A mathematical model for predicting nonuniform electrochemical impregnation of nickel hydroxide

This paper presents a mathematical model for the electrochemical impregnation of nickel hydroxide in the type of porous nickel plaque used commonly in the positive electrode of Ni/Cd and Ni/H{sub 2} cells. The model includes the transport of Ni{sup 2+}, NO{sub 3}{sup {minus}}, and H{sup +} ions, the electrochemical reduction of NO{sub 3}{sup {minus}}, and the homogeneous acid/base and precipitation reactions. The effect of plaque thickness, plaque tortuosity, and applied current density on the time-dependent porosity distribution and on the uniformity of Ni(OH){sub 2} loading are discussed. Predictions for both constant-current and constant-potential operating conditions are presented. It is shown that a change in tortuosity from 1.6 to 3.0 can yield a 27% decrease in uniformity at 30 mA/cm{sup 2} for a 0.075 cm thick plaque at 1.6 g/cm{sup 3} of void loading. It is also shown that when the current density is changed from 30 to 40 mA/cm{sup 2}, the uniformity decreased by 20% for a plaque of the same thickness with a tortuosity of 1.6 at the same loading. The model could be used to develop quality-control tools to insure a uniformly loaded plaque during electrochemical impregnation.