Semimetallic state transition in Two-Dimensional carbon nitride covalent networks and enhanced electrocatalytic...

Single-atom catalysts (SACs), due to their maximum atomic utilization rate, show tremendous potential for application in the electrocatalytic synthesis of ammonia from nitrate. Yet, the development of superior supports that preserve the high selectivity, activity, and stability of SACs remains an imperative challenge. In this work, based on first-principles calculations and tight-binding (TB) model analysis, a new two-dimensional (2D) carbon nitride monolayer, C₇N₆, is proposed. The C₇N₆ structure exhibits a strong covalent network, with dynamical, thermal, and mechanical stability. Surprisingly, the structural transition from C₉N₄ to C₇N₆ corresponds to a semimetallic state transition. Further symmetry analysis unveils that the Dirac states in C₇N₆ are protected by space-time inversion symmetry, and the physical origin of the Dirac cone was confirmed using the TB model. Additionally, a non-zero Z₂ invariant and significant topological edge states demonstrate its topologically nontrivial nature. Considering the excellent structural and topological properties of C₇N₆, a three-step screening strategy is designed to identify eligible SACs for electrochemical nitrate reduction reaction (NO₃RR), and Ti@C₇N₆ is identified as possessing the best activity, with the last proton-electron coupling step *NH₂→*NH₃ being the potential-determining step (PDS), for which the limiting potential is 0.48 V. Moreover, a free energy diagram shows that the *NOH reaction pathway is energetically preferred on Ti@C₇N₆, and ab initio molecular dynamics (AIMD) calculations at 500 K confirm its good thermal stability. Our study not only provides excellent CN-based support material but also offers theoretical guidance for constructing highly active and selective SACs for nitrate reduction.