Thermochemical CO₂ dissociation using Ce_0.8Zr_0.15Sc_0.05O_2-δ

Two-step thermochemical cycle for CO₂ splitting has been studied using cerium oxide (CeO_2-δ) as a redox material. Since oxygen release capacity of CeO_2-δ increases by doping ZrO₂, a lot of studies have been conducted on CeO₂-ZrO₂ solid solutions as state of the art redox materials for this cycle. In this study, further doping of Sc³⁺ into CeO₂-ZrO₂ solid solution was attempted with an expectation to increase amount of available oxygen vacancies which could improve redox kinetics. Scandium was chosen since the ionic radius is close to Zr⁴⁺ and oxygen vacancy is introduced by the aliovalent doping. Oxygen nonstoichiometry of Ce_0.8Zr_0.15Sc_0.05O_2-δ was investigated by a coulometric titration method and chemical diffusion coefficient and surface reaction rate constant by an electrical conductivity relaxation method. Oxygen vacancy formation of Ce_0.8Zr_0.15Sc_0.05O_2-δ upon reduction was similar to that of Ce_0.8Zr_0.2O_2-δ, while chemical diffusion coefficient in a high oxygen partial pressure region became larger by introducing Sc³⁺. Those were discussed in correlation with the reaction kinetics in the thermochemical cycle.