Mg 2 FeH 6 -BASED NANOCOMPOSITES WITH HIGH CAPACITY OF HYDROGEN STORAGE PROCESSED BY REACTIVE MILLING

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1 Mg 2 FeH 6 -BASED NANOCOMPOSITES WITH HIGH CAPACITY OF HYDROGEN STORAGE PROCESSED BY REACTIVE MILLING A. A. C. Asselli (1)*, C. S. Kiminami (2), A. M. Jorge Jr. (2), T.T. Ishikawa (2), and W. J. Botta (2) (1) Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Universidade Federal de São Carlos UFSCar, São Carlos SP, Brazil. (2) Departamento de Engenharia de Materiais, Universidade Federal de São Carlos UFSCar, São Carlos SP, Brazil. Rodovia Washington Luis, Km 235, , São Carlos SP, Brazil. * asselli@gmail.com ABSTRACT The compound Mg 2 FeH 6 was synthesized from a 2Mg-Fe mixture in a single process by high-energy ball milling under hydrogen atmosphere at room temperature. The complex hydride was prepared from Mg powder and granulated or powdered Fe using a planetary mill. The phase evolution during different milling times was performed by X-rays diffraction technique. The dehydrogenation behavior of the hydride was investigated by simultaneous thermal analysis of differential scanning calorimetry and thermogravimetry coupled with mass spectrometry. The use of powdered iron as starting material promoted conversion to complex hydride at shorter milling times than when granulated iron was used, nevertheless, after 24 hours of milling the 2Mg-Fe (powdered or granulated) mixtures presented similar dehydrogenation behavior. The gravimetric capacity of hydrogen was on average 3.2 wt.%, however, changing the proportions between the reagents to 3Mg-Fe a Mg 2 FeH 6 -based nanocomposite with high capacity of hydrogen storage (5.2 wt.%) was obtained. Keywords: hydrogen storage materials, magnesium complex hydrides, mechanochemical synthesis INTRODUCTION Magnesium is an attractive material to hydride forming due to several advantages, such as its high gravimetric density of hydrogen (7.6 wt. %), its abundance on the Earth and its low cost. However, its main drawbacks are its high stability and low hydrogen sorption kinetics. In this context, the magnesium complex hydrides appear as an interesting alternative, compromising hydrogen capacity for better absorption desorption kinetics. In this compounds group, the Mg 2 FeH 6 stands out because it presents the highest known volumetric density of 150 kg of H 2 /m 3, which is more than two times higher than liquid hydrogen (70.8 kg of H 2 /m 3 )

2 However, as Mg and Fe do not form any intermetallic compound 2, the synthesis of this complex hydride is not trivial. In the first report concerning the synthesis of this hydride, sintering processes of Mg and Fe powders under H 2 pressure were used to obtain Mg 2 FeH 6, however, high pressures, elevated temperatures and several days were required 2. In addition, half of the elemental powder remained non-reacted and the sintered sample had to be purified. A processing route to reduce these severe conditions is the mechanical alloying of precursory materials 4, furthermore, a direct synthesis of hydride can be obtained when a hydrogen pressure is applied (Reactive Milling RM) 5. This mechanically activated method can reduce the grain and particles sizes to nanometric scale and improve the hydrogen sorption kinetics of hydrides 6,7. Therefore, the use of reactive milling as a single step procedure for Mg 2 FeH 6 synthesis is very interesting, but, to the present time the highest yield of this complex hydride was achieved by this method only using MgH 2 -Fe mixtures as starting materials 8. In this paper we systematically evaluated the synthesis and the influence of the form of reagent Fe on the formation of Mg 2 FeH 6 through reactive milling of 2Mg-Fe mixtures. The decomposition behavior and the hydrogen gravimetric capacity were also studied. EXPERIMENTAL The complex hydride Mg 2 FeH 6 was synthesized from a 2Mg-Fe mixture by highenergy ball milling under hydrogen pressure. Magnesium powder ( mesh, 99.8%), iron powder (-22 mesh, %) and iron granules (1 2 mm, 99.98%), were provided by Alfa Aesar. Magnesium was mixed with granulated or powdered iron and loaded into a stainless steel vial of 160 cm 3. After three atmosphere cleaning cycles, consisting of evacuation and argon injection, the milling vial was filled with hydrogen (99.999%) at 3MPa of pressure. In all experiments, the ball-to-powder weight ratio was 40:1. The reactive millings were carried out in a Fritsch pulverissette P6 planetary mill at rotation speed of 600 rpm during different milling times (1 up to 72 hours). The vial was not recharged with hydrogen during the milling experiments and all samples were manipulated inside an argon filled glove box. 6419

3 X-ray diffraction (XRD) measurements were performed on a Rigaku Geigerflex difractometer equipped with a graphite monochromator with Cu Kα radiation. The X- rays diffraction patterns were also used for calculation of the mean crystallite size through Scherrer analysis 9. The dehydrogenation behavior of the hydride was investigated by simultaneous thermal analysis of differential scanning calorimetry (DSC) and thermogravimetry (TGA) coupled with quadrupole mass spectrometer (QMS) at heating rate of 10 K min -1. RESULTS AND DISCUSSION Synthesis of Mg 2 FeH 6 The Fig. 1 presents the phases evolution in the X-rays powder diffraction patterns of the 2Mg-Fe (Fe powder or granulated) mixtures milled under hydrogen pressure as function of milling time. The analysis of the Fig. 1a, when powdered iron is used as starting material, shows that the tetragonal β-mgh 2 (JCPDS ) is already present in the mixture after 1h of milling. The formation of Mg 2 FeH 6 (JCPDS ) starts after 4 hours of milling and the metastable orthorhombic γ-mgh 2 (JCPDS ) is also detected. In the mixture milled for 6 hours almost all Mg (JCPDS ) is consumed for the formation of β-mgh 2. After 12 hours of milling, the mixture is constituted mainly by Mg 2 FeH 6 and Fe. No change in the phase evolution can be observed by X-rays diffraction with the increase of the milling time. In the case of the 2Mg-Fe (Fe granulated), the mixtures reactively milled during 4 up to 12 hours were obtained as sheets (Fig. 2) and the X-rays diffraction patterns (Fig. 1b) show diffraction peaks preponderantly due to the planes of Mg. This is related to the use of granulated iron as precursory material and the tendency of the particles weld together in the shorter milling times 10. After 24 hours of milling, the product was obtained as powder formed mainly by Mg 2 FeH 6 and Fe. Despite 72 hours of milling, a remaining iron was identified in the sample by X-ray diffraction. 6420

4 Figure 1. X-rays diffraction patterns of the 2Mg-Fe mixtures milled during different milling times. Fe powder. Fe granulated. Figure 2. Sheets obtained through the reactive milling of 2Mg-Fe (Fe granuled) mixture during 12 hours. Table 1 presents the calculated crystallite sizes for the complex hydride Mg 2 FeH 6 from the broadening of its respective X-ray diffraction peaks. After 24 hours of milling, the mean crystallite size of the complex hydride range from 12 to 14 nanometers, independently of the form of reagent Fe (powder or granules). These interesting results show that is possible to achieve a comparable material in terms of mean crystallite size using a much cheaper precursory material (Fe granulated) by reactive milling. 6421

5 Table 1. Estimated crystallite size of Mg 2 FeH 6 as function of milling time of 2Mg-Fe mixtures Milling Time (h) Mg 2 FeH 6 Crystallite Size (nm) 2Mg-Fe (Fe powder) 2Mg-Fe (Fe granulated) Dissociation of Mg 2 FeH 6 The dehydrogenation behavior of 2Mg-Fe mixtures reactively milled during different milling times was studied by simultaneous thermal analysis of differential scanning calorimetry and thermogravimetry and the curves are presented in Fig. 3 and Fig. 4, respectively. Results obtained for MgH 2 prepared by reactive milling using the same parameters are shown in Fig. 5 for comparison. The 2Mg-Fe mixtures (Fe powder and granules) milled during 6 up to 72 hours had a comparable dehydrogenation behavior. The endothermic peaks related to the release of hydrogen take place around 300 C (Fig. 3), which are much lower than the value observed for MgH 2 (Fig. 5b). However, the gravimetric capacity for the 2Mg-Fe mixtures milled was on average only 3.2 wt.% (Fig. 4), in others words, 60% of the theoretical hydrogen capacity of Mg 2 FeH 6 (5.4 wt. %) 6422

6 Figure 3. Differential scanning calorimetry curves of 2Mg-Fe mixtures milled during different milling times. Fe powder and Fe granulated. Figure 4. Thermogravimetry curves of 2Mg-Fe mixtures milled during different milling times. Fe powder and Fe granulated. 6423

7 Figure 5. X-ray diffraction pattern and differential scanning calorimetry, thermogravimetry and quadrupole mass spectrometry curves of MgH 2 prepared by reactive milling. The results of X-ray powder diffraction and thermogravimetric analysis agree with the reaction sequence for the formation of Mg 2 FeH 6 proposed by Gennari 5 : (1) Mg ( s) + H 2 MgH ( g ) 2 and (2) 3MgH ( s) 2 + Fe ( ) ( s) Mg s 2 FeH + Mg ( s). Following these equations we change the proportion between the reagents to 3Mg:Fe and this composition was milled during 48 hours under hydrogen pressure using the same conditions applied to the 2Mg-Fe mixtures. The X-rays diffraction pattern and the thermal analysis results are presented in Fig. 6. After reactive milling, there is no remaining iron in the mixture and the powder obtained is composed mainly by Mg 2 FeH 6 (Fig. 6a). The hydrogen desorption starts around 200 C reaching the transformation peak at 337 C. Furthermore, the sample released almost 5.2 wt. % of hydrogen, as shown in Fig. 6b. This is the highest hydrogen gravimetric capacity reported using a single step procedure by reactive milling under hydrogen pressure of magnesium and iron as precursory material. 6 ( s) 6424

8 Figure 6. 3Mg-Fe (Fe powder) mixture milled under hydrogen pressure. X-ray diffraction pattern. DSC, TGA and QMS curves. CONCLUSIONS In this paper we studied the formation of the complex hydride Mg 2 FeH 6 through reactive milling of 2Mg-Fe mixtures under hydrogen pressure. The use of powdered iron as starting material promoted conversion to complex hydride at shorter milling times than when granulated iron was used, nevertheless, after 24 hours of milling the 2Mg-Fe (Fe powder or granules) mixtures presented similar mean crystallite size and dehydrogenation behavior. Even though 72 hours of milling, a remaining iron was identified by X-ray diffraction and the gravimetric capacity of hydrogen was on average 3.2 %, however, changing the proportions between the reagents to 3Mg:Fe a Mg 2 FeH 6 -based nanocomposite was obtained with higher hydrogen storage capacity (5.2 wt. %) ACKNOWLEGMENTS We would like to thank the Brazilian agencies FAPESP, CNPq and CAPES for their financial support to this paper. 6425

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