Mixed-conducting ceramic-carbonate membranes exhibiting high CO2/O2 permeation flux and stability at high temperatures

Abstract

This investigation demonstrates the feasibility to fabricate high quality ceramic-carbonate membranes based on mixed-conducting ceramics. Specifically, it is reported the simultaneous CO₂/O₂ permeation and stability properties of membranes constituted by a combination of ceramic and carbonate phases, wherein the microstructure of the ceramic part is composed, in turn, of a mixture of fluorite and perovskite phases. These ceramics showed ionic and electronic conduction, and at the operation temperature, the carbonate phase of the membranes is in liquid state, which allows the transport of ${\mathrm{C}\mathrm{O}}_3^{2-}$ and O²⁻ species via different mechanisms. To fabricate the membranes, the ceramic powders were uniaxially pressed in a disk shape. Then, an incipient sintering treatment was carried out in such a way that a highly porous ceramic was obtained. Afterwards, the piece is densified by the infiltration of molten carbonate. Characterization of the membranes was accomplished by SEM, XRD, and gas permeation techniques among others. Thermal and chemical stability under an atmosphere rich in CO₂ was evaluated. CO₂/O₂ permeation and long-term stability measurements were conducted between 850 and 940 °C.

The best permeation-separation performance of membranes of about 1 mm thickness, showed a maximum permeance flux of about 4.46×10⁻⁷ mol·m⁻²·s⁻²·Pa⁻¹ for CO₂ and 2.18×10⁻⁷ mol·m⁻²·s⁻¹·Pa⁻¹ for O₂ at 940 °C. Membranes exhibited separation factor values of 150–991 and 49–511 for CO₂/N₂ and O₂/N₂ respectively in the studied temperature range. Despite long-term stability test showed certain microstructural changes in the membranes, no significant detriment on the permeation properties was observed along 100 h of continuous operation.