Research Progress of Cathode Materials for Solid Oxide Fuel Cells

Nowadays，energy crisis and environmental pollution are two major problems affecting the development of society. Human beings are imperatively looking for efficient and environmentally friendly energy utilization methods. It is urgent to develop clean，envi⁃ ronmentally friendly，stable and reliable new energy to meet the countrýs major needs in energy structure adjustment，efficient and clean use of fossil fuels and the realization of carbon peaking and carbon neutrality goals. Fuel cells have become the most promising new generation of power generation technology because of their high energy conversion rate，low pollution emissions and wide fuel sources. There are five mature fuel cell power generation systems classification by electrolyte，including phosphoric acid fuel cell （PAFC），alkaline fuel cell（AFC），molten carbonate fuel cell（MCFC），proton exchange membrane fuel cell（PEMFC）and solid ox⁃ ide fuel cell（SOFC）. Among them，SOFC is an all-solid，green，economical，pollution-free and fuel-selective power generation de⁃ vice，which has great development prospects in efficiency conversion and environment-friendly energy applications. The studies have shown that SOFC had a series of problems such as slow start，high cost，components diffusion and aging，high decay rate of perfor⁃ mance，sealing and connection difficulties working at high-temperature（800~1000 ℃）. In contrast，medium and low temperature SOFC has good stability，low cost，fast start，and wide selection of sealing materials. Therefore，it is necessary to carry out technology to develop and research of medium and low temperature SOFC. SOFC is assembled with cathode，anode，electrolyte and connecting material. The electrolyte ohmic loss and electrode polarization loss will be increased sharply when operating in the medium and at 600~ 800 ℃. At present，electrolyte ohmic loss has been effectively solved by electrolyte thin film technology. However，the slow dynam⁃ ics，instability and polarization loss of oxygen reduction reaction（ORR）in the cathode are the main challenges restricting the develop⁃ ment of SOFC technology. The mismatch between the thermal expansion coefficient of cathode materials and electrolyte materials，the problems of Cr poisoning，CO2 poisoning and SO2 poisoning also have a great impact on the low temperature process in SOFC. Develop⁃ ing high performance cathodes is the key to lower the working temperature of SOFC. SOFC with excellent performance was closely relat⁃ ed to the performance of cathode materials. It is obligatory for researchers to develop cathode materials with excellent performance，good structural stability and high efficiency that can meet the needs of medium and low temperature SOFC. This work reviewed the re⁃ cent research progress of cathode materials with different composition and microstructure. The cathode materials could be divided into perovskite（ABO3）structure，double perovskite（A2B2O6）structure，perovskite-like（An+1BnO3n+1）structure，spinel structure（AB2O4），other structural cathode materials and composite cathode materials according to the structural characteristics. Perovskite based oxides were widely favored by researchers due to their special crystal structure，high ionic conductivity，good oxygen permeability and excel⁃ lent catalytic activity. ABO3 perovskite oxide was a typical ion-electron hybrid conductor and had been widely used as a cathode materi⁃ al for SOFC. According to whether it contained cobalt elements，it could be divided into cobalt-based cathode materials and cobalt-free cathode materials. Perovskite structure cathode could not satisfy the demands of high-performance cell. Compared with the perovskite structure，double perovskite oxides had many advantages. Cathode with double perovskite structure showed higher ionic electron con⁃ ductivity，better oxygen ion diffusion and surface exchange dynamics than the perovskite oxide with the same elements in the medium and low temperature region，and had a great development prospect. Perovskite-like structural cathode materials had the advantages of stable structure，low thermal expansion coefficient，high diffusion coefficient，oxygen exchange capacity and strong surface adsorption force，etc. They had been successfully applied in medium and low temperature solid oxide fuel cells，and had become a research hot⁃ spot of SOFC cathode materials in recent years. Spinel oxide and other structural materials with some fantastic properties had become a good choice for preparing new cathode materials. In addition，the composite cathode material prepared by complex processes had fan⁃ tastic performance than the single cathode material，which could promote the progress of medium and low temperature SOFC technolo⁃ gy，and it was also another research direction for the development and preparation of SOFC cathode materials in the future. However，there were still some structural and performance defects of these several cathode materials，which prevented them from being widely used in SOFC，thus limiting the further large-scale development of SOFC. Therefore，it was still an urgent mission to study the cathode materials in-depth for medium-low temperature SOFC，so as to promote the commercialization of SOFC application. Finally，this re⁃ view looked forward to the strategies and methods for optimization cathode performance and development new cathode materials of solid oxide fuel cells in the future. In order to improve the electrochemical properties of cathode materials，it could be considered from the following aspects：（1）Modify the existing cathode materials.（2）Accelerate the development of new cathode materials.（3）Using a series of processes to prepare composite cathode materials.（4）Structural design and performance prediction of cathode materials by using first-principle calculation and machine learning methods. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.