CO2 electrolysis to produce O-2 has been investigated at the Pt/YSZ interface by means of the solid oxide electrolysis in preparation for future human exploration of Mars. With the aid of the current interruption method, the activation overpotentials, which are free of ohmic losses, were measured in the temperature range from 1023 to 1123 K. Both the CO2 electrolysis performance curve and the activation overpotential curve show three distinct regions. Only impurity O-2 is electrolyzed in regions I and II, which occur at an applied cell voltage well below the threshold value of the open circuit voltage for CO2 electrolysis, and the reaction is controlled by O-2 gas diffusion on the cathode side. Active electrolysis of CO2 to form CO and oxygen anion occurs in the third region, where both the magnitudes of the anode and cathode activation overpotentials increase with the current density, and CO2 electrolysis is controlled kinetically. The large magnitudes of the anode and cathode activation overpotentials indicate that significant room exists for improvement of the cell's performance. (C) 2004 Elsevier B.V. All rights reserved.