Study On Synthesis And Stability Of The Perovskite Cathode Materials For IT-SOFC

Solid oxide fuel cell?SOFC?is a power generation device which can convert the chemical energy of the fuel gas?such as natural gas,coal gas,etc.?into electrical energy efficiently.It has attracted more and more attention and been considered as one of the most promising ways to generate electricity in the future,because of its advantages,such as high energy conversion efficiency,clean,environmental protection,low noise,and so on.It is an inevitable trend to develop the key materials which can be used at low temperature so as to reduce the operating temperature and cost of SOFC.During the process of searching for SOFC cathode materials used at medium and low temperature,La_1-xSr_xCo_1-yFe_yO_3-??LSCF?materials have been widely studied,which are a series of perovskite-type oxides with mixed ionic and electronic conductivity?MIEC?and high electrocatalytic activity for oxygen reduction reaction?ORR?.However,the long-term performance degradation is a major obstacle to its application in SOFC.The long-term stability and performance degradation mechanisms of LSCF cathode materials were studied in this paper.La_1-xSr_xCo_0.2Fe_0.8O_3-??x=0,0.4,0.6,1?powders were prepared by sol-gel method and the effects of Sr content and thermal effect on electrochemical performance of LSCF cathodes were investigated systematically.Results showed that La_1-xSr_xCo_0.2Fe_0.8O_3-?had a cubic perovskite structure.With the increase of Sr content,the unit cell volumes reduced from 59.6573?³ to 57.5795?³,the thermal expansion coefficients increased from 12.98×10^-6 K^-1 to 20.16×10^-6 K^-1,while the structural and chemical stability of La_1-xSr_xCo_0.2Fe_0.8O_3-?oxides decreased.The chemical composition and valence states of the elements on the surface of the samples before and after annealing in air were probed which indicated that Sr segregation occurred at the preparation process of the samples,Sr and Co segregated towards the surface continuously upon annealing.With further Sr enrichment,the second phase particles formed on the surface and were proved to be SrCO₃ phase.Thermal effect and the interaction forces in the crystal lattice were important driving forces for Sr segregation towards the surface.With a small amount of Sr doping,the oxygen vacancy concentration in the crystal was increased which tended to improve the electrochemical performance of LSCF cathodes.However,excessive Sr doping could reduce the electronic conductivity of LSCF cathodes which was detrimental to the electrochemical performance.All the cathodes showed performance degradation in the long-term tests,and the degradation rates increased with the increase of Sr content.As the most instability of La_1-xSr_xCo_0.2Fe_0.8O_3-??x=1,SCF?materials,the possibility of Nb substitution to improve the stability and the effects of Nb content on the performance were studied.With the increase of Nb content,the unit cell volumes of SrCo_0.2Fe_0.8-yNb_yO_3-?increased,the thermal expansion coefficients decreased,and the electrical conductivity had a trend of increasing first and then decreasing.TPD,TG and SEM results of undoped and doped materials indicated that the structural and chemical stability of SrCo_0.2Fe_0.8-yNb_yO_3-?oxides were improved with the increase of Nb substitution.A small amount of Nb doping improved the electrochemical performance of SrCo_0.2Fe_0.8-yNb_yO_3-?cathodes,while additional Nb doping was proved to reduce the performance.The cathode with y=0.05 had the optimal electrochemical performance.