Carbon dots decorated zinc cobaltite nanowires-assembled hierarchical arrays supported on nickel foam as binder-free electrodes for high performance supercapacitors

Hierarchical and binder-free electrode materials with large electroactive sites and low contact resistance are benefit for electrochemical capacitors. Herein, this paper reports the binder-free and hierarchical ZnCo2O4 arrays decorated with carbon dots (CDs/ZnCo2O4) on nickel foam by hydrothermal route. The CDs/ZnCo2O4 arrays exhibit the nanowires-assembled hierarchical grass-like structure. The outward directed nanowires yield high surface-to-volume ratio. The nanowires exhibit the average length of 1 μm and diameter of 50 nm. The specific capacitance of the CDs/ZnCo2O4 can reach 1937 F/g higher than the 1192 F/g of bare ZnCo2O4 at 1 A/g. And 78.5% capacitance retention rate is achieved while the current density increases from 0.5 to 8 A/g. 90% of the capacitance retention over 5000 charge-discharge cycles at 15 A/g indicates high cycling stability. These boosted electrochemical performances are attributed to the structural feature with exposed electroactive sites, high electrical conductivity assisted by CDs, and opened interspaces for electrolyte ion diffusion. Asymmetric two-electrode system assembled with CDs/ZnCo2O4 and activated carbon delivers a high-energy density of 54.94 W h/kg at 425.3 W/kg and retains an energy density of 13.23 W h/kg at 6804 W/kg. This study provides a new approach to design high-performance electrode materials for supercapacitors.