DETERMINATION OF CHEMICAL DIFFUSION-COEFFICIENTS OF MIXED IONIC-ELECTRONIC CONDUCTORS WITH ARBITRARY ELECTRONIC TRANSPORT NUMBERS - APPLICATION OF IMPEDANCE, POTENTIOSTATIC, AND GALVANOSTATIC METHODS TO AG1.92TE

The solution of the diffusion equation for chemical diffusion processe s in a mixed conductor located between an electronically blocking elec trode and an ionically blocking electrode in an asymmetric electrochem ical cell (coulometric titration cell) has been extended to materials with arbitrary electronic transport numbers. This solution allows the derivation of evaluation formulas for impedance spectroscopy as well a s potentiostatic and galvanostatic polarization experiments in order t o determine the chemical diffusion coefficient of mixed-conductivity c ompounds with comparable ionic and electronic conductivities. Impedanc e measurements as well as potentiostatic and galvanostatic polarizatio n experiments are applied to determine the chemical diffusion coeffici ent of Ag1.92Te as a function of composition at 160 degrees C using th e cell Aa \ AgI \ Ag1.92Te \ Pt. Ag1.92Te may be regarded as a typical example of a mixed ionic-electronic conductor with comparable ionic a nd electronic conductivities. The electronic transport numbers of Ag1. 92Te needed for the evaluation of the chemical diffusion coefficient m easurements are obtained from separate measurements of the partial con ductivities by applying a de four-point van der Pauw method. Typical v alues of the electronic transport number are in the range between 0.8 and 0.9 at 160 degrees C. The chemical diffusion coefficient of Ag1.92 Te increases slightly with Increasing silver content of the sample, wi th typical values around 0.003 cm(2) s(-1) In the case of potentiostat ic polarization experiments, a non-Cottrell behavior of the chemical d iffusion process is observed. Agreement between the experimental resul ts of the three methods is remarkably good.