Determination of Chemical Diffusion Coefficients of Mixed Ionic‐Electronic Conductors with Arbitrary Electronic Transport Numbers: Application of Impedance, Potentiostatic, and Galvanostatic Methods to

Abstract

The solution of the diffusion equation for chemical diffusion processes in a mixed conductor located between an electronically blocking electrode and an ionically blocking electrode in an asymmetric electrochemical 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 as potentiostatic and galvanostatic polarization experiments in order to determine the chemical diffusion coefficient of mixed-conductivity compounds with comparable ionic and electronic conductivities. Impedance measurements as well as potentiostatic and galvanostatic polarization experiments are applied to determine the chemical diffusion coefficient of Ag1 92Te as a function of composition at 160°C using the cell Ag I AgI I Ag, 92Te I Pt. Ag1 92Te may be regarded as a typical example of a mixed ionic-electronic conductor with comparable ionic and electronic conductivities. The electronic transport numbers of Ag1 92Te needed for the evaluation of the chemical diffusion coefficient measurements are obtained from separate measurements of the partial conductivities by applying a dc four-point van der Pauw method. Typical values of the electronic transport number are in the range between 0 8 and 0.9 at 160°C. The chemical diffusion coefficient of Ag1 92Te increases slightly with increasing silver content of the sample, with typical values around 0.003 cm2 s In the case of potentiostatic polarization experiments, a non-Cottrell behavior of the chemical diffusion process is observed. Agreement between the experimental results of the three methods is remarkably good.

Spatially limited diffusion coupled with ohmic potential drop and/or slow interfacial exchange: a new method to determine the diffusion time constant and external resistance from potential step (PITT) experiments

“…The possibility of applying the Cottrell equation in mixed ionic-electronic conductors with arbitrary electronic transport numbers was recently analyzed [20,21].…”

Sensitivity Enhancement in Double-Step Voltcoulometry as a Consequence of the Changes in Redox Kinetics on the Microelectrode Exposed to Low Frequency Sound

“…Small deviations of the redox kinetics from the ideal one in a narrow time interval of observation are certainly correctly approximated by a power-law dependence. We have drawn an analogy with models describing migration and diffusion (hopping) of charged species in solids [16,17] by a powerlaw dependence, supposing that the rate-determining step in the redox process is diffusion towards a clean electrode. This might hold only if there is no undesired adsorbed species on the electrode which would prevent the diffusing species (ions) to exchange charge with the working electrode.…”