Ionic Liquids as Electrolytes for Reduced Graphene Oxide Based Supercapacitors

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Authors
Hawrylow, Matthew J.
Keyword
Supercapacitor , Graphene , Hydrogel , Ionic Liquid
Abstract
Supercapacitors comprised of graphene hydrogel electrodes and ionic liquid electrolytes are prepared and characterized in terms of material properties and device performance. Thermal reduction of aqueous graphene oxide dispersions yields poroelastic hydrogels with the surface area required of an electric double layer capacitor (EDLC), while the use of imidazolium tetrafluoroborate ionic liquid (IL) provides much wider electrochemical stability window than conventional aqueous based electrolytes. The study focuses on the influence of a poly(ionic liquid) (PIL) on the properties of an IL electrolyte as well as the performance of an EDLC device. Of particular interest is the relationship between the viscosity and the ionic conductivity of an IL electrolyte. In general, those ILs that provide high ion conductivity have low fluid viscosity, which can complicate the design of flexible EDLCs. However, small amounts of a high molecular weight PIL are shown to affect IL viscosity disproportionately over ionic conductivity. Adding 5 wt% of poly(vinyl butyl imidazolium) tetrafluoroborate to 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF4) increases the IL viscosity by 328% at the expense of only a 20% decline in conductivity. The higher viscosity provided by a PIL+ IL solution alters the stress response of a hydrogel when it is subjected to compression, as it provides increased resistance to flow of electrolyte from hydrogel pores. In addition, the more viscous medium provides better elastic recovery when a hydrogel is subjected to repeated compression. The addition of PIL to the electrolyte of a graphene-based hydrogel supercapacitor results in a 20% decrease in capacitance, due to ionic conductivity losses. As such, EDLC design requires optimization of capacitance at the expense of mechanical robustness. Compression of a flexible supercapacitor increases performance by reducing equivalent series resistance, giving a maximum capacitance of 133 F/g at 1A/g and 62.5% strain. However, at 87.5% strain densification of the hydrogel leads to a reduction in effective surface area and an 18% decrease in capacitance.
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