A nanocomposite Zr/Sm-codoped ceria electrolyte coated with K2CO3/Na2CO3 was synthesized by a coprecipitation method. The electrochemical study of the two-phase nanocomposite electrolytes with carbonate coated on the doped ceria shows high oxygen ion mobility at low temperatures (300–600 °C). The interface between the two constituent phases was studied by electrochemical impedance spectroscopy. Ionic conductivities were also measured with electrochemical impedance spectroscopy. The morphology and structure of composite electrolyte were characterized using field-emission scanning electron microscopy and X-ray diffraction. The fuel cell power density is 700 mW cm−2, and an open-circuit voltage of 1.00 V is achieved at low temperatures (400–550 °C). This codoped approach with a second phase provides a good indication regarding overcoming the challenges of solid oxide fuel cell technology. Copyright © 2014 John Wiley & Sons, Ltd.