Fast hydrogen absorption/desorption kinetics in reactive milled Mg-8 mol% Fe nanocomposites

https://doi.org/10.1016/j.ijhydene.2020.02.213Get rights and content

Highlights

  • MgH2–Mg2FeH6 nanocomposites were produced by RM, applying 10 and 24 h.

  • The hydrogen desorption was investigated by in-situ synchrotron X-ray diffraction.

  • Extended milling times increases the Mg2FeH6 complex hydride fraction.

  • The hydrogen sorption kinetics are extremely fast in relatively mild conditions.

Abstract

This study aims to better understand the Fe role in the hydrogen sorption kinetics of Mg–Fe composites. Mg-8 mol% Fe nanocomposites produced by high energy reactive milling (RM) for 10 h resulted in MgH2 mixed with free Fe and a low fraction of Mg2FeH6. Increasing milling time to 24 h allowed formation of a high fraction of Mg2FeH6 mixed with MgH2. The hydrogen absorption/desorption behavior of the nanocomposites reactive milled for 10 and 24 h was investigated by in-situ synchrotron X-ray diffraction, thermal analyses and kinetics measurements in Sieverts-type apparatus. It was found that both 10 and 24 h milled nanocomposites presents extremely fast hydrogen absorption/desorption kinetics in relatively mild conditions, i.e., 300–350 °C under 10 bar H2 for absorption and 0.13 bar H2 for desorption. Nanocomposites with MgH2, low Fe fraction and no Mg2FeH6 are suggested to be the most appropriate solution for hydrogen storage under the mild conditions studied.

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

References (50)

  • V.V. Bhat et al.

    High surface area niobium oxides as catalysts for improved hydrogen sorption properties of ball milled MgH2

    J Alloys Compd

    (2008)
  • V.V. Bhat et al.

    Catalytic activity of oxides and halides on hydrogen storage of MgH2

    J Power Sources

    (2006)
  • I.E. Malka et al.

    Catalytic effect of halide additives ball milled with magnesium hydride

    Int J Hydrogen Energy

    (2010)
  • A.R. Yavari et al.

    Improvement in H-sorption kinetics of MgH2 powders by using Fe nanoparticles generated by reactive FeF3 addition

    Scripta Mater

    (2005)
  • Y. Luo et al.

    Hydrogen sorption kinetics of MgH2 catalyzed with NbF5

    J Alloys Compd

    (2008)
  • S. Deledda et al.

    H-sorption in MgH2 nanocomposites containing Fe or Ni with fluorine

    J Alloys Compd

    (2005)
  • N. Recham et al.

    Reduction of hydrogen desorption temperature of ball-milled MgH2 by NbF5 addition

    J Alloys Compd

    (2008)
  • M. Danaie et al.

    TEM analysis of the microstructure in TiF3-catalyzed and pure MgH2 during the hydrogen storage cycling

    Acta Mater

    (2012)
  • M. Jangir et al.

    Catalytic effect of TiF4 in improving hydrogen storage properties of MgH2

    Int J Hydrogen Energy

    (2016)
  • J.A. Puszkiel et al.

    Hydrogen storage properties of MgxFe (x: 2, 3 and 15) compounds produced by reactive ball milling

    J Power Sources

    (2009)
  • F.C. Gennari et al.

    Synthesis of Mg2FeH6 by reactive mechanical alloying: formation and decomposition properties

    J Alloys Compd

    (2002)
  • S. Li et al.

    Controlled mechano-chemical synthesis of nanostructured ternary complex hydride Mg2FeH6 under low-energy impact mode with and without pre-milling

    J Alloys Compd

    (2004)
  • Y. Wang et al.

    Preparation and characterization of nanocrystalline Mg2FeH6

    J Alloys Compd

    (2010)
  • D.W. Zhou et al.

    Mechanical alloying and electronic simulations of 2Mg-Fe mixture powders for hydrogen storage

    Mater Sci Eng

    (2006)
  • J. Puszkiel et al.

    Sorption behavior of the MgH2- Mg2FeH6 hydride storage system synthesized by mechanical milling followed by sintering

    Int J Hydrogen Energy

    (2013)
  • Cited by (22)

    • Effect of ternary transition metal sulfide FeNi<inf>2</inf>S<inf>4</inf> on hydrogen storage performance of MgH<inf>2</inf>

      2023, Journal of Magnesium and Alloys
      Citation Excerpt :

      In Xie's work, they found that NiS reduced the activation energy of hydrogen desorption of MgH2 by 64.71 kJ mol−1 [44]. In addition, transition metal iron (Fe) is also emerged as a highly effective catalyst for hydrogen absorption and desorption of Mg/MgH2 system [45,46]. As reported by the deepgoing study of Zhang's group, both Fe3S4 and FeS2 could remarkably increase the re/dehydrogenation kinetics of Mg/MgH2 system and lower its initial dehydrogenation temperature [47,48].

    • Achieve high-efficiency hydrogen storage of MgH<inf>2</inf> catalyzed by nanosheets CoMoO<inf>4</inf> and rGO

      2022, Journal of Alloys and Compounds
      Citation Excerpt :

      Among various methods, catalyst doping modification is considered to be the most simple and effective way to overcome the thermodynamic and kinetic defects of MgH2. Transition metals such as Ti, Ni, Co, and Mn have all been proved to significantly reduce the de/rehydrogenation temperature of MgH2 and improve its kinetic properties [13–15]. In addition, researchers have also conducted extensive research on various transition metal compounds, especially transition metal oxides, which are not only simple to prepare, but also have excellent stability in air.

    • Remarkable catalytic effect of Ni and ZrO<inf>2</inf> nanoparticles on the hydrogen sorption properties of MgH<inf>2</inf>

      2022, International Journal of Hydrogen Energy
      Citation Excerpt :

      Reducing the particle size via ball milling has also been very effective in enhancing the hydrogen sorption kinetics compared to unmilled MgH2 [12–14]. Further kinetic improvements and reduction in activation energies have been obtained using certain transition metals [3,6,15,16], their oxides [17–20], halides [21–23], and carbon nanomaterials [24,25] as catalysts or dopants during the ball milling process. Catalysts are very crucial because they play an important role of destabilizing the Mg–H bonding which is known to accelerate hydrogenation and dehydrogenation kinetics of MgH2 [26,27].

    • Enhanced catalytic effect of TiO<inf>2</inf>@rGO synthesized by one-pot ethylene glycol-assisted solvothermal method for MgH<inf>2</inf>

      2021, Journal of Alloys and Compounds
      Citation Excerpt :

      As shown in Fig. 7, there are many large particles (>2 µm) in the sample milled for 2 h. Further prolonging the milling time to 5 h and 10 h, the large particles become fewer and average particle size gets smaller. Additionally, the particle size does not decrease obviously when the time reaches 20 h, demonstrating the nonlinear relationship between particle size and milling time [33,34]. Isothermal hydrogen desorption/absorption measurements of MgH2-40TiO2@rGO-EG composite milled for different periods were carried out at different temperatures under 0.0004 MPa and 3 MPa hydrogen pressure.

    View all citing articles on Scopus
    View full text