Beyond conventional 3D-perovskite oxides, layered n = 2 Ruddlesden–Popper (RP) phases are attracting increasing interest as catalysts for the oxygen evolution reaction (OER). Here, Sr3Fe1.75M0.25O7-δ (M = Fe, Nb, Ta, and Ru) were synthesized by conventional solid-state reaction and evaluated for catalyzing the OER in alkaline media. All samples exhibited oxygen non-stoichiometry (δ ≈ 0.5) and moderate specific surface areas (≈1.3 m2 g−1), yielding modest OER activity. Among the investigated compositions, M = Fe and Ru exhibited the highest electrocatalytic activity (η ≈ 0.5 V at 2.5 mA cm−2, Tafel slope ≈140 mV·dec−1) whereas d0-cation substitution (M = Nb, Ta) further decreased activity. XRD, SEM-EDS, TEM-EDS and ICP-OES analyses revealed that these n = 2 RP ferrates suffer extensive cation leaching, surface carbonation (SrCO3) and structural degradation in alkaline media, occurring both under active OER operation and by chemical aging in the absence of an applied potential. The specific degradation mechanisms depend on the nature of M. While M = Fe, Ru phases undergo severe structural hydration and form Sr3Fe2(OH)12, substitution with high-valence d0 cations (Nb5+, Ta5+) suppresses the hydration pathway. These results contribute to a broader understanding of the factors that govern the performance and durability of layered Fe-based oxides as OER catalysts.
