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Abstract

The oxygen permeation fluxes from p′O2 to pnO2 (p′O2>pnO2) across cobalt-containing perovskite ceramic membranes La1−xSrxCoO3−δ and SrCo0.8Fe0.2O3−δ were measured by gas chromatography as functions of oxygen chemical potential gradient, temperature, thickness, and catalytic activity on the surface. Power indexes 0.5>n>0 for uncatalyzed La1−xSrxCoO3−δ and 1>n>0.5 for SrCo0.8Fe0.2O3−δ were obtained when JO2 vs. p′nO2−p'′nO2 was plotted as a straight line. The results clearly indicate an overall permeation process controlled by both surface oxygen exchange and bulk oxygen diffusion for uncatalyzed La1−xSrxCoO3−δ and SrCo0.8Fe0.2O3−δ. Application of a thin layer of catalytically active SrCo0.8Fe0.2O3−δ on the feeding-gas surface of La0.5Sr0.5CoO3−δ under the condition of a fixed p′O2=0.21 atm and a varied p′'O2 not only increases remarkably the overall oxygen flux, but also changes a mixed control to a bulk diffusion control. This enables evaluation of the bulk transport properties of the mixed conductors. A coat of SrCo0.8Fe0.2O3−δ on the permeate side has little catalytic effect, especially at low p′'O2 range, due to the formation of a poorly conducting brownmillerite phase. The results explicitly show a higher activation energy for the surface exchange kinetics than for the ambipolar transport in the mixed conductors. The mechanism of the surface exchange is discussed, and an analytic expression that agrees well with the experimental results is obtained.

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