Study On The Electrical Property And Modification Of Double Perovskite Electrode Material Sr₂CoNbO_6-δ

The electrode materialsare the key factors that affect the solid oxide fuel cell(SOFC) performance and have been researched in recent years. Double perovskite materials have such advantages as high mixed conductivity, high catalytic activity, redox stability, etc., therefore double perovskte materials can be the candidate electrode materials applied in SOFC. Through ltetature research, this paper identifies the double perovskite-type Sr2CoxNb2-xO6-δ(SCN) material as the research system.(1) The powders of Sr2CoxNb2-xO6-δwere synthesized by the solid-phase method, and the conductivities of SCN were tested by the four-terminal method in air atmosphere. The results show that the conductivities of Sr2CoxNb2-xO6-δfirst increased and then decreased with the increasing of the content of Co. When x is 1.0, its conductivity becomes maximum with the value of 6.26S·cm-1 at 850 oC in air.The relationship between the conductivity of SCN and temperature was further analyzed and it was discovered that it complied with the small polaron hopping mechanism. The states of SCN were analysised by XPS,the results show that three valence states of Co,(Co2+, Co3+, Co4+) and two valence states of Nb,(Nb4+, Nb5+) exit in SCN,and the disproportionation reaction of Co3+ and the charge balance reaction among Co3+、Co4+、Nb4+ and Nb5+ are the two modes of charge transfer in SCN.(2) The conductivities of Sr2Co(Mn,Fe,Ni)Nb O6-δ were predicted by tolerate factor, electronegativity, and the results show that Sr2 Mn Nb O6-δhas the highest conductivity.The conductivities of Sr2Co1-xMnxNb O6-δ(SCMN) first increased and then decreased with the increasing of the content of Mn. When x is 0.1, its conductivity becomes maximum with the value of 7.26S·cm-1 at 850 oC in air, and When x is 0.5, its conductivity becomes maximum with the value of 7.26 S·cm-1 at 850 oCin 5%H2/Ar. The conductivity mechanism of Sr2CoxNb2-xO6-δwas studied, and it was discovered that it complied with the small polaron hopping mechanism. The states of Mn in the SCMN were analysised by XPS, the results show that there exit three valence states of Mn,(Mn2+, Mn3+, Mn4+), Mn can not only produce disproportionation reaction, but also appear the charge balance reaction with Co and Nb. The lattice constants of SCMN were calculated by the Rietveld method, the lattice constants of SCMNfirst increased and then decreased with the increasing of the content of x, When x is 0.1, its lattice constants becomes minimum in air, while x is 0.5, its lattice constants becomes minimum in 5%H2/Ar.(3) Double perovskite SCMNas anode have been introduced to SOFC. Testing results show that the powder density of SCMN in SOFC first increased and then decreased with the increasing of the content of x. When x is 0.5, its powder density becomes maximum with the value of 464 m W·cm-2 at850 oC.There are two reation pathways in SCMN anode interior and surfaceand it can extend to the entire SCMN anode. The higher the conductivity is, and the stronger SCMN electrode reaction becomes. While Sr2Co1-xMnxNb O6-δ as cathode in SOFC, x is 0.1, its powder density becomes maximum with the value of 478 m W·cm-2 at 850 oC. The higher conductivity of SCMN makes the largr electrochemical reation space and SOFC will have higher powder density accordingly.