Performance Study Of Cobalt-based Perovskite Cathode Materials BaCaCoFeNbO For IT-SOFC

Energy is the foundation of social development,traditional power system disadvantages such as low efficiency and high pollution,solid oxide fuel cell（SOFC）has several advantages,high conversion efficiency and low pollution,which is able to realize efficient large-scale power generation based on the existing energy supply system of new energy technologies.The problem of early development of SOFC is the operating temperature（YSZ electrolyte operating temperature is 1000 ℃）is too high,thus caused material aging,high cost and inconvenient maintenance and a series of problems.The first practical problem SOFC needs to solve is its operating temperature is too high,the current common goal is lower the operating temperature to the medium temperature（600-800 ℃）,but reduce the operating temperature increases the ohmic losses of electrolytes and the cathode polarization loss.As the research and development of new electrolyte materials,ohmic losses have been lower,reduce the thickness of the electrolyte materials can effectively reduce the electrolyte ohmic losses,the development of membrane technology in recent years have larger contribution to solve the problem of electrolyte ohmic losses.Therefore,improve the performance of the cathode material under the condition of the medium temperature,reduce the polarization loss of cathode material under the condition of the medium temperature,improve the catalytic activity under the condition of the medium temperature has become the key of the IT-SOFC development.This paper aims to through the development of new cathode materials to improve the performance of the IT-SOFC.The A-site and B-site cation of ABO3 type cobalt base perovskite oxide can be partly replace by other different valence state of cation,To maintain the system’s electrically neutral and produce oxygen vacancy,to get a high electronic conductivity at the same time has good oxygen ion conductive ability,is one of very important cathode materials of IT-SOFC.Taking Ba1-x Cax Co0.7Fe0.2Nb0.1O3-δ（BCCFN）cathode materials and BCCFN-SDC electrolyte composite cathode materials as the research object,analysed the cathode materials,including phase structure,microstructure,thermal expansion coefficient,half cell impedance spectrum and single cell power output,etc.Through adjusting the compositions of material,trying to find the material to reduce the cathode polarization impedance,to study the feasibility to be IT-SOFC cathode materials,hope to improve the performance of the IT-SOFC.The main research content is as follows:1.Cobalt-based perovskite oxide Ba Co0.7Fe0.2Nb0.1O3-δ（BCFN）is a kind of IT-SOFC cathode material have better electrochemical performance,which has excellent electronic-ionic conductivity.But its problem is the poor thermal suitability with electrolyte,in order to further improve the performance of this material as IT-SOFC cathode materials,It is necessary to reduce its thermal expansion coefficient,to improve the matching with electrolyte material of thermal resistance,guarantee the stability in its working temperature,reduce its polarization impedance under medium temperature（600-800℃）,improve electrode performance,so as to improve the maximum power density of single battery.Therefore,we choose to doping Ca²⁺ in A-site trying to improve its performance of IT-SOFC cathode materials.Use solid-phase synthesis to preparation Ba1-x Cax Co0.7Fe0.2Nb0.1O3-δ（B1-x Cx CFN,x = 0.0,0.1,0.2,0.3,0.4）cathode material,through the analysis of XRD diffraction’s we found that B1-x Cx CFN cathode material sintering in 1000 oC after 10 h,formed a single-phase cubic perovskite structure.Through the analysis of electron microscope images,B0.9C0.1CFN formed porous microstructure,and combined with SDC electrolyte well,no signs of peeling off.After doped Ca²⁺ in A-site and reduce the thermal expansion coefficient of cathode materials.Analysis of half cell impedance found that doped Ca²⁺ in A-site of BCFN materials reduces the cathode materials of polarization resistance（RP）,when the amount of doped Ca²⁺ in A-site is x=0.1,weh have a minimum value of RP,in 800℃,the polarization resistance value of half cell with B0.9C0.1CFN cathode is 0.4458 Ωcm2.Proper doping of Ca²⁺ rise B-site cation from +3 to +4,make the B3+-O-B4 + small polaron concentration increased,reduced the polarization resistance of materials,the electrochemical properties been improced,when further improvement on the doping amount of Ca2 +,number of B-site cation rise from + 3 to +4 is too much,number of +3 valence decrease,increase the polarization resistance of materials.Single cel power density test show the structure of B1-x Cx CFN/SDC/Ni0.9Cu0.1-SDC single cell cathode A-site Ca²⁺ doped,power density rise,when the doping content of Ca²⁺ is x=0.1,ther single cell with B0.9C0.1CFN cathode materials has the best output power density curve,of maximum power density in the 800 oc is 338 m Wcm,when further improve the amount of Ca²⁺ doping,power density of the single cell decline,B0.9 C0.1 CFN cathode showed the best electrochemical performance.2.B_0.9C_0.1CFN cathode materials showed excellent comprehensive performance,become a kind of promising IT-SOFC cathode materials.In order to further enhance the electrochemical properties of materials and the matching with SDC electrolyte resistance,improve the oxygen reduction reaction rate,we improved B0.9C0.1CFN materials,mixed the B0.9C0.1CFN（BCCFN）and SDC electrolyte materials composite made composite cathode materials,is expected to further reduce its thermal expansion coefficient,improve the oxygen reduction reaction process of cathode materials,improve its performance.The results show that the sintering in 1000 oC after 10 h,BCCFN and SDC and keep their phase structure,good chemical compatibility.Electron microscopy（SEM）images show BCCFN-30 SDC expression a lower porosity and smaller particle size,which is beneficial to form a continuous SDC ion diffusion channel and continuous BCCFN electronic conductive channel.Composite SDC can effectively reduce the thermal expansion coefficient of cathode material,make the thermal expansion coefficient closer to the electrolyte SDC,improves the heat matching of cathode materials and electrolyte materials.BCCFN cathode materials compound SDC can improve the electrochemical properties of cathode materials,BCCFN-x SDC（x=0,20,30,40）composite cathode materials half cell impedance tests show that with the increase of the amount of compound SDC electrolyte,cathode polarization resistance（Rp）and then decreases,and when the content of compound x = 30 wt %,the polarization resistance become the lowest,Rp value reached 0.2984 Ωcm2 at 800 o C,when SDC content increased further,BCCFN phase continuity lower,affects the cathodic oxygen reduction reaction,the polarization resistance increase.BCCFN-x SDC as cathode material electrolyte support structure BCCFN-x SDC | SDC | Ni0.9 Cu0.1-SDC single cell test results show that the content of compound SDC is 30 wt %,single cells showed better power density curve,800 oC BCCFN-30 SDC as SOFC cathode maximum power density 351 m Wcm-2,indicating that BCCFN-30 SDC is a good development prospect of IT-SOFC cathode materials.