The typical electrochemical method for CO2 conversion under high temperature is introduced in Chapter 6. To be specific, the operating principle and features, and the fundamentals including thermodynamics and kinetics of CO2/H2O co-electrolysis using a solid oxide electrolysis cell (SOEC), are summarized in this chapter. The microstructure of SOECs includes the same three compartments, and one of the most common configurations of SOECs to be investigated is Ni-YSZ|YSZ|LSM-YSZ. During SOEC operation, the electrode materials are used to offer active sites for significant electrochemical reactions, including oxygen evolution reaction and the reduction of H2O and/or CO2. Electrode materials also provide pathways for transport of electrons, ions, reactants, and products.