Defects with Variable Charges: Influence on Chemical Diffusion and on the Evaluation of Electrochemical Experiments

Abstract

It is emphasized that the concept of chemical diffusion holds formally if and only if the deviations from stoichiometry (more generally: conservative ensembles) are considered, since the source terms in the equations of continuity cancel under these conditions. It is shown that, if partially ionized or neutral defects play a role in the transport processes, non-trivial modifications appear with respect to the chemical diffusion coefficient, to the cell voltage, as well as to the evaluation formulae of electrochemical or chemical polarization experiments. The importance is highlighted particularly for oxygen concentration cell experiments and for electrochemical polarizations with blocking electrodes as characteristic examples. In the first case, EMF values may be obtained that are greater than the Nernst EMF expected for fully ionized defects only. In the latter case internal dissociations of (or associations to) ionic-electronic defect-complexes can counteract the blocking effects. Particularly in the steady state (Wagner-Hebb-polarization) the apparent partial conductivities are not identical with the values in the non-polarized state.