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Ferrocenylsulfonyl(trifluoromethylsulfonyl)imide [FcNTf] is reported as a redox anionic species used to obtain an electroactive ionic liquid with a typical alkylimidazolium cation. Here, several of these redox ionic liquids (RILs) were prepared by combining FcNTf with alkylimidazolium cations in order to understand the influence of symmetry and alkyl chain length on the RIL system. The physicochemical and electrochemical properties are characterized for [C x C y Im][FcNTf], where x and y were varied between 1 and 8, both in the pure, undiluted state and in solutions of acetonitrile. In pure form, the ionic conductivity of [C x C y Im][FcNTf] was found to range between 0.22 and 0.42 mS cm −1 at 60 °C, and the alkyl chain length was found to have a similar effect on viscosity as in conventional imidazolium-NTf 2 ionic liquids. While an increase in alkyl chain length increases viscosity and decreases mass transport, it was found to have no effect on the redox potential of the ferrocene center. The cyclic voltammetry of [C x C y Im][FcNTf] solutions diluted in acetonitrile exhibited behavior which depends on concentration and on the presence of a supporting electrolyte. At concentrations above 50 wt.% and in the absence of any supporting electrolyte, the electroactive anion was found to deposit as a dense film on the electrode upon oxidation. This behavior is linked to formation of Fc + NTf − zwitterions, which accumulates in the double-layer and precipitates on the electrode surface. This study also investigates the electrochemical properties of film deposition. [ABSTRACT FROM AUTHOR]

Here we present redox ionic liquid supercapacitors (RILSCs) which use electrolytes made from ionic liquids modified with an electroactive function to increase the energy density of activated carbon electrodes via faradaic reactions. More specifically, two different ionic liquids were made by modifying either the imidazolium cation or the bis(trifluoromethanesulfonyl)imide anion with ferrocene in order to determine the importance of the electroactive ion's polarity. The functionalization of an ionic liquid with ferrocene led to high concentrations of redox moieties in the electrolyte (2.4 M) and a large maximum operating voltage (2.5 V). An energy density of up to 13.2 Wh per kg (both electrodes) was obtained which represents an 83% increase vs. the unmodified ionic liquid. When the ionic liquid's anion is modified with ferrocene, the self-discharge at the positive electrode is fully suppressed due to the deposition of a film on the electrode. The results presented herein demonstrate that electroactive ionic liquids constitute a promising alternative to conventional solute in solvent electrolytes found in energy storage devices, and are particularly well-suited for redox-active electrolyte supercapacitors. [ABSTRACT FROM AUTHOR]