Electrocatalytic HER Enhancement of C₃ N₄ in NiCo₂ O₄

Generally, C₃ N₄ was widely used in the field of photocatalytic hydrogen production, but only few studies focused on electrocatalytic hydrogen production field in Ni/Co based systems. Herein, to investigate the enhanced HER property of C₃ N₄ , NiCo₂ O₄ @C₃ N₄ were fabricated successfully via a simple mixture method. With the introduction of C₃ N₄ , NiCo₂ O₄ @C₃ N₄ showed better HER property with small overpotential (89 mV, η₁₀  mA cm^-2 ), low Tafel slope (146 mV dec^-1 ) and superior long-term stability, thereby indicating the excellent synergistic effect between NiCo₂ O₄ and C₃ N₄ . Besides, compared with NiCo₂ O₄ , the large electrochemically active surface areas of NiCo₂ O₄ @C₃ N₄ (ca. 126 cm² ) indicated the abundant active sites, directly proofing the HER enhancement of C₃ N₄ . According to Density Functional Theory (DFT) calculation results, when NiCo₂ O₄ combined with C₃ N₄ , the E_H2O* of NiCo₂ O₄ @C₃ N₄ decreased from 3.80 to 2.76 eV and, accordingly, the heterostructure possessed an appropriate ΔG_H* (0.19 eV, close to Pt), suggesting the unique synergistic effect between NiCo₂ O₄ and C₃ N₄ . Meanwhile, H₂ O interacted with the d orbital of the Co atom and induced more electrons to transfer from NiCo₂ O₄ to C₃ N₄ and H₂ O sequentially, thereby accumulating plenty of electrons on H₂ O molecule and further activating H₂ O molecule. Accordingly, the H-O-H bond angle of H₂ O was narrowed from 104.3° to 103.2°, meanwhile, the O-H bond length was extended from 0.975 to 0.989 Å, demonstrating the HER enhancement of C₃ N₄ .