Influence of porous structures on O 2 flux of BSCF asymmetric membranes

Abstract

Abstract Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ (BSCF) membranes were prepared in three configurations, consisting of a simple dense membrane (500 μm), an asymmetric membrane containing dense (500 μm) and thin porous (40 μm) layers, and a second asymmetric membrane containing a porous substrate (500 μm) in addition to dense (500 μm) and thin porous (40 μm) layers. Activated carbon (i.e. porogen agent) at 20 wt% was mixed with BSCF to form porous structures. The membranes were processed by dry pressing followed by sintering at high temperatures. The best oxygen fluxes of 2.86 ml min −1 cm −2 at 850 °C were achieved with the thin porous layer/dense membrane configuration, which delivered a 42% higher oxygen flux than the simple dense membrane. This improvement was attributed to the larger contact area with the air feed conferred by the thin porous layer. However, the thin porous layer/dense membrane/porous substrate configuration resulted in a 21% decrease of oxygen flux as compared to the thin porous layer/dense membrane shape, which strongly suggested that the porous substrate contributed extra resistance to the transport of oxygen. Computer tomography scan analysis of the porous substrate showed that the pore volume was concentrated in the center of the porous substrate, with a reduced pore volume contribution closer to the external surface. 3D analysis revealed the connectivity of the regions closer to the external surface was over three orders of magnitude lower than that of the center of the porous substrate, thus creating a bottle-neck. Although all pores were large (>1 μm), the expected resistance should be very low, however, the bottle-neck region closer to the external surface provided additional resistance for the transport of oxygen from the membrane interface to the permeate side.