Electrode kinetics of some iron complexes in an imide-type room-temperature ionic liquid

The electrode kinetics of [Fe (bpy)3] 3+ [Fe (bpy)3] 2+ (bpy=2, 2′ -bipyridine), [Fe (Cp)2] + [Fe (Cp)2] (Cp- =cyclopentadienyl), [FeCl4] - [FeCl4] 2- and [Fe (CN)6] 3- [Fe (CN)6] 4- on a platinum electrode was investigated over the temperature range from 25 to 60°C in a room-temperature ionic liquid, 1- n -butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI). The average activation energy for the diffusion coefficients of these complexes was ∼24 kJ mol-1, which was close to the activation energy for the viscosity of BMPTFSI. The diffusion coefficients of these complexes were found to be affected not only by the viscosity of BMPTFSI but also by the charge of the complexes. On the other hand, the heterogeneous rate constants of these redox reactions were estimated to be ∼ 10-4 cm s-1 by electrochemical impedance spectroscopy. The apparent heterogeneous rate constants of [Fe (bpy)3] 3+ [Fe (bpy)3] 2+, [Fe (Cp)2] + [Fe (Cp)2], and [Fe (CN)6] 3- [Fe (CN)6] 4- were smaller than those in other conventional solvents.