Compared to Co-based cathode materials, Fe-based perovskite materials exhibit superior thermal stability and electrochemical stability, but they have lower conductivity and redox activity. The Fe-based perovskite materials are regarded as one of the cathode materials with a great development potential for SOFCs due to their lower thermal expansion coefficient and cost. In the LnBaFe₂O_5+δ (Ln = lanthanide elements, Y) double perovskite materials, PrBaFe₂O_5+δ (PBF) has the lowest activation energy for the redox reaction, making it a promising alternative material for SOFC cathodes. For the PBF materials, there are a few reports on their crystal structure and redox reaction mechanisms. There is still a debate regarding the tetragonal and cubic structures of PBF materials. Also, the mechanism of the redox reaction remains unclear and requires a further in-depth investigation. This study was to prepare the PBF cathode materials by a sol-gel method. In addition, the synthesis process, crystal structure, and redox reaction mechanisms of PBF materials were also investigated.

The average binding energy of the possible structures of PBF was calculated by the Cambridge Sequential Total Energy Package (CASTEP) module in Material Studio software. The reflex module in Materials Studio was used to simulate the X-ray diffraction (XRD) patterns after optimizing the structures. The PBF powder was prepared by a sol-gel method, and the precursors were analyzed by thermgravity-differential thermal analysis (TG-DTA), X-ray diffraction (XRD) and transmission electron microscopy(TEM）. The calcined samples were treated in air, 20% (in volume) H₂(H₂+N₂) atmosphere, and pure H₂ at 800 ℃ for 20 h. In addition, the electrochemical impedance spectra of the PBF|SDC|PBF symmetrical cell were measured, and analyzed via distribution of relaxation times (DRT).

The simulated XRD patterns show distinct differences in the XRD peaks of the four PBF structures with the main difference between the tetragonal and simple cubic structures after 80°. The characteristic double peak of the double perovskite is attributed to an increased lattice distortion caused by an increase in oxygen vacancy defects, while its structural symmetry remains unchanged. The tetragonal structure has the lowest average binding energy, while the simple cubic structure has the highest. This indicates that the likelihood of PBF adopting a simple cubic structure is minimal, whereas the likelihood of it adopting a tetragonal structure is maximal.

The results of TG-DTA and XRD indicate that the precursors form BaFeO₃ and PrFeO₃ heat-treated at 800 ℃and a subsequent two-phase solid solution process after 800 ℃. Pure PBF is obtained after roasting in air at 1200 ℃ for 5 h. In a 20%H₂(H₂+N₂) atmosphere, pure PBF can be achieved at 800 ℃, indicating that the 20%H₂(H₂+N₂) treatment accelerates the solid solution of BaFeO₃ and PrFeO₃. The PBF is structurally stable in both air and a 20%H₂(H₂+N₂) atmosphere, but decomposes into Fe, BaFeO₃, and Pr₂O₃ in pure H₂, indicating that PBF material decomposes under pure hydrogen and is not suitable for use on the anode side. Also, the occupancy of O in the [Pr-O] layer decreases, indicating that more oxygen vacancies are generated after reduction. The analysis of the crystal structure shows that the unit cell is elongated in a 20%H₂(H₂+N₂) atmosphere due to the formation of more oxygen vacancies in the [Pr-O] layer.

Based on the different relaxation times of various cathodic reactions, the DRT analysis indicates that the limiting step of the ORR at 400−550 ℃ involves the ionization of oxygen atoms and the transport of oxygen ions, while that at 600−700 ℃ involves the chemisorption of oxygen and the dissociation of oxygen molecules. At 700−800 ℃, the limiting step is related to the physisorption of oxygen. Clearly, the conclusions drawn from time-domain analysis differ from those obtained through conventional impedance spectroscopy. At 750−800 ℃, time-domain analysis indicates that the limiting step affecting the ORR is the physisorption of oxygen, whereas conventional impedance spectroscopy indicates that it is the ionization of oxygen atoms and the transport of oxygen ions. This study posits that both sets of results can serve as a basis for optimizing the oxygen catalytic performance of PBF materials.

This study investigated the formation process, structure, stability, and oxygen catalytic performance of PBF materials by DFT calculations, XRD, TEM, EIS, and DRT analysis. The PBF synthesized through the solid solution formation of PrFeO₃ and BaFeO₃ at 800 ℃ exhibited a tetragonal structure (i.e., space group: P4/mmm). The crystal structure remained stable in air and 20%H₂(H₂+N₂) atmosphere, but partially decomposed into Fe, BaFeO₃, and Pr₂O₃ in pure hydrogen. The limiting steps of the oxygen reduction reaction (ORR) were identified as the ionization of oxygen atoms and the transport of oxygen ions at 400−550 ℃, chemisorption of oxygen and dissociation of oxygen molecules at 600−700 ℃, and physisorption of oxygen at 700−800 ℃.