Electrochemical and physicochemical properties of redox ionic liquids using electroactive anions: influence of alkylimidazolium chain length

• NTf2 anion commonly used in ionic liquids is modified with ferrocene.
• Combining Fc-modified NTf anion with alkylimidazolium provides redox ionic liquids.
• Longer alkyl chains increase viscosity and decrease conductivity.
• Electrochemistry in pure phases shows transport by diffusion and migration.
• A film is formed upon oxidation of the Fc-anion, which is removed by reduction.

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 [CxCyIm][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 [CxCyIm][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-NTf2 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 [CxCyIm][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.