Structure and property relationship in the mixed-conducting Sr-Fe-Co-O system

Mixed-conducting ceramic oxides have potential uses in high-temperature electrochemical applications such as solid oxide fuel cells, advanced batteries, sensors, and oxygen-permeable membranes. The Sr-Fe-Co-O 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 syngas. Samples of Sr2Fe3-xCoxOy (with x = 0, 0.6, 1.0, and 1.4) were prepared by solid-state reaction in atmospheres with various oxygen partial pressures (pO(2)) and were characterized by X-ray diffraction, scanning electron microscopy, and electrical conductivity measurements. Phase components of the samples are dependent on cobalt concentration and synthesis pO(2). Total conductivity increases with increasing temperature and cobalt content in the material. Higher ionic transference numbers have been observed in samples with lower cobalt contents. 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 Sr2Fe2CoOy Oxygen permeability measurements at various temperatures showed that oxygen permeability increases with increasing temperature, as expected. At 900 degrees C, an oxygen permeation flux of 2.5 scc.cm(-2).min(-1) was obtained for a Sr2Fe2CoOy disk membrane of 2.9 mm thickness.