(559d) Electric Field Enhanced Thermochemical CO2 Splitting

Thermochemical gas splitting has been identified as a promising method of harnessing solar energy to produce solar fuels. In thermochemical CO₂ splitting metal oxides, MO_x are thermally reduced (T > 1200 °C) to MO_x-δ using concentrated solar energy, followed by an oxidation step with CO₂ to regenerate MO_x and form CO, which can be mixed with H₂ to form liquid fuels. Ceria has been identified as the state-of-the-art material due to high stability and fast oxidation kinetics. However, solar thermochemical processes are plagued by high solar field costs due to the extraordinarily high temperatures required. Methods of lowering the reduction temperature of ceria have been well-studied, but can have detrimental effects on the conversion of CO₂ to CO. Previous studies have demonstrated the ability to reduce ceria under only the application of a strong electric field in a reversible manner at 25 °C. The E-field induced reduction of ceria changes the chemical potential of lattice oxygen, making the formation of O vacancies more favorable. As changes to the formation of O vacancies is tunable with the applied potential, removal of the potential from the system, allows ceria to re-establish its original thermodynamics and oxidation favorability. One major challenge to the usage of an electric field in thermochemical systems is the method by which a sufficiently large field is applied on a large-scale. In this work we use molten salts to generate an electronic double layer (EDL) at the electrolyte-ceria interface. The EDL acts as a parallel plate capacitor, with nanometer scale plate spacing, allowing for large specific capacitances to amplify low applied voltages into large electric fields to drive the reduction of ceria. We demonstrate an electrically enhanced two step thermochemical CO₂ splitting process, where a low potential is applied to the ionic liquid to generate an electric field via EDL in order to reduce ceria, then the field is removed and ceria is oxidized using CO₂, producing CO. Specifically, we show molten salt compositions compatible with generating the electronic double layer with CeO₂ and their ability to generate high electric fields. We also demonstrate the ability of these EDL electric fields to reduce ceria at temperatures below 1000 °C and perform isothermal CO₂ splitting cycles. By allowing for significant reductions in the needed temperatures, the E-field enhanced CO2 splitting process significantly drives down the CO production costs as the number of heliostats needed decreases, the radiative losses are lowered, and the constraints on materials of construction are loosened.