Mechanism study of highly ionic conductivity for F-doped CaO-Gd2O3-CeO2 solid electrolyte

CeO₂ is a promising electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Studies have found that F-doping improves its ionic conductivity, but the underlying mechanisms remain unclear. Therefore, this study employs a DFT + U method to calculate geometrical and electronic structures and oxygen ion migrations of Gd-, Ca- and F-dopings co-doped CeO₂. The calculated results are verified by our experiment. DFT calculations find that oxygen vacancies (V) are easily formed at next-neighboring sites near the F⁻ ions, different with the case of Gd- and Ca-doped CeO₂. Gd-, Ca- and F-dopings strengthen Ce-O bonds, inconstant with the result from the reported experimental hypothesis for the F-doping. The F-doping restrains the change of Ce⁴⁺ to Ce³⁺. Migration energy barriers E_m of oxygen ions are mainly affected by the migration distances and peripherally done by the oxygen vacancy cavity size and the defect association. Due to the lager oxygen vacancy cavity size and the smaller defect association in CGOF and CGOCF, the F-dopings have the smaller E_m. This is contrary to the speculation of the reported experimental study based on the electronegative comparison of F and O. Our experiment finds that Ca and F co-doping significantly increases the conductivity, 3.9 times of the conductivity of Gd-doped CeO₂. The study clarifies the nature mechanism of the ionic conductivity in doped CeO₂, offering atomic-level insights to aid the development of high-performance CeO₂-based electrolytes.