Electronic Origin of Enhanced Selectivity through the Halogenation of a Single Mn Atom on Graphitic C₃N₄ for Electrocatalytic Reduction of CO₂ from First-Principles Calculations

The electrochemical reduction of CO₂ to valuable products is a critical process that can potentially address energy and environmental challenges. Single-metal atom catalysts have gained significant attention because of their high efficiency and potential to mitigate the challenges associated with traditional-metal nanocatalysts. In particular, non-precious-metal-based catalysts are of great interest because of their low cost and abundance in the Earth’s crust. This work is a comprehensive study to reveal the role of halogen X (X = F, Cl, Br, and I) in improving the CO₂ reduction activity and selectivity of single manganese atom-based active sites on a graphitic carbon nitride (g-C₃N₄) monolayer. Although previous experiments prove that halogenation improves the selectivity of a single Mn-atom-based catalyst, our calculations reveal the reason for the selectivity of the catalyst. The halogen-modified MnN6 active site on g-C₃N₄ has a high hydrogen evolution reaction (HER) tolerance. Hence, the selectivity due to the increased electronic stability originated from half-filled d orbitals of the Mn atom stabilized on g-C₃N₄. Also, we present the Gibbs free energy profile, onset potential (U_Min), and overpotential (η) for various C₁ products (CO, HCOOH, CH₃OH, and CH₄) at the active sites with and without halogenation. These results suggest that the MnN₆ active site of Mn-X-decorated g-C₃N₄ is a highly efficient and selective electrocatalyst for the CO₂RR against the HER. Our study provides directions for the design of a new CO₂RR catalyst with improved selectivity and efficiency.