(363f) High Temperature Electrolysis of CO2 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 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.