Theoretical Insights on the Charge State and Bifunctional OER/ORR Electrocatalyst Activity in 4d-Transition-Metal-Doped g-C3N4 Monolayers

Exploring efficient and stable electrocatalysts for the bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is vital to developing renewable energy technologies. However, due to the substantial and intricate design space associated with these bifunctional OER/ORR electrocatalysts, their development presents a formidable challenge, resulting in their cost-prohibitive nature in both experimental and computational studies. Herein, using the defect physics method, we systematically investigate the formation energies and bifunctional overpotential (η_Bi) of 4d-transition-metal (4d-TM, 4d-TM = Zr, Nb, Mo, Ru, Rh, Pd, and Ag)-doped monolayer supercell g-C₃N₄ (4d-TM@C₅₄N₇₂) based on the density functional theory (DFT) calculations. Under N-rich and C-rich conditions, we find that the formation energies of Rh_N@C₅₄N₇₁ (Rh occupation N) and Pd_N@C₅₄N₇₁ (Pd occupation N) are smaller than that of other 4d-TM_N@C₅₄N₇₁ (4d-TM occupation N site); for the 4d-TM_int@C₅₄N₇₂ (4d-TM interstitial site occupation), the lowest-formation energy defects are Pd_int@C₅₄N₇₂. These results indicate that they have better stabilities. Interestingly, for these formation energy lower systems, Pd⁰_int@C₅₄N₇₂ (η_Bi = 1.00 V) and Rh¹⁺_N@C₅₄N₇₁ (η_Bi = 0.73 V) have ultralow overpotential and can be great candidates for bifunctional OER/ORR electrocatalysts. We find the reason is that adjusting the charge states of 4d-TM@C₅₄N₇₂ can tune the interaction strength between the oxygenated intermediates and the 4d-TM@C₅₄N₇₂, which plays a crucial role in the activity of reactions. Additionally, the data obtained through machine learning (ML) application suggest that the electronegativity (N_m) and bond length of 4d-TM and coordination atoms (d_TM-OOH) are primary descriptors characterizing the OER and ORR activities, respectively. The charged defect tuning of the bifunctional OER/ORR activity for 4d-TM@C₅₄N₇₂ would enable electrocatalytic performance optimization and the development of potential electrocatalysts for renewable energy applications.