SOFCs are electrochemical devices that directly convert the electrochemical energy of a fuel into electricity. One common approach to lower the operating temperature of the SOFC is to replace the widely used yttria – stabilized zirconia (YSZ) with higher conductivity materials such as doped ceria. Doped-ceria materials such as GDC, are mixed ionic electronic conductors (MIECs) that are reduced from Ce⁴⁺ to Ce³⁺ in a fuel atmosphere, resulting in electronic conduction in the electrolyte by small-polaron hopping mechanism. In order to block the electronic leakage current through the electrolyte that induces lower OCV and power dissipation, we are developing advanced SOFCs based on ceria/bismuth oxide bilayer electrolyte, which can operate at low temperatures (450-650 °C). Stabilized-Bi₂O₃ has been reported to be an ionic conductor under oxidizing conditions that can effectively suppress the leakage current through GDC. Recently developed stabilized bismuth oxide (SBO) electrolyte material, which showed stable conductivity at lower temperatures was used to block the electronic leakage current. In this work, the effect of SBO/GDC thickness ratio and overall thickness of the electrolyte on the OCV of the cell was studied. Thickness of GDC and SBO layers was varied. The results showed the addition of SBO layer was effective in blocking electronic conduction and increased the OCV compared to the baseline single layer GDC cell. Further, the relative and total thickness of the two layers had a significant impact on the OCV of the cell at different temperatures with best performance obtained with cells with lower GDC thickness and higher SBO thickness. Lastly, long-term stability tests showed exceptional low degradation rates for both OCV and power density of the bilayer SBO/GDC cells.