Charge-State-Regulated Transition Metals Doped g-C3N3 for Electrochemical Nitrogen Reduction Reaction: Defect Physics and Constant Potential Study

Abstract

Semiconductor single-atom catalysts (SACs) show great potential in electrochemical nitrogen reduction reactions (NRR). However, previous studies often overlook the charge state of catalysts, thereby limiting the predictive accuracy of their electrocatalytic behavior. In this study, the NRR performance of 4d and 5d transition metal (TM) doped on g-C3N3 monolayer (TM@g-C3N3) is investigated by considering distinct charge states (q). Using density functional theory calculations and machine learning, we evaluated the N2 adsorption energy, NRR catalytic activity, NRR selectivity, and thermodynamic stability to screen the candidate electrocatalysts. Six charge-state-specific systems exhibit low limiting potentials (UL): Nbint0@g-C3N3 (−0.38 V), Nbint2+@g-C3N3 (−0.12 V), Taint0@g-C3N3 (−0.24 V), Taint1+@g-C3N3 (−0.37 V), Wint0@g-C3N3 (−0.37 V), and Wint1+@g-C3N3 (−0.34 V). Gradient-boosted regression and symbolic regression models indicate that the N≡N bond length strongly correlates with NRR activity and can be effectively tuned by the magnetic moment (μTotal) and the d-band center (εd) of TM@g-C3N3. Significantly, the charge state can induce μTotal and εd redistribution, thus improving the NRR performance. Further constant potential implicit solvent model under pH = 7 and U = 0 V vs. RHE (reversible hydrogen electrode) shows that the UL of Nbint@g-C3N3, Taint@g-C3N3, and Wint@g-C3N3 are −0.30 V, −0.28 V, and −0.30 V, respectively, indicative of excellent NRR activity if synthesized experimentally. These findings provide new insights into charge-state-regulated catalysis and contribute to the rational design of efficient SACs for electrochemical NH3 production.