(Invited) Nanostructured Graphene Electrodes for High Performance and Stretchability

Abstract

Chemically derived graphene oxide (GO) and its derivatives are promising candidates for a variety of carbon-based functional nanostructures and hybrid architectures, because of their unique characteristics that include high theoretical surface area, tunable electrical conductivity, excellent solution processability, and high mass producibility at low cost. However, the irreversible stacking due to the strong π–π interactions between graphene nanosheets during drying or reduction processes significantly decreases the solution processability as well as accessible surface area. In this talk, I firstly present a sea-urchin-shaped template approach for fabricating highly crumpled graphene balls in bulk quantities with a simple process. Simultaneous chemical etching and reduction process of graphene oxide (GO)-encapsulated iron oxide particles results in dissolution of the core template with spiky morphology and conversion of the outer GO layers into reduced GO layers with increased hydrophobicity which remain in contact with the spiky surface of the template. After completely etching, the outer graphene layers are fully compressed into the crumpled form along with decrease in total volume by etching. The crumpled balls exhibit significantly larger surface area and good water-dispersion stability than those of stacked reduced GO or other crumple approaches, even though they also show comparable electrical conductivity. Furthermore, they are easily assembled into 3D macroporous networks without any binders through typical processes such as solvent casting or compression molding. The graphene networks with less pore volume still have the crumpled morphology without sacrificing the properties regardless of the assembly processes, producing a promising active electrode material with high gravimetric and volumetric energy density for capacitive energy storage. For the second part, using a facile ice-templated self-assembly process with reduced graphene sheets and vanadium phosphate (VOPO4) nanosheets, we realize a vertically porous nanocomposite of layered VOPO4 and graphene nanosheets with high surface area and high electrical conductivity. The resulting 3D VOPO4–graphene nanocomposite has a much higher capacitance of 527.9 F g–1 at a current density of 0.5 A g–1, compared with ~247 F g–1 of simple 3D VOPO4, with solid cycling stability. The enhanced pseudocapacitive behavior mainly originates from vertically porous structures from directionally grown ice crystals and simultaneously inducing radial segregation and forming inter-stacked structures of VOPO4–graphene nanosheets. This VOPO4–graphene nanocomposite electrode exhibits high surface area, vertically porous structure to the separator, structural stability from interstacked structure and high electrical conductivity, which would provide the short diffusion paths of electrolyte ions and fast transportation of charges within the conductive frameworks. In addition, an asymmetric supercapacitor (ASC) is fabricated by using vertically porous VOPO4–graphene as the positive electrode and vertically porous 3D graphene as the negative electrode; it exhibits a wide cell voltage of 1.6 V and a largely enhanced energy density of 108 Wh kg–1. In the last part, we also used the ice-templated vertically porous graphene nanostructures as a stretchable supercapacitor electrode. Radially compressed honeycomb structures exhibited nearly-zero poission ratio structures and maintained their structure and electrical conductivity even at 50 % of starched states. The capacitive performance of these compressed honeycomb structures also shows fairly high over 130 F g-1 and these high performance still be maintained at highly stretched condition.