A reduced-temperature co-firing process has been utilized to prepare anode-supported solid oxide cells (SOCs) with thin bi-layer Y0.16Zr0.92O2- (YSZ) / Gd0.1Ce0.9O1.95 (GDC) electrolytes. The firing temperature reduction from 1400 to 1250oC, enabled by the addition of a Fe2O3 sintering aid, substantially reduces the thickness of the YSZ/GDC interdiffused zone that contributed to the electrolyte resistance. Furthermore, dense GDC layers are produced that prevent Sr diffusion from the La0.6Sr0.4Fe0.8Co0.2O3 (LSFC) cathode into YSZ, avoiding formation of the resistive SrZrO3 phase. The amounts of Fe2O3 added to the GDC and YSZ layers were found to affect the layer morphology; interfacial voids were observed between the layers except for the optimized case with 1mol.%Fe2O3 in the YSZ layer and 2mol.%Fe2O3 in the GDC layer. It is suggested that the optimized case yields well-matched shrinkage of the YSZ and GDC layers during co-firing, resulting in a void-free interface. The best cells yield fuel cell power density at 0.7V in air and humidified hydrogen of 1.74 Wcm-2 (800oC) and 1.0 Wcm-2 (700oC). Under electrolysis conditions, air and 50vol.%H2O-50vol.% H2, cell area specific resistance (ASR) is 0.12 cm2 at 800oC and 0.27cm2 at 700oC. Cell performance was enhanced by the reduced firing temperature, which yielded a higher Three Phase Boundary density in the Ni-YSZ anode functional layer and improved the pore connectivity in the Ni-YSZ support. Life testing under galvanostatic fuel cell and electrolysis operation at different current densities, showing reasonably stable performance, is presented.