Ionic-electronic conducting ceramic membranes for high temperature gas separation and membrane reactor

This dissertation focuses on mixed oxygen ion-electron and proton-electron conducting membranes and gives answers to the problems that have hindered the R&D progress towards applications in solid oxide fuel cells, gas separators and catalytic membrane reactors.; In the first part, the sealing problem for high temperature permeation system was solved for both fluorite and perovskite structured dense ceramic membranes. The sealing recipes achieved complete gas-tightness at temperature from 600°C to 950°C. Then, fluorite-type CeO2 and perovskite-type La0.8Sr0.2C0.06Fe0.4O3-delta (LSCF) based oxygen ion-electron conducting membranes were studied by electrical conductivity and oxygen permeation measurements. Sm and Pr doped CeO2 shows higher oxygen ion conductivity than YSZ and Bi 2O3. Sm doped CeO2 (CS) has high n-type electronic conductivity in reducing gases while Pr and Zr doped CeO2 (CZP) has both p-type and n-type electronic conduction. Preparation methods (the citrate, solid state, spray-pyrolysis and coprecipitation methods) have considerable influences on the microstructures, electrical conductivity and oxygen permeation properties of LSCF membranes. The membranes prepared by the coprecipitation method have large strontium deficiency and lead to much lower electronic conductivity and oxygen permeability than other membranes with desired stoichiometry.; In the second part, the preparation, characterization and hydrogen permeation of proton-conducting ceramic membranes were studied. Dense SrCe0.95Tb 0.05O3-delta (SCTb) and SrCe0.95Tm 0.05O3-delta (SCTm) membranes were successfully prepared with desired perovskite-type phase structure. SCTb membrane has proton conduction in the range of 10-3--10-2 S/cm, two to three orders of magnitude higher than its electronic conductivity. SCTm shows a mixed proton-electron conducting property with a hydrogen permeation flux (JH2) of 3 x 10-8 mol/cm 2.s at 900°C for a 1.6 mm thick SCTm membrane when 10% H 2/He and air were used respectively as the feed and sweeping gases. The bulk diffusion is the rate-limiting step for hydrogen permeation through thick SCTm membranes. The modeling for hydrogen permeation through proton conducting membranes was conducted based on ambipolar diffusion theory for the first time. The general equation correlates hydrogen permeation flux to concentrations and diffusion coefficients of three kinds of charge carriers. In several cases, the equation was simplified into expressions relating flux to upstream and downstream hydrogen partial pressures.