Preparation And Properties Of Cobalt-free Perovskite Cathode Materials For SOFC

Facing the two key issues of energy utilization and environmental protection,we must change the mode of energy consumption mode mainly based on fossil energy,it is necessary to vigorously develop renewable energy in the current society.Solid oxide fuel cells（SOFC）has attracted attention because of their advantages such as fuel diversification and zero-emission operation.In recent years,cobalt-based perovskite structure materials have been widely studied as SOFC cathodes.We realize the shortcomings of poor stability and high thermal expansion coefficient of cobalt-based perovskite cathodes.So we must explore cobalt-free materials with excellent performance at medium or low temperature.It is a new research direction of SOFC cathode materials.In the thesis,there are three series of cobalt-free perovskite cathode materials with A-site doping,(Bi_0.5Pr_0.5)_1-xSr_xFe O_3-δ（BPS_xF;x=0,0.2,0.3,0.4,0.5）,(Bi_0.5Ba_0.5)_1-xLa_xFe O_3-δ（BBL_xF;x=0,0.1,0.2,0.3）and La_0.8-xBi_xSr_0.2Mn O_3-δ（LB_xSM;x=0,0.1,0.2,0.3）.They were prepared by solid-state reaction method.We analyze the structure,morphology and electrochemical properties of the cathode materials by characterization techniques,such as X-ray diffraction（XRD）,coefficient of thermal expansion（TEC）,electrical conductivity,surface specific resist-ance（ASR）,and scanning electron microscopy（SEM）.The detailed results are shown as follows:（1）We select Bi_0.5Pr_0.5Fe O_3-δmaterial,which is the mixed ion-electron conductor（MIEC）perovskite structure material,by doping to improve the electrochemical properties.Doping alkaline earth element doping is the main strategy to improve the properties of perovskite oxide.BPS_xF materials has good stability,excellent CO₂tolerance and good chemical compati-bility with SDC by doping of cationic Sr³⁺into the A-site.In addition,it also improves the electrical conductivity and electrochemically active catalytic activity of the materials.At800℃,the optimal ASR value of BPS_xF cathode is 0.035Ωcm²,and the peak power density of BPS_xF cell is 710 m W cm^-2.（2）Bi_0.5Ba_0.5Fe O_3-δis a low thermal expansion material.By doping of cationic La³⁺into the A-site,we gain Fe-based perovskite structure materials,BBL_xF（x=0-0.3）cathode mate-rials.The TEC decreases with the increase of La element doping content.Besides,La element doping plays a good role in regulating the electrinic conductivity of the material.The electronic conductivity maximum value of BBL_0.3F is 4 times higher than BBL_0.0F.The area specific resistance of BBL_0.3F is only 0.035Ωcm²,and the peak power density of Ni O-GDC|GDC|BBL_0.3F cell is 587 m W cm^-2at 800℃.（3）La_0.8Sr_0.2Mn O_3-δis the classical cathode material of SOFC.The sintering temperature of the material is reduced by doping Bi element at A-site.The LB_xSM materials have good long-term stability in air.With the increase of Bi element doping content,the average TEC of LB_xSM decrease,which has a higher thermal match with SDC.The electronic conductivity increases since the oxygen vacancy concentration is increased by Bi doping.Consequently,the electrochemical performance is substantially improved and the ASR is reduced from 1.371Ωcm²to 0.360Ωcm².The perovskite with x=0.2 is suggested as the most promising compo-sition as solid oxide fuel cell cathode material since it has demonstrated high electrical conductivity and electrochemical activity.