Dual-phase mixed ion and electron conducting carbon dioxide-selective permeation membranes

The objective of this proposed research is to systematically study the chemical and physical properties of mixed carbonate-ion and oxide-ion conductor (MOCC) and mixed carbonate-ion and electron conductor (MECC) membranes for selective electrochemical CO2 separations. First, a combined "co-precipitation" and "sacrificial template" technique has been demonstrated to produce a highly efficient porous ceramic matrix containing a vast number of three-dimensional intra- and interconnected pathways as revealed by 3D XCT for fast-ion transport. The performance of thus synthesized MOCC membrane is remarkable, showing a permeation CO2 flux density from a simulated fuel gas stream two orders of magnitude higher than ceramic-carbonate systems fabricated with other methods. In addition, the CO2 flux density measured was found to increase with the partial pressure of hydrogen in the feedstock, further verifying the CO2 transport mechanism understood.;Second, a surface-modified dense silver-MC dual-phase MECC membrane for CO2 separation from flue gas has been also demonstrated with improved stability. Two pore formers were investigated to make the porous silver matrix: microcrystalline methylcellulose and carbon black. The surface modifier is Al2O3, which was prepared by coating Al2O 3 colloidal onto the exposed surfaces of a porous silver matrix. The results show that the use of 5% Al2O3 colloidal gives MECC the best flux density and stability compared to the unmodified sample. Approximately 90% of the original flux values can still be maintained after 130-hour testing for the modified membrane with microcrystalline methylcellulose pore former, whereas only one-third of the original flux values can be retained even after 60 hours for the unmodified membrane. For the surface modified MECC membrane with carbon black pore former, the CO2 flux was found to increase with time for the first 160 hours by 200%, followed by decreasing for the next 90 hours. At the 250-hour marker, the flux is still 160% of the original value. Overall, silver-molten carbonate MECC with carbon black as a pore former and Al2O3 as a surface modifier demonstrates great potential to separate CO2 from flue gas.;With the stabilized MECC membranes, we also demonstrate that the CO 2 flux follow a linear relationship with reciprocal thickness in a thickness greater than 0.84 mm, suggesting a bulk diffusion controlling mechanism. Below 0.84 mm, the flux remains flat, suggesting that the rate-limiting step has shifted to the surface exchange kinetics of CO2 and O2. We also found that the CO2 flux is proportional to the linear chemical gradient of CO2 and O2, implying that the conductivity of CO 32- is dependent of PCO2 and PO2 with a unity reaction order.;Establishing CO2 transport models and developing flux theory for MOCC and MECC membranes is another task of this thesis project. Multifunctional CO2 transport models encompassing 3PBs and 2PBs pathways are proposed for the first time. The CO2 flux equations suitable for dual-phase mixed conductors are developed from classical flux theory and verified by experimental results.;Finally, we have discovered for the first time the existence of pyrocarbonate C2O52- species on the surface of a eutectic Li2CO3 and Na2CO3 melt subject to CO2 atmosphere through a combined "DFT" and "Raman Spectroscopy" methodology. This discovery lays the foundation for the CO2 transport models established. (Abstract shortened by UMI.).