High temperature electrolysis of CO ₂ for fuel production using concentrated solar energy

The method and measurements presented here are part of an extensive research on the dissociation of CO ₂ to CO and O ₂ using solar energy, with an ultimate goal of developing an efficient and practical method for producing fuel from solar energy. Electrolysis of CO ₂ at temperatures of 600°C - 1400°C is performed and studied over a wide range of CO concentrations (0-98%). It is the first time such measurements are conducted at T >1000°C. The experimental setup is described. The measured specific differential resistances (dj/dV) of the test cell during CO ₂ electrolysis, including lateral electrodes resistance, were 44.6 Ohm·cm2 at T= 650°C, 11.9 Ohm · cm2 at 750°C, 6.7 Ohm·cm2; at 850°C, 5.1 Ohm·cm2 at 950°C, 4.0 Ohm·cm2 at 1250°C and 4.9 Ohm·cm2 at 1400°C. These measurements show an order of magnitude reduction of the resistance as T increases from 650°C to 1250°C. The best electrolysis kinetics (j-V characteristic) is observed at 1250°C, although the difference between measurements at 950°C, 1250°C and 1400°C is small. The relatively small increase followed by a gradual decline of the performance as the temperature increases from 950°C 1250°C and then to 1400°C is at least partially due to electrode's sintering. Impedance measurements were performed to distinguish between the anode, cathode and membrane resistances. The total resistance of the anode, membrane and cathode during the experiments was about 1.4Ohm, 0.09Ohm, 0.035 Ohm and 0.06Ohm at T= 650°C, 950°C, 1250°C and 1400°C, respectively. That is, a cell resistance reduction by a factor of 15-40 as the temperature increases from 650°C to 950-1400°C. This analysis excludes wires and lateral resistance of electrodes. At T=650°C and 950°C the membrane is the main resistor, causing 50% and 66% of the total cell resistance, respectively. The cathode is the main resistor at T >1000°C. It was responsible for 71% and 50% of the total resistance at T=1250°C and 1400°C respectively. The dependence of cathode resistance on oxygen concentration was measured by impedance methods. For example, at 1250°C the cathode resistance decreased from 0.75Ohm to 0.03Ohm as the oxygen volumetric concentration decreases from 1.8ppm to 3.7·10-2ppm. The cathode resistance remained almost unchanged as the oxygen concentration decreased from 2·10-2ppm to 1·10-6ppm. Hydrogen in volumetric concentrations of up to 0.25% was observed during the electrolysis experiments. Preliminary measurements and analysis indicate that it was due to electrolysis of atmospheric water vapor on the anode; thus, the YSZ membrane operated in a mixed conduction mode, conducting oxygen ions (O2-) form the cathode to the anode, and protons (H+) from the anode to the cathode.