Rationale for the implementation of reference electrodes in ionic liquids

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.