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.     

Influence of magnesium on electrochemical properties of RE3-xMgx(Ni0.7Co0.2Mn0.1)9（x=0.5-1.25） alloy electrodes

RE3-xMgx(Ni0.7Co0.2Mn0.1)9 (x=0.5-1.25) alloys were prepared by induction melting and the influence of the partial substitution of RE (where RE stands for La-rich mischmetal) by Mg on the hydrogen storage and electrochemical properties of the alloys were investigated systematically. These alloys mainly consisted of three phases, La(Ni,Mn,Co)5 phase, La2Ni7 phase and Mg2Ni phase. The P-C-T isotherms showed that with Mg content increasing in the alloys, the hydrogen storage capacity first increased and reached the maximum capacity of 1.36 wt.% when x=1.0, and then decreased with x increasing further. Electrochemical studies revealed that the discharge capacity reached the maximum value of 380 mAh/g and the alloy electrode presented better cyclic stability when RE/Mg=2. The high rate discharge ability of the alloy electrodes was also improved by the substitution of Mg for RE. The RE2Mg(Ni0.7Co0.2Mn0.1)9 alloy exhibited better hydrogen absorption kinetics (x=1.0).)     

Influence of melt spinning and annealing treatment on structures and hydrogen storage thermodynamic properties of La0.8Pr0.2MgNi3.6Co0.4 alloy

La0.8Pr0.2MgNi3.6Co0.4 alloys were prepared by induction melting,annealing and melt spinning techniques.The influences of annealing treatment and melt spinning on phase structure and hydrogen storage properties were systematically investigated.The results of X-ray diffraction determine that the as-cast and as-spun La0.8Pr0.2MgNi3.6Co0.4 alloys consist of LaMgNi4 and LaNi5 phases,while only LaMgNi4 phase is present in the as-annealed alloy.The scanning electron microscope images illustrate that the grain of the alloy is significantly refined by melt spin ning tech no logy.The gaseous hydrogen storage kinetic and thermodynamic properties were measured by using a Sievert's apparatus at different temperatures.The maximum hydrogen storage capacity of the as-cast,as?spun and as-annealed La0.8Pr0.2MgNi3.6Co0.4 alloy is 1.699,1.637 and 1.535 wt.% at 373 K and 3 MPa,respectively.The annealed alloy has flatter and wider pressure plateaus compared with the as-cast and as-spun alloys,which correspond to the hydrogen absorption and desorption process of LaMgNi4 and corresponding hydride.Furthermore,the enthalpy and entropy changes of LaMgNi4 during hydrogenation at different temperatures were calculated using Van't Hoff methods.     

Hydriding/dehydriding behaviors of La2Mg17-10 wt.%Ni composite prepared by mechanical milling

The structure and hydriding/dehydriding behaviors of La2Mg17-10 wt.%Ni composite prepared by mechanical milling were investigated. Compared with the un-milled sample, the as-milled alloys were ready to be activated and the kinetics of hydrogen absorption was relatively fast even at environmental temperature. The composite milled for 10 h absorbed 3.16 wt.% hydrogen within 100 s at 290 K. The kinetic mechanisms of hydriding/dehydriding reactions were analyzed by using a new model. The results showed that hydrogenation processes for all composites were controlled by hydrogen diffusion and the minimum activation energy was 15.3 kJ/mol H2 for the composite milled for 10 h. Mechanical milling changed the dehydriding reaction rate-controlling step from surface penetration to diffusion and reduced the activation energy from 204.6 to 87.4 kJ/mol H2. The optimum milled duration was 5 h for desorption in our trials.     

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.     

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.     

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.     

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.     

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.     

Microstructures and hydrogen storage properties of as-cast and copper-mould-cast LaMg4Ni alloys

Phase compositions, morphologies and hydrogen storage properties of the as-cast and copper-mould-cast LaMgaNi alloys were studied. The dehydriding onset temperature of the as-cast alloy hydride was about 500 K, which was at least 50 K higher than that of the copper-mould-cast one, and the copper-mould-cast alloy hydride had a faster dehydriding rate compared with as-cast one. Additionally, the copper-mould-cast alloy could uptake 2.85 wt.% hydrogen, which was 95.0% of saturated hydrogen storage capac- ity at room temperature. While only 1.80 wt.% hydrogen （60% of saturated capacity） was absorbed for the as-cast alloy under the same conditions. The reversible hydrogen storage capacities and plateau hydrogen pressures of the two alloys were close. X-ray dif- fractions and scanning electron microscopy results indicated that similar thermodynamic property of the two alloys should be ascribed to the same hydrogen storage phase, Mg and MgzNi. The better hydrogen sorption kinetics of copper-mould-cast alloy should be as- cribed to the more uniform phase distribution compared with that of the as-cast one.     

Effect of carbon coating on electrochemical properties of AB3.5-type La-Y-Ni-based hydrogen storage alloys

The superlattice La-Y-Ni-based hydrogen storage alloys have high discharge capacity and are easy to prepare.However,there is still a gap in commercial applications because of the severe corrosion of the alloys in electrolyte and poor high-rate dischargeability(HRD).Therefore,(LaSmY)(NiMnAl)_3.5 alloy was prepared by magnetic levitation induction melting,and then the alloy was coated with different contents(0.1 wt%-1.0 wt%) of nano-carbons by low-temperature sintering with sucrose as th...     

Continuous preparation of composition-controllable Y–Ni alloy in LiF–YF3–Y2O3 molten salt

The Y–Ni alloy is a primary precursor for the preparation of high-performance La–Y–Ni-based hydrogen storage materials. However, it cannot be produced continuously at low cost, which limits the wide popularization and application of La–Y–Ni-based materials. In this paper, this problem was solved perfectly using electrochemical reduction of Y₂O₃ in the LiF-YF₃ system. It is found that the reversible reduction from Y³⁺ to Y on the W electrode takes only one step, namely a significant soluble–soluble reaction controlled by Y³⁺ diffusion throughout the melt. Four typical signals of square wave voltammetry (SWV) corresponding to different kinds of Y–Ni intermetallic compounds are observed in LiF−YF₃ and LiF–YF₃–Y₂O₃ melts, and reduction potential can become positive with the addition of Y₂O₃, probably because of the formation of more complexes in the melts. Homogeneous Y–Ni alloy samples were produced continuously and prepared via galvanostatic electrolysis by using bargain-price raw material (Y₂O₃) and setting the current density at 10 A/cm² on the nickel electrode, before they were collected into a bottom receiver. A series of analyses including scanning electron microscopy-energy idspersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICP-MS), demonstrate that concentration of yttrium in Y–Ni alloy is adjustable within the wide range of 44 wt% to 72 wt% by fine-tuning the electrolysis temperature (875–1060 °C) in the LiF-YF₃ system to ensure the optimal hydrogen storage performance and economic efficiency of La–Y–Ni-based hydrogen-storage materials.     

Crystallographic and electrochemical characteristics of La0.7Mg0.3Ni5.5? x (Al0.5Mo0.5) x ( x =0 to 0.8) hydrogen storage alloys

The structure, hydrogen storage property, and electrochemical characteristics of the La_0.7Mg_0.3Ni_5.5−x (Al_0.5Mo_0.5) _x (x=0, 0.2, 0.4, 0.6, 0.8) hydrogen storage alloys have been investigated systematically. It has been found by X-ray powder diffraction and Rietveld analysis that the alloys are multiphase and consist of impurity Ni phase and two main crystallographic phases, namely, the La(La, Mg)₂Ni₉ phase and the LaNi₅ phase, and the lattice parameters and the cell volumes of both the La(La, Mg)₂Ni₉ phase and the LaNi₅ phase increase with increasing Al and Mo content in the alloys. The P-C isotherm curves indicated that the hydrogen storage capacity of the alloy first increases and then decreases with increasing x, and the equilibrium pressure decreases with increasing x. The electrochemical measurements show that the maximum discharge capacity first increases from 298.5 (x=0) to 328.3 mAh/g (x=0.6) and then decreases to 304.7 mAh/g (x=0.8). The high rate dischargeability (HRD) of the alloy electrodes increases lineally from 65.4 pct (x=0) to 86.6 pct (x=0.8) at the discharge current density of 1200 mA/g. Moreover, the exchange current density of the alloy electrodes also increases monotonously with increasing x by the linear polarization curves. The hydrogen diffusion coefficient in the alloy bulk, D, increases with increasing Al and Mo content and thus enhances the low-temperature dischargeability (LTD) of the alloy electrodes.     

Hydrogen Storage Performances of REMg11Ni (RE = Sm,Y) Alloys Prepared by Mechanical Milling

This study adopted mechanical milling to prepare Mg-based REMg₁₁Ni (RE = Sm, Y) hydrogen storage alloys. The alloy structures were examined by X-ray diffraction and transmission electron microscopy. The isothermal hydrogenation thermodynamics and kinetics were determined by an automatic Sievert apparatus. The non-isothermal dehydrogenation performance of the alloys was tested by differential scanning calorimetry and thermogravimetry at different heating rates. The results showed a nanocrystalline and amorphous tendency for the alloys. The YMg₁₁Ni alloy exhibited a larger hydrogen absorption capacity, faster hydriding rate, and lower temperature of onset hydrogen desorption than the SmMg₁₁Ni alloy. The hydrogen desorption temperatures of the REMg₁₁Ni (RE = Sm, Y) alloys were 557.6 K and 549.8 K (284.6 °C and 276.8 °C), respectively. The hydrogen desorption property of the RE = Y alloy was found superior to the RE = Sm alloy based on the time required to absorb 3 wt pct H₂, i.e., the time needed by the RE = Y alloy was reduced to 1106, 456, 363, and 180 s, respectively, corresponding to the hydrogen desorption temperatures of 593 K, 613 K, 633 K, and 653 K (320 °C, 340 °C, 360 °C, and 380 °C), compared to 1488, 574, 390, and 192 s for the RE = Sm alloy under identical conditions. The dehydrogenation activation energies were 100.31 and 98.01 kJ/mol for the REMg₁₁Ni (RE = Sm, Y) alloys, respectively, which agreed with those of the RE = Y alloy showing a superior hydrogen desorption property.

     

Impact of La3+ substitution on electrical,magnetic, dielectric and optical properties of Ni0.7Cu0.1Zn0.2LaxFe2–xO4 (0 < x < 0.035) system

The oxalate co-precipitation method was used to synthesize the La³⁺ substituted Ni–Cu–Zn (La–NCZ) nanoferrites having chemical composition Ni_0.7Cu_0.1Zn_0.2La_xFe_2–xO₄ (x = 0, 0.015, 0.025 and 0.035). DC resistivity study of nanoferrites shows both the conducting and semiconducting behaviour. The room temperature DC electrical resistivity of Ni–Cu–Zn (NCZ) nanoferrites decreases, whereas Curie temperature increases with increasing La³⁺ content. In the temperature range of 30–170 °C nanoferrites show p-type semiconducting behavior except x = 0.015; thereafter, they show n-type behaviour. The frequency dispersive initial permeability (μ_i) associated with its real and imaginary (μ′ and μ") parts are attributed to the domain wall movement and magnetic spin resonant. The μ_i, μ′ and μ" of La–NCZ nanoferrites are higher than those of pure NCZ nanoferrite. Dielectric constant (ε′), dielectric loss (ε″) and AC resistivity (ρ_AC) of La–NCZ nanoferrites show normal dielectric behaviour. It is found that ε′ of NCZ nanoferrites decreases with the increasing content of La³⁺ ions. The bandgap energy of La–NCZ nanoferrites is achieved in the range 1.36–1.70 eV confirming the semiconducting nature of materials.     

Effect of scandium and zirconium alloying on microstructure and gaseous hydrogen storage properties of YFe3

RM₃ compounds (R = rare earth metals, M = transition metals) have rarely been studied for gaseous hydrogen storage applications because of unfavorable thermodynamics. In this work, the hydrogen storage properties of a single-phase YFe₃ alloy were improved by non-stoichiometric composition and alloying with Sc and Zr. Only the Y_1.1–ySc_yFe₃ (y = 0.22, 0.33) alloys consist of a single rhombohedral phase. The Sc substitution for Y leads to the reduction in the unit cell volume of the YFe₃ phase, and thus significantly increases the dehydriding equilibrium pressure and decreases the dehydrogenation temperature. The alloy Y_0.77Sc_0.33Fe₃ delivers a decomposition enthalpy change of 33.54 kJ/mol and a lowest dehydrogenation temperature of 135 °C, in comparison with 38.99 kJ/mol and 165 °C for the alloy Y_1.1Fe₃. The Zr substitution causes a similar thermodynamic destabilization effect, but the composition and microstructure of Y–Zr–Fe alloys need to be further optimized.     

Hot deformation behavior and finite element simulation of Mg–8.3Gd–4.4Y–1.5Zn–0.8Mn alloy

To study the hot deformation behavior of Mg–8.3Gd–4.4Y–1.5Zn–0.8Mn (wt%) alloy, hot compression tests were conducted using a Gleeble–3500 thermal simulator at temperatures ranging from 653 to 773 K, true strain rates of 0.001–1 s^?1, and a deformation degree of 60%. Results of hot compression experiments show that the flow stress of the alloy increases with the strain rate. The true stress–true strain curves are corrected by correcting the effect of temperature rise in the deformation process. Activation energy, Q, equal to 287380 J/mol and material constant, n, equal to 4.59 were calculated by fitting the true stress–true strain curves. Then, the constitutive equation was established and verified via finite element simulation. Results of the hot processing map show that the probability of material instability increases with the degree of deformation, which indicates that the material is not suitable for large deformation in a single pass. On the whole, the alloy is appropriate for multipass processing with small deformation and a suitable processing temperature and strain rate are 733 K and 0.01 s^?1, respectively.     

Effect of Compositional Variation on the Microstructural Evolution and the Castability of Al–Mg–Si Ternary Alloys

This study examined the microstructural evolution and castability of Al–Mg–Si ternary alloys with varying Si contents. Al–6Mg–xSi alloys (where x = 0, 1, 3, 5, and 7; all compositions in mass pct) were examined, with Al–6 mass pct Mg as a base alloy. The results showed that in the ternary alloys with Si ≤ 3 pct, the solidification process ended with the formation of eutectic α-Al–Mg₂Si phases generated by a univariant reaction. However, in the case of ternary alloys with Si > 3 pct, solidification was completed with the formation of α-Al–Mg₂Si–Si ternary eutectic phases generated by a three-phase invariant reaction. In addition to the eutectic Mg₂Si phases, the primary Mg₂Si phases formed in each of the ternary alloys, and the size of both sets of phases increased with increasing Si content. The two-phase eutectic α-Al–Mg₂Si nucleated from the primary Mg₂Si phases. The inoculated Al–6Mg–1Si alloy had the smallest grain size. Moreover, the grain-refining efficacy of the Al–5Ti–B master alloy in the ternary alloys decreased with increasing Si content in the alloys. Despite the poisoning effect of Si on the potency of TiB₂ compounds in the inoculated Al–6Mg–1Si alloy, the grain size of the alloy was slightly smaller than that of the Al–6Mg binary alloy. This resulted from the increasing growth restriction factor (induced by Si addition) of the Al–6Mg–1Si alloy. In terms of the castability, the examined alloys showed different levels of susceptibility to hot tearing. Among the alloys, the ternary Al–6Mg–5Si alloy exhibited the highest susceptibility to hot tearing, whereas the Al–6Mg–7Si exhibited the lowest. The severity of hot tearing initiated by the unraveling of the bifilm was determined by the freezing range, grain size, and the amount of eutectic phases at the end of the solidification process.

     

Electrochemical formation of Mg–La–Mn alloys by coreduction of Mg(II), La(III) and Mn(II) in LiCl+KCl molten salts

In the present work, cyclic voltammetry (CV), square wave voltammetry (SWV) and chronopotentiometry (CP) were used to investigate the electrochemical coreduction behaviors of La(III) and Mg(II) as well as La(III), Mg(II) and Mn(II) on Mo electrode in LiCl + KCl molten salts. CV and SWV results exhibit that the coreduction mechanism of La (III) and Mg(II) on Mo electrode is that La(III) is reduced and Mg–La intermetallic compound is formed, leading to the deposition potential of La(III) shifting to more positive one. The electrochemical signals pertaining to the formation of metallic La, Mg and Mn as well as Mg–La intermetallic compound are also observed by coreduction of La(III), Mg(II), and Mn(II) in LiCl + KCl molten salt on the inert Mo electrode. However, the electrochemical signals associated with the formation of La–Mn and Mg–Mn alloys are not observed, which means that the depolarization effect of La(III) and Mg(II) does not occur on pre-deposited Mn electrode. The Mg–La–Mn alloys were formed by co-deposition of La(III), Mg(II), and Mn(II) on Mo electrode at various concentration ratios of La(III) and Mg(II). The results of scanning electron microscopy equipped with energy dispersive spectroscopy and X-ray diffraction displays that the Mg–La–Mn alloys are comprised of La–Mg compound Mg₁₇La₂, Mg and Mn phases. The concentration ratio of La(III) and Mn(II) has few effects on the alloy composition.