Dimensionality and Compositional Effects on Sr-Fe-Based Ruddlesden-Popper Oxides for Oxygen Catalysis

Understanding how structural and compositional features influence the Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER) in oxygen electrocatalysis is crucial for the rational design of efficient catalysts. The O p-band center, obtained from density functional theory (DFT) calculations, serves as a predictive electronic descriptor linking the composition and structure of the oxide catalyst to ORR and OER activity. Ruddlesden-Popper oxides Sr _n+1Fe _n O_3n+1 (1 < n < ∞) provide a versatile platform for tuning this descriptor. Here, we systematically evaluate the effects of dimensionality, Fe substitution, and oxygen nonstoichiometry in the Sr _n+1Fe _n(1-x)M _nx O_3n+1-δ series (n = 1, 2, ∞; M = 3d-metal; x = 1/8; δ = 0, 1/8). Both increasing slab thickness (n = 1 → ∞) and Fe substitution with more electronegative transition metal elements enhance metal-oxygen hybridization, shifting the O p-band center toward the Fermi level by up to 0.2 and 0.45 eV, respectively, whereas 12% oxygen deficiency shifts it downward by up to 0.45 eV. Across the series, the combined effects of composition and structure span a ∼0.7 eV range in the O p-band center, implying only modest intrinsic variations in ORR/OER activity, often surpassed by extrinsic factors such as morphology and microstructure.