Fast hydrogen absorption/desorption kinetics in reactive milled Mg-8 mol% Fe nanocomposites
Introduction
Magnesium is a light and hydride-forming metal with a great potential as a solid hydrogen storage material. It has a high availability and low relative cost [1]. Its hydride has a high gravimetric capacity (7.6 wt% H2), good reversibility [2] and cyclic stability depending on the cycling parameters [3]. However, like other hydride-forming materials, conventional (microcrystalline) Mg requires high reaction temperatures (~400 °C) and have slow kinetics during the hydrogen absorption and desorption reactions [4]. In addition to the possibility of hydrogen storage, Mg-based hydrides may also be potential options as thermal energy storage media [5,6], generally at temperatures and pressures higher than those expected for the hydrogen storage. For instance, the Mg2FeH6 complex hydride has been studied and exhibited good potential as thermal energy storage material, presenting operation temperature range between 450 and 550 °C and theoretical thermal energy density per volume of 5447 kJ dm−3 [6].
Mechanical milling has been demonstrated to be a good strategy for processing Mg-based hydrides for hydrogen storage application. High-energy ball milling is useful to improve the dispersion of mixing elements and reducing particles sizes, which usually results in improvements in the hydrogen absorption/desorption kinetics [7]. Another alternative is the direct synthesis by reactive milling, which occurs under H2 atmosphere [8]. This synthesis route may bring advantageous effects like nanostructuring and good intermixing of final products [9,10].
The combination of additive elements and/or compounds has also shown to be an important strategy to improve the absorption/desorption kinetics of MgH2. Several additives such as transition metals [[11], [12], [13], [14]], oxides [[15], [16], [17]], chlorides [18,19] and fluorides [[20], [21], [22], [23], [24], [25], [26], [27]] have been studied. Among them, Fe is considered an efficient and low-cost additive to improve the hydrogen storage kinetics properties of MgH2 [28]. Puszkiel and co-authors [29] studied the thermo-kinetic properties of Mg–Fe based materials for hydrogen storage. They prepared MgxFe (x: 2, 3 and 15) mixtures from elemental powders via low energy ball milling under 0.5 MPa (5 bar) hydrogen atmosphere. The milling parameters were 180 rpm for 150 h and 44:1 ball-to-powder ratio (BPR). After milling, the Mg15Fe mixture yielded 90 wt% MgH2, 7 wt% Fe and 3 wt% Mg. In dynamic conditions, the Mg15Fe has shown better hydrogen capacity (4.85 wt% at 350 °C absorbed in less than 10 min, after 100 absorption/desorption cycles), reasonably good absorption/desorption times and cycling stability, than the other studied compositions.
In another study, Leiva and co-authors [23] milled a 2 Mg–Fe powder composition in a planetary mill using 600 rpm. Milling time was varied from 1 to 72 h. The BPR and the initial hydrogen pressure were 40:1 and 3 MPa (30 bar), respectively. The sample milled for 7.5 h resulted in rich fractions of both MgH2 and Mg2FeH6 hydrides, presenting also an important fraction of unreacted iron. Due to the low desorption temperatures observed in the DSC, the sample milled for 7.5 h was selected for H-sorption kinetics measurements. Ultra-fast H-sorption kinetics was verified. The results were reproducible for the first five cycles. As example, at 300 °C and 15 bar of H2 pressure for absorption and 0.3 bar for desorption, the sample capacity reached 4.2 wt% H2, absorbing and desorbing it in 6 and 2 min, respectively.
As reported in previous investigations [23,29], when Mg–Fe composites are produced for hydrogen storage applications, Fe can either remain free, in a mixture with MgH2 (because of its very low solubility in Mg [30]), or the Mg2FeH6 complex hydride can be formed. The specific role of Fe on the hydrogen absorption/desorption kinetics of MgH2 is not yet completely clear. Moreover, it is not clear if the formation of the Mg2FeH6 complex hydride has a beneficial effect on the hydrogen storage properties of MgH2.
This work aims to give a step further on the understading of the role of Fe on the hydrogen absorption/desorption kinetics of Mg–Fe composites. Mg-8mol%Fe nanocomposites were produced by RM under 30 bar of hydrogen pressure using two different milling times, i.e., 10 and 24 h. The hydrogen desorption sequence of such composites was investigated by in-situ synchrotron X-ray diffraction and thermal analyses. To the best of our knowledge, this is first time that the decomposition of Mg–Fe composites produced only by high energy reactive milling has been analyzed by in-situ XRD synchrotron technique. The hydrogen absorption/desorption kinetics as well the cycling properties of the Mg–Fe composites were measured using volumetric techniques in a Sieverts-type apparatus. The nanocomposites produced by RM present extremely fast hydrogen absorption/desorption kinetics in relatively mild conditions (absorption at 300-350 °C under 10 bar of H2 and desorption at 300-350 °C in a closed system with 0.13 bar of H2 pressure).
Section snippets
Materials and methods
A magnesium ingot (99.8% purity) was supplied by RIMA Industrial S/A. Pieces were extracted through the saw cut, which produced chips as starting material for RM. Iron powder (99.998% purity), Puratronic # 22 Mesh (0.774 mm) was supplied by Alfa Aesar. Argon and hydrogen 5.0 (analytical grade) were used for the manipulation of materials and during RM, respectively. The powdered materials were handled and maintained in a MBraun/LabMaster 130 glovebox in an argon inert atmosphere with low
Results and discussion
Two nanocomposites containing Mg and 8 %mol Fe were produced by high-energy reactive milling with milling times of 10 and 24 h, respectively. Both processes resulted in nanocomposites with mixtures of MgH2 and Mg2FeH6 phases.
Several studies have shown that Mg2FeH6 forms during RM from the reaction of MgH2 + Fe + H2 [23,29,[32], [33], [34], [35]], even if the starting materials are MgH2 + Fe or Mg + Fe + H2 [9]. Baum and co-authors [36] verified that Mg2FeH6 could be formed during RM of powder
Conclusions
Two types of nanocomposites containing mixtures of MgH2, free Fe and Mg2FeH6 phases were produced by high-energy reactive milling by the variation of the milling time (10 and 24 h). In-situ synchrotron X-ray diffraction measurements during desorption of the as-reactive milled samples showed that for both samples (Mg8Fe-RM10h and Mg8Fe-RM24h), MgH2 is the first hydride to decompose. It was also shown that increasing milling time (Mg8Fe-RM24h sample) increases the amount of Mg2FeH6 complex
Acknowledgements
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. The authors would like also to thank the Brazilian institutions Fundação de Amparo à Pesquisa do Estado de S. Paulo - Brazil (FAPESP) (grant number 2013/05987–8) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the financial support, and the Laboratory of Structure and Characterization of the Federal University of São Carlos
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