Microstructures and electrochemical performances of La2Mg(Ni0.85Co0.15)9Crx (x = 0–0.2) electrode alloys prepared by casting and rapid quenching

In order to improve the electrochemical cycle stability of La–Mg–Ni system (PuNi₃-type) hydrogen storage alloy, a trace of Cr was added and rapid quenching techniques were employed. The electrochemical performances and microstructures of the as-cast and -quenched alloys were determined and measured. The effects of Cr content and quenching rate on the microstructures and electrochemical properties of the alloys were investigated in detail. The obtained results show that the as-cast and -quenched alloys are composed of the (La, Mg)Ni₃ phase (PuNi₃-type structure) and the LaNi₅ phase as well as the LaNi₂ phase. The amount of the LaNi₂ phase increases with the increase of Cr content. The addition of Cr enhances the cycle stability of the as-cast and -quenched alloys, but decreases the discharge capacities of the alloys. The cycle lives of the alloys increase with the increase of the quenching rate. The as-cast and -quenched alloys have an excellent activation performance.     

Influence of the substitution of La for Mg on the microstructure and hydrogen storage characteristics of Mg20−xLaxNi10 (x = 0–6) alloys

In order to enhance the glass forming ability of the Mg₂Ni-type hydrogen storage alloy, the Mg in the alloy was partially substituted by La. The alloys Mg_20−xLa_xNi₁₀ (x = 0, 2, 4, 6) were prepared by casting and rapid quenching. The structures and morphologies of the as-cast and the quenched alloys were studied by XRD, SEM and HRTEM. It was found that no amorphous phase was formed in the as-quenched La-free alloy. But the as-quenched alloys containing La held a major amorphous phase, confirming that the substitution of La for Mg significantly enhances the glass forming ability of the alloys. When La content x ≤ 2, the major phase in the as-cast alloys is Mg₂Ni phase, but with the further increase of La content, the major phase of the as-cast alloys changes into (La,Mg)Ni₃ + LaMg₃ phase. Thermal stability of the as-quenched alloys was studied by DSC, showing that La content engenders a negligible influence on the crystallization temperature of the amorphous phase. The hydrogen absorption and desorption kinetics of the as-cast and the quenched alloys were measured by an automatically controlled Sieverts apparatus. The results showed that the hydrogen absorption and desorption capacities and kinetics of the as-cast alloys clearly rise with increasing La content. For La content x = 2, the as-quenched alloy displays an optimal hydrogen desorption kinetics at 200 °C. The electrochemical measurement showed that the discharge capacities of the as-cast alloys rose with the increase of La content, but those of the as-quenched alloys obtained the maximum values with the variation of La content. The cycle stability of the as-cast and the quenched alloys significantly improved with increasing La content.     

Effect of boron additive on electrochemical cycling life of La–Mg–Ni alloys prepared by casting and rapid quenching

In order to improve the electrochemical performances of La–Mg–Ni system (PuNi₃-type) hydrogen storage alloy, a trace of B was added in La₂Mg(Ni_0.85Co_0.15)₉ alloy. La₂Mg(Ni_0.85Co_0.15)₉B_x${}_x$ (x=0,0.05,0.1,0.15,0.2$x=0,0.05,0.1,0.15,0.2$) hydrogen storage alloys were prepared by casting and rapid quenching. The electrochemical charging-discharging cycling lives and microstructures of the as-cast and quenched alloys were measured and analyzed. The effects of B additive on the microstructures and cycling lives of as-cast and quenched alloys were investigated in detail. The results show that the as-cast and quenched alloys are composed of the (La, Mg)Ni₃ phase (PuNi₃ structure), the LaNi₅ phase and the LaNi₂ phase. A trace of the Ni₂B phase exists in the as-cast alloys containing B. The Ni₂B phase in the alloys containing B nearly disappears after rapid quenching and the relative ratio of each phase in the alloys changes with the variety of the quenching rate. The addition of B obviously enhances the charging-discharging cycling stabilities of the as-cast and quenched alloys. When B content x$x$ increases from 0 to 0.2, the cycling lives of the as-cast and quenched at 20 m/s alloys were increased from 72 to 94 cycles and from 86 to 104 cycles, respectively.     

Structures and electrochemical hydrogen storage behaviours of La0.75−xPrxMg0.25Ni3.2Co0.2Al0.1 (x = 0–0.4) alloys prepared by melt spinning

In order to improve the electrochemical performance of the La–Mg–Ni system A₂B₇-type electrode alloys, La in the alloy was partially substituted by Pr and melt spinning technology was used for preparing La_0.75−xPr_xMg_0.25Ni_3.2Co_0.2Al_0.1 (x = 0, 0.1, 0.2, 0.3, 0.4) electrode alloys. The microstructures and electrochemical performance of the as-cast and spun alloys were investigated in detail. The results obtained by XRD, SEM and TEM show that the as-cast and spun alloys have a multiphase structure which consists of two main phases (La, Mg)Ni₃ and LaNi₅ as well as a residual phase LaNi₂. The substitution of Pr for La leads to an obvious increase of the (La, Mg)Ni₃ phase and a decrease of the LaNi₅ phase in the alloys. The results of the electrochemical measurement indicate that the discharge capacity of the alloys first increases and then decreases with variation of the Pr content. The cycle stability of the alloy monotonically rises with increasing Pr content. When the Pr content rises from 0 to 0.4, the discharge capacity increases from 389.4 (x = 0) to 392.4 (x = 0.1) and then drops to 383.7 mAh/g (x = 0.4) for the as-cast alloy. Discharge capacity increases from 393.5 (x = 0) to 397.9 (x = 0.1), and then declines to 382.5 mAh/g for the as-spun (5 m/s) alloys. The capacity remaining after 100 cycles increases from 65.32 to 79.36% for the as-cast alloy, and from 73.97 to 93.08% for the as-spun (20 m/s) alloy.     

Cooperative effect of Co and Al on the microstructure and electrochemical properties of AB3-type hydrogen storage electrode alloys for advanced MH/Ni secondary battery

The crystal structure and electrochemical properties of the La₂MgMn_0.3Ni_8.7−x(Co_0.5Al_0.5)_x (x = 0, 1.0, 2.0 and 3.0, at%) hydrogen storage alloys are investigated systematically. The results show that all the alloys consist of (La, Mg)Ni₃ and LaNi₅ phases, the cyclic stability S₆₀ increases from 61.2% (x = 0) to 78.7% (x = 3.0) after 60 charge/discharge cycles, and the peak high rate dischargeability (HRD) at the discharge current density of 1200 mA/g appears at the alloy of x = 2.0 with the value of 68.3%. Moreover, the electrochemical kinetic properties of the alloys are also improved at different extent with increasing x. All the results indicate that the substitution of Co and Al for Ni in AB₃-type hydrogen storage alloys is effective to improving the overall electrochemical properties, and the optimum content is x = 2.0.     

Improved hydrogen storage behaviours of nanocrystalline and amorphous Mg2Ni-type alloy by Mn substitution for Ni

The nanocrystalline and amorphous Mg₂Ni-type alloys with nominal compositions of Mg₂Ni_1−xMn_x (x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized by melt spinning technique. The structures of the as-cast and spun alloys were characterized by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage performances were tested by an automatic galvanostatic system. The results show that the as-spun (x = 0) alloy holds a typical nanocrystalline structure, whereas the as-spun (x = 0.4) alloy displays a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni facilitates the glass formation in the Mg₂Ni-type alloy. The hydrogen absorption capacity of the alloys first increases then decreases with rising Mn content, but the hydrogen desorption capacity of the alloys grows with increasing Mn content. Furthermore, the substitution of Mn for Ni significantly improves the electrochemical hydrogen storage performances of the alloys, involving both the discharge capacity and the electrochemical cycle stability. With an increase in the amount of Mn from 0 to 0.4, the discharge capacity of as-spun (30 m/s) alloy grows from 116.7 to 311.5 mAh/g, and its capacity retaining rate at 20th charging and discharging cycle rises from 36.7 to 78.7%.     

Effect of Sm content on activation capability and hydrogen storage performances of TiFe alloy

Element substitution is an efficient method to enhance the activation property of TiFe alloys. In this paper, Zr, Mn and Ni were utilized to replace Fe in the alloy partially, and different content rare earth Sm substitute Ti in the alloy. The alloys with nominal compositions of Ti_1.1-xFe_0.6Ni_0.1Zr_0.1Mn_0.2Sm_x (x = 0–0.08) were made through vacuum induction melting. The microstructure, composition and hydrogen storage property of alloys were measured in detail by X-ray diffraction, scanning electron microscope, high-resolution transmission electron microscopy and automatically Sievert apparatus. The results reveal that the as-cast alloys contain TiFe as major phase and Ti₂Fe as secondary phase. Sm addition refines the grain of alloys obviously. All alloys have good activation properties and can be completely activated without any heat treatment. The activation performance can be further improved by partially replacing Ti with Sm, and the incubation period of activation can be shortened greatly.     

Effect of boron additive on the cycle life of low-Co AB5-type electrode consisting of alloy prepared by cast and rapid quenching

In order to modify the cycle stability of low-Co AB₅-type alloy, a trace of boron was added in MmNi_3.8Co_0.4Mn_0.6Al_0.2 hydrogen storage alloy. The low-Co AB₅-type alloys MmNi_3.8Co_0.4Mn_0.6Al_0.2B_x(x=0, 0.1, 0.2, 0.3, 0.4) were prepared by cast and rapid quenching. The cycle lives and microstructures of the as-cast and quenched alloys were measured and analyzed. The effects of boron additive on the microstructures and cycle lives of as-cast and quenched alloys were investigated comprehensively. The obtained results showed that the addition of boron could dramatically enhance the cycle lives of the as-cast and quenched alloys. When boron content x increases from 0 to 0.4, the cycle lives of the as-cast alloys were increased from 118 to 183 cycles, and for as-quenched alloys with quenching rate of 38 m/s from 310 to 566 cycles.     

Electrochemical hydrogen storage characteristics of Mg–Pd–Ni ternary alloys

Mg_1?xPd_xNi (x = 0.03, 0.05, 0.06, 0.07, 0.08, 0.10, 0.20) type alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. The XRD studies showed that Mg₆Pd and Mg–Pd phases form in the high-Pd-containing alloys. The discharge capacities of the alloys increased sharply up to 15 h milling. Further increase up to 25 h did not cause change in the discharge capacities considerably. Among the Mg_1?xPd_xNi ternary alloys, Mg_0.93Pd_0.07Ni alloy was observed to exhibit the best charge/discharge cyclic performance. The hydrogen storage capacity of the high-Pd-containing alloys (Mg_1?xPd_xNi; x > 0.07) deteriorated as Pd content of the alloy increased. This observation was attributed to the Mg–Pd phase formations. According to the EIS experiments as Pd atomic ratio increased up to 0.07 the charge transfer resistances of the alloys decreased. Further increase in Pd atomic ratio, however, brought about the increase in the charge transfer resistances.     

Investigation on microstructures and electrochemical performances of hydrogen storage alloys

In order to improve the cycle stability of La–Mg–Ni system (A₂B₇-type) alloy electrode, a small amount of Co was added in the La_0.75Mg_0.25Ni_3.5 alloy. The effects of Co content on the microstructures and electrochemical performances of the alloys were investigated in detail. The results by XRD and SEM show that the alloys have a multiphase structure which is composed of the LaNi₅, (La,Mg)₂Ni₇ two major phases and a small amount of the LaNi₂ phase. The cell volumes of the LaNi₅ and phases enlarge with the increasing Co content in the alloys. With the increasing Co content, some electrochemical properties and kinetic parameters of the alloy, involving the discharge capacity, high-rate discharge ability (HRD), the polarization resistance (R_p), the loss angle (ψ) and the limiting current density (I_L), first increase and then decrease. The addition of Co slightly improves the cycle stabilities of the alloy electrodes. The mechanism of the efficiency loss of the experimental alloy was investigated by means of SEM and X-ray photoelectron spectroscopy (XPS). The results indicate that the fundamental reasons for the capacity decay of the La_0.75Mg_0.25Ni_3.5Co_x (x=0,0.2,0.4,0.6) alloy electrodes are the pulverization of the alloy particle and corrosion/oxidation of La and Mg in alkaline electrolyte.     

Electrochemical hydrogen storage characteristics of nanocrystalline and amorphous Mg20Ni10-xCox(x=0−4) alloys prepared by melt spinning

Nanocrystalline and amorphous Mg₂Ni-type alloys with nominal compositions of Mg₂₀Ni_10-xCo_x (x = 0, 1, 2, 3, 4) were synthesized by melt-spinning technique. The microstructures of the as-cast and spun alloys were characterized by XRD, SEM and HRTEM. The electrochemical hydrogen storage characteristics of the as-cast and spun alloys were measured. The obtained results show that the substitution of Co for Ni does not change the major phase of Mg₂Ni, but it leads to the formation of secondary phase MgCo₂ and Mg. No amorphous phase forms in the as-spun alloy (x = 0), whereas the as-spun alloy (x = 4) holds a nanocrystalline and amorphous structure, confirming that the substitution of Co for Ni significantly heightens the glass forming ability of the Mg₂Ni-type alloy. The substitution of Co for Ni and melt spinning significantly improve the electrochemical hydrogen storage performances of the alloys. When Co content x increases from 0 to 4, the maximum discharge capacity of the as-cast alloy increases from 30.3 to 113.3 mAh/g, and from 135.5 to 402.5 mAh/g for as-spun (30 m/s) alloy. The capacity retaining rate of the as-cast alloy after 20 cycles rises from 36.71 to 37.04%, and from 27.06 to 83.35% for as-spun (30 m/s) alloy, respectively.     

A study on the hydrogen-storage properties of La2−xTixMgNi9 (x = 0.1, 0.2, 0.3, 0.4) alloys

The La_2−xTi_xMgNi₉ (x = 0.1, 0.2, 0.3, 0.4) alloys were prepared by magnetic levitation melting under Ar atmosphere. The effects of partial substitution Ti for La on the phase structures, hydrogen-storage properties and electrochemical characteristics of the alloys were investigated systematically. For La_2−xTi_xMgNi₉ (x = 0.1, 0.2, 0.3, 0.4) alloys, LaNi₅, LaNi₃ and LaMg₂Ni₉ are the main phases, the maximum hydrogen-storage capacity is 1.51, 1.36, 1.35 and 1.22 wt%, respectively. The absorption–desorption plateau pressure of the alloys first decreases and then increases with increase of Ti content, and the La_1.8MgTi_0.2Ni₉ alloy has the lowest absorption–desorption plateau pressure. The discharge voltage of the alloy electrodes rises with increasing the amount of Ti content. The La_1.8Ti_0.2MgNi₉ alloy electrode presents good electrochemical performance.     

Characteristics of A2B7-type LaYNi-based hydrogen storage alloys modified by partially substituting Ni with Mn

LaY₂Ni_10.5?xMn_x (x = 0.0, 0.5, 1.0, 2.0) alloys are prepared by a vacuum induction-quenching process followed by annealing. The structure, as well as the hydriding/dehydriding and charging/discharging characteristics, of the alloys are investigated via X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), pressure-composition isotherms (PCI), and electrochemical measurement. The alloys have multiphase structures mainly composed of Gd₂Co₇-type (3R) and Ce₂Ni₇-type (2H) phases. Partial substitution of Ni by Mn clearly increases the hydrogen storage capacity of the alloys. The x = 0.5 alloy exhibits a maximum hydrogen storage capacity of 1.40 wt % and a discharge capacity of 392.9 mAh g^?1, which are approximately 1.5 and 1.9 times greater than those of the x = 0.0 alloy, respectively. The high-rate dischargeability (HRD) of the x = 0.5 alloy is higher than that of the other alloys because of its large hydrogen diffusion coefficient D, which is a controlling factor in the electrochemical kinetic performance of alloy electrodes at high discharge current densities. Although the cyclic stability of the x = 0.5 alloy is not as high as that of the other alloys, its capacity retention ratio is as high as 56.3% after the 400th cycle. The thermodynamic characteristics of the x = 0.5 alloy satisfy the requirements of the hydride electrode of metal hydride–nickel (MH–Ni) batteries.     

Hydrogenation and dehydrogenation behaviours of nanocrystalline Mg20Ni10−xCux (x = 0−4) alloys prepared by melt spinning

In order to improve the hydriding and dehydriding kinetics of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Cu, and the nanocrystalline Mg₂Ni-type Mg₂₀Ni_10−xCu_x (x = 0, 1, 2, 3, 4) alloys were synthesized by melt-spinning technique. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results show that the substitution of Cu for Ni does not change the major phase Mg₂Ni. The hydrogen absorption capacity of the alloys first increases and then decreases with rising Cu content, but the hydrogen desorption capacity of the alloys grows with increasing Cu content. The melt spinning significantly improves the hydrogenation and dehydrogenation capacity and kinetics of the alloys.     

Effect of Bismuth on hydrogen storage properties of melt-spun LaNi4.7-x Al0.3Bix (x = 0.0, 0.1, 0.2, 0.3) ribbons

The LaNi₅ intermetallic compound is an AB5 type hydrogen storage alloy which exhibits low operating temperature, easy activation, low pressure and tolerance to impurities. In this study, LaNi_4.7-x Al_0.3Bi_x (x = 0.0, 0.1, 0.2, 0.3) alloys were produced by melt-spinning technique and the effects of Al and Bi additions on the microstructure, thermal and hydrogen storage properties of LaNi₅ were investigated. The results showed that substitution of Ni with Al led to a desired decrease in absorption/desorption plateau pressure and hysteresis without a decrease in hydrogen storage capacity. In contrast, Bi substitution with Ni increased the absorption/desorption plateau pressure, reduced the hydrogen capacity and increased pulverization resistance of the alloy due to the formation of BiLa and AlNi₃ intermetallic phases at the grain boundaries.     

Investigations on gaseous hydrogen storage performances and reactivation ability of as-cast TiFe1-xNix (x=0, 0.1, 0.2 and 0.4) alloys

In this paper, Fe is partly substituted by Ni for improving the hydrogen storage properties of the TiFe alloy, such as the activation performance, hydrogen storage capacity, reactivation ability, optimum temperature range, thermodynamics and kinetics. The as-cast TiFe alloy contains the majority phase of TiFe and the minority phases of Ti₂Fe and TiFe₂. Increasing Ni content causes the majority phase of TiFe to increase firstly and then decrease again. The activation temperature reduces from 573 K for the TiFe alloy to 523 and 443 K for the TiFe_0·8Ni_0.2 and TiFe_0·6Ni_0.4 alloys respectively. Substituting Fe with Ni partly can lower the platform pressure for the P-C-T curves and increase the dehydrogenation enthalpy (ΔH_des). The TiFe_0·8Ni_0.2 alloy possesses the highest hydrogenation capacity. Adding Ni also is beneficial to expand the optimum temperature range, corresponding to the hydrogenation capacity higher than 0.800 wt%, which is 313–383, 313–503 and 313–573 K for the TiFe_1-xNi_x (x = 0.1, 0.2 and 0.4) alloys, respectively. All the alloys can be activated again at 573 K after being exposed to air for 5 min.     

Influence of annealing treatment on the hydrogen storage properties of La2−xTixMgNi9 (x = 0.2, 0.3) alloys

La_2−xTi_xMgNi₉ (x = 0.2, 0.3) alloys have been prepared by magnetic levitation melting under an Argon atmosphere, and the as-cast alloys were annealed at 800 °C, 900 °C for 10 h under vacuum. The effects of annealing on the hydrogen storage properties of the alloys were investigated systematically by XRD, PCT and electrochemical measurements. For the La_2−xTi_xMgNi₉ (x = 0.2, 0.3) alloys, LaNi₅, LaMg₂Ni₉ and LaNi₃ are the main phases and a Ti₂Ni phase appears at 900 °C. The effective hydrogen storage capacity increases from 1.10, 1.10 wt.% (as-cast) to 1.22, 1.16 wt.% (annealed 800 °C) and 1.31, 1.27 wt.% (annealed 900 °C), respectively. The annealing not only improves the hydrogen absorption/desorption kinetics but also increases the maximum discharge capacity and enhances the cycling stability. The La_1.8Ti_0.2MgNi₉ alloy annealed at 900 °C exhibits good electrochemical properties, and the discharge capacities decrease from 366.1 mA h/g to 219.6 mA h/g after 177 charge-discharge cycles.     

Electrochemical properties of the CaNi5−xMnx electrodes synthesized by mechanical alloying

In this article, we exhaustively examined the effects of Mn substitution for Ni on the structures and electrochemical characterization of the CaNi_5−xMn_x (x = 0.2, 0.3, 0.5, 1) alloys prepared by mechanical synthesis for 40 hours at ball to powder weight ratio of 8:1. The characterization of electrodes was examined by X-ray diffraction, scanning electron microscope and electrochemical tests. In this context, the structural properties for each alloy have two major phases Ni, Ca₂Ni₇; Ni, CaNi₃; Ni, CaNi₅; Ni, CaNi₃ where x = 0.2, 0.3, 0.5 and 1 respectively. The powder micrograph shows the existence of agglomerates has average particle size between 22 and 35 μm. In addition, the quantification by energy dispersive spectroscopy has been indicated the chemical composition of the all produced alloys is near their nominal composition. All electrodes are activated during the first cycle, independent of the Mn substitution rate. The highest values of discharge capacity and reversibility are obtained for x = 0.3 (125 mAh g⁻¹, 0.17 V) and x = 0.5 (119 mAh g⁻¹, 0.19 V) despite their low cycle stability. The evolution of D_H/a² ratio and the I₀ exchange current density for the different Mn substitution rates is in accordance with that of the evolution of discharge capacities. The better kinetic properties is observed for x = 0.5 during electrochemical cycling.     

Hydrogen storage properties of amorphous and nanocrystalline (Mg24Ni10Cu2)100-xNdx (x = 0–20) alloys

The as-cast and spun (Mg₂₄Ni₁₀Cu₂)_100-xNd_x (x = 0–20) alloys were prepared in this experiment to study their hydrogen storage performances. The alloys were tested on composition, structure, de-/hydriding kinetics and electrochemical property in many ways including XRD, SEM, TEM, Sieverts apparatus and automatic galvanostatic system. It was found that each as-cast alloy is multi-phased, including Mg₂Ni-type major phase and some second phases of Mg₆Ni, Nd₅Mg₄₁ and NdNi, the proportion of which obviously increases with Nd content rising. After being melt spinning, the structure of no-Nd-added alloy is nanocrystalline, while the Nd-added alloy holds an amorphous and nanocrystalline structure, whose amorphization degree rises significantly with Nd content increasing. The hydrogen storage kinetics of alloys can be improved by the addition of Nd significantly. Besides, adding Nd results in the electrochemical discharge capacity of the as-spun alloy augments at first and declines later as well as cycle stability.     

Electrochemical characterization of LaNi5−xAlx (x = 0.1–0.5) in the absence of additives

The electrochemical performance of LaNi_5−xAl_x (x = 0.1–0.5) hydrogen storage alloys was quickly and systematically evaluated by powder microelectrode (PME) technique. X-ray diffraction (XRD) and X-ray photo-electron spectroscopy (XPS) studies were also carried out for a better understanding of the effect of Al partial substitution for Ni on the alloy's performance. PME study results show that Al partial substitution for Ni improves both the cycling performance and the anti-electro-oxidation ability of the alloys; however, it prolongs the alloy activation process, decreases the maximum discharge ability and enhances the polarization of the alloy electrode. The alloy decay mainly behaves as the capacity reduction with the time, but the maximum discharge ability almost keeps constant during the service life. The changes of both the physical and the chemical properties of the alloys resulted from Al partial substitution for Ni are the main factors which lead to the changes of the electrochemical performance of the alloys.