Mo Dopant-Mediated Oxygen Vacancy Engineering for Enhanced Cathodic Activity in Protonic Ceramic Fuel Cells

Protonic ceramic fuel cells (PCFCs) have attracted more interest than solid oxide fuel cells (SOFCs) due to their potential to be operated at lower temperatures, therefore, addressing the limitation of high-temperature oxygen ion conducting SOFC. Layered double perovskite with triple conducting (H⁺/O^2-/e^-) properties, such as PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF), have demonstrated outstanding performance as cathodes in PCFCs. Our investigation on molybdenum-doped (Mo-doped) PBSCF showed that high valence Mo doping is an effective strategy for substantially promoting the formation of oxygen vacancy and accelerating the overall cathodic reaction. The improved oxygen reduction reaction (ORR)-related kinetics are reflected by the reduced polarization resistance (R_p) of PBSCFM01 (0.052 Ω·cm² at 650 °C) and the enhanced peak power density (PPD) of the PBSCFM01 cell (817 mW·cm^-2 at 650 °C), corresponding to a 75% decrease in R_p and a 44% increase in PPD compared with pure PBSCF. Mo doping promotes the formation of oxygen vacancies, which optimizes the lattice structure and facilitates oxygen ion transport. In addition, Mo incorporation improves the charge transfer kinetics and overall electrical conductivity, with the PBSCFM01 sample exhibiting a maximum conductivity of 1051 S/cm at 250 °C─about 15% higher than that of pure PBSCF. Density functional theory calculations were further performed to elucidate the role of Mo incorporation in modulating oxygen vacancy formation. The theoretical results are in excellent agreement with the experimental observations, providing atomic-scale insight into the enhanced cathodic activity. This work demonstrates Mo as effective dopant for layered double perovskite materials to develop high oxygen reduction reaction activity cathode material PCFCs.