Decorating Graphene Oxide with Ionic Liquid Nanodroplets: An Approach Leading to Energy-Dense, High-Voltage Supercapacitors.

A major stumbling block in the development of high energy density graphene-based supercapacitors has been maintaining high ion-accessible surface area combined with high electrode density. Herein, we develop an ionic liquid (IL)-surfactant microemulsion system that is found to facilitate the spontaneous adsorption of IL-filled micelles onto graphene oxide (GO). This adsorption distributes the IL over all available surface area and provides an aqueous formulation that can be slurry cast onto current collectors, leaving behind a dense nanocomposite film of GO/IL/surfactant. By removing the surfactant and reducing the GO through a low-temperature (360 °C) heat treatment, the IL plays a dual role of spacer and electrolyte. We study the effect of IL content and operating temperature on the performance, demonstrating a record high gravimetric capacitance (302 F/g at 1 A/g) for 80 wt % IL composites. At 60 wt % IL, combined high capacitance and bulk density (0.76 g/cm3), yields one of the highest volumetric capacitances (218 F/cm3, at 1 A/g) ever reported for a high-voltage IL-based supercapacitor. While achieving promising rate performance and cycle-life, the approach also eliminates the long and costly electrolyte imbibition step of cell assembly as the electrolyte is cast directly with the electrode material.