Phase-inversion Tape-casting And Property Of Dual-phase Oxygen Separation Membrane

Oxygen finds important applications in metallurgy, chemical industries and medication. The ceramic oxygen-permeable membrane holds promise to reduce the oxygen production cost by30%compared with the cryogenic distillation process. The membrane can also be employed in chemical reactors to integrate the oxygen separation and chemical reactions in a single space membrane, resulting in significant economic and environmental benefits. A major barrier to the realization of these applications lies in the lack of membranes possessing both high stability and high oxygen permeability under stringent reactor conditions. Some oxygen-deficient perovskite-structured complex oxides have been found to exhibit high oxygen permeability, but their chemical and mechanical stabilities are far from satisfactory. Dual-phase composites comprising a stable oxygen ionic conductor and a stable electronic conductor exhibits satisfactory stability, but their oxygen permeability is poorer than that of the single-phase membrane. The oxygen permeation performance of the dual-phase composite membrane can be improved by fabricating them into an asymmetric structure with a thin dense layer to facilitate oxygen separation and a thick porous layer to provide mechanical strength. The phase-inversion tape casting has been demonstrated to be a simple and effective method in our laboratory for fabrication of asymmetric planar membranes. Based on the above-described considerations, this thesis is thus mainly devoted to the study of phase-inversion tape casting and surface modification, oxygen permeability and stability of dual-phase composite membranes as well their applications in membrane reactors.In Charter1, a review of ceramic oxygen-permeable dense membranes is presented with respect to the concepts and theories of oxygen permeation, materials systems and preparation methods and membrane-based processes.In Chapter2, a study on the dual-phase composite membrane comprising Ce0.9Gd0.101.95(CGO10) as oxygen ion conducting phase and Lao.6Sro.4Coo.2Feo.803-s (LSCF6428) as electron conducting phase is described. A green tape of the composite membrane was formed using phase-inversion tape casting method, and converted into a ceramic disk by sintering at1400℃. The as-prepared membrane possessed asymmetric structure with a dense layer of thickness100μm and finger-like porous layer of thickness～900μm. The surfaces of the membranes were modified with La0.6Sr0.4CoO3-δ (LSC). The oxygen permeability of the membrane was measured by exposing the porous support side to atmospheric air and sweeping the dense layer side with helium to carry away the permeated oxygen. An appreciable oxygen permeation flux of0.45mL cm-2min-1was observed with the asymmetric membrane at900℃, which is about30times higher than that for the symmetric dense membrane of the same thickness. The greater oxygen permeability of the asymmetric membrane is attributed to its unique structure. The relatively small thickness of the dense layer allowed fast transport of oxygen, and the presence of porous LSC layer enhanced the surface oxygen exchange. Moreover, the support layer of the membrane consisted of finger-like straight pores, imposing a small resistance to the transport of the gas and thus promoting the overall oxygen permeation process.In Charter3, a study on the Gd0.1Ce0.9O1.95-δ-La0.6Sr0.4FeO3-δ composite membrane is presented. The membrane with an asymmetric structure was prepared by phase inversion tape casting, and its surface at the dense layer side was modified with a porous layer of the same composition. An oxygen permeation flux as large as1.41mL cm-2min-1was observed at900℃by exposing the dense layer side of the membrane to air and the porous support side to flowing helium, showing that the membrane is promising for production of pure oxygen from air. The surface modification of the membrane at the dense layer side led to a significant increase in oxygen permeation flux, whereas modification at the porous support side did not result in appreciable gain in oxygen permeation flux, indicating that the surface oxygen exchange at the dense layer side limited the overall oxygen permeation process. A much larger oxygen permeation flux of10.3mL cm-2min-1was obtained for the membrane at900℃by feeding CO into its porous support side to react with the permeated oxygen. Analysis of the membrane after the oxygen permeation measurement by XRD and SEM revealed that the La0.6Sr0.4FeO3-δ grains in contact with CO side had decomposed into a porous structure, indicating that the membrane may not be able to meet the stability requirement for membrane reactor applications.In Charter4, a study of the oxygen permeability and stability of La0.8Sr0.2CrxFe1-xO3-δ (LSCrxF) and its composite with Zr0.84Y0.16O1.92(YSZ) is reported. The oxygen permeability of LSCrxF decreased with increasing Cr content. The oxygen permeation flux under air/CO gradient at900℃was measured to be4.47,0.96,0.37mL·cm-2·min-1for Cr content of0,0.3,0.5, respectively. The stability of LSCrxF increased with increasing Cr content, but even when the Cr content reached0.5, the material still could not remain intact in highly reducing atmosphere (H2, CO). However, the stability of LSCrxF could be improved significantly by making composite with YSZ. The YSZ-LSCr0.5F composite exhibited satisfactory stability and relatively low oxygen permeability (0.22mL cm-2min-1at900℃). The composite is promising for fulfilling the stability requirement for POM membrane reactor applications, while its oxygen permeability needs to be further improved which can be achieved by reducing the thickness of the membrane.Charter5is concerned with the preparation and POM membrane reactor application of Zr0.84Y0.16O1.92-La0.8Sr0.2Cr0.5Fe0.5O3-δ dual-phase composite membrane. The membrane was prepared by phase inversion tape casting technique. It possessed an asymmetric structure with a120μm thick dense layer and a～900μm thick porous support layer. An appreciable oxygen permeation flux of0.41mL cm-2min-1was observed with the membrane under Air/CO at900℃, and0.66mL cm-2min-1at a higher temperature of950C, significantly higher than that with the symmetric membrane of the same thickness (0.22mL cm-2min-1at950℃). When a porous layer of the same composition was applied to the dense layer side of the membrane, a much higher oxygen permeation flux of1.57mL cm-2min-1was attained at900℃. A reactor comprising a membrane of effective area4.5×4.5cm2in the presence of Ni/LSCrF catalyst demonstrated desired performance. At875℃and CH4feed rate30ml min-1, CH4throughput conversion92.3%, CO selectivity92.2%and H2selectivity93.9%was attained.hapter6is devoted to the study of preparation and electrochemical performance of the composite anode of NiO and yttria-stabilized zirconia (YSZ). The anode was prepared by phase-inversion tape casting method. In the as-prepared green tape, its top and middle layers were derived from a slurry of NiO and YSZ, while the bottom layer from a slurry of graphite. The graphite layer was eliminated by calcination at elevated temperatures, leaving the finger-like porous layer exposed to the gas phase. A cell supported on the as-prepared anode substrate exhibited a moderate electrochemical performance with a maximum power density of512mW cm-2at800℃. The cell did not show a convex-up curvature in Ⅰ-Ⅴ plots at high current density as often observed for most anode-supported cells, indicating the absence of concentration polarization which was in turn attributed to the open pore structure of the phase-inversion derived anode. The cell electrochemical performance is suggested to be limited by the activation polarization, as indicated by the concave-up curvature at low current density. It is expected that the cell performance can be improved by optimization of the cell structure such as adding a functional layer between the electrolyte and the anode.