Flexible nuclear power for synthetic fuel production through high temperature co-electrolysis technology (HTCE) using solid oxide electrolysis cell (SOEC) has recently received increasing international interest in the large-scale, highly efficient and carbon neutral energy storage field. It is of great importance to enhancing the understanding of co-electrolysis process and the related mechanism. In this paper, CO2 behavior and its effect on the performance of SOEC were examined by the electrochemical characterization and impedance analysis to determine the proper operating conditions, such as H2O, CO2, H-2, CO, operation temperature and electrolysis current. The polarization mechanism is also investigated by the experimental and modeling results. It was found that the electrolysis of CO2 is much harder than that of H2O, and the ASR of pure CO2 electrolysis is about three times that of H2O. When the CO2 content decreases from 50% to 10%, the ASR decreases from 1.59 to 0.90 Omega cm(2). Increasing the H2O content could also improve the electrolysis efficiency to some degree, while the CO addition in the inlet gas was not favorable for the process. Mechanism study shows that the diffusion impedance of CO2 should be the restricted step for the polarization energy loss. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.