Nanocrystalline Ce0.8Sm0.2O1.9 (SDC) has been synthesized by a combined EDTA-citrate complexing sol-gel process for low temperature solid oxide fuel cells (SOFCs) based on composite electrolyte. A range of techniques including X-ray diffraction (XRD), and electron microscopy (SEM and TEM) have been employed to characterize the SDC and the composite electrolyte. The influence of pH values and citric acid-to-metal ions ratios (C/M) on lattice constant, crystallite size and conductivity has been investigated. Composite electrolyte consisting of SDC derived from different synthesis conditions and binary carbonates (Li2CO3-Na2CO3) has been prepared and conduction mechanism is discussed. Water was observed on both anode and cathode side during the fuel cell operation, indicating the composite electrolyte is co-ionic conductor possessing H+ and O2- conduction. The variation of composite electrolyte conductivity and fuel cell power output with different synthesis conditions was in accordance with that of the SDC originated from different precursors, demonstrating O2- conduction is predominant in the conduction process. A maximum power density of 817 mW cm(-2) at 600 degrees C and 605 mW cm(-2) at 500 degrees C was achieved for fuel cell based on composite electrolyte. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.