Efficient Reduction of CO2 in a Solid Oxide Electrolyzer

When the economy is based on renewable energy resources, such as wind and solar, the major source of H2 for chemical production and energy storage will be from the electrolysis of water. The ability to reduce CO2 efficiently by a similar process could also play a role in reducing greenhouse gas emissions and moving us toward a more sustainable economy. 1 CO produced in this manner could be used in chemical production or reacted with H2 to produce liquid fuels via the Fischer‐Tropsch reaction. 2 Solid oxide electrolyzers SOEs, which are essentially solid oxide fuel cells SOFCs operated in reverse, are capable of higher water electrolysis efficiencies compared to solution-based electrolysis cells because they operate at higher temperatures 925 K. The higher operating temperatures result in a lower Nernst potential, the thermodynamic potential required for water splitting, and in lower electrode overpotentials. 3 The electrode overpotential is the difference between the actual electrode potential and the Nernst potential, and is a measure of the lost efficiency in the cell. SOEs also differ from low-temperature, solution-based electrolyzers in that the electrolyte membrane conducts oxygen anions, rather than protons. The material most often used for the electrolyte is yttria-stabilized zirconia YSZ, a material that is a good oxygen-anion conductor and an electronic insulator. In an SOE, the cathode the fuel-side electrode reaction for water electrolysis is the electrochemical dissociation of steam to produce H2 and O 2 anions, Reaction 1, while recombination of the oxygen ions to O2, Reaction 2, occurs at the anode the air-side electrode H2 O+2 e  → O 2 +H 2