Two-Dimensional, Porous Nickel-Cobalt Sulfide for High-Performance Asymmetric Supercapacitors.

High specific surface area, high electrical conductivity, and abundant channels have been recognized to favor pseudocapacitors, but their realization at the same time is still a great challenge. Here, we report on nickel-cobalt sulfide nanosheets (NSs) with both ultrathin thickness and nanoscale pores for supercapacitors. The porous Ni-Co sulfide NSs were facilely synthesized through micelle-confined growth and subsequent sulfuration. The NSs are as thin as several nanometers and have a large number of pores with a mean size of ∼7 nm, resulting in ultrahigh atom ratio at surface with unique chemical and electronic structure. Therefore, fast diffusion of ions, facile transportation of electrons and high activity make great synergistic contributions to the surface-dependent reversible redox reactions. In the resulted supercapacitors, a specific capacitance of 1304 F g(-1) is achieved at a current density of 2 A g(-1) with excellent rate capability that 85.6% of the original capacitance is remained at 20 A g(-1). The effects of crystallinity and self-doping are optimized so that 93.5% of the original capacitance is obtained after 6000 cycles at a high current density of 8 A g(-1). Finally, asymmetric supercapacitors with a high energy density of 41.4 Wh/kg are achieved at a power density of 414 W/kg.