The electrocatalytic HER enhancement of C3N4 in NiCo2O4.

Generally, C3N4 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 C3N4, NiCo2O4@C3N4 were fabricated successfully via a simple mixture method. With the introduction of C3N4, NiCo2O4@C3N4 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 NiCo2O4 and C3N4. Besides, compared with NiCo2O4, the large electrochemically active surface areas of NiCo2O4@C3N4 (ca. 126 cm2) indicated the abundant active sites, directly proofing the HER enhancement of C3N4. According to Density Functional Theory (DFT) calculation results, when NiCo2O4 combined with C3N4, the EH2O* of NiCo2O4@C3N4 decreased from 3.80 to 2.76 eV and, accordingly, the heterostructure possessed an appropriate ΔGH* (0.19 eV, close to Pt), suggesting the unique synergistic effect between NiCo2O4 and C3N4. meanwhile, H2O interacted with the d orbital of the Co atom and induced more electrons to transfer from NiCo2O4 to C3N4 and H2O sequentially, thereby accumulating plenty of electrons on H2O molecule and further activating H2O molecule. Accordingly, the H-O-H bond angle of H2O 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 C3N4.