Surface activation of asymmetric CaTi1−xFexO3−δ tubular membranes for oxygen separation

• Asymmetric tubular membranes are coated with porous layers to enhance oxygen flux.
• Activation layers showed good adhesion with CaTi0.9Fe0.1O3−δ membrane.
• Oxygen flux was significantly increased by activation layers.
• An improvement of 188% was obtained with an La0.8Sr0.2CoO3−δ coating at 850 °C.
• La0.8Sr0.2CoO3−δ activated membrane remained stable for more than 100 h operation.

Tubular asymmetric mixed ionic-electronic conducting CaTi0.9Fe0.1O3−δ (CTF) membranes can be used for oxygen separation from air. However, the oxygen permeation flux through thin CTF supported membranes may be limited by surface oxygen exchange kinetics and gas diffusion limitation in the porous support. To overcome surface exchange limitation, the membranes can be coated with an activation layer, which can either be a porous layer of the membrane material itself to increase the active surface area, or a porous layer of a material exhibiting superior oxygen exchange properties. In this study, asymmetric tubular CTF membranes were prepared by plastic extrusion and dip-coating. The membranes were further coated by spray-coating with both types of activation layers. La0.8Sr0.2CoO3−δ (LSC) or La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) were selected based on similar thermal expansion and high mixed ion and electron conductivity. We report here on the effects of spray-coating parameters and annealing treatment on both the adhesion and the porosity of the spray-coated layers. In addition, we discuss results of oxygen flux and permeability in the temperature range 750–1000 °C in relation with potential limitations from surface exchanges and gas diffusion in the porous tubular supports.