Synthesis And Electrochemistry Performance Assessment Of Copper Cobalt Spinel As Composite Cathodes Of SOFC

The intertemperation has become the main research and development trend in the solid oxide fuel cells(SOFC)field in recent years.However,the polarization loss of the cells,especially the cathode polarization loss,will be greatly increased,which greatly restricts the development of intermediate-temperate(IT)SOFC.In view of this,the modification of cathode materials at intermediate temperature and the improvement of catalytic activity are of great importance.Perovskite materials usually have high conductivity and catalytic activity,but they also have defects such as poor thermal expansion match with electrolytes and easy occurrence of side reactions.Recently,our research group have developed a new type of CuCo₂O₄ spinel cathode material,which has good performance in electronic conductivity and thermal expansion matching with electrolytes.However,due to its low ionic conductivity,this material needs to be further improved in the electrochemical performance.Furthermore,the reaction mechanism of this material also remain unclear.In this dissertation,through the combination of the CuCo₂O₄ spinel with high oxygen ionic conduction Er_0.4Bi_1.6O_3-?(ESB)material,surface modification and constructing one-dimensional nanocomposite cathode,the ionic conductivity in the electrode is improved and the number of active sites for three-phase reaction are increased.In addition,by means of electrochemical impedance spectroscopy and electrochemical relaxation time distribution,the mechanism of oxygen reduction reaction and rate control mechanism have been investigated in depth,and the influence law of electrode microstructure on its performance is revealed,which lays a solid foundation for the development of high-performance intermediate-temperature cathode materials based on spinel.Firstly,the CuCo₂O₄ spinel material and ESB material have been prepared by citric acid-EDTA combustion method.The thermogravimetric-differential thermal analysis has been used to study the weightlessness with temperature of the precursors of that two materials,and the optimum roasting temperatures of the CuCo₂O₄ spinel and ESB materials have been determined respectively.The phase structures of the two materials are determined to be CuCo₂O₄ spinel material and ESB material by the XRD characterization.The CuCo₂O₄-ESB composite cathode material has been prepared by the mechanical mixing method.After continuous calcination for 6 h at 800? in air,the chemical compatibility of the above two phases is good,and there is no obvious agglomeration.The conditions of electrode preparation have been optimized.The sintering temperature of the composite cathode is 800? and the content of ESB is 35 wt.%.The CuCo₂O₄-ESB composite cathode has obtained a polarization resistance of 0.07?·cm² at 800?,which is reduced by 42%compared with the reported single-phase CuCo₂O₄ spinel material.The oxygen reduction reaction(ORR)rate of CuCo₂O₄-ESB composite cathode is controlled by charge transfer process and oxygen adsorption process,which is indicated by the impedance spectroscopy tests at various oxygen partial pressures.The CuCo₂O₄/ESB nanofiber composite cathode has been prepared by means of the electrospinning and rapidly sintering methods.The calcinating temperature of composite fiber precursor material is determined by the thermogravimetric analysis.The phase structure of the CuCo₂O₄/ESB nanofiber composite cathode material is determined by the XRD characterization,which is composited of two phases of CuCo₂O₄ spinel and ESB.SEM characterization results show that the precursor samples form nanofibers composed of nanoparticles with uniform size after rapid sintering at 700? in air,and the diameter of the fibers is about 400nm.After mechanical mixing of the composite fiber cathode and Scandium stabled zirconia(SSZ)electrolyte and continuous respectively roasting at 800,850 and900? for 6 h in air,there is no reaction between the two phase materials,indicating good chemical compatibility.The interface stability and compatibility of CuCo₂O₄/ESB nanofiber composite cathode and SSZ electrolyte in symmetrical cells are characterized by the SEM and EDS.After experimental optimization,the electrode sintering temperature is 850? and the ESB content is 35 wt.%.At800? in air,the CuCo₂O₄/ESB nanofiber composite cathode has obtained the polarization resistance of 0.021?·cm².The polarization resistance change over time of the CuCo₂O₄/ESB nanofiber composite cathode is tested at 800?.The constant temperature test at 800? in air proves that the composite cathode possesses good thermal stability.Electrochemical impedance spectrum tests under various oxygen partial pressures confirm that the ORR rate-limiting step is mainly the process of oxygen adsorption and dissociation.By comparison,it is found that the ORR reactions corresponding to the CuCo₂O₄/ESB nanofiber composite cathode have a shorter relaxation time and a faster reaction rate.In order to optimize the microstructure of the cathode,the CuCo₂O₄-decorated-ESB nanocomposite material has been prepared by the sol-gel method.The weightlessness and appropriate calcining temperature of the CuCo₂O₄-decorated-ESB nanocomposite material are determined by the thermogravimetric analysis.By the XRD characterization,the phase structure of the CuCo₂O₄-decorated-ESB nanocomposite material is determined to be composed of two phases of the CuCo₂O₄ spinel and the ESB.The microstructure of the CuCo₂O₄-decorated-ESB nanocomposite powders has been determined by SEM and TEM characterization,indicating that CuCo₂O₄ nanoparticles are evenly distributed on the ESB matrix surface.The good interface stability and compatibility between the electrode and the electrolyte have been characterized by the SEM-EDS.After experimental optimization,the sintering temperature of the composite cathode is850? and the content of ESB is 35 wt.%.At 800?,the polarization resistance of the CuCo₂O₄-decorated-ESB nanocomposite cathode is 0.016?·cm²,which is reduced by 78%compared with the composite powder cathode.The constant temperature test at 800? in air proves that the CuCo₂O₄-decorated-ESB nanocomposite cathode possesses good thermal stability.Impedance spectroscopy tests at different oxygen partial pressures and the analysis of the electrochemical relaxation time distribution reveal that the rate of the ORR process is mainly limited by the charge transfer process on the cathode,and the oxygen adsorption process on the cathode/gas interface also possesses a partial limitation.