A MATHEMATICAL MODEL FOR THE POROUS NICKEL HYDROXIDE ELECTRODE.

摘要

A one-dimensional model of the porous nickel hydroxide electrode has been presented, based on porous electrode theory. The specific equations used in the model were the equation for current in the solution, the kinetic expression, the charge balance equation, the overall material balance equation, the material balance in the solid state, and the material balance for a minority species. An activation controlled rate expression for the charge-discharge reaction; (beta)-Ni(OH)(,2) + OH('-) (DBLARR) (beta)-NiOOH = H(,2)O + e('-); is proposed. The apparent electrochemical transfer coefficients(alpha)(,a), (alpha)(,c) and the exchange current density i(,0) were determined from potentiostatic polarization of film electrodes. The electrodes were prepared by electrolytic impregnation in a 1.8M nickel-nitrate-0.2M cobalt nitrate solution in 50% ethanol-50% water. The values determined were 0.17, 0.10 and 6.1 x 10('-5) A/cm('2) for (alpha)(,a), (alpha)(,c), and i(,0), respectively, in 30% KOH at 25(DEGREES)C. Experiments in 20% KOH were also conducted, yielding a value of 5.8 x 10('-5) A/cm('2) for i(,0).; Using the values of these coefficients and other data reported in the literature, the porous electrode model was solved with an IBM 3033 computer. Galvanostatic charging and discharging of electrodes at various currents corresponding to 1-hour, 2-hour, 5-hour, 10-hour, and 20-hour rates were simulated. The variables predicted by the model were nickel hydroxide reaction rate, current and potential in the solution, volume average flow rate of electrolyte, electrode porosity, and electrolyte concentration as functions of time and electrode width. The model reproduces charge-discharge cycles and hysteresis satisfactorily and also calculates the charging and ampere-hour efficiencies. The important results obtained were that the uniformity of the rate of reaction across the electrode width increases with decreasing charge rate, the loss due to hysteresis increases with increasing charge rate and with decreasing specific area, the charging and ampere-hour efficiencies increase with the charge rate, and the electrode capacity decreases with increasing discharge rate. The results obtained were consistent with experimental data reported in the literature.