Potential Dependent Orbital Inversion Tunes CO ₂ -to-CO Electroreduction on CoN 4-x S x Single-Atom Catalysts

Electrochemical CO 2 -to-CO conversion offers a sustainable route for carbon utilization. However, a major challenge in catalyst design is that most DFT studies rely on the constant-charge approach, which does not explicitly account for electrochemical conditions under a constant potential. To unravel the effect of constant potential, here, we investigated the electrochemical CO 2 -to-CO reduction reaction on atomically dispersed Co–N–C catalysts with systematic S substitution (CoN 4-x S x ). Going beyond the constant-charge model, grand-canonical DFT (GC-DFT) captures potential-induced changes in active-site electronic structure and provides potential-dependent free energy profiles for *CO 2 activation and subsequent proton-coupled electron transfer steps. We find that the Lewis acidity of the Co site, tuned by ligand electronegativity and Co p/d hybridization, emerges as a chemically intuitive descriptor for interfacial charge transfer and intermediate stabilization under applied potential. Among the studied catalysts, CoN 3 S is identified as the most balanced site for CO formation within the relevant potential region. As the potential becomes more negative, *COOH is consistently stabilized across all four CoN 4-x S x catalysts. In contrast, *CO binding exhibits a catalyst-dependent response with comparatively modest stabilization observed on CoN 3 S. This behavior, driven by the potential-induced inversion of the Co d z2 orbital, facilitates favorable CO desorption and suppresses the competing hydrogen evolution reaction (HER).