Ion transfer processes at the room temperature ionic liquid|aqueous solution interface supported by a hydrophobic carbon nanofibers – silica composite film

Abstract

A thin film of carbon nanofibers embedded into a hydrophobic sol–gel material and deposited onto tin-doped indium oxide substrate electrodes is employed as a support for liquid|liquid redox systems. The system 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide|aqueous electrolyte is studied with t -butylferrocene as an electrochemically active component in the ionic liquid phase. Redox processes within the ionic liquid are coupled to ion transfer processes at the ionic liquid|water interface. The carbon nanofiber electrode material provides an ideal porous support and allows both high Faradaic and high capacitive currents to be achieved. The capacitive current obtained after immobilisation of 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and immersion into aqueous electrolyte indicates that most (ca 65%) of the real carbon nanofiber surface is active, electrically connected to the substrate electrode, and in direct contact with the ionic liquid. The reversible oxidation and re-reduction of t -butylferrocene in 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide occurs at a potential which depends on the composition and nature of the aqueous electrolyte as well as on the type of electrode. The position of differential pulse voltammetric oxidation peak potentials is shown to depend on the type of anion present in the aqueous solution phase with most significant effects for the hydrophobic ClO 4 - and PF 6 - anions. Although only the Faradaic voltammetric response allows the ion transfer mechanism to be analysed, anion transfer is proposed to be coupled to both Faradaic and capacitive charge transfer.