Electrocatalytic HER Enhancement of C3N4 in NiCo2O4

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 EH2O* of NiCo2 O4 @C3 N4 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 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 .