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Effects of CNTs on the hydrogen storage properties of MgH2

and MgH2-BCC composite

A. Ranjbar a, M. Ismail a, Z.P. Guo a,b,c,*, X.B. Yu a,b,d, H.K. Liu a,b

a Institute for Superconducting & Electronic Materials, University of Wollongong, Wollongong, NSW 2522, AustraliabCSIRO National Hydrogen Materials Alliance, CSIRO Energy Centre, 10 Murray Dwyer Circuit, Steel River Estate, Mayfield West,

NSW 2304, AustraliacSchool of Mechanical, Materials & Mechatronics Engineering, University of Wollongong, NSW 2522, AustraliadDepartment of Materials Science, Fudan University, Shanghai 200433, China

a r t i c l e i n f o

Article history:

Received 18 December 2009

Received in revised form

14 May 2010

Accepted 14 May 2010

Available online 17 June 2010

Keywords:

Hydrogen storage

Magnesium hydride

Body centred cubic (BCC) alloys

Carbon nanotubes (CNTs)

* Corresponding author at: Institute for SupAustralia. Fax: þ61 2 4221 5731.

E-mail address: zguo@uow.edu.au (Z.P. G0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.05.080

a b s t r a c t

MgH2 with 10 wt.% Ti0.4Mn0.22Cr0.1V0.28 alloy (termed the BCC alloy for its body centred

cubic structure) and 5 wt.% carbon nanotubes (CNTs) were prepared by planetary ball

milling, and its hydrogen storage properties were compared with those of the pure MgH2

and the binary mixture of MgH2 and the BCC alloy. The sample with CNTs showed

considerable improvement in hydrogen sorption properties. Its temperature of desorption

was 125 �C lower than for the pure sample and 59 �C lower than for the binary mixture. In

addition, the gravimetric capacity of the ternary sample was 6 wt.% at 300 �C and 5.6 wt.%

at 250 �C, and it absorbed 90% of this amount at 150 s and 516 s at 300 �C and 250 �C,

respectively. It can be hypothesised from the results that the BCC alloy assists the disso-

ciation of hydrogen molecules into hydrogen atoms and also promotes hydrogen pumping

into the Mg/BCC interfaces, while the CNTs facilitate access of H-atoms into the interior of

Mg grains.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction catalysts, such as transition metals [8e10], metal oxides

The storage of hydrogen in solids seems to be the most effi-

cient and safestmethod for application in fuel cells and hybrid

vehicles [1]. Among the different solids,magnesiumhydride is

a very promising material for this purpose because of its high

capacity (7.6 wt.%), good reversibility, and low cost [2].

However, its practical application is greatly hindered by its

sluggish hydrogenation/dehydrogenation kinetics and high

operation temperature [3]. Over the last decade intensive

investigations has been dedicated to overcoming these

barriers. These efforts include surface activation and reduc-

tion of grain size by ball milling [4e7], as well as the use of

erconducting & Electron

uo).ssor T. Nejat Veziroglu. P

[11,12], body centred cubic alloys [13,14], and different kind of

carbons [15,16].

In our previous work [17], it was demonstrated that ball

millingmagnesiumwithTi0.4Mn0.22Cr0.1V0.28 alloy improvedthe

hydrogenstoragepropertiesofMgH2.On theotherhand, carbon

nanotubes (CNTs) exhibited excellent catalytic effects on the

hydrogen sorption properties of magnesium based composites

[18]. A combination of CNTs with transition metals has been

found to lead to an especially great enhancement of hydrogen

dissociation and diffusion into Mg nanoparticles [19e21].

In this work, the effect of the combination of 10 wt.% of

Ti0.4Mn0.22Cr0.1V0.28 alloy, hereafter referred to as the BCC

ic Materials, University of Wollongong, Wollongong, NSW 2522,

ublished by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 7 8 2 1e7 8 2 67822

alloy due to its body centred cubic structure, with a small

amount of CNTs (5 wt.%) on the hydrogen storage properties

of MgH2 has been investigated and compared with the

performance of pure MgH2 and MgH2 þ 10 wt.%

Ti0.4Mn0.22Cr0.1V0.28.

2. Experimental details

The BCC alloy sample was prepared by magnetic levitation

melting of the constituent metals (Beijing YiTianhui Institute

of Metallic Materials, all with purity of more than 99.9%). A

50 g ingot was turned over and remelted four times to ensure

higher homogeneity, and then it was quenched on a water-

cooled rotating molybdenum disc that was spinning at the

rate of 20 m/s. After preparation of the BCC alloy, 0.9 g of pure

MgH2 (SigmaeAldrich, H-storage grade powders) and 0.1 g of

the BCC alloy were introduced into stainless steel vials of

a QM-3SP2 planetary ball-milling instrument. The samples

were ball milled under Ar atmosphere for 24 h with a ball to

powder weight ratio of 20 and a speed of 400 rpm. In the last

step, the MgH2 þ 10 wt.% BCC þ 5 wt.% CNT (termed Mg-BCC-

CNT) was prepared by adding 5 wt.% multiwall carbon nano-

tubes (MWCNTs, SigmaeAldrich,> 99.9%) into the ball milling

vial with MgH2 þ 10 wt.% BCC composite and milling the

mixture for 2 h. For comparison, pure MgH2 andMgH2 þ 10 wt.

% BCC (termedMg-BCC) samples were also prepared using the

same ball-milling technique for 26 h. All handling of the

powders, before and after ball milling, was performed in an

argon filled glove box (with oxygen and water content each

below 1 ppm) in order to prevent any oxidation of the samples.

X-ray diffraction (XRD) patterns of the as-prepared and

dehydrogenated samples were obtained with a GBC-MMA

X-Ray diffractometer using Cu Ka radiation (l¼ 0.15418 nm) at

40 kV and 20 mA. The morphology of the composites and the

distribution of the elements were characterized by using

scanning electronmicroscopy (SEM; JEOL JSM-6460A) followed

by energy dispersive spectroscopy (EDS). Backscattered elec-

tron composition (BEC) images were also obtained on this

instrument. The desorption temperature and hydrogen

capacity of the as-prepared composites were determined by

using an AMC gas reactor controller in profile releasemode, in

which the samples were heated up from room temperature to

a maximum of 500 �C with a heating rate of 10 �C/min and

a pressure of 1 atm. The onset temperature (Tonset), the

temperature at which hydrogen starts to be released from the

sample, the desorption temperature (Tdes), the temperature at

which the hydrogen is completely released from the sample,

and the hydrogen content were measured and graphed using

the software package supplied with the instrument. The

hydriding kinetics, under a hydrogen pressure of 30 atm, and

the dehydriding kinetics, under a hydrogen pressure of

0.1 atm, were studied by Sievert’s method at 250 �C and 300 �Cusing the AMC gas reactor controller.

3. Results and discussion

XRD patterns of the as-milled samples are shown in Fig. 1. The

main peaks match b-MgH2 (JCPDS-12-0697), but there are

small peaks corresponding to g-MgH2 (JCPDS 351184), the BCC

alloy, MgO, and even Mg. The presence of the orthorhombic

g-phase is a result of alteration in the microstructure because

of the ball milling [22]. Despite the low quantity of the BCC

alloy in the composite, this phase could be observed as is in

agreement with Reference [13]. However, there are no peaks

which indicate formation of any new alloy phase as a result of

reaction between MgH2 and the BCC alloy. The existence of

the weak Mg peak indicates that the commercial magnesium

hydride was partially dehydrogenated during ball milling.

Diffraction peaks of the composites are considerably broad-

ened as a consequence of reduction of particle size, as well as

the increase in defects and the mechanical strains created

within the lattice by the ball milling [14].

Fig. 2 shows SEM images of all the as-prepared samples. It

can be explicitly seen that the particle size of the samples

decreases in the following order: pure MgH2 (Fig. 2(a)), MgH2-

BCC (Fig. 2(b)), and MgH2-BCC-CNT (Fig. 2(c)). The particle size

distribution of the pure MgH2 sample is concentrated in the

range of 10 mme30 mm, but from 2 mm to 20 mm for the MgH2-

BCC and from a few hundred nanometres to 5 mm for the

MgH2-BCC-CNT.

In addition, agglomeration is lower for the samples with

CNTs than in the samples without them, which is because of

the lubricant effect of the carbon as reported in the literature

[23,24]. To monitor the dispersion of the BCC alloy particles,

EDSwas conducted, and a backscattered electron composition

(BEC) image of the Mg-BCC sample with the EDS element

analysis (inset) is shown in Fig. 3. The larger gray particles

(between 5 mm and 30 mm) are MgH2, and the white particles,

with sizes of several hundred nanometres, are the BCC alloy. It

can be clearly observed that the BCC alloy particles are

distributed uniformly among the MgH2 particles. Although by

BEC it is possible to see only the surface of the powders, it can

be hypothesized that the BCC phase is distributed both on the

surface of the Mg hydride grains and in the interior of the

particles. Therefore, based on the morphology, it is predict-

able that the hydrogen sorption properties of the samples

with the BCC alloy and CNTs will be improved as a result of

a clearly smaller particle size, less agglomeration, and

homogeneous dispersion of the catalysts among the MgH2.

Fig. 4 shows the hydrogen capacity and both Tonset and

Tdes for the as-milled samples, which were measured by

hydrogen release with the gas reactor controller in profile

mode. Before the measurement, the sample holder was

evacuated at room temperature for 30 min. Based on the

graphs, Tdes for both doped sample was reduced significantly.

The onset temperature of hydrogen release, Tonset, for the

pure sample is 350 �C, while for the MgH2-BCC and MgH2-

BCC-CNT samples, Tonset is 280 �C and 245 �C, respectively.Moreover Tdes for the pure MgH2 is 430 �C, but it is 361 �C for

the binary sample and 301 �C for the ternary one. Therefore,

Tdes was reduced by 125 �C and 66 �C for the MgH2-BCC-CNT

and MgH2-BCC samples, respectively, in comparison with the

pure MgH2. In addition the differences between Tdes and

Tonset show that hydrogen release from the sample including

CNTs was much easier. Moreover, based on the figure, the

hydrogen contents for all samples are almost the same and

approximately 6 wt.%. It was expected that the gravimetric

capacity would decrease in the order of pure MgH2,

25 30 35 40 45 50 55 60 65 70 75 802θ (degree)

In

ten

sity (a. u

.)

MgH2-BCC

MgH2-BCC-CNT

MgH2

&

&

##

# #

Δ Δ× × O

# β-MgH2

× γ-MgH2

& BCC

O MgO

Δ Mg

Fig. 1 e XRD patterns for as-milled samples.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 7 8 2 1e7 8 2 6 7823

MgH2 þ BCC, and MgH2þBCC þ CNT because the BCC phase is

a hydrogen active phase with mass higher than that of

magnesium hydride and a lower gravimetric capacity, and

CNTs have not been demonstrated to absorb/adsorb

hydrogen under these experimental conditions. Therefore,

the common value of the hydrogen capacity of these samples

indicates that the hydrogen sorption efficiency is increased in

the presence of the additives.

Fig. 2 e SEM images of all samples: a) pu

After dehydrogenation under profile mode and for each

sample, the temperature of the sample holder was set at

300 �C (and then 250 �C), and the sample chamber was

evacuated for 1 h to remove any remaining hydrogen des-

orbed by the sample and to fix the temperature. Then, the

hydriding/dehydriding kinetics were studied at these

temperatures at a hydrogen pressure of 30 atm and 0.1 atm,

respectively.

re MgH2, b) Mg-BCC, c) Mg-BCC-CNT.

Fig. 3 e Backscattered electron composition (BEC) image of

the Mg-BCC sample, with its EDS element analysis shown

in the inset.

0

1

2

3

4

5

6a

b

0 500 1000 1500 2000 2500Time (s)

%)

tw

(t

ne

tn

oc

-H

Mg-BCC-CNTMg-BCCMg

-6

-5

-4

-3

-2

-1

0

0 50 100 150 200 250 300 350 400 450 500 550 600Time (s)

)%

tw

(t

ne

tn

oc

-H

MgH2-BCC-CNTMgH2-BCCMgH2

Fig. 5 e a) Hydrogen absorption at 300 �C and 30 atm, and b)

desorption at 300 �C and 0.1 atm for all samples.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 7 8 2 1e7 8 2 67824

The absorption kinetics results at 300 �C (shown in Fig. 5(a))

indicate that both doped samples absorbed hydrogen more

rapidly than the pure one and that MgH2-BCC-CNT has the

fastest absorption rate. The maximum hydrogen absorbed

(Hmax) by pure MgH2 at this temperature and after 2500 s is

3.46 wt.%. This quantity for binary MgH2-BCC is 5.7 wt.% and

6 wt.% for ternary MgH2-BCC-CNT. MgH2-BCC-CNT soaks up

90% of itsHmax in 150 s while the times for MgH2-BCC and pure

MgH2 are 195 s and 1680 s, respectively.

The desorption kinetic curves at 300 �C (Fig. 5(b)) illustrate

that 90% ofHmax release occurs at 500 s, 179 s, and 93 s for pure

MgH2, MgH2-BCC, and MgH2-BCCeCNT, respectively. Fig. 6(a)

and (b) presents the corresponding absorption/desorption

kinetics at 250 �C.Hmax, at this temperature and after 2500 s, is

2.8 wt.% for pure MgH2, and 4.7 wt.% and 5.6 wt.% for MgH2-

BCC and MgH2-BCC-CNT, respectively. At this temperature,

the pure sample has absorbed 90% of its Hmax at 1932 s, while

this time for the binary sample is 823 s and 516 s for the

ternary one.

As both 300 �C and 250 �C are lower than Tdes of pure MgH2,

the kinetics of this phase is very sluggish at these tempera-

tures, although its kinetics are improved significantly as

a result of the addition of the BCC and BCC-CNT alloys.

-7

-6

-5

-4

-3

-2

-1

0

1

200 250 300 350 400 450

Temperature (°C)

)%

tw

(t

ne

tn

oC

-H

MgH2MgH2-BCC-CNTMgH2-BCC

245°C

280°C

346°C

301°C

361°C430°C

Fig. 4 e Initial hydrogen release of all as-prepared samples

in profile release mode.

Fig. 7 shows the XRD patterns for the dehydrogenated

samples. It can be observed that the major phase after dehy-

drogenation is magnesium, however, there are peaks corre-

sponding to the BCC alloy and MgO which overlap in the

MgH2-BCC pattern. Although all the sample handling was

done in an argon filled glove box, oxidation due to periodic

opening of the milling jar and the AMC sample holder in the

glove box could be the reason for the appearance of MgO

phase. The peaks became sharper for the dehydrided samples

in comparison with the as-milled samples, which indicates

a reduction in defects and mechanical strain through the

hydriding/dehydriding processes. In addition, there is a slight

shift in the peaks (as is shown in the Fig. 7 inset for the most

intense peak), indicating extra enlargement in the lattice

parameters of the unit cells for both MgH2-BCC and MgH2-

BCC-CNT composites. This enlargement could be due to the

formation of a solid solution between themagnesium hydride

and the additives.

The results demonstrate that 1) the BCC alloy improves the

hydriding/dehydryding kinetics and maximum hydrogen

capacity, 2) theMg-BCC-CNT system shows superior hydrogen

storage properties, and so 3) the BCC alloy and CNTs have

synergistic effects on all aspects of hydrogen sorption prop-

erties. On the other hand, the hydriding and dehydriding

mechanisms could be the keys to insight into the reasons for

these special effects. It is accepted both theoretically [25] and

experimentally [19,26] that: 1) dissociation of H2 molecules

into H-atoms on the surface of the magnesium, 2) diffusion of

H-atoms along the grain boundaries, and 3) hydrogen

absorption/desorption of the catalyst and transformation of

0

1

2

3

4

5

6a

b

0 500 1000 1500 2000 2500Time (s)

)%

tw

(t

ne

tn

oc

-H

MgH2-BCC-CNT

MgH2

MgH2-BCC

-6.5

-5.5

-4.5

-3.5

-2.5

-1.5

-0.5

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Time (s)

)%

tw

(t

ne

tn

oc

-H

MgH2

MgH2-BCC-CNT

MgH2-BCC

Fig. 6 e a) Hydrogen absorption at 250 �C and 30 atm, and b)

desorption at 250 �C and 0.1 atm for all samples.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 7 8 2 1e7 8 2 6 7825

Mg atoms into MgH2 molecules at catalyst/Mg interfaces, are

the three critical steps of the hydrogenation process. The

energy barrier for H2 dissociation without any substrate is

4.52 eV, and this value reduces to 1.15 eV in the presence of

magnesium [27], which still requires more than 400 �C for

dissociation. However, this energy barrier will decrease even

more in the presence of a catalyst. For example, it is 0.201 eV

and 0.103 eV for V and Ti, respectively [28]. Although further

investigation seems to be necessary to find the exact energy

25 30 35 40 45 50

2 (de

In

ten

sit

y (

a. u

.) O

&

&

Fig. 7 e XRD patterns for the samples after dehydrogenation. The

highest intensity.

barrier for H2 dissociation on the BCC alloy’s surface, as it is

a composite of transitionmetals, the barrier could be very low.

Therefore, the main catalytic effect of the BBC could be

dissociation of hydrogen molecules at low temperature and

improvement of the first step of the hydrogenation

mechanism.

In addition, as described in our previous work [17] and

other references [13,29,30], the BCC alloy can absorb the H-

atoms at low temperature to form BCC hydride by the

following reaction:

ðBCCÞ þ x2H20ðBCCÞHx (1)

with the BCC hydride then releasing hydrogen at the Mg/BCC

interfaces. Therefore, the second effect of the BCC alloy is to

work as an atomic hydrogen bridge which facilitates the

bridge which facilitates the pumping of H atoms into the

Mg/BCC interfaces and so, improves the 3rd step of the

hydrogenation process.

On the other hand, because the outer regions of the Mg

particles and their grain boundaries are completely hydro-

genated, the centres of grains may be unreachable for

hydrogen atoms. After a 2 h ball milling, CNTs retain their

nanotube structure, and they aggregate along the grain

boundaries of magnesium particles [28], so they may facili-

tate access of H-atoms to the magnesium grains, which

increases both the absorption kinetics and the hydrogen

storage capacity. Therefore, for the MgH2-BCC-CNT sample,

the effect of the CNTs is enhancement of the 2nd step by

improvement of the diffusion path of H-atoms along the

grain boundaries.

The desorption mechanism is the reverse process to the

absorption. The H-atoms interact with the BCC alloy to form

“BCC hydride” on the magnesium particle surfaces. In addi-

tion, CNTs (for the MgH2-BCC-CNT sample) facilitate the

emergence of hydrogen from the grain boundaries. Finally,

the BCC alloy recombines the H-atoms into hydrogen mole-

cules, with a resulting improvement in the dehydriding

kinetics and desorption temperature.

55 60 65 70 75 80

gree)

MgH2-BCC-CNT

MgH2-BCC

MgH2

Mg

MgO

*

O

& BCC

enlarged area in the inset shows the shift of the peak with

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 7 8 2 1e7 8 2 67826

4. Conclusion

Acomparisonbetween theeffects of theadditionof 10wt.%BCC

(Ti0.4Mn0.22Cr0.1V0.28) alloyand10wt.%BCCþ5wt.%CNTsonthe

hydrogen storage properties of MgH2 were investigated. The

results showed that homogeneously distributed catalysts

significantly improve the desorption temperature, the

maximum absorbed hydrogen and the hydriding/dehydriding

kinetics. The results showed that these additives have different

catalytic effects on the hydrogenation/dehydrogenation

processesofmagnesiumhydride. Theeffectsof theBCCalloyon

the hydrogenation could be reduction of the barrier energy for

dissociation of hydrogen molecules into hydrogen atoms and

also promotion of hydrogen pumping into the Mg/BCC inter-

faces, while CNTs (for the MgH2-BCC-CNT sample) facilitate

access of H-atoms to the interior of Mg grains. Furthermore,

during dehydrogenation, the “BCC hydride” forms on the Mg/

BCC interfaces, and then the BCC alloy releases H-atoms more

rapidly thanMg,while it also facilitates the recombination ofH-

atoms into H2 molecules. CNTs facilitate the emergence of

hydrogen from the grain boundaries during desorption.

Acknowledgments

This work was financially supported by the Australian

Research Council (ARC) through Discovery project grant

DP0771193. The authors would like to thank Dr. Tania Silver

and Dr. Darren Attard for their great helps.

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