Proton-conducting Oxides

■ Abstract The structural and chemical parameters determining the formation and mobility of protonic defects in oxides are discussed, and the paramount role of high-molar volume, coordination numbers, and symmetry are emphasized. Symmetry also relates to the structural and chemical matching of the acceptor dopant. Y-doped BaZrO 3-based oxides are demonstrated to combine high stability with high proton conductivity that exceeds the conductivity of the best oxide ion conductors at temperatures below about 700 • C. The unfavorably high grain boundary impedances and brittleness of ceramics have been reduced by forming solid solutions with small amounts of BaCeO 3 , and an initial fuel cell test has demonstrated that proton-conducting electrolytes based on Y-doped BaZrO 3 provide alternatives for separator materials in solid oxide fuel cells (SOFCs). These materials have the potential to operate at lower temperatures compared with those of conventional SOFCs, and the appearance of chemical water diffusion across the electrolyte at typical operation temperatures (T = 500–800 • C) allows the use of dry methane as a fuel.