Intrinsic Electronic Structure and OER Thermodynamics of the Co‐N 4 Motif: A Molecular DFT Study of a Single‐Atom Catalyst

Abstract

Understanding the electronic structure, spin states, and interactions of single‐atom catalysts (SACs) is crucial for the rational design of efficient electrocatalysts for water splitting. Herein, density functional theory (DFT) calculations were performed for Co–N 4 SAC at the B3LYP‐D3BJ/def2‐TZVP level, with comprehensive spin‐resolved investigations of OER intermediates and explicit consideration of water coordination. Mulliken spin analysis indicates that the unpaired electron (+1.138 μB) is predominantly localized on Co, confirming a doublet ground state ( S = 1/2, ⟨S 2 ⟩ = 0.803). The first and second axial water molecules exhibit binding energies of −1.32 and −0.68 eV, respectively, supporting the formation of a thermodynamically favorable pseudo‐octahedral Co–N 4 –(H 2 O) 2 complex and a possible proton‐solvation relay. The calculated HOMO–LUMO gap of 1.95 eV (635 nm) suggests visible‐light activity, while TD‐DFT identifies a strong absorption band at 476 nm ( f = 0.0438) associated with metal‐to‐ligand charge‐transfer transitions. Spin‐state analysis reveals ground‐to‐quartet energy differences of 9.2–6.5 kcal/mol, with quartet OOH stabilized by 2.2 kcal/mol, supporting spin crossover during O–O bond formation. The *O → *OOH step is potential‐determining (Δ G = 1.62 eV), yielding an overpotential of 0.39 V. These results establish Co–N 4 as a promising dual electro/photo catalyst for sustainable hydrogen and oxygen production.