A-Site Cs ⁺ Doping toward High-Performance Cobalt-Free PrBaFe ₂ O _5+δ Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells

Cs+ doping has been mainly investigated in cathodes for proton-conducting solid oxide cells (P-SOCs), yet its application in oxide-ion conducting solid oxide fuel cells (O-SOFCs) remains scarce. Herein, A-site Cs+-substituted PrBa1-xCsxFe2O5+δ (x = 0, 0.05, and 0.1) cathode materials were synthesized to investigate the role of Cs+ doping in the structural and electrochemical properties of PrBaFe2O5+δ (PBF). Cs+ substitution effectively increases the oxygen vacancy concentration, simultaneously enhancing both surface exchange kinetics and bulk oxygen diffusion. The optimized composition PrBa0.95Cs0.05Fe2O5+δ (PBCF5) exhibits a polarization resistance (Rp) of 0.063 Ω·cm2 at 750 °C, a 25% reduction from 0.084 Ω·cm2 for undoped PBF. In an anode-supported single cell, the PBCF5 cell delivers a peak power density (PPD) of 1.74 W·cm−2, representing an 83% enhancement over PBF (0.95 W·cm−2), and maintains stable operation for 220 h under a constant current density of 0.5 A·cm−2. This work establishes A-site Cs+ doping as an effective strategy for enhancing the ORR kinetics and electrochemical performance of Fe-based double perovskites cathodes for O-SOFC applications.