A novel ionic conductor, BaCe0.8Sm0.2O3-delta-Ce0.8Sm0.2O2-delta (BCS-SDC, weight ratio 1:1), is reported as an electrolyte material for solid oxide fuel cells (SOFCs). Homogeneous BCS-SDC composite powders are synthesized via a one-step gel combustion method. The BCS and SDC crystalline grains play a role as matrix for each other in the composite electrolyte. The composite avoids the typical drawbacks of BCS and SDC, showing not only a better chemical stability than the single phase of BCS but much higher open circuit voltages (OCVs) than the single phase of SDC under the fuel cell conditions. Moreover, BCS-SDC exhibits mixed oxygen ionic and protonic conduction. A total conductivity of 0.0204S cm(-1) at 700 degrees C is achieved in wet hydrogen (3% H2O), the value of which is comparable with the state-of-the-art proton conductor BaZr0.1Ce0.7Y0.2O3-delta (BZCY). The peak power density achieves 505 mW cm(-2) at 700 degrees C with a 30-mu m-thick BCS-SDC electrolyte using wet H-2 as the fuel. Resistances of the tested cell under open circuit conditions at different operating temperatures are also investigated by impedance spectroscopy. (c) 2010 Elsevier B.V. All rights reserved.