Study Of Anodes In Ceria Based Solid Oxide Fuel Cells Based On Elements Diffusion

High power efficiency,non-pollution in theory,wide application and high security make solid oxide fuel cell(SOFC)technology becomes one of the reliable paths to solve the current energy and environment problems.Nowadays,SOFC technology is still in the key stage of laboratory research.To promote the widespread application of the SOFC technology,developing more efficient,stable,low-cost sofc electrodes and electrolytes are the most important parts in bringing it out of the lab and achieving the industrialization of this technology.Currently,in the preparation of SOFC,sintering at high temperature becomes a common progress in order to obtain the electrolyte with high density.However,in the process of high temperature sintering,the elements diffusion between electrode and electrolyte becomes an inevitable problem.The elements diffusion brings the chaos of material internal function and impurities between electrode and electrolyte,which directly reduces the performances of cells,destroys the cells or even causes security problems.Thus,controlling and taking advantage of the elements diffusion become an urgent scientific problem.So far doped ceria has been widely researched which can be used in intermediate temperature solid oxide full cell(IT-SOFC)as electrolyte with extensive application prospect.However,the defect of high electronic conductivity makes it useless under high voltage,which limits its practical application.Elements diffusion induced by proton conductor has been successfully reported to be an effective approach to suppress the reduction of DCO electrolyte by forming a Ba-containing functional layer over the electrolyte surface.Although some reports have successfully reported the methods in reducing the electronic conductivity in ceria-based SOFCs,the research about the factors that affect the diffusion of elements,the internal structure in diffusion layer and the theoretic mechanism related to the performance still rarely appear.The attention to the performance degradation in SOFC caused by element diffusion has not been extensively paid.The comprehensive research of elements diffusion in SOFC and the use of elements diffusion in improving the performance of SOFC are still very few.This thesis focuses on the key issue of high electronic conductivity in rare earth-doped ceria-based electrolytes.Fabricate a functional layer which can reduce the electronic conductivity with the help of elements diffusion by co-firing the half cells at high temperature.Study the formation mechanism of the functional layer,the internal structure and the influence mechanism to the performance of cells.Exploiting several typical anodes and revealing the key mechanism of the elements diffusion can provide referential values for the other researches.This thesis includes:Chapter 1.Summarize the typical features of SOFC,provide the development history,the research status and the future of SOFC in detail.Introduce the recent typical progresses in SOFC materials research,including SOFC anodes,electrolytes and cathodes.Then describe the key problems in the research of electrodes and electrolytes separately.Introduce the types of element diffusion;analyze the microscopic reasons of elements diffusion.Provide the typical problems and representative achievements in the development of electrodes and electrolytes.Illuminate the importance and value in the research of elements diffusion in SOFC.Provide the practical pathways in taking advantage of elements diffusion and analyze the future of elements difusion-based SOFC.Chapter 2.Develop a kind of typical and functional anode(NiO-BZCG)which can reduce electronic conductivity of ceria-based SOFCs.This material contains three typical features:including proton conductor(BZCG),severe element diffusion and including the elements which is similar to Gd0.1Ce0.9O2-? electrolyte.This three characteristic provide a reliable reference for the study of the reduction of electronic conductivity in electrolyte caused by the diffusion of Ba and Zr elements.Through a series of experimental study,the use of this anode in ceria-based SOFC can supply diffusion elements effectively,and inhibit the electronic conductivity in GDC electrolyte.The study also found that,to achieve the purpose of forming the optimal functional layer,simplely controlling the sintering process between 1300 and 1400?can regulate the thickness of this functional layer.The results of SEM directly show the elements distribution in the functional layer between the anode and electrolyte,which provide valuable experiences in controlling the functional layer efficiently.Chapter 3.The process of controlling the functional layer can efficiently improve the ceria-based SOFC performance by adopting the component in anode.Elements diffusion can be used to control the performances of ceria-based SOFC,but the effective approach are still few.Controlling the component in anodes maybe is a simple and effective method.This chapter introduces oxygen-ion conductor into anode forming "electron-proton ion-oxygen ion" composite conductor.The composite anodes can efficiently increase the ceria-based SOFC performance.At the same time,through adjusting the proportion of oxygen-ion conductor in composite anodes,the functional layer can be saved or even controlled efficiently.Chapter 4.The results in Chapter 3 show that 20%weight of proton conductor in the functional anode is a proper ratio.Through finding the best proportion of oxygen-ion conductor in the composite anodes,the performance of ceria-based SOFC can be greatly increased.This chapter explores a novel composite anode NiO-(1-x)BZC Y-xYSZ,a type of "electron-proton ion-oxygen ion" mixing conductor,and verifies the results obtained in Chapter 3.The results that need to be verifies are:the proper value of x at different anodes;the different elements diffusion behavior while the anode component are changed;Research results show that the composite anodes can adjust the thickness of diffusion layer between the electrolyte and anode,and then control the performance of ceria-based SOFC.In this study,the value of x=20%is the same as the value in Chapter 3.Therefore,the proton conductor can be identified as the source of elements diffusion in the composite anode.At the same time,oxygen-ion conductor can effectively control the diffusion layer and improve the performance of cells.The series of research provides effective supports for the same research in controlling elements diffusion experimentally.Chapter 5 is about the microscopic mechanism research.Through the systematic experiments,this chapter aims to:explore the source of elements diffusion,detect the microstructure and chemical composition at anode|electrolyte interface,and then obtain the the key results of the diffusion mechanism in the study of elements diffusion in ceria-based SOFC.In this chapter,we adopt the NiO-BZC Y anode,study the type,pathway,destination and explore the reason of the functional microstructure formation by elements diffusion at anode|electrolyte interface by the sintering process at high temperature.Our results indicate:elements diffuse from anodic side,form a new composition with orthorhombic phase BaCeO3-based oxide by in-situ reaction at anode|electrolyte interface,which is the reason why the functional layer can block the transmission of electron.Chapter 6 contains the summary and prospect of this thesis.The prospective researches of elements diffusion in ceria-based SOFCs and the new field which can further be researched in the improvement of the cell performance are provided.The researches of elements diffusion in SOFCs based on this research are pointed out.This chapter also provides a general description of the application for the mechanism of elements diffusion,and points out the direction for further researches on the mechanism of elements diffusion.