LaNi5 Phase Research Articles

Effect of Mg substitution for La on microstructure, hydrogen storage and electrochemical properties of La1−xMgxNi3.5 (x=0.20, 0.23, 0.25 at%) alloys

The effect of Mg substitution for La on microstructure, hydrogen storage and electrochemical properties of the annealed La1−xMgxNi3.5 (x=0.20, 0.23, 0.25at%) alloys have been studied. All the samples were mainly composed of (LaMg)2Ni7, (LaMg)Ni3, and LaNi5 phases. Mg substitution for La changed the phase abundance, but did not change the constitution of all phases, which is confirmed by the results of back-scattered SEM images and EDS analysis. The P–C isotherms indicated that the La1−xMgxNi3.5 alloys reversibly absorbed and desorbed hydrogen smoothly at 298K. The hydrogen absorption cyclic stabilities of La0.77Mg0.23Ni3.5 alloy after 5 hydrogen absorption/desorption cycles reached the maximum values of 91.9% and 96.0% at 298K and 323K, respectively. The hydrogen desorption capacity and plateau pressure for the La0.77Mg0.23Ni3.5 alloy reached the maximum values of 1.055 H/M and 0.074MPa, respectively. The desorption capacities of La0.77Mg0.23Ni3.5 reached 0.193 H/M and 0.565 H/M in the first minute at 298K and 323K, respectively. Electrochemical property measurement indicated that La1−xMgxNi3.5 (x=0.20, 0.23, 0.25at%) alloys possessed excellent activation capability and were completely activated within 3 cycles. Discharge capacities of La1−xMgxNi3.5 alloys reached 378.2mAh/g (x=0.20at%), 342.7mAh/g (x=0.23at%), and 369.6mAh/g (x=0.25at%), respectively. Moreover, energy density of La0.77Mg0.23Ni3.5 alloy was much larger than that of La0.80Mg0.20Ni3.5 alloy and nearly approaches the maximum value of La0.75Mg0.25Ni3.5. Thus, the La0.77Mg0.23Ni3.5 alloy exhibits optimum comprehensive properties of hydrogen storage and electrochemistry.

Effect of annealing temperature on microstructures and electrochemical performances of La0.75Mg0.25Ni3.05Co0.2Al0.1Mo0.15 hydrogen storage alloy

The effect of annealing temperature on microstructures and electrochemical performances of La0.75Mg0.25Ni3.05Co0.2Al0.1Mo0.15 hydrogen storage alloy is investigated. The alloys was prepared via vacuum induction melting followed by annealing treatment for 8 h at the temperatures of 1073, 1123, 1173, and 1223 K, respectively. The major phases in the as-cast and annealed alloys are consisted of (La, Mg)2Ni7, (La, Mg)5Ni19 and LaNi5 phase while the residual phase MoNi4 existing in the as-cast alloy decomposes into Mo phase after heat treatment. Annealing treatment facilitates LaNi5 phase transforms into (La, Mg)2Ni7 and (La, Mg)5Ni19 phase, but the tendency is suppressed as annealing temperature increases. Furthermore, the annealed alloys have more homogeneous phase distribution and better crystallization. Electrochemical experiments manifest that annealing treatment is beneficial for the increase of the maximum discharge capacity while impairs the activation ability of the alloy electrodes. The cycle life is prolonged in consequence of the homogeneous phase distribution created by annealing treatment at 1073–1173 K. Nonetheless, annealing treatment distinctly deteriorates the kinetic property of the alloys electrodes which is mainly ascribed to the disappearance of MoNi4 phase with high-efficiency electrocatalytic activity. The comprehensive electrochemical properties of the annealed alloy at 1173 K presents a good balance between superior discharge capacity, kinetic property and remarkably improved cyclic stability.

Preparation and electrochemical properties of La0.70MgxNi2.45Co0.75Al0.30 (x = 0, 0.30, 0.33, 0.36, 0.39) hydrogen storage alloys

The as-cast alloy with the composition of La0.70Ni2.45Co0.75Al0.30 was prepared by vacuum arc melting. La–Mg–Ni-based La0.70MgxNi2.45Co0.75Al0.30 hydrogen storage alloy has been synthesized by high-energy vibratory milling blending of the La0.70Ni2.45Co0.75Al0.30 as-cast alloy and elemental Mg, followed by an isothermal annealing. The microstructures and electrochemical properties of the La0.70Ni2.45Co0.75Al0.30 and La0.70MgxNi2.45Co0.75Al0.30 alloys were investigated by XRD, SEM and electrochemical measurements. The XRD analysis and Rietveld refinement showed that the as-cast La0.70Ni2.45Co0.75Al0.30 alloy consists of single LaNi5 phase, whereas the La0.70MgxNi2.45Co0.75Al0.30 alloys contain the LaNi5 and (La, Mg)2Ni7. The electrochemical measurements indicated that the maximum discharge capacity and discharge potential characteristic of the La0.70MgxNi2.45Co0.75Al0.30 alloys increases first and then decreases with increasing x. The maximum discharge capacity and discharge potential characteristic of alloy reaches the optimum when x is 0.36. The cyclic stability of the La0.70MgxNi2.45Co0.75Al0.30 alloy with a smaller quantity of Mg is better than that of the alloy with a larger quantity of Mg.

Microstructural evolution and electrochemical properties of the ultra-high pressure treated La0.70Mg0.30Ni3.3 hydrogen storage alloy

This research highlighted the effects of ultra-high pressure (UHP) technique (1–5 GPa) on microstructural evolution and electrochemical performances of La0.70Mg0.30Ni3.3 alloy. Structure analysis showed that the as-cast alloy consisted of (La,Mg)Ni3, (La,Mg)2Ni7, (La,Mg)5Ni19, LaNi5 and LaMgNi4 phases. For all the UHP treated alloys, (La,Mg)5Ni19 phase disappeared via converting into (La,Mg)2Ni7 and LaNi5 phases. At 1 GPa, LaNi5 phase and LaMgNi4 phase fully transformed into (La,Mg)Ni3 and (La,Mg)2Ni7 phases by the peritectic reactions. However, when the pressure was 3 GPa and 5 GPa, the peritectic reactions were uncompleted for the atom diffusion was hindered, which not only caused the minor residual of LaNi5 and LaMgNi4 phases in the alloys, but also resulted in the increase of (La,Mg)Ni3 phase and the decrease of (La,Mg)2Ni7 phase. Electrochemical measurements displayed that the cyclic stability of alloy electrode treated at 1 GPa increased by 8.7% comparing with that of the as-cast alloy electrode, while the cyclic stability reduced when the pressure increased to 3 GPa and 5 GPa. This indicated that the appropriate UHP treatment was favorable for the improvement of the cycle life of alloy electrode. The high rate dischargeability (HRD) of alloy electrodes gradually decreased with the pressure increasing, which mainly resulted from the decrease of the hydrogen diffusion coefficient.

Structural and Hydriding Properties of the LaZr2Mn4Ni5-AB3 Type Based Alloy Prepared by Mechanical Alloying from the LaNi5 and ZrMn2 Binary Compounds

In this study, we report on the synthesis of a new AB3-type compound LaZr2Mn4Ni5 (with a content of 40 wt %) at room temperature during 5h of mechanical alloying. This compound was synthesized from the two binary compounds LaNi5 (CaCu5-type structure, P6/mmm space group) and ZrMn2-Laves phase (MgZn2-type structure, P63/mmc space group) in order to take advantage of the hydrogen absorption properties of these two types of intermetallic compounds. Structural properties were investigated using X-ray diffraction (XRD). The surface morphology of the cycled electrode was observed by a scanning electron microscope (SEM). The electrochemical properties of the LaZr2Mn4Ni5–based alloy were determined using the chrono-potentiometry method. The experimental results indicate that the discharge capacity reaches a maximum value of 300 mAh/g. Solid-gaz reaction shows that this compound is able to form the LaZr2Mn4Ni5H13 hydride at room temperature at an absorption plateau pressure of about 7 bar.

Molten salt synthesis of La(Ni1−xCox)5 (x = 0, 0.1, 0.2, 0.3) type hydrogen storage alloys

La(Ni1−xCox)5 (x = 0, 0.1, 0.2, 0.3) alloys were synthesized directly from sintered mixture of La2O3 + NiO + CoO in the molten CaCl2 electrolyte by the electro-deoxidation method at 850 °C and the electrochemical hydrogen storage characteristics of the synthesized alloys were observed. Sintering (at 1200 °C for 2 h) converted the hygroscopic La2O3 (by the reaction with NiO) into the non-hygroscopic La2NiO4, LaNiO3, La3Ni2O6.5, La3Ni2O6.84 and La4Ni3O9 depending on the Co content of the oxide mixture. The X-ray diffraction peaks indicated that La2NiO4 was the main La–Ni–O phase to initiate the LaNi5 phase formation. The increase in the Co content of the sample was observed to delay the La2NiO4 formation and thus the reactions initiated by La2NiO4 reduction. LaOCl formed chemically was the last oxygen including phase remained in the alloy before obtaining the final alloy structure. The porous alloy structure was beneficial for higher hydrogen storage capacity and it was observed that La(Ni1−xCox)5 (x = 0, 0.1, 0.2, 0.3) alloys had promising discharge capacities changed between 223 mA h g−1 (LaNi5) and 325 mA h g−1 (La(Ni0.7Co0.3)5). This work clearly indicated that the electro-deoxidation was a very effective method in the synthesis of the hydrogen storage materials.

Effect of Pr on phase structure and cycling stability of La–Mg–Ni-based alloys with A2B7- and A5B19-type superlattice structures

Fast capacity degradation is a significant drawback hindering La–Mg–Ni-based alloys from wide application as anode electrode materials of nickel metal hydride batteries. Herein, the effect of Pr element on the phase structure and cycling stability of La0.8−xPrxMg0.2Ni3.4Al0.1 (x=0, 0.1, 0.2 and 0.3) alloys is investigated. All the alloys contain (La,Mg)2Ni7 and (La,Mg)5Ni19 main phases as well as LaNi5 minor phase. It is found that Pr tends to form [AB5] subunits more than [A2B4] subunits, thus increasing the content of (La,Mg)5Ni19 phase with higher [AB5]/[A2B4] ratio in sacrifice of (La,Mg)2Ni7 phase. The increased (La,Mg)5Ni19 phase network with good structural stability increases the anti-pulverization and anti-amorphization resistance of the alloys. After 100 charge/discharge cycles, part of the superlattice phases in La0.6Mg0.2Ni3.4Al0.1 alloy decomposes into amorphous phase and LaNi5 phase, leading to the decrease in superlattce phase abundance; while little amorphization is observed for La0.6Pr0.2Mg0.2Ni3.4Al0.1 alloy and its phase contents remain almost unchanged. Correspondingly La0.6Pr0.2Mg0.2Ni3.4Al0.1 alloy electrode has a slighter oxidation degree after 100 cycles in alkaline electrolyte and exhibits good electrochemical cycling stability with a discharge capacity of 340mAhg−1 and a cycling stability of 90.7% at the 100th cycle.

Effect of WFe substitution on microstructures and electrochemical hydrogen storage properties of LaNi3.70Co0.2–xMn0.30Al0.15Cu0.65(W0.42Fe0.58)x alloys

W0.42Fe0.58 alloy, instead of pure W and Fe, was used to substitute Co in LaNi3.70Co0.2Mn0.30Al0.15Cu0.65 alloy to improve the overall electrochemical properties with the decrement of the cost. Microstructures and electrochemical characteristics of LaNi3.70Co0.2–xMn0.30Al0.15Cu0.65(W0.42Fe0.58)x (x=0–0.20) hydrogen storage alloys were characterized. X-ray diffraction patterns and backscattered electron images indicated that the pristine alloy was LaNi5 phase, while the alloys containing W0.42Fe0.58 were made of LaNi5 matrix phase and W phase. The relatived abundance of W phase increased with the increase in x value. Lattice parameters a, c, c /a and cell volume V of LaNi5 phase increased with increasing x value. Activation property of the alloy electrodes was improved by substituting Co by W0.42Fe0.58. As x increased from 0 to 0.20, maximum discharge capacity of alloy electrodes decreased from 335.4 (x=0) to 320.7 mAh/g (x=0.20). The high-rate dischargeability at the discharge current density of 1200 mA increased from 59.8% (x=0) to 76.8% (x=0.10), and then decreased to 64.7% (x=0.20). The cycling capacity retention rate at the 100th cycle decreased from 80.4% (x=0) to 55.8% (x=0.20), which should be ascribed to the degradation of the corrosion resistance and electrochemical kinetics of alloy electrodes.

Effect of the temperature on electrode performance of the as cast La0.7Mg0.3(NiMnCo)3.5 alloy

A series of experiments were performed to investigate the effect of the temperature on electrode performance of the as-cast La0.7Mg0.3(NiMnCo)3.5 alloy. XRD and SEM show that La0.7Mg0.3(NiMnCo)3.5 alloy has LaNi5 and La2Ni7 phase. The activation curves show that the as-cast La0.7Mg0.3(NiMnCo)3.5 alloy electrode can be activated in three times and the maximum discharge capacity is 358.74 mA h/g at 273 K. With increasing the temperature, the cyclic stability of the alloy electrode decreases obviously. And the capacity retention decreases from 96.14 to 63.53% when the temperature increases from 238 to 303 K. It also can be seen that the as-cast La0.7Mg0.3(NiMnCo)3.5 alloy electrode has the best self-discharge ability at 238 K and the best high-rate discharge ability at 273 K.

Effect of Temperature on Structure and Electrode Properties of LaNi3.8Co0.6Mn0.3M0.3 Hydrogen Storage Alloys

Abstract A series of experiments have been performed to investigate the temperature effects on the structure and the electrochemical properties of LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al, Cu) hydrogen storage alloys at different temperatures of 238, 273, 303 and 323 K. Samples A, B and C were used to represent LaNi4.1Co0.6Mn0.3 (Ni substituted alloy), LaNi3.8Co0.6Mn0.3Al0.3 (Al substituted alloy) and LaNi3.8Co0.6Mn0.3Cu0.3 (Cu substituted alloy), respectively. The structures and the electrochemical properties of A, B and C hydrogen storage alloys were investigated by XRD and simulated battery test, respectively. The results reveal that all of the alloys are composed of the homogeneous LaNi5 phase with a CaCu5-type hexagonal structure. The low-temperature properties of the Cu substituted alloy are improved, and the high-temperature discharge capacity of the Al substituted alloy is enhanced. Electrochemical impedance spectroscopy (EIS) analysis shows that the improvement of high-temperature discharge capacity of alloy electrode B is attributed to the formation of dense oxide film to protect the active material from corrosion, which is not favorable to charge-transfer reaction at the interface of electrode-electrolyte. The deterioration in the high-rate dischargeability (HRD) of alloy electrodes B and C are attributed to the degradation of electrochemical kinetics, including both charge-transfer reactions on the electrode surface and hydrogen diffusion in the bulk. Furthermore, the good low-temperature performance of alloy electrode C is due to the improvement of charge-transfer reaction.

Effects of La-addition to the structure, hydrogen storage, and electrochemical properties of C14 metal hydride alloys

The structure of a series AB2 metal hydride alloys, in which La substitutes for Zr (Ti12Zr22.8−xV10Cr7.5Mn8.1Co7.0Ni32.2LaxAl0.4, x=0 to 5), is studied and correlated to various hydrogen storage properties in both gaseous phase and electrochemical systems. La, instead of entering the main Laves phase, forms a LaNi secondary phase which has very limited solubility with other elements. This phase improves activation of both capacity and high-rate dischargeability through a dramatic increase in surface area due to pulverization. The presence of LaNi phase also improves the high-rate dischargeability, facilitating the bulk diffusion of protons through synergetic effects with the main Laves phase. La-content in the alloys also suppresses the TiNi secondary phase due to competition of Ni with the LaNi phase. The lattice constants and unit cell volume of the main C14 phase decreases as more Zr (relatively large) is substituted out, which leads to a less stable hydride with higher equilibrium plateau pressure, higher ΔH, and lower hydrogen storage capacity. With the addition of La in the alloy samples, C14 and LaNi phase abundances increase while C15 and TiNi phase abundances decrease. At −40°C, the charge-transfer resistance of the La-containing alloys decreases as a result of increased catalytic ability in the main phase. The lowest overall charge-transfer resistance obtained from electrochemical impedance spectroscopy analysis measured at −40°C is 11Ω g.

Effects of stoichiometric ratio La/Mg on structures and electrochemical performances of as-cast and annealed La–Mg–Ni-based A2B7-type electrode alloys

In order to investigate the influences of the stoichiometric ratio of La/Mg (increasing La and decreasing Mg on the same mole ratio) on the structure and electrochemical performances of the La–Mg–Ni-based A2B7-type electrode alloy, the as-cast and the annealed ternary La0.8+xMg0.2−xNi3.5 (x=0−0.05) electrode alloys were prepared. The characterization of electrode alloys by X-ray diffraction (XRD) and scanning electron microscopy (SEM) shows that all the as-cast and the annealed alloys hold two major phases of (La,Mg)2Ni7 and LaNi5 as well as a residual phase of LaNi3. Moreover, the increase of La/Mg ratio brings on a decline of (La,Mg)2Ni7 phase and a rise of LaNi5 and LaNi3 phases. The variation of La/Mg ratio gives rise to an evident change of the electrochemical performances of the alloys. The discharge capacities of the as-cast and the annealed alloys evidently decrease with growing the La/Mg ratio, while the cycle stabilities of the alloys visibly augment under the same condition. Furthermore, the high rate discharge ability (HRD), the electrochemical impedance spectrum (EIS), the Tafel polarization curves, and the potential step measurements all indicate that the electrochemical kinetic properties of the alloy electrodes increase with the La/Mg ratio rising.

Microstructures and electrochemical characteristics of LaNi3.70Co0.2−xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x hydrogen storage alloys

Electrochemical hydrogen storage properties of LaNi3.70Co0.2Mn0.30Al0.15Cu0.65 alloy are improved by substituting Co with Mo0.46Fe0.54, rather than pure Mo and Fe. Microstructures and electrochemical properties of LaNi3.70Co0.2−xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x (x=0–0.20) hydrogen storage alloys are investigated. X-ray diffraction and backscattered electron results indicate that the pristine alloy is LaNi5 phase with a hexagonal CaCu5-type structure, while the alloys containing Mo0.46Fe0.54 consist of LaNi5 matrix phase and Mo secondary phase. The relative abundance of Mo phase increases with the increase in x value. The lattice parameters a, c, c/a and cell volume V of LaNi5 phase increase with increasing x value. As x increases from 0 to 0.20, maximum discharge capacity of the alloy electrodes monotonically decreases from 335.4 (x=0) to 324.2mAh/g (x=0.20). The high-rate dischargeability of the alloy electrodes at the discharge current density of 1200mA/g first increases from 59.8% (x=0) to 69.6% (x=0.15), and then decreases to 64.0% (x=0.20). The cycling capacity retention rate at the 100th cycle decreases from 80.4% (x=0) to 61.9% (x=0.20), which should be ascribed to the deterioration of the corrosion resistance of alloy electrode with increasing x value.

Influence of Zr Addition on Structure and Performance of Rare Earth Mg-Based Alloys as Anodes in Ni/MH Battery

In this study, the substitution of Mg with Zr in La0.7Mg0.3(Ni0.85Co0.15)3.5 was carried out with the purpose of improving the electrochemical performances. The structural and hydrogen storage properties in both gas-solid reaction and the electrochemical system were systematically studied on La0.7(Mg0.3−xZrx)(Ni0.85Co0.15)3.5 (x = 0.05, 0.1, 0.2, 0.3) alloys. Each tested alloy is composed of LaNi3 phase, LaNi5 phase and ZrNi3 phase with different phase abundances. The electrochemical studies indicated that all Zr-substituted anodes possessed a much higher cycling capacity retention than pristine La0.7Mg0.3(Ni0.85Co0.15)3.5. However, the maximum discharge capacity was reduced with the increase of Zr content. The potential-step tests showed that the diffusion of hydrogen atoms inside the anodes was decelerated after the introduction of Zr.

Improved electrochemical kinetic performances of La-Mg-Ni-based hydrogen storage alloys with lanthanum partially substituted by yttrium

Yttrium (Y) has been used as the partial substitution element for lanthanum (La) to improve the electrochemical kinetic performances of La-Mg-Ni-based hydrogen storage alloys. La0.80–xYxMg0.20Ni2.85Mn0.10Co0.55Al0.10 (x=0.00, 0.05 and 0.10) alloys were prepared by the inductive melting technique. The alloys were composed of LaNi5 and (La,Mg)2Ni7 phases, the introduction of Y promoted the formation of (La,Mg)2Ni7 phase, and thus the Y-substituted alloy electrodes exhibited higher discharge capacities. Y substitution was also found to be effective to improve the discharge kinetics of the alloy electrodes. When the Y content x increased from 0.00 to 0.10, the high-rate dischargeability of the alloy electrodes at a discharge current density of 1800 mA/g (HRD1800) increased from 23.6% to 39.7% at room temperature. In addition, the measured HRD1800 showed a linear dependence on both the exchange current density and the hydrogen diffusion coefficient at different temperatures, respectively.

Effect of Sn substitution for Co on microstructure and electrochemical performance of AB5 type La0.7Mg0.3Al0.3Mn0.4Co0.5–xSnxNi3.8 (x=0–0.5) alloys

The effects of substitution of Sn for Co on the microstructure, hydrogen storage and electrochemical discharge capacity of La0.7Mg0.3Al0.3Mn0.4Co0.5–xSnxNi3.8 (x=0, 0.1, 0.2, 0.3 and 0.5) alloys were investigated using X-ray diffraction (XRD), pressure composition isotherm (PCT) and electrochemical discharge cycle. XRD, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) tests showed that all of alloys are mainly composed of LaNi5 and MgNi2 phases, but when increasing the content of Sn in alloys, the LaNiSn phase appears and microstructure is refined. The PCT showed that increasing substitution of Sn for Co results in decrease of the maximum hydrogen storage capacity from 1.48% (x=0) to 0.85% (x=0.5). The electrochemical tests indicated that the maximum discharge capacity decreases from 337.1 mA·h/g (x=0) to 239.8 mA·h/g (x=0.5); however, the discharge capacity retention at the 100th cycle increases from 70.2% (x=0) to 78.0% (x=0.5).

Electrochemical hydrogen storage performances of the Si added La-Mg-Ni-based A2B7-type electrode alloys for Ni/MH battery application

The casting and annealing technologies were applied to fabricate the La0.8Mg0.2Ni3.3Co0.2Six (x = 0–0.2) electrode alloys. The effects of Si content and annealing temperature on the structure and electrochemical performances of the alloys were investigated systematically. The analyses of XRD and SEM show that all the alloys possess a multiphase structure, involving two main phases (La, Mg)2Ni7 and LaNi5 as well as a residual phase LaNi3. The addition of Si brings on an evident increase in the LaNi5 phase and a decrease in the (La, Mg)2Ni7 phase, without altering the main phase component of the alloy, which also makes the lattice constants and cell volumes of the alloy enlarged. Likewise, the annealing treatment engenders the same action on the lattice constants and cell volumes as adding Si. Simultaneously, it gives rise to the variation of the phase abundance and the coarsening of the alloy grains. The electrochemical measurements indicate that the addition of Si ameliorates the cycle stability of the as-cast and annealed alloys significantly, but impairs their discharge capacities clearly. Similarly, the annealing treatment makes a positive contribution to the cycle stability of the alloy evidently, and the discharge capacity of the alloy shows a maximum value with annealing temperature rising. Furthermore, the high rate discharge ability (HRD) first augments and then declines with the rising of Si content and annealing temperature.

Study on the phase structures and electrochemical performances of La0.6Gd0.2Mg0.2Ni3.15-xCo0.25Al0.1Mnx (x = 0-0.3) alloys as negative electrode material for nickel/metal hydride batteries

La0.6Gd0.2Mg0.2Ni3.15-xCo0.25Al0.1Mnx(x=0–0.3) hydrogen storage alloys are prepared by induction melting followed by annealing treatment at 1173K for 8hours and the effects of partial substitution Mn for Ni on the phase structures and electrochemical properties are studied. The phase structure analyses show that (La, Mg)2Ni7 phase with Ce2Ni7-type structure, (La, Mg) Ni3 phase with PuNi3-type structure and LaNi5 phase with CaCu5-type structure are the main phases. The increase in Mn content results in the expansion of the lattice parameters and the cell volumes of the alloys. The electrochemical analyses show that a proper amount of Mn substitution for Ni (x=0.1) can increase the maximum discharge capacity and improve the cyclic stability of the alloy electrodes. Additionally, the kinetic properties of the alloy electrodes are improved because of the lower charge transfer resistance on alloy surface and the faster hydrogen diffusion rate in alloy bulk after Mn partial substitution for Ni.

Synthesis of La2Ni7 hydrogen storage alloy by the electro-deoxidation technique

La2Ni7 alloy was synthesized in the molten CaCl2 electrolyte by the electro-deoxidation method at 850 °C and the electrochemical hydrogen storage characteristics of the synthesized alloy was observed. Sintering at 1200 °C for 3 h was determined as the optimum condition to have the electrode pellet with enough porosity and mechanical strength. The hydroscopic La2O3 disappeared and the non-hydroscopic La2NiO4 formed during the sintering. The X-ray diffraction peaks indicated that the sinter product La2NiO4 and NiO reduced to LaNi5 and Ni, respectively, within 4 h electro-deoxidation process. The reduction kinetics of LaOCl was relatively slow. This sluggish reduction caused formation of the target La2Ni7 phase only after 6 h electro-deoxidation. The final La2Ni7 alloy structure with small amount of retained LaNi5 was obtained after 10 h electro-deoxidation and this structure did not change up to 25 h. It was observed that the synthesized alloy had maximum discharge capacity of 207 mA h g−1 and the alloy kept more than 90% of its discharge capacity within 20 charge/discharge cycles. According to the gathered EIS data La2Ni7 alloy was estimated to have no considerable surface barrier layer after discharging to hinder the atomic hydrogen diffusion on its surface.

Effect of stoichiometry and Cu-substitution on the phase structure and hydrogen storage properties of Ml-Mg-Ni-based alloys

To improve the electrochemical properties of rare-earth-Mg-Ni-based hydrogen storage alloys, the effects of stoichiometry and Cu-substitution on the phase structure and thermodynamic properties of the alloys were studied. Nonsubstituted Ml0.80Mg0.20(Ni2.90Co0.50-Mn0.30Al0.30)x (x = 0.68, 0.70, 0.72, 0.74, 0.76) alloys and Cu-substituted Ml0.80Mg0.20(Ni2.90Co0.50−yCuyMn0.30Al0.30)0.70 (y = 0, 0.10, 0.30, 0.50) alloys were prepared by induction melting. Phase structure analysis shows that the nonsubstituted alloys consist of a LaNi5 phase, a LaNi3 phase, and a minor La2Ni7 phase; in addition, in the case of Cu-substitution, the Nd2Ni7 phase appears and the LaNi3 phase vanishes. Thermodynamic tests show that the enthalpy change in the dehydriding process decreases, indicating that hydride stability decreases with increasing stoichiometry and increasing Cu content. The maximum discharge capacity, kinetic properties, and cycling stability of the alloy electrodes all increase and then decrease with increasing stoichiometry or increasing Cu content. Furthermore, Cu substitution for Co ameliorates the discharge capacity, kinetics, and cycling stability of the alloy electrodes.