Synthesis and decomposition mechanisms of Mg₂FeH₆ studied by in-situ synchrotron X-ray diffraction and high-pressure DSC

Abstract

Synthesis and decomposition mechanisms of ternary Mg₂FeH₆ were investigated using in-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and high-pressure differential scanning calorimetry (HP-DSC). Two routes for synthesis of Mg₂FeH₆ were studied. The first utilizes a ball-milled homogeneous MgH₂–Fe powder mixture and the second uses a mixture of Fe and Mg formed by decomposition of the ternary hydride, Mg₂FeH₆. In both cases the reaction mixture was sintered in a temperature range from RT to 500 °C under a hydrogen pressure of 100–120 bar. The reaction mechanisms were established using in-situ SR-PXD. The formation of Mg₂FeH₆ consists of two steps with MgH₂ as an intermediate compound, and the presence of magnesium was not observed. In contrast, the decomposition of Mg₂FeH₆ was found to be a single-step reaction. Additionally, both reactions were investigated using HP-DSC under similar conditions as in the SR-PXD experiments in order to estimate reaction enthalpies and temperatures. Mg₂FeH₆ was found to form from MgH₂ and Fe under hydrogen pressure regardless of whether the MgH₂ was introduced in the mixture or formed prior to creation of the ternary hydride.

Introduction

There is plenty of available renewable energy sources, e.g., sun- and wind-energy, but unfortunately, they are unevenly distributed both geographically and over time. Therefore, efficient storage and distribution of renewable energy is of significant importance, and hydrogen has been suggested as an attractive future energy carrier [1]. The only drawback is that the lightest element of all, hydrogen, occurs as a gas at ambient conditions and is difficult to store in a compact and efficient way [2]. Solid-state hydrogen storage appears to be the only method with sufficient potential storage capacity to realize the so-called ‘hydrogen-economy’ [1], [3].

Due to moderate gravimetric (5.47 wt%) and extremely high volumetric (150 kg/m³) hydrogen storage capacities, the complex magnesium iron hydride, Mg₂FeH₆, is considered as an attractive material for hydrogen storage [4]. Furthermore, Mg₂FeH₆ is also remarkable, due to low cost and high abundance of magnesium and iron. Many ternary intermetallic hydrides are used today in commercial applications, e.g., LaNi₅H₆ and FeTiH₂, despite their relatively lower hydrogen storage capacities of ∼2 wt% and ∼100 kg/m³ [5]. Complex hydrides formed by covalently bonded hydrogen constitute an attractive alternative class of materials and boron and aluminum hydrides, such as BH₄⁻ and AlH₄⁻, currently receive much attention [6], [7], [8]. Several other new complex ions have recently been described, e.g., Sc(BH₄)₄⁻, Zn₂(BH₄)₅⁻, [Zn(BH₄)₃]_nⁿ⁻, Zn(BH₄)Cl₂⁻ [9], [10], [11], [12]. This work explores the synthesis and characterization of the ternary magnesium iron hydride Mg₂FeH₆ based on the FeH₆⁴⁻ complex.

The synthesis of Mg₂FeH₆ is complicated by the lack of an Mg₂Fe intermetallic phase in the equilibrium magnesium–iron phase diagram. Several approaches have been utilized for synthesis of Mg₂FeH₆, often based on mechanical milling, reactive mechanical alloying in a hydrogen atmosphere, or sintering at elevated temperatures and hydrogen pressures, typically in the range 300–550 °C and 20–120 bar. In general, the yield of Mg₂FeH₆ fabricated by sintering, mechanical milling, or reactive mechanical milling is >50% depending on both the initial materials and the processing conditions [4], [13], [14].

Recently, Polanski et al. [15] successfully synthesized Mg₂FeH₆ by a novel synthetic route combining high energy mechanical ball-milling of 2MgH₂–Fe under argon, sintering in Sievert's apparatus under hydrogen pressures of 85–100 bar, and heat treatment in the range from RT to 500 °C with heating rates of 5–20 °C/min. Despite a relatively short reaction time in the range of 20 min to 2 h for the complete process, a yield of >90% product was reached.

Bogdanović et al. [16] performed accurate and systematic thermodynamic and microstructural investigations of Mg₂FeH₆ with the use of combined high resolution transmission electron microscopy (TEM) and Energy Dispersive X-ray Spectroscopy (EDS). Based on ex-situ results, they concluded that the ternary Mg–Fe–H hydride is most likely formed from elemental Mg, Fe and H₂ with no MgH₂ as an intermediate phase. They also demonstrated good cycling behavior of the Mg–Fe–H system. The energy and electronic structure of Mg₂FeH₆ were studied by Zhou et al. [17] using first-principles plane-wave pseudopotential methods. The authors stated (contrary to Bogdanović) that Mg₂FeH₆ is created by a direct reaction of MgH₂ with Fe.

In this study, further knowledge of the synthesis and decomposition of Mg₂FeH₆ is obtained using in-situ synchrotron radiation X-ray diffraction. The ternary Mg₂FeH₆ is synthesized at a synchrotron beamline from a pre-milled MgH₂ + Fe mixture, and the same sample is then studied during hydrogen release and uptake. Thermal effects occurring during the reaction are studied by (HP-DSC) under similar conditions. The combination of these two methods gives unique results, adding more details to the state-of-the-art in research in this area.