π-π Conjugated g-C₁₆N₁₉ Skeleton-Supported Single-Atom Catalyst for Nitrogen Reduction Reaction: A "Lock and Key"...

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.