electrocatalysts, which contain both HER and OER activities, has advantages in terms of simplified symmetric cell design and reduced cost. [3] However, the practical applications of noble metal-based materials are still restricted due to high price, rarity, poor durability, and lopsided bifunctional activity.Recently, transition metal-based materials have been exploited as HER or OER electrocatalysts to replace existing noble metals. [4] Transitional metal sulfides (TMSs) consisting of ternary compounds exhibit the synergistic cooperation of the different components for high electrocatalytic activity. [5,6] Nevertheless, bifunctional activity and durability of TMSs still remain unsatisfactory for overall water splitting. [7] Moreover, the binderfree self-supported configuration can reduce the contact resistance between the current collector and the electrocatalysts, minimizing the loading of inactive species. [6,8] Herein, we design vertically aligned NiCo 2 S 4 nanosheets deposited on nitrogen-incorporated reduced graphene oxide (N-rGO) coated nickel foam (NCS@N-rGO/NF) for overall water splitting. The resulting binder-free free-standing electrodes exhibit the vertical growth of NiCo 2 S 4 nanosheets on the modified surface of the N-rGO/Ni foam, which ensures the exposure of the active sites and facile charge transfer. [9] Moreover, a rapid mass transport and facile access to active sites is accomplished by 3D interconnected porous structure. The electrocatalytic activity can be further improved due to the synergistic effects of the interaction between TMS with rGO in a hierarchical structure. [10] Furthermore, heteroatom-doped carbon materials such as N-rGO can improve the electron conductivity and electrolysis durability of TMSs. [11]
Result and DiscussionThe binder-free NCS@N-rGO/NF electrodes were synthesized in three-steps as shown in Figure 1. First, rGO was coated on NF using a hydrothermal method at 393 K for 2 h. Then, N atoms were incorporated to fabricate N-rGO/NF electrode through chemical vapor deposition (CVD) using urea as N precursor. The CVD was performed at 623 and 773 K, successively, where urea is gasified at 623 K and adsorbed on rGO/NF, then N doping is done at 773 K. Finally, NiCo 2 S 4 nanosheets Developing non-noble metal electrocatalysts with high catalytic activity and stability is very important for practical water splitting technology. Herein, electrocatalytically active, bifunctional, and durable electrodes, are reported where NiCo 2 S 4 nanosheets deposited onto nitrogen-incorporated reduced graphene oxide (N-rGO) are assembled onto 3D Ni foam. These freestanding binder-free electrodes exhibit excellent bifunctional catalytic activity for both hydrogen evolution and oxygen evolution reactions in alkaline condition, as well as overall water splitting reaction measured in a two-electrode configuration. The cell voltage is as low as 1.63 V to achieve a current density of 10 mA cm −2 and this value is well preserved with negligible overpotential for more than 50 h. The results open ...