Incorporation of protons and hydroxide species in BaZrO₃ and BaCeO₃

Barium zirconate (BaZrO₃ or BZO) and barium cerate (BaCeO₃ or BCO) are among the best-performing proton-conducting oxides used as electrolytes in all-solid-state fuel and/or electrolysis cells. During synthesis, they are seeded with oxygen vacancies (V²⁺_O), which charge-compensate with acceptor dopants such as yttrium (Y⁻_Zr) and, upon exposure to water vapor, are replaced by interstitial protons (H⁺_i). Here, we investigate this and alternative processes for protonation by calculating defect formation energies, concentrations, and migration barriers for several relevant species, including H⁺_i, V²⁺_O, interstitial oxygen (O²⁻_i), and interstitial hydroxide (OH⁻_i), using density functional theory. We confirm that V²⁺_O are favorable under typical operating conditions, although at lower partial pressures of H₂ gas and wet conditions, H⁺_i becomes the dominant donor species. Higher H⁺_i concentrations in BCO than in BZO under comparable conditions help to explain the higher conductivity measured in BCO. OH⁻_i species are present in low concentrations in the bulk (particularly in BZO; they may incorporate in BCO under wet conditions), and their migration is slow; however, they may form at surfaces and help seed materials with H⁺_i. Alloying BZO and BCO improves ionic conduction in general, although the presence of native defects tends to impede kinetics. Our results show that high ionic conductivity can be achieved through optimizing synthesis conditions to maximize the concentrations of H⁺_i, as well as reducing defect-rich regions such as grain boundaries.