Kinetics of the reaction between Mg2Ni and hydrogen

The reaction between Mg₂Ni and hydrogen was investigated by volumetric method. The reaction was divided into two stages; the initial stage was a very rapid reaction whose rate could not be measured, the later stage was the slower step whose rate was expressed by the equation

$\text{dn/DT}\text{=}\text{K′}\text{(}\text{P}\text{?}\text{P}{}_{\mathrm{eq}}\text{/}\text{t}$

where k′ is the constant, P_eq and P are hydrogen pressures at equilibrium and at time t. The reaction rate of the later stage did not depend upon temperature, content of hydrogen in the alloy, and directions of the reaction, desorption and absorption. The amount of reacted hydrogen, Δn, in the initial stage was expressed by

$\text{Δ}\text{n}\text{=k(}\text{P}{}_{\mathrm{o}}\text{?}\text{P}{}_{\mathrm{eq}}\text{)(}\text{n}{}_{\mathrm{s}}\text{?}\text{n}{}_{\mathrm{o}}$

where P₀ is the initial hydrogen pressure, n_s is a constant around 4, n₀ is a ratio of H to Mg₂Ni at the initiation of the run, and k is a constant. The apparent activation energy of the reaction was nearly zero. It is considered that the reaction between the alloy and gaseous hydrogen takes place on metallic Mg₂Ni in the initial stage and in the later stage reaction proceeds on the deactivated site.     

Prediction of hydrogen desorption performance of Mg2Ni hydride reactors

This work performs the simulation of hydrogen desorption processes with Mg₂Ni hydrogen storage alloy to investigate the canister designs. Reaction rates and equilibrium pressures of Mg₂Ni alloy were calculated by fitting experimental data in literature using least squares regression. The obtained reaction kinetics was used to model the thermalfluid behavior of hydrogen desorption. Since the alloy powders will expand and shrink during the absorption and desorption cycle, the canisters considered are comprised of expansion volume atop the metal bed. In order to enhance the heat transfer performance of the canister, an air pipe is equipped at the canister centre line with/without internal fins. Detailed equations that describe the force convection of the heat exchange pipe and the natural convection at the reactor wall are carefully incorporated in the model. Simulation results show that the bare cylindrical canister can not complete the desorption process in 2.8 h, while the canister equipped with the concentric heat exchanger pipe and fins can complete desorption within 1.7 h.Results also demonstrate that the reaction rates can be further increased by increasing the pipe flow velocity and/or increasing the fin volume.     

Development of nanostructured Mg2Ni alloys for hydrogen storage applications

The Mechanically Activated Self-propagating High temperature Synthesis (MASHS) has been employed to obtain nanostructured Mg₂Ni alloys. MASHS process has been further improved by controlling the electrical parameter measurements during the combustion reaction under the thermal explosion mode. The samples were hydrogenated at 20  bar and 300 °C by means of a Pressflow Gas Controller while the dehydrogenation was conducted in a Differential Scanning Calorimetry (DSC) equipped with an H₂ detector of the purged gas. Nanostructured Mg₂Ni demonstrated hydrogen storage capacity around 3.5 wt%. The desorption temperature was about 250 °C at 3 °C/min. The activation energy for dehydrogenation, calculated by the Kissinger method, was about 100  kJ/mol.     

Catalytic effect of Ni, Mg2Ni and Mg2NiH4 upon hydrogen desorption from MgH2

MgH₂ is a perspective hydrogen storage material whose main advantage is a relatively high hydrogen storage capacity (theoretically, 7.6 wt.% H₂). This compound, however, shows poor hydrogen desorption kinetics. Much effort was devoted in the past to finding possible ways of enhancing hydrogen desorption rate from MgH₂, which would bring this material closer to technical applications. One possible way is catalysis of hydrogen desorption. This paper investigates separate catalytic effects of Ni, Mg₂Ni and Mg₂NiH₄ on the hydrogen desorption characteristics of MgH₂. It was observed that the catalytic efficiency of Mg₂NiH₄ was considerably higher than that of pure Ni and non-hydrated intermetallic Mg₂Ni. The Mg₂NiH₄ phase has two low-temperature modifications below 508 K: un-twinned phase LT1 and micro-twinned phase LT2. LT1 was observed to have significantly higher catalytic efficiency than LT2.     

Hydrogen absorption in beryllium substituted Mg2Ni

The alloys Mg₂Ni_1–xBe_x (x = 0.15 and 0.25) retain the Mg₂Ni structure showing a lattice dilation proportional to the beryllium content. The pressure-composition isotherms are reported for the dissociation of hydrided samples. The results suggest that there are two type of interstices able to absorb up to 4 H atoms per formula unit. The heats of formation obtained from the van't Hoff relationship show an increased stability for the hydrides of the beryllium substituted alloys compared to the pure Mg₂Ni. The results suggest that the electronic factors are more important for hydride stability than variations in the unit cell volume.     

Study on chemical synthetic method to prepare Mg2Ni hydrogen absorbing alloy

The processes of chemical synthetic method to prepare Mg₂Ni hydrogen absorbing alloy were studied. Ni was first deposited onto Mg by stirring DMF which contained Mg particle and Ni ions. Mg₂Ni was obtained by heating the Ni-deposited Mg subsequently. Here the stirring condition which was among the most important factors influencing the alloy formation was mostly studied. It seemed that there were two steps in the stirring process for the alloy formation, i.e. the Ni deposition (adsorption) onto Mg and the precursor formation. The stirring speed and temperature affected the two steps intricately and the different behaviors were observed at the stirring temperature of 40 °C from the cases at 20 °C and 80 °C. The obtained alloy samples had rather the mixture forms and exhibited the hydrogen absorption of ∼3 wt% at above 250 °C.     

Induction thermal plasma synthesis of Mg2Ni nanoparticles

A study was carried out into possibility of thermal plasma synthesis of Mg₂Ni nanoparticles. Both prealloyed powders and elemental powders were used as precursors in an inductively coupled thermal plasma incorporating two injection probes located axially in the reactor one from the top and the other from the bottom. The study has shown that the use of prealloyed Mg₂Ni as precursor leads to its disintegration in the plasma condensing into separate phases and therefore was not suitable for the synthesis of Mg₂Ni. The study further showed that Mg₂Ni can be synthesized quite successfully with the use of elemental powders provided that elements are maintained into the plasma at carefully controlled positions. While the fraction of Mg₂Ni was quite substantial, it co-existed with other phases and therefore additional treatments would be necessary for separation. It was shown that a substantial size reduction was possible with thermal plasma where Mg₂Ni could be produced in sizes around 100 nm.     

Effect of Mg3MnNi2 on the electrochemical characteristics of Mg2Ni electrode alloy

Mg₂Ni–x mol% Mg₃MnNi₂ (x = 0, 15, 30, 60, 100), the novel composite alloys employed for hydrogen storage electrode, have been successfully synthesized by a method combining electric resistance melting with isothermal evaporation casting process (IECP). X-ray diffraction (XRD) analysis results show that the composite alloys are composed of Mg₂Ni phases and the new Mg₃MnNi₂ phases. It is found on the electrochemical studies that maximum discharge capacities of the composite alloys increase with the increasing content of the Mg₃MnNi₂ phase. The discharge capacity of the electrode alloy is effectively improved from 17 mAh g⁻¹ of the Mg₂Ni alloy to 166 mAh g⁻¹ of the Mg₃MnNi₂ alloy. Among these alloys, the Mg₃MnNi₂ phase possesses a positive effect on the retardation of cycling capacity degradation rate of the electrode materials. Cyclic voltammetry (CV) results confirm that the increasing content of the Mg₃MnNi₂ phase effectively improves the reaction activity of the electrode alloys. Surface analyses indicate that the Mg₃MnNi₂ phase can enhance the anti-corrosive performance of the particle surface of these composite alloys.     

Stability and hydrogen adsorption properties of Mg/Mg2Ni interface: A first principles study

The stability and hydrogen adsorption behaviors of Mg/Mg₂Ni interface were studied by first principles calculations. Results demonstrated that the interaction between Ni (from Mg₂Ni compound) and Mg (from Mg metal) is the key factor stabilizing the interface, and the interface provides a medium to capture hydrogen atoms originating from the accumulation of electrons in the interface zone by the formation of the interface. Hydrogen atoms adsorbed in the interface zone tend to form covalent bonds with metal atoms (Ni and Mg atoms), which deliver negative adsorption energies in the range of ?0.831 to ?0.019 eV for most possible adsorption sites. However, the strength of the H-metal bonds depends on the environment the H located. The present study illustrates that the Mg/Mg₂Ni layered structure could be a potential medium for reversible de/hydrogenation processes.     

Structure changes and optical properties of Mg2Ni switchable mirrors

A multilayer film of Mg and Ni was prepared by dc/ac magnetron sputtering and annealed below 623 K in vacuum to obtain polycrystalline Mg₂Ni thin films. The phase transformation during heating process and optical switching properties of the films were investigated. The influence of the original crystalline state of Mg₂Ni films on optical switching properties such as transmission, optical band gaps and the cycle times was discussed. The indirect optical band gaps of the fully hydrogenated amorphous Mg₂Ni films were estimated by linear extrapolation.     

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.     

Hydriding properties of a mechanically alloyed mixture with a composition Mg2Ni

Hydriding properties of a mechanically alloyed 2Mg + Ni mixture have been investigated and compared with those of the Mg₂Ni alloy prepared by melting and sintering. The mechanically alloyed 2Mg + Ni mixture was estimated as an excellent hydrogen storage material.     

Engineering the Mg–Mg2Ni eutectic transformation to produce improved hydrogen storage alloys

The technique of trace element doping to modify the solidification mechanism of faceted/non-faceted eutectics has been applied to the Mg–Mg₂Ni alloy system. It is demonstrated that the micro- and nano-structure of cast hypoeutectic Mg–Mg₂Ni alloys can be varied by trace additions of Na, Ca or Eu to the liquid prior to solidification. As a result, the reversible hydrogen absorption capability was in excess of 90% of the theoretical value of 6.8 wt.% under the absorption parameters of 350 °C and 1 MPa for 24 min and subsequent desorption at 0.2 MPa for 24 min after activation. The hydrogen absorption kinetics have been dramatically improved under realistic industrial conditions, and show no sign of reduced capacity over 200 cycles. This processing route results in a non-pyrophoric material that may be produced in large quantities at comparatively low cost.     

Hydrogen storage properties of Mg2Ni affected by Cr catalyst

The effect of Cr as a catalyst in different proportions was investigated to monitor the hydrogen storage properties of Mg₂Ni including their thermodynamic aspects. The P–C–T isotherms for absorption/desorption were measured at 225 °C, 250 °C, 275 °C and 300 °C temperatures. A significant increment in the plateau pressures at different temperature was observed, which shows the positive impact of Cr content in the formation of less stable hydrides. The active sites produced by the ball milling may be the reason for the formation of less stable hydrides. Decrements in the storage capacity with the Cr content were attributed to the formation of MgNi₂ phase which does not absorb hydrogen at the employed temperature-pressure conditions. XRD and SEM technique were used to identify the structural and morphological changes induced by the hydrogenation cycles.     

Synchrotron EXAFS studies of Ti-doped Mg2Ni alloy on the cycling behavior

In this study, Mg_1.9NiTi_0.1 alloy was synthesized by mechanical alloying and its cyclic hydrogen storage behaviors were investigated. It was found that titanium substituting magnesium in Mg_1.9NiTi_0.1 alloy notably improves the absorption/desorption kinetics. Upon cycling, the kinetic rates of absorption/desorption further increase, whereas the hydrogen storage capacity decreases. To identify the micro-structural evolution of Mg_1.9NiTi_0.1 alloy during cycling, we used X-ray diffraction, scanning electron microscope, and extended X-ray absorption fine structure. After ball milling, the decrease of Mg–Ni atomic interaction lowers the stability of Ti-doped phases and has positive effect on the absorption/desorption behaviors. After 20 cycles, the decrease in the cycling capacity may be attributed to the increasing MgNi₂ content. Further studies revealed that some amounts of Mg₂Ni transform into MgNi₂, which result in great decrease in effective hydrogen storage capacity.     

Enhancement of hydrogen-storage performance of MgH2 by Mg2Ni formation and hydride-forming Ti addition

MgH₂, rather than Mg, was used as a starting material in this work. A sample with a composition of MgH₂–10Ni–4Ti was prepared by reactive mechanical grinding. Activation of the sample was completed after the first hydriding cycle. At n = 1, the sample desorbed 2.53 wt% H for 10 min, 3.99 wt% H for 20 min, 4.58 wt% H for 30 min, and 4.68 wt% H for 60 min at 593 K under 1.0 bar H₂. At n = 2, the sample absorbed 3.59 wt% H for 5 min, 4.55 wt% H for 25 min, and 4.60 wt% H for 45 min at 593 K under 12 bar H₂. The inverse dependence of the hydriding rate on the temperature in the initial stage and the normal dependence of the hydriding rate on the temperature in the later stage were discussed. The rate-controlling step for the dehydriding reaction of activated MgH₂–10Ni–4Ti was analyzed as the chemical reaction at the hydride/α-solid solution interface.     

Effect of Cu catalyst on the hydrogenation and thermodynamic properties of Mg2Ni

Hydrogenation performance of Mg₂Ni can be improved by introducing nano-crystalline microstructures into the bulk and the modifications of surface through catalysts as well. The challenge with solid state catalysts is to be dispersed on the surface homogeneously. If the dispersion is not homogeneous somehow, it could be compensated by increasing the amount of catalyst, but at the cost of storage capacity. The use of catalyst fasten the sorption process and vanish the need of long activation process even after the exposure of powder sample to air. Ball milling is one of the efficient method to introduce both of the above effects together. The present work describe the effect of Cu catalyst in varying amount (Cu = 0, 2, 5, 10 wt%) on the crystallographic, morphologic and hydrogen sorption behavior of Mg₂Ni including the thermodynamic aspects. Effect of Cu is found to be positive in terms of forming comparatively unstable hydrides. Hydrogen storage capacity and enthalpy of formation of Mg₂Ni with 10 wt% Cu reduces to 1.81 wt% and 26.69 KJ (mol H)⁻¹ from 3.56 wt% and 54.24 KJ (mol H)⁻¹ for pure Mg₂Ni at 300 °C respectively.     

Hydriding and dehydriding kinetics of Mg2Ni above and below the structural phase transition

Mg₂Ni exhibits a structural phase transition at ca 510K. At 580K and 10 bar, hydrogen is absorbed up to Mg₂NiH_3.6 in 160 s, but at 480K, below the phase transition, and 10 bar, the absorption kinetics are much faster, the same amount of H₂ being absorbed in only 70 s. Desorption of hydrogen from Mg₂NiH₄ to Mg₂NiH_0.4 at 10^?4 bar takes 22 min at 480K and 2 min at 580K.     

Dual-tuning effect of In on the thermodynamic and kinetic properties of Mg2Ni dehydrogenation

Mg₂In_0.1Ni solid solution with an Mg₂Ni-type structure has been synthesized and its hydrogen storage properties have been investigated. The results showed that the introduction of In into Mg₂Ni not only significantly improved the dehydrogenation kinetics but also greatly lowered the thermodynamic stability. The dehydrogenation activation energy (E_a) and enthalpy change (ΔH) decreased from 80 kJ/mol and 64.5 kJ/mol H₂ to 28.9 kJ/mol and 38.4 kJ/mol H₂, respectively. The obtained results point to a method for improving not only the thermodynamic but also the kinetic properties of hydrogen storage materials.     

Pressure DSC study of the hydrogenation and dehydrogenation of some intermetallic compounds Mg2Ni

Pressure differential scanning calorimetry (DSC) has been applied to a study of the hydrogenation and dehydrogenation of some intermetallic compounds Mg₂Ni. The effects of hydrogen pressure, pulverized compound's sizes as well as chemical composition of the compound, partial substitution of Mg in Mg₂Ni by Al, and the second phase MgNi₂ dispersed in Mg₂Ni on the hydrogenation and dehydrogenation of the intermetallic compound Mg₂Ni are elucidated in some detail by this experimental technique. It is emphasized that a pressure DSC is available as a rapid and convenient experimental means for assessing hydrogen absorption and desorption properties of hydrogen storage materials.