Preparation And Oxygen Permeation Stability Of New Dual-phase Mixed Conducting Ceramic Membranes

The mixed-conducting ceramic membrane,also known as mixed ionic-electronic conducting oxygen transporting membrane?MIEC?,is a kind of efficient oxygen separation membrane with conductivity of both oxygen ion and electronic.Oxy-fuel combustion based on MIEC membranes is on of the most promising technology for CO₂ capture,but most of the MIEC membranes can't both high oxygen permeability and excellent stability,still have a gap from the industrial application.In this thesis,based on Pr_0.6Sr_0.4Co_0.5Fe_0.5O_3-??PSCF?single-phase perovskite membrane,which has excellent stability in containing CO₂ atmosphere,using fluorite-type oxide Ce_0.8Gd_0.2O₂-?as ionic phase and conbining with PSCF to get Ce_0.8Gd_0.2O₂-?–Pr_0.6Sr_0.4Co_0.5Fe_0.5O3-??CG–PSCF?dual-phase membrane.The effects of CG content on oxygen permeability,stability in pure CO₂ atmosphere and rate-determining step in He atmosphere were systematically investigated.Under the best two phase weight ratio,the effects of Nb-doped content on long-term stability in pure CO₂ atmosphere of Ce0.8Gd0.2O₂-?–Pr0.6Sr0.4Co0.5Fe0.5-xNbxO3-??CG–PSCF0.5-xNx?dual-phase memebranes were also studied in this work.PSCF single-phase perovskite membrane was successfully synthesized by the sol-gel method.The best burning temperature were established after comparing the microcosmic morphology under different sintering systems.H₂-TPR,O₂-TPD,TG-DSC and other test methods were used to systematically investigate the stability of PSCF membrane.Test results indicated that PSCF had excellent thermal stability,reducing resistance and CO₂ tolerance.The oxygen permeation flux of PSCF didn't decrease in containing CO₂ atmosphere,whose stability was higher than other single-phase membranes.While after the long-term oxygen permeation test in pure CO₂atmosphere at 900 ^oC,the stability of PSCF was not good enough.The results of XRD,SEM and EDXS indicated that there was SrCO₃ on membrane surface,owing to the reaction between CO₂ and PSCF.SrCO₃ destroyed the phase structure and hindered the surface oxygen exchange,which reduced the oxygen permeability.A series of CG–PSCF dual-phase powder with different weight ratios were synthesized by the sol-gel method,and the density dual-phse membranes were obtained after sintering.The influence of CG content on oxygen permeability and stability under pure CO₂ atmosphere,though XRD,SEM,BSEM,XPS,oxygen permeability model under He atmosphere,oxygen permeation test and other test methods.The results showed that appropriate CG content was benefit for enhancing oxygen oxygen ion bulk transport and exchange at the interface,and increasing oxygen permeation flux.But too much CG increased oxygen bulk transport resistance and changed the rate-determining step,which would reduce oxygen permeability of CG-PSCF membranes.Because CG wouldn't react with CO₂,the stability of CG-PSCF in pure CO₂ atmosphere would be improved with CG content increasing.Overall consideration of oxygen permeability and CO₂ resistance,the beat weight ratio of CG and PSCF should be 60:40.TG-DSC experiment in pure CO₂ atmosphere also confirmed that 60CG–40PSCF dual-phase membrane had better thermal stability and CO₂ resistance than PSCF single-phase membrane.Long-term oxygen permeability in pure CO₂ atmosphere at 900 ^oC indicated that after introducting fluorite oxide CG,the decrease of oxygen permeation flux changed from 30.1%to 19.5%.Compared with PSCF oxygen permeability hadn't been stable in the 200 h testing range,the oxygen permeability of 60CG-40PSCF membrane maintained stabe around 100 h after the start,the stability of PSCF membrane was obviously improved.A series of CG–PSCF_0.5-xN_x?x=0,0.025,0.025,0.05,0.1 and 0.125?dual-phase mixed conducting oxygen membranes were successfully prepared by the sol-gel method.TG-DSC,XPS,and oxygen permeation test showed that the increase of Nb-doped content made valence-variable element Co/Fe tend to low price and reduced surface basicity of CG–PSCF_0.5-xN_x membranes,which could improve thermal stability and CO₂ resistance of materials.Considering oxygen permeation performance and stability in pure CO₂ atmosphere,CG–PSCF_0.4N_0.1 was the best one among these samples,whose permeation flux decrease of 200 h oxygen permeation test in pure CO₂atmosphere under 900 ^oC was only 3.9%,better than most of other dual-phase membranes.Through oxygen permeation model analysis,the decrease of thickness could reduce oxygen ion bulk transport,which changed the rate-determining step of CG–PSCF_0.4N_0.1 membrane.Based on maintaining the stability of CG–PSCF_0.4N_0.1 in pure CO₂ atmosphere,reducing the thickness could further improve its oxygen permeability.The test result showed that the oxygen permeation flux of stable 0.5 mm thickness CG-PSCF_0.4N_0.1 membrane in pure CO₂ atmosphere under 900 ^oC was 0.36m L·min^–1·cm^–2,which had good application value in oxy-fuel combustion.