Oxygen-permeable ceramic membranes for gas separation

Mixed-conducting oxides have a wide range of applications, including fuel cells, gas separation systems, sensors, and electrocatalytic equipment. Dense ceramic membranes made of mixed-conducting oxides are particularly attractive for gas separation and methane conversion processes. Membranes made of Sr-Fe-Co oxide, which exhibits high combined electronic and oxygen ionic conductivities, can be used to selectively transport oxygen during the partial oxidation of methane to synthesis gas (syngas, i.e., CO + H{sub 2}). The authors have fabricated tubular Sr{sub 2}Fe{sub 2}CoO{sub 6+{delta}} membranes and tested them (some for more than 1,000 h) in a methane conversion reactor that was operating at 850--950 C. An oxygen permeation flux of {approx} 10 scc/cm{sup 2} {center_dot} min was obtained at 900 C in a tubular membrane with a wall thickness of 0.75 mm. Using a gas-tight electrochemical cell, the authors have also measured the steady-state oxygen permeability of flat Sr{sub 2}Fe{sub 2}CoO{sub 6+{delta}} membranes as a function of temperature and oxygen partial pressure(pO{sub 2}). Steady-state oxygen permeability increases with increasing temperature and with the difference in pO{sub 2} on the two sides of the membrane. At 900 C, an oxygen permeability of {approx} 2.5 scc/cm{sup 2} {center_dot} min was obtained in a 2.9-mm-thick membrane. This value agrees with that obtained in methane conversion reactor experiments. Current-voltage (I-V) characteristics determined in the gas-tight cell indicate that bulk effect, rather than surface exchange effect, is the main limiting factor for oxygen permeation of {approx} 1-mm-thick Sr{sub 2}Fe{sub 2}CoO{sub 6+{delta}} membranes at elevated temperatures (> 650 C)