Supercapacitors, also known as electric double-layer capacitors (EDLCs), are able to store energy rapidly and reversibly through the formation of a double-layer of electronic and ionic charge, closely spaced, at the electrode/electrolyte interface.^[1-2] Due to the combination of various advantageous properties, such as efficient operation at high power density, long cycle life and improved safety compared to Li-ion batteries,^[3,4] supercapacitors are being increasingly used as alternative power sources to rechargeable batteries. However, the implementation of supercapacitors in practical application is still restricted by the limited energy density, typically 5-8 Wh/L,^[5] which is much lower than that of lead-acid batteries which can achieve ~50-90 Wh/L.^[6] Considering a symmetric configuration, the volumetric energy density (E_V) of a supercapacitor is directly proportional to the volumetric capacitance (C_V) of a single electrode and the square of operating voltage (U) following the equation, Ev = 1/8(C_V·U²). Therefore, in such a system, there are two ways to improve energy density: boosting capacitance and extending cell voltage window.

The operating voltage of EDLCs is typically limited by the stability of electrolyte and thus room temperature ionic liquids (ILs) with large electrochemical stability windows (> 3-4 V) have become promising next-generation electrolytes. However, the relatively high viscosity of ILs results in lower ionic conductivity compared to traditional aqueous or organic electrolytes and also leads to challenges with pore wetting. On the electrode side, materials with a high intrinsic capacitance (C_DL) per area and a large ion-accessible surface area (SSA) are needed to achieve high gravimetric capacitance (C_G) since C_G = C_DL·SSA. The potentially high electrical conductivity, surface area, and chemical stability of graphene-based materials make them promising candidate electrode.^[7,8] Theoretically, single layer graphene can exhibit SSA as high as 2675 m²/g. While pristine graphene is limited by its low quantum capacitance leading to C_DL ~3-4 mF/cm², more defective and functionalized graphene produced by the chemical or thermal reduction of graphene oxide (GO) have been shown to exhibit C_DL > 17 mF/cm² in non-aqueous electrolyte leading to theoretical gravimetric capacitance, C_{G,theoretical} > 450 F/g if all of graphene’s surface area could be made ion-accessible.

To prevent aggregation and restacking of graphene-based materials into lower SSA structures, we demonstrated an IL microemulsion system (Figure 1) that spontaneously assembles on the surface of GO, placing nanometer-sized droplets of a high-performance, hydrophobic IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) directly onto the available surface of well-dispersed single GO layers. We first demonstrate that a common non-ionic surfactant, Tween 20 is capable of forming a stable microemulsion with EMImTFSI with a particle size on the order of several nanometers. These surfactant stabilized nano-droplets (microemulsion particles) spontaneously adsorb to GO sheets yielding a dispersion which can be cast directly onto current collectors leading to a dense nanocomposite of GO/IL/Tween 20. Tween 20 is then removed by evaporation, while the GO is thermally reduced leading to what we refer to as layered IL-mediated reduced GO (IM-rGO) electrodes. The approach is found to yield high ion-accessible SSA as evidenced by one of the highest C_G ever reported (302 F/g) when the composite contains 80 wt% IL. These results indicate that the microemulsion particles formed were better able to deploy IL as a spacer to prevent rGO sheets from restacking. Reducing the IL content to 60 wt%, resulted in dense electrodes that exhibited a C_V = 218 F/cm³, which is the highest value reported to date among all graphene-based supercapacitors leading to exceptional volumetric energy density.

Reference

(1) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. Springer Science & Business Media 2013.

(2) Miller, J. R.; Simon, P. Electrochemical capacitors for energy management. Science 2008, 321, 651.

(3) Du Pasquier, A.; Plitz, I.; Menocal, S.; Amatucci, G. A comparative study of Li-ion battery, supercapacitor and nonaqueous asymmetric hybrid devices for automotive applications. J. Power Sources 2003, 115, 171.

(4) Khaligh, A.; Li, Z. Battery, ultracapacitor, fuel cell, and hybrid energy storage systems for electric, hybrid electric, fuel cell, and plug-in hybrid electric vehicles: State of the art. IEEE transactions on Vehicular Technology 2010, 59, 2806.

(5) Burke, A. R&D considerations for the performance and application of electrochemical capacitors. Electrochim. Acta 2007, 53, 1083.

(6) Linden, D. Handbook of batteries. Fuel and Energy Abstracts 1995, 265.

(7) Pope, M. A.; Aksay, I. A. Four-Fold Increase in the Intrinsic Capacitance of Graphene through Functionalization and Lattice Disorder. J. Phys. Chem. C 2015, 119, 20369.

(8) Stoller, M. D.; Park, S.; Ruoff, R. S. Graphene-based ultracapacitors. Nano Lett. 2008, 8, 3498.