Study On Strontium Segregation Behavior And Suppression Strategies Of Lanthanum Strontium Cobalt Iron Based Cathodes In SOFC

Solid oxide fuel cell（SOFC）has many advantages such as high energy conversion efficiency,diverse fuels,solid-state and environmentally friendly.Under the background of carbon neutrality development,SOFC has promising development prospects and practical value.However,the degradation of performance during long-term operation,especially the degradation of cathode performance,limits its further commercial application.ABO₃-type perovskite oxide La_1-xSr_xCo_1-yFe_yO_3-δ（LSCF）has received extensive attention from researchers for the advantages such as mixed ionic-electronic conductivity,excellent catalytic activity,chemical stability and thermal stability matched with other components.In this study,the Sr segregation behavior and performance degradation of different LSCF-based cathodes were studied before and after the polarization,which aimed to exploring the strategies for suppressing Sr surface segregation behavior of cathode.The sol-gel method was used to prepare LSCF nanoparticles with different A-site,B-site composition and A-site deficiency LSCF nanoparticles which are sintered as LSCF cathodes subsequently.The Sr segregation behavior and performance degradation of LSCF cathodes were explored through electrochemical impedance spectroscopy（EIS）and constant current polarization methods.The results show that with the increase of Sr content at A-site and Co content at B-site,the oxygen reduction activity of the LSCF cathodes gradually increases,while the performance stability gradually decreases.Besides,both oxygen reduction activity and performance stability of the cathodes can be improved via introducing the Sr deficiency at A-site.The surface element analysis results show that obvious Sr surface segregation phenomenon occurred in LSCF cathodes during the constant current polarization process,and the degree of Sr surface segregation decreases with the decrease of Sr content at A-site,Co content at B-site and the introduction of A-site deficiency,which is consistent with the impedance results before and after the constant current polarization,proving that the Sr element surface segregation is the main factor causing the performance degradation of the LSCF cathodes.In order to explore the strategies for suppressing the Sr surface segregation,on the basis of previous LSCF cathodes performance stability research,the Sr segregation behavior and performance degradation of La_0.8Sr_0.2Co_0.2Fe_0.8O_3-δ-Ce_0.8Gd_0.2O_1.9（LSCF-GDC）composite cathodes were further explored.Electrospinning method and electrode solution infiltration method were used to prepare LSCF-GDC composite cathodes with different LSCF skeletons.The impedance results show that the optimal GDC mass ratios for the LSCF-GDC composite cathodes with LSCF nanoparticles,200 nm LSCF nanofibers,and 280 nm LSCF nanofibers as skeletons are 1:0.18,1:0.3,1:0.76,corresponding polarization impedances at 750℃are 0.12,0.07,and 0.06Ωcm²,respectively.Whereafter,constant current polarization with current density of 100m A/cm² for 144 h are applied on LSCF-GDC composite cathodes with optimal GDC mass ratios,and corresponding polarization resistances at 750℃increase to 0.34,0.39,and 0.21Ωcm².In addition,the optimal GDC mass ratio of the composite cathodes can be further improved by adding pore former into the cathode slurry,which optimizes the electrochemical performance and performance stability of composite cathodes after pore-forming.Combining the results of XPS and XRD,it can be preliminarily concluded that the improvement of the optimal GDC mass ratio can introduce larger compressive stress on LSCF surface,which leads to the gradual decrease of Sr segregation on LSCF surface,thus effectively suppressing the Sr segregation behavior.In view of the actual production requirements and the process simplicity,based on the inhibition effect of compressive stress on Sr element segregation,the core-shell-structured LSCF@GDC composite cathodes were prepared via external coating method,and the Sr segregation behavior and performance degradation were further explored.Compared with the electrode solution infiltration method,the optimal GDC mass ratio of nanoparticle and nanorod（200 nm）core-shell-structured LSCF@GDC composite cathodes prepared via external coating method are further improved to 1:0.4 and 1:0.6,and the performance stability are also improved.For the nanorod core-shell-structured LSCF@GDC（1:0.6）composite cathode,the resistances remain unchanged after polarization at 100 m A/cm² and 200 m A/cm² for 144 h.What’s more,after polarization at 100 m A/cm² for 1242 h,the polarization resistance decreased from 0.15Ωcm² to 0.13Ωcm²,revealing no performance degradation occurs during long-term constant current polarization.XPS results show that after the constant current polarization process,the Sr segregation degree of core-shell-structured LSCF@GDC composite cathodes is lower than that of LSCF-GDC composite cathodes prepared by the electrode solution infiltration method.The stress distribution at the LSCF@GDC interface during the sintering process is simulated by molecular dynamics.Combined with the results of XPS and XRD,it can be proved that the compressive stress which is generated at LSCF-GDC interface during the sintering process can introduce the lattice strain into LSCF lattice.As the GDC coating amount increases,the compressive stress and lattice strain at LSCF surface increase,resulting in the increase of Sr surface segregation energy,which improves the Sr segregation suppression capability of core-shell-structured composite cathodes.Based on the results in Chapters 4 and 5 that lattice strain can be introduced into the LSCF lattice by applying external stress,large radius Nb⁵⁺ions were doped into the B-site of LSCF material,which can introduce strain into the lattice and thus achieve the purpose of suppressing Sr surface segregation.La_0.8Sr_0.2Co_0.2Fe_0.8-xNb_xO_3-δ（LSCFNb,x=0,0.02,0.04,0.06,0.08,0.1）nanoparticle with different Nb contents were prepared by solid phase method and sol-gel method.The results show that LSCFNb cathodes prepared by sol-gel method reaches the minimum polarization resistance of 0.27Ωcm² at 750°C when x=0.04.Moreover,the performance stability of the LSCFNb cathodes gradually increases with the Nb content in B-site.The surface element analysis results show that doping large radius ions Nb⁵⁺at B-site can effectively inhibit the Sr segregation.Combining the synergistic effect of external stress and internal stress,the La_0.8Sr_0.2Co_0.2Fe_0.76Nb_0.04O_3-δ@Ce_0.8Gd_0.2O_1.9（LSCFNb4@GDC）composite cathode were prepared by the external coating method.The polarization resistance and surface Sr distribution of the LSCFNb4@GDC composite cathode remains unchanged before and after polarization,indicating the LSCFNb4@GDC composite cathode possesses excellent Sr segregation inhibition ability and performance stability.