Preparation And Properties Of Cobalt-based And Non-cobalt-based Layered Perovskite Cathode Materials For SOFC

In view of the increasingly severe problems of energy shortage and environmental pollution,solid oxide fuel cells?SOFC?has huge application prospects because of its green,high efficiency and other characteristics.However,the conventional SOFC has a high operating temperature?800?-1000??,which is not conducive to its further commercialization,so lowering its operating temperature?medium and low temperature?has become an inevitable trend for the future development of SOFC.In the process of decreasing the working temperature,the polarization impedance of the cathode material will increase,which leads to a serious decline in the electrical properties of SOFC.Therefore,it is very important to develop a cathode material that has high reaction activity,high electronic conductivity,and a thermal expansion coefficient matching with that of commonly used electrolytes at low and medium temperatures?400?-800??.In order to find the cathode material suitable for medium and low temperature SOFC,SmBa_0.5Sr_0.25Ca_0.25Co₂O_5+??SBSCC?was prepared by EDTA-citric acid combined complexation method,and SmBa_0.5Sr_0.25Ca_0.25CoMeO_5+??Me=Fe,Mn,Ni,Cu?materials were prepared by doping Fe,Mn,Ni and Cu in equal proportion.The phase structure,microstructure and electrochemical properties of the cathode powder were tested by means of X-ray diffraction?XRD?,scanning electron microscope?SEM?,DC four-electrode method and thermal expansion coefficient?TEC?test,in order to explore the effect of transition metal elements doping.The XRD analysis shows that after the cathode powder of SBSCCMe?Me=Fe,Mn,Ni,Cu?was calcined at 1000?for 6h,all of them were single phase except for the impurity phase of Ni doped materials.The microstructure of scanning electron microscopy?SEM?shows that Cu doping has the best morphology,the agglomeration of Mn doped materials is obvious,and the sintering of Ni is high.The test of conductivity shows that in the temperature range of 400-800?,the conductivity of other samples is decreasing except for the increase of the conductivity of Fe element.Only Cu samples meet the requirement of not less than 100S.cm^-1,the maximum value is180.891S.cm^-1,and the doping effect of other elements is poor.The doping of Fe element increases the thermal expansion coefficient of the material,while the doping of Mn,Ni and Cu element decreases the thermal expansion coefficient.The doping range is determined starting from the Cu element doping with the best performance that has been discussed above,and the doping region is fixed within 0 to 1.A series of SmBa_0.5Sr_0.25Ca_0.25Co_2-xCu_x O_5+?solid oxide fuel cell cathode materials with different Cu element doping ratios are obtained by sol-gel method.And the corresponding performance of the prepared cathode materials are tested by X-ray diffraction analysis?XRD?,field emission scanning electron microscope?FESEM?,thermal dilatometer,electrochemical workstation and other instruments.The results show that the SBSCCC series cathode materials prepared by sol-gel method show an ordered cubic perovskite structure,and when the doping ratio is 1,the microstructure of cathode materials is better,the materials with x=0.2 and 0.6 have certain sintering,and the materials with x=0.4 and0.8 exhibit partial agglomeration.In the process of increasing the temperature from 400?to 800?,the conductivity of SBSCCC when x=0.4 increases at first and then decreases,the conductivity of small polaron changes into metal-like conductivity,while the electrical conductivity of x=0.2 increases gradually,which accords with the conductive mechanism of small polaron.The conductivity of x=0.6,0.8 and 1.0 decreases all the time,which is consistent with the conductive mechanism of metal-like materials.The thermal expansion coefficient of SBSCCC series cathode materials decreases with the increase of doping of Cu elements.The Co site is completely replaced by transition metal elements,and the cathode material without Co base is prepared and tested by the same characterization method as above.The results show that all these cathode materials form pure perovskite phase,and the addition of transition metal reduces the electrical conductivity and thermal expansion coefficient of the materials.In general,the best Co-free cathode material is SmBa_0.5Sr_0.25Ca_0.25Cu₂O_5+?.