Electrochemical characterisation of oxygen nonstoichiometry and transport in mixed conducting oxides : Application to La0.4Ba0.6Fe0.8Co0.2O3

Electrochemical techniques were applied to a coulometric titration cell to study oxygen nonstoichiometry and transport in the perovskite-type oxide La0.4Ba0.6Fe0.8Co0.2O3-delta. Slow scan voltammetry (3mV/s) was used to obtain oxygen nonstoichiometry vs. the oxygen partial pressure (p(O2)) data. The voltammograms were further analysed using a simple defect model to yield the absolute value of the oxygen nonstoichiometry. Relaxation measurements were performed to obtain chemical diffusion and surface exchange coefficients. In particular, the suitability of converting the relaxation data to the frequency domain for analysis purpose was examined. Impedance spectroscopy measurements were also performed on the same cell to allow direct comparison. A satisfactory agreement was obtained for the chemical diffusion coefficients but the surface exchange coefficient values were systematically different by a factor 2 to 3. This discrepancy was attributed to the short-time extrapolation used in the numerical conversion procedure. Finally, other transport coefficients (ionic conductivity, DV and DO) were calculated from the chemical diffusion and nonstoichiometry data.