Electrocatalytic HER Enhancement of C 3 N 4 in NiCo 2 O 4 .

Generally, C 3 N 4 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 C 3 N 4 , NiCo 2 O 4 @C 3 N 4 were fabricated successfully via a simple mixture method. With the introduction of C 3 N 4 , NiCo 2 O 4 @C 3 N 4 showed better HER property with small overpotential (89 mV, η 10  mA cm -2 ), low Tafel slope (146 mV dec -1 ) and superior long-term stability, thereby indicating the excellent synergistic effect between NiCo 2 O 4 and C 3 N 4 . Besides, compared with NiCo 2 O 4 , the large electrochemically active surface areas of NiCo 2 O 4 @C 3 N 4 (ca. 126 cm 2 ) indicated the abundant active sites, directly proofing the HER enhancement of C 3 N 4 . According to Density Functional Theory (DFT) calculation results, when NiCo 2 O 4 combined with C 3 N 4 , the E H2O* of NiCo 2 O 4 @C 3 N 4 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 NiCo 2 O 4 and C 3 N 4 . Meanwhile, H 2 O interacted with the d orbital of the Co atom and induced more electrons to transfer from NiCo 2 O 4 to C 3 N 4 and H 2 O sequentially, thereby accumulating plenty of electrons on H 2 O molecule and further activating H 2 O molecule. Accordingly, the H-O-H bond angle of H 2 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 C 3 N 4 .