Gadolinia-doped ceria, or GDC, (Gd0.4Ce0.6O2–δ, where the value of δ in this material varies, depending on the temperature and oxygen concentration in the atmosphere in which it is being used) has shown promise as a cathode material for high- temperature electrolysis of carbon dioxide in solid oxide electrolysis cells. The polarization resistance of a GDC electrode is significantly less than that of an otherwise equivalent electrode made of any of several other materials that are now in use or under consideration for use as cathodes for reduction of carbon dioxide. In addition, GDC shows no sign of deterioration under typical temperature and gas-mixture operating conditions of a high-temperature electrolyzer.
Electrolysis of CO2 is of interest to NASA as a way of generating O2 from the CO2 in the Martian atmosphere. On Earth, a combination of electrolysis of CO2 and electrolysis of H2O might prove useful as a means of generating synthesis gas (“syngas”) from the exhaust gas of a coal- or natural-gas-fired power plant, thereby reducing the emission of CO2 into the atmosphere. The syngas — a mixture of CO and H2 — could be used as a raw material in the manufacture, via the Fisher-Tropsch process, of synthetic fuels, lubrication oils, and other hydrocarbon products.
An electrolysis cell of the type of interest here consists of a solid oxide electrolyte coated on each side with a porous electrode (see Figure 1). A suitable electric potential is applied to the electrodes, causing electrolysis of CO2 to proceed via overall reaction CO2 → CO + O. At the cathode, carbon dioxide is reduced to carbon monoxide and negative oxygen ions in the half-cell reaction CO2 + 2e– →CO + O2– . The carbon monoxide leaves the cell in the carbon dioxide stream. The solid oxide electrolyte conducts the O2– ions (that is to say, the O2– ions diffuse across the solid oxide electrolyte). Upon reaching the anode, the oxygen ions combine to form oxygen molecules, the overall half-cell reaction sequence being 2O2–→ 2e–+ O2. If the solid oxide electrolyte is made of yttria-stabilized zirconia (YSZ), then it conducts only oxygen ions (but not CO2 or CO), so that essentially pure oxygen is produced on the anode side.
This process is energy-intensive: The enthalpy of the overall reaction is positive and very large (+283 kJ/mole) — comparable to the enthalpy for the electrolysis of water (+286 kJ/mole). In order to make this process economically viable, it is necessary to minimize any irreversible energy losses, notably including those associated with over-potentials and ohmic resistances.
In most of the previous work on electrolysis of CO2 in cells containing YSZ solid electrolytes, both the cathodes and the anodes have been made of porous Pt. The choice of GDC as the present candidate cathode material was prompted by several considerations:
- GDC is much less expensive (about $1,200 per kilogram of GDC versus about $39,000 per kilogram of Pt at 2005 market prices).
- Ceria is known to enhance oxidation of CO when used as a catalyst support in automobile catalytic converters.
- Ceria appears to be an effective an electrocatalyst for oxidation of CO/H2 mixtures and low molecular weight hydrocarbons (methane) in solid oxide fuel cells.
- GDC acts as a mixed conductor: it conducts both ions and electrons, enabling CO2-reduction reactions to occur over a larger electrode area and volume than would otherwise be available for such reactions, thereby reducing the polarization resistance below that of a comparable electrode made of a different material.
Therefore, the performance of GDC as a candidate cathode material, in comparison with the performance of porous Pt as a cathode material, was investigated via electro-chemical measurements on symmetric GDC/YSZ/ GDC cells and compared with similar measurements on a symmetric Pt/YSZ/Pt cell. Each GDC/YSZ/GDC cell was fabricated by brushing a GDC/organic-binder mixture onto both faces of a thick cylindrical YSZ pellet, then sintering the cell at a temperature of 1,250 °C. The experiments involved several different CO/CO2 mixtures and temperatures from 700 to 950 °C. In the experiments, electrochemical impedance spectroscopy (EIS) was performed. Area-specific resistance (ASR) values were calculated from the EIS measurements. The ASR values for the cells containing GDC electrodes were found to be substantially lower than that of the cell containing Pt electrodes under identical test conditions, as shown in Figure 2. Lower ASR values are expected to translate to lower overpotentials and, hence, lower power consumption in electrolyzers.
On the basis of limited published data, it appears that the ASRs of GDC electrodes may also be lower than those of electrodes made of Ni-YSZ, a cermet electrode material commonly used in anodes of state-of-the-art solid-oxide fuel cells. Testing of one of the cells containing GDC electrodes was continued for more than 300 hours, and no degradation of performance was observed; on the contrary, the ASR decreased slightly during the test. In addition, there was no sign of coking (deposition of carbon), which is known to occur in other electrode materials under the gas-mixture and temperature conditions used in the tests.
This work was done by Robert D. Green of Glenn Research Center and Stuart B. Adler of the University of Washington.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18211-1.