Effect of rare earth elements on electrochemical properties of La–Mg–Ni-based hydrogen storage alloys

La_0.60R_0.20Mg_0.20(NiCoMnAl)_3.5 (R = La, Ce, Pr, Nd) alloys were prepared by inductive melting. Variations in phase structure and electrochemical properties due to partial replacement of La by Ce, Pr and Nd, were investigated. The alloys consist mainly of LaNi₅ phase, La₂Ni₇ phase and LaNi₃ phase as explored by XRD and SEM. The maximum discharge capacity decreases with Ce, Pr and Nd substitution for La. However, the cycling stability is improved by substituting Pr and Nd at La sites, capacity retention rate at the 100th cycle increases by 13.4% for the Nd-substituted alloy. The electrochemical kinetics measurements show that Ce and Pr substitution improves kinetics and thus ameliorates the high rate dischargeability (HRD) and low temperature dischargeability. The HRD at 1200 mA g⁻¹ increases from 22.1% to 61.3% and the capacity at 233 K mounts up from 90 mAh g⁻¹ to 220 mAh g⁻¹ for the Ce-substituted alloy.     

Electrochemical performance studies on cobalt and nickel electroplated La–Mg–Ni-based hydrogen storage alloys

A novel electroplating treatment was applied onto La–Mg–Ni-based La_0.88Mg_0.12Ni_2.95Mn_0.10Co_0.55Al_0.10 alloy powders. The effect of cobalt or nickel metallic coating on morphological and electrochemical properties was studied. FESEM results showed that a dense layer of spherical cobalt particles with uniform radius and an undulate layer of lamellar nickel formed on the surface of the Co- and Ni-coated alloys, respectively. These coatings enhanced the conductivity and the catalytic activity, besides acting as a protective layer, thereby improving the electrochemical properties. The maximum discharge capacity increased from original 316 mAh/g to 335 mAh/g on Co-coated alloys and 336 mAh/g on Ni-coated ones, the cycling stability was enhanced and the self-discharge was suppressed. The high rate dischargeability (HRD) was ameliorated remarkably, and the HRD value at 1500 mA/g rose by 10% and 17%, for cobalt- and nickel-coated alloy electrodes respectively, which is believed to be ascribed to the improved kinetics from the metallic coatings on the surface.     

Hydrogenation and electrochemical studies of La–Mg–Ni alloys

La–Mg–Ni alloys are potential candidates for hydrogen storage materials. In this study, mechanical alloying with subsequent annealing under an argon atmosphere at 973 K for 0.5 h, were used to produce La_2-xMg_xNi₇ alloys (x = 0, 0.25, 0.5, 0.75, 1). Shaker type ball mill was used. An objective of the present study was to investigate an influence of amount of Mg in alloy on electrochemical, hydrogenation and dehydrogenation properties of La–Mg–Ni materials. X-ray diffraction analyses revealed formation of material with multi-phase structure. Obtained materials were studied by a conventional Sievert's type device at 303 K. It was observed that electrochemical discharge capacity and gaseous hydrogen storage capacity of La–Mg–Ni alloys increases with Mg content to reach maximum for La_1.5Mg_0.5Ni₇ alloy. Moreover, all of La–Mg–Ni alloys were characterized by improved hydrogen sorption kinetics in comparison to La–Ni alloy.     

Electrochemical hydrogen storage performance of Mg–Ti–Zr–Ni alloys

Mg_1.5Ti_0.5−xZr_xNi (x = 0, 0.1, 0.2, 0.3, 0.4), Mg_1.5Ti_0.3Zr_0.1Pd_0.1Ni and Mg_1.5Ti_0.3Zr_0.1Co_0.1Ni alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. X-ray diffraction studies showed that all the replacement elements (Ti, Zr, Pd and Co) perfectly dissolved in the amorphous phase and Zr facilitated the amorphization of the alloys. When the Zr/Ti ratio was kept at 1/4 (Mg_1.5Ti_0.4Zr_0.1Ni alloy), the initial discharge capacity of the alloy increased slightly at all the ball milling durations. The further increase in the Zr/Ti ratio resulted in reduction in the initial discharge capacity of the alloys. The presence of Zr in the Ti-including Mg-based alloys improved the cyclic stability of the alloys. This action of Zr was attributed to the less stable and more porous characteristics of the barrier hydroxide layer in the presence of Zr due to the selective dissolution of the disseminated Zr-oxides throughout the hydroxide layer on the alloy surface. Unlike Co, the addition of Pd into the Mg–Ti–Zr–Ni type alloy improved the alloy performance significantly. The positive contribution of Pd was assumed to arise from the facilitated hydrogen diffusion on the electrode surface in the presence of Pd. As the Zr/Ti atomic ratio increased, the charge transfer resistance of the alloy decreased at all the depths of discharges. Co and Pd were observed to increase the charge transfer resistance of the Mg–Ti–Zr–Ni alloys slightly.     

Effect of particle size on the performance of rare earth–Mg–Ni-based hydrogen storage alloy electrode

Rare earth–Mg–Ni-based hydrogen storage alloy has been synthesized by vacuum induction levitation melting and sieved into five particle size fractions from 120 mesh to below 800 mesh. The effect of particle size on the electrochemical behaviors has been investigated. It was found that the alloy electrode with the particle size of 220–325 mesh exhibited better cyclic stability and high rate dischargeability than the larger or smaller alloy powders. The pulverization and the surface oxidation/corrosion have been studied by SEM, AES, and XPS methods. The results showed that the pulverization rate became faster with the increase of the particle size. The formation of an oxide layer with proper thickness during cycling can effectively improve the cyclic stability for the 220–325 mesh alloy electrode. The capacity degradation and the electrochemical kinetics of the alloy electrodes of different particle sizes are determined by the pulverization rate and the oxidation of active components on the alloy surface during cycling in the alkaline electrolyte.     

Structures and properties of Mg–La–Ni ternary hydrogen storage alloys by microwave-assisted activation synthesis

It is a challenge to prepare a material meeting two conflicting criteria – absorbing hydrogen strongly enough to reach a stable thermodynamic state and desorbing hydrogen at moderate temperature with a fast reaction rate. With the guide of the Mg–La–Ni phase diagram, microwave sintering (MS) was successfully applied to preparing Mg–La–Ni ternary hydrogen storage alloys from the powder mixture of Mg, La and Ni. Their phase structures, morphologies and hydrogen absorption and desorption (A/D) properties have been studied by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), pressure-composition-isotherm (PCI) and differential scanning calorimetry (DSC). The metal hydride of 70 Mg–9.72 La–20.28 Ni (wt pct) has the best comprehensive hydriding and dehydriding (H/D) properties, which can absorb 4.1 wt.% H₂ in 600 s and desorb 3.9 wt.% H₂ in 1500 s at 573 K. The DSC results reveal its onset temperatures of hydrogen A/D are the lowest among all the samples, which are 671.4 and 600.9 K. Its activation energy of dehydriding reaction is 113.5 kJ/mol H₂, which is the smallest among all the samples. Also, Chou model was used to analyze the reaction kinetic mechanism.     

Hydrogen storage thermodynamics and kinetics of RE–Mg–Ni-based alloys prepared by mechanical milling

The nanocrystalline/amorphous NdMg₁₁Ni + x wt.% Ni (x = 100, 200) composite hydrogen storage alloys were synthesized by ball milling, and the effects of Ni content and milling time on the hydrogen storage thermodynamics and dynamics of the alloys were systematically investigated. The results reveal that the variation of the Ni content has a slight effect on the thermodynamic properties of the alloys, but it significantly improves their absorption and desorption kinetics performance. The variation of the milling time clearly affects the hydrogen storage properties of the alloys. Hydrogen absorption capacity and hydrogen absorption saturation ratio have maximum values with milling time varying. But hydrogen desorption ratio always increases with milling time prolonging. It is found that the hydrogen desorption activation energy of the alloys clearly decreases with increasing Ni content and milling time, which is responsible for the improved hydrogen desorption kinetics of the alloys.     

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.     

Effect of Y element on cyclic stability of A2B7 -type La–Y–Ni-based hydrogen storage alloy

The La_3-xY_xNi_9.7Mn_0.5Al_0.3 (x = 1, 1.5, 1.75, 2, 2.25, 2.5) alloys were prepared by magnetic suspension induction melting method and annealed at 1273 K for 24 h. The alloys were tested using electrochemical measurements, X-ray diffraction (XRD) and scanning electron microscope with energy-dispersive X-ray diffraction spectroscope (SEM-EDS). With the increase of Y content, the main phase of the alloys changed from Gd₂Co₇ phase to Ce₂Ni₇ phase, and Ce₂Ni₇ phase increased gradually. The maximum discharge capacity of alloys increased from 279.3 mA h/g (x = 1) to 383.8 mA h/g (x = 2.5). The high-rate dischargeabilitiy at the discharge current density of 1200 mA/g increased from 56.98% (x = 1) to 83.76% (x = 2.5). The maximum capacity retention rate first increased from 50.13% (x = 1) to 69.43% (x = 2), and then decreased to 21.35% (x = 2.5). The results showed that the structural stability of the alloys was improved due to the increase of Y content. However, with the increase of Y content, the corrosion, pulverization, and the dissolution of Al element aggravated, which deteriorated the cyclic stability of the alloy electrodes.     

Effect of crystal transformation on electrochemical characteristics of La–Mg–Ni-based alloys with A2B7-type super-stacking structures

La_0.75Mg_0.25Ni_3.5 alloys with hexagonal (2H-) and rhombohedral (3R-) (La,Mg)₂Ni₇ phase were created by powder metallurgy. Partial crystal transformation of 2H- into 3R-type allotropes was realized by heat treatment and introducing LaNi₅ compound. It was found that the alloy annealed within 1073–1223 K kept (La,Mg)₂Ni₇ phase and obvious crystal transformation of 2H- into 3R-type occurred as annealing temperature reached 1223 K. Electrochemical study showed similar discharge capacity and degradation behavior for La_0.75Mg_0.25Ni_3.5 alloys with different amounts of 2H- and 3R-type allotropes while HRD was promoted by increasing 3R-type phase abundance. Introducing LaNi₅ into La_0.75Mg_0.25Ni_3.5 alloy increased 3R- to 2H-type phase ratio and led to an additional plateau in P–C isotherms. LaNi₅ introduction improved HRD, however it accelerated cycling degradation. Rietveld analysis indicated that after hydrogenation, the cell expansion of 2H- and 3R-type (La,Mg)₂Ni₇ phase was similar while the cell expansion of LaNi₅ phase was smaller than that of (La,Mg)₂Ni₇ phase. This caused discrete cell expansion between (La,Mg)₂Ni₇ and LaNi₅ phases, leading to severe pulverization and oxidation.     

Properties of mechanically alloyed Mg–Ni–Ti ternary hydrogen storage alloys for Ni-MH batteries

MgNiTi_x, Mg_1−xTi_xNi and MgNi_1−xTi_x (with x varying from 0 to 0.5) alloys have been prepared by high energy ball milling and tested as hydrogen storage electrodes. The initial discharge capacities of the Mg–Ni–Ti ternary alloys are inferior to the MgNi electrode capacity. However, an exception is observed with MgNi_0.95Ti_0.05, which has an initial discharge capacity of 575 mAh/g compared to 522 mAh/g for the MgNi electrode. The Mg–Ni-Ti ternary alloys show improved cycle life compared to Mg–Ni binary alloys with the same Mg/Ni atomic ratio. The best cycle life is observed with Mg_0.5Ti_0.5Ni electrode which retains 75% of initial capacity after 10 cycles in comparison to 39% for MgNi electrodes, in addition to improved high-rate dischargeability (HRD). According to the XPS analysis, the cycle life improvement of the Mg_0.5Ti_0.5Ni electrode can be related to the formation of TiO₂ which limits Mg(OH)₂ formation. The anodic polarization curve of Mg_0.5Ti_0.5Ni electrode shows that the current related to the active/passive transition is much less important and that the passive region is more extended than for the MgNi electrode but the corrosion of the electrode is still significant. This suggests that the cycle life improvement would be also associated with a decrease of the particle pulverization upon cycling.     

Elaboration and electrochemical characterization of Mg–Ni hydrogen storage alloy electrodes for Ni/MH batteries

Mg–Ni hydrogen storage alloy electrodes with composition of Mg–33, 50, 67 Ni at. % in amorphous phase were prepared by means of mechanical alloying (MA) process using a planetary ball mill. The electrochemical hydrogen storage characteristics and mechanisms of these electrodes were investigated by electrochemical measurements, X–ray diffraction (XRD) and scanning electron microscope (SEM) analyses. The relationship between alloy composition and electrochemical properties was evaluated. In addition, optimum milling time and composition of Mg–Ni hydrogen storage alloy with acceptable electrochemical performance were determined. XRD results show that the alloys exhibit dominatingly amorphous structures after milling of 20 h. The electrochemical measurements revealed that the discharge capacity of Mg₃₃Ni₆₇ and Mg₆₇Ni₃₃ alloy electrodes reached a maximum when alloys were prepared after 20 h of milling time (260 and 381 mAhg^?1, respectively). The maximum discharge capacity of Mg₅₀Ni₅₀ alloy was observable after 40 h milling (525 mAhg^?1). It was also found that the cyclic stability of the alloys increased with increasing Ni content. Among these alloys, the amorphous Mg₅₀Ni₅₀ alloy presents the best overall electrochemical performance. In this paper, electrode process kinetics of Mg₅₀Ni₅₀ alloy electrode was also studied by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. The impedance spectra of electrodes were measured at different depths of discharge (DODs). The observed spectra were fit well with the equivalent circuit model used in the paper. The electrochemical parameters calculated from electrochemical impedance were also compared. The electrochemical discharge and cyclic performance of 20, 40 and 60 h milled Mg₅₀Ni₅₀ alloy electrodes were demonstrated by the fitted charge transfer resistance and Warburg impedance obtained at various DODs. It was further observed that the controlling-step of the discharge process changed from a mixed rate-determining process at lower DODs to a mass-transfer controlled process at higher DODs. The fitted results demonstrated that charge–transfer resistance (R_ct) increased with DOD. The R_ct of 40 h milled Mg₅₀Ni₅₀ alloy (29.27 Ω) was lower than that of 20 h (41.89 Ω) and 60 h milled alloys (92.43 Ω) at fully discharge state.     

Effect of Al content on the structural and electrochemical properties of A2B7 type La–Y–Ni based hydrogen storage alloy

LaY_1.9Ni_10.2−xAl_xMn_0.5 (x = 0–0.6) hydrogen storage alloys have been prepared using a vacuum induction-quenching furnace and annealed at 1148 K for 16 h. The alloys are composed of Ce₂Ni₇- and Gd₂Co₇-type phases and an extra Pr₅Co₁₉-type phase appears when x = 0.6. Aluminum tends to enter the inner AB₅ slabs of Ce₂Ni₇- and Gd₂Co₇-type phases and promotes the generation of new AB₅ slabs. The maximum discharge capacity of the alloy electrodes is stable at approximate 375 mA h/g as x increases from 0 to 0.4 and then decreases to 364.2 mA h/g (x = 0.6). The cycling capacity retention rate at the 300^th cycle is 59.4%, 62.0%, 62.7% and 58.7% for x = 0, 0.2, 0.4 and 0.6, respectively, indicating that the function of aluminum on improving the cyclic stability of the alloy electrodes is limited. The main reason is that the similar pulverization degrees of the alloys are presented during the charge/discharge cycles.     

The effect of Mn substitution for Ni on the structural and electrochemical properties of La0.7Mg0.3Ni2.55−xCo0.45Mnx hydrogen storage electrode alloys

The structures, hydrogen storage property and electrochemical properties of the $\text{La}{}_{0.7}\text{Mg}{}_{0.3}\text{Ni}{}_{\mathrm{2.55-x}}\text{Co}{}_{0.45}\text{Mn}{}_{\mathrm{x}}\text{(x=0.0,0.1,0.2,0.3,0.4,0.5)}$ electrode alloys has been studied systematically. It can be found that, by X-ray powder diffraction, the alloys are all consisted of the (La,Mg)Ni₃ phase and the LaNi₅ phase, and the lattice parameters and cell volumes of both the (La,Mg)Ni₃ phase and the LaNi₅ phase increase with increasing Mn content in alloys. The P–C isotherms curves indicate that the hydrogen storage capacity first increases and then decreases with increasing x, and the equilibrium pressure decreases. The electrochemical measurements show that the maximum discharge capacity increases from $\text{342.6}\text{(x=0.0)}$ to $\text{368.9}\text{mA}\text{h}\text{/}\text{g}\text{(x=0.3)}$ and then decreases to $\text{333.5}\text{mA}\text{h}\text{/}\text{g}\text{(x=0.5)}$. For the discharge current density of $\text{1000}\text{mA}\text{/}\text{g}$, the high rate dischargeability (HRD) of the alloy electrodes increases from 55.8% (x=0.0) to 72.3% (x=0.4) and then decreases to 70.0% (x=0.5). Moreover, according to the electrochemical impedance spectroscopy, linear polarization and anodic polarization measurements, the exchange current density I₀ and the limiting current density I_L of the alloy electrodes also all increase first and then decrease with increasing Mn content in alloys.     

Effect of graphite (GR) content on electrochemical hydrogen storage performances of nanocrystalline and amorphous La9Ce1Mg80Ni5–Ni–GR composites synthesized by mechanical milling

The Mg–Ni-based alloy La₉Ce₁Mg₈₀Ni₅ was fabricated by a vacuum induction furnace with high purity helium gas. The surface modification of the as-cast alloys was operated by mechanical coating Ni and graphite (GR). The composites La₉Ce₁Mg₈₀Ni₅-200 wt% Ni-x wt.% GR (x = 0–4) with nanocrystalline and amorphous structures were synthesized by mechanical milling. Adding appropriate GR brings on the enhancement of ball-milling efficiency and inhibits the agglomeration of alloy powders. Furthermore, the discharge capacity of the composites obtains the maximal values with an optimized GR percentage. Increasing GR content from 0 to 4, the capacity retention rate at 20th cycle (S₂₀ = C₂₀/C_max) of the 20 h milled composite improves from 72.2% to 74.3% and that of the 80 h milled specimen changes from 54.4% to 56.9%. Electrochemical tests indicate that with the optimization of GR percentage, the composites can get the best electrochemical kinetic property, such as the highest HRD value, the highest hydrogen diffusion coefficient and the lowest charge transfer resistance.     

Superior electrochemical hydrogen storage properties of binary Mg–Y thin films

Mg–Y thin films capped with Pd have been prepared by direct current magnetron co-sputtering system. It is found that Mg alloyed with Y in film state forms ultrafine nanocrystalline intermetallic compounds. The structure together with the catalytic effect of Y gives rise to a high electrochemical hydrogen storage capacities and superior activation properties. It is worthy to note that Mg₇₈Y₂₂ film achieves a high discharge capacity of 1590 mAh g⁻¹ without requiring activation process. Moreover, Mg alloyed with Y effectively improves the cyclic stability of Mg-based films ascribing to the anti-corrosion role of Y. For Mg₃₇Y₆₃ film, more than 92％ of the maximum discharge capacity can be maintained after 100 charge–discharge cycles.     

Structure and electrochemical hydrogen storage properties of A2B-type Ti–Zr–Ni alloys

Elemental substitution of part Ti by Zr has been carried out for Ti₂Ni alloy to form Ti_2−xZr_xNi (x = 0, 0.2, 0.4) alloys. Mechanical milling and subsequent heat treatment have been used to prepare non-equilibrium Ti–Zr–Ni alloys. The effects of Zr addition on the structure and discharge properties of Ti₂Ni alloy were investigated. The addition of Zr could enhance the discharge capacity of the non-equilibrium Ti₂Ni alloy at electrolyte temperatures of 313 and 333 K. For instance, the non-equlibrium Ti_1.6Zr_0.4Ni alloy had a stable discharge capacity of about 210 mAh/g at 313 K. However, the protective surface layer formed during heat treatment was destroyed at a high electrolyte temperature of 333 K, and thus a severe capacity loss during cycling.     

Hydrogen absorption and desorption kinetics of Ag–Mg–Ni alloys

The hydrogen absorption/desorption (A/D) kinetics of hydrogen storage alloys Mg_2−xAg_xNi (x=0.05, 0.1) prepared by hydriding combustion synthesis in two-phase (α–β) region in the temperature range of 523–$\text{573}\text{K}$ have been investigated. The hydriding/dehydriding (H/D) reaction rate constants were extracted from the time-dependent A/D curves. The obtained hydrogen A/D kinetic curves were fitted using various rate equations to reveal the mechanism of the H/D processes. The relationships of rate constant with temperature were established. It was found that the three-dimensional diffusion process dominates the hydrogen A/D. The apparent activation energies of 63±5 and $\text{61±7}\text{kJ/mol}\text{H}{}_2$ in Mg_1.95Ag_0.05Ni alloy and $\text{52±2}\text{kJ/mol}\text{H}{}_2$ and of $\text{50±2}\text{kJ/mol}\text{H}{}_2$ in Mg_1.9Ag_0.1Ni alloy were found for the H/D processes in two-phase (α–β) coexistence region from 523 to $\text{573}\text{K}$, respectively. With the increasing content of Ag in Ag–Mg–Ni alloys, the apparent energy was decreased and the reaction rate was faster. It is reasonable to explain that the hydriding kinetics of Mg₂Ni was improved by adding Ag.     

New La–Fe–B ternary system hydrogen storage alloys

The new La₈Fe₂₈B₂₄-, La₁₅Fe₇₇B₈- and La₁₇Fe₇₆B₇-type alloys have multiphase structures including LaNi₅, La₃Ni₁₃B₂ and (Fe, Ni) phases. The amount of La₃Ni₁₃B₂ phase increased and that of (Fe, Ni) phase decreased with an increasing La/(Fe + B) atomic ratio. The measurement of P–C–I curves revealed that the maximum hydrogen capacity exceeded 1.12 wt% at 313 K in the pressure range of 10⁻³ MPa–2.0 MPa. The alloys exhibited good absorption/desorption kinetics at room temperature, and electrochemical experiments showed that all of the alloy electrodes exhibited good activation characteristics, high-rate dischargeability (HRD) and low-temperature (233 K) dischargeability (LTD).