| Solid oxide fuel cell(SOFC),as an all-solid energy conversion device,has become one of the key research directions in the energy field due to its high efficiency,environment friendliness,easy assembly,and many other characteristics.Although the higher operating temperature bring some advantages,but the problems coexisted with the high temperature limit its commercial application.The realization of intermediate or low temperature is the inevitable demand of SOFC.Proton-conducting SOFC(HSOFC)is an effective way for development of intermediate temperature due to its low proton transport cativation energy.At present,the high-quality cathode designed for HSOFC is scarce,and the reaction mechanism of the cathode and the relationship of the structure-electrochemical performance are still unclear.In this paper,based on H-SOFC,a series of design,phase analysis and electrochemical performance comparison studies were carried out.The first chapter introduces the history of fuel cells and the characteristics and working principle of SOFC,summarizes the development and status of various key materials,focuses on the research and existing problems about electrolyte and cathode of H-SOFC.In chapter 2,the electrochemical properties of a series of homologous R-P Srn+1FenO3n+1(SFO,n=1,2,3)materials are studied.Sr3Fe2O7(SFO2)exhibits high electron conductivity and oxygen transport kinetics.Fe in SFO2 shows the lowest average valence state,indicating the higher oxygen vacancy concentration,which is conductive to the catalytic capacity of oxygen reduction reaction(ORR)and oxygen incorporation kinetics of the material.The electrochemical performance of intermediate temperature SOFC(IT-SOFC)with BaZr0.1Ce0.7Y0.2O3-δ(BZCY)electrolyte was studied by using SFO series cathode.At 700℃,the maximum power density(MPD)of SFO2 battery reaches 703.8 mWcm-2,higher than that of the other two components.The comparative analysis of the polarization resistance of the single cells shows that the high frequency impedance decreases with the increase of n,but it is not the main factor affecting the power output of the single cell.The low frequency impedance corresponding to the ORR has correlation with the oxidation state of Fe in the materials.Understanding the relationship between the structure and properties of materials is helpful for us to recognize the mechanism of electrode reaction and to research and design high-performance electrode materials.In chaper 3,a strategy to improve the electrode hydrophobicity and enhance the electrocatalytic capacity is proposed.Water is generated at the cathode side in H-SOFC mode.The traditional proton-conducting composite cathode has strong water absorption,which leading to the reduction of the open circuit voltage and decline in electrocatalytic activity.A cubic fluorite structure La2Ce2O7(LCO)and pyrochlore structure Sm2Zr2O7(SZO)were selected to prepare composite cathode instead of traditional BaCe(Zr)O3-based materials.Although the proton conductivity of these two materials is much lower than that of BZCY,the electrochemical performance is still slightly higher than that of the BZCY composite cathode,especially the LSF-LCO cathode shows possible potential in the lower temperature region.The increase of cathodic hydrophobicity is beneficial to increase the open-circuit voltage(OCV)and electrocatalytic activity of H-SOFC.In chapter 4,based on the previous work,the alkali metal doped La1.85M0.15Ce2O7(LMC,M=K,Rb,Cs)materials were applied to H-SOFC composite cathode,and the excellent performance of H-SOFC composite cathode material was obtained.The results of XPS experiment show that the introduction of low valence cation increases the oxidation state of Ce element,and more oxygen vacancies are introduced to improve the ionic conductivity in general.At 550℃,the LRC cell has an MPD of 233 mW cm2 and an RP of 0.943 Ω cm2.Compared with many proton-conducting and protonblocking composite cathodes,the La0.7Sr0.3FeO3-LRC composite cathode exhibits better electrochemical performance.The long-term stability test shows that the cell with La0.7Sr0.3FeO3-LRC cathode has good stability.It is a successful strategy to use LMC series material with oxygen ion and proton conductivity as the cathode of H-SOFC,improving the cathodic hydrophobicity,promoting the electrocatalytic performance and stability in the intermediate temperature region.Chapter 5 introduces the exploration and research of various R-P structure and layered perovskite structure materials by synchrotron radiation X-ray absorption fine structure(XAFS)technology.The average valence state of Fe and local structure of different components in the R-P Srn+1FenO3n+1(SFO,n=1,2,3)were explored and studied.The relationship between the structure and property of the materials was analyzed combining with the result of electrochemical performance test in Chapter 2.The causes of the increase of the electrical conductivity in the system of Lan+1NinO3n+1(LNOn,n=1,2,3)with the increase of n were studied.The effect of Fe content in the system of La1.2Sr0.8Ni1-xFexO4 on the structure was analyzed.The asymmetry and valence state of Co in the layered perovskite MBaCo2O5+δ(MBC,M=Y,Gd,Pr)with high performance prepared by 500℃ annealing and 900℃ quenching were compared. |
