Electrode Effects in the Measurement of Ionic Conductivity

Even with thermodynamically reversible electrodes, slow processes inhibiting the discharge of ions at the interface between a solid ionic conductor and the electrode can give rise to overvoltages when current is passed or to an interface impedance in ac measurements. Some of the more important of the mechanisms responsible for this inhibition, i.e. boundary layers, slow charge-transfer processes with double-layer formation, product storage in the electrodes with slow mass transport to the ultimate thermodynamic reservoirs, and rough or porous surfaces, can be understood in a nonrigorous physical way with the help of simple models. The properties of such systems can often be expressed in terms of equivalent circuits, and these are then very useful in analyzing the behavior of specific systems. Examples from studies of materials such as CaO-stabilized ZrO2 (oxygen-ion conductor) and β-Al2O3 (sodium-ion conductor) illustrate these effects.