One-step synthesis of surface-enriched nickel cobalt sulfide nanoparticles on graphene for high-performance supercapacitors

Binary metal sulfide@ graphene composites have held great promising as electrode materials for supercapacitors (SCs). However, synthesis of nanosized binary metal sulfides that uniformly distribute on graphene in a simple way is still an enormous challenge. Herein we report a one-step solvothermal method employing poly (acrylic acid) (PAA) additive to fabricate well-dispersed nickel-cobalt sulfide nanoparticles on graphene (marked as NiCo-S@ G) used in both traditional and flexible supercapacitors. The Ni-Co-S@ G traditional electrode exhibits high specific capacitance at high rate (1021 F g(-1) at 20 A g(-1)) and high capacitance retention (92.1% after 5000 cycles at 10 A g(-1)). Such outstanding performances are benefited from its unique integrated nanostructure which offers rich electroactive surface sites as well as favourable conductivity. Furthermore, asymmetric supercapacitor (ASC) of Ni-Co-S@ G//reduced graphene hydrogels (RGH) delivers high energy density (39.5 Wh kg(-1) at power density of 1778 W kg(-1)) as well as excellent cycle stability (84.4% capacitance retention after 15,000 cycles). Besides, binder-free flexible electrode with satisfied electrochemical properties was successfully fabricated by directly growing Ni-Co-S@ G on active carbon fibre cloth (ACC). This work provides a facile and effective way to optimize the capacitive performances of binary metal sulfide@ graphene composites,