Effect of graphene thickness on the morphology evolution of hierarchical NiCoO2 architectures and their superior supercapacitance performance

The rational design of electrode materials with special hierarchical architectures which possess both high surface area and conductivity is significant to enhance the performance of supercapacitors. Herein, hierarchical NiCoO2 architectures assembled by ultrathin mesoporous nanosheets are in-situ grown on graphene@Ni foam (G@NF) by a template-free solvothermal route and subsequent annealing process, which is used as self-supported and binder-free supercapacitor electrodes. The effect of graphene thickness on morphology evolution of NiCoO2 is investigated. Benefiting from the synergistic effect between graphene with remarkable conductivity and hierarchical NiCoO2 architectures with high specific capacity, the NiCoO2/G@NF electrodes show greatly improved electrochemical performance compared to NiCoO2@NF and G@NF. The optimized NiCoO2/G@NF has a specific capacitance of 1220 F/g at 1 A/g. While the NiCoO2@NF and G@NF are only 565 and 151 F/g, respectively. The optimized NiCoO2/G@NF remains 840 F/g at 20 A/g, revealing a remarkable rate performance (69% capacity retention from 1 to 20 A/g). Moreover, an outstanding cyclic stability of 80% capacitance retention can be obtained after 5000 charge/discharge cycles at 10 A/g, whereas the NiCoO2@NF is only 46.5%. These results suggest that the hierarchical NiCoO2 architectures/graphene hybrids are good candidates as effective electrode materials for supercapacitors.