Designing 3d Transition‐Metal Doped g‐C ₃ N ₄ Monolayer for Enhanced Bifunctional Oxygen Evolution/Reduction Reactions Activity: Defect Physics and Constant‐Potential Study

ABSTRACT Graphitic carbon nitride (g‐C 3 N 4 ) doped with 3 d transition metals (3 d ‐TM@g‐C 3 N 4 ) has gained attention as a noble metal‐free alternative for oxygen evolution and reduction reactions (OER/ORR). Yet the key mechanisms driving its performance remain debated, especially across different charge states. In this study, we investigate the electrocatalytic performance of 3 d ‐TM@g‐C 3 N 4 (TM = V, Cr, Mn, Fe, Co, Ni, and Cu) in different charge states using a defect physics method based on density functional theory (DFT). We find that 33 unique charge states of 3 d ‐TM@g‐C 3 N 4 are thermodynamically stable. Among them, the Co‐substituted nitrogen site in the +1 charge state (Co N 1+ @C 54 N 71 , η OER / η ORR = 0.57/0.50 V) and the Ni interstitial site in the +1 charge state (Ni int 1+ @C 54 N 72 , η OER / η ORR = 0.43/0.35 V) exhibit lower overpotentials ( η ). Beyond the choice of dopant, key factors influencing the OER/ORR activity of 3 d ‐TM@C 3 N 4 include formation energy, charge state, and the Fermi level of the defective system. Furthermore, machine learning results reveal that the first ionization energy and the atomic radius of TMs are critical descriptors for predicting η OER and η ORR , respectively. Constant‐potential DFT calculations further confirm that Co N @C 54 N 71 and Ni int @C 54 N 72 exhibit excellent bifunctional activity at pH = 0 and U = 0 V versus RHE, with η OER / η ORR values of 0.71/0.57 V and 0.72/0.56 V, respectively—demonstrating strong potential for experimental synthesis. The study proposes a new category of bifunctional oxygen catalysts and introduces an innovative approach to optimize their electrocatalytic performance by engineering charge states and electronic structures.