Effects of support pore structure on carbon dioxide permeation of ceramic-carbonate dual-phase membranes

Dual-phase membranes consisting of an ionic or mixed conducting ceramic phase and a molten carbonate phase are permeable only to carbon dioxide at high temperatures. This paper studies the effects of the pore structure of ceramic supports on carbon dioxide permeation properties of dual-phase membranes consisting of La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF) and eutectic Li₂CO₃-Na₂CO₃-K₂CO₃ carbonate phase. Porous LSCF supports with different pore structures were prepared by pressing LSCF powders followed by sintering at various temperatures. The porosity to tortuosity and solid fraction to tortuosity ratios for the porous LSCF supports were characterized by helium permeation and electrical conductivity measurements. The direct infiltration method allows complete filling of the ceramic support pores by the carbonate. CO₂ permeance of the dual-phase membranes increases, and after reaching a maximum, decreases with increasing support sintering temperature or support porosity (carbonate fraction). A total conductance in terms of the effective carbonate and ionic conductivities (intrinsic conductivity modified by the carbonate or solid fraction to tortuosity ratio) is introduced and used to explain experimental results. The results show that the CO₂ permeance of the dual-phase membrane is controlled not only by the intrinsic carbonate and oxygen ionic conductivities of the carbonate and ceramic phases, but also the carbonate or solid fraction to tortuosity ratio. Adjusting pore and solid microstructure of ceramic support is critical to maximize CO₂ permeance of the dual-phase membranes.