Heptazine vs Triazine: Phase-Dependent Activity of V-Doped C3N4 Catalysts in Electrochemical N2 Reduction.

Electrochemical nitrogen reduction reaction (ENRR) offers a sustainable pathway for ambient ammonia synthesis, yet its practical implementation is hindered by low efficiency and selectivity. Herein, we report a phase-engineered strategy for tailoring the structure of vanadium-doped graphitic carbon nitride (V@C3N4) catalysts. By modulating the molecular precursors, we synthesized heptazine- and triazine-based frameworks and demonstrated that the heptazine-based V@C3N4 achieves a threefold higher NH3 yield of 4.605μg·h-1·mgcat-1 at -0.3 V vs RHE, with a Faradaic efficiency of 10.7%, compared to its triazine counterpart. Density functional theory (DFT) calculations reveal that vanadium binds more stably to heptazine units and facilitates nitrogen activation through a lower energy barrier and a stronger donor-acceptor interaction. Charge-density and electronic-structure analyses confirm the back-donation from vanadium 3d to N2 π2p orbitals, validating the proposed mechanism. Our results highlight the synergistic role of structural engineering and atomic-level doping in the design of efficient ENRR catalysts.