Polarons and electrical leakage in BaZrO3 and BaCeO3

The proton conductors barium zirconate (BaZrO3, or BZO), barium cerate (BaCeO3, or BCO), and their alloys have attracted considerable interest as solid-state electrolytes in solid-oxide fuel and electrolysis cells. However, the reasons for their non-negligible electrical conductivity, which can limit their performance, are not fully understood. To address that question, we use first-principles calculations based on density functional theory to study the properties of hole and electron polarons in BZO and BCO. We confirm that hole polarons form in both materials, slightly more favorably in BCO. Electron polarons, on the other hand, are stable only in BCO and in Ce-containing alloys; in pure BZO, electron polaron states are unfavorable relative to free carriers, though they become accessible when Ce impurities are introduced. In general, doped BZO and BCO will have negligible electron concentrations, but larger concentrations of holes and hole polarons may be present. Avoiding extreme O-rich conditions and limiting dopant concentrations are key strategies for reducing significant p-type electrical leakage. Our results provide physical insights into the different electronic behaviors of BZO and BCO, which can be used to optimize their performance as pure ionic conductors.