Thin film electrolytes of bilayer bismuth oxide/ceria are developed for intermediate temperature solid oxide fuel cells. Y0.25Bi0.75O1.5 is deposited via Direct Current magnetron sputtering technique on an Sm0.2Ce0.8O1.90 electrolyte film which is prepared by a dry-pressing process on an NiO-Sm0.2Ce0.8O1.90 substrate. La0.85Sr0.15MnO3-delta 6-Y0.25Bi0.75O1.5 composite is applied onto the Y0.25Bi0.75O1.5 film as the cathode to form a single cell. Cells with 6-mu m-thick Y0.25Bi0.75O1.5 and 26-mu m-thick Sm0.2Ce0.8O1.90 bilayer electrolytes exhibit improved open circuit voltages and power density compared with those obtained with only Sm0.2Ce0.8O1.90 electrolytes. The open circuit voltages are comparable and power densities are higher than those previously reported for solid oxide fuel cells with thick bilayer electrolytes using noble metals such as Pt as the electrodes. Impedance spectra show that the change of electrolyte resistance is negligible while the cathodic interfacial polarization resistance decreased significantly when the Y0.25Bi0.75O1.5 layer is added to form the Sm0.2Ce0.8O1.90/Y0.25Bi0.75O1.5 bilayer electrolytes. (C) 2010 Elsevier Ltd. All rights reserved.