Synthesis And Characterization Of Double-lanthanide-doped Double Perovskite La_1-xPr_xBaCo₂O_5+?

Solid oxide fuel cell?SOFC?is a type of green energy technologies with great application potentials.Due to its high working temperature?>800??,the traditional SOFC has many problems in thermochemical and thermomechanical compatibilities between the components,which hinders its commercial development.Therefore,development of intermediate temperature SOFC has become the research trend in this field.However,lowering the temperature also leads to the reduction of catalytic activity of traditional cathode materials.It is therefore important to develop a new type of high-efficiency cathode material for intermediate temperature SOFC?IT-SOFC?.At present,Ln BaCo₂O_5+??Ln BCO,Ln=La,Pr,Gd,etc.?with double perovskite structure are considered as potential IT-SOFC cathode materials due to their excellent electrical properties and oxygen catalytic reduction performance in the middle and low temperature range.According to the literature research,it is found the thermochemical stability,thermomechanical compatibilities of Ln BCO with electrolyte and other materials can be optimized through proper B-component regulation,and the degradation of the cathode performance can be suppressed as well.The related studies so far are mainly focused on A-sitesingle-lanthanide-doped double perovskite cathode materials.This thesis is then proposing a new type of double perovskite cathode materials with double lanthanide doping,taking the advantages of the corresponding single-lanthanide-doped Ln BCOs.To date,there are very few works related to A-site double lanthanide doped Ln BCOs.Questions,such as how the constitution and stoichiometric ratio of lanthanide elements regulate the structural,electrochemical,thermochemical and thermomechanical properitesof Ln BCO materials,need to be further clarified.In this thesis,the synthesis process of A-site double-lanthanide-doped Ln_1-xPr_xBaCo₂O_5+?cathode materials are studied and optimized,and the corresponding electricalproperties and the thermoschemical and thermomechanical compatibilitiesof such materials with GDC(Ce_0.9Gd_0.1O_2-?)are also investigated and discussed.Ln_1-xPr_xBaCo₂O_5+?cathode materials were prepared by traditional solid-state reaction method.The calcination temperature,calcination cycles,sintering temperature,cobalt excess and other factors were optimized to obtain cathode materials.The properties of samples were investigated through X-ray diffraction,scanning electron microscopy,energy spectrum analysis,thermal expansion test,electrochemical impedance spectroscopy,etc.The influence of the La/Pr ratio on the structure and properties of La_1-xPr_xBaCo₂O_5+?were then discussed.By analyzing the XRD of La_1-xPr_xBaCo₂O_5+?materials calcined at different temperatures,it is found that the cathode materials with single pure phase can be synthesized after calcined at no less than 1100?for 12 h,and all the samples are homogeneous cathode materials with La³⁺and Pr³⁺randomly distributed at A site,rather than simple mixtures of La BCO and Pr BCO.Through the analysis of phase and micro morphology of sintered samples,it is found 1150?is the ideal sintering temperature to obtain dense samples.In addition,this study also investigated the use of excessive cobalt?to make up for the loss of cobalt during the calcination process?,and determined excess of 2wt%Co₃O₄ is sufficient enough for the sample synthesis.Further analysis on the phase structure of La_1-xPr_xBaCo₂O_5+?samples showed that the crystal structure changes from single perovskite crystal structure to double perovskite crystal structurewith the increase of Pr content x.From the sintering experiments of GDC and La_1-xPr_xBaCo₂O_5+?materials,it is found that good thermochemical compatibility between these two materials can be obtained when the co-sintering temperature is not more than1100?.By studying the thermal expansion of La_1-xPr_xBaCo₂O_5+?in different oxygen partial pressure environments,it is found that the TEC?thermal expansion coefficient?of the samples with same composition increases with decreasing the oxygen partial pressure p O₂,while TEC decreases with the increase of x in the composition under same conditions.Except for La BCO,the other samples shows significant metallicity,i.e.,the conductivity decreases with the increase of temperature.In addition,the electrical conductivityof La_1-xPr_xBaCo₂O_5+?decrease with the decrease of p O₂,but increase first and then decreases with x.Among all the samples under investigate,La_0.5Pr_0.5BaCo₂O_5+??x=0.5?demonstrates the largest conductivity.The impedance analysis of La_1-xPr_xBaCo₂O_5+?/GDC/La_1-xPr_xBa-Co₂O_5+?symmetrical cells discovered ASR of La_1-xPr_xBaCo₂O_5+?materrials increases with the decrease of temperature or oxygen partial pressure,but decreases with the increase of Pr content x.