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 additions on AB5-type hydrogen storage alloy in MH–Ni battery application

The AB₅-type hydrogen storage alloy of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3 were synthesized and mixed with PVA (Polyvinyl Alcohol) or different percentage Ni powder as the test samples. The cycle stabilities of the composites were tested in 6 M KOH electrolyte through electrochemical method. The results indicated that all the samples with Ni powder have better cycle stabilities and flatter discharge voltage platform. The sample of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3 + 200 wt.% Ni has the highest capacity conservation rate of 80.5% and the longest discharge time of 5.2 h. The SEM images show that the particle diameters of the alloy decreased by 2 μm and the surface smoothed without sharp edges after adding Ni powder. It can be presumed that adding Ni can improve the cycle stability of the alloy of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3 in the alkaline electrolyte and enhance the reaction rate in the charge/discharge cycles.     

Electrochemical properties of the Mm0.3Ml0.7Ni3.55Co0.75Mn0.4Al0.3 alloy mixed with Ni powder

Composites of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3 alloy and Ni powders were mechanically synthesized and electrochemically tested in 6 M KOH electrolyte. In this work, the electrochemical properties of the alloys were greatly improved by mixing them with Ni, which plays a corrosion-resistance role in the alkali electrolyte and helps electron conduction. It has been shown that the numbers of activation cycles decreased compared with the alloys without Ni powder. All the alloys were activated after the second cycle. Improvements of the maximum discharge capacities were also found in this work. A maximum discharge capacity of 358 mAh g⁻¹ was measured in the Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3 + 250 wt.% Ni composite. In addition to that, adding Ni was found to enhance the high-rate discharge ability of the alloys, which appear to be good candidates for the realization of MH battery electrodes.     

The electrochemical performances of Ti–V-based hydrogen storage composite electrodes prepared by ball milling method

In order to overcome the inherent disadvantages of Ti–V-based hydrogen storage alloys, such as poor activation behavior and low high-rate dischargeability, the novel composites Ti_0.17Zr_0.08V_0.35Cr_0.1Ni_0.3–x wt.% La_0.7Mg_0.3Ni_2.75Co_0.75 (x = 0, 5, 10 and 20) were successfully synthesized by ball milling method in the present study. And the structure and overall electrochemical properties of as-prepared composites are investigated systemically. The electrochemical studies show that the maximum discharge capacity of the composite electrodes displays no variation with the increase of La_0.7Mg_0.3Ni_2.75Co_0.75 content, whereas the high-rate dischargeability (HRD) and the activation behavior are distinctly improved with increasing x. The electrochemical hydrogen kinetics of composite electrodes is also studied by means of electrochemical impedance spectroscopy (EIS), linear polarization (LP), anodic polarization (AP) and potential-step measurements. It is found that the charge-transfer reaction resistance R_ct is decreased with increasing the amount of La_0.7Mg_0.3Ni_2.75Co_0.75 while exchange current density I₀, limiting current density I_L and hydrogen diffusion coefficient D are all increased with increasing the amount of La_0.7Mg_0.3Ni_2.75Co_0.75. These results suggest that the formation of composite with La_0.7Mg_0.3Ni_2.75Co_0.75 alloy is a promising strategy for improving the HRD, activation behavior and electrochemical kinetics of Ti–V-based alloy electrodes.     

Effect of vanadium content on electrochemical properties of la-based AB5-type metal hydride electrodes

Commercial La–Ni–Al–Co–Mn–V hydrogen storage alloys have been investigated to examine the effect of non-stoichiometry on the microstructure and electrochemical properties. It is found that for the stoichiometric ‘B’-rich compound, single phase with CaCu₅-type exists. However, for B-poor compounds, there is principally a CaCu₅-type phase with a small amount of V-rich type phase and the amount of V-rich phase reduces with vanadium. With the increase of Vy⩽0.1 content, hydrogen storage capacity is enhanced, whereas when y=0.2–0.3 it is decreased. The discharge capacity and cyclability are increased considerably by addition of vanadium in the range 0.02–0.1 with a maximum value at about 0.02%. The decrease of capacity for high V content was also correlated with the amount of V-rich phase. The V-rich phase is consisted of La_0.1Ni_2.6Al_0.2Co_2.0Mn_0.6V_1.3. The improvement of kinetics is due to the catalytic effect, grain boundary diffusion effect or more pronounced alloy pulverization upon cycling. This can be explained because the improvement of capacity for alloys with low V content is due to better kinetics. These alloys have been subjected to analysis by EDS, SEM and XRD. In order to determine the hydrogen storage capacity, the pressure composition isotherms (PCT curves) have been used. The metal hydride electrodes were characterized by galvanostatic cycling test.     

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.     

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).     

Studies on the diffusion coefficient of hydrogen through metal hydride electrodes

The potentiostatic discharge method was used to determine the hydrogen diffusion coefficient in MmNi₅, MmNi_4.5Mn_0.5, MmNi_3.55Co_0.75Mn_0.4Al_0.3 (Mm = mischmetal), ZrVNi, ZrV_0.6Ni_1.2Mn_0.2 and Zr_0.5Ti_0.5 V_0.6Ni_1.0Mn_0.4 alloy electrodes. The values obtained were of the order of 10⁻¹⁰cm²/s for the MmNi₅ system alloys and 10⁻¹¹cm²/s for the Zr-based Laves phase alloys. It was found that the higher the content of Mn in the alloy, the larger the value of the hydrogen diffusion coefficient. The effect of the Ni:alloy ratio in the mixture (alloy and Ni powder) on the diffusion coefficient and on the high rate of discharge were studied. The advantages of the potentiostatic discharge method is also discussed.     

Pre-conditioned layered electrodes for lithium batteries

Layered Li(Co_0.33Mn_0.33Ni_0.33)O₂ and composite 0.3Li₂MnO₃·0.7LiMn_0.5Ni_0.5O₂ electrode powders have been treated with NH₃ and HNO₃ prior to cell assembly. These pre-conditioning reactions improve the electrochemical properties of the electrodes when charged to high electrochemical potentials (≥4.45 V versus Li⁰) in lithium cells. NH₃-treatment of Li(Co_0.33Mn_0.33Ni_0.33)O₂ increases electrode capacity and improves the coulombic efficiency of the electrochemical reaction after the first charge/discharge cycle, whereas acid-treatment of 0.3Li₂MnO₃·0.7LiMn_0.5Ni_0.5O₂ electrodes significantly improves the coulombic efficiency of the initial charge/discharge reaction.     

Molybdenum effect on the kinetic behavior of a metal hydride electrode

In the present investigation, the effect of the Mo in the LaNi_3,6Co_0,7Mn_(0.4−x)Al_0,3Mo_x AB5-type hydrogen storage alloys was studied. The alloys structural and microstructural characterizations were performed by means of X-ray diffraction phase analysis and scanning electron microscopy.     

Effect of substituting Al for Co on the hydrogen-storage performance of La0.7Mg0.3Ni2.6AlxCo0.5−x (x = 0.0–0.3) alloys

La_0.7Mg_0.3Ni_2.6Al_xCo_0.5−x (x = 0.0–0.3) alloys were prepared by induction melting, and the effects of partially substituting Al for Co on the structure and hydrogen-storage properties of the alloys were investigated systematically. It is found that La(Ni, Co, Al)₅ phase with hexagonal CaCu₅-type structure, LaNi₃ phase with PuNi₃ structure and MgNi₂ phase exist as the main phases in La_0.7Mg_0.3Ni_2.6Al_xCo_0.5−x (x = 0.0–0.3) alloys, and the cell volume of the La(Ni, Co, Al)₅ phase increases with the amount of Al added. The results show that the substitution of Al for Co can reduce the plateau pressure and the hysteresis between hydrogen absorption and desorption, and improve the hydrogen-absorption capacity and thermal stability of the hydride. Moreover, the addition of Al can delay the oxidation of the surface layer of the alloy electrodes in electrolyte, slow down the capacity degradation and prolong the cycling lifetime, and enhance the electrocatalytic activity of the hydrogen-storage electrodes for hydrogen oxidation.     

Nickel-graphene nanocomposite with improved electrochemical performance for La0.7Mg0.3(Ni0.85Co0.15)3.5 electrode

A Ni-rGO nanocomposite was synthesized by a hydrothermal process and La_0.7Mg_0.3(Ni_0.85Co_0.15)_3.5, an AB_3.5-type hydrogen storage alloy, was prepared by magnetic levitation melting under argon atmosphere. The influences of the Ni-rGO nanocomposite on the hydrogen storage and electrochemical performance of the La_0.7Mg_0.3(Ni_0.85Co_0.15)_3.5 alloy were investigated via pressure composition isotherms (PCT) and electrochemical measurements. The PCT curves revealed that the addition of the Ni-rGO nanocomposite improved the reversibility of hydrogen absorption and desorption for the La_0.7Mg_0.3(Ni_0.85Co_0.15)_3.5 alloy. The electrochemical measurements showed that the electrochemical impedance of the La_0.7Mg_0.3(Ni_0.85Co_0.15)_3.5 alloy electrode was significantly reduced, the high rate dischargeability, HRD₁₂₀₀, increased from 60% to 86%, the limiting current density, I_L, increased from 1216.7 mA·g^?1 to 2287.6 mA·g^?1, and the hydrogen diffusion coefficient, D, increased with the added Ni-rGO nanocomposite. These improvements to the electrochemical performance are mainly attributed to the Ni-rGO framework, with the large specific surface area of the graphene, and to the high conductivity of metal nickel.     

Performance analysis of metal hydride based simultaneous cooling and heat transformation system

The performance of metal hydrides based simultaneous cooling and heat transformation system (MHCHT) using a combination of La_0.9Ce_0.1Ni₅–MmNi_4.4Al_0.6–MmNi_3.7Co_0.7Mn_0.3Al_0.3 hydrides is evaluated. The MHCHT is thermodynamically analysed using statically and dynamically measured PCIs and thermodynamic properties. In addition, a set of governing equations is solved in order to study the heat and hydrogen transfer between the reaction beds. The experimental PCI measurement data are compared with the numerical results and a reasonably good agreement is observed between them. From the results, the slope and hysteresis factors are determined for further thermal analyses. It is observed that the performance parameters i.e. cooling capacity, heat transformation capacity and coefficient of performance (COP) of MHCHT are significantly decreased by 42.4%, 26.7% and 19.1% respectively when dynamic property data are considered compared to static property data. In addition, the thermodynamic cycle is analysed by considering the variation in pressure during hydrogen transfer process between the metal hydride beds.     

Preparation and electrochemical properties of La–Mg–Ni-based La0.75Mg0.25Ni3.3Co0.5 multiphase hydrogen storage alloy as negative material of Ni/MH battery

La–Mg–Ni-based La_0.75Mg_0.25Ni_3.3Co_0.5 hydrogen storage alloy was synthesized by high-energy mechanical milling blending of the La_0.75Ni_3.3Co_0.5 as-cast alloy prepared by vacuum arc melting and elemental Mg, and subsequent isothermal annealing. The chemical compositions, microstructures and electrochemical properties of the as-cast La_0.75Ni_3.3Co_0.5 alloy, the milled and annealed La_0.75Mg_0.25Ni_3.3Co_0.5 alloys were investigated, respectively, by inductively coupled plasma, X-ray diffraction, differential scanning calorimetry, X-ray photoelectron spectroscopy and electrochemical measurements. The results show that single LaNi₅ phase exists in the as-cast La_0.75Ni_3.3Co_0.5 alloy. The milled La_0.75Mg_0.25Ni_3.3Co_0.5 alloy contains multiphase structure, besides the main LaNi₅ phase, a small amount of (La,Mg)Ni₃ and (La,Mg)₂Ni₇ new phases are observed as well. The annealed La_0.75Mg_0.25Ni_3.3Co_0.5 alloy is composed of LaNi₅ and (La,Mg)₂Ni₇ phases. Annealing treatment can result in (La,Mg)Ni₃ phase converting into (La,Mg)₂Ni₇ phase. The electrochemical measurements indicate that the maximum discharge capacity and discharge potential characteristic of the as-cast La_0.75Ni_3.3Co_0.5 alloy are better than those of the milled La_0.75Mg_0.25Ni_3.3Co_0.5 alloy, whereas worse than those of the annealed La_0.75Mg_0.25Ni_3.3Co_0.5 alloy. The cyclic stability of the milled La_0.75Mg_0.25Ni_3.3Co_0.5 alloy is slightly better than that of the as-cast La_0.75Ni_3.3Co_0.5 alloy, whereas obviously worse than that of the annealed La_0.75Mg_0.25Ni_3.3Co_0.5 alloy. Overall, the annealed La_0.75Mg_0.25Ni_3.3Co_0.5 alloy performs the best in the maximum discharge capacity, discharge potential characteristic and cycling stability.     

Properties of La0.2Y0.8Ni5−xMnx alloys for high-pressure hydrogen compressor

Hydrogen absorption/desorption properties of La_0.2Y_0.8Ni_5−xMn_x (x = 0.2, 0.3, 0.4) alloys for high-pressure hydrogen compression application were investigated systematically. The Pressure–Composition isotherms and absorption kinetics were measured at 293, 303 and 313 K by the volumetric method. XRD analyses showed that all the investigated alloys presented CaCu₅ type hexagonal structure and the unit cell volume increased in both a and c lattice axes with Mn substitution. Hydrogen absorption/desorption measurements revealed that Mn could lower the plateau pressure effectively, improve the hydrogen storage capacity and absorption kinetics but slightly increase the slope of the pressure plateau and hysteresis. The study results suggest that La_0.2Y_0.8Ni_4.8Mn_0.2 is suitable for the high-pressure stage compression of the hydrogen compressor and the other two alloys, La_0.2Y_0.8Ni_4.7Mn_0.3 and La_0.2Y_0.8Ni_4.6Mn_0.4, for the preliminary stage.     

MmNi3.55Co0.75Mn0.4Al0.3B0.3 hydrogen storage alloys for high-power nickel/metal hydride batteries

Non-stoichiometric La-rich MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 hydrogen storage alloys using B–Ni or B–Fe alloy as additive and Ce-rich MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 one using pure B as additive have been prepared and their microstructure, thermodynamic, and electrochemical characteristics have been examined. It is found that all investigated alloys show good activation performance and high-rate dischargeability though there is a certain decrease in electrochemical capacities compared with the commercial MmNi_3.55Co_0.75Mn_0.4Al_0.3 alloy. MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 alloys using B–Ni alloy as additive or adopting Ce-rich mischmetal show excellent rate capability and can discharge capacity over 190 mAh/g even under 3000 mA/g current density, which display their promising use in the high-power type Ni/MH battery. The electrochemical performances of these MmNi_3.55Co_0.75Mn_0.4Al_0.3B_0.3 alloys are well correlated with their microstructure, thermodynamic, and kinetic characteristics.     

Electrocatalytic activity of crystalline Ni–Co–M (M = Cr,Mn, Cu) alloys on the oxygen evolution reaction in an alkaline environment

Ternary Ni₆₀Co₃₀M₁₀ (M = Cr, Mn, Cu) crystalline alloys have been characterized by means of microstructural and electrochemical techniques in view of their possible applications as electrocatalytic materials for oxygen evolution reaction (OER). The electrochemical efficiency of the electrodes has been studied on the basis of electrochemical data obtained from steady-state polarization and electrochemical impedance spectroscopy (EIS) techniques in 1 M NaOH solution at 298 K. The results were compared with those obtained on a Ni₆₀Co₄₀ commercial alloy. The overall experimental data indicate that alloying Ni–Co with Cr, Mn and Cu leads to an increase of electrocatalytic activity in oxygen evolution with respect to the Ni–Co alloy. High catalytic efficiencies were achieved on Ni₆₀Co₃₀Mn₁₀ and Ni₆₀Co₃₀Cr₁₀ electrodes, the latter being the best electrocatalyst for the OER.     

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.     

Influence of the synthesis route on hydrogen sorption properties of La2MgNi7Co2 alloy

Solid-gas and electrochemical hydrogenation properties of La₂MgNi₇Co₂ alloy are presented. Hydrogen concentration of 1.90 wt% at hydrogen pressure of 10 bar has been reached. The influence of the fabrication technology of La₂MgNi₇Co₂ alloy on electrochemical performance of the hydride electrode were studied and discussed. To evaluate electrochemical characteristics of La₂MgNi₇Co₂ electrodes including discharge capacity, self-discharge and kinetic parameters the galvanostatic charge/discharge technique was used. The studied samples were a multiphase. The presence of Mg-enriched phases (La₂MgO_x, (La, Mg)Ni₃ and LaMgNi₄) raises hydrogen capacity and makes an electrode less susceptible for the self-discharge effect. On the other hand Mg-presence in MH electrodes lowers the hydrogen desorption rate. It was found that, the dominant abundance of the LaNi₅ phase in the tested materials has a positive effect on the kinetic parameters of the hydride electrode.