π-π Conjugated g-C16N19 Skeleton-Supported Single-Atom Catalyst for Nitrogen Reduction Reaction: A "Lock and Key" Coevaluation of Activity and Selectivity Involved in Constant Potential and pH Effect.

The efficient development of electrocatalysts for the nitrogen reduction reaction (NRR) under ambient conditions has been greatly challenging due to the high stability of the N≡N bond and the competitive interference from the hydrogen evolution reaction (HER). This work innovatively designs a graphitic carbon nitride substrate (g-C₁₆N₁₉) with a super-large pore structure based on the self-doping strategy and first-principles calculations, and constructs TM@g-C₁₆N₁₉ (TM = Ti ∼ Au) single-atom catalyst systems. By establishing a "Four-Step" screening model, it is found that W@g-C₁₆N₁₉ exhibits the best NRR catalytic performance, with an ultralow limiting potential (U_L) of -0.23 V. Combining electron structure analyses such as charge density difference, ICOHP value, Bader charge, and spin magnetic moment, the micro-mechanism of W@g-C₁₆N₁₉ effectively activating the σ/π bonds of N₂ molecules through strong d-p orbital hybridization is revealed. Furthermore, based on the pH- and potential-dependent adsorption-free energy results calculated by the constant potential model, pH-potential coupling analysis shows that the configuration has a decisive influence on activity: the E-I/S-V mode dominates in alkaline media, while the E-O/S-O pathway is predominant under acidic conditions. In the E-I configuration, the energy barrier of the rate-determining step (PDS) of NRR decreases with the decrease of the electrode potential; conversely, the energy barrier of the PDS of NRR in the E-O/S-V/S-O configuration increases with the decrease of the electrode potential.