Research On Modification Of Solid Oxide Fuel Cell Based On La_0.6Sr_0.4Co_0.2Fe_0.8O₃ Electrode

Solid Oxide Fuel Cell（SOFC）gains widespread attentions due to their high power generation efficiency,high fuel selectivity,long working life,low power generation costs and other advantages.In order to accelerate the commercialization of SOFC,it’s key to decrease the operation temperature of SOFC.Many researchers at home and abroad are devoted to reducing the operating temperature of SOFC from high temperature（800-1000°C）to intermediate temperature（600-800°C）.However,the catalytic activity of the SOFC cathode decreases as the operating temperature decreases,which greatly affects the output power of the entire SOFC system ultimately.Therefore,it is very meaningful to develop cathode with high performance and stability in the intermediate temperature range.There are many kinds of SOFC cathodes,perovskite cathode materials such as La_1-x-x Sr_xMnO₃,La_1-xSr_xCo_1-yFe_yO₃,Ba_1-xSr_xCo_1-yFe_yO₃,Ruddlesden-Popper perovskite cathode materials,such as La₂NiO₄,double perovskite cathode materials,like PrBa_0.5Sr_0.5Co_1.5Fe_0.5O₅,and so on.Among them,La_0.6Sr_0.4Co_0.2Fe_0.8O₃（LSCF）cathode has been widely concerned due to its good electrical conductivity,high catalytic activity and good stability,but there is still a certain distances meeting commercial needs.It is necessary to further improve the electrochemical performance and stability of LSCF cathode.Among those methods,element doping is favored by many researchers because of easy implementation and low cost.In this thesis,Ce and Mo were selected as doping elements.By comparing results of X-ray diffraction（XRD）,scanning electron microscopy（SEM）and electrochemical impedance spectroscopy（EIS）before and after doping,the mechanism improving the performance of LSCF cathode with Ce and Mo doping was obtained.Finally,Comsol software was applied to bulid the single cell model and the actual model was further considered to compare the effect of doping on the output power of the whole cell.Three main conclusions are drawned in this thesis as follows:（1）Cathode powders with different Ce doping ratios were prepared by the sol-gel method and results of XRD and refinement show that Ce doping caused lattice expansion.The electrochemical results show that the catalytic activity of LSCF cathode is enhanced with the increase of Ce content.La_0.54Ce_0.06Sr_0.4Co_0.2Fe_0.8O₃（LCSCF06）electrode exhibits the best catalytic activity and its polarization resistance is 0.09?·cm²at 750°C.The enhancement of electrochemical performance of LSCF cathodes could be ascribed to the increase of the concentration of oxygen vacancies obtained from the thermogravimetric results,which leads to increased oxygen surface exchange performance and oxygen adsorption performance,resulting in an acceleration of oxygen reduction reaction process,improving the electrochemical performance of LSCF cathodes at finally.The stability of the Ce-doped LSCF cathode shows that the stability is not significantly improved（slightly better than the LSCF electrode）.（2）Cathode powders with different Mo doping ratios were prepared by the sol-gel method,and it is found that Mo doping causes lattice expansion by XRD and refinement results.The EIS test results show that the doping of Mo increases the catalytic activity of the cathode in the Mo doping range.The thermal stability test results show that the thermal stability of the La_0.6Sr_0.4Co_0.2Fe_0.75Mo_0.05O_3-δ（LSCFM05）cathode is greatly improved compared to the LSCF cathode under open circuit voltage for 30 hours.The value of polarization resistance increases from 0.125?·cm² to0.14?·cm² within 30 hours.The degradation rate is almost negligible.The results of X-ray photoelectron spectroscopy（XPS）show that the increase of stability is due to the fact that the doping of Mo inhibits the precipitation of Sr and stabilizes the cathode structure,stabilizing the cathode performance finally.（3）A three-dimensional single cell model based on LSCF cathode was established in Comsol.The polarization curve（I-V）and power density curve（I-P）of cell at different temperatures show the same trend with the experiment data.Namely values of power density increase with the increase of the temperature.By replacing the commonly used LSCF cathode with LCSCF06 cathode and comparing the EIS diagrams obtained from simulation and experimental results in Chapter 3,it is found that the value of polarization resistance shows the same trend.The value of the polarization resistance decreases as increasing doping contents of Ce at 750°C.By comparing the I-V and I-P diagrams of the cells with LSCF and LCSCF06 cathodes at 750°C,it shows that the output power density of the single cell with LCSCF06 cathode is higher than that with LSCF cathode at the same current density.If the LCSCF06 cathode studied in this thesis replaced the commercialized LSCF cathode,the output power of the SOFC could be further improved,which has a positive significance for further promoting the application of the high performance SOFC in the fuel cell vehicle.