Hierarchical Nanohybrids with Graphene Tubes-Networks Decorated Microspherical Graphene Assemblies for Supercapacitors
Authors: Kim Young Hwan , Park Byung Hoon , Lee Geon-Woo , Kim Kwang-Bum 
Published: May 2020

Graphene belongs to an emerging class of ultrathin carbon membrane materials with a high specific surface area, chemical stability, and high electrical and thermal conductivities. Owing to these intrinsic physicochemical characteristics, graphene has been extensively investigated for widespread applications in nanodevices, sensors, catalysis, energy storage systems, and biomedicine as an alternative to porous carbon. In particular, graphene has received increasing attention as an electrode material for electrical double-layer capacitors (EDLCs) due to its high specific surface area and high intrinsic electrical conductivity. Recently, there have been tremendous achievements for improving the gravimetric capacitance of graphene.

However, due to the 2D nature of the graphene sheet, graphene can easily restack to form lamellar microstructures on the current collector during the electrode fabrication process. The restacking of the graphene sheets may greatly reduce the utilization of the electrode material and limit the electron and mass transport at the interface of electrode, which as a result leads to decreased capacitor performance.

Taking these points into consideration, one strategy for preventing the restacking of graphene sheets is to use CNTs as nanospacers, given their high electrical pathways and large surface area. Recently, several approaches have been reported for fabricating graphene/CNT composites. The most popular approach for fabricating graphene/CNT composites is the preparation of an aqueous solution of GO and CNTs with the use of a surfactant such as sodium dodecylbenzenesulfonate (SDBS) to improve the dispersibility of CNTs, followed by vacuum filtration or a hydrothermal process. Another approach is the use of cationic surfactant polymers such as cetyltrimethylammonium bromide (CTAB) and polyethyleneimine (PEI) to induce electrostatic attraction by introducing a surface charge on CNTs or graphene. However, the use of insulating polymers is not desirable, since it is difficult to completely remove the surfactant polymers, and the amorphous carbon formed after heat treatment of the polymer under an inert atmosphere could deteriorate the electrical conductivity. Furthermore, such composites showed a marginal improvement of electrochemical properties.

Another points, to further explore the macroscopic applications of graphene, an essential prerequisite is the controlled large-scale assembly of twodimensional (2D) graphene building blocks and the transfer of their inherent properties into three-dimensional (3D) structures with a high packing density. Furthermore, graphene needs to be assembled into a microsized powder form considering the powder morphology of activated carbon, which is currently used as an electrode material for EDLCs. Despite the recent progress in realizing graphene assemblies using well-established strategies such as vacuum filtration, layer-by-layer assembly, Langmuir–Blodgett assembly, hydrothermal assembly followed by oven drying, and spray drying, the controllable and scalable assembly of such graphene into graphene microsized powders with a high packing density remains a significant challenge

In this regards, herein, we report the novel integrated Graphene tube @ Graphene Microsphere by using cobalt acetate and dicyandiamide. Graphene tube @Graphene Microspheres (GT@GMs) show a high gravimetric capacitance of 229.8 F g-1 at 0.1 A g-1 in 1 M TEABF4/ACN electrolyte. The GT@GMs exhibited excellent rate capabilities of 94.3 % (gravimetric capacitance at a current density of 2 A g-1 compared with gravimetric capacitance at a current density of 0.1 A g-1) with time constants in the range of 0.4 to 0.8 s owing to the favorable formation of pathway by formation of graphene tubes; this allowed the electrolyte to easily penetrate the graphene assembly and facilitated ion transport. Furthermore, all the A-GMs exhibited excellent cycling stability (95.1 % of the initial capacitance retained after 100,000 charge/discharge cycles at a current density of 2 A g–1) owing to their low oxygen content as well as structural stability originating from the compact assembly of graphene micropores. This study provides new insights regarding the introducing of 1D carbon into graphene to produce graphene-based electrode materials with both high gravimetric capacitances and rate capabilities. Detailed synthetic procedure, electrochemical properties will be discussed at the meeting.