Rationale for the implementation of reference electrodes in ionic liquids

Abstract

A thorough investigation of the reference electrodes for electrochemical measurements is presented in three ionic liquids (ILs), namely 1-butyl-3-butylimidazolium bis(trifluoromethanesulfonyl)imide [BMIM][Tf₂N], 1-octyl-3-butylimidazolium bis(trifluoromethanesulfonyl)imide [OMIM][Tf₂N] and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide [BMPyrr][Tf₂N]. The influence of technical aspects such as internal resistance, scan rate and concentration of the active species on cyclic voltammetry has been investigated using three quasi-reference electrodes (QREs) (AgCl, Pt, and Pd) and three reference electrodes (REs) (Fc⁺/Fc/Pt, Ag⁺/Ag, and Cl⁻/AgCl/Ag). The results show that the internal resistance has to be taken dynamically into account during any cyclic voltammetry experiment. Quasi-reference electrodes can be used only for qualitative electrochemical studies. Reference electrodes based on Ag⁺/Ag or Fc⁺/Fc dissolved in an IL have a stable potential over 15 h and thus can be used to perform reliable electrochemical experiments in ionic liquids and to get quantitative information such as potentials.