Formation and hydrogen storage properties of in situ prepared Mg–Cu alloy nanoparticles by arc discharge

Mg–Cu alloy nanoparticles were in situ prepared by a physical vapor condensation method (arc discharge) in a mixture of argon and hydrogen. Four crystalline phases, Mg, Mg₂Cu, MgCu₂ and MgO, were formed simultaneously during the arc-discharge evaporation. Detailed experiments revealed that nanostructured hydrogen-active phases of Mg₂Cu and Mg exhibit enhanced hydrogen absorption kinetics possibly due to the small grain size and surface defects. The maximal hydrogen storage contents of Mg–Cu alloy nanoparticles can reach 2.05 ± 0.10 wt% at 623 K.     

Characterization of Mg–20 wt% Ni–Y hydrogen storage composite prepared by reactive mechanical alloying

Mg–20 wt% Ni–Y composite was successfully prepared by reactive mechanical alloying (RMA). X-ray diffraction (XRD) measurement showed that both MgH₂ and Mg₂NiH₄ co-exist in the milled composite. The composite exhibits excellent hydrogen sorption kinetics and does not need activation on the first hydrogen storage process. It can absorb 3.92 and 5.59 wt% hydrogen under 3.0 MPa hydrogen pressure at 293 and 473 K in 10 min, respectively, and desorb 4.67wt% hydrogen at 523 K in 30 min under 0.02 MPa hydrogen pressure. The equilibrium desorption pressure of the composite are 0.142, 0.051 and 0.025 MPa at 573, 543 and 523 K, respectively. The differential scanning calorimetry (DSC) measurement showed that dehydrogenation of Mg–20 wt% Ni–Y composite was depressed about 100 K comparing to that of milled pure MgH₂. It is deduced that both the catalysis effect of Mg₂Ni and YH₃ distributed in Mg substrate and the crystal defects formed by RMA are the main reason for improving hydrogen sorption kinetics of the Mg–20 wt% Ni–Y composite.     

Hydrogenation and degradation of Mg–10 wt% Ni alloy after cyclic hydriding–dehydriding

Hydrogenation and degradation properties of Mg–10 wt% Ni hydrogen storage alloys were investigated by cyclic hydriding–dehydriding tests. Mg–10 wt% Ni alloy was synthesized by rotation-cylinder method (RCM) under 0.3% HFC-134a/air atmosphere and their hydrogenation and degradation properties were evaluated by pressure-composition-isotherm (PCI) measurement. Hydrogen storage capacities gradually increased following 160 hydriding–dehydriding cycles and thereafter started to decrease. Measured maximum hydrogen capacity of Mg–10 wt% Ni alloy is 6.97 wt% at 623 K. Hydriding and dehydriding plateau pressure were kept constant for whole cycles. Reversible hydrogen capacity started to descend after 280 hydriding–dehydriding cycles. The lamellar eutectic structure of Mg–Ni alloy consists of Mg-rich α$\alpha$-phase and β$\beta$-_Mg2Ni${\mathrm{Mg}}_2\mathrm{Ni}$. It is assumed that the lamellar eutectic structure enhances hydrogenation properties.     

Hydrogen-storage property characterization of Mg–15 wt%Ni–5 wt%Fe2O3 prepared by reactive mechanical grinding

Mg–15 wt%Ni–5 wt%Fe₂O₃ (Mg155) was prepared by reactive mechanical grinding (RMG). Mg155 exhibited high hydriding and dehydriding rates even at the first cycle, and its activation was completed after only two hydriding–dehydriding cycles. The activated Mg155 absorbed 5.06 and 5.38 wt% of hydrogen, respectively, for 5 and 60 min at 573 K under 12 bar H₂. It desorbed 1.50 and 5.28 wt% of hydrogen, respectively, for 5 and 60 min at 573 K under 1.0 bar H₂. The initial hydrogen absorption rate decreased, but the hydrogen desorption rate increased rapidly with an increase in temperature from 563 K to 603 K. The rate-controlling step for the dehydriding reaction in a range from F ? 0.20 to F ? 0.75 is considered to be the chemical reaction at the Mg hydride/α-solid solution interface. The absorption and desorption PCT curves exhibited two plateaus at 573 K. The hydrogen-storage capacity of the activated Mg155 was about 6.43 wt% at 573 K.     

Hydrogen-storage properties of Mg–23.5Ni–(0 and 5)Cu prepared by melt spinning and crystallization heat treatment

Mg–23.5 wt% Ni and Mg–23.5 wt% Ni–5 wt% Cu alloys for hydrogen storage were prepared by melt spinning and crystallization heat treatment. The alloys were ground by a planetary ball mill for 2 h in order to obtain a fine powder. The activated Mg–23.5Ni and Mg–23.5Ni–5Cu alloys absorbed 4.34 and 4.84 wt% H, respectively, at 573 K under 12 bar H₂ for 60 min. The activated Mg–23.5Ni and Mg–23.5Ni–5Cu alloys desorbed 4.27 and 4.81 wt% H, respectively, at 573 K under 1.0 bar H₂ for 30 min. The hydriding rates of the alloys are quite high, even at 473 K, while the dehydriding rates of the samples at 473 K are nearly zero.     

Enhancement of the hydrogen storage characteristics of Mg by reactive mechanical grinding with Ni,Fe and Ti

Mg-10wt%Ni-5wt%Fe-5wt%Ti powder was prepared by reactive mechanical grinding using a planetary ball mill. The Mg-10wt%Ni-5wt%Fe-5wt%Ti powder exhibited high hydriding and dehydriding rates even at the first cycle, and its activation was completed after two hydriding–dehydriding cycles. After the reactive mechanical grinding, the particle size of the powder was reduced, as compared with those of the starting materials. The hydrogen storage properties were measured at temperatures of 473 K, 573 K and 623 K. The activated Mg-10wt%Ni-5wt%Fe-5wt%Ti powder absorbed 5.31 wt% and 5.51 wt% of hydrogen for 5 min and 1 h, respectively, at 573 K under 12 bar H₂. It desorbed 5.18 wt% of hydrogen at 573 K under 1.0 bar H₂ for 1 h. The initial hydrogen absorption rate increased when passing from 473 K to 573 K, but decreased at 623 K. The hydrogen desorption rate increased rapidly with increasing temperature from 473 K to 623 K. The hydrogen storage capacity was about 6.72 wt% at 573 K.     

Hydriding/dehydriding properties of MgH2/5 wt.% Ni coated CNFs composite

In this paper, we reported that the prepared nickel coated carbon nanofibers (NiCNFs) by electroless plating method exhibited superior catalytic effect on hydrogen absorption/desorption of magnesium (Mg). It is demonstrated that the nanocomposites of MgH₂/5 wt.% NiCNFs prepared by ball milling could absorb hydrogen very fast at low temperatures, e.g. absorb ∼6.0 wt.% hydrogen in 5 min at 473 K and ∼5.0 wt.% hydrogen in 10 min even at a temperature as low as 423 K. More importantly, the desorption of hydrogen was also significantly improved with additives of NiCNFs. Diffraction scanning calorimetry (DSC) measurement indicated that the peak desorption temperature decreased 50 K and the on-set temperature for desorption decreased 123 K. The composites also desorbed hydrogen fast, e.g. desorb 5.5 wt.% hydrogen within 20 min at 573 K. It is suggested that the new phase of Mg₂Ni, and the nano-sized dispersed distribution of Ni and carbon contributed to this significant improvement. Johnson–Mehl–Avrami (JMA) analysis illustrated that hydrogen diffusion is the rate-limiting step for hydrogen absorption/desorption.     

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.     

Hydrogen-storage properties of gravity cast and melt spun Mg–Ni–Nb2O5 alloys

95%(gravity cast Mg–23.5Ni)–-5%Nb₂O₅ alloy was prepared by horizontal ball milling in n-hexane of gravity cast Mg–23.5wt%Ni with Spex milled Nb₂O₅. Melt spun Mg–23.5wt%Ni after heat treatment at 523 K for 1 h was also ground by planetary ball milling with finer Nb₂O₅ prepared by milling with NaCl. The activated 90%(melt spun Mg–23.5Ni)–10%Nb₂O₅ alloy shows higher hydriding and dehydriding rates than the activated 95%(gravity cast Mg–23.5Ni)–5%Nb₂O₅ alloy, thanks to the homogeneous distribution of fine Mg₂Ni phase in melt spun Mg–23.5Ni and the finer Nb₂O₅ addition to melt spun Mg–23.5Ni, which leads to the effective diminution of the Mg particle size. The activated 90%(ms Mg–23.5Ni)–10%Nb₂O₅ alloy absorbs 4.70 wt%H at 573 K under 12 bar H₂ for 10 min, and desorbs 4.75 wt%H at 573 K under 1.0 bar H₂ for 25 min.     

Hydrogen storage properties of Mg–TM–La (TM = Ti,Fe, Ni) ternary composite powders prepared through arc plasma method

For the first time, Mg based Mg–Transition metal (TM) –La (TM = Ti, Fe, Ni) ternary composite powders were prepared directly through arc plasma evaporation of Mg–TM–La precursor mixtures followed by passivation in air. The composition, phase components, microstructure and hydrogen sorption properties of the composite powders were carefully investigated. Composition analyses revealed a reduction in TM and La contents for all powders when compared with the compositions of their precursors. It is observed that the composites are all mainly composed of ultrafine Mg covered by nano La₂O₃ introduced during passivation. Based on the Pressure–Composition–Temperature measurements, the hydrogenation enthalpies of Mg are determined to be −68.7 kJ/mol H₂ for Mg–Ti–La powder, −72.9 kJ/mol H₂ for Mg–Fe–La powder and −82.1 kJ/mol H₂ for Mg–Ni–La powder. Meantime, the hydrogen absorption kinetics can be significantly improved and the hydrogen desorption temperature can be reduced in the hydrogenated ternary Mg–TM–La composites when compared to those in the binary Mg–TM or Mg–RE composites. This is especially true for the Mg–Ni–La composite powder, which can absorb 1.5 wt% of hydrogen at 303 K after 3.5 h. Such rapid absorption kinetics at low temperatures can be attributed to the catalytic effects from both Mg₂Ni and La₂O₃. The results gathered in this study showed that simultaneous addition of 3d transition metals and 4f rare earth metals to Mg through the arc plasma method can effectively alter both the thermodynamic and kinetic properties of Mg ultrafine powders for hydrogen storage.     

Hydrogen generation by the hydrolysis of magnesium–aluminum–iron material in aqueous solutions

Highly activated Mg–Al–Fe materials are prepared from powder by mechanical ball milling method for hydrogen generation. The hydrolysis characteristics of Mg–Al–Fe materials in aqueous solutions under different experimental conditions are carefully investigated. The results show that the hydrolysis reactivity of Mg–Al–Fe material can be significantly improved by increasing the ball milling time and Fe content. The increase of NaCl solution concentration and initial temperature is also found to promote the hydrogen generation reaction. At 25 °C, the Mg60–Al30–Fe10 (wt%) material ball-milled for 4 h shows the best performance in 0.6 mol L⁻¹ NaCl solution, and the reaction can produce 1013.33 ml g⁻¹ hydrogen with a maximum hydrogen generation rate of 499.50 ml min⁻¹ g⁻¹. In comparison to NaCl solution, natural seawater is found to have an inhibiting effect on the hydrolysis of Mg–Al–Fe material. Especially, the presence of Mg²⁺ and Ca²⁺ in seawater can greatly reduce the hydrogen conversion yield, and the SO₄²⁻ can decrease the hydrogen generation rate.     

Preparation of Mg–23.5Ni–10(Cu or La) hydrogen-storage alloys by melt spinning and crystallization heat treatment

Mg–23.5 wt%Ni–10 wt%Cu and Mg–23.5 wt%Ni–10 wt%La alloys were prepared by melt spinning and crystallization heat-treatment. The Mg–23.5Ni–10Cu alloy (5.18 wt%) after planetary ball milling has a higher hydrogen-storage capacity than the Mg–23.5Ni–10La alloy (4.98 wt%) at 573 K. The activated Mg–23.5Ni–10Cu alloy has a slightly lower hydriding rate in the beginning, but a higher hydriding rate after about 5 min than the activated Mg–23.5Ni–10La alloy at 573 K. The activated Mg–23.5Ni–10Cu alloy has a much higher dehydriding rate and a larger quantity of the decomposed hydrogen than the activated Mg–23.5Ni–10La alloy at 573 K.     

Investigations on hydrogenation behaviour of CNT admixed Mg2Ni

The aim of the present paper is to report results on hydrogenation behaviour of the new composite material Mg₂Ni: CNT. Admixing of carbon nanotubes (CNT) in storage material Mg₂Ni leads to noticeable enhancement in desorption kinetics as well as storage capacity. We have found that the composite material Mg₂Ni–2 mole% CNT is the optimum material. The Mg₂Ni–CNT composite exhibits hydrogen desorption rate of 5.7 cc/g/min as against 3.0 cc/g/min for Mg₂Ni alone (enhancement of ∼ 90%) and storage capacity of ∼ 4.20 wt% in contrast to ∼3.20 wt% for Mg₂Ni alone (increase of ∼ 31%). Feasible mechanisms for the enhancement of hydrogen desorption kinetics and storage capacity have been put forward.     

Structural investigation and hydrogen storage properties of Ca3-xLaxMg2Ni13 alloys

The phase structures and hydrogen storage properties of the Ca_3-xLa_xMg₂Ni₁₃ alloys were investigated. It was found that the La substitution is unfavorable for the formation of the Ca₃Mg₂Ni₁₃-type phase. The La-substituted alloys consist of multiple phases. Increasing La content to x = 2.25 leads to a disappearance of Ca₃Mg₂Ni₁₃-type phase. Among these alloys, the Ca_1.5La_1.5Mg₂Ni₁₃ alloy has highest equilibrium pressures of hydrogen absorption–desorption and a highest hydrogen desorption capacity of 1.34 wt.% at 318 K. The discharge capacity decreases for La-substituted alloys. However, the cycling capacity retention rate (S₃₀) increases from 13.7 to 67.6% when x increases from 0 to 3.     

Hydrogen storage in rapidly solidified and crystallized Mg–Ni-(Y,La)–Pd alloys

Amorphous-crystalline composite ribbons of quaternary Mg–Ni–(Y,La)–Pd alloys are produced via rapidly solidification and used as precursors for creating nanocrystalline hydrogen storage materials. The resulting materials demonstrate relatively high hydrogen capacity of around 4.5 mass% H and excellent absorption/desorption kinetics at 573 K. Additionally, the alloys demonstrate reversible hydrogen storage at 473 K. A composition of Mg₈₅Ni₁₀Y_2.5Pd_2.5 fully absorbs and desorbs 4.6 mass% H in 90 min. The cyclability of the quaternary alloys demonstrates good stability, with little loss in maximum capacity through 8–10 cycles. This has been attributed to the improved stability of the nanocrystalline structure attained via the Y and La additions. Thermodynamically, the enthalpy of the hydrogen absorption reaction is reduced by 5 kJ/mol in the quaternary alloys, compared to Mg-MgH₂; while the entropy of reaction is also reduced.     

Ternary compounds in the magnesium–titanium hydrogen storage system

Mg–Ti–H samples were mechano-chemically synthesized by ball milling in argon atmosphere or under elevated hydrogen pressure. The detailed reaction mechanism during hydrogen release and uptake during continuous cycling was investigated by in-situ synchrotron radiation powder X-ray diffraction (SR-PXD) experiments. The thermal behaviour of the samples and hydrogen desorption properties were examined by simultaneous thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and mass spectrometry (MS) measurements. A ternary Ti–Mg–H compound with a fcc lattice form during mechano-chemical sample preparation in hydrogen atmosphere using metal powders, but not using metal hydrides as reactants. The amount of β-MgH₂ increases during the first hydrogen absorption cycle at 300 °C at the expense of the high-pressure polymorph, γ-MgH₂ and the amount of β-MgH₂ remain constant during the following hydrogenations. This study reveals that the ternary compound tends to absorb increasing amounts of magnesium in the dehydrogenated state during cycling. A strong coupling between the amounts of magnesium in the ternary Ti–Mg–H phase and the formation of magnesium and magnesium hydride during hydrogen release and uptake at 300 °C is observed. The composition and the amount of the Ti–Mg–H phase appear to be similar in the hydrogenated state. Fast absorption–desorption kinetics at 300 °C and lower onset temperatures for hydrogen release is observed for all investigated samples (lowest onset temperature of desorption T_on = 217 °C).     

Hydrogenation properties of Mg-based composites prepared by reactive mechanical alloying

A nanocrystalline composite of Mg–LaNi₃–Cu has been prepared by reactive mechanical alloying of Mg, Cu and LaNi₃ powders after 60 h ball-milling under a hydrogen atmosphere. This composite desorbed 1.06 mass% of hydrogen at 533 K under a hydrogen pressure of 0.1 MPa. Addition of Cu promotes the hydrogen desorption.     

Hydriding and dehydriding characteristics of nanocrystalline and amorphous Mg20Ni10−xCox (x = 0–4) alloys prepared by melt-spinning

In order to improve the hydriding and dehydriding performances of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Co, and melt-spinning technology was used for the preparation of the Mg₂₀Ni_10−xCo_x (x = 0–4) hydrogen storage alloys. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. Thermal stability of the as-spun alloys was researched by DSC. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results showed that no amorphous phase formed in the as-spun Co-free alloy, but the as-spun alloys containing Co showed certain amount of amorphous phase. The hydrogen absorption capacities of the as-cast alloys first increase and then decrease with the variety of Co content. The hydrogen desorption capacities of as-cast and spun alloys rise with increasing Co content. The rapid quenching significantly improved the hydrogenation and dehydrogenation capacities and the kinetics of the alloys. When the quenching rate increased from 0 (as-cast was defined as spinning rate of 0 m/s) to 30 m/s, the hydrogen absorption capacity of the alloys (x = 0) at 200 °C and 1.5 MPa in 20 min rose from 1.39 to 3.12 wt%, and from 1.91 to 2.96 wt% for the alloy (x = 4). The hydrogen desorption capacity of the alloy (x = 0) in 20 min increased from 0.19 to 0.89 wt%, and from 1.39 to 2.15 wt% for the alloy (x = 4).     

Hydrogen storage in binary and ternary Mg-based alloys: A comprehensive experimental study

This study focused on hydrogen sorption properties of 1.5 μm thick Mg-based films with Al, Fe and Ti as alloying elements. The binary alloys are used to establish as baseline case for the ternary Mg–Al–Ti, Mg–Fe–Ti and Mg–Al–Fe compositions. We show that the ternary alloys in particular display remarkable sorption behavior: at 200 °C the films are capable of absorbing 4–6 wt% hydrogen in seconds, and desorbing in minutes. Furthermore, this sorption behavior is stable over cycling for the Mg–Al–Ti and Mg–Fe–Ti alloys. Even after 100 absorption/desorption cycles, no degradation in capacity or kinetics is observed. For Mg–Al–Fe, the properties are clearly worse compared to the other ternary combinations. These differences are explained by considering the properties of all the different phases present during cycling in terms of their hydrogen affinity and catalytic activity. Based on these considerations, some general design principles for Mg-based hydrogen storage alloys are suggested.     

Effect of in-situ formed Mg2Ni/Mg2NiH4 compounds on hydrogen storage performance of MgH2

Magnesium-based hydrogen storage materials (MgH₂) are promising hydrogen carrier due to the high gravimetric hydrogen density; however, the undesirable thermodynamic stability and slow kinetics restrict its utilization. In this work, we assist the de/hydrogenation of MgH₂ via in situ formed additives from the conversion of an MgNi₂ alloy upon de/hydrogenation. The MgH₂–16.7 wt%MgNi₂ composite was synthesized by ball milling of Mg powder and MgNi₂ alloy followed by a hydrogen combustion synthesis method, where most of the Mg converted to MgH₂, and the others reacted with the MgNi₂ generating Mg₂NiH₄, which produced in situ Mg₂Ni during dehydrogenation. Results showed that the Mg₂Ni and Mg₂NiH₄ could induce hydrogen absorption and desorption of the MgH₂, that it absorbed 2.5 wt% H₂ at 473 K, much higher than that of pure Mg, and the dehydrogenation capacity increased by 2.6 wt% at 573 K. Besides, the initial dehydrogenation temperature of the composite under the promotion of Mg₂NiH₄ decreased greatly by 100 K, whereas it is 623 K for MgH₂. Furthermore, benefiting from the catalyst effect of Mg₂NiH₄ during dehydrogenation, the apparent activation energy of the composite reduced to 73.2 kJ mol⁻¹ H₂ from 129.5 kJ mol⁻¹ H₂.