Doping And Electrochemical Properties Of Ln₂CuO_4+δ Perovskite-Like Oxides As Cathode Materials For Medium And Low Temperature SOFC

Solid oxide fuel cells（SOFC）has received extensive attention due to their high conversion rate and environmental friendliness.At the same time,with its outstanding thermal stability and small thermal expansion coefficient（TEC）,A₂BO₄-type perovskite-like is expected to become a cathode material for SOFC.While systematically studying the preparation,composition and electrochemical properties of new Ln₂Cu O4+δ（Ln=Pr,Nd,Sm）perovskite-like materials,this thesis improves the material properties by doping its A site.First,Y³⁺is doped at A site.Ln_2-xY_xCu O4+δ（Ln=Pr,Nd,Sm;x=0,0.025,0.05,0.1）cathode materials are synthesized by sol-gel method after calcination at 1000℃for 24 h.At the same time,the phase structure,the average TEC,electrical conductivity and electrochemical AC impedance of cathode materials are characterized and analyzed.The X ray diffraction（XRD）test results show that the increase of Y³⁺doping amount will lead to the decrease of the unit cell parameters of each cathode material.In addition,compatibility tests show that all cathode materials maintain good chemical compatibility with SDC electrolytes.Thermal expansion test showed that the increase of Y³⁺doping decreased the average TEC of Ln₂Cu O4+δ.Among the three A-site dopings,the Pr-doped material exhibits the lowest average TEC value.Among them,the average TEC value of Pr_1.9Y_0.1Cu O4+δis the smallest,which is 12.789×10^-6K^-1.Conductivity test shows that Pr_2-xY_xCu O4+δhas higher conductivity than Nd_2-xY_xCu O4+δand Sm_2-xY_xCu O4+δ.Meanwhile,the doping of Y³⁺improves the conductivity of Ln₂Cu O4+δ.Among them,Pr_1.975Y_0.025Cu O4+δhas the highest conductivity,and its value is 256 Scm^-1 at800℃.The AC impedance test show that the doping of Y³⁺promotes the oxygen reduction activity of Ln₂Cu O4+δ,and the ASR value of Pr_2-xY_xCu O4+δis smaller than that of the other two materials.At 800℃,Pr_1.975Y_0.025Cu O4+δhas a minimum ASR value of 0.204Ωcm²,showing good electrochemical activity.Secondly,Ca²⁺is doped at A site.Ln_2-xCa_xCu O4+δ（Ln=Pr,Nd,Sm;x=0,0.025,0.05,0.1）cathode materials are synthesized by sol-gel method after calcination at 900℃for 12 h.XRD test results show that the increase of Ca²⁺doping makes the unit cell volume of the cathode material decrease gradually.Compatibility tests show that each cathode material has good compatibility with the SDC electrolyte.Thermal expansion tests show that Ca²⁺doping reduces the average TEC of Ln₂Cu O4+δwithin a certain doping range.The cathode doped with Pr as the A site has a lower average TEC value than Nd and Sm.Among them,the average TEC value of Pr_1.95Ca_0.5Cu O4+δis the smallest,which is 12.723×10^-6K^-1.Conductivity tests show that Ca²⁺doping increases the conductivity of Ln₂Cu O4+δ.Among them,Pr_2-xCa_xCu O4+δhas higher conductivity than Nd_2-xCa_xCu O4+δand Sm_2-xCa_xCu O4+δ.At 800℃,Pr_1.95Ca_0.05Cu O4+δhas the highest conductivity,which is 265 Scm^-1.The AC impedance test showed that the doping of Ca²⁺reduced the oxygen reduction activity of Ln₂Cu O4+δ.Among them,the ASR value of Pr_2-xCa_xCu O4+δis smaller than the other two series of cathodes.Pr_1.975Ca_0.025Cu O4+δhas the smallest ASR value at 800℃after doping,which is 0.336Ωcm².Finally,Sr²⁺doping and co-doping of Sr²⁺and Ca²⁺are performed at the A site.Two materials,Pr_1.95Sr_0.05Cu O4+δand Pr_1.9Sr_0.05Ca_0.05Cu O4+δare synthesized by sol-gel method after calcination at 900℃for 12 h,and the same tests are carried out.XRD test results show that Sr²⁺doping and co-doping of Sr²⁺and Ca²⁺reduce the volume of cathode unit cells.Compatibility testing found that both materials had good compatibility with SDC electrolyte.Thermal expansion test results show that Sr²⁺doping and co-doping of Sr²⁺and Ca²⁺reduce the average TEC of Pr₂Cu O4+δ.The average TEC values are 13.514×10^-6K^-1 and 13.490×10^-6K^-1,respectively.Conductivity test results show that Sr²⁺doping and co-doping of Sr²⁺and Ca²⁺improve the conductivity of Pr₂Cu O4+δ.At 800°C,the conductivities are 148 Scm^-1 and 140Scm^-1,respectively.The AC impedance test showed that Sr²⁺doping and co-doping of Sr²⁺and Ca²⁺reduced the oxygen reduction activity of Pr₂Cu O4+δ.The ASR values at 800℃are 0.918Ωcm²and 1.220Ωcm²,respectively.In summary,under the same synthesis conditions,Ca²⁺doping is superior to Sr²⁺doping in suppressing TEC,increasing electrical conductivity,and improving oxygen reduction activity.Through the electrochemical performance test,it can be found that appropriate doping of Y³⁺and Ca²⁺will make the material show higher electrical conductivity and lower area-specific resistance value.This result indicates that the introduction of two elements can enhance the performance of Ln₂Cu O4+δas SOFC cathode material.At the same time,Pr_1.975Y_0.025Cu O4+δand Pr_1.95Ca_0.05Cu O4+δcathode materials with good electrochemical properties can also be obtained.The results of this study also provide a basis for related research.