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 (UL = 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 UL = –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.