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Improved hydrogen storage performance of MgH2eNaAlH4

composite by addition of TiF3

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

a Institute for Superconducting and Electronic Materials, University of Wollongong, 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 17 November 2011

Received in revised form

4 February 2012

Accepted 20 February 2012

Available online 17 March 2012

Keywords:

MgH2

NaAlH4

TiF3Catalytic effect

* Corresponding author. Department of PhysKuala Terengganu, Malaysia.** Corresponding author. Institute for SuperAustralia. Fax: þ61 2 4221 5731.

E-mail addresses: mohammadismail@um0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2012.02.117

a b s t r a c t

In a previous paper, it was demonstrated that a MgH2eNaAlH4 composite system had

improved dehydrogenation performance compared with as-milled pure NaAlH4 and pure

MgH2 alone. The purpose of the present study was to investigate the hydrogen storage

properties of theMgH2eNaAlH4 composite in the presence of TiF3. 10 wt.% TiF3 was added to

the MgH2eNaAlH4 mixture, and its catalytic effects were investigated. The reaction mech-

anism and the hydrogen storage properties were studied by X-ray diffraction, thermogra-

vimetric analysis, differential scanning calorimetry (DSC), temperature-programmed-

desorption and isothermal sorption measurements. The DSC results show that

MgH2eNaAlH4 composite milled with 10 wt.% TiF3 had lower dehydrogenation tempera-

tures, by 100, 73, 30, and 25 �C, respectively, for each step in the four-step dehydrogenation

process compared to the neat MgH2eNaAlH4 composite. Kinetic desorption results show

that the MgH2eNaAlH4eTiF3 composite released about 2.4 wt.% hydrogen within 10 min at

300 �C, while the neat MgH2eNaAlH4 sample only released less than 1.0 wt.% hydrogen

under the same conditions. From the Kissinger plot, the apparent activation energy, EA, for

the decomposition of MgH2, NaMgH3, and NaH in the MgH2eNaAlH4eTiF3 composite was

reduced to 71, 104, and 124 kJ/mol, respectively, compared with 148, 142, and 138 kJ/mol in

the neat MgH2eNaAlH4 composite. The high catalytic activity of TiF3 is associated with in

situ formation of a microcrystalline intermetallic TieAl phase from TiF3 and NaAlH4 during

ball milling or the dehydrogenation process. Once formed, the TieAl phase acts as a real

catalyst in the MgH2eNaAlH4eTiF3 composite system.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction hydrogen and liquid hydrogen storage, particularly in terms

Solid state hydrogen storage offers several benefits over

other means of storing hydrogen such as compressed

ical Sciences, Faculty of

conducting and Electroni

t.edu.my (M. Ismail), yue2012, Hydrogen Energy P

of safety, cost, and high volumetric and gravimetric densities

[1,2]. Among the solid state hydrogen storage materials,

reversible metal hydrides, especially MgH2, show great

Science and Technology, Universiti Malaysia Terengganu, 21030

c Materials, University of Wollongong, Wollongong, NSW 2519,

@uow.edu.au (Y. Zhao).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 7 ( 2 0 1 2 ) 8 3 9 5e8 4 0 18396

potential as suitable hydrogen storage materials due to high

hydrogen storage capacities (7.6 wt.% for MgH2) and low cost

[3,4]. However, its high decomposition temperature and

sluggish sorption kinetics hinder the use of MgH2 as

a hydrogen storage material [1,5,6]. Many studies have shown

that the dehydrogenation properties of MgH2 were improved

when it was mixed with certain other elements or

compounds, such as Si [7], LiAlH4 [8e11], and Li3AlH6 [12]. It

was proposed that the formation of an intermediate phase,

such as Mg2Si [7], Li0.92Mg4.08, or Mg17Al12 [8,10,12], was

beneficial for destabilizing MgH2 and improving its thermo-

dynamic properties.

Recently we have demonstrated that the hydrogen

desorption properties of MgH2 were improved after mixing

with NaAlH4 [13]. The mutual destabilization of MgH2 and

NaAlH4 in the reactive hydride composite MgH2eNaAlH4 is

attributed to the formation of intermediate compounds,

namely, NaMgH3 and Mg17Al12. 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 as shown in Eq. (1).

NaAlH4 þMgH2/NaMgH3 þAlþ 1:5H2 (1)

During the second dehydrogenation stage, the Al phase

reacts with MgH2 to formMg17Al12 phase, accompanied by the

self-decomposition of the excessiveMgH2, as shown in Eqs. (2)

and (3).

17MgH2 þ 12Al/Mg17Al12 þ 17H2 (2)

MgH2/Mg þH2 (3)

NaMgH3 goes on to decompose to NaH during the third

dehydrogenation stage, and the last stage is the decomposi-

tion of NaH, as shown in Eqs. (4) and (5).

NaMgH3/NaHþMg þH2 (4)

NaH/Naþ 1=2H2 (5)

X-ray diffraction patterns indicate that the second, third,

and fourth stages are fully reversible under w3 MPa of H2 at

300 �C. Although the MgH2eNaAlH4 composite system

exhibits improved hydrogen storage properties, the decom-

position temperature is still too high (over 150 �C), and the

composite shows slow de/rehydrogenation kinetics. So, this

composite system still needs further investigation in order to

meet the requirements of a suitable candidate for solid state

hydrogen storage.

In this study, TiF3 was added to the MgH2eNaAlH4

composite by ball milling to improve its hydrogen storage

properties. The hydrogen storage properties of the

MgH2eNaAlH4 composite in the presence of TiF3 were inves-

tigated by temperature-programmed-desorption (TPD), ther-

mogravimetric analysis/differential scanning calorimetry

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

diffraction (XRD) was used to clarify the reaction mechanism

during the de/rehydrogenation process. The possible mecha-

nism behind the catalytic effect of TiF3 in the MgH2eNaAlH4

composite is also discussed herein.

2. Experimental details

The milling experiments were performed in a planetary ball

mill (QM-3SP2), by first milling for 0.5 h, resting for 6 min, and

then milling for another 0.5 h in a different direction at the

speed of 400 rpm, using hardened stainless steel milling tools

and an initial ball-to-powder ratio of 40:1. The starting mate-

rials, MgH2 (hydrogen storage grade), NaAlH4 (hydrogen

storage grade, 93% purity), and TiF3, were purchased from

SigmaeAldrich and were used as received with no further

purification. Themolar ratio of MgH2 and NaAlH4 in this study

is 4:1, and this composite will be referred as MgH2eNaAlH4 for

simplicity. 10 wt.% TiF3 was mixed with MgH2eNaAlH4 under

the same conditions to investigate the catalytic effects. Pure

MgH2 and NaAlH4 were also prepared under the same condi-

tions for comparison purposes. All handling of the powder,

including weighing and loading, were performed in an argon

atmosphere MBraun Unilab glove box.

For the temperature-programmed-desorption (TPD) and

the sorption measurements, the sample was loaded into

a sample vessel and sealed inside the glove box. The experi-

ments were performed in a Sieverts-type pressure-composi-

tion-temperature (PCT) apparatus (Advanced Materials

Corporation). This system covers the temperature range from

room temperature to 500 �C at hydrogen pressures up to

10 MPa. The heating rate for the TPD measurement was 5 �C/min, and samples were heated from room temperature to

450 �C. The re/dehydrogenation kinetics measurements were

performed at the desired temperature with initial hydrogen

pressures of 3.0 MPa and 0.001 MPa, respectively.

X-ray diffraction (XRD) samples were also prepared in the

glove box. To avoid exposure to air during the measurement,

the sample was spread uniformly on the sample holder and

covered with plastic wrap. The powders at different experi-

mental stages were characterized by a GBC MMA X-ray

diffractometer with Cu Ka radiation. The patterns were scan-

ned in steps of 0.02� (2q) over diffraction angles from 25� to 80�

with a speed of 2.00�/min.

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 presents the combined TGA/DSC curves of the

MgH2eNaAlH4eTiF3 composite. From the DSC curve, there are

four distinct endothermic peaks during the heating process.

Based on weight loss from the TGA curve, the first endo-

thermic peak at 175 �C should be associated with the first

dehydrogenation stage. After the first endothermic process,

three overlapping endothermic reactions occurred due to the

Fig. 1 e TGA/DSC traces of the MgH2eNaAlH4eTiF3composite. Heating rate: 15 �C minL1, argon flow: 30 ml/

min (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 7 ( 2 0 1 2 ) 8 3 9 5e8 4 0 1 8397

second, third, and fourth dehydrogenation stages (peaks at

290, 350, and 375 �C, respectively). The four endothermic

processes in the DSC curve agree well with the four dehy-

drogenation stages shown by the TGA curve.

The dehydrogenation behavior of as-milled MgH2e

NaAlH4eTiF3 samples was further investigated by a Sieverts-

type pressure-composition-temperature (PCT) apparatus.

Fig. 2 presents the temperature-programmed-desorption

(TPD) curve for the dehydrogenation of the as-milled

MgH2eNaAlH4eTiF3 composite. As can be seen, the four

stages of dehydrogenation occur during the heating process

with a total liberation amount of 6.7 wt.% H2. The first stage

proceeds in the temperature range from 60 �C to 200 �Creleases about 1.8 wt.% H2 (theoretically 3.7 wt.% H2, Eq. (1));

the second stage takes place from 200 �C to 315 �C and about

3.2 wt.% H2 are released (theoretically 4.9 wt.% H2, Eqs. (2) and

Fig. 2 e Temperature-programmed-desorption (TPD) curve

of the MgH2eNaAlH4eTiF3 composite. (I, II, III, and IV refer

to the first, second, third, and fourth dehydrogenation

stage, respectively).

(3)); the third stage starts at 315 �C and is completed at 370 �C is

releasing about 1.3 wt.% H2 (theoretically 3.9 wt.% H2, Eq. (4));

and the last stage runs after 375 �C releases about 0.4 wt.% H2

(theoretically 1.9 wt.% H2, Eq. (5)). From the results, the

experimental capacities are lower than that of theoretical is

reasonable. The four stages of the dehydrogenation process

agree well with the TGA/DSC results (Fig. 1). From the TPD

result, one finds that the onset decomposition temperature in

the TGA/DSC curves (Fig. 1) is slightly higher than in the TPD

curve. These differences may result from the fact that the

dehydrogenation measurements were run under different

conditions in these two cases, as was the case in our previous

report on the MgH2eHfCl4, FeCl3 system [14]. The TGA/DSC

measurements were conducted under 1 atm argon flow with

a 15 �C/min heating rate, while the TPDmeasurements started

from 0.1 atm under vacuum with a 5 �C/min heating rate.

Obviously, the onset decomposition temperature will shift to

higher values when the heating rate increases from 5 to 15 �C/min. On the other hand, the low pressure environment is

favorable for the hydrogen release reaction, resulting in the

reduced decomposition temperature.

In order to clarify themechanism in each dehydrogenation

stage, XRD measurements were employed. Fig. 3 shows the

XRD patterns of theMgH2eNaAlH4eTiF3 composite before and

after dehydrogenation at different temperatures. For the as-

milled sample, MgH2 and NaAlH4 phases are detected, but

no Ti or F related phases were observed, probably owing to the

small amount or the amorphous state. After heating to 200 �C,the NaAlH4 phase disappeared, and the pattern indicates the

presence of a perovskite-type hydride, NaMgH3, and Al, as

well as un-reacted MgH2, suggesting that the first endo-

thermic peak of the DSC curve (Fig. 1) is due to the reaction of

NaAlH4 and MgH2, as shown in Eq. (1). After heating to 315 �C,the MgH2 and Al phases disappeared, and Mg17Al12 alloy and

Mg were formed. The NaMgH3 phase was still apparent,

indicating that the hydrogen released in the second stage is

due to a mixed decomposition from (i) the reaction of MgH2

with Al and (ii) the decomposition of the excessive MgH2, as

shown in Eqs. (2) and (3). In addition, a new phase, Al3Ti, was

Fig. 3 e XRD patterns of the MgH2eNaAlH4eTiF3 composite

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

200 �C, (c) 315 �C, (d) 370 �C, and (e) 385 �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 7 ( 2 0 1 2 ) 8 3 9 5e8 4 0 18398

identified after dehydrogenation at 315 �C, indicating that

a reaction between NaAlH4 and the TiF3 had occurred during

ball milling or the dehydrogenation process. However, no

phase containing F was detected, which may due to the low

concentration or amorphous phase. From the results, we

speculate that the second endothermic peak of the DSC curve

(Fig. 1) is due to the MgH2-relevant decomposition. On further

heating to 370 �C, the peaks for NaMgH3 disappear, and new

peaks corresponding to NaH are observed, demonstrating that

the hydrogen release in the third stage is from NaMgH3. The

last dehydrogenation stage, starting at 370 �C and ending at

385 �C, is due to decomposition of NaH. From Fig. 3(e), the

peaks of Na can be detected, and the intensity of the Al3Ti

phase becomes strong. Based on the above results, the last

two endothermic peaks of the DSC curve (Fig. 1) are speculated

to be due to the decomposition processes of NaMgH3 and NaH,

respectively, which occur through the reactions in Eqs. (4)

and (5).

In order to investigate the effects of TiF3 on the dehydro-

genation temperature of MgH2eNaAlH4 composite, DSC

curves of MgH2eNaAlH4 composite with and without TiF3were compared, as shown in Fig. 4. For the undoped

MgH2eNaAlH4 composite, there are six peaks, one peak cor-

responding to an exothermic process and five peaks corre-

sponding to endothermic processes. The exothermic peak at

170 �C can be assigned to the presence of surface hydroxyl

impurities in the NaAlH4 powder, as reported in our previous

papers [15e17] on LiAlH4. The first endothermic peak at 185 �Ccorresponds to the melting of NaAlH4 [18], and the second

endothermic peak at 275 �C corresponds to the reaction of

NaAlH4 and MgH2 (first stage dehydrogenation). The third,

fourth, and fifth overlapping endothermic peaks at 363, 380,

and 390 �C respectively, are due to the decomposition of MgH2,

NaMgH3, and NaH, as proved in our previous paper [13]. After

doping with TiF3, the first endothermic effect, corresponding

to the melting of NaAlH4, disappears, indicating that NaAlH4

decomposes without melting in the presence of TiF3. The

disappearance of the melting process is likely to be due to the

Fig. 4 e TGA/DSC traces of the MgH2eNaAlH4 with and

without TiF3. Heating rate: 15 �C minL1, argon flow: 30 ml/

min.

fact that the decomposition temperature of the NaAlH4 in

MgH2eNaAlH4eTiF3 is lower than the melting temperature of

pure NaAlH4. The first endothermic peak at 175 �C is attributed

to the reaction of NaAlH4 and MgH2 (first stage dehydrogena-

tion), which takes place at a temperature 100 �C lower than for

undoped MgH2eNaAlH4 composite. The second endothermic

event appear at 290 �C, corresponding to the decomposition of

MgH2, which is broad and 73 �C lower than the decomposition

of MgH2 in the undoped MgH2eNaAlH4 composite. Further-

more, the third and fourth endothermic peaks are due to the

decomposition processes of NaMgH3 and NaH, respectively,

which occur at temperatures 30 and 25 �C lower than for the

undopedMgH2eNaAlH4 composite. This result shows that the

dehydrogenation temperatures for all stages in the

MgH2eNaAlH4 composite were improved after the addition of

TiF3.

Fig. 5 compares the dehydrogenation kinetics of

MgH2eNaMgH3 in the MgH2eNaAlH4 and MgH2eNaAlH4eTiF3samples after a rehydrogenation process under w3 MPa of H2

at 300 �C. The dehydrogenation of pure MgH2 was also

examined for comparison under the same conditions. The

MgH2eNaAlH4eTiF3 sample desorbed 2.8 wt.% hydrogen after

15 min, which is higher than for the MgH2eNaAlH4 (1.5 wt.%

H2) andmuch higher than for the pureMgH2 (0.3 wt.% H2). The

results indicate that the dehydrogenation kinetics of

MgH2eNaMgH3 composite is significantly improved by adding

TiF3.

The kinetics enhancement is related to the energy barriers

for H2 release. In order to investigate the kinetics enhance-

ment of the MgH2eNaAlH4eTiF3 composite in more detail, we

used DSC curves at different heating rates to calculate the

activation energy for the MgH2-relevant decomposition (Eqs.

(2) and (3)) and the decomposition processes for NaMgH3 (Eq.

(4)) and NaH (Eq. (5)). Figs. 6 and 7 shows DSC traces for the

neat MgH2eNaAlH4 and the MgH2eNaAlH4eTiF3 composite at

different heating rates. The activation energy, EA, for the

hydrogen desorption was obtained by performing a Kissinger

analysis [19], according to the following equation:

Fig. 5 e Comparison of dehydrogenation kinetics of the

MgH2, the MgH2eNaAlH4, and the MgH2eNaAlH4eTiF3samples at 300 �C.

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 7 ( 2 0 1 2 ) 8 3 9 5e8 4 0 1 8399

lnhb=T2

p

i¼ �EA=RTp þA (6)

Fig. 7 e DSC traces of MgH2eNaAlH4eTiF3 composite at

different heating rates.

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 ln½b=T2p� versus 1000/Tp. Kissinger analysis was

applied to the second, third, and fourth endothermic peaks, as

shown in Fig. 8 for the neat MgH2eNaAlH4 and the

MgH2eNaAlH4eTiF3 composite. The apparent activation

energy estimated from Kissinger analysis for MgH2-relevant

decomposition and for the decomposition of NaMgH3 andNaH

in the neat MgH2eNaAlH4 sample was found to be 148, 142,

and 138 kJ/mol, respectively, and these values decreased to 71,

104, and 124 kJ/mol when 10 wt.% TiF3 was introduced into

MgH2eNaAlH4. These results provide quantitative evidence

for decreased kinetic barriers during the dehydrogenation

process, and moreover, for improved dehydrogenation prop-

erties of the MgH2eNaAlH4eTiF3 composite.

Fig. 9 presents the rehydrogenation kinetics of the MgH2,

the MgH2eNaAlH4 composite, and the MgH2eNaAlH4eTiF3composite at 300 �C and under 3 MPa hydrogen pressure. The

MgH2eNaAlH4eTiF3 sample shows slow rehydrogenation

kinetics compared to the MgH2eNaAlH4 and MgH2 samples.

After 10 min rehydrogenation, the MgH2eNaAlH4eTiF3 only

absorbed about 3.0 wt.% hydrogen compared to about 4.0 and

5.0 wt.%, respectively, for the MgH2eNaAlH4 and MgH2

samples. This phenomenon is quite similar to what was

reported in our previous paper on the MgH2eLiAlH4 (4:1)

system [20], in which the addition of titanium-based metal

halides (TiF3 and TiCl3$1/3AlCl3) to aMgH2eLiAlH4 (4:1) sample

did not result in any improvement in the rehydrogenation

kinetics measurements.

To determine the rehydrogenation product, XRD

measurement were carried out on the MgH2eNaAlH4eTiF3sample after the rehydrogenation process at 300 �C under

3 MPa hydrogen pressures, as shown in Fig. 10. Clearly, the

phases MgH2, NaMgH3, Al, and Al3Ti can be detected, as well

as small peaks of Mg2Al3. The disappearance of Mg17Al12indicates that recovery of MgH2, Al, and Mg2Al3 from AleMg

Fig. 6 e DSC traces of MgH2eNaAlH4 composite at different

heating rates.

alloy had been achieved, as reported by Chen et al. [10] as

follows:

Mg17Al12þð17�2yÞH2/yMg2Al3þð17�2yÞMgH2þð12�3yÞAl(7)

The appearance of NaMgH3 phase confirms that reactions

(4) and (5) are reversible, as reported by Ikeda at al. [21]. In

addition, although in the presence of TiF3, no trace of NaAlH4

can be obtained after hydrogenation at 300 �C under 3 MPa

hydrogen pressure. One also finds that hardly any Mg2Al3 is

transformed into MgH2 and Al under the present conditions,

even in the presence of the catalyst. The slow rehydrogena-

tion kinetics (Fig. 9) may due to the formation of the stable

phase Mg2Al3.

As mentioned previously, the formation of Al3Ti may be

due to the reaction of NaAlH4 and TiF3 during the ball milling

or the heating process, indicating that the TiF3 component in

the MgH2eNaAlH4eTiF3 sample plays a catalytic role through

Fig. 8 e Kissinger plots of the hydrogen desorption reaction

for (a) MgH2, (b) NaMgH3, and (c) NaH in the neat

MgH2eNaAlH4 composite, and for (d) MgH2, (e) NaMgH3,

and (f) NaH in the MgH2eNaAlH4eTiF3 composite.

Fig. 9 e Comparison of rehydrogenation kinetics of the

MgH2, the MgH2eNaAlH4, and the MgH2eNaAlH4eTiF3samples at 300 �C and under 3 MPa.

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 7 ( 2 0 1 2 ) 8 3 9 5e8 4 0 18400

the formation of F-containing or Ti-containing catalytic

species. However, no phase containing F was detected before

or after rehydrogenation, which may be due to the low

concentration or amorphous phase. Many studies have been

reported on the catalytic effects of TiF3 on the decomposition

of NaAlH4, where the formation of TieAl clusters [22] and the

active function of the F anion [23,24] play the catalytic roles,

which leads to improved hydrogen storage performance. The

catalytic effect of Ti-containing species and the active func-

tion of the F anion have also been proved to be important in

improving the hydrogen sorption properties of MgH2 [25e27].

Based on the experimental results, we conclude that the TiF3component in the MgH2eNaAlH4eTiF3 sample plays a cata-

lytic role through the formation of Ti-containing and F-con-

taining catalytic species, which may promote the interaction

of NaAlH4 and MgH2, and thus further improve the dehydro-

genation of the MgH2eNaAlH4 system.

Fig. 10 e XRD pattern of the MgH2eNaAlH4eTiF3 composite

after rehydrogenation at 300 �C and under 3 MPa.

4. Conclusion

In the present work, TiF3 effectively improved the dehydro-

genation properties of the MgH2eNaAlH4 composite system.

During the dehydrogenation process, the DSC measurements

showed that all the endothermic peaks in the

MgH2eNaAlH4eTiF3 composite were shifted to lower

temperature, reduced by 100, 73, 30, and 25 �C compared with

the neat MgH2eNaAlH4 composite. Furthermore, the dehy-

drogenation kinetics of the MgH2eNaAlH4 was also improved

in the presence of TiF3. Kissinger analysis shows that the

activation energy of the dehydrogenation of MgH2-relevant

compounds, NaMgH3, and NaH in the MgH2eNaAlH4eTiF3 is

decreased by about 77, 38, and 14 kJ/mol, respectively,

compared with the neat mixture. These improvements are

mainly attributed to the active TieAl phases formed in situ

during the ball milling or the dehydrogenation process, which

accelerate the reactions in the MgH2eNaAlH4eTiF3 composite

system. However, the addition of TiF3 to the MgH2eNaAlH4

composite did not result in any improvement in the rehy-

drogenation kinetics measurement, indicating that TiF3 has

a negligible influence on the rehydrogenation process.

Acknowledgments

The authors thank the University of Wollongong for financial

support of this research. M. Ismail acknowledges the Ministry

of Higher Education Malaysia for his PhD scholarship. Many

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

manuscript.

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