Influence of electrode thickness on the performance of composite electrodes for SOFC

Measurements on half-cells consisting of yttria-stabilized zirconia (YSZ) electrolyte pellets and slurry-coated cathodes of different thickness were performed in order to determine the active area for oxygen reduction in composite cathodes of lanthanum strontium manganite (LSM) and YSZ. Electrochemical impedance spectroscopy was used to evaluate the main electrochemical parameters of the cathodic process. The temperature range was between 500 and 900 degrees C. The experimental results show a remarkable effect of the electrode thickness on the overall reaction rate in all the temperature range. At 750 degrees C a change in the controlling regime of the oxygen reduction is detectable and has been ascribed to the transition of the rate-determining step from a charge transfer to a mass transfer of the ionic species. A simplified theoretical model of the cathode that accounts for charge transfer and ionic conduction was developed to give insight into the experimental results. The model simulations compared satisfactorily with the experimental data confirming that the behaviour experimentally observed could be approached with the proposed model.