Derek O. Northwood

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.

Enhanced electrochemical properties of ball-milled Mg2Ni electrodes

Abstract The characteristics and mechanisms of enhanced electrochemical properties for a Mg 2 Ni alloy produced by a ball-milling treatment were investigated by electrochemical measurements and XRD analysis. The particle size decreases, the internal strain increases, and the alloy gradually becomes amorphous on ball-milling. With the accompanying increased diffusion rate of hydrogen into the ball-milled Mg 2 Ni alloy, the limiting current density, i crit , is decreased and the exchange current density, i 0 , is increased. As a result, the specific discharge capacity is significantly increased compared to that of the as-received Mg 2 Ni alloy and reaches a maximum for a ball-milling time of 25 h .

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.

A new empirical formula for the bainite upper temperature limit of steel

AbstractThe definition of the practical upper temperature limit of the bainite reaction in steels is discussed. Because the theoretical upper temperature limit of bainite reaction, B0, can neither be obtained directly from experimental measurements, nor from calculations, then, different models related to the practical upper temperature limit of bainite reaction, BS, are reviewed and analyzed first in order to define the B0 temperature. A new physical significance of the BS and B0 temperatures in steels is proposed and analyzed. It is found that the B0 temperature of the bainite reaction in steels can be defined by the point of intersection between the thermodynamic equilibrium curve for the austenite→ferrite transformation by coherent growth (curve Z

Room temperature creep of a high strength steel

\gamma \to \overrightarrow \alpha

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

) and the extrapolated thermodynamic equilibrium curve for the austenite→cementite transformation (curve ES in the Fe-C phase diagram). The BS temperature for the bainite reaction is about 50–55 °C lower than the B0 temperature in steels. Using this method, the B0 and BS temperatures for plain carbon steels were found to be 680 °C and 630 °C, respectively. The bainite reaction can only be observed below 500 °C because it is obscured by the pearlite reaction which occurs prior to the bainite reaction in plain carbon steels. A new formula, BS(°C) =, 630-45Mn-40V-35Si-30Cr-25Mo-20Ni-15W, is proposed to predict the BS temperature of steel. The effect of steel composition on the BS temperature is discussed. It is shown that BS is mainly affected by alloying elements other than carbon, which had been found in previous investigations. The new formula gives a better agreement with experimental results than for 3 other empirical formulae when data from 82 low alloy steels from were examined. For more than 70% of these low alloy steels, the BS temperatures can be predicted by this new formula within ±25°C. It is believed that the new equation will have more extensive applicability than existing equations since it is based on data for a wide range of steel compositions (7 alloying elements).

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

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.

Temperature, cycling, discharge current and self-discharge electrochemical studies to evaluate the performance of a pellet metal-hydride electrode

Abstract The time dependent deformation at room temperature of a high strength steel was investigated. The room temperature creep tests showed that creep can occur below 1/3 σ0.2 (yield strength at 0.2% offset). The resulting creep behavior consists of only two stages, including primary creep and steady-state creep, each of which has its own distinctive strain–time features. The effects of creep stress, creep time, steel hardness and heat treatment schedule on the room temperature creep were investigated. It is believed that the increased creep strains can be attributed to higher applied stresses, longer creep times, lower hardnesses and the existence of an inhomogeneous microstructure. However, increasing the number of cycles in cyclic creep tests at room temperature resulted in a decrease in creep strain. Possible room temperature creep mechanisms have been proposed and discussed.