Modeling the Effects of Ion Association on Direct‐Current Polarization of Solid Polymer Electrolytes

Abstract

Considerable experimental evidence indicates that ion association occurs in solid polymer electrolytes. This work provides a thorough theoretical analysis of the effect of ion association on the conductivity, general current‐potential behavior, and limiting current density in a solid polymer electrolyte. The model employs dilute solution theory to describe the fluxes of cations, anions, and ion pairs in a motionless continuum but neglects higher order association. The predictions of the model highlight the effects of the relative diffusion coefficients and dimensionless association constant on concentration distributions of simple ions and ion pairs, the limiting current density, and the potential drop required to drive a specified current density. If ion pairs have a diffusivity comparable to those of cations and anions, increasing ion association leads to a continuous decrease in molar conductivity and current density at constant applied potential. If the ion pairs have a diffusivity that is large compared to cations, the situation is quite different. In this case, ion association increases the limiting current density to values that may be several times that found in the case of full dissociation. Furthermore, the model predicts maxima in the molar conductivity and current density at fixed potential drop as the degree of ion association increases. © 2000 The Electrochemical Society. All rights reserved.