Synthesis of Mg2FeH6 by reactive mechanical alloying: formation and decomposition properties
Introduction
Magnesium based transition metal hydrides such as Mg2FeH6, Mg2CoH5 and Mg2NiH4 are attractive materials for hydrogen storage due mainly to their high hydrogen storage capacity. The hydrogen to metal weight ratios are 5.8 wt.% for Mg2FeH6, 4.7 wt.% for Mg2CoH5 and 3.7 wt.% for Mg2NiH4. Despite their weight efficiency for hydrogen storage, the high enthalpy of dissociation of Mg2FeH6 (98 kJ/mol H2) and Mg2CoH5 (86 kJ/mol H2) makes them less promising for energy storage applications than Mg2NiH4 (64 kJ/mol H2) and MgH2 (76 kJ/mol H2). In addition, both Mg2FeH6 or Mg2CoH5 are difficult to synthesize from Mg–Fe and Mg–Co alloys due mainly to the absence of intermetallic compounds of the type Mg2X (X=Fe, Co) in the binary systems [1].
The traditional way of preparing both Mg2FeH6 or Mg2CoH5 is by sintering at high temperature (450–520 °C) under high hydrogen pressure (2–12 MPa) [2], [3], [4]. Didisheim et al. [2] have prepared Mg2FeH6 from elemental powder mixtures by a sintering technique at 500 °C under 2–12 MPa hydrogen pressure. In a similar way, Zolliker et al. [3] have synthesized Mg2CoH5 by the same technique at 350–500 °C and hydrogen pressures between 4 and 6 MPa. However, the yield of the sintering technique is about 50% [2], [3] and the unreacted elements need to be separated through a complicated process. An optimization of the sintering process was reported by Selvam and Yvon [4]. They have synthesized both metal hydrides in a single process involving the sintering of mixtures of fine metal powders at 450–500 °C under 9 MPa of hydrogen pressure.
Different studies have shown an improvement in the synthesis methods of these hydrides associated to the use of mechanical alloying (MA) [5], [6], [7], [8]. Ivanov et al. [5] have reported the formation of Mg2FeH6 and Mg2CoH5 by MA the elemental powders and subsequently hydrogenating and dehydrogenating the material. However the hydrides yield was not given. Similarly, Konstanchuk et al. [6] have synthesized Mg2FeH6 by MA an Mg–25%Fe mixture for 5 min and successively hydrogenating and dehydrogenating at high temperature. Huot et al. prepared Mg2CoH5 and Mg2FeH6 by MA during 20 h under argon or hydrogen atmospheres followed by sintering at 350 °C under 5 MPa of hydrogen [7]. The highest yield of Mg2FeH6 (65 wt.%) was obtained when the milling was performed under hydrogen, whereas for the Mg2CoH5 the yield was about 25–30%, independently of the milling atmosphere. In a recent work, Huot et al. [8] have produced Mg2FeH6 by high-energy MA MgH2 and Fe under argon atmosphere without subsequent sintering. The method gave a yield of 56 wt.% of Mg2FeH6 after 60 h of milling [8], leaving Fe and Mg as unreacted phases. Up to now, this is the only work that presents an alternative route for Mg2FeH6 production through one process without sintering.
In this work we present the formation of Mg2FeH6 involving only one process by means of reactive mechanical alloying (RMA) without sintering. The Mg2FeH6 is formed performing a low energetic ball milling of Mg and Fe elemental powders at room temperature under hydrogen atmosphere. A scheme of the formation mechanism and decomposition is reported and the thermal stability and the effect of unreacted Fe on the decomposition of the hydrides formed during the RMA is analyzed.
Section snippets
Experimental
Elemental magnesium powder (99+%) and iron granules (99+%) were mechanically milled under a hydrogen atmosphere, using a Uni-Ball-Mill II apparatus (Australian Scientific Instrument). The 2Mg–Fe mixture together with ferromagnetic steel balls were placed in a stainless steel container and enclosed in an argon glove box. The container was then evacuated to 10−5 MPa prior to filling with 0.5 MPa of high-purity hydrogen (99.995%). The samples were milled for different times up to a total of 100 h
Results and discussion
Fig. 1 shows the XRD patterns of the 2Mg–Fe mixture after RMA under hydrogen as a function of milling time. The Mg diffraction peaks (JCPDS Powder Diffraction Data Card No. 35-0821) become smaller and broader with increasing milling time, as can be seen on the XRD patterns up to 20 h. Tetragonal MgH2 (JCPDS Powder Diffraction Data Card No. 12-0697) is formed during the initial 10 h of milling and its diffraction peaks broaden as milling time increases. After 40 h of milling, the diffraction
Conclusion
In the present work, we report the synthesis of Mg2FeH6 hydride from a 2Mg–Fe mixture involving as the only process RMA under hydrogen atmosphere. The formation of Mg2FeH6 comprises two steps that involve MgH2 as an intermediate compound. In opposition, the decomposition of Mg2FeH6 consists of only one step and does not follow the inverse route. The global mechanisms can be depicted as:
We also analyze the thermal stability of the material as a function of milling time. We observe that the
Acknowledgements
The authors thank Fundación Balseiro for the partial financial support to carry out this work.
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