Study On Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-?-based Core-shell Structure Cathode For Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell?SOFC?is an energy conversion device by converting chemical energy of fuels into electric energy directly with the advantages of high efficient and clean.Its development and application are of great significance to alleviate energy crisis and environmental pollution.The operating temperature of traditional SOFC is in the range of 800¹000 °C and the high operating temperature has a negative impact on the cost and stability of SOFC stack.Recently,the traditional high temperature SOFC gradually develops to the intermediate-low temperature range in order to accelerate the commercialization of SOFC.The decrease of operating temperature can not only greatly reduce the cost and expand the range of materials selection,but also greatly improve the stability.In addition,when the operating temperature is below 600 °C,SOFC can achieve rapid start and stop,making SOFC with wider application in the field of mobile power supply and transportation.Nevertheless,the decrease of the operating temperature will lead to the increase of resistance,especially the polarization impedance?Rp?of the cathode.Therefore,the development of new cathode materials with excellent electrochemical performance in the intermediate-low temperature range is a key step for the commercialization of SOFC.SOFC cathode materials must have good electron-ion mixed conductivity?MIEC?,oxygen reduction reaction?ORR?catalytic activity,good stability and physical and chemical compatibility with adjacent materials.As the most common cathode material in traditional high temperature SOFC,La1-xSrxMn O3-??LSM?has excellent comprehensive performance.With the decrease of operating temperature,however,its negligible oxygen ion conductivity will cause a sharp deterioration in electrochemical performance,and the single-phase LSM cathode can hardly meet the performance requirements of intermediate temperature solid oxide fuel cell?IT-SOFC?.Recently,the most studied cathode material for IT-SOFC are the perovskite oxides with MIEC.The high ionic conductivity can extend the ORR active site from the three-phase boundary to the whole cathode system,and thus these perovskite oxides have excellent electrochemical performance in intermediate-low temperature range.Ba1-xSrxCo1-yFeyO3-??BSCF?is a typical MIEC perovskite oxide with high oxygen diffusion coefficient,oxygen surface exchange coefficient and ORR catalytic activity.However,it is easily poisoned by CO₂ in the air because of the presence of Ba and Sr in A site.In addition,Fe-Cr alloy is the common interconnect for SOFC stack.During the operating process,the Cr evaporation will diffuse to cathode and deposit.Ba and Sr can easily react with Cr element,resulting in severe deterioration of the cathode electrochemical performance.Up to now,the single-phase cathode material can hardly meet all the requirements for IT-SOFC cathode.Therefore,BSCF is chosen as the backbone and precursor solution impregnation method is carried out to decorate the backbone.After introducing LSM and La Co O3-??LC?shell,LSM@BSCF,LC@BSCF and LC@BSCF-Gd0.1Ce0.9O1.95?GDC?core-shell structure cathode are successfully prepared,respectively.The electrochemical performance,ORR mechanism,CO₂-poisoning resistance,Cr-poisoning resistance and long-term stability of these cathodes are studied.The following conclusions can be drawn:?1?In order to obtain LSM@BSCF?or LC@BSCF?core-shell structure cathode,LSM?or LC?precursor solution is infiltrated onto BSCF backbone.LSM?or LC?can cover the backbone completely and compactly.The core-shell structure cathodes with different shell thickness are obtained by different impregnation times.The results of electrochemical tests show that the core-shell structure cathode with 2 times impregnation has the most excellent electrochemical performance.?2?The electrochemical tests and ORR mechanism investigation of BSCF and?2?The electrochemical tests and ORR mechanism investigation of BSCF and LSM@BSCF cathode are carried out,and the influence of the LSM shell on the electrochemical performance of cathode system is analyzed.LSM has excellent oxygen adsorption performance and ORR catalytic activity.The introduction of LSM shell can promote the oxygen surface process of the cathode system,which leads to the improvement of the electrochemical performance.The ORR speed controlling step of the cathode system also changes from the oxygen surface adsorption process to the oxygen ion transmission process.?3?After the treatment in 10 vol%CO₂,the electrochemical performance of single-phase BSCF cathode has sharp deteriorate.Even after the removal of CO₂,the Rp still increases 4.7 times comparing with the initial value.While for the LSM@BSCF cathode,the Rp presents only a small increase after the treatment in 10 vol%CO₂,and the Rp almost recovers to the initial value after the removal of CO₂.By comparison,LSM@BSCF core-shell structure cathode has remarkable CO₂-poisoning resistance.?4?The electrochemical performance of BSCF and LC@BSCF cathode are analyzed,and electrical conductivity relaxation is used to measure the oxygen surface exchange coefficients of BSCF and LC@BSCF.After the introduction of the LC shell,the oxygen surface exchange coefficient of cathode system increases from 5×10-4 cm s^-1 to 1 × 10-3 cm s^-1,thus explaining the excellent electrochemical performance of LC@BSCF cathode.With the protection of LC shell,the long-term stability of the cathode system is also greatly improved.?5?The CO₂-poisoning resistance of BSCF and LC@BSCF cathode are tested under different CO₂ concentrations.With the increase of CO₂ concentration,the CO₂-poisoning degree of BSCF cathode is more and more serious.While for the LC@BSCF cathode,the excellent CO₂-poisoning resistance can be maintained in each CO₂ concentrations.?6?Based on the LC@BSCF cathode,the LC@BSCF-GDC core-shell structure cathode is prepared.The Cr-poisoning effect of BSCF-GDC and LC@BSCF-GDC cathode are studied in detail.When SUS430 is used as the current collect material,Cr-poisoning of BSCF-GDC cathode is serious and causes serious deteriorate of electrochemical performance.Moreover,Cr deposition mainly occurs on the surface layer of the cathode.After introducing the LC shell,however,the cathode system presents obvious Cr-poisoning resistance.