Structure and property relationships in mixed-conducting Sr4(Fe1-xCox)6O13±δ materials

Mixed-conducting ceramic oxides have potential uses in high-temperature electrochemical applications such as solid-oxide fuel cells, batteries, sensors, and oxygen-permeable membranes. The Sr-4(Fe1-xCox)(6)O13+/-delta system combines high electronic/ionic conductivity with appreciable oxygen permeability at elevated temperatures. Dense ceramic membranes made of this material can be used to separate high-purity oxygen from air without the need for external electrical circuitry or to partially oxidize methane to produce synthesis gas (syngas, CO + H-2). Samples of Sr-4(Fe1-xCox)(6)O13+/-delta (where x = , 0.1, 0.2, 0.333, and 0.467) were prepared by a solid-state reaction method in atmospheres with various oxygen partial pressure (p(O2)) and were characterized by powder X-ray diffraction, scanning electron microscopy, and electrical conductivity testing. The proportion of phase components in the samples is dependent on both cobalt content and p(O2). The total conductivity increases with both temperature and cobalt substitution in the material. Current-voltage characteristics determined in a gas-tight cell indicate that a bulk effect, rather than a surface exchange effect, is the main limiting factor for oxygen permeation through membranes made of Sr4Fe4Co2 O13+/-delta (x = 0.333 sample). Oxygen permeability measurements at various temperatures showed that, as expected, permeability increases with increasing temperature. At 900 degrees C, an oxygen permeation flux of 2.5 scc.cm(-2).min(-1) was obtained from an Sr4Fe4Co2O13+/-delta disk membrane that was 2.9 mm thick. (C) 1998 Academic Press.