Novel chemically stable Ba₃Ca_1.18Nb _1.82-xY_xO_9-δ proton conductor: Improved proton conductivity through tailored cation ordering

Simple perovskite-structured proton conductors encounter significant challenges to simultaneously achieving excellent chemical stability and proton conductivity that are desirable for many important applications in energy conversion and storage. This work demonstrates that Y-doped complex-perovskite- structured Ba₃Ca_1.18Nb_1.82-xY_xO _9-δ materials possess both improved proton conductivity and exceptional chemical stability. Neutron powder diffraction refinement revealed a Fm3̄m perovskite-structure and increased oxygen vacancy concentration due to the Y doping. High-resolution TEM analysis confirmed the perturbation of the B site cation ordering in the structure for the Ba₃Ca _1.18Nb_1.82-xY_xO_9-δ materials. Such combined effects led to improved proton conductivity with a value of 5.3 × 10^-3 S cm^-1 at 600 °C for Ba ₃Ca_1.18Nb_1.52Y_0.3O _9-δ (BCNY0.3), a value 2.4 times higher compared with that of the undoped Ba₃Ca_1.18Nb_1.82O _9-δ. The Ba₃Ca_1.18Nb _1.82-xY_xO_9-δ materials showed remarkable chemical stability toward water and demonstrated no observable reactions to CO₂ exposure. Ionic transport number studies showed that BCNY0.3 had predominantly proton conduction below 600 °C. Solid oxide fuel cells using BCNY0.3 as an electrolyte demonstrated cell power output of 103 mW cm ^-2 at 750 °C. These results suggest that a doping strategy that tailors the cation ordering in complex perovskites provides a new direction in the search for novel proton conducting ceramics.