F. Feng

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

Abstract 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 AB 2 and AB 5 ), 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.

The application of mathematical models to the calculation of selected hydrogen storage properties (formation enthalpy and hysteresis) of AB2-type alloys

Abstract Two mathematical models have been applied to AB2-type hydrogen-absorbing alloys. The first model is for the calculation of hydride formation enthalpy and the second model allows for the calculation of P-C-T curves. Certain physical parameters (activity coefficient of hydrogen (γ), partial molar volume of hydrogen (VH), solution heat of hydrogen (ΔHs), enthalpy (ΔH) and entropy (ΔS) of formation of a hydride, slope factor (fs) of a plateau and the variation rate (k) of slope factor with respect to temperature in a plateau region of P-C-T curves) for these intermetallic compounds and their hydrides are estimated from these models. From the second model, the relationship between the hysteresis factor (RT ln Pa/Pd) and temperature, hydrogen concentration and slope factor of the plateau region for the P-C-T curves has been obtained.

Charging/discharging stability of a metal hydride battery electrode

The metal hydride (MH) alloy powder for the negative electrode of the Ni/MH battery was first pulverized and oxidized by electrochemically charging and discharging for a number of cycles. The plate of the negative electrode of an experimental cell in this study was made from a mixture of a multicomponent AB{sub 5}-based alloy powder, nickel powder, and polytetra fluoroethylene (PTFE). The characteristics of the negative electrode, including discharge capacity, exchange current density, and hydrogen diffusivity, were studied by means of the electrochemical experiments and analysis in an experimental cell. The exchange current density of a Mm{sub 0.95}Ti{sub 0.05}Ni{sub 3.85}Co{sub 0.45}Mn{sub 0.35}Al{sub 0.35} alloy electrode increases with increasing number of charge/discharge cycles and then remains almost constant after 20 cycles. A microcracking activation, resulting from an increase in reaction surface area and an improvement in the electrode surface activation, increases the hydrogen exchange current densities. Measurement of hydrogen diffusivities for Mm{sub 0.95}Ti{sub 0.05}Ni{sub 3.85}Co{sub 0.45}Mn{sub 0.35}Al{sub 0.35} alloy powder shows that the ratio of D/a{sup 2} (D = hydrogen diffusivity; a = sphere radius) increases with increasing number of cycles but remains constant after 20 cycles.

Electrochemical measurements of a metal hydride electrode for the Ni/MH battery

Abstract The characteristics of the Ni/MH battery, including discharge voltage, high-rate discharge capability and charge/discharge cycle lifetime are mainly determined by the construction of the negative and positive electrodes and the composition of the hydrogen-absorbing alloy. The metal hydride (MH) alloy powder for the negative electrode of the Ni/MH battery was first pulverized and oxidized by electrochemically charging/discharging for a number of cycles. A multicomponent AB5-based alloy Mm0.95Ti0.05Ni3.85Co0.45Mn0.35Al0.35 was developed as the hydrogen-absorbing alloy. The discharge characteristics of the negative electrode, including discharge capacity, cycle lifetime and exchange current density, were studied by means of the electrochemical experiments and analysis in the experimental cell. The polarization measurements show that the exchange current density of the MH electrode increases with increasing number of charge/discharge cycles and then it almost remains constant after 30 cycles. A microcracking activation, resulting from an increase in reaction surface area and an improvement in the electrode surface activation, decreases the polarization resistance and increases the hydrogen exchange current densities. Measurement of the hydrogen diffusivities for the Mm0.95Ti0.05Ni3.85Co0.45Mn0.35Al0.35 alloy shows that the ratio D/a2 (D=hydrogen diffusivity; a=sphere radius) increases with increasing number of cycles up to 20 cycles and it remains constant after further cycling.

Mathematical model for the plateau region of P–C-isotherms of hydrogen-absorbing alloys using hydrogen reaction kinetics

Abstract A theoretical treatment is presented to account for the two-phase (α–β) region of pressure-composition (P–C) isotherms of hydrogen-absorbing alloys, i.e., the dependence of equilibrium pressure on the hydrogen concentration in hydrogen-absorbing alloys at different temperatures. The model is based upon a consideration of the hydrogen–metal reaction kinetics (i.e., static equilibrium condition between adsorbed and absorbed hydrogen at the particle interface) and the hydrogen–hydrogen (H–H) interaction kinetics. The relationship between the slope, the pressure level and the length of plateau region of the P–C isotherms can be derived from this expression and this then enables us to understand the hydrogen reaction mechanism in the alloy powders. The model is found to fit very well with experimental data obtained on LaNi4.7Al0.3. It can be used to analyse the enthalpies for the hydride decomposition and hydride formation processes, and the slope changes for the two-phase (α and β) coexistence region. The average enthalpies (H/M=0.02 to 0.9) for hydride decomposition and hydride formation of the LaNi4.7Al0.3 alloy are ΔH d =34.5 kJ mol H 2 −1 and ΔH a =−31.8 kJ mol H 2 −1 . The effect of lateral H–H interactions on the values of equilibrium pressure and variation of slope with temperature are discussed. H–H interactions contribute to the slope, and the slope of P–C isotherms will become lower when the degree of H–H interaction is high in the LaNi4.7Al0.3 alloy. The H–H interactions do not contribute to the hysteresis of P–C isotherms for hydrogen-absorbing alloys. It is proposed that hysteresis originates from energy differences rather than from entropy differences.

Electrochemical characteristics of the interface between the metal hydride electrode and electrolyte for an advanced nickel/metal hydride battery

Abstract The characteristics of the negative electrode of a Ni/MH (metal hydride) battery are related to the charge transfer and mass transfer processes at the interface between the MH electrode and the electrolyte. With increasing number of charge/discharge cycles, the MH alloy powders micro-crack into particles that are several microns in diameter and this then influences the exchange current density. A polarization experiment was used to analyze the charge transfer and mass transfer processes. The exchange current densities of uncoated and Pd-coated Mm 0.95 Ti 0.05 Ni 3.85 Co 0.45 Mn 0.35 Al 0.35 alloy electrodes increase with increasing number of charge/discharge cycles before reaching a constant value after 20–30 cycles.

Hydrogen-absorbing alloys for the NICKEL–METAL hydride battery

Abstract In recent years, owing to the rapid development of portable electronic and electrical appliances, the market for rechargeable batteries has increased at a high rate. The nickel-metal hydride battery (Ni/MH) is one of the more promising types, because of its high capacity, high-rate charge/discharge capability and non-polluting nature. This type of battery uses a hydrogen storage alloy as its negative electrode. The characteristics of the Ni/MH battery, including discharge voltage, high-rate discharge capability and charge/discharge cycle lifetime are mainly determined by the construction of the negative electrode and the composition of the hydrogen-absorbing alloy. The negative electrode of the Ni/MH battery described in this paper was made from a mixture of hydrogen-absorbing alloy, nickel powder and polytetrafluoroethylene (PTFE). A multicomponent MmNi5-based alloy (Mm0.95Ti0.05Ni3.85 Co0.45Mn0.35Al0.35) was used as the hydrogen-absorbing alloy. The discharge characteristics of the negative electrode, including discharge capacity, cycle lifetime, and polarization overpotential, were studied by means of electrochemical experiments and analysis. The decay of the discharge capacity for the Ni/MH battery (AA size, 1 Ah) was about 1% after 100 charge/discharge cycles and 10% after 500 charge/discharge cycles.

Cyclic Voltametry Investigation of a Metal Hydride Electrode for Nickel Metal Hydride Batteries

In this study, an -type hydrogen-absorbing alloy, was used as the negative electrode material. The metal-hydride (MH) electrode was charge/discharged over 200 cycles. The discharge capacity of the alloy was 250 mAh g−1. The specific discharge capacity as a function of discharge current density and temperature effect is also discussed. Hydrogen diffusion reaction in the MH alloy powder of the negative MH alloy electrode dominates the high-rate discharge capability of the metal hydride electrode in a nickel metal hydride (Ni/MH) battery. Cyclic voltametry technique was used to analyze the charge-transfer reactions at the electrode/electrolyte interface and hydrogen surface coverage capacity. The charge-transfer reaction in the negative electrode reflects a capability of hydrogen reduction and oxidation reactions at the electrode/electrolyte interface.

Charge transfer and mass transfer reactions in the metal hydride electrode

Abstract 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 AB 5 -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.

Anodic polarization and galvanostatic investigation of a metal hydride alloy electrode

Abstract Hydrogen diffusion in the metal hydride dominates the high-rate discharge capability of the metal hydride electrode in a nickel metal hydride (Ni/MH) battery. In this study, an AB5-type hydrogen-absorbing alloy was used as the metal hydride electrode material. The oxidation and the rate-determining mass transfer reactions in the metal hydride electrode were studied and analyzed using anodic polarization measurements. The diffusion coefficient of hydrogen in the metal hydride alloy was determined using both anodic polarization measurements at varying hydrogen concentrations and a galvanostatic technique. The diffusion coefficient of hydrogen in the MH alloy is of the order of 10 −11 cm 2 s −1 assuming that the average radius of the alloy particles is 5 μm . The anodic polarization performance of an electrode without hydrogen in the alloy is related both to the oxidation of the alloy powder and to the formation of a passive layer on the surface of the alloy powder.