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

Abstract

Non-perovskite SrFeCo_0.5O _x (SFC2) was found to have high electronic and ionic conductivities as well as structural stability. At 800°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 ≈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, ≈75 wt% ${\text{Sr}}_{4} {\left( {{\text{Fe}}_{{1 - x}} {\text{Co}}_{x} } \right)}_{6} {\text{O}}_{{13 \pm \delta }}$, ≈20 wt% perovskite ${\text{Sr}}{\left( {{\text{Fe}}_{{1 - x}} {\text{Co}}_{x} } \right)}{\text{O}}_{{3 - \delta }}$, and ≈5 wt% rock salt CoO. Argon-annealed SFC2 contains brownmillerite Sr₂(Fe_1−x Co _x )₂O₅ and rock salt CoO. Dense SFC2 membranes were able to withstand large pO₂ 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 ≈2.5 cm³(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 ≈10 cm³(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.