Rational design of nickel-cobalt sulfide nanorods grown on graphene with high performance for supercapacitors

Nickel cobalt sulfides have exhibited great promise as supercapacitor electrode materials owing to the abundant redox states and considerable theoretical capacities. However, they still suffer from sluggish reaction kinetics, resulting in undesirable capacitance, cyclic and rate performances. Herein, electrochemical exfoliated graphene with high dispensability was utilized as the matrix and uniform nickel-cobalt sulfide nanorods grown on graphene (Ni-Co-S/G) was achieved by a facile two-step strategy for enhanced capacitive performances. This wellconstructed Ni-Co-S/G can provide plentiful active sites for electrochemical reaction, offer more efficient and rapid electron/ion transfer pathways during the charge/discharge process, and the tight contact of nickel-cobalt sulfide nanorods and graphene can improve the stability. Consequently, the obtained Ni-Co-S/G electrode exhibits a high specific capacity of 1579.68 F g-1 at 1 A g-1, splendid rate capability (1240 F g-1 at 20 A g-1) and excellent cycling stability with capacity retention of 91.5 % after 5000 cycles at 5 A g-1. Additionally, the assembled asymmetric supercapacitor (ASC) device Ni-Co-S/G//AC with Ni-Co-S/G and commercial active carbon as positive and negative electrode displays an energy density of 75.3 W h kg-1 at the power density of 1125 W kg-1, still retaining 50.5 Wh kg-1 even at 16.9 kW kg-1. This work provides a potential strategy to design high-performance nickel-cobalt sulfides electrode for advanced supercapacitors.