Synthesis of La2MgNi9 hydrogen storage alloy in molten salt

La₂MgNi₉ alloy is synthesized directly from the sintered mixture of La₂O₃ + NiO + MgO in the molten CaCl₂ electrolyte by the electro-deoxidation method at 740 °C and the electrochemical hydrogen storage characteristics of the synthesized alloy are observed. Sintering (at 1200 °C for 2 h) converts the hygroscopic La₂O₃ (by the reaction with NiO) into the non-hygroscopic La₂NiO₄ and La₃Ni₂O₇ phases. The X-ray diffraction peaks indicate that the electro-deoxidation causes LaOCl, Ni, LaNi₅ and even target phase La₂MgNi₉ to form within 2 h process time. The molten salt synthesis process ends up with the final alloy structure of 79% La₂MgNi₉ and 21% retained LaNi₅. The porous alloy structure (with approximately 31.66 m²g⁻¹ specific surface area) is beneficial for higher hydrogen storage capacity and it is observed that La₂MgNi₉ alloy has promising discharge capacity which is approximately 280 mAhg⁻¹. This work clearly indicates that the electro-deoxidation is a very effective method in the synthesis of the hydrogen storage materials.     

Superior electrochemical performances of LaMgNi alloys with A2B7/A5B19 double phase

Here, phase transformation and electrochemical characteristics of non-stoichiometry La₄MgNi_x (x = 16, 17 and 18) hydrogen storage alloys were studied. It is found that after annealed at 1223 K for 24 h, the minor AB₃ and AB₅ phases in La₄MgNi₁₆ alloy transform into A₂B₇ phase by a peritectic reaction and the La₄MgNi₁₆ alloy shows a A₂B₇ single phase structure. Double phase structures of A₂B₇/A₅B₁₉ are obtained in La₄MgNi₁₇ and La₄MgNi₁₈ alloys after annealed at the same condition. The abundance of A₅B₁₉ phase increases as x increases, indicating the increasing x value contributes to the formation of A₅B₁₉ phases. Electrochemical studies show that the maximum discharge capacity and capacity retention at the 100^th charge/discharge cycles (S₁₀₀) of A₂B₇ single phase La₄MgNi₁₆ alloy is 373 mAh g⁻¹ and 78.4%, respectively. The appearance of A₅B₁₉ (minor) phase in the La₄MgNi₁₇ alloy makes a remarkable improvement in the discharge capacity from 373 mAh g⁻¹ to 388.8 mAh g⁻¹, as well as the S₁₀₀ from 78.4% to 90.1%. It is believed that the LaMgNi-based alloy with the A₂B₇(main)/A₅B₁₉(minor) phase structure possesses the good overall electrochemical properties and is applicable to the high-power and long-cycle life negative electrode material for nickel metal hydride batteries.     

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.     

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).     

Characteristics of A2B7-type LaYNi-based hydrogen storage alloys modified by partially substituting Ni with Mn

LaY₂Ni_10.5?xMn_x (x = 0.0, 0.5, 1.0, 2.0) alloys are prepared by a vacuum induction-quenching process followed by annealing. The structure, as well as the hydriding/dehydriding and charging/discharging characteristics, of the alloys are investigated via X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), pressure-composition isotherms (PCI), and electrochemical measurement. The alloys have multiphase structures mainly composed of Gd₂Co₇-type (3R) and Ce₂Ni₇-type (2H) phases. Partial substitution of Ni by Mn clearly increases the hydrogen storage capacity of the alloys. The x = 0.5 alloy exhibits a maximum hydrogen storage capacity of 1.40 wt % and a discharge capacity of 392.9 mAh g^?1, which are approximately 1.5 and 1.9 times greater than those of the x = 0.0 alloy, respectively. The high-rate dischargeability (HRD) of the x = 0.5 alloy is higher than that of the other alloys because of its large hydrogen diffusion coefficient D, which is a controlling factor in the electrochemical kinetic performance of alloy electrodes at high discharge current densities. Although the cyclic stability of the x = 0.5 alloy is not as high as that of the other alloys, its capacity retention ratio is as high as 56.3% after the 400th cycle. The thermodynamic characteristics of the x = 0.5 alloy satisfy the requirements of the hydride electrode of metal hydride–nickel (MH–Ni) batteries.     

Hydrogen storage material based on LaNi5 alloy produced by mechanical alloying

The electrochemical characteristics of the La_0.8Ce_0.2Ni_2.5Co_1.8Mn_0.4Al_0.3 compound, produced by mechanical alloying, are investigated for hydrogen storage in nickel-metal hydride (NiMH) batteries by discharging tests at constant current and by calculating equilibrium pressure of hydrogen from the equilibrium potentials. It is shown that the alloy produced by mechanical alloying, followed by annealing and activation exhibits high specific capacity at the stable potential plateau, even at the high discharge rate (10 mA cm⁻²), and low hydrogen equilibrium pressure. The alloy of such composition gives low capacity loss during cycling, which enables its application for metal hydride battery production.     

Investigations of AB5-type negative electrode for nickel-metal hydride cell with regard to electrochemical and microstructural characteristics

In the present investigation, AB₅-type hydrogen storage alloys with compositions Mm_0.8La_0.2Ni_3.7Al_0.38Co_0.3Mn_0.5Mo_0.02 and Mm_0.75Ti_0.05La_0.2Ni_3.7Al_0.38Co_0.3Mn_0.5Mo_0.02 are synthesized by radio-frequency induction melting. The electrochemical properties are studied through the measurements of discharge capacity, activation process, rate capability, self-discharge rate and cyclic stability of both the electrodes. Pressure-composition isotherms are plotted by converting the electrode potential into the hydrogen pressure following the Nernst equation. The structural and microstructural characterizations are performed by means of X-ray diffraction phase analysis and scanning electron microscopy of as-fabricated and electrochemically tested electrodes. An attempt is made to correlate the observed electrochemical properties with the structural–microstructural characteristics.     

Hydrogen storage and electrochemical properties of annealed low-Co AB5 type intermetallic compounds

The structure, hydrogen storage and electrochemical properties of annealed low-Co AB₅-type intermetallic compounds have been investigated. La-alloy, Nd-alloy and Cr-alloy are used to represent La_0.8Ce_0.2Ni₄Co_0.4Mn_0.3Al_0.3, La_0.6Ce_0.2Nd_0.2Ni₄Co_0.4Mn_0.3Al_0.3 and La_0.6Ce_0.2Nd_0.2Ni_3.8Co_0.4Mn_0.3Al_0.3Cr_0.2, respectively. The XRD results indicated that annealed samples are all single-phase alloys with CaCu₅ type structure. The maximum of both hydrogen content and discharge capacity is obtained for La-alloy 1.23 wt%H₂ and 321.1 mA h/g, respectively. All the investigated alloys are quiet stable with ΔH of hydrogen desorption about 36–38 kJ/mol H₂. Cycle life of alloy electrode has been improved by partial substitution of La for Nd and Ni for Cr. The highest capacity retention of 92.2% after 100 charge/discharge cycles at 1C has been observed for Nd-alloy. The hydrogen diffusion coefficient measured by PITT is higher at the start of charging process and dramatically reduces by 2–3 order of magnitude with saturation of β-hydride. The highest value 6.9 × 10^?13 cm²/s is observed for La alloy at 100% SOC. Partial substitution La for Nd and Cr for Ni in low-Co AB₅ metal hydride alloys slightly reduces maximum discharge capacity, HRD performance and hydrogen diffusion kinetics. Low-Co alloys show good overall electrochemical properties compared to high-Co alloys and might be perspective materials for various electrochemical applications.     

Effects of surface coating with Cu-Pd on electrochemical properties of A2B7- type hydrogen storage alloy

La_0.75Mg_0.25Ni_3.2Co_0.2Al_0.1 hydrogen storage alloy, the nickel-metal hydride (MH/Ni) secondary battery negative electrode, was modified by CuSO₄ solution (3 wt% in Cu in contrast with alloy weight) and PdCl₂ solution varied from 1 wt% to 4 wt% in Pd in contrast with alloy weight with a simplified pollution-free replacement plating method, aiming at improving its comprehensive electrochemical properties. The XRD analysis and SEM images combined with EDS results reveal that Cu and Pd nanoparticles are uniformly plated on the pristine alloy surface. The relative amount of Pd on the Cu-Pd coated alloy surface increases notably as the PdCl₂ concentration increases in the plating solution. Electrochemical tests indicate that alloy electrodes modified by Cu-Pd composite coating show perfect activation performance, which achieve the maximum discharge capacity at the first charge-discharge cycle. Moreover, alloy electrodes coated with Cu-Pd perform dramatically enhanced high rate dischargeability (HRD). The enhancement increases firstly and then decreases as the content of Pd increases in the Cu-Pd coating. Meanwhile, the cycle life of modified alloys is also improved significantly. Among all the samples, the Cu-Pd coated alloy with 3 wt% Pd content in the PdCl₂ solution reinforces the comprehensive electrochemical properties most sufficiently, with dischargeability of 86.4% under 1500 mA/g and remaining capacity of 82.7% after 100 cycles.     

Effect of Al content on structure and electrochemical properties of LaNi4.4 − xCo0.3Mn0.3Alx hydrogen storage alloys

The structure and electrochemical properties of LaNi_4.4 − xCo_0.3Mn_0.3Al_x hydrogen storage alloys have been investigated by XRD and simulated battery test, including maximum capacity, cyclic stability, self-discharge, high-rate dischargeability (HRD). Samples A, B, C and D were used to represent alloys LaNi_4.4Co_0.3Mn_0.3Al, LaNi_4.3Co_0.3Mn_0.3Al_0.1, LaNi_4.2Co_0.3Mn_0.3Al_0.2 and LaNi_4.1Co_0.3Mn_0.3Al_0.3 respectively. The results indicated that as-prepared LaNi_4.4 − xCo_0.3Mn_0.3Al_x alloys are all single-phase alloys with hexagonal CaCu₅ type structure. The maximum discharge capacity is 330.4 mAh g⁻¹ (Alloy C). With the increase of Al content from A to D, cycle life of alloy electrode has been improved. Higher capacity retention of 89.29% (after 50 charge/discharge cycles) has been observed for electrode D, while with a smaller capacity loss of 12.5% in its self-discharge test. Better high-rate charge/discharge behaviors have been observed in electrode B, and the maximum data is 54.7% at charge current of 900 mA/g) and 68.54% at discharge current of 1800 mA/g). Furthermore, the electrochemical impedance spectroscopy (EIS) analysis shown that the reaction of alloy electrode is controlled by charge-transfer step. The addition of Al results in the formation of protective layer of aluminum oxides on the surface of the alloy electrode, which is good for the improvement of electrode properties in alkaline solution and is detrimental for the charge-transfer process. Therefore, a suitable addition of Al is needed to improve its electrode properties.     

A study on the hydrogen-storage properties of La2−xTixMgNi9 (x = 0.1, 0.2, 0.3, 0.4) alloys

The La_2−xTi_xMgNi₉ (x = 0.1, 0.2, 0.3, 0.4) alloys were prepared by magnetic levitation melting under Ar atmosphere. The effects of partial substitution Ti for La on the phase structures, hydrogen-storage properties and electrochemical characteristics of the alloys were investigated systematically. For La_2−xTi_xMgNi₉ (x = 0.1, 0.2, 0.3, 0.4) alloys, LaNi₅, LaNi₃ and LaMg₂Ni₉ are the main phases, the maximum hydrogen-storage capacity is 1.51, 1.36, 1.35 and 1.22 wt%, respectively. The absorption–desorption plateau pressure of the alloys first decreases and then increases with increase of Ti content, and the La_1.8MgTi_0.2Ni₉ alloy has the lowest absorption–desorption plateau pressure. The discharge voltage of the alloy electrodes rises with increasing the amount of Ti content. The La_1.8Ti_0.2MgNi₉ alloy electrode presents good electrochemical performance.     

Effect of electrolyte on electrochemical characteristics of MmNi3.55Co0.72Al0.3Mn0.43 alloy electrode for hydrogen storage

A series of experiments have been performed to investigate the effects of three electrolytes of different compositions (EO, EA and EM) on the electrochemical characteristics of MmNi_3.55Co_0.72Al_0.3Mn_0.43 hydrogen storage alloy electrode. Electrolytes EA and EM were obtained by adding appropriate amounts of Al₂(SO₄)₃ and MnSO₄ to the original electrolyte EO (6 M KOH + 1 wt% LiOH), respectively. Electrode activation, maximum capacity, cycle life, self-discharge and high-rate discharge characteristics have been studied. It was found that a maximum capacity of about 260 mA h/g has been obtained for the alloy electrodes in all these electrolytes after 5–7 cycles of charging/discharging. The alloy electrodes have a good durability in electrolytes EA and EM, especially after 175 cycles. Using the capacity retention as an indication of self-discharge resistance, almost identical degree of capacity retention (82% after 4 days and 45% after 16 days) has been observed at 298 K, regardless of the electrolytes used. When tested at higher temperature, however, a higher capacity retention (46% after 3 days) at 333 K has been observed for electrodes in electrolyte EA, and about 32% for electrodes in both electrolytes EO and EM. As to high-rate discharge behavior of the results of high-rate discharge tests indicated that about 50% of discharge efficiencies were obtained in the three electrolytes at 333 K by continuous-model high-rate discharge method, at a discharge rate of 7C, and 22% in 298 K. The alloy electrode in electrolyte EM has the best durability, in which about 50% of discharge efficiency at DC = 9C was obtained by step-model high-rate discharge method at 333 K, which was even higher than that at 298 K.     

Effects of annealing temperature on the structure and properties of the LaY2Ni10Mn0.5 hydrogen storage alloy

The effects of annealing at 1123, 1148, 1173 and 1198 K for 16 h on the structure and properties of the LaY₂Ni₁₀Mn_0.5 hydrogen storage alloy as the active material of the negative electrode in nickel–metal hydride (Ni–MH) batteries were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy linked with an energy dispersive X-ray spectrometer (SEM–EDS), pressure-composition isotherms (PCI) and electrochemical measurements. The quenched and annealed LaY₂Ni₁₀Mn_0.5 alloys primarily consist of Ce₂Ni₇- (2H) and Gd₂Co₇-type (3R) phases. The homogeneity of the composition and plateau characteristics of the annealed alloys are significantly improved, and the lattice strain is effectively reduced. The alloys annealed at 1148 K and 1173 K have distinctly greater hydrogen storage amounts, 1.49 wt% (corresponding to 399 mAh g^?1 in equivalent electrochemical units) and 1.48 wt%, respectively, than the quenched alloy (1.25 wt%, corresponding to 335 mAh g^?1 in equivalent electrochemical units). The alloys annealed at 1148 K and 1173 K have relatively good activation capabilities. The annealing treatment slightly decreases the discharge potentials of the alloy electrodes but markedly increases their discharge capacity. The maximum discharge capacities of the annealed alloy electrodes (372–391 mAh g^?1) are greater than the extreme capacity of the LaNi₅-type alloy (370 mAh g^?1). The cycling stability of the annealed alloy electrodes was improved.     

Effects of pretreatment of LM-Ni3.9Co0.6Mn0.3Al0.2 alloy powders in a KOH/NaBH4 solution on the electrode characteristics and inner pressure of nickel-metal-hydride secondary batteries

The discharge capacities of lanthanum-rich mischmetal (LM)-Ni_3.9Co_0.6Mn_0.3Al_0.2 alloy electrodes are significantly degraded by an increase in the C rate. Nevertheless, the discharge capacity of alloy electrodes pretreated with KOH/NaBH₄ is maintained higher than that of raw alloy electrodes, with the difference in discharge capacities between the raw and pretreated alloy electrodes more prominent at higher C rates. The charge retention of the electrodes decreases with increasing rest time. In particular, the charge retention of the pretreated alloy electrode is lower than that of the raw alloy electrode due to the higher self-discharge rate. The overvoltage for hydrogen evolution of the pretreated alloy electrode is superior to that of the raw alloy electrode, particularly at higher temperatures. This phenomenon indicates that the charge efficiency of the electrode was significantly improved by the surface pretreatment, resulting from its high surface catalytic activity. Repeated charge-discharge increases the inner pressure of the battery. Nevertheless, due to its higher charge efficiency and faster recombination rate, the inner pressure of the battery made using the pretreated alloy electrode is much smaller than that of the battery made using a raw alloy electrode.     

Enhanced electrochemical and hydrogen storage properties of La–Mg–Ni-based alloy electrode using a Ni and N co-doped reduced graphene oxide nanocomposite as a catalyst

To enhance the electrochemical property of a La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy, a three-dimensional (3D) reduced graphene oxide (rGO)-supported nickel and nitrogen co-doped (Ni–N@rGO) nanocomposite is fabricated by an impregnation method and introduced into the La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy. The results show that the reversible hydrogen storage property and the comprehensive electrochemical performance of the La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy are enhanced effectively when it is modified by the Ni–N@rGO nanocomposite. The high-rate dischargeability values at a discharge current density of 1500 mA g⁻¹ for the La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy and Ni–N@rGO-modified samples are 0.0% and 70.5%, respectively. Additionally, the anodic peak currents for the unmodified alloy electrode is 892 mA g⁻¹. Under the catalytic action of the Ni–N@rGO nanocomposite, the value increases to 2307 mA g⁻¹, which is 2.59 times larger than that of unmodified samples. The results also indicate that the diffusion ability of the hydrogen atom in the alloy electrode body enhances significantly when modified by the Ni–N@rGO nanocomposite. The hydrogen diffusion coefficient for the La_0.7Mg_0.3(Ni_0.9Co_0.1)_3.5 alloy electrode increases from 3.93 × 10⁻¹⁰ cm² s⁻¹ to 6.15 × 10⁻¹⁰ cm² s⁻¹ when is modified by Ni–N@rGO nanocomposite. These improvements in the comprehensive electrochemical properties are mainly attributed to the excellent electrochemical activity and conductivity of the Ni–N@rGO nanocomposite.     

Electrochemical properties of Ti2Ni hydrogen storage alloy

In this paper, the long cycling behavior, the kinetic and thermodynamic properties of Ti₂Ni alloy used as negative electrode in nickel-metal hydride batteries have been studied by different electrochemical techniques. Several methods, such as, galvanostatic charge and discharge, the constant potential discharge and the potentiodynamic polarization are applied to characterize electrochemically the studied alloy. The studied electrodes are observed before and after electrochemical tests at different temperatures by scanning electron microscopy.The amorphous Ti₂Ni is activated after five cycles and the achieved maximum discharge capacity is about 67 mAh g^?1 at ambient temperature. Despite the low values of the maximum discharge capacity and the cycling stability (17%) and the steep decrease of the discharge capacity after activation, this alloy conserves a good stability lifetime during a long cycling. A good correlation is observed between the evolution of the discharge capacity and those of the redox parameters during a long cycling.The enthalpy change, the entropy change and the activation energy of the formation reaction of the Ti₂Ni metal hydride are evaluated electrochemically. The found values of the enthalpy change, the entropy change and the activation energy are about ?43.3 kJ mol^?1, 51.7 J K^?1 mol^?1 and 34.9 kJ mol^?1, respectively.     

Preparation of porous polyaniline/RGO composite and its application in improving the electrochemical properties of Co9S8 hydrogen storage alloy

The sea urchin-like porous polyaniline (PPANI) is prepared by a facile saturated solution synthetic route. The porous polyaniline/reduced graphene oxide composite (PPANI/RGO) is synthesized via a solution-assisted self-assembly method. Mechanical alloying is used to obtain the Co₉S₈ alloy. Composites of Co₉S₈ mixed with PPANI and PPANI/RGO are fabricated through ball-milling to improve the electrochemical performance of Co₉S₈ alloy. The structures and morphologies of the composite alloys are studied by XRD, SEM and BET. The electrochemical properties of alloys are tested as negative electrodes of Ni-MH batteries by the LAND CT2001A tester and three-electrode system. For comparison, Co₉S₈ alloys doped with conventional polyaniline (CPANI) and RGO are also prepared. Ultimately, the Co₉S₈ + PPANI composite shows preferable discharge capacity compared with CPANI modified Co₉S₈ and matrix alloy. In addition, the PPANI/RGO composite modified Co₉S₈ electrode exhibits superior discharge capacity than separate PPANI and RGO coated alloys. A maximum discharge capacity (701.4 mAh/g) is achieved for Co₉S₈ + PPANI/RGO electrode. Furthermore, the Co₉S₈ + PPANI/RGO composite materials exhibit preferable high-rate dischargeability, improved corrosion and oxidation resistance and excellent kinetics properties. The PPANI material with special porous structure and unique morphology displays better performance than CPANI. Moreover, a synergistic effect between PPANI and RGO species in the PPANI/RGO material may provide a rapid passageway for charge transfer and accelerate the hydrogen transmission. Accordingly, the electrochemical activity and kinetic properties are improved for Co₉S₈ + PPANI/RGO composite electrode.     

The different effect of Co3O4 or/and carbon fiber originated from biomass on the electrochemical hydrogen storage performance of Co2B alloy

Co₂B alloy was synthesized via the method of high temperature solid phase. Carbon fiber (CF) was prepared from cotton by calcination process. The addition of carbon fiber and Co₃O₄ improves corrosion resistance and charge transfer speed of the composite material electrode. The R_ct value of Co₂B + 1 wt.% CF was 360 mΩ, lower than the other composite electrode could reduce charge transfer resistance. The overall electrochemical performance of Co₂B + 2 wt.% Co₃O₄ + 1 wt.% CF was best among all the electrodes, and its C_max could reach 715.3 mAh/g. The high conductivity and multiple reaction sites provided by carbon fiber and the catalytic effect of Co₃O₄ may be the main reasons for the improvement of electrochemical performance, which enhance the kinetic performance of electrochemical reactions. The synergistic effect of carbon fiber and Co₃O₄ improves the electrochemical hydrogen storage properties of Co₂B alloy. This work presents a simple and effective method to improve the electrochemical hydrogen storage performance of cobalt-boron alloys by adding transition metal oxides and carbon materials derived from biomass.     

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.     

Crystal structure and hydrogen absorption− desorption property of La5Co19

An intermetallic compound, La₅Co₁₉ is synthesized successfully for hydrogen storage, and its crystal structure is determined by X-ray diffraction. The alloy is formed by annealing the precursor at 1073 K for 10 h, and it has a Ce₅Co₁₉-type structure (space group R-3m, 3R) with a = 0.5130(1) nm and c = 4.882(1) nm. Its maximum hydrogen capacity reaches 0.92 H/M, but 0.40 H/M of hydrogen remains in the sample after the first desorption. Its reversible hydrogen capacity is 0.51 H/M. The formed hydride phases, phase I (La₅Co₁₉H₁₀) and phase II (La₅Co₁₉H₂₂) also have the Ce₅Co₁₉-type crystal structure; the hydride phases retain the same metal sublattice as that of the original alloy. Phase I is formed through anisotropic expansion of the La₅Co₁₉ lattice, while the unit cell, the MgZn₂-type and CaCu₅-type cells, of phase II is formed by the isotropic expansion of the La₅Co₁₉ lattice.