The method and measurements presented here are part of an extensive research on the
dissociation of CO 2 to CO and ½O2 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 CO2
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=
650ºC, 950ºC, 1250ºC and 1400ºC, 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=650ºC and 950ºC 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=1250ºC and 1400ºC respectively. The
dependence of cathode resistance on oxygen concentration was measured by
impedance methods. For example, at 1250ºC 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 (O 2-) form the cathode
to the anode, and protons (H +) from the anode to the cathode.
See more of this Group/Topical: International Congress on Energy 2011