Axial Coordination Effect on the Oxygen Reduction Reaction of FeN₄ Electrocatalysts Based on Grand Canonical Density Functional Theory

The Fe–N–C materials are promising noble-free alternatives to Pt-based oxygen reduction reaction (ORR) electrocatalysts. Identifying the actual active structure of FeN₄-based moieties is helpful for the regulation and design of high-performance electrocatalysts. However, the current theoretical researches were mostly based on the charge-neutral model (CNM), which cannot accurately describe the electrochemical interface. Herein, we employed the constant potential based grand canonical density functional theory (GC-DFT) computations to study the ORR mechanism of axially decorated FeN₄ electrocatalysts. We studied around ten types of axial ligands, and the constant potential energetics and microkinetic modeling demonstrated that the Fe center can exhibit excellent activity for boosting the four-electron ORR via covalently linked −NH₂ ligand. The lowering of the antibonding d_z²–p_z orbital is responsible for weakening the adsorption of oxygen in FeN₄–Ls to promote ORR activity, and the −NH₂ decoration led to the lowest antibonding orbital energy. In particular, the adsorption free energy (ΔG*O₂) and O–O bond length (L_O–O) of the adsorbed O₂ reactant can be used as the effective energetic and geometric descriptors to describe the ORR activity of FeN₄–Ls electrocatalysts. Our results not only elucidate the axial coordination effect on the ORR performance of FeN₄ SACs but also demonstrate the importance of electrode potential in computational electrochemistry.