Determination of surface exchange and diffusion coefficient in mixed conductors using EMF measurements

We discuss a new technique for determining the chemical diffusion coefficient, Dchem, as well as the coefficient Kchem of surface chemical exchange, in mixed ionic electronic conductors (MIECs). The method is based on EMF measurements following the response to chemical step changes. Expressions for the dependence of VOC(t), the open circuit voltage (EMF), on time after chemical step are developed. The interpretation of Dchem and Kchem depends on the defect model of the MIEC and the surface reaction process. The analytic solution for VOC(t) assumes constant Dchem and Kchem. However, the solution can be applied to cases with non-constant Dchem and Kchem provided the composition changes are done in small steps. The method is demonstrated by applying it to LSM (La0.8Sr0.2MnO3 − x) as the MIEC.