Defect and hydration regulation in Ba/Fe co-doped Ruddlesden–Popper nickelate for reversible protonic ceramic cells

The practical deployment of reversible protonic ceramic cells requires oxygen electrodes that can operate efficiently and reversibly without relying on cobalt- and strontium-containing perovskites, that often suffer from thermal mismatch, surface segregation, and durability degradation. Here, we report a Sr/Co-free Ba/Fe co-doped Ruddlesden–Popper nickelate, Pr 1.8 Ba 0.2 Ni 0.9 Fe 0.1 O 4+δ (PBNF0.1), as an efficient and stable oxygen electrode. Ba incorporation stabilizes the layered Pr 2 NiO 4+δ framework, while Fe substitution regulates the Ni valence state, promotes oxygen-vacancy formation, and enhances hydration capability, thereby improving oxygen-electrode kinetics. Consequently, PBNF0.1 exhibits reduced polarization resistance compared with Pr 1.8 Ba 0.2 NiO 4+δ (PBN) and delivers excellent reversiblecell performance, achieving 1.28 W cm −2 in fuel-cell mode and −2.47 A cm −2 at 1.3 V in electrolysis mode at 700 °C. Stable operation under fuel-cell, electrolysis, and reversible cycling conditions further highlights Ba/Fe co-doping as an effective strategy for durable Sr/Co-free oxygen electrodes.