Research On Cobalt-free PBSCF Composite Cathode Materials For H-SOFCs

The research on cathode material is always one of the key issues in the field of proton conductor-based solid oxide fuel cell（H-SOFCs）.Although the traditional Co-based cathode materials have high catalytic properties,they have the disadvantages of high thermal expansion coefficient,high temperature volatilization and high cost etc.Using Fe-based cathode materials is becoming the trend in H-SOFCs.In addition,the electrochemical performance of H-SOFCs with BaZr_0.1Ce_0.7Y_0.2O_3-δ（BZCY）electrolyte can be further improved by the proton blocking composite cathode,which consists of Fe-based mixed ionic-electronic conductor（MIEC）and conventional oxygen ion conductor Ce_0.8Sm_0.2O_2-δ（SDC）.In this thesis,the effects of（Pr,Ba,Sr）FeO_3-δon the performance of single cell are observed by doping Pr at A-site of ABO₃ perovskite Ba_0.5Sr_0.5FeO_3-δ（BSF）MIEC.Based on the results,the performance of single cell with（Pr,Ba,Sr）（Fe,Cu）O_3-δcathode is comparatively studied by doping Cu at B-site of ABO₃.Four kinds of MIECs,i.e.,BSF,Pr_0.5Sr_0.5FeO_3-δ（PSF）,Pr_0.5Ba_0.5FeO_3-δ（PBF）and Pr_0.5Ba_0.25Sr_0.25FeO_3-δ（PBSF）,were prepared by combined solid-phase reaction and ethylenediaminetraacetic（EDTA）-acid-citric acid complexation method.XRD analyses show that four kinds of MIEC powders are all single cubic perovskite phase.The lattice constants of BFS,PSF,PBF,and PBSF are 3.9410,3.8680,3.9308 and3.9012?,respectively,while the corresponding Goldschmidt tolerance factors are1.011,0.958,0.987 and 0.972,respectively.A good chemical compatibility is observed among BSF（PSF,PBF or PBSF）,SDC and BZCY electrolyte.Co-free proton blocking MIEC-SDC composite cathodes were prepared by mixing BSF（PSF,PBF or PBSF）with SDC powders,respectively.Four kinds of single cell of NiO-BZCY|BZCY|MIEC-SDC（MIEC=BSF,PSF,PBF or PBSF）configuration were fabricated with NiO-BZCY as anode and BZCY as electrolyte.The results of electrochemical performance show that BSF-SDC single cell has the best maximum power density(MPD,176±18 mW?cm^-2)at 600^oC while PBSF-SDC single cell has better MPD at 650^oC and higher.The MPD of PBSF-SDC single cell at 600,650,700 and 750^oC are 153±11,303±17,486±20 and 865±38 mW?cm^-2,respectively.According to our knowledge,the MPD at 750^oC is the highest among the reported single cells with Fe-based（composite）cathode.Electrochemical impedance spectroscopy（EIS）analyses show that the polarization process of PBSF-SDC single cell should be mainly controlled by oxygen reduction reaction accompanied with electron transfer,while that of other three kinds of single cells should be mainly controlled by oxygen ion migration.In addition,in the range of50-750^oC,the thermal expansion coefficients of dense BSF-SDC,PSF-SDC,PBF-SDC and PBSF-SDC are 16.2×10^-6,15.8×10^-6,16.8×10^-66 and 15.4×10^-66 K^-1,respectively.They are all apparently smaller than that of Co-containing perovskite materials(²1×10^-66 K^-1),which is beneficial to improve the high temperature stability of single cells.Based on the better performance of PBSF-SDC single cell here and the novel PrBa_0.5Sr_0.5Cu₂O_5-δ（PBSC）-SDC single cell recently found by our group,the B-site（Fe）of PBSF was doped with Cu to further improve the performance of single cell.The Pr_0.5Ba_0.25Sr_0.25Fe_1-xCu_xO_3-δ（x=0.25,0.5,0.75,denoted as PBSFC0.25,PBSFC0.5 and PBSFC0.75,respectively）MIECs were prepared.XRD analyses show that three kinds of powders are still cubic perovskite structure although some unidentified weak XRD peaks appear.The lattice constant of PBSFC powders decreases slightly with the increase of Cu content.Unfortunately,the chemical compatibility of PBSFC with SDC and BZCY is some poor than that of PBSF without Cu.The performance of single cells decreases by partially substituting Fe of PBSF with Cu,which may be associated with the poor chemical compatibility.Among the three kinds of single cells containing Cu,the PBSFC0.75 single cell has best performance.The MPDs of PBSFC0.75 single cell at 600,650,700 and 750^oC are 89±10,243±12,342±15 and 462±21 mW?cm^-2,respectively,which are some lower than those of PBSF without Cu or PBSC without Fe.In addition,the PBSFC0.5single cell shows the best temperature stability of performance.Its MPD only decreases from 331±18 mW?cm^-22 to 205±8 mW?cm^-22 for operating temperature changed from 750^oC to 600^oC.In short,among the four kinds of Co-free（Pr,Ba,Sr）FeO_3-δMIECs in this thesis,the single cell with proton-blocking PBSF-SDC composite cathode shows better electrochemical performance.However,the performance of single cell decreases for further doping Cu at B-site of PBSF.As a whole,PBSF-SDC proton-blocking composite cathode should have a potential application prospect in H-SOFCs.