The design and development of highly efficient and durable non-precious metal catalysts are of great significance for the HER and the OER, especially bifunctional catalysts for overall water splitting. Here, a series of novel composites of 3D nitrogen-doped graphene foams with homogeneously dispersed CoS1.097 nanoparticles (NPs) are synthesized through hydrazine-induced foaming and thermal-treatment process at different temperatures. Notably, the composite thermally treated at 750
C (CoS1.097/NGF-750) which possesses continuous 3D porous structures exhibits super-hydrophilic properties and ultrahigh catalytic activity, providing a current density of 10 mA cm2 at a low overpotential of 240 mV for the OER and 124 mV for the HER, respectively. In addition, a self-supported two-electrode electrolyzer

assembled using CoS1.097/NGF-750 as both the cathode and anode exhibits the current densities of 10 and 20 mA cm2 at potentials of 1.56 V and 1.65 V, respectively, and long-term stability (95% of the activity remained after 45 h). Furthermore, this novel catalyst can be produced on a large scale, therefore it could be promising for practical applications in overall water splitting