Metal Ion Migration: A New Insight into the H⁺/O^2– Dual-Ion Transport in Perovskite–Fluorite Composites

Ionic conduction (H⁺ and O^2–) in composites composed of perovskite- and fluorite-type oxides remains an outstanding issue in the research of dual-ion-conducting electrolyte materials for solid oxide fuel cells (SOFCs). In this work, perovskite–fluorite composite ceramic pellets are fabricated, and the ionic conduction in the composites is comprehensively analyzed according to the electrochemical impedance spectra under different atmospheres. The results demonstrate that the grain boundary resistance aroused by ionic diffusion resistance across the interfaces is the dominant part in the impedance spectra of the hybrid. Additionally, composites consisting of perovskite oxides with different A-site cation ratios are synthesized and tested. Strong Ba ion migration is observed with high relevance for the interpretation of the increased grain boundary resistances. Transmission electron microscopy (TEM) results indicate that the in situ reaction occurs at the perovskite/fluorite interface depending on the Ba ratio in the perovskites, which plays an important role in the internal H⁺ and O^2– transport across the interfaces.