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

Area of Science:

• Materials Science
• Electrochemistry
• Computational Chemistry

Background:

• Oxygen electrocatalysis, including the Oxygen Reduction Reaction (ORR) and Oxygen Evolution Reaction (OER), is vital for energy technologies.
• The O p-band center, derived from density functional theory (DFT), is a key electronic descriptor linking catalyst properties to ORR/OER activity.
• Ruddlesden-Popper oxides (Sr$_{n+1}$Fe$_n$O$_{3n+1}$) offer a tunable platform for investigating these structure-property relationships.

Purpose of the Study:

• To systematically evaluate how dimensionality, Fe substitution, and oxygen nonstoichiometry affect the O p-band center in Sr$_{n+1}$Fe$_{n(1-x)}$M$_{nx}$O$_{3n+1-\delta}$ oxides.
• To understand the intrinsic electronic variations and their impact on oxygen electrocatalysis activity across a series of Ruddlesden-Popper oxides.

Main Methods:

• Utilized density functional theory (DFT) calculations to determine the O p-band center for various Ruddlesden-Popper oxide compositions.
• Investigated the Sr$_{n+1}$Fe$_{n(1-x)}$M$_{nx}$O$_{3n+1-\delta}$ series with varying 'n' (1, 2, ∞), 3d-metal substitutions (M), and oxygen deficiencies (δ).

Main Results:

• Increasing slab thickness (n=1 to ∞) and substituting Fe with more electronegative metals shifted the O p-band center towards the Fermi level (up to 0.2 eV and 0.45 eV, respectively).
• A 12% oxygen deficiency shifted the O p-band center downwards by up to 0.45 eV.
• The overall O p-band center variation across the series was approximately 0.7 eV, indicating modest intrinsic electronic tuning.

Conclusions:

• Structural and compositional modifications in Ruddlesden-Popper oxides lead to predictable shifts in the O p-band center, a crucial descriptor for oxygen electrocatalysis.
• The intrinsic electronic variations achieved through these modifications are relatively modest and can be surpassed by extrinsic factors like morphology and microstructure.