Ni-YSZ/YSZ/GDC/LSCF6428-GDC10 solid oxide cells were fabricated and investigated in high-temperature H2O/CO2 co-electrolysis using various inlet fuel compositions at 800 degrees C. Furthermore, to see the effect of infiltration on co-electrolysis, the precursor solution for a 10% gadolinium-doped ceria (GDC10) catalyst was dispersed into a fuel electrode backbone by a controlled urea/cation infiltration method. The influence of the inlet gas composition on the syngas product ratio was investigated by in-line gas chromatography. To understand the high-temperature H2O/CO2 co-electrolysis cell performance, electrochemical impedance spectroscopy (EIS) under open-circuit voltage and I-V characteristics, by the electrochemical reactions occurring at the electrodes of prepared solid oxide electrolysis cell (SOEC) were examined. When the inlet gas composition was N-2 63.5%, H-2 5%, CO2 11.5% and H2O 20% under electrolysis current density at -0.6 A cm(-2), the H-2/CO ratio of infiltrated cell was improved from 2.1 to 2.8 compared with the non-infiltrated cell. The thermodynamic factors determining the size, morphology and shape of the infiltrated particles were discussed. (C) The Author(s) 2018. Published by ECS.