On the effect of electrode material in electrical conductivity relaxation - An alternative interpretation of the two-fold relaxation behavior in La_5.4WO_11.1-δ

Oxygen and proton transport properties of La_5.4WO_11.1-δ are investigated using electrical conductivity relaxation. Chemical diffusion coefficients and surface exchange coefficients of both hydrogen and oxygen presumed to govern conductivity relaxation are extracted from fitting the two-fold conductivity relaxation observed after hydration/dehydration steps at fixed oxygen partial pressure to the model equations. Surprisingly, the kinetic parameters obtained from fitting are found to depend on the magnitude of the current used in the measurements. A two-fold relaxation behavior with characteristics depending on the magnitude of the current is also observed after oxidation/reduction steps under dry conditions. Furthermore, at fixed temperature, oxygen and water partial pressures, the conductivity is found to exhibit relaxation behavior after a current step, while the apparent steady-state conductivity is found to depend on the applied current. The observations are attributed to the use of gold electrodes in the experiments and associated interfacial capacitances. It is obvious from this study that the use of partially ion-blocking electrodes in conductivity and conductivity relaxation measurements on mixed ionic-electronic conducting oxides with prevailing ionic conduction may lead to erroneous results and interpretations.