Rare-earth- and cobalt-free Ruddlesden-Popper phases Sr3Fe2-xNixO7-δ (x = 0, 1) as air electrodes for reversible solid oxide cells

The development of cobalt- and rare-earth-free air electrodes is essential to improve the sustainability, cost, and scalability of reversible solid oxide cells (rSOCs). Here, the n = 2 Ruddlesden–Popper oxides Sr 3 Fe 2 O 7–δ (SF) and Sr 3 FeNiO 7–δ (SFN) are investigated in the first systematic study linking structure, defect chemistry, transport properties, and cell performance of this ferrite-based RP family for reversible operation. Nickel substitution significantly modifies oxygen non-stoichiometry and redox behaviour, producing a fourfold increase in total electrical conductivity for SFN compared to SF, reaching ∼200 S cm −1 in O 2 at 350 °C. Impedance spectroscopy demonstrates that performance is governed by a composition-dependent balance between electronic and ionic transport in RP–CGO composites. Optimal polarisation resistance is obtained at 50:50 wt% SF:CGO (0.24 and 0.06 Ω cm 2 at 700 and 800 °C, respectively) and at higher CGO fractions for SFN (30:70 wt%, 0.30 and 0.10 Ω cm 2 at 700 and 800 °C). These trends are reflected in single-cell tests, yielding peak power densities of ∼455 and 360 mW cm −2 at 800 °C for SF- and SFN-based electrodes, respectively. Confocal Raman microscopy is employed as a diagnostic tool to resolve interfacial and compositional evolution before and after operation, revealing distinct phase and vacancy redistribution mechanisms. • Cobalt- and rare-earth-free Ruddlesden–Popper ferrites as air electrodes for rSOCs. • Ni substitution enhances conductivity but limits electrochemical performance. • Confocal Raman reveals electrode–electrolyte interfacial evolution after operation.