Varying the amount of specific interface area in the CeO2-Na2CO3 nanocomposite fuel cell electrolyte helped reveal the role of interfaces in ionic conductivity. We mixed ceria particles with micrometer or nanometer size distributions to obtain a specific surface area (SSA) in the composite from 47 m(2) g(-1) to 203 m(2) g(-1). Microstructural investigations of the nanocomposite showed that the Na2CO3 phase serves as the glue in the microstructure, while thermal analysis revealed a glass transition-like behavior at 350 degrees C. High SSA enhanced the ionic conductivity significantly at temperatures below 400 degrees C. Moreover, the activation energy for the Arrhenius conductivity (sigma T) of the composites was lower than that of the Na2CO3 phase. This difference in the activation energies is consistent with the calculated dissociation energy of the carbonate phase. The strong dependence of conductivity on the SSA, along with differences in the activation energies, suggests that the oxide surface acted as a dissociation agent for the carbonate phase. A model for the solid composite electrolyte is proposed: in the nanocomposite electrolyte, the oxide surface helps Na2CO3 dissociate, so that the "liberated" ions can move easily in the interaction region around the oxide particles, giving rise to high ionic conductivities. (C) 2014 Elsevier B.V. All rights reserved.