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

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.     

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

The hydrogen sorption behavior of the Mg₂FeH₆–MgH₂ hydride system is investigated via in-situ synchrotron and laboratory powder X-ray diffraction (SR-PXD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), particle size distribution (PSD) and volumetric techniques. The Mg₂FeH₆–MgH₂ hydride system is obtained by mechanical milling in argon atmosphere followed by sintering at high temperature and hydrogen pressure. In-situ SR-PXD results show that upon hydriding MgH₂ is a precursor for Mg₂FeH₆ formation and remained as hydrided phase in the obtained material. Diffusion constraints preclude the further formation of Mg₂FeH₆. Upon dehydriding, our results suggest that MgH₂ and Mg₂FeH₆ decompose independently in a narrow temperature range between 275 and 300 °C. Moreover, the decomposition behavior of both hydrides in the Mg₂FeH₆–MgH₂ hydride mixture is influenced by each other via dual synergetic-destabilizing effects. The final hydriding/dehydriding products and therefore the kinetic behavior of the Mg₂FeH₆–MgH₂ hydride system exhibits a strong dependence on the temperature and pressure conditions.     

High-pressure synthesis of Mg2FeH6 complex hydride

We have designed a new synthesis method for the ternary metal hydride Mg₂FeH₆ based on the direct reaction of simple hydrides under high-pressure conditions. Well-crystallized samples were prepared in a piston-cylinder hydrostatic press at 2 GPa and temperatures around 750 °C from mixtures of MgH₂ and Fe enclosed in gold or platinum capsules. Seven different samples have been prepared under different conditions. X-ray powder diffraction analysis was used to identify and assess the purity of the samples, through Rietveld analyses of the crystal structure (K₂PtCl₆-type). Mg₂FeH₆ shows a cubic symmetry with space group Fm-3m. SEM images show an average particle size of 1–2 μm for Mg₂FeH₆; the microcrystals present well-grown faces and display a high homogeneity of shapes and sizes. Thermogravimetric analysis has been carried out to determine not only the hydrogen desorption temperature but also the hydrogen contents.     

Bulk modulus and thermal expansion coefficient of mechano-chemically synthesized Mg2FeH6 from high temperature and high pressure studies

Mg₂FeH₆ was synthesized by ball milling MgH₂ and Fe (2:1 molar ratio) mixture for 72 h followed by heating at 400 °C under H₂ pressure. The hydride formation, its structure and homogeneity were investigated by scanning electron microscopy, X-ray diffraction, transmission electron microscopy and Raman spectroscopy. High pressure in situ synchrotron X-ray diffraction and Vienna ab initio simulation were used to determine bulk modulus of the sample. The bulk modulus of Mg₂FeH₆ was found to be 75.4(4) GPa by optimized experiment and 76.3 GPa by theoretical simulation. From high temperature in situ X-ray diffraction study the volumetric thermal expansion coefficient of Mg₂FeH₆ was found to be α_v = 5.85(3) × 10⁻⁵ + 7.47(7) × 10⁻⁸ (T − T_o)/°C. Decomposition of Mg₂FeH₆ was observed at 425 °C and the decomposition products were Mg, Fe and H₂.     

Reactivity of complex hydrides Mg2FeH6, Mg2CoH5 and Mg2NiH4 with lithium ion: Far from equilibrium electrochemically driven conversion reactions

The electrochemical reaction of lithium ion with Mg₂FeH₆, Mg₂CoH₅ and Mg₂NiH₄ complex hydrides prepared by reactive grinding is studied here. Plateaus at an average potential of 0.25 V, 0.24 V and 0.27 V corresponding to discharge capacities of 6.6, 5.5 and 3.6 Li can be achieved respectively for Mg₂FeH₆, Mg₂CoH₅ and Mg₂NiH₄. From in situ X-ray diffraction (XRD) characterizations of complex hydride based electrodes, dehydrogenation leads to a decrease of the intensities of the diffraction peaks suggesting a strong loss of crystallinity since formation of Mg and M (M = Fe, Co, Ni) peaks is not observed. ⁵⁷Fe Mössbauer spectroscopy confirms the formation of nanoscale Fe or an amorphous Mg–Fe alloy during the decomposition of Mg₂FeH_6. Interestingly, lattice parameter variations suggest phase transitions in the Mg₂NiH₄ system involving the formation of low hydrogen content hydride Mg₂NiH, while an increase of lattice parameters of Mg₂CoH₅ hydride could be attributed to the formation of a Mg₂CoH₅Li_x solid solution compound up to x = 1.     

Mg2FeH6–LiBH4 and Mg2FeH6–LiNH2 composite materials for hydrogen storage

Two composite hydrogen storage materials based on Mg₂FeH₆ were investigated for the first time. The Mg₂FeH₆–LiBH₄ composite of molar ratio 1:5 showed a hydrogen desorption capacity of 5.6 wt.% at 370 °C, and could be rehydrogenated to 3.6 wt.% with the formation of MgH₂, as the material was heated to 445 °C and held at this temperature. The Mg₂FeH₆–LiNH₂ composite of 3:10 molar ratio exhibited a hydrogen desorption capacity of 4.3 wt.% and released hydrogen at 100 °C lower then the Mg₂FeH₆–LiBH₄ composite, but this mixture could not be rehydrogenated. Compared to neat Mg₂FeH₆, both composites show enhanced hydrogen storage properties in terms of desorption kinetics and capacity at these low temperatures. In particular, Mg₂FeH₆–LiNH₂ exhibits a much lower desorption temperature than neat Mg₂FeH₆, but only Mg₂FeH₆–LiBH₄ re-absorbs hydrogen.     

Enhanced electrochemical lithium storage performance of Mg2FeH6 anode with TiO2 coating

Light-weight metal hydrides are potential high-capacity conversion anode materials for lithium-ion batteries, but the poor reaction reversibility and cyclic stability of hydride anodes need to be improved. In this work, the ternary hydride Mg₂FeH₆ was composited with the graphite (G) by ball-milling, and the Mg₂FeH₆-G composite electrode was further coated with amorphous TiO₂ film by magnetron sputtering. The resultant Mg₂FeH₆-G/TiO₂ electrode exhibited a stable charge capacity of 412 mAh g^?1 over 100 cycles, which is much higher than 46 mAh g^?1 at 20th cycle for the pure Mg₂FeH₆ electrode, or 185 mAh g^?1 at 100th cycle for the Mg₂FeH₆-G electrode. There is only little capacity degradation after 20 cycles for the Mg₂FeH₆-G/TiO₂ electrode and the charge capacity retention is 84.7% after 100 cycles. The remarkable improvement in the cyclic stability of Mg₂FeH₆-G/TiO₂ electrode is mainly attributed to the dense TiO₂ coating that maintains the structural integrity of electrode during cycling. The TiO₂ coating also prevents the direct contact of high active LiH/MgH₂ with the liquid electrolyte, and thus ensures the high reversibility of conversion reaction of MgH₂ during cycling.     

Mg2NiH4 synthesis and decomposition reactions

A ternary Mg₂NiH₄ hydride was synthesized using method that relies on a relatively short mechanical milling time (one hour) of a 2:1 MgH₂–Ni powder mixture followed by sintering at a sufficiently high hydrogen pressure (>85 bar) and temperature (>400 °C). The ternary hydride forms in less than 2.5 h (including the milling time) with a yield of ∼90% as a mixture of two polymorphic forms. The mechanisms of formation and decomposition of ternary Mg₂NiH₄ under different hydrogen pressures were studied in detail using an in situ synchrotron radiation powder X-ray diffraction (SR-PXD) and high pressure DSC. The obtained experimental results are supported by morphological and microstructural investigations performed using SEM and high resolution STEM. Additionally, effects occurring during the desorption reaction were studied using DSC coupled with mass spectrometry.     

Hydrogen storage performance of 5LiBH4 + Mg2FeH6 composite system

The hydrogen storage properties of 5LiBH₄ + Mg₂FeH₆ reactive hydride composites for reversible hydrogen storage were investigated by comparing with the 2LiBH₄ + MgH₂ composite in the present work. The dehydrogenation pathway and reaction mechanism of 5LiBH₄ + Mg₂FeH₆ composite were also investigated and elucidated. The self-decomposition of Mg₂FeH₆ leads to the in situ formation of Mg and Fe particles on the surface of LiBH₄, resulting in a well dispersion between different reacting phases. The formation of FeB is observed during the dehydrogenation of 5LiBH₄ + Mg₂FeH₆ composite, which might supplies nucleation sites of MgB₂ during the dehydrogenation process, but is not an ascendant catalyst for the self-decomposition of LiBH₄. And FeB can also transform to the LiBH₄ and Fe by reacting with LiH and H₂ during the rehydrogenation process. The dehydrogenation capacity for 5LiBH₄ + Mg₂FeH₆ composite still gets to 6.5 wt% even after four cycles. The X-ray diffraction analyses reveal the phase transitions during the hydriding and dehydriding cycle. The formed FeB in the composite maintains a nanostructure after four hydriding-dehydriding cycles. The loss of hydrogen storage capacity and de-/rehydrogenation kinetics can be attributed to the incomplete generation of Mg₂FeH₆ during the rehydrogenation process.     

Formation and hydrogen storage behavior of nanostructured Mg2FeH6 in a compressed 2MgH2–Fe composite

The present study focuses on enhancing the yield of Mg₂FeH₆ and its hydrogen storage performances through a novel high-pressure compression approach. For which, MgH₂ and Fe powders are first mechanically milled in a molar ratio of 2:1 and subsequently compressed to a cylindrical pellet. Due to the compression, the yield of Mg₂FeH₆ in the compressed 2MgH₂–Fe pellet (90%) has been increased by 24% as compared to the reference ball-milled powder (66%). The thermodynamic destabilization of Mg₂FeH₆ in the pelletized sample is observed through measuring the pressure-composition isotherms, resulting in the reduced ab/desorption enthalpy for the pellet sample (−68.34 and 75.61 kJ/mol H₂, respectively). The hydrogen uptake and release kinetics of Mg₂FeH₆ is remarkably fast, and it can store/release about 5 wt% H in less than 2.5 min at 400 °C. The faster hydrogen ab/desorption kinetics corresponds to the lower activation energies (36 and 95 kJ/mol H₂, respectively). The excellent yield of Mg₂FeH₆ and its improved hydrogen storage properties for the compressed pellet are primarily attributed to the microstructural modifications upon high-pressure compression, and also the obtained results for Mg₂FeH₆ ternary hydride are linked to the literature data based on theoretical calculations.     

The influence of the milling time on the yield of Mg2FeH6 from a two-step synthesis conducted in a custom-made reactor

Dimagnesium iron hydride was synthesized by mechanical milling of a MgH₂/Fe mixture followed by sintering under a high hydrogen pressure (120 bar). The influence of the milling time on the synthesis yield was observed. Properly chosen processing parameters led to a 94–97 % reaction yield (depending on the measurement method) for the formation of Mg₂FeH₆. Milling times that were too short or too long proved to be ineffective. A custom-made reactor for synthesis of a batch of up to 20 g is presented. Synthesized samples were characterized by XRD analysis and PCT measurements were performed.     

Thermal stability of Vale Inco nanonometric nickel as a catalytic additive for magnesium hydride (MgH2)

The structure stability of nanometric-Ni (n-Ni) produced by Vale Inco Ltd. Canada as a catalytic additive for MgH₂ has been investigated. Each n-Ni filament is composed of nearly spherical interconnected particles having a mean diameter of 42 ± 16 nm. After ball milling of the MgH₂ + 5 wt.%n-Ni mixture for 15 min the n-Ni particles are found to be uniformly embedded within the particles of MgH₂ and at their surfaces. Neither during ball milling of the MgH₂ + 5 wt.%n-Ni mixture nor its first decomposition at temperatures of 300, 325, 350 and 375 ^°C the elemental n-Ni reacts with the elemental Mg to form the Mg₂Ni intermetallic phase (and eventually the Mg₂NiH₄ hydride). The n-Ni additive acts as a strong catalyst accelerating the kinetics of desorption. From the Arrhenius and Johnson–Mehl–Avrami–Kolmogorov theory the activation energy for the first desorption is determined to be ∼94 kJ/mol. After cycling at 300 ^°C the activation energy for desorption is determined to be ∼99 kJ/mol. This is much lower than ∼160 kJ/mol observed for the undoped and ball milled MgH₂. During cycling at 275 and 300 ^°C the n-Ni additive is converted into Mg₂Ni (Mg₂NiH₄). The newly formed Mg₂NiH₄ has a nanosized grain on the order of 20 nm. Its catalytic potency seems to be similar to its n-Ni precursor. The formation of Mg₂Ni (Mg₂NiH₄) may be one of the factors responsible for the systematic decrease of hydrogen capacity observed upon cycling at 275 and 300 ^°C.     

Destabilization of LiAlH4 by nanocrystalline MgH2

In this work, we report the synthesis, characterization and destabilization of lithium aluminum hydride by ad-mixing nanocrystalline magnesium hydride (e.g. LiAlH₄ + nanoMgH₂). A new nanoparticulate complex hydride mixture (Li–nMg–Al–H) was obtained by solid-state mechano-chemical milling of the parent compounds at ambient temperature. Nanosized MgH₂ is shown to have greater and improved hydrogen performance in terms of storage capacity, kinetics, and initial temperature of decomposition, over the commercial MgH₂. The pressure–composition isotherms (PCI) reveal that the destabilized LiAlH₄ + nanoMgH₂ possess ∼5.0 wt.% H₂ reversible capacity at T ≤ 350 °C. Van't Hoff calculations demonstrate that the destabilized (LiAlH₄ + nanoMgH₂) complex materials have comparable enthalpy of hydrogen release (∼85 kJ/mole H₂) to their pristine counterparts, LiAlH₄ and MgH₂. However, these new destabilized complex hydrides exhibit reversible hydrogen sorption behavior with fast kinetics.     

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

This work deals with the assessment of the thermo-kinetic properties of Mg–Fe based materials for hydrogen storage. Samples are prepared from Mg_xFe (x: 2, 3 and 15) elemental powder mixtures via low energy ball milling under hydrogen atmosphere at room temperature. The highest yield is obtained with Mg₁₅Fe after 150 h of milling (90 wt% of MgH₂). The thermodynamic characterization carried out between 523 and 673 K shows that the obtained Mg–Fe–H hydride systems have similar thermodynamic parameters, i.e. enthalpy and entropy. However, in equilibrium conditions, Mg₁₅Fe has higher hydrogen capacity and small hysteresis. In dynamic conditions, Mg₁₅Fe also shows better hydrogen capacity (4.85 wt% at 623 K absorbed in less than 10 min and after 100 absorption/desorption cycles), reasonably good absorption/desorption times and cycling stability in comparison to the other studied compositions. From hydrogen uptake rate measurements performed at 573 and 623 K, the rate-limiting step of the hydrogen uptake reaction is determined by fitting particle kinetic models. According to our results, the hydrogen uptake is diffusion controlled, and this mechanism does not change with the Mg–Fe proportion and temperature.     

Reversible de-/hydriding characteristics of a novel Mg18In1Ni3 alloy

The present work demonstrates the reversible hydrogen storage properties of the ternary alloy Mg₁₈In₁Ni₃, which is prepared by ball-milling Mg(In) solid solution with Ni powder. The two-step dehydriding mechanism of hydrogenated Mg₁₈In₁Ni₃ is revealed, namely the decomposition of MgH₂ is involved with different intermetallic compounds or Ni, which leads to the formation of Mg₂Ni(In) solid solution or unknown ternary Mg–In–Ni alloy phase. As a result, the alloy Mg₁₈In₁Ni₃ shows improved thermodynamics in comparison with pure Mg. The Ni addition also results in the kinetic improvement, and the minimum desorption temperature is reduced down to 503 K, which is a great decrease comparing with that for Mg–In binary alloy. The composition and microstructure of Mg–In–Ni ternary alloy could be further optimized for better hydrogen storage properties.     

Synthesis and decomposition mechanisms of ternary Mg2CoH5 studied using in situ synchrotron X-ray diffraction

A ternary Mg₂CoH₅ hydride was synthesized using a novel method that relies on a relatively short mechanical milling time (1 h) of a 2:1 MgH₂-Co powder mixture followed by sintering at a sufficiently high hydrogen pressure (>85 bar) and heating from RT to 500 °C. The ternary hydride forms in less than 2.5 h (including the milling time) with a yield of ∼90% at ∼300 °C. The mechanisms of formation and decomposition of ternary Mg₂CoH₅ were studied in detail using an in situ synchrotron radiation powder X-ray diffraction (SR-PXD). The obtained experimental results are supported by morphological and microstructural investigations performed using SEM and high-resolution STEM. Additionally, thermal effects occurring during the desorption reaction were studied using DSC. The morphology of as-prepared ternary Mg₂CoH₅ is characterized by the presence of porous particles with various shapes and sizes, which, in fact, are a type of nanocomposite consisting mainly of nanocrystallites with a size of ∼5 nm. Mg₂CoH₅ decomposes at approximately 300 °C to elemental Mg and Co. Additionally, at approximately 400 °C, MgCo is formed as precipitates inserted into the Mg-Co matrix. During the rehydrogenation of the decomposed residues, prior to the formation of Mg₂CoH₅, MgH₂ appears, which confirms its key role in the synthesis of the ternary Mg₂CoH₅.     

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

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.     

Studies on de/rehydrogenation characteristics of nanocrystalline MgH2 co-catalyzed with Ti,Fe and Ni

In the present study, we have investigated the combined effect of different transition metals such as Ti, Fe and Ni on the de/rehydrogenation characteristics of nano MgH₂. Mechanical milling of MgH₂ with 5 wt% each of Ti, Fe and Ni for 24 h at 12 atm of H₂ pressure lead to the formation of nano MgH₂-Ti5Fe5Ni5. The decomposition temperature of nano MgH₂-Ti5Fe5Ni5 is lowered by 90 °C as compared to nano MgH₂ alone. It is also found that the nano MgH₂-Ti5Fe5Ni5 absorbs 5.3 wt% within 15 min at 270 °C and 12 atm hydrogen pressures. However, nano MgH₂ reabsorbs only 4.2 wt% under identical condition. An interesting result of the present study is that mechanical milling of MgH₂ separately with Fe and Ni besides refinement in particle size also leads to the formation of alloys Mg₂NiH₄ and Mg₂FeH₆ respectively. On the other hand, when MgH₂ is mechanically milled together with Ti, Fe and Ni, the dominant result is the formation of nano particles of MgH₂. Moreover the activation energy for dehydrogenation of nano MgH₂ co-catalyzed with Ti, Fe and Ni is 45.67 kJ/mol which is 35.71 kJ/mol lower as compared to activation energy of nano MgH₂ (81.34 kJ/mol). These results are one of the most significant in regard to improvement in de/rehydrogenation characteristics of known MgH₂ catalyzed through transition metal elements.     

Thermochemical transformations in 2MNH2–3MgH2 systems (M = Li or Na)

Thermochemical reactions between alkali metal amides and magnesium hydride taken in 2:3 molar ratios have been investigated using pressure-composition-temperature, X-ray powder diffraction and residual gas analysis measurements. The thermally induced reactions in both title systems are stoichiometric and proceed as a following solid state transformation: 2MNH₂ + 3MgH₂ → Mg₃N₂ + 2MH + 4H₂↑. A total of 6.45 wt.% of hydrogen is released by the 2LiNH₂–3MgH₂ system beginning at 186 °C, and a total of 5.1 wt.% H₂ is released by the 2NaNH₂–3MgH₂ system starting at 130 °C. Combined structure/property investigations revealed that the transformation in the lithium containing system proceeds in two steps. In the first step, lithium amide reacts with MgH₂ to form Li₂Mg(NH)₂ and hydrogen. In the second step, reaction between Li₂Mg(NH) and MgH₂ leads to the formation of the Mg₃N₂ nitride, lithium hydride and additional gaseous hydrogen. The transformation in the sodium containing system appears to proceed through a series of competing solid state processes with formation of Mg(NH₂)₂ and NaMgH₃ intermediates. Partial rehydrogenation in 190 bar hydrogen pressure leading to formation of the MgNH imide was observed in the dehydrogenated 2NaNH₂–3MgH₂ system at 395 °C.