A systematic investigation of the catalytic performances of monolayer carbon nitride nanosheets C1−xNx

Abstract

Graphitic carbon nitrides (CNs) are potential candidate materials for the electro-catalytic industry due to their unique physical and chemical properties. However, to date, a full understanding of the electro-catalytic properties of CNs is still lacking. Herein, by using density functional theory calculations, we systematically investigate the catalytic performances in the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), N₂ reduction reaction (NRR), and CO₂ reduction reaction (CO₂RR) of monolayer graphitic carbon nitrides (C_1−xN_x), C₃N (x = 1/4), C₂N (x = 1/3), and g-C₃N₄ (x = 4/7). We also evaluated the NRR activity of B doped C_1−xN_x, and the CO₂RR activity of Cu and Pd modified C_1−xN_x. The cohesive energy and ab initio molecular dynamics (AIMD) results show that C₃N, C₂N, and g-C₃N₄ are stable at room temperature. The C₃N–C1 site is predicted to deliver the best HER catalytic performance with a reaction Gibbs free energy (ΔG_H*) of −0.03 eV (close to the ideal value (0 eV)). Among the studied C_1–xN_x materials, the C₃N–C2 site is predicted to possess a favorable η_OER of 0.82 V for OER. Pure C₃N, C₂N, and g-C₃N₄ are not suitable for NRR and CO₂RR. Due to the strong hybridization between the N 2p orbital and the B 2p orbital, the NRR performances of B doped B_N–C₂N, B_N–C₃N, and B_N–g-C₃N₄ are greatly enhanced, with corresponding overpotential η_NRR of 0.57 V, 0.70 V, and 0.72 V, respectively. The transition metals Cu and Pd can enhance the CO₂RR activity of C₃N, C₂N, and g-C₃N₄. The limiting potentials U_L of pure C₃N, C₂N, and g-C₃N₄ are 0.96 V, 0.86 V, and 2.37 V, respectively, while these values are 0.63 V, 0.68 V, and 0.77 V with Cu or Pd modification. This work provides deep understanding of the catalytic properties of monolayer C_1−xN_x and guidance for synthesizing higher activity catalysts in the future.