Solid oxide fuel cells that contain mixed ionic-electronic conductors (MIECs) as electrolytes exhibit maximum efficiency at high ionic current density due to a small electronic leakage current. At low ionic current density the electronic current increases due to partial reduction of the electrolyte and efficiency decreases. This leads to restrictions in the optimum cell operating range and cell design. Provided the necessary materials and cell design parameters are known, the optimum cell design, especially the electrolyte thickness, can be predicted. The optimization of fuel cells with a mixed-conducting ceria electrolyte is outlined. The cell optimization considered in this study includes operating range (V-T), MIEC thickness, and the electrochemical performance of the electrodes. Defect chemical considerations predict an optimum in MIEC thickness depending on the operating temperature.