Modifying The Physicochemical And Electrochemical Properties Of Perovskite Cathode Materials By Tailoring Their A-site Cation Composition

Because the relatively low electrochemical performance of the cathode materials in the temperature range of 600-800°C is one of the key factors that limit the commercialization of solid oxide fuel cells（SOFC）and the performance of cathode materials are very sensitive to their A-site cation composition,the effects of A-site cation composition on the physicochemical and electrochemical properties of double perovskites SmBa_1-x-x Ca_xCoCuO_5+δ（x=0.0-0.4）and perovskites Ln_0.5Sr_0.5Fe_0.8Cu_0.2O_3-δ（Ln=La,Pr,and Nd）are studied systematically in this thesis.The main contents and conclusions are as follows:（1）Double perovskites SmBa_1-xCa_xCoCuO_5+δ（x=0.0-0.4）were synthesized by a sol-gel method and the effect of Ca²⁺content on their physicochemical and electrochemical properties were studied systematically.The results demonstrated that:SmBa_1–xCa_xCoCuO_5+δ（x=0.0–0.3）exhibited a highly crystalline tetragonal phase with A-site cation ordering along c-axis,and the cell volume decreased when x was increased from 0.0 to 0.3,Minor Ca_0.828CuO₂ phase was detected for samples for which x=0.4.At a given temperature,the oxygen content decreased from 5.54 to 5.45 when x was increased 0.0 to 0.3.The average TEC of SmBa_1–xCa_xCoCuO_5+δdecreases as Ca²⁺content increased,the average TEC value of SmBa_0.7Ca_0.3CoCuO_5+δas low as15.3×10^-6°C^-11 in air from 30°C to 800°C.SmBa_1–xCa_xCoCuO_5+δ–GDC mixtures were calcined at 950°C for 4 h in air and the XRD patterns were indexed to the SmBa_1–xCa_xCoCuO_5+δand GDC phases without any additional peaks,suggesting a good chemical compatibility between the cathode and electrolyte materials at 950℃.The area specific resistances at 700°C decrease by approximately 50%when the calcium content is increased from x=0.0（0.173Ωcm²）to x=0.3（0.086Ωcm²）.The maximum power densities of SmBa_1–xCa_xCoCuO_5+δ-based single cells at 800°C increase from 635 mW cm^-2（x=0.0）to939 mW cm^-2（x=0.3）.The oxygen content in SmBa_1–xCa_xCoCuO_5+δdecreased at a given temperature as the level of Ca doping increased from x=0.0 to x=0.3.This improvement in the oxygen surface exchange and bulk transport kinetics of mixed ionic–electronic conducting perovskites and ORR catalytic activity at higher x values was ascribed to the formation of additional oxygen vacancies with increasing Ca doping level.（2）Perovskite cathodes Ln_0.5Sr_0.5Fe_0.8Cu_0.2O_3-δ（LnSFC,Ln=La,Pr,and Nd,where Ln=La is written as LSFC,Ln=Pr is written as PSFC,and Ln=Nd is written as NSFC）were synthesized via a sol-gel method and the effect of Ln on the physicochemical and electrochemical properties of LnSFC was systematically systermatically.The results are as follows:All of the the samples exhibit an orthorhombic perovskite structures,and their cell volumes decrease as the ionic radius of Ln³⁺decreases.Both the oxygen vacancy content and the magnitude of lattice oxygen release per formula unit increase in the temperature range from370°C to 850°C as the ionic radius of Ln decreases.LnSFC–GDC mixtures were calcined at950°C for 4 h in air and the XRD patterns were indexed to the LnSFC and GDC phases without any additional peaks,suggesting a good chemical compatibility between the cathode and electrolyte materials at 950°C.In the temperature range of 600°C-750°C,NSFC possesses the highest catalytic activity for the oxygen reduction reaction with area specific resistance values of 0.071Ωcm² and 0.141Ωcm² at 750°C and 700°C,respectively.The maximum power densities of the anode-supported single cells at 800°C are 1003.7 mW cm²for PSFC and 944.5 mW cm² for NSFC.