Synthesis of CoO-Decorated Graphene Hollow Nanoballs for High-Performance Flexible Supercapacitors

The formation of thin and uniform capacitive layers for fully interacting with an electrolyte in a supercapacitor is a key challenge to achieve optimal capacitance. Here, we demonstrate a binder-free and flexible supercapacitor with the electrode made of cobalt oxide nanoparticle (CoO NP)-wrapped graphene hollow nanoballs (GHBs). The growth process of Co(OH)₂ NPs, which could subsequently be thermally annealed to CoO NPs, was monitored by in situ electrochemical liquid transmission electron microscopy (TEM). In the dynamic growth of Co(OH)₂ NPs on a film of GHBs, the lateral formation of fan-shaped clusters of Co(OH)₂ NPs spread over the surface of GHBs was observed by in situ TEM. This CoO-GHBs/CC electrode exhibits high specific capacitance (2238 F g^-1 at 1 A g^-1) and good rate capability (1170 F g^-1 at 15 A g^-1). The outstanding capacitive performance and good rate capability of the CoO-GHBs/CC electrode were achieved by the synergistic combination of highly pseudocapacitive CoO and electrically conductive GHBs with large surface areas. A solid-state symmetric supercapacitor (SSC), with CoO-GHBs/CCs used for both positive and negative electrodes, exhibits high power density (6000 W kg^-1 at 8.2 Wh kg^-1), high energy density (16 Wh kg^-1 at 800 W kg^-1), cycling stability (∼100% capacitance retention after 5000 cycles), and excellent mechanical flexibility at various bending positions. Finally, a serial connection of four SSC devices can efficiently power a red light-emitting diode after being charged for 20 s, demonstrating the practical application of this CoO-GHBs/CC-based SSC device for efficient energy storage.