Steady state current flow in solid binary electrolyte cells. Part 2. The effect of ion association

The steady state current flow following the application of a constant voltage to cells of the form M(s)< M⁺X^-(s)|M(s) where M⁺X^-(s) is a binary solid electrolyte, such as a polymer electrolyte, containing mobile M⁺ cations, X^- anions and MX ion pairs, is discussed. General equations are derived for the effective conductivity of the electrolyte in terms of the diffusion coefficients of the three species and the association constant. A number of special cases which are of particular importance to the study of polymer electrolytes are considered briefly. It is shown (i) that measurements of steady state currents can indicate when significant concentrations of ion pairs are present, and (ii) that this technique may provide an excellent method of evaluating the practical merit of these materials for applications such as power sources or electrochromic devices. It is also noted that transference numbers cannot be obtained from this or the related ac technique when mobile ion pairs are present. © 1989.