High temperature membranes for CO₂ separation can potentially lead to more efficient energy conversion systems and more effective means of CO₂ capture in power plants. A novel technology has been successfully demonstrated for the separation of carbon dioxide, CO₂, in the temperature range of 600-900°C. The transport of CO₂ is accomplished with a dual-ion transport mechanism between carbonate ions in a molten carbonate phase and oxide ions in an oxide conducting ceramic coupled with a surface reaction converting CO₂ to CO₃^2- with O^2- from an oxide crystal lattice. The transport of such a system was modeled, and an analytical expression was derived for the flux of CO₂ in a bulk diffusion limited system.

Dual-phase membranes were fabricated by first creating a porous solid oxide structure using tape casting techniques. The structure was engineered to immobilize the molten carbonate phase in the pore space. Membranes comprised of either 8-mol% yttria stabilized zirconia (YSZ) or 10-mol% gadolinia doped ceria (CGO) and a tertiary mixture of alkali metal carbonates (Li₂CO ₃,Na₂CO₃,K₂CO₃) were able to selectively permeate CO₂ at temperatures over 600°C. The flux of CO₂ across these membranes increased exponentially with temperature, reaching permeabilities of 1.0 x 10^-11 mol m ^-1 s^-1 Pa^-1 (or permeance of 3.6 x 10 ^-8 mol m^-2 s^-1 Pa^-1) with YSZ based membranes and 7.0 x 10^-12 mol m^-1 s^-1 Pa^-1 (or permeance of 2.3 x 10^-8 mol m-² s^-1 Pa^-1) with CGO based membranes at 850°C. It was also discovered that alumina, Al₂O₃, a non-oxide conducting ceramic, was unable to selectively permeate CO₂, providing support for the role of an oxide conducting phase in the transport mechanism.

Finally, the chemical reactivity between YSZ and CGO with various mixtures of alkali metal carbonates was examined with thermogravimetric (TGA) and x-ray diffraction (XRD) analysis in order to understand the chemical reactivity and how it relates to the performance of these materials as composite, CO ₂ selective membranes. It was revealed that a lack of reactivity between electrolyte pairs does not preclude these materials from functional separation membranes, yet irreversible chemistry can negatively impact long-term CO ₂ permeance.