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Mg2Ni is considered as a promising material for both storage hydrogen and catalyze MgH2 decomposition. Using Mg2Ni–Mg as a matrix material may achieve the combination of both advantages. Herein, hypoeutectic and hypereutectic Mg100-xNix (x = 11, 15, 22) alloys are prepared in this work to investigate the relationship between Mg2Ni in different microstructures and hydrogen storage performances. The results indicate that Mg2Ni connected Mg–Mg2Ni eutectics are formed besides the lamellar eutectic in Mg85Ni15 and Mg78Ni22 alloys because Mg2Ni precipitates firstly in eutectics, which are different from the α-Mg connected Mg–Mg2Ni eutectics in Mg89Ni11 alloy. Mg100-xNix alloys show rapid hydrogen absorption rates at ranges of both 125–175 °C and 275–325 °C because the presents of abundant interfaces generated by nano-Mg2Ni particles. Mg89Ni11 alloy with hydrogenation capacities of 3.6 wt% at 125 °C due to the large hydrogenation driving force of α-Mg. The Mg78Ni22 hydride with the highest proportion nano-Mg2Ni phase presents the fastest dehydrogenation rates. Two independent decomposition peaks are observed in the DSC curves of Mg100-xNix hydrides and the desorption temperature of Mg2NiH4 becomes lower with increasing Mg2Ni content due to Mg2Ni nanoparticles providing channels for the rapid diffusion of H atoms, and the reduced H concentration is conducive to the decomposition of MgH2.

Ultrasonic treatment has great contributions on modifying the morphology, dimension and distribution of constituent phases during solidification, which serve as dominate factors influencing the hydrogen storage performance of Mg-based alloys. In this research, ultrasonic treatment is utilized as a novel method to enhance the de-/hydriding properties of Mg-2Ni (at.%) alloy. Due to ultrasonic treatment, the microstructure of as-cast alloy is significantly refined and homogenized. Ascribing to the increased eutectic boundaries and shortened distance inside α-Mg for hydrogen atoms diffusion, the hydrogen uptake capacities and isothermal de-/hydriding rates improve effectively, especially at lower temperature. The peak desorption temperature reduces from 392.99 °C to 345.56 °C, and the dehydriding activation energy decreases from 101.93 kJ mol−1 to 88.65 kJ mol−1. Weakened hysteresis of plateau pressures and slightly optimized thermodynamics are determined from the pressure-composition isotherms. Owing to the refined primary Mg, a larger amount of hydrogen with the higher hydriding proportion is absorbed in the first stage when hydrides nucleate in eutectic region and grow on primary Mg periphery subsequently before MgH2 colonies impinging, resulting in the enhancement of hydrogenation rates and capacities.

Ultrasonic treatment has great contributions on modifying the morphology, dimension and distribution of constituent phases during solidification, which serve as dominate factors influencing the hydrogen storage performance of Mg-based alloys. In this research, ultrasonic treatment is utilized as a novel method to enhance the de-/hydriding properties of Mg-2Ni (at.%) alloy. Due to ultrasonic treatment, the microstructure of as-cast alloy is significantly refined and homogenized. Ascribing to the increased eutectic boundaries and shortened distance inside α-Mg for hydrogen atoms diffusion, the hydrogen uptake capacities and isothermal de-/hydriding rates improve effectively, especially at lower temperature. The peak desorption temperature reduces from 392.99 °C to 345.56 °C, and the dehydriding activation energy decreases from 101.93 kJ mol−1 to 88.65 kJ mol−1. Weakened hysteresis of plateau pressures and slightly optimized thermodynamics are determined from the pressure-composition isotherms. Owing to the refined primary Mg, a larger amount of hydrogen with the higher hydriding proportion is absorbed in the first stage when hydrides nucleate in eutectic region and grow on primary Mg periphery subsequently before MgH 2 colonies impinging, resulting in the enhancement of hydrogenation rates and capacities. [ABSTRACT FROM AUTHOR]

In order to reduce the obstacle influence of coarse Mg 2 Ni phase on hydrogen absorption kinetics in Mg–Ni alloys, aluminum was doped and Mg 77 Ni 23- x Al x (x = 0, 3, 6, 9) alloys were prepared. The results show that AlNi phase was formed when Al was added, the size of primary Mg 2 Ni phase decreases with increasing Al content till 6 at.%, while primary Mg 2 Ni phase was diminished and primary Mg phase was formed when Al content increased to 9 at.%. The initial hydrogenation rates of Mg 77 Ni 23- x Al x alloys were increased, which is resulted from the refined primary Mg 2 Ni and the catalytic AlNi phase. More importantly, the hydrogenation rates and capacities were significantly improved at 150 °C, especially for the Mg 77 Ni 17 Al 6 alloy. The apparent activation energy of the Mg 77 Ni 17 Al 6 alloy for hydrogenation was reduced to 73.68 kJ/mol from 102.27 kJ/mol of the Mg 77 Ni 23 alloy. Its enthalpy changes for hydrogenation at low and high platforms are 72.3 kJ/mol and 53.9 kJ/mol, respectively. The multiple channels and short distance for hydrogen atoms diffusion provided by refined primary Mg 2 Ni phase, the solid dissolution of Al in Mg 2 Ni lattice, and catalytic effect of AlNi on hydrogenation, leading to the improvement of the hydrogen storage properties. • The primary Mg 2 Ni phase surrounded by AlNi was formed by doping Al. • Hydrogen storage capacity of Mg 77 Ni 17 Al 6 alloy at 125 °C reaches 3.01 wt%. • The hydrogenation E a of Mg 77 Ni 17 Al 6 alloy is decreased to 73.68 kJ/mol. • The catalyst of AlNi is critical to reduce the hydrogenation temperature. [ABSTRACT FROM AUTHOR]

We have analyzed crystallization process of the MgxNi10 (x = 20.5 – 26.5) alloys using phase diagram of Mg-Ni system. Their structure, atomic arrangement and crystal defects were tested by XRD, SEM and HRTEM, respectively. Hydrogenation and dehydrogenation behaviors were measured by pressure-composition-isotherm measurement. The results show that the crystallization processes of the MgxNi10 (x = 20.5 and 22.5) alloys are unlike those of the MgxNi10 (x = 24.5 and 26.5) alloys, but their room temperature microstructure all contain Mg2Ni and eutectic structure of Mg2Ni + α-Mg. The alloys are composed of Mg2Ni phase and α-Mg phase. The addition of Mg is beneficial to the formation of eutectic structure. The alloys have all good activation property. At a lower temperature, such as 200 and 250 °C, the hydrogen absorption rate and hydrogen saturation ratio are significantly lower than those of the alloy at the higher temperatures, such as 300 and 350 °C. At 350 °C, the hydrogen absorption capacity of the alloy increase and the hydrogen release efficiency of the alloy decreases with the increase of Mg content. The time of 90% of the amount of saturated hydrogen absorption and desorption of the alloys is not more than 10 and 2 minutes, respectively. The hydrogen desorption rate of the Mg22.5Ni10 alloy in the four investigated alloys is relatively large and up to 7.170 wt.%.min-1.

We have analyzed crystallization process of the MgxNi10 (x = 20.5 - 26.5) alloys using phase diagram of Mg-Ni system. Their structure, atomic arrangement and crystal defects were tested by XRD, SEM and HRTEM, respectively. Hydrogenation and dehydrogenation behaviors were measured by pressure-composition-isotherm measurement. The results show that the crystallization processes of the MgxNi10 (x = 20.5 and 22.5) alloys are unlike those of the MgxNi10 (x = 24.5 and 26.5) alloys, but their room temperature microstructure all contain Mg2Ni and eutectic structure of Mg2Ni + α-Mg. The alloys are composed of Mg2Ni phase and α-Mg phase. The addition of Mg is beneficial to the formation of eutectic structure. The alloys have all good activation property. At a lower temperature, such as 200 and 250 °C, the hydrogen absorption rate and hydrogen saturation ratio are significantly lower than those of the alloy at the higher temperatures, such as 300 and 350 °C. At 350 °C, the hydrogen absorption capacity of the alloy increase and the hydrogen release efficiency of the alloy decreases with the increase of Mg content. The time of 90% of the amount of saturated hydrogen absorption and desorption of the alloys is not more than 10 and 2 minutes, respectively. The hydrogen desorption rate of the Mg22.5Ni10 alloy in the four investigated alloys is relatively large and up to 7.170 wt.%.min-1. [ABSTRACT FROM AUTHOR]

To increase the hydrogen de-/absorption rate and reduce the stability of MgH 2 , element indium (In) is introduced into the hypereutectic Mg 85 Ni 15 alloy. The results show that solid solution phases of Mg(In) and Mg 2 Ni(In) are formed simultaneously, and the lattice parameters of Mg crystals decrease while that of Mg 2 Ni crystals increase. The hydrogen storage capacity of Mg 85 Ni 14.5 In 0.5 alloy reaches 3.5 wt% at 125 ℃ with the hydrogenation rates accelerated significantly, which is higher than the 2.4 wt% of Mg 85 Ni 15 alloy. For dehydrogenation, Mg 85 Ni 14 In 1.0 hydride can release 4.1 wt% hydrogen at 225 ℃ in 15 min. The main peak temperature in DSC test for dehydrogenation is decreased to 226.6 ℃ of Mg 85 Ni 13.5 In 1.5 hydride from 246.1 ℃ of Mg 85 Ni 15 hydride and decomposition enthalpy (ΔH) of Mg 85 Ni 13.5 In 1.5 hydride is decreased to 60.9 kJ/mol. The rapid hydrogenation rate is owing to the dissolved In atoms in Mg 2 Ni lattice causing lattice expansion, which acts as fast diffusion path for hydrogen atoms. In Mg(In)H 2 phase, the strong interaction between Mg and In weakens the affinity of Mg and H, which accounts for the reduced ΔH of Mg(In)H 2. The weakened strength of Ni-H bond in Mg 2 Ni(In)H 4 also results in the decreased stability of hydride. • Mg(In) and Mg 2 Ni(In) solid solutions are prepared simultaneously in Mg-Ni alloy. • Hydrogenation capacity of Mg 85 Ni 14.5 In 0.5 alloy reaches 3.5 wt% at 125 ℃ in 60 min. • The desorption ΔH value of Mg 85 Ni 13.5 In 1.5 hydride decreases to 60.9 kJ/mol. • The two solid solutions play a role in hydrogenation and desorption, respectively. [ABSTRACT FROM AUTHOR]

To investigate the effect of microstructure on the better de-/hydrogenation property of Mg-based alloy, hypo-eutectic Mg-8Ni (at. %) alloy and hyper-eutectic Mg-15Ni alloy are prepared by metallurgy method. The phase constitutions and microstructures are characterized by XRD and SEM/EDS. Mg-8/15Ni alloy is composed of primary Mg/Mg 2 Ni and eutectic Mg-Mg 2 Ni. In isothermal sorption test, Mg-15Ni alloy shows preferable activation performance and faster de-/hydrogenation rates than Mg-8Ni alloy. The respective hydrogen uptake capacity in 165min is 5.62 wt% and 5.76 wt% H 2 at 300 °C 3 MPa. Intersections of Mg-Mg 2 Ni eutectic phase boundaries with particle surface provide excellent sites and paths for the dissociation and permeation of hydrogen. The de-/hydrogenation enthalpy and entropy values are determined by PCI measurement. Based on the DSC curves at different heating rates, the desorption behavior of Mg-8/15Ni hydride is revealed and the respective activation energy is calculated to be 134.67 kJ mol −1 and 88.34 kJ mol −1 H 2 by Kissinger method. Synergic dehydrogenation occurs in eutectic MgH 2 -Mg 2 NiH 4 , which facilitates the primary MgH 2 in Mg-8Ni hydride to decompose at a lower temperature. The rapid H diffusion and synergic effect in eutectic MgH 2 -Mg 2 NiH 4 collectively contribute to the lower dehydrogenation energy barrier of Mg-15Ni hydride. [ABSTRACT FROM AUTHOR]

Mg–Ni hydrogen storage alloy electrodes with composition of Mg–33, 50, 67 Ni at. % in amorphous phase were prepared by means of mechanical alloying (MA) process using a planetary ball mill. The electrochemical hydrogen storage characteristics and mechanisms of these electrodes were investigated by electrochemical measurements, X–ray diffraction (XRD) and scanning electron microscope (SEM) analyses. The relationship between alloy composition and electrochemical properties was evaluated. In addition, optimum milling time and composition of Mg–Ni hydrogen storage alloy with acceptable electrochemical performance were determined. XRD results show that the alloys exhibit dominatingly amorphous structures after milling of 20 h. The electrochemical measurements revealed that the discharge capacity of Mg 33 Ni 67 and Mg 67 Ni 33 alloy electrodes reached a maximum when alloys were prepared after 20 h of milling time (260 and 381 mAhg −1 , respectively). The maximum discharge capacity of Mg 50 Ni 50 alloy was observable after 40 h milling (525 mAhg −1 ). It was also found that the cyclic stability of the alloys increased with increasing Ni content. Among these alloys, the amorphous Mg 50 Ni 50 alloy presents the best overall electrochemical performance. In this paper, electrode process kinetics of Mg 50 Ni 50 alloy electrode was also studied by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. The impedance spectra of electrodes were measured at different depths of discharge (DODs). The observed spectra were fit well with the equivalent circuit model used in the paper. The electrochemical parameters calculated from electrochemical impedance were also compared. The electrochemical discharge and cyclic performance of 20, 40 and 60 h milled Mg 50 Ni 50 alloy electrodes were demonstrated by the fitted charge transfer resistance and Warburg impedance obtained at various DODs. It was further observed that the controlling-step of the discharge process changed from a mixed rate-determining process at lower DODs to a mass-transfer controlled process at higher DODs. The fitted results demonstrated that charge–transfer resistance (R ct ) increased with DOD. The R ct of 40 h milled Mg 50 Ni 50 alloy (29.27 Ω) was lower than that of 20 h (41.89 Ω) and 60 h milled alloys (92.43 Ω) at fully discharge state. [ABSTRACT FROM AUTHOR]

Abstract: It is of high technical importance to consider the loading and unloading dynamics of hydride-based hydrogen storage tanks, which are mainly influenced by the heat and gas transfer properties inside the reaction bed. In this regard, hydride-graphite composites offer improved heat transfer properties and higher volumetric storage capacities compared to commonly used powder beds. In this contribution, we report on the cycle stability of densified hydride-graphite composites based on melt-spun Mg90Ni10. The results reveal superior heat conduction properties compared to loose Mg90Ni10 flakes. Furthermore, this work deals with the hydrogenation behavior of such composites and their evolution throughout cyclic hydrogenation. Cycles at different temperatures and hydrogen pressures were conducted. High gas permeability in radial direction and sufficient thermal conductivity in combination with a stable composite structure underline the potential of such composites for hydrogen storage applications with high un-/loading dynamics. [Copyright &y& Elsevier]

Abstract: Promising hydrogen storage can be obtained by hydride composites, the present investigation employed high currency pulsed electron beam (HCPEB) to deposit nano Ni powder on a commercial purity magnesium producing Mg-Ni alloy of multi-phases. Different roughness surface of magnesium were made and subjected to HCPEB treatment firstly, a content of Ni powder were coated on magnesium bases subsequently. Acceleration voltage of 25kV and 10 pulses were used for Mg-Ni alloying by HCPEB. XRD results show a composite of Mg, Mg2Ni and MgNi2 after HCPEB processing. It concludes that the deformation introduced by HCPEB treatment and adhesion of Ni powder are favorable for the formation of hydrogen storage phase Mg2Ni. [Copyright &y& Elsevier]

Abstract: The dehydrogenation process of hydrogenated switchable mirrors using magnesium–nickel alloy thin film was studied in situ using spectroscopic ellipsometry. Ellipsometric angles Ψ and Δ of the switchable mirrors varied drastically as a result of dehydrogenation, which is a transformation from transparent to reflective states. The process was analyzed by dividing into the following three phases. The first phase was the dehydrogenation process of a thin Mg4Ni layer with several nanometers at a hydrogenated Pd/Mg4Ni interface. The second phase was the dehydrogenation processes of the hydrogenated Mg4Ni layer, which proceeded from the Pd/Mg4Ni interface to the substrate. The final phase was the desorption process of hydrogen, which was absorbed in Mg4Ni as solid solution and the dehydrogenation process was terminated. [Copyright &y& Elsevier]

Abstract: Mg–Ni multilayer films with sequential Mg and Ni layers were prepared by direct current magnetron sputtering. The substrate temperature influences the microstructure of the films greatly. The film deposited at 298K exhibits multilayered structure, while the film shows nanocrystalline/amorphous composite structure at the deposition temperature of 473K. The optical properties between hydrogenation/dehydrogenation states of the films were performed using spectrophotometer in visible light region. The film deposited at 473K can switch from mirror-like metallic state towards brownish yellow transparent state under 0.6MPa H2 at 298K, and the optical transmittance modulation reaches up to 20% both at a wavelength of 770nm and IR region, while the film deposited at 298K exhibits low optical change, and the optical switching behavior can hardly be found. The extra free energy stored in the boundary of the nanocrystallines benefits the formation of magnesium-based hydride, resulting in the enhancement of the optical switching properties of the Mg–Ni film deposited at 473K. [Copyright &y& Elsevier]

Abstract: Optical switching processes of Mg–Ni based switchable mirrors were studied “in situ” using spectroscopic ellipsometry. Ellipsometric angles Ψ and Δ of the switchable mirrors varied drastically as a result of hydrogenation and dehydrogenation. These variations are due to transformation between metal and semiconductor of Mg–Ni layers. We have clarified the switching processes by analyzing these variations using the following two steps. First, we evaluated dielectric functions of five materials: metal, hydrogen-solid-solution, hydride states of Mg–Ni alloy, and metal and hydride states of Pd. Using these dielectric functions, we then varied thickness and concentration of each layer. [Copyright &y& Elsevier]

Abstract: Nanocrystalline Mg–Ni alloy powder was produced via the magnetron co-sputtering technique. The samples obtained showed crystallisation of Mg and Mg2Ni and an overall morphological heterogeneity with different enriched zones, as evidenced by microstructural analysis. A number of pressure-composition-isotherms (PCI) were carried out on these samples at a range of temperatures. Following hydrogenation, different hydrogen sorption behaviours were observed depending on the samples’ relative composition in terms of Mg and Mg2Ni. Hydrogenation/dehydrogenation cycling is believed to have caused modification of the nanocrystalline materials investigated. The derived dissociation enthalpies of the corresponding hydrides, MgH2 and Mg2NiH4, were found to be coherent with values reported in literature, with the exception of two samples. In addition, one of these samples showed a third plateau pressure pointing to the presence of a hydrogenated metastable phase. [Copyright &y& Elsevier]

Abstract: Mg–Ni films were grown on a silicon substrate using two magnetron sputter deposition sources and simultaneous Ar ion irradiation. X-ray diffraction microstructure and phase composition, EDX elemental composition and atomic force microscopy surface topography analysis showed that under low-energy Ar ion irradiation (bias voltages from 0 to −120V), the Mg2Ni phase was dominant and on the contrary with the increase of ion energy (bias voltages from −120 to −200V), the MgNi2 phase appeared. The Mg content changed from 63at% down to 42at% in films grown under bias voltages of 0 and −200V, respectively. During hydrogenation at 8bar, 270°C for 3h, films with a dominant phase of Mg2Ni were transformed into Mg2NiH4. Hydrogen in MgNi2 films was mainly in interstitials and tended to form bubbles. [Copyright &y& Elsevier]

Abstract: The optical properties, the switching kinetics and the lifetime of hydrogen switchable mirrors based on Mg–Ni alloys are determined with particular regard to the composition of the optically active metal-hydride layer in combination with the thickness of the catalytic capping layer. For this, a high-throughput experiment is introduced. The switching kinetics and the reversibility of switchable mirrors are strongly thickness dependent, though the details hinge on the fine structure of the clustered capping layer. Therefore, the kinetics is correlated with the surface structures of Pd on Mg y Ni1−y as investigated by scanning tunneling microscopy. The results are explained by the so-called strong metal–support interaction (SMSI) state, characterized by a complete encapsulation of the capping layer clusters by oxidized species originating from the support. The SMSI-effect is less important with increasing Pd-layer thickness, and is suppressed by a good wetting of the Pd-clusters on the optically active film. This explains the critical thickness for the catalyzed hydrogen uptake observed in many switchable mirror systems. Moreover, the degradation of the kinetics during cycling is found to depend on the Pd-layer thickness and on the gas environment. Only films, covered with at least 15nm Pd, show small degradation caused by the SMSI-effect. The SMSI-effect is partly reversible: after changing the gas environment from hydrogen to oxygen, the oxide on the Pd-clusters can be partly removed. [Copyright &y& Elsevier]

Abstract: The temperature dependence of the electrochemical property of Mg–Ni alloy codeposited from aqueous solution was studied. Results show that not only solvating temperature, deposition temperature but also temperature variation in deposition temperature controlling will affect Mg–Ni codeposition and electrochemical hydrogen storage property of the Mg–Ni deposit synthesized. The temperature combination of solvating temperature C and deposition temperature 47.9  C is favourable for Mg–Ni codeposition, deposit synthesized under this temperature combination has a electrochemical capacity of 67 mAh/g. [Copyright &y& Elsevier]