High temperature performance of La0.6Ce0.4Ni3.45Co0.75Mn0.7Al0.1 hydrogen storage alloy for nickel/metal hydride batteries

La_0.6Ce_0.4Ni_3.45Co_0.75Mn_0.7Al_0.1 hydrogen storage alloy has been prepared and its electrochemical characteristics and gas hydrogen absorption/desorption properties have been investigated at different temperatures. X-ray diffraction results indicated that the alloy consists of a single phase with CaCu₅-type structure. It is found that the investigated alloy shows good cycle performance and high-rate discharge ability, which display its promising use in the high-power type Ni-MH battery. The exchange current density and the diffusion coefficient of hydrogen in the bulky electrode increase with increasing temperature, indicating that increasing temperature is beneficial to charge-transfer reaction and hydrogen diffusion. However, the maximum discharge capacity, the charge retention and the cycling stability degrade with the increase of the temperature.     

Effects of H2O2 addition to the cell balance and self-discharge of Ni/MH batteries with AB5 and A2B7 alloys

In this paper we have compared nickel/metal hydride batteries made from AB₅ and Nd-only A₂B₇ alloys with or without addition of hydrogen peroxide (H₂O₂). The biggest advantages Nd-only A₂B₇ alloys have over AB₅ alloys are: a higher positive electrode utilization rate, lower initial internal resistance and less resistance increase after a 60 °C storage, and higher capacity and resistance degradation during cycling. The hydrogen peroxide was used as an oxidation agent and was added into the electrolyte before closing the cells. The H₂O₂ can oxidize both Co(OH)₂ in the positive electrode and MH alloy in the negative electrode. From the test results, H₂O₂ oxides the MH alloy preferentially over the Co(OH)₂ in the case of AB₅ alloy. This preferential oxidation is reversed in the case of the A₂B₇ alloy in which Co(OH)₂ is oxidized first. In cells made from both alloys, the addition of H₂O₂ prevented the venting of cells during formation, increased the utilization of positive electrode, improved the 60 °C charge retention, and increased the mid-point voltage after 300 cycles. Additionally the H₂O₂ also improved the cell balance for A₂B₇ alloy by decreasing the over-discharge reservoir in the negative electrode and reducing the capacity degradation in A₂B₇ alloy. However, the addition of H₂O₂ in cells made with AB₅ alloy deteriorated the cell balance by increasing the over-discharge reservoir in the negative electrode. The different cell balance and failure mechanisms for the two alloy compositions and H₂O₂ additive were compared and discussed.     

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.     

Design and optimization of single particle electrodes for the kinetic analysis of hydrogen evolution and absorption on hydrogen storage alloys

In the literature, there is a large discrepancy between reported values of electrochemical, kinetic and transport parameters of hydrogen storage alloys. These discrepancies arise, because in most cases, electrodes are prepared with the powdered alloy supported within a porous matrix, constituted by carbon and additive binders such as PTFE. The main drawback, of this preparation technique, for the identification of kinetic parameters, is the uncertainty in the specific active area value, where the hydrogen evolution and absorption processes take place. To overcome the disadvantages described, a new type of electrode, was designed, using a single particle of AB₅ and AB₂ hydride forming alloys. The data obtained from electrochemical impedance measurements were adjusted in terms of a physicochemical model that takes into account the processes of hydrogen evolution and absorption coupled to hydrogen diffusion. From the study it can be concluded that the differences in the behavior of the AB₅ and AB₂ alloys, presenting the first best performance during the activation and operation at high discharge currents, are mainly due to higher values of the exchange current density and the diffusion coefficient of H for the AB₅ alloy.     

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.     

Investigations of synthesis and characterization of MmNi4.3Al0.3Mn0.4 AND MmNi4.0Al0.3Mn0.4Si0.3, hydrogen storage materials through thermal and spin melting processes

The present study deals with investigations on the synthesis and characterization of negative electrode material for high energy density Ni-MH battery. The hydrogen storage material (MH) has been synthesized through thermal and spin melting techniques. A comparative study of materials synthesized by these two techniques with emphasis on the characteristics relevant to battery electrode applications has been carried out. In the present study, the modified composition of AB₅-type corresponds to the spin as well as thermal melted versions of MmNi_4.3Al_0.3Mn_0.4 and MmNi_4.0Al_0.3Mn_0.4Si_0.3.Structural characterization has revealed that, whereas for the spin melted MmNi_4.3Al_0.3Mn_0.4 the dominant growth is perpendicular to the c-axis, it is parallel to the c-axis for MmNi_4.0Al_0.3Mn_0.4Si_0.3. The hydrogenation behaviour of these materials has been monitored through P-C-T and kinetic curves. Attempts have been made to establish a correlation between the structure and hydrogenation behaviour. The spin melted material (MmNi_4.3Al_0.3Mn_0.4) exhibits reduced pulverization and hence is expected to have increased cycle life. This version of the material also exhibits higher storage capacity, faster kinetics and faster activation as compared to the conventionally prepared bulk form. The bulk version of the alloy with silicon has been found to undergo easy activation (2nd cycle) as compared to the bulk version of the alloy without silicon (6th cycle). The spin melted version of the material with silicon leads to smaller (finer) particle size material compared to the alloy form without silicon.     

Structural,hydrogen storage,and electrochemical performance of LaMgNi4 alloy and theoretical investigation of its hydrides

Structural, hydrogen storage, and electrochemical properties of LaMgNi₄ alloy were investigated in this study to determine whether it can be used as an active material of the negative electrode in nickel–metal hydride (Ni/MH) batteries. X-ray diffraction study showed that amorphization occurs at the first dehydrogenation cycle and was recovered crystallization after 873 K annealing.Maximum hydrogen storage capacity reached 1.4 wt% in the first hydrogenation under 373 K. The reannealed alloy showed improved reversible hydrogen storage capacity at ～0.9 wt% due to more LaNi₅ phase composition. Electrodes prepared from the investigated alloy showed maximum discharge capacities of ～340 mAh/g at 10 mA/g. The LaMgNi₄ alloy electrode exhibited satisfactory cycling stability remaining 47% of its initial capacity after 250 cycles. The negative cohesive energy indicated the exothermic process and stable compound structures of the LaMgNi₄ alloy and its hydrides via Density functional theory calculations.     

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.     

Metal hydride hydrogen storage and compression systems for energy storage technologies

Along with a brief overview of literature data on energy storage technologies utilising hydrogen and metal hydrides, this article presents results of the related R&D activities carried out by the authors. The focus is put on proper selection of metal hydride materials on the basis of AB₅- and AB₂-type intermetallic compounds for hydrogen storage and compression applications, based on the analysis of PCT properties of the materials in systems with H₂ gas. The article also presents features of integrated energy storage systems utilising metal hydride hydrogen storage and compression, as well as their metal hydride based components developed at IPCP and HySA Systems.     

Charge transfer and mass transfer reactions in the metal hydride electrode

Charge transfer reaction across the electrode/electrolyte interface and hydrogen diffusion in the negative MH alloy electrode dominate the high-rate discharge capability of the metal hydride electrode in a nickel metal hydride (Ni/MH) battery. The mass transfer process in the MH electrode mainly involves hydrogen diffusion in the bulk MH alloy. The charge transfer reaction in the negative electrode reflects the capability of hydrogen reduction and oxidation reactions at the surface of the MH alloy powder. In this study, an AB₅-type hydrogen-absorbing alloy was used as the negative electrode material. The rate-determining mass transfer process in the bulk MH alloy electrode was studied and analyzed using anodic polarization measurements. The exchange current density, which is related to the charge transfer reaction, was analyzed by using the hydrogen equilibrium pressure. The estimation of hydrogen diffusion coefficient in the MH alloy is strongly dependent on the value of the effective reaction area of charge transfer reaction at the surface of the alloy powder.     

Influence of rare earth content on electrode kinetics in Misch metal-based AB5 MH alloys – Cyclic voltammetric investigations

The influence of lanthanum/cerium ratio (0.42–14.14) in Misch metal (Mm)-based AB₅-type hydrogen storage alloys has been investigated. The samples were subjected to X-ray florescence (XRF) and cyclic voltammetry (CV) studies. The metal hydride electrodes were assembled, and their discharge capacity was determined. These alloys delivered discharge capacity between 118 and 266 mAh/g. CV investigations threw light on charge-transfer reactions at the electrode/electrolyte interface and hydrogen surface coverage capacity. The CV parameters in general indicate that the battery activity increases with lanthanum/cerium ratio. At low lanthanum/cerium values, the discharge reactions proceed at potentials that are more negative. Furthermore, electrochemical reversibility of the hydrogen absorption–desorption on the alloy surface is enhanced at optimum lanthanum/cerium ratio as revealed by decreasing values of peak separation. The slopes of graphical plots of I_peak (anodic) vs. ν^1/2 for the fully charged samples reverse direction at very high lanthanum/cerium values. The results suggest that the discharge plateau is optimum at lanthanum/cerium ratio around 12.     

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.     

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.     

Study of hydrogen storage properties of oxygen modified Ti- based AB2 type metal hydride alloy

A multi component AB₂ type hydrogen storage intermetallic alloy (A = Ti_0.85Zr_0.15, B₂ = Mn_1.22Ni_0.22Cr_0.2V_0.3Fe_0.06; was investigated in this work. The intermetallic specified above was modified by oxygen to yield the composition AB₂O_0.05. The oxygen was introduced by adding TiO₂ to the charge, with corresponding decrease of the Ti amount, followed by arc melting and annealing at the same conditions as for the oxygen free AB₂-type alloy. The addition of oxygen to the alloy did not change much the PCT properties; the only difference was that the plateau pressure for the oxygen-modified alloy increased slightly. Both alloys have shown to be excellent candidates for H₂ storage, particularly for utility vehicles, due to their relatively high reversible H₂ storage capacity (1.6 wt%) and low plateau pressure at room temperature (<5 bar). The addition of oxygen improved hydrogen absorption kinetics in the AB₂ alloy allowing it to immediately absorb H₂ without activation while for the non-modified sample an incubation period (30 min) was observed at the same conditions.     

Improvement in the electrochemical properties of gas atomized AB2 metal hydride alloys by hydrogen annealing

The effects of high temperature hydrogen annealing were studied on powders made by gas atomization of both conventional vanadium-containing AB₂ metal hydride alloys and new vanadium-free AB₂ alloys designed for high power and low self-discharge applications. In both alloy systems, annealing in 950 °C hydrogen for 30 min was proven to be effective in improving the capacity, formation, high power, and low temperature performance in the nickel metal hydride battery compared to previous gas atomization trials where each property was reduced. The advantage in improving the cycle life by gas atomization was further extended by the hydrogen annealing process. Reduction in the surface oxide was confirmed by the use of Auger electron spectroscopy depth profiling and magnetic susceptibility. Metallic elements were reduced from the oxide state by hydrogen to react with the metallic nickel particulates originally embedded in the surface oxide in a high temperature environment and created a new surface free of oxygen.     

Modelling and experimental results of heat transfer in a metal hydride store during hydrogen charge and discharge

A numerical model for the transient hydrogen charge/discharge rates and thermal behaviour of metal hydride stores was developed and verified against experiments using a cylindrical reactor filled with AB₅-type metal hydride. The model assumes local thermal equilibrium between the gas and solid phases, and incorporates the pressure and temperature-dependent hydrogen reaction rates, as well as heat transfer in the porous metal hydride bed. The model was verified through experimental data. The experiments were performed using a unit with hydrogen storage capacity of 130 Nl H₂; the store was submerged in an isothermal water bath. Experiments at different water bath temperatures and charge/discharge hydrogen pressures indicated a relation between charge/discharge time and these parameters. The reactor's ability to deliver a constant hydrogen flow at different water bath temperatures was experimentally investigated. During simulations it was found that the model applied is sensitive to perturbations of some of its parameters; activation energy of absorption, effective conductivity and heat of reaction were found to be the most important ones. The charge and discharge performances of the store are controlled by the reaction rate in the first half-part of the H absorption/desorption experiments and by a heat transfer in the second half-part of charge/discharge.     

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.     

Investigations on calculation of heat of formation for multi-element AB5-type hydrogen storage alloy

With development in hydrogen energy research, more and more applications of hydrogen storage materials have been put forward. This requires synthesis of new materials for specific purpose. In context to designing of metal hydride bed, thermodynamic parameter ‘Heat of formation’ (ΔH) for hydrogen storage alloy is very important. Theoretical calculation of ΔH for binary compound or ternary hydride is accomplished by well known ‘Miedema's Rule of Reverse Stability’. Experimentally ΔH may be determined using Van't Hoff Equation. So far, theoretical calculation of ΔH for multi-element alloy is not known. In the present investigation simple phenomenological formulae have been proposed to calculate ΔH for multi-element alloy including AH_m, BH_m, AB_n, AB_nH_2m, AB_n-xC_x, AB_n-xC_xH_2m, AB_n-x-yC_xD_y, AB_n-x-yC_xD_yH_2m and so on. The calculated values of ΔH in present investigation have been compared with the experimental reported value or calculated by any other model reported in literature. An excellent agreement has been observed between the two.     

Carbon nanotube buckypaper/MmNi5 composite film as anode for Ni/MH batteries

An efficient and cost-effective method has been developed to produce high quality buckypapers from multi-walled carbon nanotubes. The buckypapers were then used as a substrate for AB₅ hydrogen storage alloy electrodes, and electrochemical performances of these composite films in Ni/MH batteries have been investigated. AB₅ alloy was coated on the buckypaper using magnetron sputtering. The buckypapers prepared by our approach were thin, highly flexible and provided sufficient strength as substrates for the hydrogen storage alloy film. A good contact between the buckypaper and the MmNi₅ alloy was established. The electrochemical results show that the buckypapers can be a versatile replacement for conventional metal substrates for the anode in Ni/MH batteries. They provide exceptional electrical conductivity and significant reduction in weight and cost. The obtained maximum discharge capacity of 276 mAh/g for BM-1 electrode is higher than what was previously obtained on electrode with metal substrate of 220 mAh/g. Amongst the two different thickness of AB₅ film studied, it was found that the reduction of MmNi₅ layer thickness enhanced the discharge capacity of the electrode, but the high rate discharge capability was irrelevant to the film thickness. However, the thicker film exhibits better chargeability and cycle stability. Thus all these are beneficial for the miniaturisation of the Ni/MH batteries.     

Hydriding behavior in Zr-based AB2 alloy by gas atomization process

The hydriding characteristics of Zr-based AB₂ alloy produced by gas atomization have been investigated during its absorption–desorption reaction with hydrogen gas. Its gas-phase hydrogenation properties are different from those of specimens prepared by conventional methods. For the particle morphology of the as-cast and gas-atomized powders, it can be seen that the mechanically crushed powders are irregular, while the atomized powder particles are spherical. In PCT (Pressure–Composition–Temperature) measurements, for the gas-atomized particles smaller than 50 μm, the hydrogen storage capacity is dramatically decreased and the hysteresis loop becomes larger than that of the gas-atomized particles larger than 50 μm. In addition, the increase of jet pressure of gas atomization results in the decrease of hydrogen storage capacity and the slope of plateau pressure significantly increases. TEM and EDS studies showed the increase of jet pressure in the atomization process accelerated the phase separation within grain of the gas-atomized alloy, which brought about a poor hydrogenation property. In the measurements of hydrogen absorption–desorption kinetics, the improvement of desorption kinetics of gas-atomized AB₂ alloys was mainly caused by the higher plateau pressure, which is attributed to the smaller grain size and higher site energy for hydrogen in the gas-atomized alloys.