust gas wascontinuously recorded by a Hiden HPR-20 QIC massspectrometer.

In situ XRD measurements were performed at the MAXII Synchrotron in Lund, Sweden, at Beamline I711 [13].The measurement time was chosen to be 30 or 60 s for fixedexposure time mode and varied between 50 and 100 s fordose mode (collecting the same dose of intensity for all

U. Bosenberg et al. / Acta Materialia 55 (2007) 3951–3958 3953

scans). The wavelength was either 1.001 or 1.1072 A. Forcomparison, all XRD data are referring to 4psinh/k, relat-ing the reflections to the normalized reciprocal lattice vec-tors. The diffracted intensity was measured by a Mar165CCD detector.

The hydride powder was placed into airtight sapphirecapillaries in a glovebox under an argon atmosphere. Dur-ing the experiments the hydrogen pressure was kept con-stant. The sample was heated by a tungsten wire wrappedaround the capillary and the temperature was controlledwith an external PID regulator and a thermocoupleinserted into the powder-bed. A schematic diagram of theexperimental setup is shown in Fig. 1. The applied heatingrates and pressures are noted in the correspondingexperiments.

3. Results and discussion

3.1. Investigation of the desorption of MgH2–LiBH4

composites

In the following, the desorption of MgH2–LiBH4 com-posites is analyzed using DSC, in situ XRD and a Sie-vert-type apparatus.

Fig. 2 displays different simultaneous signals for thedesorption of uncatalysed MgH2–LiBH4 composite: theDSC signal, the hydrogen signal and the signal ofthe integrated thermogravimetric analyser (TGA) plottedover the temperature. This measurement was performedunder an argon atmosphere and reveals four distinct ther-mal events, denoted A, B, C and D. However, a signifi-cant release of hydrogen and a significant loss of weightis only observed for the reactions that correspond to

Fig. 2. Simultaneous DSC, TG and mass spectrometry of the desorption reactemperature of 465 �C; argon flow of 200 ml min�1.

peaks C and D, indicating that the desorption has atwo-step character.

In order to investigate the influence of the starting mate-rial, further HP-DSC experiments were performed. As for-mation of MgB2 is only ensured with a residual hydrogenpressure during the desorption reaction, the measurementswere performed under 3 bar of hydrogen (see Fig. 3). Scan1 shows the desorption of uncatalysed material, which wasmilled in the hydrided state. Scan 2 shows the desorptionreaction of uncatalysed material that was milled in thedehydrogenated state and had been hydrogenated priorto this measurement. Scan 3 shows the DSC measurementof the first desorption of catalysed material, which wasmilled in the hydrided state using titanium isopropoxideas an additive. As can be seen from scans 2 and 3, thecycling and doping significantly influenced the kinetics ofthe desorption reaction, which is indicated by a shift ofthe desorption peaks C and D towards lower temperatures.This behaviour might be due to activation during theabsorption reaction, as has been observed for many otherhydrides [14]. Suitable additives such as titanium isoprop-oxide can enhance the desorption reaction significantlywithout such a previously performed activation process.However, it is noted that the desorption reaction stilloccurs in two steps.

To further characterize the desorption reaction, in situXRD measurements were performed. Fig. 4 shows theresults of the desorption reaction under a hydrogen pressureof �5 bar, starting from milled hydrides and doped withtitanium isopropoxide as additive. As indicated by DSC, afirst phase transformation occurs at around 120 �C. TheXRD measurements reveal that this reaction correspondsto a phase transformation (indexed A) of LiBH4. The

tion of MgH2–2LiBH4 composite. Heating rate 5 K min�1 to a maximum

Fig. 3. HP-DSC of the desorption reaction of MgH2–2LiBH4 compositesafter milling (1), after initial hydrogenation (2) and after milling withadditive (3). Heating rate 5 K min�1; 20 ml H2 min�1; 3 bar H2.

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second peak, labeled B in Figs. 2 and 3, can be related to themelting of LiBH4 as the reflections of LiBH4 disappear at270 �C, the melting point of this hydride. The maximumintensities of the MgH2 reflections increase at temperaturesabove 250 �C due to recrystallization and grain coarsening.Under the applied conditions, desorption of MgH2 to Mgoccurs at about 390 �C. After formation of Mg, MgB2

forms at temperatures above 400 �C. At the same time,the Mg peak intensities decrease. Due to the liquid state

Fig. 4. In situ XRD measurement of the desorption reaction of MgH2–2LiBH4

with a heating rate of 5 K min�1 to 450 �C. A and B correspond to the polym

of LiBH4 at these temperatures, desorption of this hydridecan only be observed by the formation of LiH. Due to thelow intensity of the Bragg diffraction of LiH, the peaksappear to rise up later than the MgB2 peaks, although theformation should occur simultaneously.

It is interesting to note that in the HP-DSC measure-ments no distinct exothermic reaction corresponding tothe formation of MgB2 can be observed. However, MgB2

is clearly formed, as proven by the in situ XRDmeasurements.

During the desorption reaction, none of the predictedintermediate phases of LiBH4 [15–17] is observed, but thismay be due to the liquid state of the Li–B–H phases atthese elevated temperatures. We observe novel reflectionsduring the desorption reaction; an XRD scan of the sampleat 450 �C is shown in Fig. 5 as an example. However, asonly one clear peak arises without overlapping, the identi-fication of this novel peak is very uncertain and has there-fore not been performed.

The effect of different milling parameters and additiveson the desorption reaction was investigated in more detailin a Sievert-type apparatus. As additives, different oxides,borides, chlorides and metal-organic compounds on thebasis of light transition metals have been used. For clarity,in Fig. 6 only a few examples with 5 at.% of VCl3, titaniumisopropoxide and SiO2 are compared to the non-catalysedmaterial. All samples were heated up to 400 �C and thenkept under isothermal and isobaric conditions of approxi-mately 400 �C and 5 bar hydrogen. The two-step desorp-tion reaction is clearly visible. As observed in the XRDmeasurements, hydrogen is first released from the MgH2.

composite doped with 5 at.% titanium isopropoxide under 5 bar hydrogenorphic transformation and to the melting of LiBH4, respectively.

Fig. 5. XRD measurement of the desorption reaction at temperaturesT = 450 �C and residual hydrogen pressure of 5 bar.

U. Bosenberg et al. / Acta Materialia 55 (2007) 3951–3958 3955

Thereafter, hydrogen desorption nearly stops and thedesorption reaction of LiBH4 proceeds after a certain incu-bation period. Variation in milling parameters and theaddition of suitable additives have a significant effect onthe length of the incubation period as well as on the reac-tion kinetics of the second reaction step. The best kineticswere observed using titanium isopropoxide as an additive;the positive effect of this additive on the sorption kineticshas also been observed for other RHC systems [18–20].The differences in the hydrogen storage capacities corre-spond to the added weight of the additives.

Fig. 6. Kinetic measurements of the desorption reaction of MgH2–2LiBH4 comup to 400 �C. Desorption was performed under 5 bar hydrogen pressure. pm,

The incubation period may be caused by a nucleationrestriction, e.g. a large critical nucleus size. In addition,the low mobility of atoms and ions and the low reactivityof the complex hydride in combination with large diffusionpath lengths, due to large grains from improper millingparameters, can contribute to such an incubation period.In order to determine the rate-limiting step, microstruc-tural characterization is currently in progress to clarifyphase distribution, particle and crystallite sizes, and themorphology of the composite.

3.2. Absorption of hydrogen by the MgB2–LiH system

Fig. 7 shows the HP-DSC results of a full cycle of acatalysed LiBH4 + MgH2 reaction. In scan 1 the desorp-tion reactions of the catalysed material, milled in the hyd-rided state, are presented. Reactions A and B againcorrespond to the polymorphic transformation and meltingof LiBH4, and reactions C and D to the desorption ofMgH2 and LiBH4, respectively. Scan 2 presents the resultsof the subsequent absorption reaction at 50 bar hydrogenpressure, showing two exothermic reaction peaks. The sec-ond desorption of the sample indicates, by the occurrenceof peaks A and B, that LiBH4 has been formed duringabsorption. The desorption of LiBH4 (reaction D) isenhanced and proceeds at lower temperatures.

The two peaks of the absorption reaction suggest alsothat absorption takes place in two steps. The correspond-ing XRD measurements (see Fig. 8) performed at low tem-peratures show the formation of MgH2 at T = 265 �C.However, cooling this sample from T = 265 �C to room

posite with additive in a Sievert-type apparatus. The samples were heatedpre-milling; PBM, planetary ball mill; Spex, Spex Mixer Mill.

Fig. 7. HP-DSC of a full cycle of MgH2–2LiBH4 composite doped with5 at.% titanium isopropoxide, after initial hydrogenation and after millingwith additive. Heating rate 5 K min�1; 3 bar H2 for desorption and 50 barH2 for absorption. A and B corresponding to the polymorphic transfor-mation and to the melting of LiBH4, respectively, C to the desorption ofMgH2 and D to the desorption of LiBH4.

Fig. 9. XRD pattern at T = 30 �C of the sample used in the previousexperiment (see Fig. 8). 2LiH–MgB2 composite under 150 bar hydrogenafter heating to 265 �C and holding for 5.5 h.

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temperature also reveals the formation of LiBH4. In addi-tion to LiBH4, an unknown phase with one clear peak(indicated with a question mark) arises. The correspondingpattern is shown in Fig. 9. Therefore, the two exothermicpeaks in the HP-DSC do not correspond to MgH2 andLiBH4, respectively, as both hydrides seem to form duringthe first reaction.

Fig. 8. In situ XRD measurements of 2LiH–MgB2-composite under 150 bar hyfor several hours.

The hydrogenation in the in situ XRD setup was contin-ued with the same sample at higher temperatures toobserve the reaction corresponding to the second HP-DSC peak. The results are shown in Fig. 10. A significantincrease in the formation of MgH2 occurs at temperaturesabove 380 �C. This corresponds to an equally significantdecrease in MgB2. As there is still MgB2 left at the end,the reaction is not complete. During the heating, theunknown phase of the low-temperature absorption can stillbe observed; it disappears at a temperatures of �190 �C,

drogen, heating with 5 K min�1 to 265 �C and keeping at this temperature

Fig. 10. In situ XRD measurement of 2LiH–MgB2 composite under 150 bar hydrogen, heating with 10 K min�1 to 400 �C. This experiment is acontinuation of the experiment shown in Figs. 8 and 9; the sample was heated to 265 �C, held for 5.5 h and cooled to room temperature.

Fig. 11. Comparison of several absorption reactions in the HP-DSCapparatus. Scan 1: the results of 2LiH–MgB2 composite; scans 2 and 3: theresults of the absorption reaction of 2LiBH4–MgH2 composite doped with5 at.% VCl3, or 5 at.% titanium isopropoxide after initial dehydrogena-tion. A and B correspond to the polymorphic transformation and to thesolidification of LiBH4, respectively.

U. Bosenberg et al. / Acta Materialia 55 (2007) 3951–3958 3957

i.e. between the temperatures of the structural phase trans-formation and the melting point of LiBH4.

Just before the enhanced formation of MgH2 at a tem-perature of �370 �C, another unknown phase (also indi-cated with a question mark) arises. The unidentifiedphases each show only one well-resolved reflection thatdoes not overlap with other peaks. As pointed out above,this drawback makes the identification of the correspond-

ing phases very uncertain. However, it should be pointedout that the observed reflections do not match any interme-diate phases, neither calculated nor experimentallyobserved, from LiBH4 [15–17].

For the absorption reaction the effect of different addi-tives has also been investigated in the HP-DSC (seeFig. 11). Scan 1 presents the first hydrogenation of uncatal-ysed material. Scans 2 and 3 show the absorption reactionsof catalysed material after the first desorption. VCl3 andtitanium isopropoxide were selected as additives. Theabsorption reaction for the material doped with titaniumisopropoxide proceeds strongest at low temperatures incomparison. The phase transitions are clearly also influ-enced by the additives. However, the origin of the changein kinetics is not yet understood.

4. Conclusions

Using the powerful tools of HP-DSC and in situ XRD,sorption reactions in MgH2–2LiBH4/MgB2–2LiH RHCshave been investigated in detail. The desorption proceedsin two steps following the scheme MgH2 + 2LiBH4!Mg + 2LiBH4 + H2!MgB2 + 2LiH + 4H2. In the firststep, desorption of MgH2 takes place and Mg is formed.Thereafter, LiBH4 desorbs and LiH and MgB2 are formed.Although the DSC signal also suggests a two-step absorp-tion process, in situ XRD measurements reveal that duringabsorption, MgH2 as well as LiBH4 are both formed at veryearly stages and low temperatures. While in case of the pureLiBH4 hydrogenation from the elements only takes place atvery high temperatures and pressures of 600 �C and 350 bar

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hydrogen [5], MgB2–LiH composites start to absorb hydro-gen even at 250 �C and 50 bar hydrogen.

The current work shows that by the use of suitable addi-tives, the absorption and desorption temperatures of aRHC such as MgH2–2LiBH4 composites can be consider-ably lowered and a significant enhancement of sorptionkinetics obtained. Among the tested catalysts, titanium iso-propoxide as well as VCl3 show the best effect on sorptionkinetics and temperatures.

The in situ XRD measurements hint at the existence ofpotential intermediate phases. However, due to the meltingof LiBH4 as well as the limited number of peaks ofunknown phases, a clear determination is not yet possible.

Further work has to focus on identifying the basic mech-anisms and rate-limiting steps in such systems as well as onimproving reaction kinetics.

Acknowledgements

The authors thank O. Metz and M. Herrich for the tech-nical support. Partial funding by the European Commis-sion DG Research (Contract SES6-2006-518271/NESSHY) as well as by the Helmholtz Initiative ‘‘Func-tional Materials for Mobile Hydrogen Storage’’ are grate-fully acknowledged by the authors.

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