La and Zr co-doped Ruddlesden–Popper air electrode materials for PCFCs

The development of high-performance air electrodes is critical for the practical application of protonic ceramic fuel cells (PCFCs). In this work, a series of Ruddlesden–Popper (RP)-type cathodes with a nominal composition of La_0.3Sr_3.7Fe_3−xZr_xO_10−δ (x = 0.3, 0.4, 0.5) is synthesized via a modified combustion method. The influence of Zr doping on phase evolution, defect chemistry, and electrochemical activity is systematically investigated. The results reveal that increasing the Zr content gradually reduces the oxygen vacancy concentration, promotes the reduction of Fe⁴⁺ to Fe³⁺, and introduces SrZrO₃ precipitation (x≥0.4). Nevertheless, among the prepared samples, the La_0.3Sr_3.7Fe_2.6Zr_0.4O_10−δ (LSFZr0.4) cathodes exhibit the best electrochemical performance. At 700 °C, the polarization resistance of LSFZr0.4 is 0.14 Ω cm², approximately 26.3%, 22.2%, and 12.5% lower than that of the undoped sample, x = 0.3, and x = 0.5, respectively. The superior electrochemical performance of LSFZr0.4 mainly benefits from its well-matched thermal expansion coefficient, which strengthens the cathode–electrolyte interface and accelerates charge transfer and surface exchange kinetics. In single-cell tests, the LSFZr0.4-based cell delivers a peak power density of 1098.0 mW cm⁻² at 700 °C, which is more than 200 mW cm⁻² greater than those of the x = 0.3 and x = 0.5 cathodes. This work provides a promising air electrode for PCFCs and demonstrates that moderate Zr doping is an effective strategy for optimizing the performance of RP-type cathodes.