Study On Modification Of Cobalt-based Double Perovskite SOFC Cathode Material

Solid oxide fuel cells（SOFCs）,as a new class of energy conversion devices,have attracted much attention and research because of their clean and pollution-free nature and high conversion efficiency.In recent years there has been an increasing amount of research on medium temperature solid oxide fuel cells（IT-SOFC）,which have the advantages of lower cost,wider material selection and longer lifetime compared to conventional SOFCs.It is crucial to develop SOFC cathode materials suitable for operation in medium-temperature conditions.The double perovskite structure oxide Ln Ba Co₂O_5+δ（Ln=La,Pr,Nd,etc.）has good electrochemical performance in medium-and low-temperature conditions,but there are also problems such as high-temperature mismatch with electrolyte materials,so in this paper,Pr Ba Co₂O_5+δand Nd Ba Co₂O_5+δare used as the parent materials,by using different sintering temperatures,compounding or doping to carry out the modification study.First,Pr Ba Co₂O_5+δ（PBC）was synthesized using the sol-gel method,sintered at950℃,1000℃,1050℃and 1100℃,and its electrochemical properties were investigated serially.The results showed that the higher sintering temperatures were favorable to the formation of a single perovskite structure of PBC,but did not improve the electrochemical properties of the material.Sintering temperatures of 1000℃,1050℃and 1100℃all resulted in the formation of pure perovskite phase.The conductivity increases with the increase of sintering temperature,and the lowest conductivity of 1000℃calcined PBC at 850℃is 552.5 S cm^-1,which is much higher than the minimum requirement of conductivity of cathode material;the polarization impedance value of the material increases with the increase of sintering temperature,and the lowest polarization impedance of 1000℃calcined PBC at 800℃is 0.069Ωcm²,and the higher the sintering temperature,the higher the impedance provided by the charge transfer between the cathode and the electrolyte.To further optimize the electrochemical properties of PBC cathode materials,the PBC cathode calcined at 1000℃was compounded with the electrolyte material SDC,and four samples of PBC-10%SDC,PBC-20%SDC,PBC-30%SDC and PBC-40%SDC were prepared by mechanical synthesis according to the mass ratio of the two materials.The experiments showed that PBC and SDC have good chemical compatibility at high temperature,and the conductivity of the cathode material decreased with the increase of the amount of SDC composite,and the lowest conductivity of PBC-40%SDC was108.05 S cm^-1 at 850℃.The polarization impedance of the composite cathode decreases with increasing SDC loading in the test range from 600 to 750℃.But at800℃,the polarization impedance first decreases and then increases with the composite amount,and the lowest impedance value is 0.027Ωcm² at PBC-30%SDC.In addition,the effect of Nb doping on the electrochemical properties of perovskite Nd Ba Co_2-xNb_xO_5+δcathode materials was prepared by the sol-gel method,and the Nb doping amount on the electrochemical properties of Nd Ba Co₂O_5+δ（NBCO）.The results show that the solubility limit of Nb element in NBCO oxide is x=0.02,and the pure perovskite phase cannot be formed by increasing the annealing temperature and sintering times when the solubility limit is exceeded.For this reason,the structure and electrochemical properties of the Nd Ba Co_2-xNb_xO_5+δ（x=0,0.01,0.02）cathode were investigated.In the test range of 200～850℃,all samples exhibited metal-like conductive properties,and the conductivity of the materials increased with the increase of Nb doping at the same test temperature,and at 850℃x=0,0.01 and 0.02 have the lowest conductivity of 370,627 and 735 S cm^-1,respectively.By electrochemical impedance spectroscopy test,it was found that the impedance values of the samples decreased with the increase of Nb doping,and at 800℃,the impedance values of x=0,0.01 and 0.02 were 0.078,0.058 and 0.048Ωcm²,respectively,and the price state of the transition metal and oxygen content of the material at room temperature were found to increase with the increase of doping after iodine titration test.