Enhanced hydrogen desorption/absorption properties of magnesium hydride with CeF3@Gn

In order to improve the hydrogen storage performance of MgH₂, graphene and CeF₃ co-catalyzed MgH₂ (hereafter denoted as MgH₂+CeF₃@Gn) were prepared by wet method ball milling and hydriding, which is a simple and time-saving method. The effect of CeF₃@Gn on the hydrogen storage behavior of MgH₂ was investigated. The experimental results showed that co-addition of CeF₃@Gn greatly decreased the hydrogen desorption/absorption temperature of MgH₂, and remarkably improved the dehydriding/hydriding kinetics of MgH₂. The onset hydrogen desorption temperature of Mg + CeF₃@Gn is 232 °C,which is 86 °C lower than that of as-milled undoped MgH₂, and its hydrogen desorption capacity reaches 6.77 wt%, which is 99% of its theoretical capacity (6.84 wt%). At 300 °C and 200 °C the maximum hydrogen desorption rates are 79.5 and 118 times faster than that of the as-milled undoped MgH₂. Even at low temperature of 150 °C, the dedydrided sample (Mg + CeF₃@Gn) also showed excellent hydrogen absorption kinetics, it can absorb 5.71 wt% hydrogen within 50 s, and its maximum hydrogen absorption rate reached 15.0 wt% H₂/min, which is 1765 times faster than that of the undoped Mg. Moreover, no eminent degradation of hydrogen storage capacity occurred after 15 hydrogen desorption/absorption cycles. Mg + CeF₃@Gn showed excellent hydrogen de/absorption kinetics because of the MgF₂ and CeH_2-3 that are formed in situ, and the synergic catalytic effect of these by-products and unique structure of Gn.     

Effect of V,Nb, Ti and graphite additions on the hydrogen desorption temperature of magnesium hydride

In this study the effects of mechanical milling with 5 wt.% of additives (V, Nb, Ti and Graphite) on the hydrogen desorption temperature of the magnesium hydride (MgH₂) were studied. The powder mixtures were mechanically milled for 2 h. X-ray diffraction (XRD), scanning electron microscope (SEM), and optical microscope (OM) techniques were used for the structural and morphological characterization of powders. Differential scanning calorimeter (DSC) was used to investigate the effects of the mechanical milling with additives on the hydrogen desorption temperature of the magnesium hydride powder. DSC results show that the hydrogen desorption temperatures of mechanically milled MgH₂ with additives are depressed about ∼40–50 °C compared with that of as-received MgH₂. The particle size analysis results indicate that decrease of the particle size of powders leads to a decrease of the hydrogen desorption temperature. Moreover, increasing specific surface area can also contribute to a decrease on the hydrogen desorption temperature.     

Effect of cobalt on the microstructure and hydrogen sorption performances of TiFe0.8Mn0.2 alloy

The microstructures and the hydrogen sorption performances of TiFe_0.8Mn_0.2Co_x (x = 0, 0.05, 0.10, 0.15) and TiFe_0.8Mn_0.2-yCo_y (y = 0.05, 0.10) alloys have been investigated. For TiFe_0.8Mn_0.2Co_x alloys, the lattice parameters of TiFe phase decreased and the Laves phase contents increased with the addition of Co. With the increase of Co content in TiFe_0.8Mn_0.2Co_x alloys, the maximum hydrogen storage capacities of TiFe_0.8Mn_0.2Co_0.05 and TiFe_0.8Mn_0.2Co_0.10 alloys decreased, but the effective hydrogen capacities increased, which is ascribed to the improved flatness of the α-β desorption plateau. Substitution of Co for Mn in TiFe_0.8Mn_0.2-yCo_y alloys can effectively lead to single phase of TiFe alloys. Therefore, TiFe_0.8Mn_0.2-yCo_y alloy showed a deteriorated activation property, but its effective hydrogen capacity increased remarkably due to the obviously improved flatness of the α-β desorption plateau. The addition of Co might adjust the change of the octahedral intersitial environment caused by Mn doping in TiFe phase, which contributes to the improved flatness of the α-β desorption plateau and hence the increased effective hydrogen capacity.     

Catalytic effect of halide additives ball milled with magnesium hydride

The influence of various halide additives milled with magnesium hydride (MgH₂) on its decomposition temperature was studied. The optimum amount of halide additive and milling conditions were evaluated.     

Iron and niobium based additives in magnesium hydride: Microstructure and hydrogen storage properties

In this study, powder mixtures of MgH₂ + 2 mol.% X, with X = Nb, Nb₂O₅, NbF₅, Fe, Fe₂O₃, FeF₃, were processed by mechanical milling at liquid nitrogen temperature (cryomilling). The effect of additives on crystalline structure, thermal properties and hydrogen storage properties of the mixtures were investigated. Morphological investigations indicated a heterogeneous particle size distribution of the powder mixtures and a fine dispersion of additive particles (FeF₃) in the MgH₂ matrix. High resolution synchrotron radiation X-ray diffraction (SR-XRD) data followed by Rietveld refinements showed a significant reduction on crystallite size for the samples containing fluorides (11 nm) in comparison with the pure MgH₂ sample (29 nm). This was related to the mechanical behavior of fluorides during milling with MgH₂, which act as a lubricant, dispersing and/or cracking agent during milling, and thus helping to further reduce MgH₂ particle size. DSC analysis revealed that fluorides (NbF₅, FeF₃) are much more effective than oxides (Nb₂O₅, Fe₂O₃) and the transition metals (Nb and Fe), respectively, in reduction the desorption temperature. Furthermore, Nb₂O₅ is more efficient than Fe₂O₃. Finally, the best results for desorption kinetics were observed for the fluorides: NbF₅ and FeF₃ (equivalent effect and consistent to the DSC analysis) followed by the oxides: Nb₂O₅, Fe₂O₃ and Nb. The addition of Fe was not efficient in comparison with the pure cryomilled sample.     

Oxygen vacancy in magnesium/cerium composite from ball milling for hydrogen storage improvement

The hydrogen sorption performance of Mg is constrained by the difficulties of hydrogen dissociation on particle surface and mass transfer in particle bulk. This work focuses on oxygen vacancy and its effect on the performance of Mg-xCeO₂ (x = 0.7, 1.5, 3, and 6 mol.%) from ball milling for hydrogen storage. The HRTEM observation shows that the crystal domains of Mg from ball milling are reduced to nanoscale by the addition of hard CeO₂ nanoparticles. The XRD and XPS characterization shows that during heating for hydrogenation, some O atoms in CeO₂ transfer to Mg and form MgO, and CeO₂ converts to Ce₆O₁₁ with oxygen vacancies. The isothermal absorption (p-c-T) analysis shows that the hydrogen capacity of the materials increases with the increase of CeO₂ additive, and the optimum addition is 3.0 mol.%. The DSC analysis shows that with the addition of 3.0 mol.% of CeO₂, the hydrogen desorption peak temperature is 35 °C lower than that of pure MgH₂, and the calculated activation energy deceases by 31.3 kJ/mol. The improvement of hydrogen sorption performance is mainly attributed to the formation of oxygen vacancies.     

Mg2(Fe,Cr, Ni)HX complex hydride synthesis from austenitic stainless steel and magnesium hydride

Mg₂(Fe, Cr, Ni)H_X synthesis was successfully performed using magnesium hydride and 316L austenitic stainless steel. It was proven that despite being widely used and considered inert, under some conditions, this steel could react with magnesium hydride and be used as a reaction substrate for complex hydride synthesis. The properties of the product are different from those of pure Mg₂FeH_6, which suggests the partial substitution of iron atoms with chromium and nickel. The influence of milling time on hydrogen content and phase composition was studied and compared to the reference material. It was found to be likely that mechanically induced martensite formed in austenitic stainless steel during ball milling may enable the reaction between steel and magnesium hydride.     

First hydrogenation kinetics of Zr and Mn doped TiFe alloy after air exposure and reactivation by mechanical treatment

In this paper, effect of air exposure on first hydrogenation kinetics of TiFe +4 wt% Zr + 2 wt% Mn alloy was studied. After 7 days of air exposure, the first hydrogenation kinetics of the alloy was slow with a long incubation time. An air exposure of 30 days made the alloy totally inert to hydrogen. In an attempt to recover the hydrogen absorption ability of the alloy, it was mechanically treated using cold rolling and ball milling processes. It was found that the air exposed alloy could be successfully hydrogenated after ball milling and after cold rolling with some loss in hydrogen storage capacity. The loss in storage capacity was more important after ball milling than after cold rolling.     

Effects of SiC nanoparticles with and without Ni on the hydrogen storage properties of MgH2

In this work, MgH₂–SiC–Ni was prepared by magneto-mechanical milling in hydrogen atmosphere. Scanning electron microscope mapping images showed a homogeneous dispersion of both Ni and SiC among MgH₂ particles. Based on the differential scanning calorimetry traces, the temperature of desorption is reduced by doping MgH₂ with SiC and Ni. Hydrogen absorption/desorption behaviour of the samples was investigated by Sievert's method at 300 °C, and the results showed that both capacity and kinetics were improved by adding SiC and Ni. The hydrogen desorption kinetic investigation indicated that for pure MgH₂, the rate-determining step is surface controlled and recombination, while for the MgH₂–SiC–Ni sample it is controlled as described by the Johnson–Mehl–Avrami 3D model (JMA 3D).     

Enhancement of hydrogen sorption in magnesium hydride using expanded natural graphite

Magnesium hydrogenation reaction being exothermic and limited by heat removal, the thermal conductivity of ball-milled magnesium hydride (BM MgH₂) powders has to be improved. The compression of BM MgH₂ associated to Expanded Natural Graphite (ENG) to form compacted disks has been investigated. Using BM MgH₂ without ENG, its compression reduces the porosity and increases its volumetric hydrogen storage capacity. Incorporating ENG before compression drastically improves the thermal conductivity in the direction normal to compression axis. Moreover, the thermal conductivity increases linearly with ENG content, and can be adjusted to fulfill the loading time requirements. The thermodynamic properties and intrinsic sorption kinetics remain unchanged. However, both compression and ENG incorporation reduce the hydrogen permeability, especially in the direction parallel to the compression axis, which imposes a limit to the disk thickness. A small-size instrumented tank has been loaded with either pure BM MgH₂ powder or with disks having different ENG contents. The results obtained for both cases are compared.     

Improved ball milling method for the synthesis of nanocrystalline TiFe compound ready to absorb hydrogen

In this study, we propose a method to produce nanocrystalline TiFe powder by high-energy ball milling, in order to avoid the common sticking problem of the material to the milling tools, assuring a material prompt to absorb hydrogen as well. The method consists of making a preliminary milling operation with the elemental powders (50:50 stoichiometric ratio) to form a strong adhered layer of the milled material on the surfaces of the vial and balls. The main milling operation is then performed with a new powder charge (same composition as before), but now adding a process control agent (stearic acid). Various processing times - 2, 6, 10 and 20 h - were used in the milling experiments. Nanocrystalline TiFe was synthesized in this way with low oxygen contamination, full yields for milling times of 6 h or over, requiring no heat treatments for the first hydrogen absorption. Hydrogen storage capacity of 1.0 wt% at room temperature under 20 bar was attained by the sample milled for 6 h. Kinetic data from samples milled for 2 h and 6 h agreed with Jander model for the rate limiting step of the hydriding reaction, which is based on diffusion with constant interface area.     

Enhancement of hydrogen sorption properties of MgH2 with a MgF2 catalyst

Effect of a MgF₂ catalyst, prepared by ball-milling, on the hydrogen desorption ability of commercial MgH₂ was investigated. When MgH₂ was catalyzed with a MgF₂ composite, it exhibited good cyclability and sharp faceting, with a small grain size (around 10 nm), which differs from those of pure MgH₂. The addition of the MgF₂ catalyst suggests that the F anion could significantly contribute to the cyclability of Mg particles and aid in the inhibition of MgH₂ grain growth.     

Enhanced hydrogen storage properties and mechanisms of magnesium hydride modified by transition metal dissolved magnesium oxides

Magnesium hydride (MgH₂) is a promising on-board hydrogen storage material due to its high capacity, low cost and abundant Mg resources. Nevertheless, the practical application of MgH₂ is hindered by its poor dehydrogenation ability and cycling stability. Herein, the influences and mechanisms of thin pristine magnesium oxide (MgO) and transition metals (TM) dissolved Mg(TM)O layers (TM = Ti, V, Nb, Fe, Co, Ni) on hydrogen desorption and reversible cycling properties of MgH₂ were investigated using first-principles calculations method. The results demonstrate that either thin pristine MgO or Mg(TM)O layer weakens the MgH bond strength, leading to the decreased structural stability and hydrogen desorption energy of MgH₂. Among them, the Mg(Nb)O layer exhibits the most pronounced destabilization effect on MgH₂. Moreover, the Mg(Nb)O layer presents a long-acting confinement effect on MgH₂ due to the stronger interfacial bonding strength of Mg(Nb)O/MgH₂ and the lower brittleness of Mg(Nb)O itself. Further analyses of electronic structures indicate that these thin oxide layers coating on MgH₂ surface reduce the bonding electron number of MgH₂, which essentially accounts for the weakened MgH bond strength and enhanced hydrogen desorption properties of modified MgH₂ systems. These findings provide a new avenue for enhancing the hydrogen desorption and reversible cycling properties of MgH₂ by designing and adding suitable MgO based oxides with high catalytic activity and low brittleness.     

Enhanced electrochemical kinetics of magnesium-based hydrogen storage alloy by mechanical milling with graphite

Magnesium nickel alloy (Mg₂Ni) which used as the negative electrode material in the nickel-metal hydride (Ni/MH) secondary battery is modified by graphite via mechanical milling. The effects of graphite on the Mg₂Ni are systematically investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and a series of electrochemical tests. The results show that the cycle stability of the Mg₂Ni alloy is improved with the addition of 10 wt.% graphite and the discharge capacity at the 20th cycle increase from 116.9 mA g^?1 to 178.5 mA g^?1. The Tafel polarization test indicates better corrosion resistance of the Mg₂Ni/graphite composite. Meanwhile, the results of electrochemical tests indicate that both the charge-transfer reaction rate on the surface of the alloy and the hydrogen diffusion rate inside the bulk of alloy are boosted with the introduction of graphite.     

Improved desorption temperature of magnesium hydride via multi-layering Mg/Fe thin film

Hydrogen sorption property of Mg in Pd-capped thin film nanoconfined with Fe is investigated. Two methods of depositing the thin films were utilised, i.e., resistive heating method and pulsed laser deposition (PLD) method. In the thinnest Mg film prepared by resistive heating, hydrogen content was observed to be the highest among all samples and the hydrogen desorption temperature is 230 °C. Using pulsed laser deposition method, Mg/Fe nanoconfined multilayers are easily prepared. The hydrogen desorption temperature of Mg film with 12 Mg/Fe layers prepared via PLD method was significantly reduced to 155 °C, and the hydrogen storage capacity is improved as compared to the Mg/Fe with only several layers of same overall thickness. This study showed that the desorption temperatures correlate with the film thickness, thinner films react with hydrogen at lower temperatures. In addition, multi-layering Mg with Fe improves the desorption temperatures and hydrogen capacity, due to the higher grain boundary density, which acts as diffusion pathways for Pd in hydrogenation and dehydrogenation process.     

Improved hydrogen storage properties of MgH2 by the addition of KOH and graphene

Mg hydride is a competitive candidates for hydrogen storage based on its high gravimetric hydrogen capacity and accessibility. In this study, a small amount of KOH and graphene were added into MgH₂ by high energy ball milling. MgH₂ doped with both KOH and graphene has a greatly improved hydrogen storage performance. The existence of graphene and the in-situ formed KMgH₃ and MgO decreased activation energy of MgH₂ to 109.89 ± 6.03 kJ/mol. The both KOH and graphene doped sample has a reversibly capacity of 5.43 wt % H₂ and can released H₂ as much as 6.36 times and 1.84 times faster than those of undoped sample and only KOH doped sample at 300 °C, respectively. The addition of graphene not only can provide more “H diffusion channels”, but also can disperse the catalyst.     

Formation of BCC TiFe hydride under high hydrogen pressure

We investigated the hydrogenation of a binary TiFe alloy at 5 GPa and 600 °C by in situ synchrotron radiation X-ray diffraction measurements. After formation of a solid solution of hydrogen in TiFe, an order–disorder phase transition in the metal lattice of TiFe occurred, which yielded a BCC TiFe hydride. The unit cell volume of the BCC hydride increased by 21.0% after the hydrogenation reaction. The volume expansion was larger than that of a γ-hydride TiFeH_1.9 prepared by hydrogenation near ambient conditions.     

Microstructure and first hydrogenation properties of TiFe alloy with Zr and Mn as additives

In this paper, the effect of Zr and Mn on the microstructure and first hydrogenation kinetic of TiFe alloy is reported. TiFe alloy to which Zr, Mn or a combination of both have been added were synthesized by induction melting. First hydrogenation of all alloys was performed at room temperature under 20 bar of hydrogen. We found that addition of manganese makes possible activation at room temperature, but kinetics was very sluggish. Alloy with 2 wt% Zr did not absorb hydrogen. However, with addition of 4 wt% Zr, the alloy absorbed 1.2 wt% of hydrogen. A synergetic effect was found when zirconium was added along with manganese. Alloy with 1 wt% Mn and 2 wt% Zr had better kinetics than the alloy having only Mn or only Zr. The maximum hydrogen capacity was also greater at ~1.8 wt% after 7 h. Combination of 4 wt% Zr and 2 wt% Mn absorbed 2 wt% of hydrogen in 5 h. The rate limiting step for each activated alloy was found to be diffusion controlled with decreasing interface velocity.     

Improved hydrogen desorption properties of exfoliated graphite and graphene nanoballs modified MgH2

Magnesium hydride is a leading hydrogen storage material with high hydrogen content, however, suffers with sluggish kinetics. Several methods have been adopted to improve its kinetics, out of which, the addition of catalyst is an impressive way. Carbon materials have shown their promises as catalyst for several hydrogen storage materials. The present work is devoted to investigating the catalytic effects of exfoliated graphite and graphene nanoballs on dehydrogenation kinetics of MgH₂. The lowest onset temperature of 282 °C is observed for graphene nanoballs modified MgH₂ system. Exfoliated graphite mixed MgH₂ desorbed hydrogen at onset temperature 301 °C which is also less than the dehydrogenation temperature of pure MgH₂ (410 °C). The dehydrogenation kinetics has significantly improved by the addition of these catalysts as compared to the pure MgH₂. The activation energy for the hydrogen desorption of MgH₂ was reduced from 170 (pure MgH₂) to 136 ± 2 and 140 ± 2 kJ/mol by the addition of exfoliated graphite and graphene nanoballs, respectively. The XRD results confirmed the presence of MgH₂ after milling with exfoliated graphite and graphene nanoballs that indicates that there are no reactions during the milling thus both the additives are effective to improve the dehydrogenation as a catalyst.