Solid-oxide fuel cells (SOFCs) with electrolytes that provide high open-circuit voltage (OCV) and low ohmic loss down to 550 degrees C are described. The electrolytes were bilayers consisting of a 4 to 8 mu m thick Y-doped ceria (YDC) layer with a 1 to 1.5 mu m thick Y-doped zirconia (YSZ) layer on the fuel side. The cathode/supports were La0.95Sr0.15MnO3-YSZ. The anodes consisted of thin YDC and Ni-YSZ layers. The YDC/YSZ electrolyte SOFCs yielded 85 to 98% of the theoretical OCV, compared with approximate to 50% for YDC electrolyte SOFCs. The cathode overpotential, which was a main factor limiting SOFC power density, was lower for YDC/YSZ than YSZ electrolytes. The maximum power density at 600 degrees C, 210 mW/cm(2), is higher than for previously reported SOFCs.