High temperature co-electrolysis of H₂O/CO₂on solid oxide electrolysis cell (SOEC) is one technology to produce syngas using carbon dioxide. In this study, Ni-8YSZ/8YSZ/LSCF6428-SDC button cells were fabricated and the effect of different heat treatment temperatures on the performance of SOEC was investigated. The SOEC with LSCF6428-SDC air electrode, SDC buffer layer, and YSZ-(LSCF- SDC) adhesion layer gave better performance in overall results and the voltage obtained for this SOEC at current density of 0.8 A.cm^-1 was from ~ 1.2 V to ~ 1.8 V. Co-electrolysis and the electrochemical performance of the cells were dependent on heat-treatment temperature of the electrode and the buffer layers. To identify the electrochemical processes occurring at the electrodes of SOEC, analysis of the electrochemical impedance (EIS) data was carried out.