Mixed-conducting dense ceramic membranes for air separation and natural gas conversion

Non-perovskite SrFeCo0.5O (x) (SFC2) was found to have high electronic and ionic conductivities as well as structural stability. At 800 degrees C in air, total and ionic conductivities of 17 and 7 S.cm(-1) were measured, respectively; the ionic transference number was calculated to be approximate to 0.4. This material is unique because of its high electronic conductivity and comparable electronic and ionic transference numbers. X-ray diffraction analysis showed that air-sintered SFC2 consists of three phase components, approximate to 75 wt% Sr-4(Fe1-xCox)(6)O-13 +/-delta, approximate to 20 wt% perovskite SrFe(Co-1-x(x))O3-delta, and approximate to 5 wt% rock salt CoO. Argon-annealed SFC2 contains brownmillerite Sr2(Fe1-xCox)(2)O-5 and rock salt CoO. Dense SFC2 membranes were able to withstand large pO(2) gradients and retain mechanical strength. A 2.9-mm-thick disk membrane was tested in a gas-tight electrochemical cell at 900 C; an oxygen permeation flux rate approximate to 2.5 cm(3)(STP).cm(-2).min(-1) was measured. A dense thin-wall tubular membrane of 0.75-mm thickness was tested in a methane conversion reactor for over 1,000 h. At 950 C, the oxygen permeation flux rate was approximate to 10 cm(3)(STP).cm(- 2).min(-1) when the SFC2 thin-wall membrane was exposed with one side to air and the other side to 80% methane balanced with inert gas. Results from these two independent experiments agreed well. The SFC2 material is a good candidate as dense ceramic membranes for oxygen separation from air or for use in methane conversion reactors.