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Improved hydrogen storage performance of MgH2–NaAlH4 composite by addition of TiF3
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 e n e r g y 3 7 ( 2 0 1 2 ) 8 3 9 5 e8 4 0 1
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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, Australia Department of Physical Sciences, Faculty of Science and Technology, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia c School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, NSW 2522, Australia d Department of Materials Science, Fudan University, Shanghai 200433, China b
article info
abstract
Article history:
In a previous paper, it was demonstrated that a MgH2eNaAlH4 composite system had
Received 17 November 2011
improved dehydrogenation performance compared with as-milled pure NaAlH4 and pure
Received in revised form
MgH2 alone. The purpose of the present study was to investigate the hydrogen storage
4 February 2012
properties of the MgH2eNaAlH4 composite in the presence of TiF3. 10 wt.% TiF3 was added to
Accepted 20 February 2012
the MgH2eNaAlH4 mixture, and its catalytic effects were investigated. The reaction mech-
Available online 17 March 2012
anism and the hydrogen storage properties were studied by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry (DSC), temperature-programmed-
Keywords:
desorption and isothermal sorption measurements. The DSC results show that
MgH2
MgH2eNaAlH4 composite milled with 10 wt.% TiF3 had lower dehydrogenation tempera-
NaAlH4
tures, by 100, 73, 30, and 25 C, respectively, for each step in the four-step dehydrogenation
TiF3
process compared to the neat MgH2eNaAlH4 composite. Kinetic desorption results show
Catalytic effect
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
Solid state hydrogen storage offers several benefits over other means of storing hydrogen such as compressed
hydrogen and liquid hydrogen storage, particularly in terms 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
* Corresponding author. Department of Physical Sciences, Faculty of Science and Technology, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia. ** Corresponding author. Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2519, Australia. Fax: þ61 2 4221 5731. E-mail addresses: [email protected] (M. Ismail), [email protected] (Y. Zhao). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.02.117
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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 thermodynamic 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 perovskitetype hydride, NaMgH3, and Al as shown in Eq. (1). NaAlH4 þ MgH2 /NaMgH3 þ Al þ 1:5H2
(1)
During the second dehydrogenation stage, the Al phase reacts with MgH2 to form Mg17Al12 phase, accompanied by the self-decomposition of the excessive MgH2, as shown in Eqs. (2) and (3). 17MgH2 þ 12Al/Mg17 Al12 þ 17H2
(2)
MgH2 /Mg þ H2
(3)
NaMgH3 goes on to decompose to NaH during the third dehydrogenation stage, and the last stage is the decomposition of NaH, as shown in Eqs. (4) and (5). NaMgH3 /NaH þ Mg þ H2
(4)
NaH/Na þ 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 decomposition 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 investigated by temperature-programmed-desorption (TPD), thermogravimetric 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 mechanism 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 materials, 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. The molar 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 conditions 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 experiments were performed in a Sieverts-type pressure-composition-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 experimental stages were characterized by a GBC MMA X-ray diffractometer with Cu Ka radiation. The patterns were scanned in steps of 0.02 (2q) over diffraction angles from 25 to 80 with a speed of 2.00 /min. Thermogravimetric analysis/differential scanning calorimetry (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 endothermic 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
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 e n e r g y 3 7 ( 2 0 1 2 ) 8 3 9 5 e8 4 0 1
Fig. 1 e TGA/DSC traces of the MgH2eNaAlH4eTiF3 composite. 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).
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 dehydrogenation stages shown by the TGA curve. The dehydrogenation behavior of as-milled MgH2e NaAlH4eTiF3 samples was further investigated by a Sievertstype 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 C releases 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).
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(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 TPD measurements 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 the mechanism in each dehydrogenation stage, XRD measurements were employed. Fig. 3 shows the XRD patterns of the MgH2eNaAlH4eTiF3 composite before and after dehydrogenation at different temperatures. For the asmilled 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 endothermic 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.
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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 dehydrogenation temperature of MgH2eNaAlH4 composite, DSC curves of MgH2eNaAlH4 composite with and without TiF3 were compared, as shown in Fig. 4. For the undoped MgH2eNaAlH4 composite, there are six peaks, one peak corresponding to an exothermic process and five peaks corresponding 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 C corresponds 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
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 dehydrogenation), 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. Furthermore, 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 undoped MgH2eNaAlH4 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 MgH2eNaAlH4eTiF3 samples 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) and much higher than for the pure MgH2 (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 enhancement 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. 4 e TGA/DSC traces of the MgH2eNaAlH4 with and without TiF3. Heating rate: 15 C minL1, argon flow: 30 ml/ min.
Fig. 5 e Comparison of dehydrogenation kinetics of the MgH2, the MgH2eNaAlH4, and the MgH2eNaAlH4eTiF3 samples 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 e n e r g y 3 7 ( 2 0 1 2 ) 8 3 9 5 e8 4 0 1
h i ln b=T2p ¼ EA =RTp þ A
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(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 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 and NaH 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 properties of the MgH2eNaAlH4eTiF3 composite. Fig. 9 presents the rehydrogenation kinetics of the MgH2, the MgH2eNaAlH4 composite, and the MgH2eNaAlH4eTiF3 composite 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 a MgH2eLiAlH4 (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 MgH2eNaAlH4eTiF3 sample 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 Mg17Al12 indicates that recovery of MgH2, Al, and Mg2Al3 from AleMg
Fig. 6 e DSC traces of MgH2eNaAlH4 composite at different heating rates.
Fig. 7 e DSC traces of MgH2eNaAlH4eTiF3 composite at different heating rates.
alloy had been achieved, as reported by Chen et al. [10] as follows: Mg17 Al12 þ ð17 2yÞH2 /yMg2 Al3 þ ð17 2yÞMgH2 þ ð12 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 rehydrogenation 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.
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4.
Fig. 9 e Comparison of rehydrogenation kinetics of the MgH2, the MgH2eNaAlH4, and the MgH2eNaAlH4eTiF3 samples at 300 C and under 3 MPa.
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 function 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 TiF3 component in the MgH2eNaAlH4eTiF3 sample plays a catalytic role through the formation of Ti-containing and F-containing catalytic species, which may promote the interaction of NaAlH4 and MgH2, and thus further improve the dehydrogenation of the MgH2eNaAlH4 system.
Fig. 10 e XRD pattern of the MgH2eNaAlH4eTiF3 composite after rehydrogenation at 300 C and under 3 MPa.
Conclusion
In the present work, TiF3 effectively improved the dehydrogenation 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 dehydrogenation 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 rehydrogenation 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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[18] Gross KJ, Guthrie S, Takara S, Thomas G. In-situ X-ray diffraction study of the decomposition of NaAlH4. J Alloys Compd 2000;297:270e81. [19] Kissinger HE. Reaction kinetics in differential thermal analysis. Anal Chem 1957;29:1702e6. [20] Ismail M, Zhao Y, Yu XB, Dou SX. Effect of different additives on the hydrogen storage properties of the MgH2eLiAlH4 destabilized system. RSC Adv 2011;1:408e14. [21] Ikeda K, Kogure Y, Nakamori Y, Orimo S. Reversible hydriding and dehydriding reactions of perovskite-type hydride NaMgH3. Scripta Mater 2005;53:319e22. [22] Leon A, Yalovega G, Soldatov A, Fichtner M. Investigation of the nature of a TieAl cluster formed upon cycling under hydrogen in Na alanate doped with a Ti-based precursor. J Phys Chem C 2008;112:12545e9. [23] Yin L-C, Wang P, Kang X-D, Sun C-H, Cheng H-M. Functional anion concept: effect of fluorine anion on hydrogen storage of sodium alanate. Phys Chem Chem Phys 2007;9:1499e502. [24] Wang P, Kang XD, Cheng HM. Improved hydrogen storage of TiF3-doped NaAlH4. ChemPhysChem 2005;6:2488e91. [25] Hanada N, Ichikawa T, Isobe S, Nakagawa T, Tokoyoda K, Honma T, et al. X-ray absorption spectroscopic study on valence state and local atomic structure of transition metal oxides doped in MgH2. J Phys Chem C 2009;113:13450e5. [26] Ma LP, Kang XD, Dai HB, Liang Y, Fang ZZ, Wang PJ, et al. Superior catalytic effect of TiF3 over TiCl3 in improving the hydrogen sorption kinetics of MgH2: catalytic role of fluorine anion. Acta Mater 2009;57:2250e8. [27] Xie L, Liu Y, Wang YT, Zheng J, Li XG. Superior hydrogen storage kinetics of MgH2 nanoparticles doped with TiF3. Acta Mater 2007;55:4585e91.
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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, Australia Department of Physical Sciences, Faculty of Science and Technology, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia c School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, NSW 2522, Australia d Department of Materials Science, Fudan University, Shanghai 200433, China b
article info
abstract
Article history:
In a previous paper, it was demonstrated that a MgH2eNaAlH4 composite system had
Received 17 November 2011
improved dehydrogenation performance compared with as-milled pure NaAlH4 and pure
Received in revised form
MgH2 alone. The purpose of the present study was to investigate the hydrogen storage
4 February 2012
properties of the MgH2eNaAlH4 composite in the presence of TiF3. 10 wt.% TiF3 was added to
Accepted 20 February 2012
the MgH2eNaAlH4 mixture, and its catalytic effects were investigated. The reaction mech-
Available online 17 March 2012
anism and the hydrogen storage properties were studied by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry (DSC), temperature-programmed-
Keywords:
desorption and isothermal sorption measurements. The DSC results show that
MgH2
MgH2eNaAlH4 composite milled with 10 wt.% TiF3 had lower dehydrogenation tempera-
NaAlH4
tures, by 100, 73, 30, and 25 C, respectively, for each step in the four-step dehydrogenation
TiF3
process compared to the neat MgH2eNaAlH4 composite. Kinetic desorption results show
Catalytic effect
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
Solid state hydrogen storage offers several benefits over other means of storing hydrogen such as compressed
hydrogen and liquid hydrogen storage, particularly in terms 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
* Corresponding author. Department of Physical Sciences, Faculty of Science and Technology, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia. ** Corresponding author. Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2519, Australia. Fax: þ61 2 4221 5731. E-mail addresses: [email protected] (M. Ismail), [email protected] (Y. Zhao). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.02.117
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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 e n e r g y 3 7 ( 2 0 1 2 ) 8 3 9 5 e8 4 0 1
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 thermodynamic 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 perovskitetype hydride, NaMgH3, and Al as shown in Eq. (1). NaAlH4 þ MgH2 /NaMgH3 þ Al þ 1:5H2
(1)
During the second dehydrogenation stage, the Al phase reacts with MgH2 to form Mg17Al12 phase, accompanied by the self-decomposition of the excessive MgH2, as shown in Eqs. (2) and (3). 17MgH2 þ 12Al/Mg17 Al12 þ 17H2
(2)
MgH2 /Mg þ H2
(3)
NaMgH3 goes on to decompose to NaH during the third dehydrogenation stage, and the last stage is the decomposition of NaH, as shown in Eqs. (4) and (5). NaMgH3 /NaH þ Mg þ H2
(4)
NaH/Na þ 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 decomposition 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 investigated by temperature-programmed-desorption (TPD), thermogravimetric 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 mechanism 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 materials, 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. The molar 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 conditions 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 experiments were performed in a Sieverts-type pressure-composition-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 experimental stages were characterized by a GBC MMA X-ray diffractometer with Cu Ka radiation. The patterns were scanned in steps of 0.02 (2q) over diffraction angles from 25 to 80 with a speed of 2.00 /min. Thermogravimetric analysis/differential scanning calorimetry (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 endothermic 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
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 e n e r g y 3 7 ( 2 0 1 2 ) 8 3 9 5 e8 4 0 1
Fig. 1 e TGA/DSC traces of the MgH2eNaAlH4eTiF3 composite. 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).
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 dehydrogenation stages shown by the TGA curve. The dehydrogenation behavior of as-milled MgH2e NaAlH4eTiF3 samples was further investigated by a Sievertstype 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 C releases 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).
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(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 TPD measurements 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 the mechanism in each dehydrogenation stage, XRD measurements were employed. Fig. 3 shows the XRD patterns of the MgH2eNaAlH4eTiF3 composite before and after dehydrogenation at different temperatures. For the asmilled 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 endothermic 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.
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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 dehydrogenation temperature of MgH2eNaAlH4 composite, DSC curves of MgH2eNaAlH4 composite with and without TiF3 were compared, as shown in Fig. 4. For the undoped MgH2eNaAlH4 composite, there are six peaks, one peak corresponding to an exothermic process and five peaks corresponding 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 C corresponds 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
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 dehydrogenation), 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. Furthermore, 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 undoped MgH2eNaAlH4 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 MgH2eNaAlH4eTiF3 samples 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) and much higher than for the pure MgH2 (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 enhancement 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. 4 e TGA/DSC traces of the MgH2eNaAlH4 with and without TiF3. Heating rate: 15 C minL1, argon flow: 30 ml/ min.
Fig. 5 e Comparison of dehydrogenation kinetics of the MgH2, the MgH2eNaAlH4, and the MgH2eNaAlH4eTiF3 samples 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 e n e r g y 3 7 ( 2 0 1 2 ) 8 3 9 5 e8 4 0 1
h i ln b=T2p ¼ EA =RTp þ A
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(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 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 and NaH 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 properties of the MgH2eNaAlH4eTiF3 composite. Fig. 9 presents the rehydrogenation kinetics of the MgH2, the MgH2eNaAlH4 composite, and the MgH2eNaAlH4eTiF3 composite 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 a MgH2eLiAlH4 (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 MgH2eNaAlH4eTiF3 sample 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 Mg17Al12 indicates that recovery of MgH2, Al, and Mg2Al3 from AleMg
Fig. 6 e DSC traces of MgH2eNaAlH4 composite at different heating rates.
Fig. 7 e DSC traces of MgH2eNaAlH4eTiF3 composite at different heating rates.
alloy had been achieved, as reported by Chen et al. [10] as follows: Mg17 Al12 þ ð17 2yÞH2 /yMg2 Al3 þ ð17 2yÞMgH2 þ ð12 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 rehydrogenation 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.
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4.
Fig. 9 e Comparison of rehydrogenation kinetics of the MgH2, the MgH2eNaAlH4, and the MgH2eNaAlH4eTiF3 samples at 300 C and under 3 MPa.
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 function 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 TiF3 component in the MgH2eNaAlH4eTiF3 sample plays a catalytic role through the formation of Ti-containing and F-containing catalytic species, which may promote the interaction of NaAlH4 and MgH2, and thus further improve the dehydrogenation of the MgH2eNaAlH4 system.
Fig. 10 e XRD pattern of the MgH2eNaAlH4eTiF3 composite after rehydrogenation at 300 C and under 3 MPa.
Conclusion
In the present work, TiF3 effectively improved the dehydrogenation 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 dehydrogenation 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 rehydrogenation 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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