Evaluation of a Ag∣Ag+ reference electrode for use in room temperature ionic liquids

Interest continues to grow in the use of room temperature ionic liquids (RTILs) as electrolytes in a range of electrochemical applications, such as lithium batteries, supercapacitors and dye-sensitized solar cells. Underpinning this growth, investigations into the electrochemical behaviour of RTILs and RTIL-based systems rely on accurate and precise data on the potentials of redox processes. While most researchers have continued the practice (developed with non-aqueous solvents) of reporting potentials relative to one of the metal-organic standards (such as ferrocene), little attention has been given to the development of a reliable reference electrode, based on an ionic liquid. Such an electrode is always valuable, especially in situations where addition of a reference material is not possible.We report a Ag∣Ag+ reference electrode, incorporating a known concentration of silver trifluoromethanesulfonate (AgTf) in 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (P14TFSI), which provides a stable and reproducible reference potential. Voltammetric monitoring of the redox potentials for ferrocene and cobalticinium hexafluorophosphate have shown that the electrode Ag∣Ag+(10 mM AgTf, P14TFSI) is stable to within a millivolt, over a period of around three weeks, when used in an argon atmosphere at room temperature. Higher concentrations of silver ion reveal close-to-Nernstian behaviour. All Ag∣Ag+ configurations were significantly more stable than a silver wire quasi-reference electrode, even when the latter was separated in a salt-bridge. Voltammetric data recorded in a range of different ionic liquids, against the Ag∣Ag+ (10 mM AgTf, P14TFSI) reference electrode, produced apparent junction potentials of a few tens of mV. Changes in sign of the junction potential are usefully discussed in terms of the relative mobilities of the anions and cations present, while the magnitude can be discussed in the terms of a classic molten salt treatment.