Investigations On The Preparation,modulation And Oxygen Permeability Of Pr₂NiO₄-based Materials With K₂NiF₄-type Structure

K₂Ni F₄-type composite oxide Pr₂Ni O₄（PNO）was chosen as the precursor,Ni element was partially substituted by Mo element.K₂Ni F₄-type Pr₂Ni_1-xMo_xO4+δ（PNM_x）ceramic powders were prepared by sol-gel method.PNMx hollow fiber membranes were prepared by phase inversion/sintering method.The crystal structure,microscopic morphology,oxygen permeability and CO₂ stability of PNMx hollow fiber membranes were investigated by XRD,SEM,TEM,GC,respectively.First-principles calculations were used to study the oxygen ion conduction mechanism in PNO materials.The results show that the PNM_x powders can form K₂Ni F₄-type structure after calcined at 1100°C in air atmosphere.With the increase of doping amount of Mo,new phases Pr₆O₁₁,Pr₄Ni₃O₁₀,Ni O and Ni Mo O₄ appeared.Oxygen permeability tests showed that the Pr₂Ni_0.95Mo_0.05O4+δ(PNM_0.05)hollow fiber membrane exhibited the highest oxygen permeability.When He sweep gas flow rate is fixed at 120m L·min^-1,the oxygen permeation flux achieves 3.54 m L·min^-1·cm^-2 at 1000°C.About 300 h oxygen permeability test shows that PNM_0.05 hollow fiber membrane is quite stable at 950°C;no significant fluctuation of oxygen flux is observed when the sweeping gas shifts from He to mixture of He-CO₂（50%-50%）during the 100 h test,the oxygen permeation flux of the PNM_0.05 hollow fiber membrane remains 2.35 m L·min^-1·cm^-2,which indicates that the PNM_0.05hollow fiber membrane is very resistant to CO₂.The first-principles calculations showed that the oxygen transfer in PNO takes interstitial oxygen mediated transport mechanism instead of oxygen vacancy mechanism.In order to improve the oxygen permeability,PNM_0.05 hollow fiber membrane with the highest oxygen permeation flux was selected to be optimized from both surface reaction and bulk diffusion proccess.On the one hand,a perovskite-type La_0.8Sr_0.2Co O_3-δ(LSC₈₂)ceramic powder was synthesized by sol-gel method.PNM_0.05 and LSC₈₂ materials were used to modify the outer surface of PNM_0.05 hollow fiber membrane,which were named as self-modified and LSC₈₂-modified PNM_0.05 to improve the surface reaction kinetics by reducing the apparent activation energy.The oxygen permeability test results showed that the oxygen permeation fluxes of PNM_0.05hollow fiber membranes were improved after surface modified,especially in the low temperature range.When temperature is 750°C,the sweep gas flow rate is 120m L·min^-1,the oxygen permeation fluxes increase from 0.43 to 1.53 and 1.27 m L·min^-1·cm^-2after self-modified and LSC₈₂-modified,respectively.The effect of the self-modified is slightly better than that of the LSC₈₂-modified.Arrhenius calculations were performed on the oxygen permeation process.The apparent activation energies of the modified membranes are greatly reduced,especially in the low temperature range.On the other hand,the microstructure of the hollow fiber membrane was adjusted by changing the types of the coagulation bath（core fluid）in the phase inversion process of the PNM_0.05 hollow fiber membrane,and the resistance of the bulk diffusion process was reduced.Thus,the oxygen permeation fluxes of the PNM_0.05 hollow fiber membranes were improved.When the core fluid is NMP-Et OH mixture,the lengths of the finger holes in the hollow fiber membrane grow with the increase of NMP concentration.When the NMP content is 70%,the microstructure of PNM-70 membrane possesses the longest finger holes,but the sintering temperature of the membrane increases,which results in the decrease of the material activity.When the NMP content is 50%,the oxygen permeation fluxes of PNM-50 membrane are higher than those of PNM-0 membrane（deionized water was used as core fluid）,and the improvement is more obvious in low temperature range.Increasing the viscosity of the spinning mixture,and NMP-Et OH-H₂O mixture is used as the core fluid,with the increase of NMP content,the inner surface of the membrane changes from dense to porous and large pores appear finally.The PNM-901 hollow fiber membrane exhibits the highest oxygen permeability,but its sintering temperature is identical to that of the PNM-0 membrane.When temperature ranges from 750°C to 1000°C,the sweep gas flow rate is 120 m L·min^-1,the oxygen permeation fluxes of PNM-0 and PNM-901 hollow fiber membranes are 0.43-3.54m L·min^-1·cm^-2and 1.74-3.60 m L·min^-1·cm^-2,respectively.Electrochemical tests on PNM_x materials show that PNO exhibits the highest conductivity with a maximum of 120 S·cm^-1at 450°C.PNM_0.05 material has the biggest surface exchange coefficients with a maximum of 6.78×10^-5S·cm^-1 at 850°C according to the electrical conductivity relaxation（ECR）measurements.