Influence of rare earth content on Mm-based AB5 metal hydride alloys for Ni-MH batteries—An X-ray fluorescence study

AB₅-type MH alloys with Mm (Misch metal) as the A part (with varied rare earth contents in Mm) were investigated for rare earth by XRF analysis and battery performance by life cycle tests with an objective of understanding the influence of rare earth content on electrochemical hydrogen storage. The La/Ce ratio was found to vary from 0.51 to 18.73. The capacity output varied between 179 and 266 mAh g⁻¹. The results show that the La/Ce ratio has a strong influence on the performance, with the best performance realized with samples having an La/Ce ratio of around 12. La enhancement facilitates easy activation due to refinement in grain size and interstitial dimensions. Also, an orderly influence on crystalline structure could be seen. The study demonstrates that the rare earth content is an essential factor in determining the maximum capacity output because of its influence on crystal orientation as well as an increase in the radius of the interstitials, lattice constants and cell volumes.     

Theoretical model on the electronic properties of multi-element AB5-type metal hydride

Nowadays, multi-element alloys are preferred over binary alloys for application point of view. The hydrogenation properties strongly depend on the thermodynamic, structural and electronic properties of the alloys. At present, no model is available which can predict the hydrogen storage properties of the multi-element alloy, before actual synthesis of the alloy. In the present investigation, efforts are made to develop a theoretical mathematical model to predict the hydrogenation properties of multi-element AB₅-type metal hydride. The present investigation deals with the various electronic parameters which may affect the hydrogenation characteristics of the metal hydride. Based on all such parameters, an electronic factor has been proposed for AB₅-type alloys. Electronic factor has been combined with the structural and thermodynamical factor to propose a new combined factor, which was further correlated with the hydrogen storage capacity of the alloy. Atomic radius and electronic configuration of substituted elements in the multi-element AB₅-type hydrogen storage alloy have been found as key players to predict the hydrogenation properties of the alloys before synthesis. It has been shown that in the case of alloy series with multiple substitutions, the combined factor is more relevant in deciding the hydrogen storage capacity in comparison to electronic factor alone. Combined factor is directly proportional to the hydrogen storage capacity. All the three factors thermodynamic, structural and electronic together may lead to the prediction of pressure-composition isotherm of the multi-element AB₅-type hydrogen storage alloy.     

The structure, hydrogen storage, and electrochemical properties of Fe-doped C14-predominating AB2 metal hydride alloys

A series of Fe-substituting cobalt C14-predoninating AB₂ alloys with the general formula Ti₁₂Zr_21.5V₁₀Cr_7.5Mn_8.1Fe_xCo_8−xNi_32.2Sn_0.3Al_0.4 (x = 0-5) were studied for the impacts of Fe to structure, gaseous, and electrochemical hydrogen storage properties. All alloys exhibit hyper-stoichiometric C14 main phase due to the formation of A-rich non-Laves secondary phases and the loss of Zr and Ti in the melt. Lattice parameters together with the unit cell volume increases and then decreases with increasing Fe-content which indicates the existence of anti-site defects. The amount of TiNi secondary phase increases with the increase of Fe-content up to 4% and shows a detrimental effect to the high-rate dischargeability of the alloys. Most of the gaseous storage characteristics remain unchanged with the addition of Fe. In the electrochemical properties, Fe-addition in the AB₂ alloys facilitates activation, increases the total electrochemical capacity and effective surface reaction area, decreases the half-cell high-rate dischargeability and bulk hydrogen diffusion, and deteriorates both −10 and −40 °C low-temperature performance. Fe-substituting Co in AB₂ alloys as negative electrode of nickel metal hydride battery can reduce the raw material cost with the trade-off being mainly in the low-temperature performance.     

An investigation on electrochemical and gaseous hydrogen storage performances of as-cast La1−xPrxMgNi3.6Co0.4 (x = 0–0.4) alloys

The as-cast RE–Mg–Ni-based AB₂-type La_1−xPr_xMgNi_3.6Co_0.4 (x = 0–0.4) alloys were prepared by vacuum induction furnace with a high purity helium gas as the protective atmosphere. The phase composition and microstructure of the as-cast alloys were characterized by XRD, SEM equipped with EDS. The results indicate that the as-cast alloys consist of two phases of LaMgNi₄ and LaNi₅. The measurements of the electrochemical properties show that the discharge capacity of the alloys slightly decreases with Pr content rising. As the Pr content grows from 0 to 0.4, the maximum discharge capacity decreases from 347.0 to 310.4 mAh/g. However, the cycle stability and the high-rate dischargeability of the alloy obviously augment with the Pr content increasing. Furthermore, the measurements of the electrochemical hydrogen storage kinetics reveal that the limiting current density (I_L) first increases then decreases whereas the exchange current density I₀ of the alloys first decreases then increases with the rising amount of Pr substitution, which indicates that the electrochemical dynamic of the alloy electrode are jointly dominated by the charge-transfer resistance and diffusion ability of hydrogen atoms. The measuring of the gaseous hydrogen storage reveals two pressure plateaus appear on each pressure–concentration–isotherm (P–C–T) curve of the as-cast alloys, which correspond to the LaMgNi₄ and LaNi₅ phases. Furthermore, we note that the pressure plateau of the P–C–T curve visibly rises with Pr content increasing.     

Effect of molybdenum content on structural, gaseous storage, and electrochemical properties of C14-predominant AB2 metal hydride alloys

The structure, gaseous storage, and electrochemical properties of Mo-modified C14-predominant AB₂ metal hydride alloys were studied. The addition of Mo expands the unit cell volume and stabilizes the metal hydride. This increased metal-to-hydrogen bond strength reduces the equilibrium plateau pressure, reversible hydrogen storage, and the high-rate dischargeability in the flooded cell configuration, but not the high-rate dischargeability in the sealed cell configuration. The low-temperature performance was improved by the addition of Mo through increases in bulk diffusion rate, surface area, and surface catalytic ability. The increase in bulk diffusion is the result of smaller crystallites and larger AB₂-AB₂ grain boundary densities. The increase in surface area is due to the high solubility of Mo in alkaline solution. Even with a higher leaching rate, the Mo-containing alloys still have strong corrosion resistance which contributes positively to both the charge retention and the cycle life performances. As the Mo-content in the alloy increases, the low temperature performance improves at the expense of a lower capacity.     

Gaseous phase hydrogen storage and electrochemical properties of Zr8Ni21, Zr7Ni10, Zr9Ni11, and ZrNi metal hydride alloys

A systematic study of four important non-Laves phase alloys, Zr₈Ni₂₁, Zr₇Ni₁₀, Zr₉Ni₁₁, and ZrNi, commonly seen in the Zr-based AB₂ metal hydride alloys was presented. In order to investigate the synergetic effect between the major and secondary phases, an annealing treatment was used to change the abundances of various phases in the alloys. The structure, gaseous phase hydrogen storage, and electrochemical properties were obtained for each of the four alloy compositions before and after annealing, and the correlations among these properties were explored. Annealing generally suppressed secondary phases except for the case of Zr₉Ni₁₁, where its secondary ZrNi phase abundance increased. As the Zr/Ni ratio in the average composition increased, the maximum gaseous phase hydrogen storage capacity increased but maximized at Zr:Ni = 9:11. Comparing the properties before and after annealing, it was established that the change in phase distribution influenced the gaseous phase storage. Through the electrochemical measurements, it was found that the highest full discharge capacity was obtained at Zr:Ni = 7:10, which represents a compromise between the hydrogen desorption/discharge rate and the theoretical maximum gaseous phase hydrogen storage. As the Zr/Ni ratio increased, the high-rate dischargeability decreased, which followed the trend of the amount of metallic Ni in the surface oxide determined by magnetic susceptibility measurement. The synergetic effect was observed in the electrochemical environment by comparing the results before and after annealing. In general, annealing deteriorated and improved the electrochemical discharge capacity and high-rate dischargeability, respectively, due to the reduction in secondary phase abundance and consequent synergetic effect. Among all alloys investigated, the unannealed Zr₇Ni₁₀ demonstrated the best overall gaseous phase hydrogen storage and electrochemical capacity and could be considered as a candidate to replace the AB₅ and AB₂ metal hydride alloys in Ni/MH battery applications. Furthermore, the unannealed Zr₈Ni₂₁ showed a good balance between high-rate dischargeability and ease of formation.     

Machine learning analysis of alloying element effects on hydrogen storage properties of AB2 metal hydrides

Zirconium-titanium-based AB₂ is a potential candidate for hydrogen storage alloys and NiMH battery electrodes. Machine learning (ML) has been used to discover and optimize the properties of energy-related materials, including hydrogen storage alloys. This study used ML approaches to analyze the AB₂ metal hydrides dataset. The AB₂ alloy is considered promising owing to its slightly high hydrogen density and commerciality. This study investigates the effect of the alloying elements on the hydrogen storage properties of the AB₂ alloys, i.e., the heat of formation (ΔH), phase abundance, and hydrogen capacity. ML analysis was performed on the 314 pairs collected and data curated from the literature published during 1998–2019, comprising the chemical compositions of alloys and their hydrogen storage properties. The random forest model excellently predicts all hydrogen storage properties for the dataset. Ni provided the most contribution to the change in the enthalpy of the hydride formation but reduced the hydrogen content. Other elements, such as Cr, contribute strongly to the formation of the C14-type Laves phase. Mn significantly affects the hydrogen storage capacity. This study is expected to guide further experimental work to optimize the phase structure of AB₂ and its hydrogen sorption properties.     

Studies of off-stoichiometric AB2 metal hydride alloy: Part 2. Hydrogen storage and electrochemical properties

In Part 2 of this two-part series of papers, gaseous hydrogen storage and electrochemical properties of three series of alloys with different combinations of Cr/Mn/Co ratios are studied and compared to the structural properties reported in Part 1. As the B/A stoichiometry in each series of alloys increases from 1.8 to 2.2, systematic trends in certain storage properties are found: the hydrogen dissociation pressure and heat of hydride formation increases; the alloy with a AB_2.0 stoichiometry has the highest electrochemical full capacity; and slightly higher and lower B-contents increase the electrochemical high-rate-dischargeability and gaseous phase maximum storage capacity, respectively. Stoichiometric or slightly hyper-stoichiometric AB₂ alloys have lower PCT hysteresis which are expected to reduce pulverization during cycling. The full and high-rate discharge electrochemical capacities correlate well with the maximum and reversible gaseous hydrogen storages, respectively. Slight hyper-stoichiometry increases the high-rate dischargeability. Open circuit voltage, an important parameter in high-power application, is also found to be more relevant to the surface reaction than to the bulk hydride stability.     

Improvement of substituting La with Ce on hydrogen storage thermodynamics and kinetics of Mg-based alloys

The rare earth elements are believed to catalyze the reversible reaction between magnesium and hydrogen and reduce the thermal stability of MgH₂ by weakening the Mg–H bond. This study focuses on investigating the effect of Ce partial substitution of La on the comprehensive hydrogen storage performances of La_10-xCe_xMg₈₀Ni₁₀ (x = 0–4) alloys (prepared by vacuum induction melting). The phase composition and microstructure were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM). The thermodynamics and kinetics of isothermal reactions were measured by the automatic Sievert apparatus. Non-isothermal dehydrogenation performance of the alloys was researched by thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC). All the experimental alloys have a large capacity of hydrogen absorption and desorption and the kinetics of the Ce containing alloys is better. The additive Ce exists in the solid solution of alloy and results in the refinement of grain, making the stability of the hydride visibly lower, which is the reason for the decline in the initial dehydrogenation temperature and enthalpy (ΔH) of the hydride. Besides, the dehydrogenation activation energy of the alloys is distinctly reduced by composition adjustment, which indicates the improved hydrogen storage performances.     

Electrocatalytic characteristics of the metal hydride electrode for advanced Ni/MH batteries

The electrocatalytic characteristics of a metal hydride (MH) electrode for advanced Ni/MH batteries include the hydrogen adsorption/desorption capability at the electrode/electrolyte interface. The hydrogen reactions at the MH electrode/electrolyte interface are also related to factors such as the surface area of the MH alloy powder and the nature of additives and binder materials. The high-rate discharge capability of the negative electrode in a Ni/MH battery is mainly determined by the mass transfer process in the bulk MH alloy powder and the charge transfer process at the interface between the MH alloy powder and the electrolyte. In this study, an AB₅-type hydrogen-absorbing alloy, Mm (Ni, Co, Al, Mn)_5.02 (where Mm denotes Mischmetal, comprising 43.1 wt.% La, 3.5 wt.% Ce, 13.3 wt.% Pr and 38.9 wt.% Nd), was used as the negative MH electrode material. The MH electrode was charged and discharged for up to 200 cycles. The specific discharge capacity of the alloy electrode decreases from a maximum value of 290–250 mAh g⁻¹ after 200 charge/discharge cycles. A cyclic voltammetry technique is used to analyze the charge transfer reactions at the electrode/electrolyte interface and the hydrogen surface coverage capacity.     

Effects of electrolytes and temperature on high-rate discharge behavior of MmNi5-based hydrogen storage alloys

A series of experiments have been performed to investigate the effects of electrolyte composition and temperature on the high-rate discharge behaviors of MmNi₅-based AB₅ hydrogen storage alloy electrodes. Two types of AB₅ electrodes have been used using different alloys: Ce-rich alloy V (La_0.26 Ce_0.44Pr_0.1Nd_0.2Ni_3.55Co_0.72Mn_0.43Al_0.3) and La-rich alloy N (La_0.58Ce_0.25Pr_0.06 Nd_0.11Ni_3.66Co_0.74Mn_0.41Al_0.18). Electrolytes EN were obtained by adding a saturated amount of Al₂(SO₄) ₃ to the original electrolyte EO (6 M KOH + 1 wt% LiOH). The electrolyte EN has previously been shown to be very effective to stop the self-discharge of the AB₅ electrodes, better charge/discharge cycle life have been observed. The electrochemical properties of the electrodes were measured by two methods: step mode high-rate discharge and continuous mode high-rate discharge. The results indicate that at 298 K and 333 K, high-rate discharge capacity of Ni–MH battery was mostly affected by the chemical composition of the electrolyte, then the type of alloy. Better dischargeabilities in high-rate discharge capacity have been observed in electrolyte EO than in electrolyte EN. The Ce-rich alloy V has a higher high-rate discharge capacity than La-rich alloy N. High-rate discharge capacity decreases in the following order: VEO > NEO > VEN > NEN (VEO denotes the combination of alloy V and electrolyte EO used in the test battery, similarly equivalent representations for NEN, VEO and VEN).     

Hybrid nickel-metal hydride/hydrogen battery

High capacity, high efficiency and resource-rich energy storage systems are required to store large scale excess electrical energy from renewable energy. We proposed “Hybrid Nickel-Metal Hydride/Hydrogen (Ni-MH/H₂) Battery” using high capacity AB₅-type hydrogen storage alloy and high-pressure H₂ gas as negative electrode active materials. It was experimentally confirmed that hydrogen gas can be utilized as an active material of negative electrode by the presence of the AB₅-type hydrogen storage alloy. The experimental average cell voltage suggested that H₂ gas passed through the alloy in the form of atoms. The calculated gravimetric energy density of this hybrid battery increased up to 1.5 times of the conventional Ni-MH battery with low content of rare-earth element which is 32 wt% of the Ni-MH battery.     

Electrode characteristics of the Cr and La doped AB2-type hydrogen storage alloys

AB₂-type alloy, a kind of hydrogen storage alloys used as an anode of Ni-MH batteries, has a large discharge capacity but still has several problems such as initial activation, cycle life and self-discharge. In this study, we have investigated the effects of Cr addition and fluorination after La addition on AB₂-type alloy with Zr_0.7Ti_0.3V_0.4Mn_0.4Ni_1.2 composition. The EPMA and SEM surface analysis techniques were used and the crystal structure was characterized by XRD analysis. Metal hydride negative was characterized by galvanostatic cycling test, electrochemical impedance spectroscopy and potentiodynamic polarization. Cr-addition is found to be effective to improve cycle life and self-discharge characteristics but ineffective to promote initial activation due to the formation of stable oxide film on alloy surface. Highly reactive particles have been formed by fluorination after La-addition to the alloys and those particles may remarkably improve the initial activation of MH-negative electrodes.     

Effect of partial substitution of La with Ce,Pr and Nd on the properties of LaNi5-based alloy electrodes

A comparison is made of the properties of LaB₅ (BNi_3.55Co_0.75Mn_0.4Al_0.3), La_0.7R_0.3B₅ (RCe, Pr, Nd) and MmB₅ (Mm is mischmetal in an atomic ratio of La:Ce:Pr:Nd = 0.7:0.2:0.05:0.05) alloy electrodes. X-ray diffraction results reveal that Ce, Pr, Nd substitute for La and decrease the unit cell volume. Pressure-composition isotherms of the electrode alloys are determined by an electrochemical method. The characteristics of the alloy electrodes, including initial activation, high-rate discharge, cycle life and self-discharge, are examined. It is found that partial replacement of La with Ce, Pr, Nd in the LaNi₅-based alloy improves greatly the activation, high-rate discharge and cycle life of the electrode, but increases the self-discharge due to a higher dissociation pressure of the metal hydride.     

Electrochemical behaviour of intermetallic-based metal hydrides used in Ni/metal hydride (MH) batteries: a review

Hydrogen storage alloys are a group of new functional intermetallics which can be used in heat pumps, catalysts, hydrogen sensors and Ni/MH batteries. The development of Ni/MH (Metal Hydride) batteries based on MH negative electrodes has seen considerable activity in recent years. Batteries based on such hydride materials have some major advantages over the more conventional lead–acid and nickel–cadmium systems. These advantages include: high-energy density; high-rate capability; tolerance to overcharge and over-discharge; the lack of any poisonous heavy metals; and no electrolyte consumption during charge/discharge cycling. The most important electrochemical characteristics of the hydrogen storage compounds used in these batteries include capacity, cycle lifetime, exchange current density and equilibrium potential. These characteristics can be changed by designing the composition of the hydrogen storage alloy to provide optimum performance of the Ni/MH batteries. The electrochemical behaviour of such intermetallics depends on the types of intermetallics (mainly AB₂ and AB₅), microstructure, the nature and amount of each element in the intermetallic compound, and the electrochemical process(es) taking place. The addition of some highly electrocatalytic materials for the hydrogen evolution reaction (h.e.r.) are beneficial in generating optimum performance for the MH electrodes. In this paper, we present some recent results on the electrochemical behaviour of such compounds and the mechanisms of the electrochemical reactions.     

Compositional effects on the hydrogen cycling stability of multicomponent Ti-Mn based alloys

Intermetallic alloys such as AB, AB₂, and AB₅ type have been studied due to their capability to reversibly store hydrogen. These alloys exhibit varying hydrogen storage properties depending on the crystal structure and composition. Compositional modification is commonly known as an effective method to modify the alloys thermodynamic and kinetics for various applications such as metal hydride batteries, metal hydrides hydrogen storage and compression. However, the effects of the compositional modification on the cyclic stability of these alloys are not usually well studied.Here, the hydrogen cycling stabilities of Ti-Mn based alloys with C14 type structure are studied. Hyper-stoichiometry, stoichiometry and hypo-stoichiometry alloys were prepared accordingly: Ti_30.6V_16.4Mn_48.7 (Zr_0.7Cr_0.8Fe_2.8) (B/A = 2.19), Ti_32.8V_15.1Mn_47.1 (Zr_0.9Cr_1.2Fe_2.9) (B/A = 1.97) and Ti_34.5V_15.4Mn_44.7 (Zr_0.9Cr_1.3Fe_3.2) (B/A = 1.87). Whilst the hyper-stoichiometry alloy showed almost a stable (about 9% capacity reduction) hydrogen capacity after 1000 cycles of hydrogenation and dehydrogenation, the stoichiometry and hypo-stoichiometry alloys failed to hydrogenate after about 950 and 500 cycles respectively. A limited reduction in the calculated crystalline size of the alloys was observed before and after the hydrogen cycling, denoting that pulverisation plays a less significant role on the observed hydrogen capacity loss. In addition, a reduction in the B/A ratio from 2.19 to 1.82 (hyper to hypo-stoichiometry) encouraged the formation of more stable hydride and a higher level of heterogeneous lattice strain. Whilst a small loss of hydrogen capacity (9%) in the hyper-stoichiometry alloy was attributed to the trapped hydrogen, the complete loss of hydrogen capacity in the stoichiometry and hypo-stoichiometry alloys seemed to originate from the formation of stable hydride and the lattice distortion.     

Optimizing Ti–Zr–Cr–Mn–Ni–V alloys for hybrid hydrogen storage tank of fuel cell bicycle

AB₂-type Ti-based alloys with Laves phase have advantages over other kinds of hydrogen storage intermetallics in terms of hydrogen sorption kinetics, capacity, and reversibility. In this work, Ti–Zr–Cr-based alloys with progressive Mn, Ni, and V substitutions are developed for reversible hydrogen storage under ambient conditions (1–40 atm, 273–333 K). The optimized alloy (Ti_0.8Zr_0.2)_1.1Mn_1.2Cr_0.55Ni_0.2V_0.05 delivers a hydrogen storage capacity of 1.82 wt%, the hydrogenation pressure of 10.88 atm, and hydrogen dissociation pressure of 4.31 atm at 298 K. In addition, fast hydrogen sorption kinetics and low hydriding-dehydriding plateau slope render this alloy suitable for use in hybrid hydrogen tank of fuel cell bicycles.     

SOC-dependent high-rate dischargeability of AB5-type metal hydride anode: Mechanism linking phase transition to electrochemical H-desorption kinetics

The current application of nickel-metal hydride (Ni-MH) batteries places a particular emphasis on the high-rate dischargeability (HRD) at varying state-of-charges (SOCs). However, most research on the HRD of AB₅-type MH anodes only considers the fully charged case but overlooks the significant impact of SOC. In this work, at first, the great SOC effect on the HRD or pulse power of AB₅-type MH anode is presented. Then, by crosschecking the SOC dependence of both ‘in situ’ polarization and ‘ex situ’ kinetic parameters, a definite SOC-dependent H-desorption kinetics for AB₅-type MH anode is acquired. Finally, a novel mechanism linking phase transition to H-desorption kinetics for AB₅-type MH anode is proposed. The HRD or pulse power of AB₅-type MH anode significantly improves when SOC decreases from 100% to an appropriate range (90-60%) and suddenly deteriorates when SOC drops below ∼20%. The former improvement relates to the formation of saturated solid solution that simultaneously facilitates both charge-transfer reaction and hydrogen diffusion. The latter deterioration is due to the complete depletion of hydride causing an insufficient supply of hydrogen atoms.     

Effect of Mn on the long-term cycling performance of AB5-type hydrogen storage alloy

Rare-earth AB₅-type hydrogen storage alloys are widely studied due to their extensive application potentials in hydrogen compressors, heat pump, Ni–MH batteries etc. However, their shortcomings such as plateau splitting and capacity degradation during hydrogen absorption/desorption hinder their practical applications. In this paper, we study the effect of Mn partial substitution for Ni on the plateau characteristics and long-term cycling performance of LaNi_5-xMn_x alloys. It is found that Mn addition expands the lattice interstitial for hydrogen accommodation, thus prohibiting the plateau splitting phenomenon. In addition, the substitution of Mn for Ni stabilizes the crystal structure of the alloys against hydrogen absorption/desorption, thus relieving the capacity degradation. The capacity retention of the alloys at the 1000th cycle (S₁₀₀₀) increases from 83.2% (x = 0) to 94.0% (x = 0.75). But when x reaches 1, the hydrogen desorption reversibility is reduced due to the low plateau pressure, resulting in a slight decrease in capacity retention.     

Microstructure and electrochemical investigations of La0.76−xCexMg0.24Ni3.15Co0.245Al0.105 (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys

The effects of substitution of Ce for La on the microstructure and electrochemical performance of La_0.76−xCe_xMg_0.24Ni_3.15Co_0.245Al_0.105 (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) analyses showed that the main phases of the alloys consist of (La, Mg)Ni₃ phase (PuNi₃-type rhombohedral structure), LaNi₅ phase (CaCu₅-type hexagonal structure) and (La, Mg)₂Ni₇ phase (Ce₂Ni₇-type hexagonal structure). The cell volume of the (La, Mg)Ni₃ phase, (La, Mg)₂Ni₇ phase and LaNi₅ phase decreased monotonously with increasing Ce content. Electrochemical investigations showed a decrease in the discharge capacity, while high rate dischargeability (HRD) first increased and then decreased with increasing Ce content. The Ce substitution for La slightly enhanced the cyclic stability of the alloy electrodes. The pressure–composition (P–C) isotherms showed that the plateau region was broadened with Ce content increased in the alloys, meanwhile, two plateaus appeared and pressure of the hydrogen absorption and desorption increased accordingly.