Supercapacitors are used as an alternative power source for rechargeable batteries due to their efficient operation at high power density, long cycle life and improved safety. Nevertheless, the limited energy density, typically of the order of 5-8 Wh/L, limits their widespread use for many practical applications. Boosting capacitance and extending window of cell voltage are the available options to impart further improvement in their energy density. Researchers at University of Waterloo, Canada and Jain University, Bangalore, India have proposed a novel approach towards the development of high voltage super capacitors with high energy density (ACS Nano, 2017, 11 (10), pp 10077–10087).
Ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) and Tween 20 (nonionic surfactant) were mixed together to obtain a stable microemulsion with nanometer sized particles. Upon mixing it with graphene oxide (GO), the surfactant stabilized microemulsion spontaneously adsorbs on the surface of GO. This dispersion was directly casted onto copper with the formation of a dense nanocomposite film of GO/IL/Tween 20. Subsequent thermal treatment leads to the removal of IL by evaporation and reduction of GO to reduced graphene oxide (rGO). The resultant electrode is referred as IL-mediated reduced graphene oxide (IM-rGO).
Fig. 1 Schematic of fabrication of IM-rGO electrode assembly: (a) spontaneous adsorption of surfactant stabilized microemulsion particles on the surface of GO; (b) enlarged view of EMImTFSI/Tween 20/H2O microemulsion particle; (c) film structure after drop-casting and water evaporation; and (d) film structure after evaporation of Tween 20 following thermal reduction
The surface morphology of the nanocomposite film reveals the presence of macropores (Fig. 2(a)) due to evaporation of water and Tween 20 during thermal treatment. Morphology at the cross-section indicates a layered structure (Fig. 2(b)), in which the sheets lay parallel to the current collector, thus providing a relatively high bulk density.
Fig. 2 Morphology of the IM-rGO film fabricated using 60% IL: (a) at the surface; and (b) at the cross-section
The electrochemical performance of the IM-rGO electrode fabricated using 60 wt% of IL at RT is depicted in Fig. 3. The formation of a dense film enabled a CV of 218 F/cm3. This electrode offered a maximum energy density of 45 Wh/L at a power density of 571.4 W/L and maintained a high energy density of 21.7 Wh/L at a power density as high as 6.04 kW/L at RT.
Fig. 3 Electrochemical performance of 60% IL electrodes at RT: (a) CVs and (b) GCDs for IM-rGO at RT; (c) specific capacitance at varying current density
Eliminating the macropores of the film still remains a challenge and elimination of macropores would help to achieve even higher bulk density. The easy adoptability of the proposed methodology provides new avenues for the manufacturing of large-scale supercapacitors.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Zimin She et al., ACS Nano, 2017, 11 (10), pp 10077–10087
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