Valence Regulation and Defect Engineering in Zr-Substituted BaFe0.9Nb0.1O3-δ Perovskites: A Promising Air Electrode for Reversible Protonic Ceramic Cells

Reversible protonic ceramic cells (R-PCCs) can function at lower temperatures due to the lower activation energy required for proton conduction. In this work, a series of perovskite materials BaZrxFe0.9-xNb0.1O3(x=0, 0.05, 0.1) are synthesized. Characterization results reveal that the introduction of Zr effectively reduced the average valence states of Fe and Nb in the original perovskite matrix, promoting an increase in surface oxygen vacancy concentration. Electrochemical tests on symmetrical cells show that Zr doping positively influence the oxygen reduction reaction (ORR) activity. When the Zr doping amount is 0.1, the polarization resistance (Rp) of the symmetrical cell decreases to 0.23 Ω cm2, a 72.3% reduction compared to the undoped sample. Additionally, the Rp remained stable under different humidity environments (PH2O=0.03-0.10 atm), while it exhibits an exponential decrease with increasing PO2, confirming that the ORR kinetics are limited by the oxygen exchange process at the electrode surface. In addition, a single cell with BaZr0.1Fe0.8Nb0.1O3 as the air electrode achieves a maximum power density of 755 mW cm-2 in fuel cell mode and a current density of 1037 mA cm⁻² under 1.3 V electrolysis mode at 700 °C, using wet H2 (3%H2O) as the fuel and wet air (30%H2O) as the oxida