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Study on the hydrogen storage properties and reaction mechanism of NaAlH4–MgH2–LiBH4 ternary-hydride system
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Study on the hydrogen storage properties and reaction mechanism of NaAlH4eMgH2eLiBH4 ternary-hydride system M. Ismail* School of Ocean Engineering, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia
article info
abstract
Article history:
In this paper, we report the hydrogen storage properties and reaction mechanism of
Received 8 January 2014
NaAlH4eMgH2eLiBH4 (1:1:1) ternary-hydride system prepared by ball milling. It was found
Received in revised form
that during ball milling, the NaAlH4/MgH2/LiBH4 combination converted readily to the
14 March 2014
mixture of LiAlH4/MgH2/NaBH4 and there is a mutual destabilization among the hydrides.
Accepted 22 March 2014
Three major dehydrogenation steps were observed in the system, which corresponds to
Available online 22 April 2014
the decomposition of LiAlH4, MgH2, and NaBH4, respectively. The onset dehydrogenation temperature of MgH2 in this system is observed at around 275 C, which is over 55 C lower
Keywords:
from that of as-milled MgH2. Meanwhile, NaBH4-relevant decomposition showed signifi-
Alanate
cant improvement, starts to release hydrogen at 370 C, which is reduced by about 110 C
Magnesium hydride
compared to the as-milled NaBH4. The second and third steps decomposition enthalpy of
Borohydride
the system were determined by differential scanning calorimetry measurements and the
Hydrogen storage
enthalpies were changed to be 61 and 100 kJ mol1 H2 respectively, which are smaller than that of MgH2 and NaBH4 alone. From the Kissinger plot, the apparent activation energy, EA, for the decomposition of MgH2 and NaBH4 in the composite was reduced to 96.85 and 111.74 kJ mol1 respectively. It is believed that the enhancement of the dehydrogenation properties was attributed to the formation of intermediate compounds, including LieMg, MgeAl, and MgeAleB alloys, upon dehydrogenation, which change the thermodynamics of the reactions through altering the de/rehydrogenation pathway. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Due to an ongoing environmental crisis and the increasingly limited supply of fossil fuels, hydrogen has attracted much attention as an alternative energy carrier [1]. However, hydrogen storage presents a significant challenge for the development of a hydrogen economy, especially in relation to hydrogen-powered vehicles. There are three methods used to
store hydrogen, specifically: high pressure storage, cryogenics, as well as chemical compounds that reversibly release H2 upon heating (solid-state storage). Among these methods, solid-state storage has become an attractive option due to its high volumetric hydrogen capacity and favorable safety considerations. However, until now, there has been no single material that can satisfy all the requirements for an on-board hydrogen storage material which is suitable for mobile applications [2].
* Tel.: þ60 9 6683336; fax: þ60 9 6683991. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.ijhydene.2014.03.166 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 9 ( 2 0 1 4 ) 8 3 4 0 e8 3 4 6
The high decomposition temperature and slow de/absorption kinetics are two major challenges facing the development of a solid-state hydrogen storage material based on chemisorptions such as metal hydride (e.g. MgH2) and complex hydride (e.g. LiBH4) [1]. For example, MgH2 requires a temperature of 300 C at 1 bar H2 to release hydrogen, while LiBH4 starts to release hydrogen at a level above 380 C. Moreover, only half of the hydrogen can be released before 600 C. This temperature is too high for practical onboard applications [3]. In an attempt to improve the hydrogen storage properties of metal hydride and complex hydride, an abundance of studies have been performed. These include, namely: reducing the size of particles by using ball milling [4,5]; utilization of additives, which can facilitate hydrogen de/ absorption kinetics [6e21]; and design of so-called reactive hydride composites (RHCs) [15,22e39]. The RHCs method could be regarded as a quite different approach. This approach is aimed at modifying the thermodynamics and kinetics of the hydrogen sorption reaction by mixing two or more hydrides [27,28,32,40e44]. Thermodynamic destabilization is achieved when the mixed hydrides react and form a new intermediate species that may facilitate thermodynamic and kinetic properties of hydrogen release and uptake [45e48]. Recently, a study by Mao et al. [49] has shown that there is a mutual destabilization between the hydrides in the ternary LiAlH4eMgH2eLiBH4 system, which exhibits superior hydrogen storage properties compared with the unary components (LiAlH4, MgH2 and LiBH4). From the XRD analysis, they found that the intermediate compounds, LieMg, MgeAl, and MgeAleB alloys, (formed during dehydrogenation), may improve the thermodynamics of reactions by changing the de/ rehydrogenation pathway. The basic idea in this study is to explore the utilization of other complex hydrides as the Al source in the MgH2eLiBH4 composite system. In a previous study, we have shown that a MgH2eNaAlH4 composite system improved dehydrogenation performance compared with as-milled pure NaAlH4 and pure MgH2 alone [22]. It is believed that the formation of the NaMgH3 and Mg17Al12 phases during the dehydrogenation process plays a critical role in the enhancement of dehydrogenation in the MgH2eNaAlH4 composite. In addition, Shi et al. [50] and Ravnsbaek et al. [51] reported a LiBH4eNaAlH4 composite, in which this mixed system is found to have initiated a transformation to LiAlH4eNaBH4. They claimed that there is mutual destabilization existing in the LiBH4eNaAlH4 composite. Therefore, in the present study, the dehydrogenation properties of the ternary-hydride system, NaAlH4eMgH2eLiBH4 was prepared and investigated, in order to improve the hydrogen storage properties through mutual interaction among the three hydrides. To the best of the author’s knowledge, no studies have been reported on the hydrogen storage properties and reaction mechanism of the NaAlH4eMgH2eLiBH4 ternary-hydride system.
Experimental details LiAlH4 (powder, reagent grade, 95%), NaAlH4 (hydrogen storage grade), MgH2 (hydrogen storage grade), NaBH4 (hydrogen storage grade, 98%), and LiBH4 (hydrogen storage grade,
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90%) were purchased from Sigma Aldrich and were used as received with no further purification. Ball milling (BM) of NaAlH4, MgH2, and LiBH4 powders in the mole ratio of 1:1:1 was performed in a planetary ball mill (NQM-0.4) for 1 h at the rate of 400 rpm. Handling of the samples was conducted in an MBraun Unilab glove box filled with high purity Ar atmosphere. Samples were put into a sealed stainless steel vial together with hardened stainless steel balls. The ratio of the weight of balls to the weight of powder was 30:1. For comparison purposes, pristine LiAlH4, MgH2 and NaBH4 were also milled for a period of 1 h. The thermal desorption performances and re/dehydrogenation kinetics experiments were performed in a Sievertstype pressure-composition-temperature (PCT) apparatus (Advanced Materials Corporation). The sample was loaded into a sample vessel in the glove box. For the thermal desorption performances experiment, all the samples were heated in a vacuum chamber, and the amount of desorbed hydrogen was measured to determine the lowest decomposition temperature. The heating rate for the thermal desorption performances experiment was 5 C min1, and samples were heated from room temperature to desired temperature. The re/de-hydrogenation kinetics measurements were performed at the desired temperature with initial hydrogen pressures of 5.0 MPa and 0.01 MPa respectively. XRD analysis was performed using a Rigaku MiniFlex II diffractometer with Cu Ka radiation. qe2q scans were carried out over diffraction angles from 25 to 80 with a speed of 2.00 min1. Before the measurement, a small amount of sample was spread uniformly on the sample holder, which was wrapped withplastic wrap to prevent oxidation. 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 min1, and different heating rates were used.
Results and discussion Fig. 1 presents the thermal desorption performances curves of the as-milled NaAlH4eMgH2eLiBH4 (molar ratio 1:1:1). From the curve, the NaAlH4eMgH2eLiBH4 sample clearly showed three major stages of dehydrogenation that occurred during the heating process. These include, namely: the first stage, which took place within the temperature range of 150e230 C; the second dehydrogenation stage, starting at approximately 275 C and completed at about 350 C; and the final stage, which occurred at a temperature of about 370 C and was completed at about 475 C. To clarify the dehydrogenation mechanism in every stage, XRD measurements were employed as shown in Fig. 2. After 1 h milling (Fig. 2(a)), it can be seen that the peaks of the starting materials, NaAlH4 and LiBH4, were absent. Further, new peaks corresponding to the formation of LiAlH4 and
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Meanwhile, after dehydrogenation at 350 C, as can be seen from the XRD result (Fig. 2(c)), the peaks for MgH2 disappear. Further, new peaks corresponding to Mg17Al12, Mg2Al3, and Li3Mg7 can be observed besides NaBH4. These results confirmed that the hydrogen released in the second stage is from the MgH2-relevant decomposition through the reactions in Eqs. (4)e(6) as illustrated below:
Fig. 1 e Thermal desorption performances curve of the NaAlH4eMgH2eLiBH4 composite.
NaBH4 can be observed besides MgH2. This result indicates that the solid state reaction between the components of the NaAlH4 and LiBH4 mixture was completed during the milling process, as shown in Eq. (1): NaAlH4 þ LiBH4 /LiAlH4 þ NaBH4
(1)
This result concurs with those reported in a previous literature study [50]. Fig. 2(b) indicates the presence of LiH and Al phases in addition to the MgH2 and NaBH4 after dehydrogenation at 230 C. The fact that no LiAlH4 phase was found indicates that the reactions in Eqs. (2) and (3) had been completed at this stage as shown below: 3LiAlH4 /Li3 AlH6 þ 2Al þ 3H2
(2)
Li3 AlH6 /3LiH þ Al þ 3=2H2
(3)
Fig. 2 e XRD patterns of the NaAlH4eMgH2eLiBH4 composite (a) after 1 h ball milling and after dehydrogenation at (b) 230 C, (c) 350 C, and (d) 475 C.
17MgH2 þ 12Al/Mg17 Al12 þ 17H2
(4)
2MgH2 þ 3Al/Mg2 Al3 þ 2H2
(5)
7MgH2 þ 3LiH/Li3 Mg7 þ 8:5H2
(6)
After further heating to 475 C (Fig. 2(d)), the NaBH4 phase disappeared, indicating that the system is fully dehydrogenated at 475 C. This concurs with the TPD results (Fig. 1). It also can be seen that the Mg17Al12 phase has disappeared and a new phase, NaH and Mg1xAlxB2 has formed. Mao et al. [49] have suggested that the new phase is Mg1xAlxB2 in their LiAlH4eMgH2eLiBH4 (molar ratio, 1:1:1) system. Based on the XRD results, the transformation after ball milling and the three-step dehydrogenation pathway for the NaAlH4eMgH2eLiBH4 system is proposed to be as follows:
NaAlH4 þ MgH2 þ LiBH4 /LiAlH4 eMgH2 eNaBH4 /Al þ LiH þ MgH2 þ NaBH4 þ 3=2H2 /ð1 21yÞ 27Mg17 Al12 þ ð5 þ 84yÞ 27Mg2 Al3 þ yLi3 Mg7 þ ð1 3yÞLiH þ NaBH4 þ ð1 þ 3=2yÞH2 /1 2Mg1x Alx B2 þ ð1 þ x 14yÞ 4Mg2 Al3 þ yLi3 Mg7 þ ð1 3yÞLiH þ NaH þ ð42y 5x þ 1Þ=4Al þ 3=2H2 (7) where x means the amount of Al which substitutes for Mg in MgB2 alloy and y means the amount of Li3Mg7 alloy [49]. In order to investigate the effects of mutual destabilization on the desorption temperature among the three hydrides, thermal desorption performances curve of as-milled LiAlH4, as-milled MgH2, as-milled NaBH4, and NaAlH4eMgH2eLiBH4 composite system were compared, as shown in Fig. 3. In addition, to confirm the phase transformation of NaAlH4eMgH2eLiBH4 to the mixture of LiAlH4eMgH2eNaBH4, the dehydrogenation curve of a mixture of LiAlH4eMgH2eNaBH4 (1:1:1) has been included in Fig. 3. The as-milled LiAlH4 starts to desorb hydrogen at about 142 C and about 173 C for the first and second stages respectively. Meanwhile, the as-milled MgH2 and as-milled NaBH4 start to release hydrogen at about 330 C and about 490 C, respectively. After mixed together, the curve shows a significant improvement for MgH2- and NaBH4-relevant decomposition in the NaAlH4eMgH2eLiBH4 composite system. After combined together, the decomposition temperature of MgH2 and NaBH4-relevant decomposition in the NaAlH4eMgH2eLiBH4 composite reduced by 55 C and 110 C respectively, compared with that of as-milled MgH2 and as-milled NaBH4. This indicates that the dehydrogenation performance of MgH2 and NaBH4 was significantly improved. However, compared with as-milled LiAlH4, the dehydrogenation temperature for the LiAlH4 in the NaAlH4eMgH2eLiBH4 composite showed no improvement. For the LiAlH4eMgH2eNaBH4 (1:1:1) mixture, the pattern of dehydrogenation curve is similar to the
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Fig. 3 e Thermal desorption performances curve of the asmilled LiAlH4, as-milled MgH2, as-milled NaBH4, NaAlH4eMgH2eLiBH4, and LiAlH4eMgH2eNaBH4 composite.
NaAlH4eMgH2eLiBH4 mixture. The curve clearly showed three major stages of dehydrogenation that corresponding to decomposition of LiAlH4, MgH2, and NaBH4. This result confirm that the NaAlH4eMgH2eLiBH4 mixture transform to LiAlH4/MgH2/NaBH4 during the milling process. The NaAlH4eMgH2eLiBH4 composite system was further investigated by TGA/DSC, as shown in Fig. 4. As can be seen, there were two distinct exothermic peaks and four distinct endothermic peaks present during the heating process. The first exothermic peak at 110 C was due to the presence of surface hydroxyl impurities in the LiAlH4 powder, as reported in our previous papers [10,12,13] and the first endothermic peak at 125 C corresponds to the melting of LiAlH4 [52]. The second exothermic peak at 180 C corresponds to the decomposition of liquid LiAlH4 (first step decomposition, Eq. (1)), while the second endothermic peak at 210 C is assigned to the decomposition of Li3AlH6 (second step decomposition, Eq. (2)). The third endothermic peak at about 330 C is due to the decomposition of MgH2, while the last endothermic peak at 450 C corresponds to the decomposition of NaBH4. The three major weight loss events in the TGA curve concur with the three major stages of dehydrogenation shown by the thermal desorption performances curve in Fig. 1. The kinetics enhancement is related to the energy barriers for H2 release. In order to investigate the kinetics enhancement of the NaAlH4eMgH2eLiBH4 composite in more detail, we used DSC curves at different heating rates to calculate the activation energy (EA) for the MgH2 and NaBH4-relevant decomposition in the system. Fig. 5 shows DSC traces for the NaAlH4eMgH2eLiBH4 composite at different heating rates (10, 15, and 20 C min1, respectively). The activation energy, EA, for the hydrogen desorption was obtained by performing a Kissinger analysis [53] according to the following Eq.: h . i ln b T2p ¼ EA RTp þ A
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Fig. 4 e TGA/DSC traces of the NaAlH4eMgH2eLiBH4 composite. Heating rate: 15 C minL1, argon flow: 30 ml minL1.
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 and third endothermic peaks, as shown in Fig. 6 for the NaAlH4eMgH2eLiBH4 composite. The apparent activation energy estimated from the Kissinger analysis for MgH2 and NaBH4-relevant decomposition was found to be 96.85 and 111.74 kJ mol1 respectively. This is greatly reduced from the reported value of pristine MgH2 (162 kJ mol1) [23] and PrF3-doped NaBH4 (235.3 kJ mol1) [54]. These results provide quantitative evidence for decreased kinetic barriers during the dehydrogenation process and, moreover, for improved dehydrogenation properties of the NaAlH4eMgH2eLiBH4 composite. As discussed in the Introduction Section, thermodynamic destabilization of mixed hydride progresses through the formation of an end product which has a lower enthalpy than that of the pure element. To determine the enthalpy (DHdec) of NaAlH4eMgH2eLiBH4 decomposition, the DSC curves were
(8)
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.
Fig. 5 e DSC traces of the NaAlH4eMgH2eLiBH4 composite at different heating rates.
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Fig. 6 e The Kissinger’s plot of the dehydrogenation for the MgH2 and NaBH4-relevant decomposition.
analyzed by STARe software. From the DSC peak areas (third endothermic event) in Fig. 4, the reaction enthalpy was integrated as 488.19 J/g (hydrides). According to the TG performance of NaAlH4eMgH2eLiBH4 (1:1:1) composite in Fig. 4, we observed that the dehydrogenation capacity (H/M in weight ratio) of the second stage is about 1.6 wt.%. This means that the enthalpy change calculated from the DSC curves is 61 kJ mol1 H2 for the MgH2-relevant decomposition (third endothermic event). By means of the same methods, the reaction enthalpies of NaBH4-relevant decompositions (last endothermic event) is determined to be 100 kJ mol1 H2. This is lower than the overall decomposition enthalpy of pure MgH2 (75.7 kJ mol1 H2) [23] and pure NaBH4 (191.9 kJ mol1 H2) [55]. This result confirmed that a mutual destabilization exists among the three hydrides. In relation to the evaluation of hydrogen storage reversibility, the fully dehydrogenated composite was subjected to a process of rehydriding under 5 MPa of hydrogen pressure at 350 C. Fig. 7 shows the isothermal rehydrogenation curve of the dehydrogenated NaAlH4eMgH2eLiBH4 composite. The dehydrogenated NaAlH4eMgH2eLiBH4 composite can absorb about 4.5 wt.% of hydrogen after 60 min of rehydrogenation. Specifically, the reversible hydrogen storage capacity is lower than the dehydrogenation amount of the as-milled sample, implying a partial reversibility. To understand the chemical events occurring in the reversible hydrogen storage process, it can be seen that the hydrogenated samples collected at 350 C were analyzed by means of XRD as shown in Fig. 8. After hydrogenation at 350 C, the characteristic diffraction peaks of MgH2, LiH and Al phases can be clearly observed. Interestingly, LiBH4 also appears as a rehydrogenation product, with no peaks corresponding to the NaBH4 phases. This phenomenon is likely due to occurrences during the cooling process (the rehydrogenation process was performed after the dehydrogenation process and the cooling process was performed under vacuum from about 530 C to 350 C). In this process, sodium hydride decomposes rapidly and molten sodium is formed, which
Fig. 7 e Isothermal rehydrogenation kinetics curve of the NaAlH4eMgH2eLiBH4 composite at 350 C and under 5 MPa.
might evaporate due to the combination of high temperature and low pressure [51]. After rehydrogenation, the Li3Mg7 peaks disappear, and peaks of MgH2 and LiH appear, indicating that Li3Mg7 can be dissociated into MgH2 and LiH (Eq. (9)) as reported by Mao et al. [49]. 2Li3 Mg7 þ 17H2 /14MgH2 þ 6LiH
(9)
However, the Mg1xAlxB2 phases still remain in the product, indicating that the recombination of MgH2 and LiBH4 is incomplete under these rehydrogenation conditions. This is illustrated in Eq. (10) [49] below: Mg1x Alx B2 þ 2LiH þ ð4 xÞH2 /ð1 xÞMgH2 þ 2LiBH4 þ xAl (10)
Fig. 8 e XRD patterns of the NaAlH4eMgH2eLiBH4 composite after rehydrogenation at 350 C and under 5 MPa.
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In relation to the Mg2Al3, if higher hydrogenation pressure was applied, Mg2Al3 would subsequently be transformed into MgH2 and Al, as shown in Eq. (11) [56]: Mg2 Al3 þ 2H2 /2MgH2 þ 3Al
(11)
In this study, Mg2Al3 was hardly to transform into MgH2 and Al due to the lower hydrogenation pressure applied (5 MPa). This phenomenon is similar to that reported by Mao et al. [49] for the LiAlH4eMgH2eLiBH4eTiF3 composite, in which their sample was rehydrogenated under 4 MPa. For comparison Chen et al. [57] reported that no Mg2Al3 phase was detected from their MgH2 þ LiAlH4 (4:1) composite sample after rehydrogenation at 350 C and under 10 MPa. This implies that the reaction (11) occurred when the higher hydrogen pressure was applied. So, in order to improve the rehydrogenation process of NaAlH4eMgH2eLiBH4 system, the higher hydrogen pressure (>10 Mpa) is needed. This is interesting issue for the future work. From these results, it can be pointed out that the NaAlH4eMgH2eLiBH4 system shows superior hydrogen storage properties compared with the unary component. These improvements could be attributed to the mutual interactions among the three hydrides. The formation of LieMg, MgeAl, and MgeAleB alloys as a reaction product during the dehydrogenation process may create a beneficial pathway for hydrogen atom diffusion to the surface and recombination. This can be attributed to the improvement of the thermodynamic properties of the NaAlH4eMgH2eLiBH4 composite system.
Conclusion In summary, it has been demonstrated that there is a mutual destabilization existing between the hydrides in the ternary NaAlH4eMgH2eLiBH4 (1:1:1) system. This system exhibits superior hydrogen storage properties compared with the unary components. Structural analyses revealed the NaAlH4eMgH2eLiBH4 combination converted to the LiAlH4eMgH2eNaBH4 mixture after ball milling. Upon dehydrogenation, LiAlH4 decomposed first to form LiH and Al with hydrogen release at below 250 C. Subsequently, these LiH and Al reacted with MgH2 to produce Mg17Al12, Mg2Al3, and Li3Mg7 and hydrogen, respectively, at 250e350 C. With a further increase to 475 C, the reaction between NaBH4 and Mg17Al12 took place to release all of the hydrogen and yield NaH and Mg1xAlxB2. DSC measurements indicate that the enthalpy change in the second and third step decomposition is recorded as 61 and 100 kJ mol1 H2, respectively. This is smaller than that of MgH2 and NaBH4 alone (76 and 191 kJ mol1 H2). The Kissinger plots for different heating rates in DSC show that the apparent activation energy, EA, for decomposition of MgH2- and NaBH4 relevant in the NaAlH4eMgH2eLiBH4 composite is reduced to 96.85 and 111.74 kJ mol1. The formation of intermediate compounds upon dehydrogenation, including LieMg, MgeAl, and MgeAleB alloys, change the thermodynamics of the reactions through altering the de/rehydrogenation pathway. Hence, it is believed that this plays a critical role in the enhancement of dehydrogenation in the NaAlH4eMgH2eLiBH4 composite.
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Acknowledgment This work was supported by The Ministry of Higher Education of Malaysia under Research Grant FRGS 59295. The author also acknowledges the Universiti Malaysia Terengganu for providing the facilities to carry out this project.
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[18] Yu XB, Guo YH, Yang ZX, Guo ZP, Liu HK, Dou SX. Synthesis of catalyzed magnesium hydride with low absorption/ desorption temperature. Scr Mater 2009;61:469e72. [19] Cui J, Wang H, Liu J, Ouyang L, Zhang Q, Sun D, et al. Remarkable enhancement in dehydrogenation of MgH2 by a nano-coating of multi-valence Ti-based catalysts. J Mater Chem A 2013;1:5603e11. [20] Zhang Y, Zhang W-S, Fan M-Q, Liu S-S, Chu H-L, Zhang Y-H, et al. Enhanced hydrogen storage performance of LiBH4eSiO2eTiF3 composite. J Phys Chem C 2008;112:4005e10. [21] Sun T, Wang H, Zhang Q, Sun D, Yao X, Zhu M. Synergetic effects of hydrogenated Mg3La and TiCl3 on the dehydrogenation of LiBH4. J Mater Chem 2011;21:9179e84. [22] Ismail M, Zhao Y, Yu XB, Mao JF, Dou SX. The hydrogen storage properties and reaction mechanism of the MgH2eNaAlH4 composite system. Int J Hydrogen Energy 2011;36:9045e50. [23] 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. [24] Ismail M, Zhao Y, Yu XB, Dou SX. Improved hydrogen storage performance of MgH2eNaAlH4 composite by addition of TiF3. Int J Hydrogen Energy 2012;37:8395e401. [25] Ismail M, Zhao Y, Dou SX. An investigation on the hydrogen storage properties and reaction mechanism of the destabilized MgH2eNa3AlH6 (4:1) system. Int J Hydrogen Energy 2013;38:1478e83. [26] Varin RA, Czujko T, Chiu C, Pulz R, Wronski ZS. Synthesis of nanocomposite hydrides for solid-state hydrogen storage by controlled mechanical milling techniques. J Alloys Compd 2009;483:252e5. [27] Pan H, Shi S, Liu Y, Li B, Yang Y, Gao M. Improved hydrogen storage kinetics of the LieMgeNeH system by addition of Mg(BH4)2. Dalt Trans 2013;42:3802e11. [28] Hu J, Weidner E, Hoelzel M, Fichtner M. Functions of LiBH4 in the hydrogen sorption reactions of the 2LiHeMg(NH2)2 system. Dalt Trans 2010;39:9100e7. [29] Li Y, Fang F, Song Y, Li Y, Sun D, Zheng S, et al. Hydrogen storage of a novel combined system of LiNH2eNaMgH3: synergistic effects of in situ formed alkali and alkaline-earth metal hydrides. Dalt Trans 2013;42:1810e9. [30] Zhou Y, Liu Y, Zhang Y, Gao M, Pan H. Functions of MgH2 in hydrogen storage reactions of the 6LiBH4eCaH2 reactive hydride composite. Dalt Trans 2012;41:10980e7. [31] Fan X, Xiao X, Chen L, Wang X, Li S, Ge H, et al. High catalytic efficiency of amorphous TiB2 and NbB2 nanoparticles for hydrogen storage using the 2LiBH4eMgH2 system. J Mater Chem A 2013;1:11368e75. [32] Pan H, Gao M, Gu J, Wang Y, Liu Y, Liang C, et al. Ca(BH4)2eLiBH4eMgH2: a novel ternary hydrogen storage system with superior long-term cycling performance. J Mater Chem A 2013;1:12285e92. [33] Zhou Y, Liu Y, Wu W, Zhang Y, Gao M, Pan H. Improved hydrogen storage properties of LiBH4 destabilized by in situ formation of MgH2 and LaH3. J Phys Chem C 2012;116:1588e95. [34] Alcantara KS, Ramallo-Lopez JM, Boesenberg U, Saldan I, Pistidda C, Requejo FG, et al. 3CaH2 þ 4MgB2 þ CaF2 reactive hydride composite as a potential hydrogen storage material: hydrogenation and dehydrogenation pathway. J Phys Chem C 2012;116:7207e12. [35] Bosenberg U, Kim JW, Gosslar D, Eigen N, Jensen TR, von Colbe JMB, et al. Role of additives in LiBH4eMgH2 reactive hydride composites for sorption kinetics. Acta Mater 2010;58:3381e9. [36] Mao J, Guo Z, Yu X, Liu H. Improved hydrogen storage properties of NaBH4 destabilized by CaH2 and Ca(BH4)2. J Phys Chem C 2011;115:9283e90.
[37] Mao JF, Yu XB, Guo ZP, Liu HK, Wu Z, Ni J. Enhanced hydrogen storage performances of NaBH4eMgH2 system. J Alloys Compd 2009;479:619e23. [38] Liu H, Wang X, Liu Y, Dong Z, Cao G, Li S, et al. Improved hydrogen storage properties of MgH2 by ball milling with AlH3: preparations, de/rehydriding properties, and reaction mechanisms. J Mater Chem A 2013;1:12527e35. [39] Zhu M, Wang H, Ouyang LZ, Zeng MQ. Composite structure and hydrogen storage properties in Mg-based alloys. Int J Hydrogen Energy 2006;31:251e7. [40] Liu Y, Li B, Tu F, Liang C, Gao M, Pan H, et al. Correlation between composition and hydrogen storage behaviors of the Li2NHeMgNH combination system. Dalt Trans 2011;40:8179e86. [41] Cao H, Zhang Y, Wang J, Xiong Z, Wu G, Qiu J, et al. Effects of Al-based additives on the hydrogen storage performance of the Mg(NH2)2e2LiH system. Dalt Trans 2013;42:5524e31. [42] Zhang Y, Tian Q-F, Liu S-S, Sun L-X. The destabilization mechanism and de/re-hydrogenation kinetics of MgH2eLiAlH4 hydrogen storage system. J Power Sources 2008;185:1514e8. [43] Yu XB, Grant DM, Walker GS. A new dehydrogenation mechanism for reversible multicomponent borohydride systems e the role of LieMg alloys. Chem Commun; 2006:3906e8. [44] Mustafa NS, Ismail M. Enhanced hydrogen storage properties of 4MgH2 þ LiAlH4 composite system by doping with Fe2O3 nanopowder. Int J Hydrogen Energy; 2014. http://dx.doi.org/ 10.1016/j.ijhydene.2014.02.118. [45] Vajo JJ, Mertens F, Ahn CC, Bowman RC, Fultz B. Altering hydrogen storage properties by hydride destabilization through alloy formation: LiH and MgH2 destabilized with Si. J Phys Chem B 2004;108:13977e83. [46] Vajo JJ, Skeith SL, Mertens F. Reversible storage of hydrogen in destabilized LiBH4. J Phys Chem B 2005;109:3719e22. [47] Zhang Y, Tian Q, Zhang J, Liu S-S, Sun L-X. The dehydrogenation reactions and kinetics of 2LiBH4eAl composite. J Phys Chem C 2009;113:18424e30. [48] Yu XB, Wu Z, Chen QR, Li ZL, Weng BC, Huang TS. Improved hydrogen storage properties of LiBH4 destabilized by carbon. Appl Phys Lett 2007;90:034106. [49] Mao J, Guo Z, Leng H, Wu Z, Guo Y, Yu X, et al. Reversible hydrogen storage in destabilized LiAlH4eMgH2eLiBH4 ternary-hydride system doped with TiF3. J Phys Chem C 2012;114:11643e9. [50] Shi Q, Yu X, Feidenhans’l R, Vegge T. Destabilized LiBH4eNaAlH4 mixtures doped with titanium based catalysts. J Phys Chem C 2008;112:18244e8. [51] Ravnsbaek DB, Jensen TR. Tuning hydrogen storage properties and reactivity: investigation of the LiBH4eNaAlH4 system. J Phys Chem Solids 2010;71:1144e9. [52] McCarty M, Maycock JN, Verneker VRP. Thermal decomposition of lithium aluminum hydride. J Phys Chem 1968;72:4009e14. [53] Kissinger HE. Reaction kinetics in differential thermal analysis. Anal Chem 1957;29:1702e6. [54] Chong L, Zou J, Zeng X, Ding W. Reversible hydrogen sorption in NaBH4 at lower temperatures. J Mater Chem A 2013;1:13510e23. [55] Stull D, Prophet H. JANAF thermochemical tables. Washington: US Department of Commerce; 1985 (Cp Fitted by CRCT, Montreal). [56] Bouaricha S, Dodelet JP, Guay D, Huot J, Boily S, Schulz R. Hydriding behavior of MgeAl and leached MgeAl compounds prepared by high-energy ball-milling. J Alloys Compd 2000;297:282e93. [57] Chen R, Wang X, Xu L, Chen L, Li S, Chen C. An investigation on the reaction mechanism of LiAlH4eMgH2 hydrogen storage system. Mater Chem Phys 2010;124:83e7.
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Study on the hydrogen storage properties and reaction mechanism of NaAlH4eMgH2eLiBH4 ternary-hydride system M. Ismail* School of Ocean Engineering, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia
article info
abstract
Article history:
In this paper, we report the hydrogen storage properties and reaction mechanism of
Received 8 January 2014
NaAlH4eMgH2eLiBH4 (1:1:1) ternary-hydride system prepared by ball milling. It was found
Received in revised form
that during ball milling, the NaAlH4/MgH2/LiBH4 combination converted readily to the
14 March 2014
mixture of LiAlH4/MgH2/NaBH4 and there is a mutual destabilization among the hydrides.
Accepted 22 March 2014
Three major dehydrogenation steps were observed in the system, which corresponds to
Available online 22 April 2014
the decomposition of LiAlH4, MgH2, and NaBH4, respectively. The onset dehydrogenation temperature of MgH2 in this system is observed at around 275 C, which is over 55 C lower
Keywords:
from that of as-milled MgH2. Meanwhile, NaBH4-relevant decomposition showed signifi-
Alanate
cant improvement, starts to release hydrogen at 370 C, which is reduced by about 110 C
Magnesium hydride
compared to the as-milled NaBH4. The second and third steps decomposition enthalpy of
Borohydride
the system were determined by differential scanning calorimetry measurements and the
Hydrogen storage
enthalpies were changed to be 61 and 100 kJ mol1 H2 respectively, which are smaller than that of MgH2 and NaBH4 alone. From the Kissinger plot, the apparent activation energy, EA, for the decomposition of MgH2 and NaBH4 in the composite was reduced to 96.85 and 111.74 kJ mol1 respectively. It is believed that the enhancement of the dehydrogenation properties was attributed to the formation of intermediate compounds, including LieMg, MgeAl, and MgeAleB alloys, upon dehydrogenation, which change the thermodynamics of the reactions through altering the de/rehydrogenation pathway. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Due to an ongoing environmental crisis and the increasingly limited supply of fossil fuels, hydrogen has attracted much attention as an alternative energy carrier [1]. However, hydrogen storage presents a significant challenge for the development of a hydrogen economy, especially in relation to hydrogen-powered vehicles. There are three methods used to
store hydrogen, specifically: high pressure storage, cryogenics, as well as chemical compounds that reversibly release H2 upon heating (solid-state storage). Among these methods, solid-state storage has become an attractive option due to its high volumetric hydrogen capacity and favorable safety considerations. However, until now, there has been no single material that can satisfy all the requirements for an on-board hydrogen storage material which is suitable for mobile applications [2].
* Tel.: þ60 9 6683336; fax: þ60 9 6683991. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.ijhydene.2014.03.166 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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The high decomposition temperature and slow de/absorption kinetics are two major challenges facing the development of a solid-state hydrogen storage material based on chemisorptions such as metal hydride (e.g. MgH2) and complex hydride (e.g. LiBH4) [1]. For example, MgH2 requires a temperature of 300 C at 1 bar H2 to release hydrogen, while LiBH4 starts to release hydrogen at a level above 380 C. Moreover, only half of the hydrogen can be released before 600 C. This temperature is too high for practical onboard applications [3]. In an attempt to improve the hydrogen storage properties of metal hydride and complex hydride, an abundance of studies have been performed. These include, namely: reducing the size of particles by using ball milling [4,5]; utilization of additives, which can facilitate hydrogen de/ absorption kinetics [6e21]; and design of so-called reactive hydride composites (RHCs) [15,22e39]. The RHCs method could be regarded as a quite different approach. This approach is aimed at modifying the thermodynamics and kinetics of the hydrogen sorption reaction by mixing two or more hydrides [27,28,32,40e44]. Thermodynamic destabilization is achieved when the mixed hydrides react and form a new intermediate species that may facilitate thermodynamic and kinetic properties of hydrogen release and uptake [45e48]. Recently, a study by Mao et al. [49] has shown that there is a mutual destabilization between the hydrides in the ternary LiAlH4eMgH2eLiBH4 system, which exhibits superior hydrogen storage properties compared with the unary components (LiAlH4, MgH2 and LiBH4). From the XRD analysis, they found that the intermediate compounds, LieMg, MgeAl, and MgeAleB alloys, (formed during dehydrogenation), may improve the thermodynamics of reactions by changing the de/ rehydrogenation pathway. The basic idea in this study is to explore the utilization of other complex hydrides as the Al source in the MgH2eLiBH4 composite system. In a previous study, we have shown that a MgH2eNaAlH4 composite system improved dehydrogenation performance compared with as-milled pure NaAlH4 and pure MgH2 alone [22]. It is believed that the formation of the NaMgH3 and Mg17Al12 phases during the dehydrogenation process plays a critical role in the enhancement of dehydrogenation in the MgH2eNaAlH4 composite. In addition, Shi et al. [50] and Ravnsbaek et al. [51] reported a LiBH4eNaAlH4 composite, in which this mixed system is found to have initiated a transformation to LiAlH4eNaBH4. They claimed that there is mutual destabilization existing in the LiBH4eNaAlH4 composite. Therefore, in the present study, the dehydrogenation properties of the ternary-hydride system, NaAlH4eMgH2eLiBH4 was prepared and investigated, in order to improve the hydrogen storage properties through mutual interaction among the three hydrides. To the best of the author’s knowledge, no studies have been reported on the hydrogen storage properties and reaction mechanism of the NaAlH4eMgH2eLiBH4 ternary-hydride system.
Experimental details LiAlH4 (powder, reagent grade, 95%), NaAlH4 (hydrogen storage grade), MgH2 (hydrogen storage grade), NaBH4 (hydrogen storage grade, 98%), and LiBH4 (hydrogen storage grade,
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90%) were purchased from Sigma Aldrich and were used as received with no further purification. Ball milling (BM) of NaAlH4, MgH2, and LiBH4 powders in the mole ratio of 1:1:1 was performed in a planetary ball mill (NQM-0.4) for 1 h at the rate of 400 rpm. Handling of the samples was conducted in an MBraun Unilab glove box filled with high purity Ar atmosphere. Samples were put into a sealed stainless steel vial together with hardened stainless steel balls. The ratio of the weight of balls to the weight of powder was 30:1. For comparison purposes, pristine LiAlH4, MgH2 and NaBH4 were also milled for a period of 1 h. The thermal desorption performances and re/dehydrogenation kinetics experiments were performed in a Sievertstype pressure-composition-temperature (PCT) apparatus (Advanced Materials Corporation). The sample was loaded into a sample vessel in the glove box. For the thermal desorption performances experiment, all the samples were heated in a vacuum chamber, and the amount of desorbed hydrogen was measured to determine the lowest decomposition temperature. The heating rate for the thermal desorption performances experiment was 5 C min1, and samples were heated from room temperature to desired temperature. The re/de-hydrogenation kinetics measurements were performed at the desired temperature with initial hydrogen pressures of 5.0 MPa and 0.01 MPa respectively. XRD analysis was performed using a Rigaku MiniFlex II diffractometer with Cu Ka radiation. qe2q scans were carried out over diffraction angles from 25 to 80 with a speed of 2.00 min1. Before the measurement, a small amount of sample was spread uniformly on the sample holder, which was wrapped withplastic wrap to prevent oxidation. 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 min1, and different heating rates were used.
Results and discussion Fig. 1 presents the thermal desorption performances curves of the as-milled NaAlH4eMgH2eLiBH4 (molar ratio 1:1:1). From the curve, the NaAlH4eMgH2eLiBH4 sample clearly showed three major stages of dehydrogenation that occurred during the heating process. These include, namely: the first stage, which took place within the temperature range of 150e230 C; the second dehydrogenation stage, starting at approximately 275 C and completed at about 350 C; and the final stage, which occurred at a temperature of about 370 C and was completed at about 475 C. To clarify the dehydrogenation mechanism in every stage, XRD measurements were employed as shown in Fig. 2. After 1 h milling (Fig. 2(a)), it can be seen that the peaks of the starting materials, NaAlH4 and LiBH4, were absent. Further, new peaks corresponding to the formation of LiAlH4 and
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Meanwhile, after dehydrogenation at 350 C, as can be seen from the XRD result (Fig. 2(c)), the peaks for MgH2 disappear. Further, new peaks corresponding to Mg17Al12, Mg2Al3, and Li3Mg7 can be observed besides NaBH4. These results confirmed that the hydrogen released in the second stage is from the MgH2-relevant decomposition through the reactions in Eqs. (4)e(6) as illustrated below:
Fig. 1 e Thermal desorption performances curve of the NaAlH4eMgH2eLiBH4 composite.
NaBH4 can be observed besides MgH2. This result indicates that the solid state reaction between the components of the NaAlH4 and LiBH4 mixture was completed during the milling process, as shown in Eq. (1): NaAlH4 þ LiBH4 /LiAlH4 þ NaBH4
(1)
This result concurs with those reported in a previous literature study [50]. Fig. 2(b) indicates the presence of LiH and Al phases in addition to the MgH2 and NaBH4 after dehydrogenation at 230 C. The fact that no LiAlH4 phase was found indicates that the reactions in Eqs. (2) and (3) had been completed at this stage as shown below: 3LiAlH4 /Li3 AlH6 þ 2Al þ 3H2
(2)
Li3 AlH6 /3LiH þ Al þ 3=2H2
(3)
Fig. 2 e XRD patterns of the NaAlH4eMgH2eLiBH4 composite (a) after 1 h ball milling and after dehydrogenation at (b) 230 C, (c) 350 C, and (d) 475 C.
17MgH2 þ 12Al/Mg17 Al12 þ 17H2
(4)
2MgH2 þ 3Al/Mg2 Al3 þ 2H2
(5)
7MgH2 þ 3LiH/Li3 Mg7 þ 8:5H2
(6)
After further heating to 475 C (Fig. 2(d)), the NaBH4 phase disappeared, indicating that the system is fully dehydrogenated at 475 C. This concurs with the TPD results (Fig. 1). It also can be seen that the Mg17Al12 phase has disappeared and a new phase, NaH and Mg1xAlxB2 has formed. Mao et al. [49] have suggested that the new phase is Mg1xAlxB2 in their LiAlH4eMgH2eLiBH4 (molar ratio, 1:1:1) system. Based on the XRD results, the transformation after ball milling and the three-step dehydrogenation pathway for the NaAlH4eMgH2eLiBH4 system is proposed to be as follows:
NaAlH4 þ MgH2 þ LiBH4 /LiAlH4 eMgH2 eNaBH4 /Al þ LiH þ MgH2 þ NaBH4 þ 3=2H2 /ð1 21yÞ 27Mg17 Al12 þ ð5 þ 84yÞ 27Mg2 Al3 þ yLi3 Mg7 þ ð1 3yÞLiH þ NaBH4 þ ð1 þ 3=2yÞH2 /1 2Mg1x Alx B2 þ ð1 þ x 14yÞ 4Mg2 Al3 þ yLi3 Mg7 þ ð1 3yÞLiH þ NaH þ ð42y 5x þ 1Þ=4Al þ 3=2H2 (7) where x means the amount of Al which substitutes for Mg in MgB2 alloy and y means the amount of Li3Mg7 alloy [49]. In order to investigate the effects of mutual destabilization on the desorption temperature among the three hydrides, thermal desorption performances curve of as-milled LiAlH4, as-milled MgH2, as-milled NaBH4, and NaAlH4eMgH2eLiBH4 composite system were compared, as shown in Fig. 3. In addition, to confirm the phase transformation of NaAlH4eMgH2eLiBH4 to the mixture of LiAlH4eMgH2eNaBH4, the dehydrogenation curve of a mixture of LiAlH4eMgH2eNaBH4 (1:1:1) has been included in Fig. 3. The as-milled LiAlH4 starts to desorb hydrogen at about 142 C and about 173 C for the first and second stages respectively. Meanwhile, the as-milled MgH2 and as-milled NaBH4 start to release hydrogen at about 330 C and about 490 C, respectively. After mixed together, the curve shows a significant improvement for MgH2- and NaBH4-relevant decomposition in the NaAlH4eMgH2eLiBH4 composite system. After combined together, the decomposition temperature of MgH2 and NaBH4-relevant decomposition in the NaAlH4eMgH2eLiBH4 composite reduced by 55 C and 110 C respectively, compared with that of as-milled MgH2 and as-milled NaBH4. This indicates that the dehydrogenation performance of MgH2 and NaBH4 was significantly improved. However, compared with as-milled LiAlH4, the dehydrogenation temperature for the LiAlH4 in the NaAlH4eMgH2eLiBH4 composite showed no improvement. For the LiAlH4eMgH2eNaBH4 (1:1:1) mixture, the pattern of dehydrogenation curve is similar to the
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Fig. 3 e Thermal desorption performances curve of the asmilled LiAlH4, as-milled MgH2, as-milled NaBH4, NaAlH4eMgH2eLiBH4, and LiAlH4eMgH2eNaBH4 composite.
NaAlH4eMgH2eLiBH4 mixture. The curve clearly showed three major stages of dehydrogenation that corresponding to decomposition of LiAlH4, MgH2, and NaBH4. This result confirm that the NaAlH4eMgH2eLiBH4 mixture transform to LiAlH4/MgH2/NaBH4 during the milling process. The NaAlH4eMgH2eLiBH4 composite system was further investigated by TGA/DSC, as shown in Fig. 4. As can be seen, there were two distinct exothermic peaks and four distinct endothermic peaks present during the heating process. The first exothermic peak at 110 C was due to the presence of surface hydroxyl impurities in the LiAlH4 powder, as reported in our previous papers [10,12,13] and the first endothermic peak at 125 C corresponds to the melting of LiAlH4 [52]. The second exothermic peak at 180 C corresponds to the decomposition of liquid LiAlH4 (first step decomposition, Eq. (1)), while the second endothermic peak at 210 C is assigned to the decomposition of Li3AlH6 (second step decomposition, Eq. (2)). The third endothermic peak at about 330 C is due to the decomposition of MgH2, while the last endothermic peak at 450 C corresponds to the decomposition of NaBH4. The three major weight loss events in the TGA curve concur with the three major stages of dehydrogenation shown by the thermal desorption performances curve in Fig. 1. The kinetics enhancement is related to the energy barriers for H2 release. In order to investigate the kinetics enhancement of the NaAlH4eMgH2eLiBH4 composite in more detail, we used DSC curves at different heating rates to calculate the activation energy (EA) for the MgH2 and NaBH4-relevant decomposition in the system. Fig. 5 shows DSC traces for the NaAlH4eMgH2eLiBH4 composite at different heating rates (10, 15, and 20 C min1, respectively). The activation energy, EA, for the hydrogen desorption was obtained by performing a Kissinger analysis [53] according to the following Eq.: h . i ln b T2p ¼ EA RTp þ A
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Fig. 4 e TGA/DSC traces of the NaAlH4eMgH2eLiBH4 composite. Heating rate: 15 C minL1, argon flow: 30 ml minL1.
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 and third endothermic peaks, as shown in Fig. 6 for the NaAlH4eMgH2eLiBH4 composite. The apparent activation energy estimated from the Kissinger analysis for MgH2 and NaBH4-relevant decomposition was found to be 96.85 and 111.74 kJ mol1 respectively. This is greatly reduced from the reported value of pristine MgH2 (162 kJ mol1) [23] and PrF3-doped NaBH4 (235.3 kJ mol1) [54]. These results provide quantitative evidence for decreased kinetic barriers during the dehydrogenation process and, moreover, for improved dehydrogenation properties of the NaAlH4eMgH2eLiBH4 composite. As discussed in the Introduction Section, thermodynamic destabilization of mixed hydride progresses through the formation of an end product which has a lower enthalpy than that of the pure element. To determine the enthalpy (DHdec) of NaAlH4eMgH2eLiBH4 decomposition, the DSC curves were
(8)
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.
Fig. 5 e DSC traces of the NaAlH4eMgH2eLiBH4 composite at different heating rates.
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Fig. 6 e The Kissinger’s plot of the dehydrogenation for the MgH2 and NaBH4-relevant decomposition.
analyzed by STARe software. From the DSC peak areas (third endothermic event) in Fig. 4, the reaction enthalpy was integrated as 488.19 J/g (hydrides). According to the TG performance of NaAlH4eMgH2eLiBH4 (1:1:1) composite in Fig. 4, we observed that the dehydrogenation capacity (H/M in weight ratio) of the second stage is about 1.6 wt.%. This means that the enthalpy change calculated from the DSC curves is 61 kJ mol1 H2 for the MgH2-relevant decomposition (third endothermic event). By means of the same methods, the reaction enthalpies of NaBH4-relevant decompositions (last endothermic event) is determined to be 100 kJ mol1 H2. This is lower than the overall decomposition enthalpy of pure MgH2 (75.7 kJ mol1 H2) [23] and pure NaBH4 (191.9 kJ mol1 H2) [55]. This result confirmed that a mutual destabilization exists among the three hydrides. In relation to the evaluation of hydrogen storage reversibility, the fully dehydrogenated composite was subjected to a process of rehydriding under 5 MPa of hydrogen pressure at 350 C. Fig. 7 shows the isothermal rehydrogenation curve of the dehydrogenated NaAlH4eMgH2eLiBH4 composite. The dehydrogenated NaAlH4eMgH2eLiBH4 composite can absorb about 4.5 wt.% of hydrogen after 60 min of rehydrogenation. Specifically, the reversible hydrogen storage capacity is lower than the dehydrogenation amount of the as-milled sample, implying a partial reversibility. To understand the chemical events occurring in the reversible hydrogen storage process, it can be seen that the hydrogenated samples collected at 350 C were analyzed by means of XRD as shown in Fig. 8. After hydrogenation at 350 C, the characteristic diffraction peaks of MgH2, LiH and Al phases can be clearly observed. Interestingly, LiBH4 also appears as a rehydrogenation product, with no peaks corresponding to the NaBH4 phases. This phenomenon is likely due to occurrences during the cooling process (the rehydrogenation process was performed after the dehydrogenation process and the cooling process was performed under vacuum from about 530 C to 350 C). In this process, sodium hydride decomposes rapidly and molten sodium is formed, which
Fig. 7 e Isothermal rehydrogenation kinetics curve of the NaAlH4eMgH2eLiBH4 composite at 350 C and under 5 MPa.
might evaporate due to the combination of high temperature and low pressure [51]. After rehydrogenation, the Li3Mg7 peaks disappear, and peaks of MgH2 and LiH appear, indicating that Li3Mg7 can be dissociated into MgH2 and LiH (Eq. (9)) as reported by Mao et al. [49]. 2Li3 Mg7 þ 17H2 /14MgH2 þ 6LiH
(9)
However, the Mg1xAlxB2 phases still remain in the product, indicating that the recombination of MgH2 and LiBH4 is incomplete under these rehydrogenation conditions. This is illustrated in Eq. (10) [49] below: Mg1x Alx B2 þ 2LiH þ ð4 xÞH2 /ð1 xÞMgH2 þ 2LiBH4 þ xAl (10)
Fig. 8 e XRD patterns of the NaAlH4eMgH2eLiBH4 composite after rehydrogenation at 350 C and under 5 MPa.
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In relation to the Mg2Al3, if higher hydrogenation pressure was applied, Mg2Al3 would subsequently be transformed into MgH2 and Al, as shown in Eq. (11) [56]: Mg2 Al3 þ 2H2 /2MgH2 þ 3Al
(11)
In this study, Mg2Al3 was hardly to transform into MgH2 and Al due to the lower hydrogenation pressure applied (5 MPa). This phenomenon is similar to that reported by Mao et al. [49] for the LiAlH4eMgH2eLiBH4eTiF3 composite, in which their sample was rehydrogenated under 4 MPa. For comparison Chen et al. [57] reported that no Mg2Al3 phase was detected from their MgH2 þ LiAlH4 (4:1) composite sample after rehydrogenation at 350 C and under 10 MPa. This implies that the reaction (11) occurred when the higher hydrogen pressure was applied. So, in order to improve the rehydrogenation process of NaAlH4eMgH2eLiBH4 system, the higher hydrogen pressure (>10 Mpa) is needed. This is interesting issue for the future work. From these results, it can be pointed out that the NaAlH4eMgH2eLiBH4 system shows superior hydrogen storage properties compared with the unary component. These improvements could be attributed to the mutual interactions among the three hydrides. The formation of LieMg, MgeAl, and MgeAleB alloys as a reaction product during the dehydrogenation process may create a beneficial pathway for hydrogen atom diffusion to the surface and recombination. This can be attributed to the improvement of the thermodynamic properties of the NaAlH4eMgH2eLiBH4 composite system.
Conclusion In summary, it has been demonstrated that there is a mutual destabilization existing between the hydrides in the ternary NaAlH4eMgH2eLiBH4 (1:1:1) system. This system exhibits superior hydrogen storage properties compared with the unary components. Structural analyses revealed the NaAlH4eMgH2eLiBH4 combination converted to the LiAlH4eMgH2eNaBH4 mixture after ball milling. Upon dehydrogenation, LiAlH4 decomposed first to form LiH and Al with hydrogen release at below 250 C. Subsequently, these LiH and Al reacted with MgH2 to produce Mg17Al12, Mg2Al3, and Li3Mg7 and hydrogen, respectively, at 250e350 C. With a further increase to 475 C, the reaction between NaBH4 and Mg17Al12 took place to release all of the hydrogen and yield NaH and Mg1xAlxB2. DSC measurements indicate that the enthalpy change in the second and third step decomposition is recorded as 61 and 100 kJ mol1 H2, respectively. This is smaller than that of MgH2 and NaBH4 alone (76 and 191 kJ mol1 H2). The Kissinger plots for different heating rates in DSC show that the apparent activation energy, EA, for decomposition of MgH2- and NaBH4 relevant in the NaAlH4eMgH2eLiBH4 composite is reduced to 96.85 and 111.74 kJ mol1. The formation of intermediate compounds upon dehydrogenation, including LieMg, MgeAl, and MgeAleB alloys, change the thermodynamics of the reactions through altering the de/rehydrogenation pathway. Hence, it is believed that this plays a critical role in the enhancement of dehydrogenation in the NaAlH4eMgH2eLiBH4 composite.
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Acknowledgment This work was supported by The Ministry of Higher Education of Malaysia under Research Grant FRGS 59295. The author also acknowledges the Universiti Malaysia Terengganu for providing the facilities to carry out this project.
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