Charge-state-regulated transition metal doped g-C3N3 for the electrochemical nitrogen reduction reaction: defect physics and constant potential study

Abstract

Semiconductor single-atom catalysts (SACs) show great potential in the electrochemical nitrogen reduction reaction (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 the 4d and 5d transition metal (TM) doped 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 candidate electrocatalysts. Six charge-state-specific systems exhibit low limiting potential (U_L): Nb_int⁰@g-C₃N₃ (−0.38 V), Nb_int²⁺@g-C₃N₃ (−0.12 V), Ta_int⁰@g-C₃N₃ (−0.24 V), Ta_int¹⁺@g-C₃N₃ (−0.37 V), W_int⁰@g-C₃N₃ (−0.37 V), and W_int¹⁺@g-C₃N₃ (−0.34 V). Gradient-boosted regression and symbolic regression models indicate that the NN bond length strongly correlates with NRR activity and can be effectively tuned by the magnetic moment (μ_total) and the d-band center (ε_d) of TM@g-C₃N₃. Significantly, the charge state can induce μ_total and ε_d redistribution, thus improving the NRR performance. Furthermore, the constant potential implicit solvent model at pH = 7 and U = 0 V vs. RHE (reversible hydrogen electrode) shows that the U_L of Nb_int@g-C₃N₃, Ta_int@g-C₃N₃, and W_int@g-C₃N₃ is −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.