Preparation And Properties Of Mn-based Novel Cathode Materials For Solid Oxide Fuel Cells

A solid oxide fuel cell（SOFC）is a device that efficiently converts chemical energy into electrical energy with many advantages.The cathode material is a key component of SOFC.At present,the common cathode materials are mostly perovskite structures,and the perovskite cathodes have problems such as large thermal expansion coefficients and element segregation.These problems are mainly caused by the change of the spin state of Co during heating and the presence of alkaline earth metals.To solve these problems,the development of other structured materials such as spinel and mullite is one of research direction.In this study,firstly,the application of Sm Mn₂O₅（SMO）mullite in SOFC cathode was studied,and the synthesis process and electrochemical properties were explored.In addition,a spinel Cu Mn_1.5Ni_0.5O₄（CMN）cathode material was developed,and its electrochemical properties and cathode reaction mechanism were investigated.The main results are as follows:（1）SMO powder was synthesized by the solid-liquid composite method,and the synthesis conditions were explored.The pure phase powder was synthesized at 950℃with 40 wt%gluconic acid as a complexing agent,and the particle size of the powder was about 200 nm.SMO has good chemical compatibility with GDC below 1100℃,while it reacts slightly with YSZ at1050℃.The average thermal expansion coefficient of SMO at 800℃is 8.12×10^-6K^-1,while that of SMO-GDC55 is 10.6×10^-6K^-1,which is very close to YSZ(10.4×10^-6K^-1),showing excellent thermal expansion matching.The conductivity at 100-800℃was tested by the four-electrode method,and the highest conductivity was about 0.1 S cm^-1,indicating that this is a possible defect of SMO in the application of SOFC cathode.（2）To improve the performance of the SMO cathode,SMO-GDC and SMO-YSZ composite cathodes were made,and the polarization resistances（Rp）of the two cathodes at different calcination temperatures were tested.The results show that the SMO-GDC composite cathode has the lowest polarization resistance and smaller activation energy at 1050℃.The composite ratio of SMO-GDC cathode was further tested,and it was found that the ratio of 50:50 wt%had the best polarization resistance below 700℃.Therefore,SMO-GDC55 was selected as the composite cathode.The performance of the button cell was tested and it has a peak power density（PPD）of818.6 m W cm^-2at 800℃.Adding Sm Mn O₃perovskite to make a three-phase cathode,and its PPD increased to 948.2 m W cm^-2.SMO has good power performance and stability in SOFC.（3）Spinel oxides have made great progress in solid oxide fuel cells due to their special characteristics different from perovskite.CMN powder was synthesized by the solid-liquid method.Rietveld refinement showed that CMN was a cubic structure and belonged to the P4₃32 space group.CMN exhibits high conductivity of about 70.0-91.2 S cm^-1and good catalytic activity for oxygen at 600-800℃.The symmetrical cell with CMN-GDC composite cathode has a low Rp of0.047Ωcm²at 800℃.The performance test results of the button cell with CMN-GDC composite cathode are excellent,and the power density is as high as 1342.4 m W cm^-2at 800℃.After 110 h of a long-term test,the cell runs stably.The cell microstructure is intact,and no damage occurs.The combined analysis of EIS and DRT shows that the rate-limiting step of the cathodic reaction of the CMN-GDC composite cathode is the charge transfer process of oxygen,which hinders the oxygen reduction reaction at a lower temperature.The CMN composite cathode has excellent catalytic performance and is a potential novel cathode material.