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Effect of LaCl3 addition on the hydrogen storage properties of MgH2
Energy 79 (2015) 177e182
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Effect of LaCl3 addition on the hydrogen storage properties of MgH2 M. Ismail* School of Ocean Engineering, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2014 Received in revised form 22 October 2014 Accepted 2 November 2014 Available online 26 November 2014
In this study, the effect of LaCl3 on the hydrogen storage properties of MgH2 prepared by ball milling was investigated for the first time. It was found that the MgH2 þ 10 wt.% LaCl3 sample started to decompose at around 300 C, which was 50 C lower than in as-milled MgH2. For desorption kinetics, the LaCl3-doped MgH2 composite sample released about 4.2 wt.% hydrogen at 320 C after 5 min dehydrogenation, while the as-milled MgH2 only released about 0.2 wt.% hydrogen for the same temperature and time. Meanwhile, a hydrogen absorption capacity of 5.1 wt.% was reached at 300 C in 2 min for the LaCl3-doped MgH2 sample. In contrast, the ball-milled MgH2 only absorbed 3.8 wt.% hydrogen at 300 C in 2 min. The activation energy of dehydrogenation was 166.0 kJ/mol for the as-milled MgH2 and 143.0 kJ/mol for the 10 wt.% LaCl3-added MgH2, indicating that the LaCl3 additive decreased the activation energy for the hydrogen desorption of the MgH2. The improved hydrogen storage properties of the MgH2 in the presence of LaCl3 is believed to be due to the catalytic effects of the LaeMg alloy and MgCl2 that were formed in situ during the heating process. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen storage Magnesium hydride Lanthanum chloride Catalytic effect
1. Introduction Compared to gaseous and liquid hydrogen storage, solid-state hydrogen storage attracts attention due to its advantages such as high gravimetric hydrogen capacity, safety and space for storage, and processing convenience [1]. Although solid-state storage is promising as a potential energy carrier for the future, the problem is finding the materials that can achieve the target by DOE (Department of Energy) such as release and absorb >6.5 wt.% H2, reversibility, and the ability to operate at moderate temperatures [2]. Among solid-state hydrogen storage materials, growing interest has been shown in MgH2 of the metal hydride family due to its large gravimetric density (7.6 wt.% H2), abundant resources, low cost and good reversibility. However, the disadvantages of MgH2 are that it is too stable, leading to desorption temperatures that are too high. Furthermore, the kinetics of the hydrogen uptake and release in Mg are poor. Many studies have been conducted to improve the hydrogen storage properties of MgH2 through the use of a catalyst [3e10], the use of ball milling to produce smaller particles [11e13] and the combination of MgH2 with other metal/ complex hydrides (destabilisation systems) [14e27]. Among these, the use of catalysts has been shown to play a vital role in reducing the decomposition temperature, enhancing the sorption kinetics,
* Tel.: þ60 9 6683487; fax: þ60 9 6683991. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.energy.2014.11.001 0360-5442/© 2014 Elsevier Ltd. All rights reserved.
and improving the reversibility of MgH2. Among catalysts, metal halide- and metal oxide-based such as TiF3 and Nb2O5 are found the most promising catalyst. The significant improvement of MgH2 sorption properties in the MgH2/TiF3 [28] and MgH2/Nb2O5 [29] is due to the catalytic effects of in-situ-generated TiH and NbH2 species which were formed during the dehydrogenation/hydrogenation process. Rare-earth chlorides as catalysts have been widely applied in the light complex metal hydrides such as NaAlH4 [30e32] and LiAlH4 [33,34]. However, the role of rare-earth chlorides as a catalyst in the hydrogen storage capability of MgH2 has not yet been extensively explored, to the best of the author's knowledge. According to Sun et al. [31], by using a series of rare-earth chlorides, namely, SmCl3, CeCl3, NdCl3, GdCl3, LaCl3, and ErCl3, as dopants, the dehydriding rate of doped NaAlH4 can be considerably improved. The highest catalytic efficiency was found for SmCl3 and CeCl3, followed by NdCl3, GdCl3, LaCl3, and ErCl3. Sun et al. used LaCl3 to further explore the mechanism of rare-earth elements and claimed that La was first hydrogenised as LaH2 and then formed some kinds of LaeAl alloy (e.g., La3Al11) when the temperature was raised to a certain level. Moreover, the reactions that occurred between LaCl3 and NaAlH4 were found to enhance the dehydrogenation kinetics of the whole system. Therefore, it is reasonable to hypothesise that LaCl3 would show great potential as a catalyst to advance MgH2 hydrogen storage performance. However, the effects of the LaCl3 additive on MgH2 have not been reported so far, to the best of the author's knowledge.
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In this work, LaCl3 was used as a catalyst precursor to study the effect on the hydrogen storage properties of MgH2 prepared by ball milling. The aim of this study is to further investigate the addition of a different type of catalyst and examine the difference in the way they take effect, and therefore, gain deeper understanding on the modification of hydrogen storage properties of the MgH2. The possible catalyst mechanism that is supported by the results is discussed accordingly. 2. Experimental details Ball milling of MgH2 and LaCl3 powders was performed in a planetary ball mill for 1 h, by milling for 0.5 h, resting for 6 min, and then milling for another 0.5 h in a different direction at the rate of 400 rpm. Handling of the samples was conducted in an MBraun Unilab glovebox filled with a high-purity Ar atmosphere. About 400 mg of MgH2 (95% pure; Sigma Aldrich) was mixed with 10 wt.% of LaCl3 (98% pure; Sigma Aldrich). The samples were put into a sealed stainless steel vial together with hardened stainless steel balls. The ratio of the weight of the balls to the weight of the powder was 30:1. The measurements for dehydrogenation and rehydrogenation were performed in a Sievert-type PCT (Pressure-composition temperature) apparatus (Advanced Materials Corporation) or also known as GRC (Gas Reaction Controller) apparatus. The GRC performs quantitative analysis of the gasesolid reaction. It admits a controlled amount of gas into the reaction chamber that holds a specimen and monitors the pressure of the gas while the temperature of the chamber is held constant or slowly changed. The amount of gas absorbed by the specimen is determined by calculating the amount of the remaining gas. The instrument is connected to a computer and controlled by software (GrcLV), which performs in fully automatic operations. About 100 mg of the sample was loaded into a sample vessel. This apparatus can operate at up to 200 atm and 900 C. For desorption purposes, all the samples were heated under a controlled vacuum of 0.1 atm. The heating rate for the desorption experiment was 5 C/min. Rehydrogenation studies were carried out after the first complete dehydrogenation, and the samples were kept at 300 C under 30 atm hydrogen pressure for 1 h in order to reabsorb hydrogen. All the temperature and hydrogen absorb/desorbs measurements reported in this paper were correct within ±2 C and ±0.1 wt.%. The measurements for dehydrogenation and rehydrogenation were repeated for three times and the average was used as a final result. The phase structures of the samples, before and after desorption, as well as after rehydrogenation, were determined by X-ray diffraction (Rigaku MiniFlex II diffractometer with Cu Ka radiation). XRD (X-ray diffraction) is a non-destructive analytical method which can yield the unique fingerprint of Bragg reflections associated with a crystal structure. The nature of the powders, whether crystalline or amorphous, can be determined using XRD. A crystalline powder is a material that has an internal structure in which the atoms are arranged in an orderly three-dimensional configuration. An amorphous powder is a non-crystalline material that has no definite order or crystallinity. X-rays with a similar wavelength to the distances between planes of the crystal structure can be reflected such that the angle of reflection is equal to the angle of incidence. This is called ‘diffraction’ and can be described by Bragg's law:
2d sin q ¼ nl
at this angle. The position of these reflections is related to the interlayer spacings of atoms in the crystal structure. Before measurement, a small amount of sample was spread uniformly on the sample holder, which was wrapped with plastic wrap to prevent oxidation. q2q scans were carried out over diffraction angles from 20 to 80 with a speed of 2.00 /min. DSC (Differential scanning calorimetry) analysis of the dehydrogenation process was carried out on a Mettler Toledo TGA (Thermogravimetry analysis)/DSC 1, with temperature range from room temperature to 1200 C and gas flow between 0 and 200 mL/ min, with the capability to switch up to 4 gases such as air, argon, nitrogen, and oxygen. DSC is another type of thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample is recorded. DSC can be used to determine the thermodynamics properties data such as on entropy and enthalpy. Approximately 2e6 mg of sample was loaded into an alumina crucible in the glovebox. The crucible was then placed in a sealed glass bottle in order to prevent oxidation during transportation from the glovebox to the DSC apparatus. An empty alumina crucible was used as the reference material. The samples were heated from room temperature to 500 C under a 1 atm flowing argon atmosphere, and different heating rates were used. 3. Results and discussion Fig. 1 shows the TPD (temperature-programmed desorption) results for the as-received MgH2, the as-milled MgH2 and the MgH2 with 10 wt.% LaCl3 added. The as-received MgH2 started to release hydrogen at about 410 C, and desorbed about 7.2 wt.% hydrogen (7.6 wt.% H2 was theoretically released). After milling, the onset desorption temperature of the MgH2 was reduced to about 350 C, indicating that the milling process also influenced the onset decomposition temperature of the MgH2. The as-milled MgH2 released about 7.4 wt.% hydrogen after 420 C. After adding 10 wt.% LaCl3, the onset decomposition temperature of the MgH2 decreased to about 300 C and the full dehydrogenation was completed below 375 C, which was 50 and 110 C lower than for the as-milled and as-received MgH2, respectively. The total amount of hydrogen release was about 6.7 wt.%. To further examine the effects of doping MgH2 with LaCl3, the isothermal dehydrogenation kinetics curve of the LaCl3-doped
(1)
where d is the interplanar spacing, q the Bragg angle, n is the order of reflection, and l is the X-ray wavelength. When Bragg's law is satisfied, constructive interference of diffracted X-ray beams occurs, and a ‘Bragg reflection’ will be detected by a detector scanning
Fig. 1. TPD patterns for the dehydrogenation of the as-received MgH2, as-milled MgH2 and MgH2 doped with 10 wt.% LaCl3.
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Fig. 2. Isothermal desorption kinetics curves for the as-milled MgH2 and the MgH2 doped with 10 wt.% LaCl3 at 320 C under vacuum.
MgH2 composite at 320 C under vacuum (after rehydrogenation under 30 atm H2 at 300 C) was collected, as shown in Fig. 2. The isothermal dehydrogenation of MgH2 was also examined for comparison purposes under the same conditions. After 5 min dehydrogenation, the LaCl3-doped MgH2 composite sample released about 4.2 wt% hydrogen at 320 C, but the MgH2 sample only released about 0.2 wt.% hydrogen at the same time and temperature. Saturation of the dehydrogenation process for the LaCl3doped MgH2 composite sample at 320 C was achieved within 15 min. This result indicates that the dehydrogenation kinetics of the MgH2 was significantly improved after doping with LaCl3. In order to investigate the isothermal rehydrogenation kinetics of the LaCl3-doped MgH2 composite, the rehydrogenation of the dehydrogenated samples was performed under 30 atm of H2 at 300 C, as shown in Fig. 3. The undoped MgH2 was also examined for comparison. The results show that adding LaCl3 caused the MgH2 to absorb as much as 5.1 wt.% hydrogen within 2 min at
Fig. 4. DSC traces of the as-milled MgH2 and the MgH2þ10 wt.% LaCl3 (Heating rate: 25 C/min, argon flow: 30 ml/min).
300 C, while the MgH2 only absorbed about 3.8 wt.% hydrogen after the same period of time. This result suggests that the LaCl3 additive also improved the rehydrogenation kinetics of the MgH2. The thermal properties of the LaCl3-doped MgH2 sample were further investigated by DSC, as shown in Fig. 4 (25 C/min heating rate). For comparison, the as-milled MgH2 was also included in the DSC investigation. The DSC curve of the as-milled MgH2 and the LaCl3-doped MgH2 sample displayed only one strong endothermic peak at approximately 442.562 C and 380.254 C, respectively, corresponding to the decomposition of the MgH2. The notable reduction of the peak temperature in the DSC results reveals that the dehydrogenation properties of MgH2 were significantly improved by adding LaCl3. However, the onset decomposition temperatures of the samples in the DSC were slightly higher than in the TPD (Fig. 1). These differences may have resulted from the fact that the dehydrogenation was conducted under different heating rates and there were different heating atmospheres in the two types of measurements, as explained in our previous papers [35e38]. The improvement of the decomposition temperature and sorption kinetic is related to the energy barrier for H2 release from MgH2. In the present study, the activation energy for decomposition of the MgH2 was reduced by adding LaCl3. To calculate the activation energy of the as-milled MgH2 and the LaCl3-added MgH2, the Kissinger plot was used. The plot was obtained from the Kissinger equation [39] as follows:
h . i ln b Tp2 ¼ EA RTp þ A
Fig. 3. Isothermal absorption kinetics measurement of the as-milled MgH2 and the MgH2 doped with 10 wt.% LaCl3 at 300 C under 30 atm hydrogen pressure.
179
(2)
where b is the heating rate, Tp is the peak temperature in the DSC curve, R is the gas constant and A is a linear constant. Thus, the activation energy, EA, can be obtained from the slope in a plot of ln [b/T2p] versus 1000/Tp. Fig. 5(a) and (b) show the DSC traces for the as-milled MgH2 and LaCl3-added MgH2 composite at different heating rates. From a Kissinger plot of the DSC data (Fig. 6), the apparent activation energy for the LaCl3-added MgH2 composite is found to be 143.0 kJ/mol, which is much lower than the activation energy of the decomposition of the as-milled MgH2 (166.0 kJ/mol). This reduction indicates that the apparent activation energy for
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Fig. 6. Kissinger's plot of the dehydrogenation for the 10 wt.% LaCl3-doped MgH2 composite as compared with the as-milled MgH2.
Fig. 5. DSC traces at different heating rates for (a) as-milled MgH2 and (b) MgH2þ10 wt.% LaCl3.
confirms the possibility of the reaction in equation (2) from the thermodynamic potentials. In the XRD pattern of the rehydrogenated sample (Fig. 7(c)), the characteristic diffraction peaks of Mg disappear and the characteristic diffraction peaks of MgH2 appear, indicating that Mg was largely transformed into MgH2 during the rehydrogenation process. In addition, the MgCl2 and Mg3La peaks remained unchanged after the rehydrogenation process. From the above analyses, the improved sorption properties of MgH2 by doping with LaCl3 could be explained by a number of reasons. The formation of the LaeMg alloy, Mg3La, that resulted from the reaction of the MgH2 and LaCl3 (Eq. (3)) during the heating process may play an important role in the enhancement of MgH2 sorption. It is well known that the dehydrogenation product in the light metal hydride-catalyst system could act as a real catalyst to facilitate the de/rehydrogenation step. These products could create surface activation and form a large number of nucleation sites at the surface of the MgH2 matrix. It is also believed that the finely
decomposition of hydrogen from the MgH2 was reduced by doping with the LaCl3. In order to clarify the phase structure of the LaCl3-doped MgH2 sample after 1 h milling, after dehydrogenation at 450 C, and after rehydrogenation at 300 C under 3 MPa hydrogen pressure, XRD was used, as shown in Fig. 7. After the ball milling processes (Fig. 7(a)), the main phases presented were the formations of the parent materials, MgH2 and LaCl3. No new compounds were formed from the mixtures. After dehydrogenation at 450 C, the XRD pattern of Fig. 7(b) reveals that there were distinct peaks of Mg, which indicates that the dehydrogenation process of MgH2 was completed. In addition, the peaks for LaCl3 disappear, and some new peaks corresponding to an LaeMg alloy (Mg3La) and MgCl2 are observed, suggesting that the reaction of MgH2 with LaCl3 may have occurred during the heating process as follows:
9MgH2 þ 2LaCl3 /3MgCl2 þ 2Mg3 La þ 9H2
(3)
The standard Gibbs free energy, DG f of MgH2, LaCl3, MgCl2 and Mg3La is 35.98 [40], 708.9 [41], -592.12 [40] and 79.65 kJ/mol [42], respectively; thus, the total change DG associated with the reaction in equation (2) will be 194.04 kJ/mol of MgH2. This
Fig. 7. XRD patterns of the 10 wt.% LaCl3-doped MgH2 (a) after milling, (b) after dehydrogenation, and (c) after rehydrogenation.
M. Ismail / Energy 79 (2015) 177e182
dispersed dehydrogenated products may contribute to kinetic de/ absorption improvement by serving as the active sites for nucleation and creating the dehydrogenation product by shortening the diffusion distance of the reaction ions [43,44]. Furthermore, the function of Cl may also introduce an extra catalytic effect on MgH2 sorption properties. As discussed in the literature [45,46], the catalytic effect of a metal halide on the hydrogen sorption of MgH2 could also be simultaneously influenced by several factors, such as the formation of MgF2 and the catalytic influence of transition metal halides (with different levels of metal oxidation state). Based on this, in this study, the chlorine-based product, MgCl2, may have also introduced an extra catalytic effect on MgH2 sorption properties, as proved in previous reports [7,47]. The catalytic effect of MgCl2 may further combine with the catalytic function of the LaeMg alloy species to generate a synergetic effect. In addition, the reaction in equation (3) could generate clean surfaces (without MgO at the MgH2 surface as proved by the XRD results in Fig. 7) and, subsequently, increase the surface reactivity and the decomposition reaction. However, further work is necessary to clarify the exact role of the LaCl3 addition in MgH2 by observation methods such as transmission electron microscopy. 4. Conclusion In summary, LaCl3 showed a good catalytic effect, giving MgH2 both a significantly decreased decomposition temperature and enhanced sorption kinetics. The addition of 10 wt.% LaCl3 led to the release of hydrogen at about 300 C, decreasing the decomposition temperature by 50 C compared to the as-milled MgH2. Furthermore, the kinetic desorption results showed that the added MgH2 released about 4.2 wt.% hydrogen within 5 min at 320 C, while the MgH2 only released 0.2 wt.% hydrogen within the same time and temperature parameters. Meanwhile, a hydrogen absorption capacity of 5.1 wt.% was reached at 300 C in 2 min for the LaCl3-doped MgH2 sample. In contrast, the ball-milled MgH2 only absorbed 3.8 wt.% hydrogen at 300 C in 2 min. The apparent activation energy for hydrogen desorption was decreased from 166.0 kJ/mol for the as-milled MgH2 to 143.0 kJ/mol by the addition of 10 wt.% LaCl3. This indicates that the catalytic effect due to the addition of LaCl3 significantly decreased the activation energy for the hydrogen desorption of MgH2. Based on the results, it is believed that the significant effects of the LaCl3 on the hydrogen storage properties of MgH2 were due to the catalytic effects of the LaeMg alloy and MgCl2 that formed in situ during the heating process. Acknowledgements The author thanks the Universiti Malaysia Terengganu for providing the facilities to carry out this project. The author also acknowledges the Malaysian Government for financial support through the Fundamental Research Grant Scheme (59295). References [1] Principi G, Agresti F, Maddalena A, Lo Russo S. The problem of solid state hydrogen storage. Energy 2009;34:2087e91. [2] Satyapal S, Petrovic J, Read C, Thomas G, Ordaz G, The US. Department of Energy's National Hydrogen Storage Project: progress towards meeting hydrogen-powered vehicle requirements. Catal Today 2007;120:246e56. [3] Mustafa NS, Ismail M. Influence of K2TiF6 additive on the hydrogen sorption properties of MgH2. Int J Hydrogen Energy 2014;39:15563e9. [4] Ranjbar A, Ismail M, Guo ZP, Yu XB, Liu HK. Effects of CNTs on the hydrogen storage properties of MgH2 and MgH2-BCC composite. Int J Hydrog Energy 2010;35:7821e6. [5] Ismail M, Zhao Y, Yu XB, Dou SX. Improved hydrogen storage properties of MgH2 doped with chlorides of transition metals Hf and Fe. Energy Educ Sci Technol A Energy Sci Res 2012;30(SPEC.ISS.1):107e22.
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Effect of LaCl3 addition on the hydrogen storage properties of MgH2 M. Ismail* School of Ocean Engineering, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 April 2014 Received in revised form 22 October 2014 Accepted 2 November 2014 Available online 26 November 2014
In this study, the effect of LaCl3 on the hydrogen storage properties of MgH2 prepared by ball milling was investigated for the first time. It was found that the MgH2 þ 10 wt.% LaCl3 sample started to decompose at around 300 C, which was 50 C lower than in as-milled MgH2. For desorption kinetics, the LaCl3-doped MgH2 composite sample released about 4.2 wt.% hydrogen at 320 C after 5 min dehydrogenation, while the as-milled MgH2 only released about 0.2 wt.% hydrogen for the same temperature and time. Meanwhile, a hydrogen absorption capacity of 5.1 wt.% was reached at 300 C in 2 min for the LaCl3-doped MgH2 sample. In contrast, the ball-milled MgH2 only absorbed 3.8 wt.% hydrogen at 300 C in 2 min. The activation energy of dehydrogenation was 166.0 kJ/mol for the as-milled MgH2 and 143.0 kJ/mol for the 10 wt.% LaCl3-added MgH2, indicating that the LaCl3 additive decreased the activation energy for the hydrogen desorption of the MgH2. The improved hydrogen storage properties of the MgH2 in the presence of LaCl3 is believed to be due to the catalytic effects of the LaeMg alloy and MgCl2 that were formed in situ during the heating process. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen storage Magnesium hydride Lanthanum chloride Catalytic effect
1. Introduction Compared to gaseous and liquid hydrogen storage, solid-state hydrogen storage attracts attention due to its advantages such as high gravimetric hydrogen capacity, safety and space for storage, and processing convenience [1]. Although solid-state storage is promising as a potential energy carrier for the future, the problem is finding the materials that can achieve the target by DOE (Department of Energy) such as release and absorb >6.5 wt.% H2, reversibility, and the ability to operate at moderate temperatures [2]. Among solid-state hydrogen storage materials, growing interest has been shown in MgH2 of the metal hydride family due to its large gravimetric density (7.6 wt.% H2), abundant resources, low cost and good reversibility. However, the disadvantages of MgH2 are that it is too stable, leading to desorption temperatures that are too high. Furthermore, the kinetics of the hydrogen uptake and release in Mg are poor. Many studies have been conducted to improve the hydrogen storage properties of MgH2 through the use of a catalyst [3e10], the use of ball milling to produce smaller particles [11e13] and the combination of MgH2 with other metal/ complex hydrides (destabilisation systems) [14e27]. Among these, the use of catalysts has been shown to play a vital role in reducing the decomposition temperature, enhancing the sorption kinetics,
* Tel.: þ60 9 6683487; fax: þ60 9 6683991. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.energy.2014.11.001 0360-5442/© 2014 Elsevier Ltd. All rights reserved.
and improving the reversibility of MgH2. Among catalysts, metal halide- and metal oxide-based such as TiF3 and Nb2O5 are found the most promising catalyst. The significant improvement of MgH2 sorption properties in the MgH2/TiF3 [28] and MgH2/Nb2O5 [29] is due to the catalytic effects of in-situ-generated TiH and NbH2 species which were formed during the dehydrogenation/hydrogenation process. Rare-earth chlorides as catalysts have been widely applied in the light complex metal hydrides such as NaAlH4 [30e32] and LiAlH4 [33,34]. However, the role of rare-earth chlorides as a catalyst in the hydrogen storage capability of MgH2 has not yet been extensively explored, to the best of the author's knowledge. According to Sun et al. [31], by using a series of rare-earth chlorides, namely, SmCl3, CeCl3, NdCl3, GdCl3, LaCl3, and ErCl3, as dopants, the dehydriding rate of doped NaAlH4 can be considerably improved. The highest catalytic efficiency was found for SmCl3 and CeCl3, followed by NdCl3, GdCl3, LaCl3, and ErCl3. Sun et al. used LaCl3 to further explore the mechanism of rare-earth elements and claimed that La was first hydrogenised as LaH2 and then formed some kinds of LaeAl alloy (e.g., La3Al11) when the temperature was raised to a certain level. Moreover, the reactions that occurred between LaCl3 and NaAlH4 were found to enhance the dehydrogenation kinetics of the whole system. Therefore, it is reasonable to hypothesise that LaCl3 would show great potential as a catalyst to advance MgH2 hydrogen storage performance. However, the effects of the LaCl3 additive on MgH2 have not been reported so far, to the best of the author's knowledge.
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In this work, LaCl3 was used as a catalyst precursor to study the effect on the hydrogen storage properties of MgH2 prepared by ball milling. The aim of this study is to further investigate the addition of a different type of catalyst and examine the difference in the way they take effect, and therefore, gain deeper understanding on the modification of hydrogen storage properties of the MgH2. The possible catalyst mechanism that is supported by the results is discussed accordingly. 2. Experimental details Ball milling of MgH2 and LaCl3 powders was performed in a planetary ball mill for 1 h, by milling for 0.5 h, resting for 6 min, and then milling for another 0.5 h in a different direction at the rate of 400 rpm. Handling of the samples was conducted in an MBraun Unilab glovebox filled with a high-purity Ar atmosphere. About 400 mg of MgH2 (95% pure; Sigma Aldrich) was mixed with 10 wt.% of LaCl3 (98% pure; Sigma Aldrich). The samples were put into a sealed stainless steel vial together with hardened stainless steel balls. The ratio of the weight of the balls to the weight of the powder was 30:1. The measurements for dehydrogenation and rehydrogenation were performed in a Sievert-type PCT (Pressure-composition temperature) apparatus (Advanced Materials Corporation) or also known as GRC (Gas Reaction Controller) apparatus. The GRC performs quantitative analysis of the gasesolid reaction. It admits a controlled amount of gas into the reaction chamber that holds a specimen and monitors the pressure of the gas while the temperature of the chamber is held constant or slowly changed. The amount of gas absorbed by the specimen is determined by calculating the amount of the remaining gas. The instrument is connected to a computer and controlled by software (GrcLV), which performs in fully automatic operations. About 100 mg of the sample was loaded into a sample vessel. This apparatus can operate at up to 200 atm and 900 C. For desorption purposes, all the samples were heated under a controlled vacuum of 0.1 atm. The heating rate for the desorption experiment was 5 C/min. Rehydrogenation studies were carried out after the first complete dehydrogenation, and the samples were kept at 300 C under 30 atm hydrogen pressure for 1 h in order to reabsorb hydrogen. All the temperature and hydrogen absorb/desorbs measurements reported in this paper were correct within ±2 C and ±0.1 wt.%. The measurements for dehydrogenation and rehydrogenation were repeated for three times and the average was used as a final result. The phase structures of the samples, before and after desorption, as well as after rehydrogenation, were determined by X-ray diffraction (Rigaku MiniFlex II diffractometer with Cu Ka radiation). XRD (X-ray diffraction) is a non-destructive analytical method which can yield the unique fingerprint of Bragg reflections associated with a crystal structure. The nature of the powders, whether crystalline or amorphous, can be determined using XRD. A crystalline powder is a material that has an internal structure in which the atoms are arranged in an orderly three-dimensional configuration. An amorphous powder is a non-crystalline material that has no definite order or crystallinity. X-rays with a similar wavelength to the distances between planes of the crystal structure can be reflected such that the angle of reflection is equal to the angle of incidence. This is called ‘diffraction’ and can be described by Bragg's law:
2d sin q ¼ nl
at this angle. The position of these reflections is related to the interlayer spacings of atoms in the crystal structure. Before measurement, a small amount of sample was spread uniformly on the sample holder, which was wrapped with plastic wrap to prevent oxidation. q2q scans were carried out over diffraction angles from 20 to 80 with a speed of 2.00 /min. DSC (Differential scanning calorimetry) analysis of the dehydrogenation process was carried out on a Mettler Toledo TGA (Thermogravimetry analysis)/DSC 1, with temperature range from room temperature to 1200 C and gas flow between 0 and 200 mL/ min, with the capability to switch up to 4 gases such as air, argon, nitrogen, and oxygen. DSC is another type of thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample is recorded. DSC can be used to determine the thermodynamics properties data such as on entropy and enthalpy. Approximately 2e6 mg of sample was loaded into an alumina crucible in the glovebox. The crucible was then placed in a sealed glass bottle in order to prevent oxidation during transportation from the glovebox to the DSC apparatus. An empty alumina crucible was used as the reference material. The samples were heated from room temperature to 500 C under a 1 atm flowing argon atmosphere, and different heating rates were used. 3. Results and discussion Fig. 1 shows the TPD (temperature-programmed desorption) results for the as-received MgH2, the as-milled MgH2 and the MgH2 with 10 wt.% LaCl3 added. The as-received MgH2 started to release hydrogen at about 410 C, and desorbed about 7.2 wt.% hydrogen (7.6 wt.% H2 was theoretically released). After milling, the onset desorption temperature of the MgH2 was reduced to about 350 C, indicating that the milling process also influenced the onset decomposition temperature of the MgH2. The as-milled MgH2 released about 7.4 wt.% hydrogen after 420 C. After adding 10 wt.% LaCl3, the onset decomposition temperature of the MgH2 decreased to about 300 C and the full dehydrogenation was completed below 375 C, which was 50 and 110 C lower than for the as-milled and as-received MgH2, respectively. The total amount of hydrogen release was about 6.7 wt.%. To further examine the effects of doping MgH2 with LaCl3, the isothermal dehydrogenation kinetics curve of the LaCl3-doped
(1)
where d is the interplanar spacing, q the Bragg angle, n is the order of reflection, and l is the X-ray wavelength. When Bragg's law is satisfied, constructive interference of diffracted X-ray beams occurs, and a ‘Bragg reflection’ will be detected by a detector scanning
Fig. 1. TPD patterns for the dehydrogenation of the as-received MgH2, as-milled MgH2 and MgH2 doped with 10 wt.% LaCl3.
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Fig. 2. Isothermal desorption kinetics curves for the as-milled MgH2 and the MgH2 doped with 10 wt.% LaCl3 at 320 C under vacuum.
MgH2 composite at 320 C under vacuum (after rehydrogenation under 30 atm H2 at 300 C) was collected, as shown in Fig. 2. The isothermal dehydrogenation of MgH2 was also examined for comparison purposes under the same conditions. After 5 min dehydrogenation, the LaCl3-doped MgH2 composite sample released about 4.2 wt% hydrogen at 320 C, but the MgH2 sample only released about 0.2 wt.% hydrogen at the same time and temperature. Saturation of the dehydrogenation process for the LaCl3doped MgH2 composite sample at 320 C was achieved within 15 min. This result indicates that the dehydrogenation kinetics of the MgH2 was significantly improved after doping with LaCl3. In order to investigate the isothermal rehydrogenation kinetics of the LaCl3-doped MgH2 composite, the rehydrogenation of the dehydrogenated samples was performed under 30 atm of H2 at 300 C, as shown in Fig. 3. The undoped MgH2 was also examined for comparison. The results show that adding LaCl3 caused the MgH2 to absorb as much as 5.1 wt.% hydrogen within 2 min at
Fig. 4. DSC traces of the as-milled MgH2 and the MgH2þ10 wt.% LaCl3 (Heating rate: 25 C/min, argon flow: 30 ml/min).
300 C, while the MgH2 only absorbed about 3.8 wt.% hydrogen after the same period of time. This result suggests that the LaCl3 additive also improved the rehydrogenation kinetics of the MgH2. The thermal properties of the LaCl3-doped MgH2 sample were further investigated by DSC, as shown in Fig. 4 (25 C/min heating rate). For comparison, the as-milled MgH2 was also included in the DSC investigation. The DSC curve of the as-milled MgH2 and the LaCl3-doped MgH2 sample displayed only one strong endothermic peak at approximately 442.562 C and 380.254 C, respectively, corresponding to the decomposition of the MgH2. The notable reduction of the peak temperature in the DSC results reveals that the dehydrogenation properties of MgH2 were significantly improved by adding LaCl3. However, the onset decomposition temperatures of the samples in the DSC were slightly higher than in the TPD (Fig. 1). These differences may have resulted from the fact that the dehydrogenation was conducted under different heating rates and there were different heating atmospheres in the two types of measurements, as explained in our previous papers [35e38]. The improvement of the decomposition temperature and sorption kinetic is related to the energy barrier for H2 release from MgH2. In the present study, the activation energy for decomposition of the MgH2 was reduced by adding LaCl3. To calculate the activation energy of the as-milled MgH2 and the LaCl3-added MgH2, the Kissinger plot was used. The plot was obtained from the Kissinger equation [39] as follows:
h . i ln b Tp2 ¼ EA RTp þ A
Fig. 3. Isothermal absorption kinetics measurement of the as-milled MgH2 and the MgH2 doped with 10 wt.% LaCl3 at 300 C under 30 atm hydrogen pressure.
179
(2)
where b is the heating rate, Tp is the peak temperature in the DSC curve, R is the gas constant and A is a linear constant. Thus, the activation energy, EA, can be obtained from the slope in a plot of ln [b/T2p] versus 1000/Tp. Fig. 5(a) and (b) show the DSC traces for the as-milled MgH2 and LaCl3-added MgH2 composite at different heating rates. From a Kissinger plot of the DSC data (Fig. 6), the apparent activation energy for the LaCl3-added MgH2 composite is found to be 143.0 kJ/mol, which is much lower than the activation energy of the decomposition of the as-milled MgH2 (166.0 kJ/mol). This reduction indicates that the apparent activation energy for
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Fig. 6. Kissinger's plot of the dehydrogenation for the 10 wt.% LaCl3-doped MgH2 composite as compared with the as-milled MgH2.
Fig. 5. DSC traces at different heating rates for (a) as-milled MgH2 and (b) MgH2þ10 wt.% LaCl3.
confirms the possibility of the reaction in equation (2) from the thermodynamic potentials. In the XRD pattern of the rehydrogenated sample (Fig. 7(c)), the characteristic diffraction peaks of Mg disappear and the characteristic diffraction peaks of MgH2 appear, indicating that Mg was largely transformed into MgH2 during the rehydrogenation process. In addition, the MgCl2 and Mg3La peaks remained unchanged after the rehydrogenation process. From the above analyses, the improved sorption properties of MgH2 by doping with LaCl3 could be explained by a number of reasons. The formation of the LaeMg alloy, Mg3La, that resulted from the reaction of the MgH2 and LaCl3 (Eq. (3)) during the heating process may play an important role in the enhancement of MgH2 sorption. It is well known that the dehydrogenation product in the light metal hydride-catalyst system could act as a real catalyst to facilitate the de/rehydrogenation step. These products could create surface activation and form a large number of nucleation sites at the surface of the MgH2 matrix. It is also believed that the finely
decomposition of hydrogen from the MgH2 was reduced by doping with the LaCl3. In order to clarify the phase structure of the LaCl3-doped MgH2 sample after 1 h milling, after dehydrogenation at 450 C, and after rehydrogenation at 300 C under 3 MPa hydrogen pressure, XRD was used, as shown in Fig. 7. After the ball milling processes (Fig. 7(a)), the main phases presented were the formations of the parent materials, MgH2 and LaCl3. No new compounds were formed from the mixtures. After dehydrogenation at 450 C, the XRD pattern of Fig. 7(b) reveals that there were distinct peaks of Mg, which indicates that the dehydrogenation process of MgH2 was completed. In addition, the peaks for LaCl3 disappear, and some new peaks corresponding to an LaeMg alloy (Mg3La) and MgCl2 are observed, suggesting that the reaction of MgH2 with LaCl3 may have occurred during the heating process as follows:
9MgH2 þ 2LaCl3 /3MgCl2 þ 2Mg3 La þ 9H2
(3)
The standard Gibbs free energy, DG f of MgH2, LaCl3, MgCl2 and Mg3La is 35.98 [40], 708.9 [41], -592.12 [40] and 79.65 kJ/mol [42], respectively; thus, the total change DG associated with the reaction in equation (2) will be 194.04 kJ/mol of MgH2. This
Fig. 7. XRD patterns of the 10 wt.% LaCl3-doped MgH2 (a) after milling, (b) after dehydrogenation, and (c) after rehydrogenation.
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dispersed dehydrogenated products may contribute to kinetic de/ absorption improvement by serving as the active sites for nucleation and creating the dehydrogenation product by shortening the diffusion distance of the reaction ions [43,44]. Furthermore, the function of Cl may also introduce an extra catalytic effect on MgH2 sorption properties. As discussed in the literature [45,46], the catalytic effect of a metal halide on the hydrogen sorption of MgH2 could also be simultaneously influenced by several factors, such as the formation of MgF2 and the catalytic influence of transition metal halides (with different levels of metal oxidation state). Based on this, in this study, the chlorine-based product, MgCl2, may have also introduced an extra catalytic effect on MgH2 sorption properties, as proved in previous reports [7,47]. The catalytic effect of MgCl2 may further combine with the catalytic function of the LaeMg alloy species to generate a synergetic effect. In addition, the reaction in equation (3) could generate clean surfaces (without MgO at the MgH2 surface as proved by the XRD results in Fig. 7) and, subsequently, increase the surface reactivity and the decomposition reaction. However, further work is necessary to clarify the exact role of the LaCl3 addition in MgH2 by observation methods such as transmission electron microscopy. 4. Conclusion In summary, LaCl3 showed a good catalytic effect, giving MgH2 both a significantly decreased decomposition temperature and enhanced sorption kinetics. The addition of 10 wt.% LaCl3 led to the release of hydrogen at about 300 C, decreasing the decomposition temperature by 50 C compared to the as-milled MgH2. Furthermore, the kinetic desorption results showed that the added MgH2 released about 4.2 wt.% hydrogen within 5 min at 320 C, while the MgH2 only released 0.2 wt.% hydrogen within the same time and temperature parameters. Meanwhile, a hydrogen absorption capacity of 5.1 wt.% was reached at 300 C in 2 min for the LaCl3-doped MgH2 sample. In contrast, the ball-milled MgH2 only absorbed 3.8 wt.% hydrogen at 300 C in 2 min. The apparent activation energy for hydrogen desorption was decreased from 166.0 kJ/mol for the as-milled MgH2 to 143.0 kJ/mol by the addition of 10 wt.% LaCl3. This indicates that the catalytic effect due to the addition of LaCl3 significantly decreased the activation energy for the hydrogen desorption of MgH2. Based on the results, it is believed that the significant effects of the LaCl3 on the hydrogen storage properties of MgH2 were due to the catalytic effects of the LaeMg alloy and MgCl2 that formed in situ during the heating process. Acknowledgements The author thanks the Universiti Malaysia Terengganu for providing the facilities to carry out this project. The author also acknowledges the Malaysian Government for financial support through the Fundamental Research Grant Scheme (59295). References [1] Principi G, Agresti F, Maddalena A, Lo Russo S. The problem of solid state hydrogen storage. Energy 2009;34:2087e91. [2] Satyapal S, Petrovic J, Read C, Thomas G, Ordaz G, The US. Department of Energy's National Hydrogen Storage Project: progress towards meeting hydrogen-powered vehicle requirements. Catal Today 2007;120:246e56. [3] Mustafa NS, Ismail M. Influence of K2TiF6 additive on the hydrogen sorption properties of MgH2. Int J Hydrogen Energy 2014;39:15563e9. [4] Ranjbar A, Ismail M, Guo ZP, Yu XB, Liu HK. Effects of CNTs on the hydrogen storage properties of MgH2 and MgH2-BCC composite. Int J Hydrog Energy 2010;35:7821e6. [5] Ismail M, Zhao Y, Yu XB, Dou SX. Improved hydrogen storage properties of MgH2 doped with chlorides of transition metals Hf and Fe. Energy Educ Sci Technol A Energy Sci Res 2012;30(SPEC.ISS.1):107e22.
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