Mg2FeH6-based nanocomposites with high capacity of hydrogen storage processed by reactive milling

Mg₂FeH₆-based nanocomposites with high capacity of hydrogen storage processed by reactive milling

Alexandre Augusto Cesario AsselliI, ^*; Alberto Moreira Jorge Junior^II; Tomaz Toshimi Ishikawa^II; Walter José Botta Filho^II

^IPrograma de Pós-Graduação em Ciência e Engenharia de Materiais, Universidade Federal de São Carlos - UFSCar, Rod. Washington Luis, Km 235, CEP 13565-905, São Carlos, SP, Brazil

^IIDepartamento de Engenharia de Materiais, Universidade Federal de São Carlos - UFSCar, Rod. Washington Luis, Km 235, CEP 13565-905, São Carlos, SP, Brazil

ABSTRACT

The compound Mg₂FeH₆ was synthesized from a 2Mg-Fe mixture in a single process through 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 through simultaneous thermal analyses of differential scanning calorimetry and thermogravimetry coupled with mass spectrometer. 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 hydrogen absorption during milling was on average 3.2 wt. (%), however, changing the proportions of the reagents to 3Mg-Fe a Mg₂FeH₆-MgH₂ based nanocomposite with higher density of hydrogen (5.2 wt. (%)) was obtained.

Keywords: hydrogen storage materials, magnesium complex hydrides, mechanochemical synthesis

1. Introduction

Magnesium is an attractive material to hydrogen storage due to several advantages, such as its abundance on the Earth's crust, its low cost and the high hydrogen storage capacity of its hydride (7.6 wt. (%)). However, its main drawbacks are its high stability and slow hydrogen sorption kinetics. In this context, the magnesium complex hydrides appear as an interesting alternative, compromising hydrogen storage capacity for better absorption - desorption kinetics. In this compounds group, the Mg₂FeH₆ stands out because it presents the highest known volumetric density of 150 kg of H₂.m^-3, which is more than two times higher than liquid hydrogen (70.8 kg of H₂.m^-3) one¹. However, as Mg and Fe do not form any intermetallic compound², 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 H₂ pressure were used to obtain Mg₂FeH₆³. Although using high hydrogen pressures, elevated temperatures and several days, the yield of the complex hydride was only 50 wt. (%) and the sintered sample had to be purified by a complicated procedure. A processing route to reduce these severe conditions is the mechanical alloying of precursory materials before sintering⁴, furthermore, a direct synthesis of the hydride can be obtained when the milling is performed under a hydrogen pressure (Reactive Milling-RM)^5,6. This mechanically activated method can reduce the grain and particles sizes to nanometric scale and improve the hydrogen sorption kinetics of the hydrides^7,8.

The synthesis of the ternary complex hydride, Mg₂FeH₆, using ball milling from the stoichiometric compositions 2Mg-Fe or 2MgH₂-Fe were also studied using different milling conditions^9-16. Regardless of the milling condition, a remaining iron was always present after milling, which represents an incomplete reaction of hydride formation and results in a lower hydrogen density.

In an attempt to improve the yield of Mg₂FeH₆ formation and the hydrogen storage capacity, several papers reported the synthesis of Mg₂FeH₆ applying ball milling procedures using different proportions of the precursors Mg-Fe or MgH₂-Fe^17-20. Nonetheless, these papers presented some controversial results. As an example, Herrich et al.¹⁸ claimed a 92 wt. (%) yield of Mg₂FeH₆ after reactive milling of 3.5MgH₂:Fe, even if the maximum theoretical yield of Mg₂FeH₆ phase at this reactant ratio is 74 wt. (%).

In the present paper we systematically evaluated the synthesis and the influence of the type of reagent Fe on the formation of Mg₂FeH₆ through reactive milling of 2Mg-Fe mixtures. The decomposition behavior and the amount of hydrogen of the hydrides were also studied. In order to obtain a complete reaction of the metallic elements and consequently a higher hydrogen gravimetric density, we also performed the reactive milling of a 3Mg-Fe mixtures under hydrogen pressure.

2. Experimental

The complex hydride Mg₂FeH₆ was synthesized from a 2Mg-Fe mixture by high-energy ball milling under hydrogen pressure. Magnesium powder (-20 + 100 mesh, 99.8%), iron powder (-22 mesh, 99.998%) 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 cm³. 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.

X-rays diffraction (XRD) measurements were performed on a Rigaku Geigerflex difractometer equipped with a graphite monochromator with Cu Kα radiation. The XRD patterns were also used to estimate the mean crystallite size through Scherrer analysis⁹. Scanning electron microscopy (SEM) images were obtained in Phillips XL 30 FEG microscope.

The dehydrogenation behavior of the hydride was investigated in a Netzsch STA 449 Jupiter® through simultaneous thermal analysis of differential scanning calorimetry (DSC) and thermogravimetry (TG) coupled with quadrupole mass spectrometer (QMS). The heating rate was 10 °C/min, using about 10 mg of sample in each experiment.

3. Results and Discussion

3.1. Synthesis of Mg₂FeH₆

The analysis of Figure 1a, when powdered iron is used as starting material, shows that the tetragonal β-MgH₂ (JCPDS 74-0934) is already present in the mixture after 1 hour of milling. The formation of the Mg₂FeH₆ phase (JCPDS 38-0843) starts after 4 hours of milling and the metastable orthorhombic Γ-MgH₂ (JCPDS 35-1184) was also detected in the XRD pattern. The Γ-MgH₂ phase formation synthesized at room temperature by ball milling was also reported in others papers^19,21. Its formation is related to the high energy impact of the balls in the high energy ball milling process. In the mixture milled for 6 hours almost all Mg (JCPDS 35-0821) is consumed for the formation of β-MgH₂. After 12 hours of milling (Figure 1b), the powder is constituted mainly by Mg₂FeH₆ and α-Fe phases, and the increase of the milling time to 24 hours promoted a slight decrease of the iron peak intensity related to the complex hydride one. No change in the phase evolution was observed through XRD increasing 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 flakes (Figure 2), and the XRD patterns, as shown in Figure 1c, show diffraction peaks preponderantly due to the planes of Mg phase. This is related to the use of granulated iron as precursory material and the tendency of the particles weld together in the shorter milling times²². After 24 hours of milling (Figure 1d), the product was obtained as powder formed mainly by Mg₂FeH₆ and α-Fe phases. Despite 72 hours of milling, a remaining iron was identified in the sample by XRD.

Regardless of the precursory iron type, the 2Mg-Fe mixtures processed through reactive milling presented a non-reacted iron even after long times of processing. This residual iron is an indicative of an incomplete reaction of hydride formation and results in a lower hydrogen gravimetric density due to its high atomic weight.

Table 1 presents the estimated mean crystallite sizes for the complex hydride Mg₂FeH₆ from the broadening of its respective XRD peaks.

After 24 hours of reactive milling, regardless of the type of reagent Fe (powder or granules), the mean crystallite sizes of the complex hydride range from 12 to 14 nm. These results show that is possible to achieve a comparable material in terms of mean crystallite size using a much cheaper precursory material (Fe granulated) through reactive milling.

The 2Mg-Fe (Fe powder) sample prepared by reactive milling during 60 hours was selected for SEM analysis. The micrographs of SEM are presented in Figure 3.

The SEM images shows that the powder has a sponge-like structure and it is formed by agglomerates of much smaller particles, with diameters which can reach even 20 nm, as seen in Figure 3. It is possible also to note in Figure 3b that the surface of the bigger particles is full of asperities. This surface morphology presented by the powders is interesting for hydrogen storage applications, since high surface specific area values benefits fast hydrogen sorption kinetics.

3.2. Dissociation of Mg₂FeH₆

The dehydrogenation behavior of the 2Mg-Fe mixtures reactively milled during different milling times was studied by simultaneous thermal analyses of DSC and TG, and the curves are presented in Figures 4 and 5, respectively. Results of XRD and simultaneous thermal analyses of MgH₂ prepared from magnesium powder through reactive milling using the same processing parameters are shown in Figure 6 for comparison.

The DSC curve of the 2Mg-Fe (Fe powder) sample milled for 6 hours presented only one endothermic peak related to the hydrogen release, as shown in Figure 4a, nonetheless, α-Fe and two hydride phases, Mg₂FeH₆ and MgH₂, were identified through XRD (Figure 1a). This decomposition thermal behavior indicates a reaction peak overlap. Analyzing the higher desorption temperature range for the MgH₂ in Figure 5b, we can suggest that the presence of α-Fe and Mg₂FeH₆ in the powder destabilized the MgH₂ structure, lowering its hydrogen desorption temperature. As reported by Zhou et al.¹⁴, the Mg₂FeH₆ can reduce the structural stability of MgH₂ and further improve its dehydrogenation properties.

Longer milling times than 6 hours promote higher conversion of the Mg₂FeH₆ from MgH₂ and α-Fe, and consequently the DSC peak is dislocated to higher temperatures up to 24 hours of milling. Additional milling times reduce the decomposition peak temperature up to 48 hours, nonetheless, the 2Mg-Fe (Fe powder) milled for 60 hours presented a higher hydrogen desorption temperature, which could be attributed to some contamination from the vial and the balls as a result of the long milling time.

The same hydrogen desorption behavior is observed for the 2Mg-Fe (Fe granulated) samples in Figure 4b, however, it does not occurs in the same milling times than the 2Mg-Fe (Fe powder) samples due to the differences in the milling times to the conversion of the Mg₂FeH₆.

The hydrogen gravimetric capacities measured through TG analyses of the 2Mg-Fe samples were on average 3.2 wt. (%), as depicted in Figure 5. This result represents only 60% of the theoretical hydrogen capacity of Mg₂FeH₆ (5.4 wt. (%)) and it is associated to the presence of the remaining iron in the powder.

3.3. Reactive milling of 3Mg-Fe

The low hydrogen gravimetric capacities measured through TG (Figure 5) are related to the presence of the remaining iron in the powders, as identified by XRD (Figure 1). These results are explained based on the reaction sequence for the formation of Mg₂FeH₆ proposed by Gennari et at.⁵:

In the second reaction, the proportion of MgH₂ and Fe is 3:1.Therefore, in a 2Mg-Fe composition, the 2MgH₂ synthesized from 2Mg will react with 2/3 of the Fe, remaining 1/3 of the Fe in the powder. Consequently, the products of the second reaction will be 2/3Mg₂FeH₆, 1/3Fe and Mg. The theoretical hydrogen gravimetric capacity of this compound is 3.6 wt. (%), which agrees with ours TG results as presented in Figure 5. We also have to consider that MgH₂ might be formed from Mg in the reactive milling process. The identification through XRD of Mg or even MgH₂ in the powder is difficult due to their small crystallite size, low volumetric fraction and low electron density in comparison to Mg₂FeH₆ and Fe.

Aiming a complete reaction of the metallic elements and a higher hydrogen gravimetric density due to the presence of MgH₂ and Mg₂FeH_6, we changed the proportion of the reagents to 3Mg:Fe. This composition was milled during 48 hours under hydrogen pressure using the same conditions applied to the 2Mg-Fe mixtures.

After the reactive ball milling, the 3Mg-Fe powder presented a greenish color while the 2Mg-Fe one had a black/dark gray color. A green color was also reported when sintering process was used to synthesize Mg₂FeH₆ from a 2Mg-Fe composition^3,4.

Figure 7a shows that the 3Mg-Fe powder is mainly constituted by the Mg₂FeH₆ phase, having a mean crystallite size of 13 nm. The β-MgH₂ phase was also identified in the XRD pattern. The most interesting result was that the α-Fe phase was kept to a minimum, indicating a high yield of the Mg₂FeH₆ synthesis. In the others papers concerning the synthesis of Mg₂FeH₆ from a 3Mg-Fe or 3MgH₂-Fe composition through ball milling^17-19, the α-Fe phase is easily identified in the XRD pattern. Moreover, in all these papers the milling times were longer than 60 hours.

The simultaneous thermal analysis curves are presented in Figure 7b. The hydrogen desorption starts at a temperature lower than 200 °C and reaches its peak at 338 °C. Furthermore, a hydride duplex phase, Mg₂FeH₆-MgH_2, was identified through XRD (Figure 7a); nonetheless, only one endothermic peak in the DSC curve (Figure 7b) represents the hydride decomposition. This thermal behavior indicates a reaction peaks overlap and it is explained by the fact of the Mg₂FeH₆ can reduce the structural stability of MgH₂ and further improve its dehydrogenation properties¹⁴.

The TG analysis curve shows that the hydrogen gravimetric capacity is 5.2 wt. (%). This is one the highest hydrogen gravimetric capacity reported using a single step procedure through reactive milling under hydrogen pressure of magnesium and iron as precursory materials.

Considering the theoretical hydrogen capacity of the 3Mg-Fe mixture (5.8 wt. (%)), the measured gravimetric capacity after the reactive milling represents a 90% yield of the hydride formation. The higher hydrogen gravimetric capacity of the 3Mg-Fe sample in comparison to the 2Mg-Fe one is related to the complete reaction of the metallic elements with the hydrogen and the presence of the MgH₂ phase.

4. Conclusions

In this paper we studied the formation of the complex hydride Mg₂FeH₆ by reactive milling of 2Mg-Fe mixtures under hydrogen pressure. The influence of type of the reagent Fe on the formation of Mg₂FeH₆ was also evaluated. The use of powdered iron as starting material promoted conversion to complex hydride at shorter milling times than when granulated iron was used, nonetheless, after 24 hours of milling the 2Mg-Fe (Fe powder or granules) mixtures presented similar mean crystallite size and dehydrogenation behavior. Even after 72 hours of milling, a remaining iron was identified through XRD and the hydrogen gravimetric capacity was on average 3.2%, however, changing the proportions of the reagents to 3Mg:Fe a Mg₂FeH₆-MgH₂ based nanocomposite was obtained with higher hydrogen storage capacity (5.2 wt. (%)).

Acknowledgements

We would like to thank the Brazilian agencies FAPESP, CNPq and CAPES for their financial support to this paper.

Received: May 23, 2011

Revised: December 13, 2011

• 1. Züttel A, Wenger P, Rentsch S, Sudan P, Mauron PH and Emmenegger CH. LiBH4 a new hydrogen storage material. Journal of Power Sources 2003; 118:1-7. http://dx.doi.org/10.1016/S0378-7753(03)00054-5
• 2. Moffatt WG. Handbook of Binary Phase Diagrams New York: Genium Publishing; 1984. v. 3
• 3. Didisheim J-J, Zolliker P, Yvon K, Fisher P, Shefer J, Gubelmann M et al. Dimagnesiumiron(II) hydride, Mg2FeH6, containing octahedral FeH64-anions. Inorganic Chemistry 1984; 23:1953-1957. http://dx.doi.org/10.1021/ic00181a032
• 4. Huot J, Hayakawa H and Akiba E. Preparation of the hydrides Mg2FeH6 and Mg2CoH5 by mechanical alloying followed by sintering. Journal of Alloys and Compounds 1997; 248:164-167. http://dx.doi.org/10.1016/S0925-8388(96)02705-3
• 5. Sai Raman SS, Davidson DJ, Bobet J-L and Srivastava ON. Investigations on the synthesis, structural and microstructural characterizations of Mg-based K2PtCl6 type (Mg2FeH6) hydrogen storage material prepared by mechanical alloying. Journal of Alloys and Compounds 2002; 333:282-290. http://dx.doi.org/10.1016/S0925-8388(01)01729-7
• 6. Gennari FC, Castro FJ and Andrade Gamboa JJ. Synthesis of Mg2FeH6 by reactive mechanical alloying: formation and decomposition properties. Journal of Alloys and Compounds 2002; 339:261-267. http://dx.doi.org/10.1016/S0925-8388(01)02009-6
• 7. Zaluska A, Zaluski L and Ström-Olsen JO. Nanocrystalline magnesium for hydrogen storage. Journal of Alloys and Compounds 1999; 288:217-225. http://dx.doi.org/10.1016/S0925-8388(99)00073-0
• 8. Huot J, Liang G and Schulz R. Mechanically alloyed metal hydride systems. Applied Physics A 2001; 72:187-195. http://dx.doi.org/10.1007/s003390100772
• 9. Varin RA, Li S, Calka A and Wexler D. Formation and environmental stability of nanocrystalline and amorphous hydrides in the 2Mg-Fe mixture processed by controlled reactive mechanical alloying (CRMA). Journal of Alloys and Compounds 2004; 373:270-286. http://dx.doi.org/10.1016/j.jallcom.2003.11.015
• 10. Castro FJ and Gennari FC. Effect of the nature of the starting materials on the formation of Mg2FeH6. Journal of Alloys and Compounds 2004; 375:292-296. http://dx.doi.org/10.1016/j.jallcom.2003.11.147
• 11. Li S, Varin RA, Morozova O and Khomenko T. Controlled mechano-chemical synthesis of nanostructured ternary complex hydride Mg2FeH6 under low-energy impact mode with and without pre-milling. Journal of Alloys and Compounds 2004; 384:231-248. http://dx.doi.org/10.1016/j.jallcom.2004.05.018
• 12. Varin RA, Li S, Wronski Z, Morozova O and Khomenko T. The effect of sequential and continuous high-energy impact mode on the mechano-chemical synthesis of nanostructured complex hydride Mg2FeH6. Journal of Alloys and Compounds 2005; 390:282-296. http://dx.doi.org/10.1016/j.jallcom.2004.08.048
• 13. Varin RA, Li S, Chiu Ch, Guo L, Morozova O, Khomenko T et al. Nanocrystalline and non-crystalline hydrides synthesized by controlled reactive mechanical alloying/milling of Mg and Mg-X (X = Fe, Co, Mn, B) systems. Journal of Alloys and Compounds 2005; 404-406:494-498. http://dx.doi.org/10.1016/j.jallcom.2004.12.176
• 14. Zhou DW, Li SL,Varin RA, Peng P, Liu JS and Yang F. Mechanical alloying and electronic simulations of 2Mg-Fe mixture powders for hydrogen storage. Materials Science and Engineering A 2006; 427:306-315. http://dx.doi.org/10.1016/j.msea.2006.04.074
• 15. Riktor MD, Deledda S, Herrich M, Gutfleisch O, Fjellvag H and Hauback BC. Hydride formation in ball-milled and cryomilled Mg-Fe powder mixtures. Materials Science and Engineering B 2009;158:19-25. http://dx.doi.org/10.1016/j.mseb.2008.12.031
• 16. Leiva DR, Villela ACS, Paiva-Santos CO, Fruchart D, Miraglia S, Ishikawa TT et al. High-Yield Direct Synthesis of Mg2FeH6 from the Elements by Reactive Milling. Solid State Phenomena 2011; 170:259-262. http://dx.doi.org/10.4028/www.scientific.net/SSP.170.259
• 17. Shang CX, Bououdina M and Guo ZX. Direct mechanical synthesis and characterisation of Mg2Fe(Cu)H6. Journal of Alloys and Compounds 2003; 356-357:626-629. http://dx.doi.org/10.1016/S0925-8388(03)00273-1
• 18. Herrich M, Ismail N, Lyubina J, Handstein A, Pratt A and Gutfleisch O. Synthesis and decomposition of Mg2FeH6 prepared by reactive milling. Materials Science and Engineering B 2004; 108:28-32. http://dx.doi.org/10.1016/j.mseb.2003.10.031
• 19. Puszkiel JA, Arneodo Larochette P and Gennari FC. Hydrogen storage properties of MgxFe (x: 2, 3 and 15) compounds produced by reactive ball milling. Journal of Power Sources 2009; 186:185-193. http://dx.doi.org/10.1016/j.jpowsour.2008.09.101
• 20. Wang Y, Cheng F, Li C, Tao Z and Chen J. Preparation and characterization of nanocrystalline Mg2FeH6. Journal of Alloys and Compounds 2010; 508:554-558. http://dx.doi.org/10.1016/j.jallcom.2010.08.119
• 21. Huot J, Boily S, Akiba E and Schulz R. Direct synthesis of Mg2FeH6 by mechanical alloying. Journal of Alloys and Compounds 1998; 280:306-309. http://dx.doi.org/10.1016/S0925-8388(98)00725-7
• 22. Suryanarayana C. Mechanical alloying and milling. Progress in Materials Science 2001; 46:1-184. http://dx.doi.org/10.1016/S0079-6425(99)00010-9

Publication Dates

• Publication in this collection
  06 Mar 2012
• Date of issue
  Apr 2012

History

• Received
  23 May 2011
• Accepted
  13 Dec 2011