Preparation And Properties Of Some Cobalt-free And Cobalt-based Perovskite Cathode Materials For Intermediate Temperature Solid Oxide Fuel Cells

As a kind of clean and efficient energy conversion device, solid oxide fuel cells(SOFC) have attracted remarkable attention due to their high energy conversionefficiency and low impact on the environment. Conventional La1xSrxMnO3(LSM)perovskite cathode has been widely used as the cathode of SOFC due to their highelectrical, catalytic properties and stability at high temperature (800-1000C) andthermal expansion compatibility with traditional electrolyte such as yttria-stabilizedzirconia (YSZ). However, the conventional LSM perovskite cathode shows pooroxide-ion conductivity and inadequate catalytic activity at intermediate temperature(IT) range (600-800C).Therefore, the cathode becomes the main limiting factor indetermining the performance of IT-SOFCs.Among IT-SOFCs cathode materials, the cobalt-based perovskite cathodesexhibited the highest electrochemical activities for oxygen reduction reaction.Unfortunately, these cobalt-based perovskite cathode materials suffer from very highthermal expansion, poor stability and high cost of cobalt element, and thus limit theirpractical applications in SOFCs. Recently, a new class of perovskite-like compoundsLnBaCo2O5+have received much attention due to their special layer structure.Because of its excellent electrical conductivity and the high oxygen vacancyconcentration, these oxides with perovskite or perovskite-related structures have beenwidely investigated as alternative cathode materials. However, the cobalt-baseddouble perovskite cathodes still suffer the problem of thermal expansion match.Therefore, the development of cobalt-free cathode materials with goodelectrochemical performance and cobalt-based cathodes with low TEC for IT-SOFCsis highly desired.The cobalt-free peroveskite Ba0.5Sr0.5Fe0.9Nb0.1O3-(BSFN), double peroveskitesLn2MnFeO6-(Ln=La, Pr, Nd, Sm) and AA’MnFeO6-(A=Ca, Sr, Ba; A’=La, Pr) andcobalt-based double peroveskite cathode materials YBaCo2-xCuxO5+(x=0.2,0.4, 0.6) were synthesized by the conventional solide state reaction and investigated ascathode material for use in IT-SOFCsFor the cobalt-free perovskite cathode BSFN, its basic physical properties wereinvestigated, such as the sintering temperature, electrical conductivity, thermalexpansion and electrochemical properties of the material. SDC and LSGMelectrolytes were used to compare the electrochemical properties. The phase evolutionin the sample sintered at various temperatures was characterized by powder X-raydiffraction. The single-phase BSFN cathode with cubic-structure was obatained bysintering the samples at1250oC for10h. The electrical conductivity of the BSFNsample changed at aroude425C, which undergoes a semiconducting-like conductionbehavior to metal-like conduction behavior. The polarization resistances of the BSFNmaterial on LSGM and SDC electrolyts are0.078and0.062cm2, respectively. ForBSFN/SDC/Ni-SDC and BSFN/LSGM/SDC/Ni-SDC single cell, the maximumpower density are414and516mW cm2, respectively. The electrochemical propertiesof BSFN on LSGM electrolyte are better than on SDC electrolyte. The results showthat BSFN oxiede can be a promising cathode material for application in IT-SOFCs.Double perovskite structure cathde materials YBaCo2-xCuxO5+(x=0.2,0.4,0.6)(YBCC) showed excellent cathode performance in the electrochemicalexperiment. We synthesized YBaCo2-xCuxO5+(x=0.2,0.4,0.6) samples andinvestigated the effect of Cu doping on the performance of cathode in the intermediatetemperature. XRD results showed that the double perovskite structureYBaCo2-xCuxO5+(x=0.2,0.4,0.6) cathode material were obtained by sintering at950°C for20h and no chemical reaction was observed between the YBCC cathodesand La0.9Sr0.1Ga0.8Mg0.115Co0.085O2.85(LSGMC) electrolyte calcined at900°C for2h.The electrical conductivity of YBCC samples decreased with the increase of Cucontent. But, the x=0.6sample keep semiconducting conduction in all temperatureranges investigated. The thermal expansion coefficients (TECs) of YBCC materialswere all relatively low, the average TEC value in the temperature range30-900C is~15×106K1. The x=0.2sample showed the best electrochemical performance, thepolarization resistance of the x=0.2sample on LSGMC electrolyte was0.012cm2 at700°C, the maximum power density reached810.96m W cm2at800°C.To develop new cobalt-free double perovskite cathode materials for applicationin IT-SOFCs, Ln2MnFeO6-(Ln=La, Pr, Nd, Sm; LnMFO) and AA’MnFeO6-(AA’MF)(A=Ca, Sr, Ba; A’=La, Pr) cathodes were prepared and characterized. The resultsshow that the average TECs of the sample LnMFO is~10×106K1in the temperature30-1000C. LnMFO cathodes showed a good chemically compatiblility with SDCand LSGM electrolytes. However, the counductivity and polarization resistances forthe LnMFO sample are relatively low compared to other cabalt-based materials. Inorder to further optimiaze electrochemical performance of LnMFO cathodes, weselect La2MnFeO6-and Pr2MnFeO6-cathodes with better research comprehensiveperformance for further research. We present a systematic investigation of thesubstitution of A=Ca，Sr，Ba for Ln in AA’MnFeO6-(A=Ca, Sr, Ba; A’=La, Pr), andtheir effects on the crystal chemistry, thermal expansion, electrical conductivity andcatalytic activity for the oxygen redunction reaction in single cell SOFC. The XRDresults show that, when doped with Ba, there are impurity phases were observed.Therefore, we choose AA’MnFeO6-(A=Ca, Sr; A’=La, Pr) cathodes as our mainresearch object. The A=Sr samples show better cathode performance compared withthe A=Ca samples. By substituting Sr for Ln, the electrical conductivity andcatalytical acticity were improved, but the TEC increased. For SrLaMnFeO6-andSrPrMnFeO6-samples, the polarization resistances on SDC electrolyte were4.29and4.97cm2at700C, respectively. Maximum power density of the single cells withSrLaMnFeO6-and SrPrMnFeO6-as cathodes attains432and392mW cm2at850C,respectively.