Improved Hydrogen Storage Properties of MgH2 Co-Doped with

Article pubs.acs.org/JPCC

Improved Hydrogen Storage Properties of MgH2 Co-Doped with FeCl3 and Carbon Nanotubes M. Ismail,* N. Juahir, and N. S. Mustafa School of Ocean Engineering, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia ABSTRACT: A MgH2/FeCl3/carbon nanotubes (CNTs) composite was prepared by dry ball milling, and its hydrogen storage properties were investigated. The CNT addition resulted in both a decreased desorption temperature and improved sorption kinetics compared to the undoped MgH2− FeCl3 composite. The desorption temperature of the 5 wt % CNT-added MgH2−FeCl3 composite was decreased to 230 °C compared with 275 °C for undoped MgH2−FeCl3. For the dehydrogenation kinetics, the 5 wt % CNT-added MgH2− FeCl3 sample released about 4.3 wt % hydrogen at 320 °C after 4 min of dehydrogenation, while the MgH2−FeCl3 composite released about 3.1 wt % hydrogen under the same conditions. Meanwhile, for the rehydrogenation kinetics, the 5 wt % CNT-added MgH2−FeCl3 sample absorbed about 5.21 wt % hydrogen at 300 °C after 1 min of rehydrogenation, but the MgH2−FeCl3 composite only absorbed about 4.8 wt % hydrogen. The apparent activation energy, Ea, for dehydrogenation decreased from 130 kJ/mol for the MgH2−FeCl3 composite to 112 kJ/mol by the addition of 5 wt % CNTs. It is believed that the enhancement of the hydrogenation performance of the MgH2/FeCl3/ CNTs composite is due to the active Fe-containing species and the function of the Cl anions, as well as the unique structure of the CNTs.

1. INTRODUCTION Storing hydrogen in a solid state has become a promising option due to the favorable safety considerations and the high volumetric hydrogen capacity of storage materials. Among the solid-state hydrogen storage material based on chemisorptions, such as metal hydrides1 and complex hydrides,2,3 MgH2 has attracted much attention due to its high hydrogen storage capacity (7.6 wt %), good reversibility, and low cost. Although MgH2 has become an attractive candidate for on-board hydrogen storage, the high thermodynamic stability and sluggish sorption kinetics hinder the practical application of MgH2. Over the past several decades, these disadvantages have been overcome by reducing the grain size,4,5 introducing a catalyst,6−8 and reactions with other metal or metal hydrides (the so-called destabilization concept).9−17 Among these three methods, the introduction of a catalyst into MgH2 has played a vital role in the development of hydrogen storage materials. Many studies have shown that by using various additives or catalysts, the hydrogenation properties of MgH2 could be improved. In these studies, different kinds of additives or catalysts have been mixed with MgH2 by ball milling, such as metals,18,19 metal oxides,20−22 metal halides,23,24 and carbon.25−28 Previous studies have shown that the carbon nanotube (CNT) is a good catalyst for MgH2.29,30 In addition, a combination of transition metals with CNTs as mixed dopants has been found to lead to significant improvement of hydrogen dissociation and diffusion in nanostructured magnesium.31−34 This indicates that the synergistic interaction among CNTs and © 2014 American Chemical Society

metals may be an effective approach to improve the hydrogen storage properties of MgH2. Although the hydrogen storage properties of MgH2 were improved, it still does not satisfy all of the requirements for practical applications. Moreover, the exact mechanism of the metal or metal halide combined with CNTs as a catalyst in the enhancement of hydrogen storage properties of MgH2 is still a matter of debate. Therefore, it is an important issue to further explore and develop the synergistic effects of other metallic or metal halide catalysts with CNTs that can improve the hydrogen storage properties of MgH2 and to gain a deeper understanding of the modification of the hydrogen sorption process of MgH2. We recently demonstrated that the hydrogenation performance of MgH2 was enhanced after catalyzing with FeCl3.35 It is believed that the significant improvement of MgH2 sorption properties in the MgH2/FeCl3 composite is due to the catalytic effects of in-situ-generated Fe species and MgCl2 that were formed during the heating process. Therefore, in this study, with the aim of combining CNTs with transition metals or metal halides, we investigate the effects of the CNTs as a codopant on the hydrogen storage properties of MgH2−FeCl3 composites. To the best of our knowledge, no studies have been reported on MgH2 co-doped with FeCl3 and CNTs. The hydrogen storage performance of the MgH2−FeCl3 composite in the presence of CNTs was investigated by temperatureReceived: May 12, 2014 Revised: July 24, 2014 Published: July 24, 2014 18878

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programmed desorption (TPD), isothermal sorption measurements, and differential scanning calorimetry (DSC). X-ray diffraction (XRD) was used to clarify the reaction mechanism after the de/rehydrogenation process, and scanning electron microscopy (SEM) was used to investigate the surface morphology. The possible mechanism behind the catalytic effect of CNTs in the MgH2−FeCl3 composite is also discussed.

2. EXPERIMENTAL SECTION All of the necessary materials, namely, MgH2 powder (≥95% pure), FeCl3 (reagent grade, 97%), and MWCNT (diameter = 110−170 nm, length = 5−9 μm, 90+%), were purchased from Sigma-Aldrich. All of the materials were used as received without further purification. All handling of the samples was conducted under an Ar atmosphere in an MBraun Unilab glovebox. About 400 mg of MgH2 was mixed with 10 wt % of FeCl3 and 5 wt % of CNT. For comparison, pure MgH2 and MgH2 + 10 wt % FeCl3 samples were also prepared under the same conditions. Each sample was put into a sealed stainless steel vial together with four hardened stainless steel balls. The sample was then milled in a planetary ball mill (NQM-0.4) for 1 h at 400 rpm, with a ball to powder ratio of 30:1. The experiments for TPD and de/rehydrogenation kinetics were performed in a Sievert-type pressure−composition temperature apparatus (Advanced Materials Corporation). About 100 mg of sample was loaded into a sample vessel in the glovebox. For TPD, the sample was heated in a vacuum chamber from room temperature to 450 °C, and the heating rate was 5 °C/min. For the rehydrogenation kinetic purposes, after complete dehydrogenation at 450 °C, the samples were kept at 300 °C under 3.0 MPa of hydrogen pressure for 1 h. The dehydrogenation kinetic measurements were conducted at 320 °C with initial hydrogen pressures of 0.01 MPa. The morphology of the samples was investigated by using a JEOL JSM-6360LA scanning electron microscope with the samples set on carbon tape and then coated with gold spray under vacuum. The phase structure for the as-milled and after de/rehydrogenation was determined by a Rigaku MiniFlex Xray diffractometer with Cu Kα radiation. The patterns were scanned in steps of 0.02° (2θ) over diffraction angles from 20 to 80° with a speed of 2.00°/min. DSC analysis was carried out using a Mettler Toledo TGA/ DSC 1. About 2−6 mg of sample was loaded into an alumina crucible in the glovebox. The samples were heated from room temperature to 500 °C under an argon atmosphere, and different heating rates were used.

Figure 1. TPD patterns for the dehydrogenation of the as-received MgH2, as-milled MgH2, MgH2 + 10 wt % FeCl3, and MgH2 + 10 wt % FeCl3 + 5 wt % CNT.

MgH2, respectively, but the amount of hydrogen released capacity slightly dropped to about 6.4 wt % H2. Furthermore, the FeCl3 and CNT co-doped MgH2 samples started to release hydrogen at about 230 °C, which was a decrease of about 45, 110, and 200 °C compared with the MgH2 + 10 wt % FeCl3, asmilled MgH2 and as-received MgH2, respectively. The FeCl3 and CNT co-doped MgH2 sample also demonstrated a total hydrogen released capacity of 6.3 wt % H2, which was almost the same as the hydrogen desorption capacity of the FeCl3doped MgH2. These results show the synergistic effects between CNT and FeCl3 as a mixed dopant. Figure 2 shows the results of the isothermal dehydrogenation kinetic measurements for the as-milled MgH2, MgH2 + 10 wt %

Figure 2. Isothermal dehydrogenation kinetics at 320 °C of the asmilled MgH2, MgH2 + 10 wt % FeCl3, and MgH2 + 10 wt % FeCl3 + 5 wt % CNT.

3. RESULTS AND DISCUSSION Figure 1 shows the TPD patterns for the dehydrogenation of the as-received MgH2, as-milled MgH2, MgH2 + 10 wt % FeCl3, and MgH2 + 10 wt % FeCl3 + 5 wt % CNT. The as-received MgH2 started to decompose at about 410 °C, with a total dehydrogenation capacity of 7.0 wt % H2 by 430 °C. After milling, the onset desorption temperature of MgH2 decreased to about 340 °C, indicating that the milling process also influenced the onset decomposition temperature of the MgH2. The curve shows that no reduction occurred in the hydrogen released capacity of the MgH2 after milling. The addition of FeCl3 markedly improved the onset decomposition temperature for the MgH2. The addition of 10 wt % FeCl3 caused decreases in the decomposition onset temperature of about 65 and 155 °C compared with that of the as-milled and as-received

FeCl3, and MgH2 + 10 wt % FeCl3 + 5 wt % CNT at a constant temperature of 320 °C. The results show that the samples doped with 10 wt % FeCl3 and 10 wt % FeCl3 + 5 wt % CNT released 3.9 wt % hydrogen and 4.8 wt % hydrogen, respectively, at 320 °C in 5 min under 0.1 MPa of pressure. In contrast, almost no hydrogen was desorbed at this temperature from the as-milled MgH2 over the same time period. This result further suggests that a synergistic catalytic effect from the combination of FeCl3 and CNT exists for MgH2. The results of the isothermal rehydrogenation kinetics at 300 °C under 3.0 MPa of hydrogen pressure (shown in Figure 3) show that the MgH2 + 10 wt % FeCl3 and MgH2 + 10 wt % FeCl3 + 5 wt % CNT samples absorbed hydrogen faster than the pure MgH2 and that the MgH2 + 10 wt % FeCl3 + 5 wt % 18879

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hydrides is improved with reduced particle agglomeration and growth.37 From the morphology results, it is speculated that the hydrogen storage properties of the MgH2 + 10 wt % FeCl3 + 5 wt % CNT sample were improved as a result of the reduced particle agglomeration. The thermal properties of the as-received and as-milled MgH2, MgH2 + 10 wt % FeCl3, and MgH2 + 10 wt % FeCl3 + 5 wt % CNT samples were further investigated by DSC, with the results as shown in Figure 5. Clearly, the curve for the as-

Figure 3. Isothermal rehydrogenation kinetics at 300 °C under 3.0 MPa of hydrogen pressure of the as-milled MgH2, MgH2 + 10 wt % FeCl3, and MgH2 + 10 wt % FeCl3 + 5 wt % CNT.

CNT had the fastest kinetics rate. The as-milled MgH2 sample absorbed 3.42 wt % at this temperature after 1 min. In contrast, the hydrogen absorbed by the MgH2 + 10 wt % FeCl3 and MgH2 + 10 wt % FeCl3 + 5 wt % CNT samples at 300 °C reached 4.8 and 5.21 wt % hydrogen, respectively, within 1 min. Taken together, these results suggest that the CNT also improves the rehydrogenation kinetics of the MgH2−FeCl3 composite. Figure 4 shows the SEM images of the as-received MgH2, the as-milled MgH2 + 10 wt % FeCl3, and the as-milled MgH2 + 10

Figure 5. DSC traces of the as-received and as-milled MgH2, MgH2 + 10 wt % FeCl3, and MgH2 + 10 wt % FeCl3 + 5 wt % CNT (heating rate: 10 °C/min; argon flow: 30 mL/min).

received MgH2 shows only one strong endothermic process, namely, a peak at 440.69 °C, which corresponds to the decomposition of the MgH2. As a whole, the DSC curves for the as-milled MgH2, MgH2 + 10 wt % FeCl3, and MgH2 + 10 wt % FeCl3 + 5 wt % CNT samples were similar to those of the as-received MgH2 sample, displaying only one endothermic peak at 424.62, 353.35, and 325.35 °C, respectively, which corresponded to the decomposition of the MgH2 but with the peaks having moved to lower temperatures. This phenomenon further suggested the synergistic effect of FeCl3 and CNT on the decomposition of MgH2. The enhancement of the dehydrogenation property is related to the energy barrier for the H2 release from the MgH2. The activation energy for dehydrogenation of the MgH2 was reduced by ball milling and doping with the catalyst. To compare the activation energy of the FeCl3-doped sample and the FeCl3 and CNT co-doped samples, the Kissinger analysis38 was used as follows

Figure 4. SEM micrographs of (a) pure MgH2, (b) MgH2 + 10 wt % FeCl3, and (c) MgH2 + 10 wt % FeCl3 + 5 wt % CNT after ball milling.

⎡ ⎤ E β ln⎢ 2 ⎥ = − a + A ⎢⎣ Tp ⎥⎦ RTp

(1)

where β, Tp, and R are the heating rate, the peak temperature, and the gas constant. Thus, the activation energy, Ea, can be obtained from the slope in a plot of ln[β/Tp2] versus 1000/Tp. Figures 6 and 7 show the DSC traces for the FeCl3-doped sample and the FeCl3 and CNT co-doped sample at different heating rates. From a Kissinger plot of the DSC data, as shown in Figure 8, the apparent activation energy, Ea, for H2 release from the MgH2 in the FeCl3-doped sample was found to be 130 kJ/mol. This value was lowered by 18 kJ/mol after being codoped with CNT and FeCl3 (Ea ≈ 112 kJ/mol). This result shows that a synergistic catalysis between the FeCl3 and CNT existed for the MgH2. In order to determine the phase structures of the MgH2 + 10 wt % FeCl3 + 5 wt % CNT sample after milling, after

wt % FeCl3 + 5 wt % CNT. The particle size of the as-received MgH2 was larger than 100 μm (Figure 4a). The SEM images showed the FeCl3-doped MgH2 powders (Figure 4b) to be irregularly shaped and agglomerated, which is the typical morphology for ball-milled powders, and their particle size was between 1 and 10 μm. Figure 4c shows the SEM images of the sample ball-milled with FeCl3 and CNT. This image confirms that the CNT was not destroyed after the short milling process, which is in accordance with the findings reported in the literature.36 The length of the nanotube was about 5−9 μm, which is in agreement with the information provided by the supplier. In addition, the results indicate that the sample with CNT appeared to have less agglomeration. It is already wellknown that the hydrogen storage properties of light metal 18880

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Figure 9. XRD patterns of the MgH2 + 10 wt % FeCl3 + 5 wt % CNT (a) after milling, (b) after dehydrogenation, and (c) after rehydrogenation.

Figure 6. DSC traces of the MgH2 + 10 wt % FeCl3 at different heating rates.

were distinct peaks of Mg, which indicates that the dehydrogenation of MgH2 was completed. A small amount of MgO was also detected in the dehydrogenation spectra, which is likely due to the slight oxygen contamination. In addition, some peaks of the MgCl2 and Fe appeared after dehydrogenation, suggesting that the reactions of MgH2−FeCl3 may have occurred, as discussed in our previous paper.35 The peak corresponding to the CNT still existed after the dehydrogenation process. For the rehydrogenated sample, it can be seen that the Mg was largely transformed into MgH2. The peaks of the MgCl2 and Fe remained unchanged alongside the CNT, together with a small peak of MgO. The cycling performance of the MgH2 co-doped with FeCl3 and CNT mixture was further characterized, as shown in Figure 10. Temperatures of 300 and 320 °C were applied in the

Figure 7. DSC traces of the MgH2 + 10 wt % FeCl3 + 5 wt % CNT at different heating rates.

Figure 8. Kissinger plot for (a) MgH2 + 10 wt % FeCl3 and (b) MgH2 + 10 wt % FeCl3 + 5 wt % CNT composites.

Figure 10. Isothermal re/dehydrogenation kinetics of the MgH2 + 10 wt % FeCl3 + 5 wt % CNT mixture in the 1st and 10th cycles.

dehydrogenation at 450 °C, and after rehydrogenation at 300 °C under 3.0 MPa of hydrogen pressure, XRD was used, with the results as shown in Figure 9. After milling, as well as with the MgH2 phases, the results show that there was a peak corresponding to the CNT. This result is in accordance with Wu et al.’s report on single-walled carbon nanotube (SWNT)doped MgH2.30 In their report, after milling for 1 h, MgH2 doped with 5 wt % SWNT also showed an XRD peak for the SWNT. In addition, neither FeCl3 nor any secondary FeClcontaining phase was detected after milling, which was probably due to the fact that the FeCl3 grains were too small to be detectable in the MgH2 matrix by XRD or because the FeClcontaining phases may have existed in an amorphous state directly after ball milling. In the dehydrogenation spectra, there

cycling study of the MgH2 + 10 wt % FeCl3 + 5 wt % CNT sample. The sorption kinetics persisted well, even after the 10th cycle, indicating that CNTs combined with FeCl3 is a good catalyst for the cycle life of MgH2. The hydrogen storage capacity after 10 min of desorption shows almost no decrease with cycling, being maintained at about 5.5 wt %. In order to examine the phases of the sample after cycling, XRD scans were performed on the MgH2 + 10 wt % FeCl3 + 5 wt % CNT sample, as shown in Figure 11. As seen in Figure 9b and c, CNT, Fe, and MgCl2 were also detected in re/dehydrogenation states after cycling. Besides, MgH2 and Mg were observed in the pattern of the hydrogenated and dehydrogenation states, with a small amount of MgO. 18881

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decreasing the desorption temperature by 45 °C compared to the undoped MgH2−FeCl3 composite. In terms of the sorption kinetics, the 5 wt % CNT-added MgH2−FeCl3 composite sample released about 4.3 wt % hydrogen at 320 °C after 4 min of dehydrogenation, while the MgH2−FeCl3 composite sample released about 3.1 wt % hydrogen under the same conditions. Meanwhile, for absorption kinetics, the 5 wt % CNT-added sample absorbed about 5.21 wt % hydrogen at 300 C after 1 min of rehydrogenation, but the undoped MgH2−FeCl3 composite sample only absorbed about 4.8 wt % hydrogen under the same conditions. The apparent activation energy, Ea, for dehydrogenation was reduced from 130 to 112 kJ/mol for the MgH2−FeCl3 composite by the addition of 5 wt % CNT. It is believed that the in situ formation of the Fe and MgCl2 species as well as the presence of the unique structure of the CNTs plays a critical role in the improvement of hydrogen storage properties in the MgH2/FeCl3/CNTs composite.

Figure 11. XRD patterns of the MgH2 + 10 wt % FeCl3 + 5 wt % CNT sample (a) after the 10th dehydrogenation and (b) after the 10th rehydrogenation.

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On the basis of the above results, it is clearly shown that the onset decomposition temperature and de/rehydrogenation kinetics of MgH2 were improved by introducing a co-dopant, namely, FeCl3 and a CNT. As discussed in our previous paper,35 the formation of the Fe particle resulting from the reaction of the MgH2 and FeCl3 during the dehydrogenation process may play an important role in the enhancement of MgH2 storage properties because it is well-known that Fe is a good catalyst for MgH2.18,39 The Fe metal may interact with hydrogen molecules, which may lead to the dissociation of the hydrogen molecules and the enhancement of the sorption kinetics.40 Apart from the speculated catalytic effects of the Fe species, the function of Cl− may also introduce an extra catalytic effect on MgH2 sorption properties. The chlorinebased product, MgCl2, may contribute to the enhancement of the de/rehydrogenation kinetics by serving as the active site for nucleation and creation of the dehydrogenated product by shortening the diffusion distance of the reaction ions.41 The catalytic effect of MgCl2 may further combine with the catalytic function of the Fe species to generate a synergetic effect. In addition, a combination of active metal nanoparticles and nanostructured carbon materials as mixed dopants is an effective catalyst for enhancement of the hydrogen storage properties of metal hydrides and complex hydrides, as reported in the literatures.31−34,42,43 The unique nanostructure of the CNT is expected to form a net-like architecture after being milled together with the host materials and acting as a diffusion channel for hydrogen, while the metal nanoparticles have high catalytic activity.44,45 In this study, the enhancement of the CNT-added MgH2−FeCl3 sample may also have been due to the hardness of the CNT. The presence of the CNT in the MgH2−FeCl3 composite prevented particle agglomeration, as shown above in Figure 4c. It is well-known that the hydrogen storage properties of light metal hydride are improved with reduced particle agglomeration.37 Therefore, in this study, it is believed that the enhancement of the hydrogenation process of MgH2 co-doped with FeCl3 and CNT is due to the combination of the active Fe-species and the function of Cl anions with the catalytic effect of the CNT.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +609-6683487. Fax: +609- 6683991. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Universiti Malaysia Terengganu for providing the facilities to carry out this project. The authors also acknowledge the Ministry of Education Malaysia for financial support through the Fundamental Research Grant Scheme (FRGS 59295). N. Juahir and N.S. Mustafa are grateful to the Ministry of Education Malaysia for a MyBrain15 scholarship.

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REFERENCES

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4. CONCLUSION In this study, CNTs showed good effect as a cocatalyst, giving the MgH2−FeCl3 composite both a decreased onset desorption temperature and improved sorption kinetics. The addition of 5 wt % CNT led to the release of hydrogen at about 230 °C, 18882

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dx.doi.org/10.1021/jp5046436 | J. Phys. Chem. C 2014, 118, 18878−18883