INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. (2009)Published online in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/er.1604

Combustion-type hydrogenation of nanostructured Mg-basedcomposites for hydrogen storage

M. V. Lototsky1,�,y, R. V. Denys2,3 and V. A. Yartys2,z,y

1South African Institute for Advanced Materials Chemistry, University of the Western Cape, Private Bag X17, Bellville 7535,South Africa

2Institute for Energy Technology, P.O. Box 40, Kjeller NO-2027, Norway3Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, 5 Naukova str., Lviv 79601, Ukraine

SUMMARY

In this study Reactive Ball Milling in hydrogen gas was used to synthesize nanostructured hydrogenated composites of Mgand V-based alloy. After hydrogen desorption, the nanocomposites exhibited a dramatic facilitation of the rate of Habsorption by Mg and reduction of the temperature of onset of hydrogenation. These favourable changes were caused by asynergy of catalytic effect of the V-based alloy on hydrogen absorption by Mg and heat release caused by exothermichydrogen absorption by the V-based alloy. When the initial interaction temperature exceeded a threshold, rather low, valueof 20–1251C, depending on the H2 pressure, composition of the sample and its total amount, a combustion-typehydrogenation took place. With optimal interaction parameters applied, H absorption was completed in just 5–70 s andwas accompanied by a significant heat release. The observed features can be utilized to reach fast recharge of the Mg-basedH stores and to develop efficient heat management systems. Copyright r 2009 John Wiley & Sons, Ltd.

KEY WORDS: hydrogen storage; metal hydride; nanocomposite; hydrogenation; magnesium hydride; BCC-vanadiumalloy; combustion-type mode; thermal desorption spectroscopy

1. INTRODUCTION

Slow rate of hydrogenation of magnesium, hightemperatures and hydrogen pressures required forhydrogen absorption, are among obstacles hinder-ing the use of Mg in H storage applications [1,2].

Improvement of the hydrogenation kinetics ofmagnesium can be achieved by preparation of theMg-based composite materials containing easily hy-drogenated alloys. Such approach was first applied[3,4] by compacting Mg powder with 10–20wt%LaNi5 resulting in absorption of 5wt% H in just

zCorrespondence to: V. A. Yartys, IFE, P.O. Box 40, Kjeller NO-2027, Norway.yE-mail: volodymyr.yartys@ife.no

*Correspondence to: M. V. Lototsky, South African Institute for Advanced Materials Chemistry, University of the Western Cape,Private Bag X17, Bellville 7535, South Africa.yE-mail: mlototskyy@uwc.ac.za

Contract/grant sponsor: Department of Science and Technology in South Africa; contract/grant number: KP3-S04Contract/grant sponsor: Norwegian Research CouncilContract/grant sponsor: South Africa–Norway Program of Research Co-operation (2007-2010); contract/grant number: 180344

Received 23 April 2009

Revised 24 June 2009

Accepted 25 June 2009Copyright r 2009 John Wiley & Sons, Ltd.

20–30min. Despite the improvements, high H2

pressure and operating temperature were still re-quired (30bar H2 and 3001C). Similar improvementsof the re-hydrogenation kinetics for the MgH2-basedsamples doped by transition metals, including vana-dium, and ball milled in argon were reported in [5,6].Mechanical grinding of Mg with catalytic additivesof the alloys of transition metals in hydrogen hasproved to be an efficient way to further accelerate there-hydrogenation process, which was completed inonly 5–10min (at 11bar H2 and 3001C) [7].

Nanotechnological approach has been shownto be very effective for the favourable modificationof the properties of the hydrogen storage materials,particularly for those MgH2-based, providing pos-sibilities for the desired optimization of the bondingconditions for hydrogen, thus allowing to controlthe behaviours [8].

Nanostructuring may significantly improve thereaction kinetics, reduce the enthalpy of hydro-genation, and decrease the H absorption and releasetemperatures via destabilization of the metal hy-drides and catalysis phenomena. A detailed reviewof different phenomena and mechanisms relatingnanostructuring and H storage is presented in [9].Further to the beneficial behaviours, nanostruc-tures can also have poor cycle stability, exhibitdeterioration of the sorption behaviours, and lead toa significant reduction of the thermal conductivity,thus making nanostructured metal hydridesimpractical for hydrogen storage. Particularly, inthe review are considered both beneficial anddeteriorating effects of such characteristics of nano-materials as increased surface area; increased grainboundaries; increased porosity; formation of nano-composites; doping with catalysts and alloying.

In the present work we have studied thehydrogen absorption and desorption performancesof the nanostructured Mg-based compositesmodified by additives of the V-based BCCalloy and synthesized by Reactive Ball Milling inHydrogen gas (HRBM) [10].

2. EXPERIMENTAL

The materials under investigation included Mg(Fluka, 991%; grit 50–150 mesh), Mg–20Ni–8Mm

eutectic alloy [11] mixed with 10–35wt% of theBCC-type V75Ti10Zr7.5Cr7.5 alloy [12] (for simplicitywe will mark it as V alloy in the paper). One samplealso contained 5wt% of graphite (Fluka, 991%;powder, p20m). The materials were ball milled for2–2.5h in hydrogen gas (PH25 30bar; rotationspeed 500 rpm) using FRITCH Pulverisette 6planetary mill equipped with a specially designedDUPLEX steel vial allowing to carry out themilling at gaseous pressures up to 50bar. Carbon–Chromium steel balls (balls-to-powder ratio 80:1)were used, and hydrogen absorption by the materialduring HRBM was periodically checked. Thepreparation and characterization details are pre-sented elsewhere [10]. The compositions and mainhydrogen sorption characteristics of the preparedmaterials are summarized in Table I.

The materials were handled in a glove box filledwith purified Ar (concentration of impurities, O2

and H2O, was below 10 ppm).Figure 1(A, B) shows typical SEM images of

the as-prepared composite (MgMmNi/35V) takenwith Hitachi S-4800 instrument in secondaryelectrons. After the HRBM, the materials con-sisted of agglomerates of submicron particles.Several cycles ‘H desorption (thermal desorptionspectroscopy (TDS))–re-hydrogenation’ resultedin some sintering of the particles (Figure 1(C)).

Hydrogen charge–discharge experiments werecarried out using combined Sieverts/TDS setup(Figure 2). The samples (m5 1.070.1 g) wereloaded into a stainless steel reactor (shown on inset)equipped with thermocouples allowing to measurethe temperatures in the sample bed (TS) and on theouter reactor wall (TW). The procedure includedthe TDS measurements (dynamic vacuum;T5 20–4001C; ramp rate 1–5deg/min) followed byre-hydrogenation of the samples (PH25 10–30bar,starting temperature, T0 5 20–2001C). Up to 20‘TDS–re-hydrogenation’ cycles were performed foreach sample; the re-hydrogenation conditions(PH2, T0) were varied from cycle to cycle within theranges specified above. A proper selection of sys-tem volumes, in combination with usage of bothabsolute (3) and deferential (4) pressure sensorsallowed us to carry out the hydrogenation experi-ments at nearly isobaric conditions (pressure dropless than 1 bar) and, at the same time, to provide

M. V. LOTOTSKY, R. V. DENYS AND V. A. YARTYS

Copyright r 2009 John Wiley & Sons, Ltd. Int. J. Energy Res. (2009)

DOI: 10.1002/er

sufficient accuracy of the determination of theamount of hydrogen absorbed by the sample(�0.5 cm3 g�1 or better than 0.01wt% H).

Since during some of the re-hydrogenationexperiments the changes of reactor wall (TW) and,especially, sample (TS) temperatures were very

fast, a correction for a thermocouple response timewas introduced as follows [13]:

T�ðtÞ ¼ TðtÞ þ exp �t

t

� �Z t

0

exps

t

� � d

dsTðsÞds

ð1Þ

Figure 1. SEM images of the MgMmNi/35V composite; A, B, after HRBM; C, after 10 ‘H desorption–re-hydrogenation’ cycles.

Table I. Hydrogen absorption characteristics of the studied composites.

wt% H

Rehydrogenation t0 (T0 (1C))�,y

Composite HRBM Slow mode (min) Fast mode (s) Slow mode Fast mode Ti (1C)z TDES

y

MgMmNi/35V 4.79 3.7 3.4 75 (25) 3 (150) 60 216Mg/35V 6.13 2.8 3.6 20 (25)z 0.5 (25) o20 203Mg/25V 6.63 4.6 5.9 12 (25) 4.5 (75) 75 211Mg/10V 6.85 3.8 5.0 3 (75) 3 (100) 100 212Mg/10V/5C 6.87 5.5 6.2 36 (75) 14 (150) 125 230

�t0—characteristic time of the hydrogenation when (1–1/e) of maximum hydrogen absorption capacity is achieved.yT0—initial temperature of the hydrogenation at PH2 5 26 bar.zTi—ignition temperature at PH2 5 26 bar.yTDES—peak temperature of hydrogen desorption in dynamic vacuum measured at a heating rate of 2 degmin�1.zHydrogenation performed at PH2 5 11 bar.

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where T�(t) and T(t) correspond to the actual(corrected) temperature values and thermocouplereadings, respectively. Time constant of the ther-mocouple, t5 4.5(6) s, was determined in separatecalibration measurements.

Mechanism and kinetics of the phase trans-formations in the materials during hydrogencharge and discharge were studied using in situSynchrotron X-ray diffraction (SR XRD). Thedata were collected at Swiss–Norwegian Beam-lines, BM01A, at ESRF, Grenoble, France usingthe wavelengths l of 0.7114(1) and 0.7243(1) A.Detailed description of the experimental setup andresults collected for some Mg-based systems aregiven in [14].

3. RESULTS

3.1. Hydrogen absorption and desorption processes

All studied materials form the hydrides during theHRBM; hydrogen content in these hydrides rangesfrom 4.8 to 6.9wt% H. During the secondhydrogenation, on hydrogen absorption after

completion of the TDS process, H storagecapacities were slightly lower; 3.5–6.2 wt% H werere-absorbed (Table I). Typical re-hydrogenationbehaviours of the composites are presented inFigure 3 as time dependencies of hydrogen contentand the temperatures of the sample bed (TS) andreactor wall (TW). Depending on the compositionof the material and hydrogenation conditions, thereaction proceeded via two different, slow or fast,dynamic modes.

The first, slow mode (typical behaviour is pre-sented in Figure 3(a)), is characterized by a rathersluggish absorption. The composite–hydrogen in-teraction takes place even at room temperature,and the rate of hydrogen absorption is increasedwith the temperature. However, even when theheating was applied, hydrogen uptake reachedsaturation only after 2–20 h resulting in achieve-ment of 50–80% of maximum hydrogen sorptioncapacity of the material compared with thosereached after the HRBM (Table I). The initialperiod of the hydrogenation in the slow mode(10–30min) is accompanied by a moderateheat release resulting in heating of the sample

∅12.5 mm

1.65 mm

205

mm

1.1

1.3

5 mm

1.4.1

H2

Evacuation

1

5

3

4

6

16 m

m

7 m

m

1.2

1.4.2

2

2.1

TS

TW

PVac

PaPd

TR

T0

PC

V1

V3

V2

V4 V5

V1=49.5 cm3

V2=39.3 cm3

V3=70.1 cm3

V4=297 cm3

V5=34.2 cm3

7

8

9

10

11

Figure 2. Schematics of the experimental setup for studies of hydrogen absorption/desorption in Mg/V composite: 1,reactor assembly; 1.1, SS reactor; 1.2, sample; 1.3, heat insulation (glass wool); 1.4, thermocouples; 2, furnace; 2.1,thermocouple; 3, pressure sensor (0–60 bar absolute, acc. 0.04% FS); 4, pressure sensor (70–1 bar differential, acc.

0.04% FS); 5, vacuum sensor; 6, buffer cylinder, 7–11, shut-off valves.

M. V. LOTOTSKY, R. V. DENYS AND V. A. YARTYS

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and the reactor by 10–401; this is followed by agradual cooling down to the environment tem-perature (T0).

In contrast, when the environment temperature(T0) exceeds a threshold (ignition) value (Ti), avery fast, combustion-type hydrogenation modewas observed for all the studied materials. Arepresentative dependence of such a type is shownin Figure 3(b). The time for the completion of this

process was very short; it varies depending on thematerial between 5 (Mg/35 V) and 70 (Mg/10 V/5 C)seconds. The hydrogenation was accompanied bya significant heat release resulting in a dramaticincrease of the sample temperature (TS), which tookplace immediately, just in a few seconds, after thebeginning of the interaction. The evaluated values(T�S ) of the maximum sample temperature, for allcases when the combustion-type hydrogenationwas observed, agree well (710–50K) with thetemperature (TEQ) corresponding to the equilibriumof Mg2MgH2 transformation at the operatinghydrogen pressure (P):

TEQ ¼DH

R lnðPÞ þ DSð2Þ

where DH5�74.5kJmol�1 H2, DS5–135.0Jmol�1

H2 K�1 are the enthalpy and entropy of the trans-

formation [2]. Such a good agreement indicatesthat during the combustion-type hydrogenationof the Mg-based composites, because of the fastkinetics of the processes, the equilibrium isestablished nearly instantly, and further reactionis controlled by dissipation of the heat into theenvironment.

Influence of the environment temperature, T0,on the hydrogenation dynamics of Mg/25 V (a)and Mg/10 V/5 C (b) composites (PH2 5 26 bar) isillustrated in Figure 4. Generally, when the environ-ment temperature is increased, the hydrogenationrate is gradually accelerated; however, when thetemperature exceeds a threshold (ignition) value, thereaction rate increases spasmodically by �2 ordersof magnitude indicating a transition from the slowto the combustion-type hydrogenation. For allstudied materials similar behaviour was observed; atypical example, Mg/25 V composite, is presented inFigure 4(a). We note that for the only carbon-con-taining composite, a transition from slow to thecombustion-type mode of hydrogenation (at 1251C;Figure 4(b)) was not so sharp as for the carbon-freematerials.

The data summarizing the hydrogenationbehaviour of the composites are given in Table I.Comparison of these data shows that the increasein the content of the vanadium alloy in the com-posite facilitates the hydrogenation. At the sametime, hydrogen absorption capacities are reduced

Figure 3. Typical hydrogen absorption behaviours of thenanocomposites prepared by dehydrogenation in vacuumduring their heating in a TDS experiment: a—slow mode(Mg/10V/5C; T05 501C; PH25 26bar; cycle 6); b—fast(combustion) mode (Mg/35V; T05 231C; PH25 26bar;cycle 5). Corrected temperature profiles of the sample(T�S ) and of the reactor wall (T�W) were evaluated byaccounting the thermocouple response time. The equili-brium van’t Hoff temperature TEQ corresponding to thetransformation Mg2MgH2 (TEQ5DH/[R lnP1DS)]) atthe operating hydrogen pressure of 26bar is shown as a

dashed line.

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due to the lower hydrogen content in the V-basedhydride (3.2 wt% H [12]) compared with MgH2

(7.6wt% H). As it can be seen, for all the samples,the re-hydrogenation process was incomplete, andthe lowest degree of transformation Mg-MgH2

was observed for the sample containing the max-imum amount of the V alloy (35wt%). Mostprobably, such a difference in the behaviour forthe last sample is caused by a quick formation of adense layer of MgH2 covering the unreacted Mgparticles and acting as a diffusion barrier duringthe re-hydrogenation.

The main results of the studies of the hydro-genation properties are summarized below.

� Hydrogen storage capacities of particularmaterials are influenced by the synthesisroute. The largest H capacity and degree ofsaturation takes place for the materialsobtained during the HRBM in hydrogengas. Limiting H absorption capacitiesachieved in the course of the com-bustion-type re-hydrogenation of the Mg-Vsamples were lower, from 60 (Mg/35 V) to87% (Mg/25 V) of the maximum valuesobserved for the HRBM.

� For the graphite-modified Mg-V sample, nosignificant differences in hydrogen sorptioncapacities after HRBM and re-hydrogena-tion, both during slow and combustion-typeprocesses (84 and 90% of the maximumvalue, respectively) were observed. However,the combustion-type hydrogenation of thissample proceeded slower than for the sampleswithout the additive of graphite.

� The hydrogenation rate gradually acceleratesfollowing the rise of the initial interactiontemperature. However, when the temperatureexceeds a threshold value, the reaction rateincreases spasmodically by �2 orders ofmagnitude indicating the transition from theslow to the ignition, or combustion-type hydro-genation mode.

� The ignition temperature decreases followingan increase in the content of the V alloy in thecomposite.

� Transition to the fast, combustion mode ofhydrogenation is facilitated at higher hydro-genation pressures. As an example, for thesample Mg/35 V the combustion-type modewas observed at PH2X20bar, while loweringthe pressure to 11bar resulted in the slow-modehydrogenation.

� The hydrogenation–dehydrogenation cyclingalso affects a transition to the combustion-mode reaction. The fast hydrogenation modeswitches to a slow hydrogenation after sur-passing a certain number of cycles. Thecombustion mode appears to be most ‘stable’in the composites containing lower amount

Figure 4. Hydrogenation of the Mg/25V: (a) and Mg/10V/5C and (b) composites at PH2 5 26 bar performed

at different temperatures.

M. V. LOTOTSKY, R. V. DENYS AND V. A. YARTYS

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DOI: 10.1002/er

of V alloy and in presence of graphite in thecomposite.

� The composite of Mg–Ni–Mm eutectic alloybehaves differently from the Mg-based ones.Mg–Ni–Mm alloy is characterized by slowerhydrogenation rate and lower H sorptioncapacity compared with the Mg–V compositecontaining the same amount of V alloy. Suchdifferences are caused by the presence ofMg2NiH4 (3.6wt% H; 35.7wt%) and RH3

(�0.7wt% H reversible; 7.7wt%), further tothe main constituent, MgH2, in the hydro-genated Mg–Ni–Mm alloy. This results in asignificant reduction of its maximum Hstorage capacity (5.7 wt% H) as comparedwith MgH2 (7.6wt% H). In addition, onecomponent of the alloy, the Mg2Ni inter-metallic compound, has much slower hydro-genation kinetics compared with those for theMg metal [11]; thus, the overall hydrogena-tion absorption rates are slowed down.

3.2. Synchrotron X-ray diffraction

The mechanism of the phase-structural transfor-mations on hydrogenation was studied using SRXRD; Mg/35V and Mg/25V composites werechosen as representative materials. During the insitu SR XRD experiments the as preparedcomposites were first dehydrogenated by heatingthe hydrides in dynamic vacuum to 4001C;obtained samples were then re-hydrogenated attemperatures 25–1001C and pressures 6–12.5 barH2. For the hydrogenated V-based BCC alloythree different hydride phases were identifiedfrom the SR XRD data: (1) b1-VH�0.5 with theBCT structure at temperatures o1001C andBCC structure at temperatures X1001C;(2) b2-VH�0.7, BCC; and (3) g-VH2, FCC (CaF2-type). Hydrogen content in the VHx (V5

V0.75Ti0.1Zr0.075Cr0.075) hydrides was estimatedfrom the values of the relative volume expansionon hydrogenation, assuming that H-inducedexpansion equals to 2.8 A3/at.H and DV/V linearlyincreases with H content; these suggestionswere based on the available crystallographicdata for VH0.56 (DV/V5 2.90 A3/at.H) and VH2

(DV/V5 2.73 A3/at.H) [12].

No new compounds were formed by Mg andBCC alloy on cycling indicating the absence of thechemical interaction between the components ofthe composites.

SR XRD analysis of the as prepared Mg/35Vand Mg/25V hydrogenated composites pointedthat during the HRBM a complete transformationof the initial metals into the nanocrystalline dihy-drides was achieved, a mixture of the conven-tional, rutile-type a-MgH2, high pressure g-MgH2

polymorph and dihydride of the V-based alloyg-VH2. In addition, two lower V hydrides,b1-VH�0.5 and b2-VH�0.7, were identified, appa-rently originating from a partial decomposition ofVH2 during its handling in Ar atmosphere. Phase-structural characteristics of the composites aftercompletion of the dehydrogenation and re-hydro-genation processes are provided in Table II. Allhydride phases decompose during their heating indynamic vacuum to 4001C. Subsequent re-hydro-genation of the composites proceeded in severalsteps. As can be seen from Figure 5, at the firststages of the hydrogenation process, an instantformation of the b1-VH�0.5 is observed; it isfollowed by a transformation into b2-VH�0.7.Hydrogenation of Mg leading to the formation ofa-MgH2 is slower in rate and proceeds afterthe vanadium monohydride is formed. Since theexperimental conditions of in situ SR XRD wereclose to equilibrium pressure–temperature condi-tions of g-VH2 formation, and because formationof the dihydride requires application of excesspressure, only rather slow and incompleteb2-VH�0.7!g-VH2 transformation was observed.

The rate of the transformation V!b2-VH�0.7

was found to be two orders of magnitude higherthan that for the Mg!MgH2 hydrogenation (seeFigure 5(b)). We note that the rehydrogenationkinetics observed in the course of the SR XRDstudy of a small, few milligram, sample, was muchslower than that during the volumetric studies of alarger (1 g) sample. This observation indicates thatthe conditions of the in situ SR XRD experiment(quartz capillary 0.5mm diameter, 0.01mm thickwall) allow for a quick dissipation of heat, thusreducing influence of thermal effect on the trans-formation V-VHx on the facilitation of thehydrogenation of Mg. Thus, influence of V alloy in

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the nanocomposite on the hydrogenation beha-viour of Mg is limited to the catalytic effect,particularly (1) source of atomic H, because of aneasy H2 dissociation on the surface of the V alloyand (2) formation of extra centres for thenucleation of MgH2 on an interface between Mgand V alloy. Not surprisingly, characteristic timet0 for the hydrogenation of Mg/25 V composite(Figure 1) in the course of in situ SR XRDexperiment (T5 801C, PH2 5 8.5 bar) is close tothe value found for the slow mode reaction(T5 251C, PH2 5 26 bar) during the volumetricstudies (see Table I).

Despite the conditions of the in situ SR XRDexperiments do not allow the hydrogenationreaction to enter the combustion mode, hydrogenabsorption by magnesium was rather fast even atlow temperatures (20–1001C); indeed, at 1001Cduring the re-hydrogenation of the Mg/35Vcomposite 83% of Mg transformed into the dihy-dride MgH2 in just 30min (together with VH0.7

hydride it yields a total hydrogen content in thecomposite of 4.7wt% H).

We note that for the Mg–V composites the peaktemperatures in the TDS spectra (T2 5 203–2301C)are significantly lower compared with the pure

Table II. Phase-structural characteristics of the RBM Mg/V-alloy composites obtained from Rietveld refinements ofthe SR XRD data at different stages of the in situ experiments.

Material Phase� Space group Unit cell parameters, A Abundancyy, wt%

Mg/25 VAs prepared T5 801C (vacuum) a-MgH2 P42/mnm a5 4.5197(5), c5 3.0187(5) 56.3(1)

g-MgH2 Pbcn a5 4.519(2), b5 5.431(2),c5 4.926(3)

20.8(2)

g-VH2.0 Fm�3m a5 4.2914(2) 8.9(1)b2-VH0.76 Im�3m a5 3.1944(5) 6.1(1)b1-VH0.56 I4/mmm a5 3.095(1), c5 3.287(3) 7.8(1)

Dehydrogenatedz, T5 801C (vacuum) Mg P63/mmc a5 3.21561(3), c5 5.22158(7) 76.83(5)

V-BCCalloy

Im�3m a5 3.0478(1) 23.2(1)

Re-hydrogenatedy T5 801C(8.5 bar H2)

Mg P63/mmc a5 3.21502(8), c5 5.2206(2) 33.3(1)

a-MgH2 P42/mnm a5 4.5202(2), c5 3.0221(2) 45.5(1)

b2-VH0.70 Im�3m a5 3.1817(2) 21.0(1)

g-VH2.0 Fm�3m a5 4.289(–) 0.21(9)Mg/35 VAs prepared T5 251C (vacuum) a-MgH2 P42/mnm a5 4.5119(6), c5 3.0150(9) 27.1(1)

g-MgH2 Pbcn a5 4.512(2), b5 5.434(3),c5 4.919(3)

38.8(1)

g-VH2.0 Fm�3m a5 4.2900(2) 12.2(1)b2-VH0.79 Im�3m a5 3.1987(3) 9.8(2)b1-VH0.54 I4/mmm a5 3.089(1), c5 3.285(2) 12.1(2)

Dehydrogenatedz T5 1001C(vacuum)

Mg P63/mmc a5 3.2158(1), c5 5.2216(2) 66.0(2)

V-BCC alloy Im�3m a5 3.0479(2) 34.0(3)Re-hydrogenatedy T5 1001C(12.5 bar H2)

Mg P63/mmc a5 3.2109(4), c5 5.214(1) 11.0(1)

a-MgH2 P42/mnm a5 4.5183(2), c5 3.0213(2) 53.7(2)

b2-VH0.67 Im�3m a5 3.1764(3) 35.3(1)

�Hydrogen content in VHx (V5V0.75Ti0.1Zr0.075Cr0.075) hydrides was estimated from the values of the relative volume increase onhydrogenation, assuming that hydrogen-induced expansion equals to 2.8 A3 / at.H and DV/V linearly increases with H content.ySmall amounts of a-Fe impurities (o1wt%; a5 2.88 A), originating from the wearing of the milling steel balls during preparation ofthe composites, were identified; this impurity is neglected in presentation of the phase-structural composition of the samples.zComposite material synthesized in course of the in situ TDS (heating in vacuum up to 4001C).yComposition of the composite evaluated after 30min of in situ hydrogenation experiment.

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magnesium hydride synthesized by the HRBM(T2 5 3041C). Detailed data giving the results ofthe studies of the thermal stability of the milledmaterials will be presented and discussed in [10].

In the case of the MgMmNi alloy, presence ofMg2Ni-based hydride in the hydrogenation pro-ducts decreases the overall thermal stability of thehydride. Typical traces of hydrogen evolutionfrom the hydrogenated MgMmNi/35V compositeare presented in Figure 6. Indeed, spectra ofhydrogen thermal desorption show an onset ofH2 evolution already at T1 5 150–1601C with a

maximum at T2 5 215–2251C. The latter value wellagrees with the temperatures of hydrogen deso-rption from MgH2 in the hydrogenated Mg–Vcomposites. We also note a good agreement withobservations of [11] (T1 5 1501C, T2 5 2001C forthe hydrogenated ball-milled and T2 5 2201C forthe unmilled Mg–Mm–Ni-based hydride).

4. DISCUSSION

For the exothermic chemical reactions, when the rateof heat generation exceeds the rate of heat dissipa-tion from the reaction volume to the environment, arunaway temperature rise takes place. For the solid-state reactions this results in the appearance of acombustion-type dynamic mode, when the fastexothermic process either takes place simultaneouslyin the entire volume of the material (similar to thethermal explosion), or spatially spreads therein (selfpropagating high temperature synthesis, SHS).Typically, combustion-type hydrogenation must beinitiated by a local thermal ignition (SHS), or by anirradiation of the hydride-forming material (thermal-radiation synthesis, TRS) [15]. Despite the highvalue of the enthalpy of formation of MgH2 (DH�–74.5kJmol H2

�1), previous attempts to hydrogenatemagnesium in a combustion-type synthesis mode,

Figure 5. (a) Evolution of the in situ SR XRD pattern ofMg/25V composite during rehydrogenation at 801C and8.5 bar H2 and (b) Hydrogenation plots for the individualcomponents of the composite (Mg- and V-based phase)and for the composite in total obtained from the Rietveldrefinements of the in situ SR XRD data. Characteristic

time t0 of the hydrogenation reactions is shown.

Figure 6. Thermal desorption spectra of H desorp-tion from the hydrogenated MgMmNi/35V comp-osite prepared by HRBM (Tmax 5 2231C; ramp rate2 deg/min); rehydrogenation in a combustion-type mode(Tmax 5 2211C) and in a slow mode (Tmax 5 2161C).

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via SHS, were unsuccessful [16] because of too slowhydrogenation kinetics.

In contrast, present study clearly showed thatthe synthesis of the nanostructured composites ofMg- and V-based alloys dramatically facilitates therates of H uptake by Mg-based materials andsignificantly decreases the reaction temperatures;in all cases 50–80% hydrogenation of Mg wasobserved already at room temperature. Thisfacilitation has been achieved as a result ofsignificant decrease in the particle size during theHRBM, thus decreasing a diffusion length forhydrogen in the material, and, also by achievingsynergy in hydrogenation of V and Mg.

H uptake by the V-based alloy (V75Ti10Zr7.5Cr7.5)used in the present work is characterized by thefast rates of transformations V-VH1�x at mildpressure–temperature conditions, thus promotingfast hydrogen exchange between H2 molecules inthe gaseous phase and H atoms accommodated bythe solid. In turn, these results in the formation ofnon-equilibrium, excited H species [17], thus,explaining a catalytic effect of the BCC-V on thehydrogenation of Mg. In addition, fast hydro-genation of the catalytic V-based additive is ac-companied by a significant heat release, whichheats Mg/Mg alloy to reach the temperature whenthe hydrogenation rate becomes high enough.

To summarize, the hydrogenation of the Mg-based nanocomposite materials was significantlyaccelerated due to both catalytic and exothermicheat effects of hydrogen absorption by the V-basedalloy. This removed the kinetic obstacles to realizesuch a process in the combustion-type mode.

An additional advantage for the application ofthe vanadium alloy as a catalyst and heatingsource promoting the hydrogenation of magne-sium is that the enthalpy of formation of VH1–x

(�70 kJ/mole H2 [18]) is close to the corres-ponding value for MgH2. Owing to that, thetemperature increase in the course of the fasthydrogenation of V alloy embedded into the com-posite does not exceed the equilibrium value forthe Mg2MgH2 transformation (TEQ5 405–4251Cat PH2520–30bar, as calculated using Equation (2)).In contrast, the use of catalytic additives havinghigher hydrogenation enthalpy (e.g. Ti for whichTEQ�8001C at the same conditions) would result

in overheating accompanied by a decompositionof MgH2, or even melting of Mg. Such a behaviourwas indeed observed in the course of SHS inthe Mg–Ti–H2 system [16]. Moreover, closevalues of thermal stabilities of MgH2 and VH1�x

(all observed TDS patterns showed a one-stagedecomposition of the hydride; see Figure 6) resultin an efficient regeneration of the V-based hydro-genation catalyst during the hydrogen desorptionprocess.

The observed combustion-type hydrogenationof the nanostructured Mg-based composites con-taining additive of the BCC-V alloy excellentlyagrees with the theory of thermal explosion[19,20]. According to the latter, combustion-typekinetics is defined by interplay between the reac-tion rate (exponentially increases with tempera-ture) and the intensity of heat removal from thereaction zone (linearly increases with tempera-ture). As a result, at the environment temperatureT04Ti, where Ti is an ignition temperature, thereaction proceeds as a self-accelerating processresulting in thermal explosion. The ignition tem-perature increases with increased heat losses be-cause of (a) reduction of the sample size; (b)facilitation of the heat exchange with environment,or (c) increase of the effective thermal conductivityof the material. In turn, acceleration of the reac-tion rate (by increasing hydrogen pressure, or bythe increase in the amount of the catalytic additivein the composite) promotes the transition to thecombustion-type mode occurring at lower ignitiontemperatures.

Indeed, for the studied composites the ignitiontemperature was increased with lowering theamount of the BCC-V alloy, or with introductionof the thermal-conductive inert to hydrogen gra-phite additive (Table I). It is interesting that in thelatter case the observed transition to the combus-tion-type mode was characterized by a time delay(induction period preceding fast hydrogenationstep, as shown in Figure 4). The observed phe-nomenon of transition from the slow reactionmode to the thermal explosion was predicted byapplying the theory of thermal explosion to themultiphase media [21].

Multiple cycling ‘hydrogen desorption–rehy-drogenation’ inhibits the combustion-type hydrogen

M. V. LOTOTSKY, R. V. DENYS AND V. A. YARTYS

Copyright r 2009 John Wiley & Sons, Ltd. Int. J. Energy Res. (2009)

DOI: 10.1002/er

absorption. Most probably, the origin of such abehaviour is in sintering/re-crystallization of thesample (see Figure 1(C)) resulting in increase of itseffective thermal conductivity. Since, as mentionedearlier, the ‘stability’ of the combustion-type hy-drogenation during the cycling was well developedfor the samples with lower content of the BCC-Valloy, we believe that sintering/re-crystallizationmainly occurs due to the overheating of the Mgparticles contacting with V-based catalyst duringthe re-hydrogenation.

Increase in the volume of the sample also pro-motes fast, combustion-type hydrogenation. Forthe composite material containing MgMmNi eu-tectic alloy together with 25wt% of the sameBCC-V alloy, studied in a similar reactor andhaving a rather large size (m5 12 g), the hydro-genation accompanied by a significant heat releasewas observed at room temperature [18], while theignition temperature for MgMmNi/35 V andMg/25 V (m5 1 g) was found in this work to beclose to 60 and 751C, respectively (Table I). Fur-ther, decrease in the size of the sample to severalmg (Mg/25 V and Mg/35 V composites studied inthe course of in situ SR XRD studies (Figure 5))resulted in suppression of the combustion-typehydrogenation dynamics even at the starting H2

absorption temperatures of T5 80–1001C.In the course of the combustion-type hydro-

genation, the fast growth of the dense MgH2 layeron the surface of the Mg particles results in low-ering of the maximum H sorption capacity ofthe composite, by preventing H diffusion towardsthe non-reacted Mg core at the late stages of theprocess.

In conclusion, the nanocomposites obtained byball milling of Mg/Mg–Mm–Ni eutectic alloy withBCC V-based alloy in H2 can be easily re-hydro-genated even at room temperature, without anyexternal heating. Under certain conditions theprocess proceeds in a very fast, combustion-typemode.

Finally, this work has resulted in the deve-lopment of the weight-efficient nanostructuredhydrogen storage materials characterized byultra-fast kinetics of H2 uptake at mild pressure–temperature conditions, which completely releaseabsorbed hydrogen at low temperatures not

exceeding 2501C. The developed materials canbe used in various hydrogen energy applications,including light weight H storage units with shortrecharge time and efficient heat managementsystems (e.g., heat transformers) operating in thetemperature range 20–3501C.

ACKNOWLEDGEMENTS

We are grateful to the Norwegian Research Council forfinancial support. The collaboration between IFE andUWC was supported by a program of bilateralcollaboration between Norway and South Africa withina project 180344 ‘Industrial Applications of MetalHydrides for Hydrogen Extraction, Storage and Com-pression’ (HYDROTECH), and Hydrogen and FuelCell Technologies Research Development and Innova-tion Programme funded by the Department of Scienceand Technology in South Africa (project KP3-S04‘On-Board Hydrogen Storage’).The authors thank Dr Jan Petter Maehlen and

Andrey Poletaev for their help in the experimental work.A skilful assistance of the personal of the SwissNorwegian Beam Lines at ESRF, Grenoble, France isgratefully acknowledged.

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DOI: 10.1002/er