Abstract
Asymmetric membranes of mixed ionic-electronic conducting perovskite-type oxides SrTi1-xFexO3-δ (STF, x = 0.3, 0.5 and 0.7) were prepared by inverse sequential tape-casting. Both porous support (~600 μm) and functional membrane layer (~20 μm) for a given membrane assembly were made from the same composition to ensure thermochemical compatibility between the layers. Oxygen fluxes were assessed in the range 650 -1020 °C, using either (non-pressurized) ambient air or pure oxygen as feed gas at the support side of the asymmetric membrane and argon as sweep gas. Notably, similar oxygen fluxes (~1.2 × 10−6 mol cm−2 s−1) are measured through the membranes of different compositions above 950 °C when using ambient air as feed gas. This observation is interpreted to reflect the major role of the support layer resistance in rate-limiting the oxygen fluxes through the STF asymmetric membranes, which conclusion is supported by comparison of the oxygen fluxes with those measured previously through asymmetric membranes of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF). A simple diffusion-convection model is used to account for the observed gas phase polarization in the porous support layers limiting the oxygen fluxes.
Highlights
With the prospects of obtaining low-cost oxygen, ceramic mixed ionic-electronic conducting (MIEC) oxygen-permeable membranes have attracted considerable attention during the past decades
The porosity in the support layers appears homogeneously distributed and emerges to be the highest for STF30. The latter is attributed to the poor sinter activity of this composition compared to STF50 and STF70
The above results make clear that significant mass transport limita tions may occur in the porous support layers of the asymmetric STF membranes if air is used as feed gas
Summary
With the prospects of obtaining low-cost oxygen, ceramic mixed ionic-electronic conducting (MIEC) oxygen-permeable membranes have attracted considerable attention during the past decades. The so-called ambipolar diffusion of the oxygen ions and electrons, being driven by the gradient in oxygen chemical potential across the membrane, may be described by Wagner’s equation [10], which pre dicts that the oxygen flux is inversely proportional to membrane thickness. Gas phase transport in the porous support layer, if present, may limit the oxygen fluxes [13,14,15]. We have identified the perovskite-type oxide SrTi1-xFexO3-δ (STF) as a promising candidate membrane material [17] Functional properties, such as ionic and electronic conductivities, thermo-mechanical [18,19] and reducibility [20], affecting the operational reliability of the STF membranes, can be tailored by varying the compositional parameter x in the series. We describe the fabrication of asymmetric planar STF membranes, using the inverse sequential tape-casting technique developed previously in our labora tory [13-15,[13,14,15], and present a detailed analysis of the data of oxy gen permeation
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