Polarons and electrical leakage in BaZrO₃ and BaCeO₃

Barium zirconate (BaZrO₃, or BZO), barium cerate, (BaCeO₃, or BCO), and their alloys have great potential as proton-conducting electrolytes in solid-oxide fuel and electrolysis cells. However, the factors leading to their non-negligible electrical conductivity, which can limit their performance, are not fully understood. We address that question using first-principles calculations based on density functional theory (DFT). First, we study the properties of hole and electron polarons in BZO and BCO. We confirm that hole polarons form favorably in both materials, while electron polarons can only be stabilized in BCO and in Ce-containing alloys. In general, doped BZO and BCO will have low electron concentrations, but larger concentrations of holes and hole polarons may be present. To reduce the resultant risk of p-type electrical leakage, we recommend avoiding extreme O-rich conditions and limiting dopant concentrations as much as possible. Our results provide physical insights into the electronic behaviors of BZO, BCO, and their alloys, which can be used to optimize their pure ionic conductivity in electrolytes.