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The hydrogen storage properties and reaction mechanismof the MgH2eNaAlH4 composite system

M. Ismail a,b, Y. Zhao a,c,*, X.B. Yu a,d,*, J.F. Mao a, S.X. Dou a

a Institute for Superconducting and Electronic Materials, University of Wollongong, Northfields Ave, Wollongong, NSW 2519, AustraliabDepartment of Physical Sciences, Faculty of Science and Technology, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, MalaysiacSchool of Mechanical, Materials and 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 14 February 2011

Received in revised form

5 April 2011

Accepted 17 April 2011

Available online 18 May 2011

Keywords:

MgH2

NaAlH4

Absorption/desorption

Perovskite-type hydride

* Corresponding authors. Institute for SupercNSW 2519, Australia. Fax: þ61 2 4221 5731.

E-mail addresses: yue@uow.edu.au (Y. Zh0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.04.132

a b s t r a c t

In this study, we report the hydrogen absorption/desorption properties and reaction

mechanism of the MgH2eNaAlH4 (4:1) composite system. This composite system showed

improved dehydrogenation performance compared with that of as-milled NaAlH4 and

MgH2 alone. The dehydrogenation process in the MgH2eNaAlH4 composite can be divided

into four stages: NaAlH4 is first reacted with MgH2 to form a perovskite-type hydride,

NaMgH3 and Al. In the second dehydrogenation stage, the Al phase reacts with MgH2 to

form Mg17Al12 phase accompanied with the self-decomposition of the excessive MgH2.

NaMgH3 goes on to decompose to NaH during the third dehydrogenation stage, and the last

stage is the decomposition of NaH. Kissinger analysis indicated that the apparent activa-

tion energy, EA, for the MgH2-relevent decomposition in MgH2eNaAlH4 composite was

148 kJ/mol, which is 20 kJ/mol less than for as-milled MgH2 (168 kJ/mol). X-ray diffraction

patterns indicate that the second, third, and fourth stages are fully reversible. It is believed

that the formation of Al12Mg17 phase during the dehydrogenation process alters the

reaction pathway of the MgH2eNaAlH4 (4:1) composite system and improves its thermo-

dynamic properties.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction [1e6]. However, the high thermodynamic stability and slow

Because it is a promising energy carrier, intensive efforts have

been made to realize the potential of hydrogen to become

a major energy carrier, for both mobile and stationary appli-

cations. Solid-state hydrogen storage has become an attrac-

tive option due to its high volumetric hydrogen capacity and

favorable safety considerations. Its large hydrogen storage

capacity (7.6 wt.%), low cost, and superior reversibility make

MgH2 a promising candidate for on-board hydrogen storage

onducting and Electronic

ao), yuxuebin@fudan.edu2011, Hydrogen Energy P

desorption kinetics [7,8] constrain the practical application of

MgH2. Many studies have been conducted to overcome these

problems, such as by preparing nanocrystalline Mg powder [7]

and adding catalysts [8e10]. Besides the studies mentioned

above, the “destabilization concept” was also introduced

[11,12]. The destabilization concept could be regarded as

a quite different approach. This approach is aimed at modi-

fying the thermodynamics and kinetics of the hydrogen

sorption reaction. Thermodynamic destabilization is achieved

Materials, University of Wollongong, Northfields Ave, Wollongong,

.cn (X.B. Yu).ublications, LLC. Published 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 6 ( 2 0 1 1 ) 9 0 4 5e9 0 5 09046

using additives that form a new compound during dehydro-

genation. Vajo et al. [13] tried to use certain elements such as

Si to destabilize MgH2. The formation of an intermediate

phase, Mg2Si, during the dehydrogenation process is benefi-

cial to MgH2 destabilization. However, the formation of Mg2Si

would reduce the gravimetric hydrogen density because Si

cannot be hydrogenated, so this intermediate phase seems

hard tomake reversible. Motivated by the studies of Vajo et al.,

many elements have been mixed with MgH2, including LiBH4

[14e18], NaBH4 [19e21], LiAlH4 [22e25], and Li3AlH6 [26].

Recently Zhang et al. [22] reported that MgH2 can be

destabilized effectively by LiAlH4 in which the onset dehy-

drogenation temperature and the enthalpies of MgH2-relevent

decomposition in MgH2eLiAlH4 composite is improved from

that of as-milled MgH2. In addition, Chen et al. [24] also

demonstrated that the combination of MgH2 and LiAlH4 by

mechanical grinding reduced the decomposition temperature

of MgH2. Both articles claimed that the formation of Al12Mg17as a reaction product during the dehydrogenation process

play a main catalytic effect in the MgH2eLiAlH4 composite

systems.

Another potential element to destabilize MgH2 is NaAlH4.

Varin et al. [27] studied the morphology and the reaction

mechanism in an MgH2 þ 73 vol.% NaAlH4 composite system

by scanning electron microscopy (SEM) and differential

scanning calorimetry. Sartori et al. [28] further reported the

reaction mechanism and hydrogen released from

NaAlH4 þ MgH2 þ Al prepared by hydrogenating a mixture of

2MgH2 þ 3Al þNaH under 160 bar and 80 �C for 12 h. However,

the hydrogen storage performances of an MgH2eNaAlH4

composite system have not been thoroughly investigated so

far. It is also necessary to verify whether the role of NaAlH4 is

similar to the role of LiAlH4 in the MgH2eLiAlH4 composite

system, since NaAlH4 and LiAlH4 are from the same alanate

family.

In the present paper, we investigated the hydrogen storage

properties and the reaction pathways of the MgH2eNaAlH4

(4:1) composite systemduring the de/rehydrogenation process

through temperature programmed desorption (TPD), Ther-

mogravimetric analysis/differential scanning calorimetry

(TGA/DSC), isothermal sorption measurements and X-ray

diffraction (XRD) measurements. The reaction mechanism of

MgH2eNaAlH4 (4:1) composite was discussed based on our

experimental results.

2. Experimental

Pure MgH2 (hydrogen storage grade) and NaAlH4 (hydrogen

storage grade, 93% purity) were purchased fromSigmaeAldrich

and were used as received with no further purification.

Approximately 2 g of a mixture of MgH2 and NaAlH4 with

a molar ratio of 4:1 was loaded into a sealed stainless steel vial

together with hardened stainless steel balls in an argon atmo-

sphere MBraun Unilab glove box. For simplicity, the mixture of

MgH2 and NaAlH4 with molar ratio of 4:1 will be referred to as

MgH2eNaAlH4 composite. The ratio of theweight of balls to the

weight of powder was 40:1. The sample was then milled in

a planetary ball mill (QM-3SP2) for 1 h, by first milling for 0.5 h,

resting for 6min, and thenmilling for another 0.5h inadifferent

direction at the rate of 400 rpm. Pure MgH2 and NaAlH4 were

also prepared under the same conditions for comparison

purposes.

The experiments for de/rehydrogenation were performed

in a Sieverts-type pressure-composition-temperature (PCT)

apparatus (Advanced Materials Corporation). The sample was

loaded into a sample vessel in the glove box. For the TPD

experiment, all the samples were heated in a vacuum

chamber, and the amount of desorbed hydrogen was

measured to determine the lowest decomposition tempera-

ture. The heating rate for the TPD experiment was 5 �Cmin�1,

and samples were heated from room temperature to 450 �C.The re/dehydrogenation kinetics measurements were per-

formed at the desired temperature with initial hydrogen

pressures of 3.0 MPa and 0.001 MPa, respectively.

XRD analysis was performed using a GBC MMA X-ray

diffractometer with Cu Ka radiation. qe2q scans were carried

out over diffraction angles from 25� to 80� with a speed of

2.00 �/min. Before the measurement, a small amount of

sample was spread uniformly on the sample holder, which

was wrapped with plastic wrap to prevent oxidation.

Thermogravimetric analysis/differential scanning calo-

rimetry (TGA/DSC) of the dehydrogenation process was

carried out on a Mettler Toledo TGA/DSC 1. The sample was

loaded into an alumina crucible in the glove box. The crucible

was then placed in a sealed glass bottle in order to prevent

oxidation during transportation from the glove box to the

TGA/DSC apparatus. An empty alumina crucible was used for

reference. The samples were heated from room temperature

to 500 �C under an argon flow of 30 ml/min, and different

heating rates were used.

3. Results and discussion

Fig. 1 displays the TPD curves of the as-milled MgH2, the as-

milled NaAlH4, and the MgH2eNaAlH4 composite. The as-

milled MgH2 starts to release hydrogen at 330 �C and desorbs

about 7.1 wt.% H2 after 420 �C. Meanwhile, the as-milled

NaAlH4 releases about 5.5 wt.% hydrogen in the dehydroge-

nation temperature range between 180 �C and 285 �C. For theMgH2eNaAlH4 composite, it is believed that there are four

stages of dehydrogenation occurring during the heating

process. The four stages of dehydrogenation process are re-

ported by Sartori et al. [28] for the dehydrogenation process of

hydrogenating 2MgH2 þ 3Al þNaH under 160 bar and 80 �C for

12 h. The first stage, which takes place within the temperature

range from 170 to 212 �C, is attributed to the reaction:

NaAlH4 þMgH2/NaMgH3 þAlþ 1:5H2 (1)

In order to clarify the dehydrogenation mechanism, XRD

was used. From the Fig. 2(a), it can be seen that, MgH2 and

NaAlH4 phases are detected in the as-milled MgH2eNaAlH4

composite. Fig. 2(b) shows the XRD pattern of the

MgH2eNaAlH4 composite after dehydrogenation at 212 �C.

Clearly, the pattern indicates the presence of perovskite-type

hydride, NaMgH3 and Al besides the unreacted MgH2. The fact

that no NaAlH4 phase was found indicates that the Eq. (1)

reaction was completed at this stage. From the result we can

Fig. 1 e TPD (temperature programmed desorption) curves

of the as-milled MgH2, the as-milled NaAlH4, and the

MgH2eNaAlH4 composite. (I, II, III, and IV refer to the first,

second, third, and fourth dehydrogenation stage,

respectively).

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 6 ( 2 0 1 1 ) 9 0 4 5e9 0 5 0 9047

see that the decomposition temperature of NaAlH4 in the

MgH2eNaAlH4 composite reduced by 10 �C compared with

that of as-milled NaAlH4 indicating that the dehydrogenation

performance of NaAlH4 is slightly improved after combining

with MgH2.

The second dehydrogenation stage, starting at about 280 �Cand completed at about 330 �C, can be attributed to the MgH2

or NaMgH3-relevent decomposition. In order to investigate the

reaction progress and mechanism, XRD measurements were

performed on the MgH2eNaAlH4 composite after dehydroge-

nation at 330 �C. As can be seen from the XRD result (Fig. 2(c)),

the peaks for MgH2 and Al disappear, and new peaks corre-

sponding to Mg17Al12 and Mg are observed. Furthermore, the

peaks for NaMgH3 still remain unchanged. These results

demonstrate that the hydrogen released in the second stage is

Fig. 2 e XRD patterns of the MgH2eNaAlH4 composite (a)

after 1 h ball milling and after dehydrogenation at (b)

212 �C, (c) 330 �C, (d) 360 �C, and (e) 375 �C.

from a mixing decomposition of (i) the reaction of MgH2 with

Al and (ii) the decomposition of the excessive MgH2. This kind

of MgH2-relevent decomposition can be speculated to be

through the reaction in Eqs. (2) and (3), which occurs at

a temperature 50 �C lower than the decomposition tempera-

ture of the as-milled MgH2.

17MgH2 þ 12Al/Mg17Al12 þ 17H2 (2)

MgH2/Mg þH2 (3)

Further heating led to the third decomposition, starting at

330 �C and completed at 360 �C, corresponding to the decom-

position of NaMgH3, as shown in Eq. (4). The last stage dehy-

drogenation is due to the decomposition of NaH, with

decomposition starting at 360 �C and ending at 375 �C (Eq. (5)).

XRD measurements at 360 and 375 �C were carried out to

clarify these dehydrogenation stages. As shown in Fig. 2(d)

and (e), the peaks of NaH appear after dehydrogenation at

360 �C, and the peaks of Na can be detected after further

heating to 375 �C. These results indicated that the reactions of

Eqs. (4) and (5) occurred in the third and fourth stages,

respectively.

NaMgH3/NaHþMg þH2 (4)

NaH/Naþ 1=2H2 (5)

Fig. 3 compares the isothermal dehydrogenation kinetics of

MgH2 þNaMgH3 in the MgH2eNaAlH4 composite at 300 �C and

320 �C. The dehydrogenation of MgH2 was also examined for

comparison under the same conditions. The dehydrogenation

kinetics test was carried out after a rehydrogenation process

under w3 MPa of H2 at 300 �C. At 320 �C, the MgH2eNaAlH4

sample desorbed about 3.5 wt.% hydrogen after 12 min, which

is much higher than for the MgH2 (desorbed about 0.4 wt.%

hydrogen). Meanwhile, after 12 min dehydrogenation, the

MgH2eNaAlH4 sample released about 1.2 wt.% hydrogen at

300 �C, but the MgH2 only released about 0.2 wt.% hydrogen at

the same time and temperature. It is important to note that

the dehydrogenation kinetics of the MgH2eNaAlH4 sample at

Fig. 3 e Isothermal dehydrogenation kinetics of the MgH2

and the MgH2eNaAlH4 composite at 300 �C and 320 �C.

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 6 ( 2 0 1 1 ) 9 0 4 5e9 0 5 09048

300 �C ismuch faster than the dehydrogenation kinetics of the

MgH2 at 320 �C. This result indicates that the MgH2-relevant

dehydrogenation kinetics is significantly improved after

combining with NaAlH4.

Fig. 4 presents the TGA/DSC traces of the MgH2eNaAlH4

composite and the as-milled MgH2, which is also shown for

comparison (inset plot). For the MgH2eNaAlH4 composite,

the first endothermic peak at 185 �C corresponds to the

melting of NaAlH4 [29], and the second endothermic peak at

275 �C corresponds to the reaction of NaAlH4 and MgH2 (first

stage dehydrogenation). Note that, before the first endo-

thermic peak, there is an extra exothermic peak. This

exothermic peak at 170 �C is most probably due to the pres-

ence of surface hydroxyl impurities in the powder as re-

ported in our previous paper [30,31] for LiAlH4. Furthermore,

after the second endothermic (first stage dehydrogenation),

three overlapping endothermic reactions occurred due to the

decomposition of MgH2, NaMgH3 and NaH. The first over-

lapping endothermic peak at 363 �C can be ascribed to MgH2-

relevent decomposition (second stage dehydrogenation), and

the rest are due to the decomposition of NaMgH3 and NaH

(third and fourth stage dehydrogenation). Ikeda et al. [32]

reported that there are two overlapping endothermic reac-

tions accompanying with continuous weight-loss in the

decomposition of NaMgH3 and NaH. Compared to the as-

milled MgH2 (inset plot), the temperature of the MgH2-rele-

vent decomposition in the MgH2eNaAlH4 composite is

clearly decreased. It starts to release hydrogen at about

315 �C, 50 �C below the temperature for the as-milled MgH2

(365 �C). This decrease in the hydrogen release temperature is

correlated to the results observed in the PCT measurement

(Fig. 1). From the TGA/DSC results, it can be seen that the

decomposition temperatures of the as-milled MgH2 and the

MgH2eNaAlH4 composite are higher than those from the

Sieverts-type PCT results (Fig. 1). This difference may be due

Fig. 4 e TGA/DSC traces of the MgH2eNaAlH4 composite

and the inset plot is TGA/DSC traces of the as-milled MgH2.

Heating rate: 15 �C minL1, argon flow: 30 ml/min. (I, II, III,

and IV refer to the first, second, third, and fourth

dehydrogenation stages).

to the different heating rates and the different heating

atmospheres between the two measurements.

The activation energy, EA, for the hydrogen desorption

mechanism of the MgH2eNaAlH4 composite was obtained by

performing a Kissinger analysis [33], according to the

following equation:

lnhb=T2

p

i¼ �EA=RTp þ A (6)

where b is the heating rate, Tp is the peak temperature in the

DSC curve, R is the gas constant, and A is a linear constant.

Thus, the activation energy, EA, can be obtained from the slope

in a plot of ln[b/Tp2] versus 1000/Tp.

Fig. 5 shows DSC traces for the MgH2eNaAlH4 composite at

different heating rates (b ¼ 10, 15, and 20 �C min�1, respec-

tively). All the peaks in the MgH2eNaAlH4 composite were

shifted when the heating rates increased. The inset of Fig. 5

shows a Kissinger’s plot of the dehydrogenation reaction for

the MgH2eNaAlH4 composite as compared with the as-milled

MgH2. (The plot refers to the MgH2-relevent decomposition,

the first of the overlapping endothermic peaks). The apparent

activation energy, EA, estimated from the Kissinger analysis

for the MgH2-relevent decomposition in MgH2eNaAlH4

composite is found to be 148 kJ/mol, which is lower than for

the as-milled MgH2 (168 kJ/mol). This indicates that mixing

with NaAlH4 reduced the activation energy and thus improved

the dehydrogenation behavior of MgH2.

To investigate the reversibility of MgH2eNaAlH4

composite, the rehydrogenation of the dehydrogenated

sample was performed under w3 MPa of H2 at 300 �C. TheMgH2 system was also examined for comparison. Fig. 6

compares the isothermal rehydrogenation kinetics of the

MgH2eNaAlH4 composite and the MgH2. From the results, the

MgH2eNaAlH4 sample shows slow absorption kinetics

compared with the MgH2. After 30 min, the MgH2eNaAlH4

sample has just absorbed 5.1 wt.% hydrogen relative to the

MgH2 sample, which can absorb as much as 6.1 wt.%

hydrogen.

Fig. 5 e DSC traces of MgH2eNaAlH4 composite at different

heating rates. The inset plot is the Kissinger’s analysis for

(a) as-milled MgH2 and (b) MgH2eNaAlH4 composite.

Fig. 6 e Isothermal rehydrogenation kinetics of the

MgH2eNaAlH4 composite and the MgH2 samples at 300 �Cand under 3 MPa.

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 6 ( 2 0 1 1 ) 9 0 4 5e9 0 5 0 9049

Fig. 7 displays the XRD patterns of the MgH2eNaAlH4

composite after rehydrogenation under w3 MPa of H2 at

300 �C. From the patterns, it can be seen that the peaks

correspond to MgH2, NaMgH3, Al and a small peak for Al3Mg2that appears after rehydrogenation. No NaAlH4 peak was

detected. The NaMgH3 phase confirms that reactions (4) and

(5) are reversible, as reported by Ikeda et al. [32]. After rehy-

drogenation, the Mg17Al12 peaks disappear, and peaks of

MgH2, Al, and small peaks of Al3Mg2 appear, indicating that

Mg17Al12 can be dissociated into Mg2Al3, MgH2, and Al (Eq. (7))

as reported by Yabe and Kuji [34], and Zhang and Wu [35].

Mg17Al12 þ ð17� 2yÞH2/yMg2Al3 þ ð17� 2yÞMgH2

þ ð12� 3yÞAl (7)

If higher hydrogenation pressure were applied, Mg2Al3would subsequently be transformed into MgH2 and Al, as

shown in Eq. (8) [36]:

Fig. 7 e XRD patterns of the MgH2eNaAlH4 composite after

rehydrogenation at 300 �C and under 3 MPa.

Mg2Al3 þ 2H2/2MgH2 þ 3Al (8)

In this study, Mg2Al3 was hardly to transform intoMgH2 and

Al due to the lower hydrogenation pressure applied (3 MPa).

This phenomenon is similar to that reported by Zhang et al.

[22] for MgH2eLiAlH4 (4:1) composite, in which their sample

was rehydrogenated under 4 MPa. For comparison, Chen et al.

[24] reported that no Mg2Al3 phase was detected from their

MgH2eLiAlH4 (4:1) composite sample after rehydrogenation at

350 �C and under 10 MPa, implying that reaction (8) occurred

when the higher hydrogen pressure was applied. The slow

absorption kinetics in the MgH2eNaAlH4 composite may due

to the yield of Mg2Al3.

From the results, it can be pointed out that the role of

NaAlH4 in this study is quite similar to the role of LiAlH4 in the

MgH2eLiAlH4 composite system. The formation of Mg17Al12phase as a reaction product of MgH2 with Al during the

dehydrogenation process may create a beneficial pathway for

hydrogen atom diffusion to the surface and recombination,

while it improves the thermodynamic properties of the

MgH2eNaAlH4 composite system, as reported by Zhang et al.

[22] and Chen et al. [24] for the MgH2eLiAlH4 (4:1) composite.

Besides Mg17Al12 phase, Zhang et al. [22] also claimed that

a catalytic effectmay also be contributed by Li0.92Mg4.08 phase,

which is formed as a reaction product of MgH2 with LiH during

the dehydrogenation process in the MgH2eLiAlH4 composite

system. In our study, the reaction of MgH2 with NaAlH4

formed a perovskite-type hydride, NaMgH3, and this product

may also play a catalytic role in enhancement of the dehy-

drogenation behavior of MgH2eNaAlH4 composite. This

hypothesis is still under investigation and will be presented in

a further report.

4. Conclusion

In summary, we have demonstrated that the hydrogen

desorption properties of MgH2 were improved after mixing

with NaAlH4. From the TPD results, theMgH2 in the composite

starts to release hydrogen at about 280 �C, compared with

330 �C for as-milled MgH2. This composite also showed strong

improvement in the dehydrogenation kinetics. The TPD

results show that the dehydrogenation process in the

MgH2eNaAlH4 (4:1) composite can be divided into four stages:

the first dehydrogenation stage can be attributed to the reac-

tion of NaAlH4 and MgH2. The second dehydrogenation stage

is due to the MgH2 react with Al accompanied with the self-

decomposition of excessive MgH2; the third dehydrogenation

stage can be ascribed to the decomposition of NaMgH3; and

the last dehydrogenation stage is due to the decomposition of

NaH. The Kissinger plots for different heating rates in DSC

show that the apparent activation energy, EA, for decompo-

sition of MgH2-relevent in theMgH2eNaAlH4 (4:1) composite is

reduced to 148 kJ/mol compared with 168 kJ/mol for the as-

milled MgH2. We believe that the formation of Mg17Al12phase during the dehydrogenation process plays a critical role

in the enhancement of dehydrogenation in MgH2eNaAlH4

composite. However, the rehydrogenation measurements

revealed that the MgH2eNaAlH4 (4:1) composite did not show

any improvement compared with the as-milled MgH2.

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 6 ( 2 0 1 1 ) 9 0 4 5e9 0 5 09050

Acknowledgments

The authors thank the University of Wollongong for financial

support of this research. M. Ismail acknowledges the Ministry

of Higher Education Malaysia for a PhD scholarship. Many

thanks also go to Dr. T. Silver for critical reading of the

manuscript.

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