Preparation And Performance Of Alkali Metal Doped Perovskite Oxygen Permeable Membrane

In recent years,the oxygen separation technology of inorganic oxygen-permeable membranes has aroused great concern from researchers,and various types of oxygen-permeable membrane materials were developed.However,these oxygen-permeable membrane materials are difficult to meet the requirements of industrial use.Studies have shown that in practical applications,oxygen-permeable membrane materials must have both high stability and high oxygen permeability,but many materials currently developed are difficult to achieve both oxygen permeability and stability,most-investigated perovskite-type oxygen membrane materials have high oxygen permeability which pocess excellent industrial advantages,but their stability is insufficient.Therefore,improving the stability of perovskite oxygen-permeable membrane materials and maintaining their high oxygen permeability have become research goals.The research goal of this paper is to develop a perovskite oxygen permeable membrane material with both stability and oxygen permeability.That is to develop a high-quality oxygen permeable membrane material that has the balance advantages of high stability and high oxygen permeability.Based on Sr Co_0.8Fe_0.2O_3-??SCF?perovskite material with high oxygen flux,a new potassium ion-doped perovskite oxygen permeable material,i.e.,K_0.1Sr_0.9Co_0.8Fe_0.2O_3-?(K_0.1SCF)and K_0.05Sr_0.95Co_0.8Fe_0.2O_3-?(K_0.05SCF),were prepared by a sol-gel method.Meanwhile,their crystal structure,microstructure,oxygen permeability and stability of these materials were systematically studied.XRD characterization shows that K_0.1SCF obtained by doping 10%potassium ions with large radius into A site of perovskite crystal cell still maitains pure cubic perovskite structure.However,doping above 10%of potassium ions into A site will induce the materials to gradually deviate from pure cubic perovskite structure,and more miscellaneous phases would appear.SEM analysis shows that K_0.1SCF membranes can be highly dense when sintered at 1220^oC.Especially K_0.1SCF still maintains high mechanical strength after long-term storage,and no powdered disintegration similar to the SCF material will occur.Oxygen permeation experiments show that the increase in operating temperature,the decrease in membrane thickness and the increase in the purge gas flow rate are all conducive to the improvement of the oxygen permeability of the membrane.The K_0.1SCF membrane with a thickness of0.5mm at 950^oC can obtain oxygen flux of 2.65 ml·cm^-2·min^-1.By investigating the rate control steps of the K_0.1SCF membrane,it can be found that when the thickness of the oxygen permeable membrane is less than 0.7 mm,the oxygen permeation process is determined by the surface exchange control.When the thickness of the membrane is higher than 0.7mm,the oxygen permeation process can be ascribed into bulk phase diffusion control.When the membrane thickness is 0.7mm,its oxygen permeation activation energy is approximately60.70 k J·mol^-1.K_0.05SCF XRD characterization shows that K_0.05SCF has pure cubic perovskite structure after A site of SCF material is doped with 5%of potassium ion.SEM analysis showed that K_0.05SCF membrane was highly dense when calcined at 1240^oC.And layered ripples appeared on the surface of the grain,which was obviously different from other perovskite oxygen-permeable materials without potassium ions.Oxygen permeation stability test of K_0.05SCF showed that the oxygen permeability of K_0.05SCF membrane?1.0mm?dropped sharply after 70 hours of reaction.The membrane surface was corroded and its stability tended to deteriorate.K_0.05SCF oxygen permeation results show that the increase of operating temperature,the decrease of the membrane thickness and the increase of the purge gas flow rate are conducive to the improvement in oxygen permeability.The oxygen permeability of K_0.05SCF can reach 2.78ml·cm^-2·min^-1.By investigating the rate control step of K_0.05SCF membrane,it can be found that when the thickness of K_0.05SCF membrane is less than 1.2 mm,their oxygen permeation process is determined by the surface exchange control.When the membrane thickness is larger than 1.2 mm,the oxygen permeation process will be transformed into bulk phase diffusion control.When the membrane thickness of K_0.05SCF is 1.2mm,its oxygen permeation activation energy is approximately 49.21 k J·mol^-1.Comparison experiment of oxygen permeability for two samples shows that oxygen permeability of K_0.05SCF membrane is higher than that of K_0.1SCF and SCF under the same test conditions,their oxygen flux are 1.44ml·cm^-2·min^-1,1.64ml·cm^-2·min^-1,1.78ml·cm^-2·min^-1at 950^oC,respectively.Stability comparison experiments show that the stability of K_0.1SCF is much better than that of K_0.05SCF.The oxygen flux of K_0.1SCF only decreases by 0.04ml·cm^-2·min^-1within 120h.The oxygen flux of K_0.05SCF began to drop sharply after 70 h.Comparative experiments of rate-determing steps show that the critical thickness of K_0.1SCF membrane is about 0.7 mm,and the critical thickness of K_0.05SCF membrane material is about 1.2mm.Through comparing the above three materials,it can be found that the doping of potassium ion in perovskite materials can significantly improve the stability of SCF-based material,and meanwhile maintain comparatively high oxygen permeability.However,excessive doping of potassium ion in SCF will destroy the perovskite cubic structure and lead to the appearance of heterophases,which is not conducive to obtain excellent oxygen permeation materials.