Dynamic synthesis of ternary Mg2FeH6

This work presents new results on the dynamic synthesis and decomposition of ternary Mg₂FeH₆. A novel synthesis method was applied for the rapid and effective synthesis of a ternary Mg–Fe hydride. This method consists of two processing routes. The first route involves high-energy ball milling of the initial MgH₂–Fe powder mixture, while the second is composed of a unique pressurizing and heating cycle route to obtain a full phase transformation within half an hour. The structural investigations carried out by X-ray diffraction revealed that almost all of the initial powder mixture transforms into the ternary hydride. Furthermore, the sample, which was synthesized, was also decomposed and reloaded with hydrogen. The formation of Mg₂FeH₆ consists of two steps that involve MgH₂ as an intermediate compound. In contrast, the decomposition of Mg₂FeH₆ consists of only one step and does not follow the inverse route. Some traces of iron were found in the reaction products. TDP results show that a desorption peak occurs at 315 °C, and this is in good agreement with DSC measurements showing only a single endothermic peak around 340 °C. Microstructural examinations revealed that the synthesized Mg₂FeH₆ powder generally exhibits a duplex structure that consists of plate-like particles larger than 1 μm in diameter and spherical particles smaller than 50 nm that show a tendency to agglomerate and form larger particles exhibiting a sponge-like structure. The formation of Mg₂FeH₆ takes place at the phase boundary between Fe seeds and the growing hydride phase. In contrast, the decomposition of the Mg₂FeH₆ phase takes place with the formation of the separate nanosized Mg and Fe phases. The dehydrogenated powder sample shows oval Fe precipitates of 10–100 nm in size that are embedded in the Mg-based matrix.     

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.     

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.     

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.     

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.     

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.     

First-principles study on the dehydrogenation properties and mechanism of Al-doped Mg2NiH4

Mg₂NiH₄, with fast sorption kinetics, is considered to be a promising hydrogen storage material. However, its hydrogen desorption enthalpy is too high for practical applications. In this paper, first-principles calculations based on density functional theory (DFT) were performed to systematically study the effects of Al doping on dehydrogenation properties of Mg₂NiH₄, and the underlying dehydrogenation mechanism was investigated. The energetic calculations reveal that partial component substitution of Mg by Al results in a stabilization of the alloy Mg₂Ni and a destabilization of the hydride Mg₂NiH₄, which significantly alters the hydrogen desorption enthalpy ΔH_des for the reaction Mg₂NiH₄ → Mg₂Ni + 2H₂. A desirable enthalpy value of ∼0.4 eV/H₂ for application can be obtained for a doping level of x ≥ 0.35 in Mg_2−xAl_xNi alloy. The stability calculations by considering possible decompositions indicate that the Al-doped Mg₂Ni and Mg₂NiH₄ exhibit thermodynamically unstable with respect to phase segregation, which explains well the experimental results that these doped materials are multiphase systems. The dehydrogenation reaction of Al-doped Mg₂NiH₄ is energetically favorable to perform from a metastable hydrogenated state to a multiphase dehydrogenated state composed of Mg₂Ni and Mg₃AlNi₂ as well as NiAl intermetallics. Further analysis of density of states (DOS) suggests the improving of dehydrogenation properties of Al-doped Mg₂NiH₄ can be attributed to the weakened Mg-Ni and Ni-H interactions and the decreasing bonding electrons number below Fermi level. The mechanistic understanding gained from this study can be applied to the selection and optimization of dopants for designing better hydrogen storage materials.     

First-principles studies of the structures and properties of Al- and Ag-substituted Mg2Ni alloys and their hydrides

The structures and properties of hydrogen storage alloy Mg₂Ni, of aluminum and silver substituted alloys Mg_2−xM_xNi (M = Al and Ag, x = 0.16667), and of their hydrides Mg₂NiH₄, Mg_2−xM_xNiH₄ (M = Al and Ag, x = 0.125) have been calculated from first-principles. Results show that the primitive cell sizes of the intermetallic alloys and hydrides were reduced by substitution of Mg with Al or Ag. Also, the interaction of Ni–Ni was weakened by the substitution. A strong covalent interaction between H and Ni atoms forms tetrahedral NiH₄ units in Mg₂NiH₄. The NiH₄ unit near the Al/Ag atom became tripod-like NiH₃ in Mg_2−xM_xNiH₄ (M = Al, Ag), indicating that the hydrogen storage capacity was decreased by the substitution. The calculated enthalpies of hydrogenation for Mg₂Ni, Mg_2−xAl_xNi and Mg_2−xAg_xNi are −65.14, −51.56 and −53.63 kJ/mol H₂, respectively, implying that the substitution destabilizes the hydrides. Therefore, the substitution is an effective technique for improving the thermodynamic behavior of hydrogenation/dehydrogenation in magnesium-based hydrogen storage materials.     

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.     

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.     

The Mg2Ni0.75M0.25 alloys (M = 3d element): Their application to hydrogen storage

The solid solutions of Mg₂Ni_1?xM_x (when M = V, Cr, Fe, Co, Cu and Zn) have a Mg₂Ni-type structure with a large homogeneity range. A comparative study of the action of hydrogen has been carried out on all alloys corresponding to an x = 0.25 formulation. The absorption-desorption process of hydrogen is reversible and after dissociation of the hydride the starting material is regenerated except for copper which has not been examined here. Hydriding leads for all other alloys to formation of quaternary hydrides. The thermal stability is very close to that of Mg₂NiH₄ stability: partial substitution of nickel by cobalt in Mg₂Ni leads at given temperature to a lower dehydriding rate.     

Ab initio calculations on energetics and electronic structures of cubic Mg3MNi2 (M = Al, Ti, Mn) hydrogen storage alloys

Mg₃MNi₂ (M = Al, Ti, Mn) ternary intermetallic compounds with cubic structure are a new type of potential hydrogen storage alloys. Using ab initio density functional theory (DFT) calculations, the energetics and electronic structures of Mg₃MNi₂ (M = Al, Ti, Mn) compounds are systematically investigated. The optimized structural parameters including lattice constants and internal atomic positions are close to experimental data determined from X-ray powder diffraction. The calculated results of formation enthalpy ΔH_form show that the stabilities of cubic Mg₃MNi₂ (M = Al, Ti, Mn) compound, compared with hexagonal Mg₂Ni, increase in the order of Mg₃MnNi₂, Mg₂Ni, Mg₃TiNi₂ and Mg₃AlNi₂, whereas the stabilities of their saturated Mg₃MNi₂H₃ (M = Al, Ti, Mn) hydrides, compared with monoclinic Mg₂NiH₄, decrease in the order of Mg₂NiH₄, Mg₃AlNi₂H₃, Mg₃TiNi₂H₃ and Mg₃MnNi₂H₃. Further calculations of hydrogen desorption enthalpy ΔH_des indicate that these cubic Mg₃MNi₂ (M = Al, Ti, Mn) compounds possess promising dehydrogenation properties for their relatively lower ΔH_des values. Among of them, the dehydrogenation ability of Mg₃TiNi₂ is the most pronounced. Analysis of electronic structures suggests that the strong covalent bonding interactions between Ni and M within cubic Mg₃MNi₂ (M = Al, Ti, Mn) are dominant and directly control the structural stabilities of these compounds.     

Hydrogen desorption performance of high-energy ball milled Mg2NiH4 catalyzed by multi-walled carbon nanotubes coupling with TiF3

The Mg₂NiH₄ complex hydrides were synthesized by high-energy ball milling (HEBM) MgH₂ + Ni mixtures. Multi-walled carbon nanotubes (MWCNTs) or TiF₃ as catalysts were added and the catalytic-dehydrogenation behaviors were investigated. All prepared samples are characterized by X-ray diffraction (XRD) spectroscopy, scanning electron microscope (SEM) and differential scanning calorimetry (DSC) to acquire information of microstructure, phase compositions, surface and dehydrogenation properties. The results indicate that the method of adding catalysts by HEBM is reasonable and the hydrogen desorption property of Mg₂NiH₄ is improved by catalysts. It is worth noting that the dehydrogenation temperature (T_D) and the activation energy (E_a) of Mg₂NiH₄ catalyzed by MWCNTs coupling with TiF₃ are reduced to 230 °C (243.6 °C of Mg₂NiH₄) and 53.24 kJ/mol (90.13 kJ/mol of Mg₂NiH₄), respectively. The addition of proper catalysts is proved to be an effective strategy to decrease T_D and E_a of Mg₂NiH₄ hydrides.     

Complex quaternary hydrides Mg2(Fe,Co)Hy for hydrogen storage

Complex ternary hydrides based in Mg and transition metals are very attractive materials for hydrogen and energy storage due to their large volumetric capacity, up to 150 kgH₂/m³ in Mg₂FeH₆ and their high dissociation enthalpies. These compounds may be produced at room temperature by mechanical milling of the constituents in H₂ atmosphere. This technique has also served to explore the synthesis of quaternary hydrides Mg₂T_1−zT’_zH_y, combining two transition metals to optimize the properties of the resulting hydride. In the present work we analyze the mechanical synthesis of the compounds Mg₂Fe_1−zCo_zH_y (z = 0, ¼, ½, ¾, 1) by mechanical alloying at room temperature Mg-Fe-Co powder mixtures in adequate proportion, at 0.3 MPa H₂. We follow the mechanosynthesis process trough the analysis of the hydrogen absorption kinetic curves. Samples obtained after a steady state was reached were characterized by X ray diffraction and Mössbauer spectroscopy. The different stages in the mechanosynthesis of these complex hydrides are discussed in terms of the composition and initial state of the powder mixture.     

Kinetics and electrochemical characteristics of Mg2NiH4-x wt.% MmNi3.8Co0.75Mn0.4Al0.2 (x = 5, 10, 20, 40) composites for Ni-MH battery

The structure, kinetics and electrochemical characteristics of Mg₂NiH₄-x wt.% MmNi_3.8Co_0.75Mn_0.4Al_0.2 (x = 5, 10, 20, 40) composites prepared by mechanical milling have been investigated in this paper. XRD results indicate that the as-milled Mg₂NiH₄ shows nanocrystalline or amorphous-like structure, and it does not react with MmNi_3.8Co_0.75Mn_0.4Al_0.2 during mechanical milling. As the amount of MmNi_3.8Co_0.75Mn_0.4Al_0.2 increases, the maximum discharge capacity decreases initially from 508 mAh/g (x = 5) to 440 mAh/g (x = 10) and then increases to 509 mAh/g (x = 40). Meanwhile, the capacity retention (R₁₀) increases from 12.8% (x = 5) to 23.4% (x = 40), and the corrosion potential of electrode (E_corr) increases from −0.930 V to −0.884 V (vs. Hg/HgO). Especially, the more MmNi_3.8Co_0.75Mn_0.4Al_0.2 content the composite contains, the higher high rate dischargeability (HRD) the electrode exhibits, which could be attributed to the catalytic reaction and reduction of the Mg₂NiH₄ grain size brought by MmNi_3.8Co_0.75Mn_0.4Al_0.2. The improvement in electrode kinetics has been depicted from the bulk hydrogen diffusion coefficient (D), the exchange current density (I₀) and the charge transfer resistance (R_ct) on the alloy surface.     

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.     

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.