Electrochemical Performance Of A Bi 0.5 Sr 0.5 FeO 3-? Based Cathode Material For A Medium Temperature Solid Oxide Fuel Cell

A novel low-cost cathode material with excellent electrocatalytic properties is one of the key components for intermediate temperature solid oxide fuel cell?IT-SOFC?.Cathode materials of high electrochemical activity are good for improving the performance of SOFCs at intermediate temperatures?500-700 ^oC?.In this dissertation,a mixed ionic-electronic conductor Bi0.5Sr0.5FeO3-_??BSF?with perovskite structure is studied.The properties of BSF are optimized by doping and recombination,including the cation(Cu²⁺/Nb⁵⁺/Sb⁵⁺)doping to B site,the cation(Ca²⁺/Ba²⁺)doping to A site and the electrolyte Ce_1.9Gd_0.1O_1.95?CGO?compounding the BSF.Their performances for the intermediate temperature solid oxide fuel cells as cathode materials were studied.Firstly,the properties of Bi_0.5Sr_0.5Fe_1-x-x Cu_x O_3-??x=0.05-0.20?,Bi_0.5Sr_0.5Fe_1-xNb_xO_3-??x=0.05-0.15?and Bi_0.5Sr_0.5Fe_1-xSb_xO_3-??x=0.05-0.15?cathode materials were synthesized by conventional high-temperature solid phase method.All samples have good chemical compatibility with CGO electrolyte at high temperature.The Cu-doped increases the conductivity and oxygen vacancy concentration of the material,and the electrochemical performance of the cathode is improved.The Bi_0.5Sr_0.5Fe_0.8Cu_0.2O_3-?cathode possesses the lowest polarization resistance?0.13?cm²?and maximum output power density(0.78 W cm^-2)at 700 ^oC.When the current density reaches-125 mA cm²,the overpotential of cathode is-37 mV.At the same time,the doping of Nb and Sb elements improve the stability and oxygen permeability of the materials,thereby increasing the electrochemical performance of the materials.Among them,the polarization resistance of Bi_0.5Sr_0.5Fe_0.9Nb_0.1O_3-?cathode is only 0.038?cm²,and the output power density is 1.51 W cm^-2 at 700 ^oC.The polarization resistance and output power density of Bi_0.5Sr_0.5Fe_0.9Sb_0.1O_3-?cathodes were 0.098?cm² and 0.95 W cm^-22 at600 ^oC,respectively.Secondly,the effect of A-site doping the electrochemical properties of the parent material was further studied.The cathode material of Bi_0.5-xCa_xSr_0.5FeO_3-??x=0.05-0.15?was synthesized by high-temperature solid phase method.It is concluded that Ca-doped increases the conductivity and decreases the thermal expansion coefficient of the materials,thereby improving the electrochemical performance and thermal compatibility.At 700 ^oC,the polarization resistance of Bi_0.4Ca_0.1Sr_0.5FeO_3-?cathode is0.052?cm²,and the power density is 1.23 W cm^-2.Then Bi_0.5-x.5-x Ba_xSr_0.5FeO_3-??x=0.05-0.10?cathode materials was also synthesized.The Ba-doped improves the structural stability of the materials,while increasing the electrochemical performance of the cathode materials.The polarization resistance of Bi_0.4Ba_0.1Sr_0.5FeO_3-?cathode is only0.078?cm² and the power density is 0.92 W cm^-2,which is lower than tthe BSF material?0.26?cm²?.Finally,Bi_0.5Sr_0.5FeO_3-?-CGO?BSF-CGO?composite cathode was synthesized by mechanical mixing method,and its physical and electrochemical properties were studied.The results display that the BSF-CGO composite cathode can effectively improve the electrode performance and the connection between electrode and electrolyte.When the amount of CGO compound reaches 30 wt.%,the minimum value of polarization resistance is 0.14?cm²,which is about 1/2 that of the parent BSF material,and the output power density is 0.71 W cm^-2.It shows that the composite CGO significantly improves the electrochemical performance of the cathode material and increases the active site at the three-phase interface.