The team of Associate Professor Fan Liangdong from the College of Chemistry and Environmental Engineering, Shenzhen University, recently published a research paper entitled "Revisiting Mo-doped SrFeO_3-δ Perovskite: the Origination of Cathodic Activity and Longevity for Intermediate-Temperature Solid Oxide Fuel Cells” on Advanced Functional Materials, and Nature indexed journal paper. Professor Fan is the only corresponding author and Shenzhen University is the sole corresponding institution for this published work.

Research background:

Solid oxide fuel cells (SOFCs) are famous for their high conversion efficiency from chemical energy to electrical energy and fuel flexibility. They are an important sustainable development technology for achieving the "dual carbon" goal. Classic SOFCs have high operating temperatures and fast kinetic rates, but they also suffer from serious problems like component degradation and performance attenuation. Reducing the operating temperature of SOFCs has become a current research trend. As the temperature decreases, the oxygen reduction catalytic activity in SOFC components decreases, and there is a compromise between performance and stability in traditional electrodes. The mixed ion/electronic conductive perovskite oxide SrFeO₃ has been an intensive research object because of its unique FeO₆ and rich valence state of Fe, as well as its very good thermomechanical and chemical compatibility with electrolyte materials. In previous studies, it was mainly used as the anode of SOFC. In the reducing anode atmosphere, to stabilize the cubic structure, it is necessary to use high-valent elements such as Mo, W, Ti, Nb, etc. with heavy doping. Then the high-valent dopant and its stable valence electron structure reduce the conductivity and catalytic activity of the material. SrFeO₃ itself has excellent electrocatalytic oxygen reduction ability. To utilize this property, the paper adopts a small amount doping method, combined with Smith acidity and average metal-oxygen bond energy theory (Figure 1) and corresponding multi-dimensional (in situ) experiments and theoretical calculations to verify that only a small amount of Mo doping is needed to stabilize its crystal structure and improve its catalytic activity, breaking the compromise between stability and performance.

Research highlights:

#1: In-situ high-temperature XRD confirmed that only a trace amount (7% vs. the classic 25%) of Fe doping is needed to stabilize the cubic perovskite crystal structure from room temperature to the SOFC operating temperature range. Trace doping effectively maintains the electrocatalytic activity and the conductivity of the material and stabilizes the electrode microstructure. In addition, based on the combined strategy of Smith acidity and average metal-oxygen bond energy  regulation, it is confirmed that a small amount of Mo (7%) doping effectively inhibits the migration and segregation of Sr to the oxide surface and significantly improves the resistance to CO₂ poisoning, effectively maintaining the new electrode stability under actual operating conditions;

#2. Combined with oxygen partial pressure and in-situ impedance spectroscopy technology, the oxygen reduction kinetics of the new electrode material were deeply studied, confirming that the first electron transfer process of oxygen reduction is the rate-controlling step. The single-phase electrode exhibits the best ORR activity and single-cell output performance among similar types of electrode materials. Its high activity is due to the small amount of Mo doping, which modifies the overlap of Fe-3d and O-2p electronic structures and reduces the oxygen vacancy formation energy and oxygen ion migration energy.

#3. The practical application potential of the SFM0.07+GDC composite electrode under actual SOFC conditions was confirmed: the composite electrode remains remarkably stable in accelerated thermal cycling operation between 700 ^oC and 400 oC at an ultra-fast ramp rate of 30 ^oC/min and operates stably and continuously at a constant voltage of 0.7 V for more than 750 h.

Perspective: This study breaks through the shackles of traditional thinking and not only provides new electrode materials for SOFC but also provides theoretical scientific references for the design and synthesis of new high-temperature catalytic materials in the future.

This study was strongly supported by the National Natural Science Foundation of China, the Basic and Applied Basic Research Project of the Guangdong Provincial Department of Science and Technology, and the Taipei University of Technology-Shenzhen University Collaborative Research Project.

Full-text weblink: https://onlinelibrary.wiley.com/doi/10.1002/adfm.202411025.