Electrochemical characteristics of metal hydride electrodes for nickel/metal hydride rechargeable batteries.

摘要

The electrochemical characteristics of LaNi_4.7Al _0.3, LaNi_4.7Al_0.3 (with Cu-coating), Mm_0.95 Ti_0.05Ni_3.85Co_0.45Mn_0.35Al _0.35 and Mm(Ni_0.71Co_0.14Al_0.08Mn _0.06)_5.02 alloy electrodes are examined in detail. The specific discharge capacity of the cell made using the LaNi_4.7Al_0.3 , Mm_0.95Ti_0.05Ni_3.85Co_0.45Mn _0.35Al_0.35 and Mm(Ni_0.71Co_0.14Al _0.08Mn_0.06)_5.02 alloys maintain 250 mAh g −1 at 100–120 mA g−1 discharge current density after 20, 40 and 200 cycles respectively. Thus with regard to cycle lifetime the Mm(Ni_0.71Co_0.14Al_0.08Mn _0.06)_5.02 and Mm_0.95Ti_0.05Ni_3.85 Co_0.45Mn_0.35Al_0.35 alloys are considerably superior to LaNi_4.7Al_0.3 alloy. At the same number of cycles, all the electrochemical properties are related to the hydrogen concentration, i.e., depth of discharge (DOD). As the hydrogen concentration decreases, the exchange current density, the apparent activation energy, the hydrogen diffusion coefficient and the symmetry factor increase. The equilibrium potential increases (i.e. becomes more positive) with decreasing hydrogen concentration.; Almost all the electrochemical properties are temperature related. As the temperature increases, both the exchange current density and the symmetry factor increase. The specific discharge capacity reaches a maximum value at room temperature for the Mm(Ni_0.71Co_0.14Al_0.08 Mn_0.06)_5.02 alloy. The equilibrium potential decreases with increasing temperature. With increasing number of cycles, the exchange current density, the ratio of D/a2 ( D = hydrogen diffusivity; a = sphere radius) and the equilibrium potential increase with cycles, stabilizing after 30–40 cycles. Cu-coating of the electrode alloys increases the exchange current density and the high-rate dischargeability of the metal hydride electrodes, and decreases the discharge potential, especially for higher discharge current densities. The discharge potentials for the Cu-coated electrode showed little change with discharge current density differences, indicating the stabilizing effect of Cu-coating on the battery performance.; A theoretical treatment is derived to account for the two-phase (α-β) region of pressure-composition (P-C) isotherms of hydrogen-absorbing alloys by considering H-H interaction kinetics. Based on electrochemical reaction kinetics, a theoretical model on the relationship between equilibrium potential and hydrogen concentration is established for the equilibrium discharge process of a MH electrode. The relationship between equilibrium potential of a metal hydride electrode reaction and hydrogen pressure in a gaseous hydrogen environment is also derived and thus, E-C-T curves can be accurately transferred to P-C-T curves and vice versa. These theoretical equations are of particular use in evaluating suitable electrode alloys.; A novel and relatively simple electrochemical method, called “Potential Step Chrono-Amperometry (PSCA)” method, is developed to determine the hydrogen diffusion coefficient and its variation with hydrogen concentration. Using this method, the value of the room temperature diffusion coefficient of hydrogen in a LaNi_4.7Al_0.3 alloy is found to be in the range of 3.1 × 10−14 to 8.6 × 10 −13 m2 s−1, and