A series of composites with nominal compositions of BaCe0.8Sm0.2O3-δ (BCS)-Ce0.8Sm0.2 O1.9 (SDC) (20:80, 35:65, 50:50, 65:35 wt.%) are synthesized by a modified citric acid-nitrate sol-gel combustion method and evaluated as the electrolytes for low-temperature solid oxide fuel cells (SOFCs). The TEM results show that doped BaCeO3 decorated SDC particles are formed after calcining at 1000 °C for 3 h. X-ray diffraction results reveal that the composites are only composed of BaCeO3-based and SDC phases without any other impurity phases. Besides, BaCeO3-based phase is uniformly distributed in the sintered electrolytes according to EDS element mapping. The open cell voltages (OCVs) of the single cells increase gradually with increasing the proportion of BaCeO3-based phase, and are higher than those for bare SDC-based cells. Besides, the power performances of the cells are superior to SDC-based cells when BaCeO3-based phase is lower than 35 wt.%. Electrochemical impedance spectroscopy analysis indicates that, in addition to blocking electronic current leakage, BaCeO3-based phase would induce higher ohmic and polarization resistance, which is detrimental to power performance. Further specific effort should be focused on synthesizing uniform SDC@BCS core-shell electrolyte powders and minimizing the proportion of BCS phase towards high-performance SOFCs with high OCVs.