Studies on Metal Complexes as Active Materials in Redox-flow Battery Using Ionic Liquids as Electrolyte: Electrochemical Properties of [Fe(L)x][Tf2N]2 (L: Multidentate Ligands, x = 2 or 3) in 1-Butyl-3-methylimidazolium Bis(Trifluoromethylsulfonyl)Imide, [BMI][Tf2N]

As part of studies for finding active materials in the redox-flow battery system using ionic liquids (ILs) as electrolyte, we have synthesized [Fe(L)x][Tf2N]2 (L = 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), or 2,2′:6′,2′′-terpyridine (terpy); Tf2N = bis(trifluoromethylsulfonyl)imide, x = 3 for bpy and phen; 2 for terpy), and examined their electrochemical properties in [BMI][Tf2N] (BMI = 1-butyl-3-methylimidazolium). From 1H-NMR and UV-vis absorption spectra, it was confirmed that three Fe(II) species exist as [Fe(L)x]2+ in [BMI][Tf2N] solution. Cyclic voltammograms (CVs) and chronoamperograms (CAs) of the [Fe(L)x]2+ species in [BMI][Tf2N] solution were measured at 25°C. In the CVs, two peaks corresponding to the redox couples of Fe(II)/Fe(III) were observed around 1 V vs. Ag/AgCl, and their redox reactions were found to be quasi-reversible. The diffusion coefficients of [Fe(L)x]2+ were estimated to be (3−9) × 10−7 cm2 s−1 by chronoamperometry. The standard rate constants for the redox reactions were estimated to be (2−6) × 10−3 cm s−1 at 25°C. Furthermore, the present Fe(II)/Fe(III) redox systems were found to be cycled stably. From these results, the [Fe(L)x]2+ species are expected to be used as cathode active materials for the redox-flow batteries using [BMI][Tf2N] as the electrolyte.