Effect of substitution of aluminum for nickel on electrochemical properties of La_(0.75)Mg_(0.25)Ni_(3.5–x)Co_(0.2)Al_x hydrogen storage alloys

La0.75Mg0.25Ni3.5–xCo0.2Alx (x=0–0.09) hydrogen storage alloys were prepared by induction melting and effect of Al substitution for Ni on phase constitution and electrochemical property was investigated.With the substitution of Al for Ni,LaNi5 and LaNi2 phases occurred and (La,Mg)2(Ni,Co,Al)7 phase with hexagonal Ce2Ni7-type structure replaced (La,Mg)2(Ni,Co)7 phase.The cell volumes of LaNi5 and (La,Mg)2(Ni,Co,Al)7 main phases increased with increasing Al content.Some electrochemical properties and kinetic parameters of the alloys,including discharge capacity,high rate discharge ability (HRD),loss angle (ψ),exchange current density (I0) and limiting current density (IL),decreased with increasing amount of substitution of Al for Ni.Substitution of Al for Ni could be favorable for positive shift in corrosion potential of the alloy electrode,and prolonged cyclic lifetime of La0.75Mg0.25Ni3.5–xCo0.2Alx (x=0–0.09) alloy electrodes.     

A study on hydrogen storage and electrochemical properties of La_（0.55）Pr_（0.05）Nd_（0.15）Mg_（0.25）Ni_（3.5） （Co_（0.5）Al_（0.5））_x （x=0.0, 0.1, 0.3, 0.5） alloys

The La0.55Pr0.05Nd0.15Mg0.25Ni3.5(Co0.5Al0.5)x(x=0.0, 0.1, 0.3, 0.5) alloys were prepared by magnetic levitation melting under an Ar atmosphere, and the effects of Co and Al on the hydrogen storage and electrochemical properties were systematically investigated by pressure composition isotherms, cyclic voltammetry, Tafel polarization and electrochemical impedance spectroscopy testing. The results showed that the alloy phases were mainly consisted of (La,Pr)(Ni,Co)5, LaMg2Ni9, (La,Nd)2Ni7 and LaNi3 phases, and the cell volumes of (La,Pr)(Ni,Co)5, LaMg2Ni9, (La,Nd)2Ni7 and LaNi3 phases expanded with Co and Al element added. The hydrogen storage capacity initially increased from 1.36 (x=0) to 1.47 wt.% (x=0.3) and then decreased to 1.22 wt.% (x=0.5). The discharge capacity retention and cycle stability of the alloy electrodes were improved with the increase of Co and Al contents. The La0.55Pr0.05Nd0.15Mg0.25Ni3.5(Co0.5Al0.5)0.3 alloy electrode possessed better electrochemical kinetic characteristic.     

Effect of the partial substitution of samarium for lanthanum on the structure and electrochemical properties of La_(15)Fe_(77)B_8-type hydrogen storage alloys

La15Fe77B8 hydrogen storage alloys were prepared using a vacuum induction-quenching furnace. The results of X-ray diffraction(XRD) and scanning electron microscopy(SEM) suggested that La15–xSmxFe2Ni76Mn5B2(x=0, 2, 4, 6) alloys had multiphase structure including the main LaNi5 phase, La3Ni13B2 phase and(Fe, Ni) phase. With the increasing substitution of Sm for La, the main phase structure of alloys did not change, while the unit cell volumes decreased, the cycle stability was improved and the maximum discharge capacity decreased, but the low temperature maximum discharge capacity of the same substitution alloy was gradually approaching the maximum discharge capacity at room temperature, which showed that La15Fe77B8 hydrogen storage alloys of the partial substitution of Sm for La had better low-temperature dischargeability(LTD). For the same substitution alloys, self-discharge characteristics and cycle stability at low temperature were better than that at room temperature. Furthermore, the high-rate dischargeability(HRD) and the exchange current density I0 first increased and then decreased with the increasing of Sm content, whereas the hydrogen diffusion coefficient D in alloy bulk decreased gradually, which indicated that appropriate substitution of Sm for La improved the electrochemical kinetics properties of the alloys. The HRD was mainly dominated by the charge-transfer rate on the alloy surface.     

Effect of annealing treatment on hydrogen storage properties of La-Ti-Mg-Ni-based alloy

The present study dealt with investigations on the effects of annealing on the hydrogen storage properties of La 1.6 Ti 0.4 MgNi 9 alloys.The experimental alloys were prepared by magnetic levitation melting followed by annealing treatment.For La 1.6 Ti 0.4 MgNi 9 alloys,LaNi 5,LaNi 3 and LaMg 2 Ni 9 were the main phases,Ti 2 Ni phase appeared at 900℃.Annealing not only enhanced the maximum and effective hydrogen storage capacity,improved the hydrogen absorption/desorption kinetics,but also increased the discharge capacity.The cyclic stability had been improved markedly by annealing,e.g.,when the discharge capacity reduced to 60% of maximum discharge capacity,the charge/discharge cycles increased from 66(as-cast) to 89(annealed at 800℃) and 127 times(annealed at 900℃).La 1.6 Ti 0.4 MgNi 9 alloy annealed at 900℃ exhibited better electrochemical properties compared to the other two alloy electrodes.     

An investigation of hydrogen storage kinetics of melt-spun nanocrystalline and amorphous MgeNi-type alloys

The nanocrystalline and amorphous Mg2Ni-type Mg2–xLaxNi (x=0,0.2) hydrogen storage alloys were synthesized by melt-spinning technique.The as-spun alloy ribbons were obtained.The microstructures of the as-spun ribbons were characterized by X-ray diffraction (XRD),high resolution transmission electronic microscopy (HRTEM) and electron diffraction (ED).The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus,and their electrochemical kinetics were tested by an automatic galvanostatic system.The electrochemical impedance spectrums (EIS) were plotted by an electrochemical workstation (PARSTAT 2273).The hydrogen diffusion coefficients in the alloys were calculated by virtue of potential-step method.The obtained results showed that no amorphous phase was detected in the as-spun La-free alloy,but the as-spun alloys substituted by La held a major amorphous phase,con-firming that the substitution of La for Mg markedly intensified the glass forming ability of the Mg2Ni-type alloy.The substitution of La for Mg notably improved the electrochemical hydrogen storage kinetics of the Mg2Ni-type alloy.Furthermore,the hydrogen storage kinetics of the experimental alloys was evidently ameliorated with the spinning rate growing.     

Study on structure and electrochemical properties of the （LaGdMg）Ni3.35-xCoxAl0.15（x=0-2.0） hydrogen storage alloys

Hydrogen storage alloys(LaGdMg)Ni3.35-xCoxAl0.15(x=0,0.1,0.3,0.5,1.0,1.5,2.0) were prepared by induction melting followed by annealing treatment in argon atmosphere.The effects of partly replacing Ni by Co element in(LaGdMg)Ni3.35Al0.15 on the phase structure and electrochemical properties of(LaGdMg)Ni3.35-xCoxAl0.15 alloys were investigated.Structure analysis showed that the alloys consisted of Ce2Ni7-type(Gd2Co7-type),CaCu5-type,Pr5Co19-type,PuNi3-type phase structure.The addition of Co element obviously reduced the contents of CaCu5-type phase and increased the contents of Ce2Ni7-type phase.However,Pr5Co19-type and CaCu5-type phase obviously increased with the high content of Co.Rietveld analysis showed that the c-axis lattice parameters and cell volumes of the component phases increased with increasing Co content.The electrochemical measurements showed that as the Co content increased,the maximum discharge capacity and the cyclic stability of the annealed alloys both first increased then decreased.The(LaGdMg)Ni3.05Co0.3Al0.15 alloy electrode exhibited the maximum discharge capacity(392.92 mAh/g),and the(LaGdMg)Ni1.85Co1.0Al0.15 alloy electrode showed the best cyclic stability(S100=96.1%).     

Influences of molybdenum substitution for cobalt on the phase structure and electrochemical kinetic properties of AB_5-type hydrogen storage alloys

The effects of the partial replacement of Co with Mo on the phase structure and electrochemical kinetic properties of La0.35Ce0.65Ni3.54Co0.80-xMn0.35Al0.32Mox (x=0.00, 0.10, 0.15, 0.20, 0.25) hydrogen storage alloys prepared by arc-melting method were sys-tematically studied in this paper. The X-ray diffraction (XRD) showed that after partial substitution of Mo for Co, the alloys remained a single LaNi5 phase with a hexagonal CaCu5-type structure. The P-C isotherms indicated that the equilibrium pressure gradually decreased with in-creasing of Mo content. Electrochemical studies showed that the substitution of Mo for Co could greatly increase discharge capacity, improve activation ability and reduce self-discharge of alloy electrodes. The alloy with x=0.25 exhibited a higher rate dischargeability (HRD1200= 50.9%). Moreover, Mo is a vital element in favor of kinetic properties of AB5-type hydrogen storage alloys. As Mo content increased, the ex-change current density I0, the hydrogen diffusion rate gradually increased.     

Hydrogen storage properties of mechanically milled La2Mg17-x wt.% Ni （x=0, 50, 100, 150 and 200） composites

Melting method was used to obtain La2Mg17 alloy,and then Ni powder was added by mechanical alloying method.The kinetics of hydriding process and electrochemical properties of La2Mg17-x wt.%Ni(x=0,50,100,150,200) composites were investigated.X-ray diffraction(XRD) and scanning electron microscopy(SEM) analyses showed that the crystal structure of composite alloy gradually transformed into amorphous phase by the effect of ball milling and Ni powders.The research of hydrogen absorption properties found that La2Mg 17-50 wt.%Ni reached the highest hydrogen absorption than other alloys with more addition of Ni content,reached to 5.796 wt.% at 3 MPa,and up to 5.229 wt.% merely in 2 min,which revealed that the amorphous phase reduced the H occupation of the lattice clearance,resulting in the decline of hydrogen absorption capacity.The electrochemical tests indicated that the maximum discharge capacity increased to 353.1 mAh/g at 30 oC,however,the cycle stability decreased considerably.A series of kinetic measurements demonstrated that the controlling steps of electrochemical process of La 2 Mg 17-x wt.%Ni alloys transferred from hydrogen diffusion on alloy bulk(x=50,100) to hydrogen diffusion on both alloy bulk and surface(x=150,200).     

Phase structure and Cycle Stabilities of Mg2Ni/Mm0.3Ml0.7Ni3.55Co0.75Mn0.4Al0.3 Composite Hydrogen Storage Alloys Prepared by Two-step re-melting

Mm0.3Ml0.7Ni3.55Co0.75Mn0.4-Al0.3 alloy has high chemical activity and favorable plateaus pressure. Mg2Ni is in favor of high hydrogen storage capacity and low weight, but it is difficult to be activated. In order to improve the capacity and cycle performances of hydrogen-storage alloy electrodes, Mm0.3Ml0.7Ni3.55Co0.75Mn0.4-Al0.3-x%Mg2Ni (x=0,5,10,30) composite hydrogen storage alloys prepared by two-step re-melting were investigated in this work. The influences of Mg2Ni content on the cycle stabilities were analyzed by electrochemical methods. It was observed by XRD that the main phase of all the alloys is LaNi5 and the crystal lattice parameters of LaNi5 are changed with the increasing of x value, i. e, α-axis and unit cell volume decrease and c-axis decreases nonlinearly. The c-axis of alloy with x=5 is larger than the others. With the increasing of x value, capacity retentions of the composite hydrogen storage alloys rise from 66.21% while x=0 to 82.04% while x=10, but the capacity retention of the composite alloy with 30% Mg2Ni declines because of its decreasing axial ratio. More over, the composite alloy with 5% Mg2Ni shows the best cycle stability and higher discharge capacity, and it is an appropriate candidate for battery materials.     

Structure and high-temperature electrochemical properties of La_（0.60）Nd_（0.15）Mg_（0.25）Ni_（3.3）Si_（0.10） hydrogen storage alloys

The structure and high-temperature electrochemical properties of the as-cast and annealed (940 °C, 8 h) La0.60Nd0.15Mg0.25Ni3.3Si0.10 hydrogen storage alloys were investigated. The X-ray diffraction revealed that the multiphase structure of the as-cast alloy with LaNi5 phase as the main phase was converted into a double-phase structure with La2Ni7 phase as the main phase after annealing. The surface morphology studied by scanning electronic microscope (SEM) showed that the annealed alloy had a much higher anti-corrosion ability than the as-cast alloy. Both alloys presented excellent activation characteristics at all test temperatures. The maximum discharge capacity of the as-cast alloy decreased when the test temperature increased, while the temperature almost had no effect on the annealed alloy. As the test temperature increased, the cyclic stability and charge retention of both alloys decreased, and these properties were improved significantly by annealing.     

Microstructure and electrochemical characteristics of La0.7Ce0.3Ni3.75Mn0.35Al0.15Cu0.75-x（V0.81Fe0.19）x hydrogen storage alloys

Microstructure and electrochemical characteristics of La0.7Ce0.3Ni3.75Mn0.35Al0.15Cu0.75-x(V0.81Fe0.19)x hydrogen storage alloys were investigated. XRD indicated that La0.7Ce0.3Ni3.75Mn0.35Al0.15Cu0.75-x(V0.81Fe0.19)x alloys consisted of a single phase with CaCu5-type structure, and the lattice parameter a and cell volume V increased with increasing x value. The maximum discharge capacity first increased from 319.0 (x=0) to 324.0 mAh/g (x=0.05), and then decreased to 307.0 mAh/g (x=0.20). The high-rate dischargeability at the discharge current density of 1200 mA/g first increased from 52.1% (x=0) to 59.1% (x=0.15), and then decreased to 55.4% (x=0.20). The hydrogen diffusion in the bulky alloy was responsible for the high-rate dischargeability. Cycling stability first increased with increasing x from 0 to 0.10 and then decreased when x increased to 0.20, which was resulted from the synthesized effect of the improvement of the pulverization resistance and the decrease of corrosion resistance.     

Microstructure and electrochemical properties of LaNi4–xFeMnx (x=0–0.8) hydrogen storage alloys

In order to reduce the cost of LaNi5 based hydrogen storage alloys, effect of substitution of Mn for Ni on structural and electro-chemical properties of LaNi4-xFeMnx(x=0-0.8) hydrogen storage alloys was studied systematically. X-ray diffraction (XRD) and scanning electron microscope (SEM) showed that LaNi5 and La2Ni7 phases were invariably present in all alloy samples, and when x≥0.4, (Fe,Ni) phase was observed. Electrochemical studies revealed that the discharge capacity reached a maximum value of 306.4 mAh/g when x=0.2 and the cycling stability decreased with the increase of x.With the increase of Mn content, hydrogen diffusion coefficient decreased, whereas high rate discharge-ability (HRD) and exchange current density first increased slowly when x≤0.2 and then decreased markedly when x=0.8,indicating that electrochemical reaction on the surface of alloy electrodes had strong influence on kinetic property.     

Structure and electrochemical hydrogen storage characteristics of the as-cast and annealed La0.8-xSmxMg0.2Ni3.15Co0.2Al0.1Si0.05 （x=0-0.4） alloys

In order to ameliorate the electrochemical cycle stability of the RE-Mg-Ni based A2B7-type electrode alloys, the Mg content in the alloy was reduced and La in the alloy was partially substituted by Sm. The La0.8-xSmxMg0.2Ni3.15Co0.2Al0.1Si0.05 (x=0, 0.1, 0.2, 0.3, 0.4) elec-trode alloys were fabricated by casting and annealing. The microstructures of the as-cast and annealed alloys were characterized by XRD and SEM. The electrochemical hydrogen storage characteristics of the as-cast and annealed alloys were measured. The results revealed that all of the experimental alloys mainly consisted of two phases: (La,Mg)2Ni7 phase with the hexagonal Ce2Ni7-type structure and LaNi5 phase with the hexagonal CaCu5-type structure. As Sm content grew from 0 to 0.4, the discharge capacity and the high rate discharge ability (HRD) first in-creased and then decreased for the as-cast and annealed alloys, whereas the capacity retaining rate (S100) after 100 cycles increased continuously.     

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 La Ni5 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 m A/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.     

Investigations on hydrogen storage properties of LaMg8.52Ni2.23M0.15 （M=Ni, Cu, Cr） alloys

LaMg8.52Ni2.23M0.15 (M=Ni, Cu, Cr) alloys were prepared by induction melting. X-ray diffraction showed that all the three alloys had a multiphase structure, consisting of La2Mg17, LaMg2Ni and Mg2Ni phases. Energy dispersive X-ray spectrometer results revealed that most of Cu and Cr distributed in Mg2Ni phase. La2Mg17 and LaMg2Ni phases decomposed into MgH2, Mg2NiH4 and LaH3 phases during the hydrogenation process. Hydriding/dehydriding measurements indicated that the reversible hydrogen storage capacities of Mg2Ni phase in LaMg8.52Ni2.23M0.15 (M=Cu, Cr) alloys increased to 1.05 wt.% and 0.97 wt.% from 0.79 wt.% of Mg2Ni phase in LaMg8.52Ni2.38 alloy at 523 K. Partial substitution of Cu and Cr for Ni decreased the onset dehydrogenation temperature of the alloy hydrides and the temperature lowered by 18.20 and 5.50 K, respectively. The improvement in the dehydrogenation property of the alloys was attributed to that Cu and Cr decreased the stability of Mg2NiH4 phase.     

Influence of elemental composition on crystal structure and hydrogen storage performance for Mg-containing alloys

The influence of elemental composition on the crystal structure, hydrogen storage and electrochemical properties for Mg-containing alloys was investigated. As La/Mg ratio decreased, the slight change of Ni content was detected. XRD results indicated that these alloys were composed of LaNi 5 and Mg-containing phases. The lattice parameters of Mg-containing phases decreased. Meanwhile, the mass fraction of Mg-containing phases varied with the change of La/Mg and Ni. The hydrogen storage capacity reached ～1.6 wt.% for La/Mg≧ 3 :1 and decreased to ～0.71 wt.% for La/Mg=1. Two hydrogen absorption processes were observed because of the existence of the multiphases for La/Mg≧ 3 :1. With decreasing La/Mg ratio, the equilibrium pressurerose due to the shrinkage of the lattice parameter. Meanwhile, one hydrogen absorption process was obviously present. The discharge capacity of these as-cast alloys was higher, but the cyclic stability was poor for La/Mg≧ 3 :1 due to the partial amorphisation. It was better for La/Mg≤2 although the discharge capacity was lower. The polarization resistance increased, contrarily the exchange current density decreased with decreasing Mg content.     

Phase structure and hydrogen storage properties of LaMg3.93Ni0.21 alloy

The phase structure and hydrogen storage property of LaMg3.93Ni0.21 alloy were studied.XRD and SEM results exhibited that LaMg3.93Ni0.21 alloy consisted mainly of LaMg3,La2Mg17 and LaMg2Ni phases;after hydriding/dehydriding process,all the three phases transformed,La3H7 phase existed and the actual hydrogen absorption phases were Mg and Mg2Ni phases.Pressure-composition-temperature (P-C-T)measurement showed that the reversible hydrogen storage capacity of LaMg3.93Ni0.21 alloy was 2.63 wt.%,and the absorption time for reaching 90%of the storage capacity was 124 s at 523 K,and it was 1850 s for deabsorbing 90%of the maximum dehydrogen capacity.The hydriding process of LaMg3.93Ni0.21 alloy followed the nucleation and growth mechanisms.The enthalpy and entropy for hydriding and dehydriding reactions of the Mg phase in LaMg3.93Ni0.21 alloy were calculated to be-66.38±1.10 kJ/mol H2,-100.96±1.96 J/(K·mol)H2 and 68.50±3.87 kJ/mol H2,98.28±5.48 J/(K·mol)H2,respectively.A comparison of these data with those of MgH2(-74.50 kJ/mol H2,-132.30 J/K·mol H2)suggested that the hydride of LaMg3.93Ni0.21 alloy was less stable than MgH2.The existence of La hydride and synergetic effect of multiphase led to higher reversible hydrogen storage capacity and better kinetic property at lower temperature for LaMg3.93Ni0.21 alloy.     

Effect of cobalt content on electrochemical performance of La-Mg-Ni system （Ce2Ni7-type） electrode alloys

In order to improve the cyclic stability of La-Mg-Ni system （Ce2Ni7-type） alloy electrode, small amount of Co was added in La0.75Mg0.25Ni3.5 alloy. The effect of Co on electrochemical performance and microstructure of the alloys were investigated in detail. XRD results showed that the alloys had multiphase structure composed of （La, Mg）2Ni7, LaNi5 and small amount of LaNi2 phases. The discharge capacity of the alloys first increased and then decreased with increasing Co content. At a discharge current density of 900 mA/g, the HRD of the alloy electrodes increased from 81.3% （x=0） to 89.2 % （x=0.2）, and then reduced to 87.8 % （x=0.6）. After 60 charge/discharge cycles, the capacity retention rate of the alloys enhanced from 52.67% to 61.32%, and the capacity decay rate of the alloys decreased from 2.60 to 2.05 mAh/g per cycle with increasing Co content. The obtained results by XPS and XRD showed that the fundamental reasons for the capacity decay of the La-Mg-Ni system （Ce2Ni7-type） alloy electrodes were corrosion and oxidation as well as passivation of Mg and Lain alkaline solution.     

Microstructures and electrochemical properties of LaNi3.8-xAlx hydrogen storage alloys

The microstructures and electrochemical properties of LaNi3.8-xAlx (x=0.0,0.1,0.2,0.3 and 0.4) alloys were studied systematically. The microstructures were identified by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The main phases were not changed with the substitution of Ni by Al, but minor phases appeared when x=0.4. With Al content increasing, the cell volume increased and the hydrogen storage capacity increased first and then decreased, and the maximum discharge capacity increased from 209.4 mAh/g (x=0.0) to 285.3 mAh/g (x=0.3) and then decreased to 241.3 mAh/g (x=0.4). Meanwhile, the exchange current density (I0) increased and the diffusion coefficient (D) decreased with the addition of Al.     

Phase structure and electrochemical properties of non-stoichiometric La0.7Ce0.3(Ni3.65Cu0.75Mn0.35Al0.15(Fe0.43B0.57)0.10)x hydrogen storage alloys

Phase structure and electrochemical characteristics of Co-free La0.7Ce0.3(Ni3.65Cu0.75Mn0.35Al0.15(Fe0.43B0.57)0.10)x (0.90≤x≤1.10) al-loys were investigated. When x was 0.90, the alloy was composed of LaNi5, La3Ni13B2 and Ce2Ni7 phases. The Ce2Ni7 phase disappeared, and the abundant of La3Ni13B2 phase decreased when x increased to 0.95. When x was 1.00 or higher the alloys consisted of LaNi5 phase. The lat-tice parameter a and the cell volume V of the LaNi5 phase decreased, and the c/a ratio of the LaNi5 phase increased with x value increasing. Maximum discharge capacity of the alloy electrodes first increased and then decreased with x value increasing from 0.90 to 1.10, and the highest value was obtained when x was 1.00. High-rate dischargeability at the discharge current density of 1200 mA/g increased from 50.7% (x= 0.90) to 64.1% (x=1.10). Both the charge-transfer reaction at the electrode/electrolyte interface and the hydrogen diffusion in the alloy were responsible for the high-rate dischargeability. Cycling capacity retention rate at 100th cycle (S100) gradually increased from 77.3% (x= 0.90) to 84.6% (x=1.10), which resulted from the increase in Ni content and the c/a ratio of the LaNi5 phase with x value increasing.