Flat-Band-Assisted Nitrogen Reduction on M-N4 Single-Atom Catalysts in a Two-Dimensional Tetraaza[8]circulene Framework

Abstract

The porphyrin-like M-N₄ coordination environment embedded in two-dimensional (2D) lattices represents a compelling design principle for single-atom electrocatalysts (SACs) targeting the electrochemical nitrogen reduction reaction (eNRR). Density functional theory (DFT) calculations were employed to investigate 2D tetraaza[8]circulene framework (2D-TAC) that provides a periodic array of well-defined M-N₄ active sites and harbours a flat-band electronic structure near the Fermi level. Systematic screening of 25 transition metals based on thermodynamic and electrochemical stability, N₂ activation, and selectivity against the hydrogen evolution reaction (HER) identified Ti, Fe, Mo, and Ta as the most viable candidates. Spin-polarized PDOS, Bader charge, crystal orbital Hamilton population (COHP), local density of states (LDOS), and local magnetic moment analyses collectively confirm that Mo and Ta achieve superior N₂ activation through a backdonation-dominated mechanism amplified by flat-band-derived electronic states. Quantitative LDOS comparison with graphene-N₄ catalysts (GN₄) provides quantitative support for the hypothesis that the flat-band character of 2D-TAC enhances the metal d-electron density at the active site by 4.04-fold for Mo and 1.31-fold for Ta relative to the same coordination on a conventional 2D support. Mo-N₄ achieves a near-zero limiting potential (U_L = 0.00 V vs RHE within the implicit solvation CHE framework) along the distal pathway, among the most favorable limiting potentials reported for 2D M-N₄ catalysts within the CHE framework, while Ta-N₄ delivers U_L = –0.19 V vs RHE. The combined electronic and mechanistic analyses yield four interconnected design principles that extend beyond the present 2D-TAC system and provide consistent guidelines for the rational design of flat-band-enabled single-atom electrocatalysts.