Simultaneous Reduction of Proton Conductivity and Enhancement of Oxide-Ion Conductivity by Aliovalent Doping in...

Hexagonal perovskite-related oxides have garnered a great deal of research interest because of their high oxide-ion conductivity at intermediate temperatures, with Ba₇Nb₄MoO₂₀ being a notable example. However, concomitant proton conduction in Ba₇Nb₄MoO₂₀ may cause a decrease in power efficiency when used as the electrolyte in conventional solid oxide fuel cells. Here, through investigations of the transport and structural properties of Ba₇Nb_4-WMoO₂₀₊ (x = 0-0.25), we show that the aliovalent substitution of Nb⁵⁺ by W⁶⁺ not only increases the oxide-ion conductivity but also dramatically lowers proton conductivity. The highest conductivity is achieved for x = 0.15 composition, with 2.2 × 10^-2 S cm^-1 at 600 °C, 2.2 times higher than that of pristine Ba₇Nb₄MoO₂₀. The proton transport number of Ba₇Nb_3.85W_0.15MoO_20.075 is smaller compared with Ba₇Nb₄MoO₂₀, Ba₇Nb_3.9Mo_1.1O_20.05, and Ba₇Ta_3.7Mo_1.3O_20.15. The structure analyses of neutron diffraction data of Ba₇Nb_3.85W_0.15MoO_20.075 at 25 and 800 °C reveal that the aliovalent W⁶⁺ doping introduces interstitial oxide ions in the intrinsically oxygen-deficient c' layers, thereby simultaneously increasing the carrier concentration for oxide-ion conduction and decreasing oxygen vacancies responsible for dissociative absorption of water. Neutron scattering length density distribution was examined using the maximum-entropy method and neutron diffraction data at 800 °C, which indicates the interstitialcy oxide-ion diffusion in the c' layers of Ba₇Nb_3.85W_0.15MoO_20.075. Ba₇Nb_3.85W_0.15MoO_20.075 exhibits extremely high chemical and electrical stability in the wide oxygen partial pressure P(O₂) region [ex. 10^-23 ≤ P(O₂) ≤ 1 atm at 903 °C]. The present results offer a strategy for developing pure oxide-ion conducting hexagonal perovskite-related oxides for possible industrial applications.