Steady state current flow in solid binary electrolyte cells

The steady state current flow following the application of a dc voltage to cells of the form M|M+X−|M, where M+X− is a binary solid electrolyte and M is an electrode electroactive towards the M+ ions, is discussed. Consideration is given to three important cases: (i) where the electrolyte is ideal and the electrode kinetics are infinitely fast, (ii) where the electrolyte is ideal, but the electrodes exhibit finite kinetics, and (iii) where the electrolyte is non-ideal, but the electrode kinetics are again infinitely fast. It is shown that in (i) a linear relationship exists between the steady state current and the applied potential difference, only for small values of the latter. Under such conditions the transport numbers of the ions may be obtained directly from current—voltage measurements. Even in the presence of ion-ion interactions, the ionic transport may still be characterised by current—voltage measurements alone under certain well-defined conditions. The theory presented is of particular relevance to binary polymer, ceramic and immobilised electrolytes.