In situ encapsulation of CoO polyhedra in graphene sheets for high-capacitance supercapacitors.

Co3O4 polyhedra were well encapsulated in reduced graphene oxide (rGO) sheets by in situ growth of Co-based zeolitic imidazolate framework (ZIF-67) polyhedra in the presence of graphene oxide followed by thermal annealing. The resultant rGO/Co3O4 composites consist of a continuously-conductive double-network constructed from graphene sheets and the derived N-doped carbons from ZIF-67, showing a large specific surface area of 523 m2 g-1. The as-fabricated symmetrical supercapacitor based on rGO/Co3O4 exhibits a high specific capacitance of 277.5 F g-1 at 25 A g-1 and an energy density of 24.7 W h kg-1 at a power density of up to 40 kW kg-1. The supercapacitor also retains 87.5% of the initial capacitance over 5000 cycles at 5 A g-1. Such large capacitance, high energy density, and excellent cycling stability for rGO/Co3O4 are attributable to the 3D double conductive network from 2D graphene sheets and porous channels of pseudo-capacitive Co3O4 polyhedra.