Solid Solution And Composite Modifications Of Rare-earth Zirconate Solid Electrolytes For SOFCs At Intermediate And Low Temperatures

Solid oxide fuel cells(SOFCs)are attracting significant attention in the research of new energy systems.However,SOFCs show some negative effect when working at high temperature,such as limited selection of electrode materials and short lifetime.Therefore,the development of novel electrolyte materials with excellent electrical properties at low-intermediate temperatures is of great importance.In the present work,solid solution and composite solid electrolyte materials were designed and constructed based on A₂Zr₂O₇ type rare-earth zirconate materials,with the aim of further improving its conductivity.Based on the study of order disorder transition and r(A³⁺)/r(B⁴⁺)critical value of A₂Zr₂O₇-type rare-earth zirconate,(Nd_1–xY_x)₂Zr₂O₇,(Sm_1–xGd_x)₂Zr₂O₇ and GdSm(Zr_1–xCe_x)₂O₇ rare-earth zirconate solid solutions were designed and prepared by pressureless sintering,with different rare-earth elements doping at A-site and Zr-site.Multi-component A₂Zr₂O₇(A=Ln1_0.2Ln2_0.2Ln3_0.2Ln4_0.2Ln5_0.2)high entropy ceramics were also prepared inspired by the idea of high entropy ceramics(HEC).Based on EMPT model,GdSmZr₂O₇-(Li_0.52Na_0.48)₂CO₃ and GdSmZr₂O₇-(Li_0.62K_0.38)₂CO₃ composite solid electrolytes for intermediate and low temperature SOFCs were constructed.Mechanical mixing method and molten salt infiltration method were used respectively to obtain dense solid electrolyte materials with multiphase.X-Ray Diffraction(XRD),Raman,Scanning Electron Microscopy(SEM)and Transmission Electron Microscope(TEM)were performed to characterize the microstructure,order-disorder transformation of rare-earth zirconate solid solution as well as the densification process of the composite ceramics.The electrical conductivity was measured by AC impedance spectroscopy and DC four-electrode technique.The conductivity of H⁺and O^2–was calculated via effective medium percolation theory(EMPT).The conductive mechanism in the composite ceramics was revealed.The results show that the microstructure of solid solutions shows an order-disorder transformation with the increase of doping ion content of Y³⁺,Sm³⁺and Ce⁴⁺.The critical value of r(A³⁺)/r(B⁴⁺)for order-disorder transformation was determined.The crystal structure of A₂B₂O₇ materials is determined by the ratio of cation radius.The larger r(A³⁺)/r(B⁴⁺)is,the higher the order degree of solid solution is.The disorder degree will further influence on the electrical conductivity of the rare-earth doped zirconates:when the solid solutions changes from a disordered to an ordered structure,the conductivity activation energy is decreased,the conductivity first increases,reaching the highest value in the pyrochlore phase,and then decreases.As for Nd₂Zr₂O₇ and Gd₂Zr₂O₇,doping Y or Sm at A-site is beneficial to the improvement of the conductivity of rare-earth zirconate.Therefore,the total conductivity of(Nd_1–xY_x)₂Zr₂O₇(0?x?1)and(Gd_1–xSm_x)₂Zr₂O₇(0?x?1)first increases and then decreases with the increase of doping amount.The total conductivity of GdSmZr₂O₇ solid solution is the highest,reaching 1.6×10^–3 S·cm^–17.9×10^–3 S·cm^–1 at 600 ^oC and 800 ^oC,respectively.The effects of carbonate type,GdSmZr₂O₇ particle size and composite process on the relative density of the composites were investigated.When nano-GdSmZr₂O₇was used by mechanical mixing method,the relative density of the composite was not affected by the type of carbonate.The relative densities of GdSmZr₂O₇-40(Li_0.62K_0.38)₂CO₃ and GdSmZr₂O₇-40(Li_0.52Na_0.48)₂CO₃ sintered at700 ^oC for 1 h are 76.1%and 74.1%,respectively.When the composite process is mechanical mixing method,the relative density of GdSmZr₂O₇-30(Li_0.52Na_0.48)₂CO₃prepared by micro-GdSmZr₂O₇ is 83.8%,which is significantly higher than that of GdSmZr₂O₇-30(Li_0.52Na_0.48)₂CO₃(64.2%).The composites obtained by molten salt infiltration method exhibit densities of more than 93%,which is obviously better than that of the composites prepared by mechanical mixing method.The conductivity of rare-earth zirconate-carbonate composites is closely related to the content of carbonate,the particle size of GdSmZr₂O₇ powder and the composite process.The conductivity of the composite increases with the increase of the carbonate content.The conductivity of GdSmZr₂O₇-(Li_0.52Na_0.48)₂CO₃ is better than that of GdSmZr₂O₇-(Li_0.62K_0.38)₂CO₃.The electrical conductivity of the composites prepared by micro-GdSmZr₂O₇ powder is obviously better than that of the composite prepared by nano-GdSmZr₂O₇ powder.When the content of carbonate is the same,the conductivity of the composite obtained by molten salt infiltration is the highest.When the carbonate content is 40wt.%,the maximum conductivity and power density of GdSmZr₂O₇-(Li_0.52Na_0.48)₂CO₃ prepared by mechanical mixing method are 0.54 S·cm^–1 and 221 m W·cm^–2 respectively at 600 ^oC.The conductivity of GdSmZr₂O₇-(Li_0.52Na_0.48)₂CO₃ prepared by molten salt infiltration method is 0.50S·cm^–1 at 600 ^oC,and the carbonate content is only 23 wt.%,which is two orders of magnitude higher than that of single-phase GdSmZr₂O₇.The conductivity of GdSmZr₂O₇-(Li_0.52Na_0.48)₂CO₃ composites was calculated by EMPT model.It is found that the conduction mechanism is oxygen ion conduction and proton conduction,and ion conduction plays a leading role in molten carbonate percolation network,which is consistent with the prediction results of EMPT model.