Electrochemistry and Mass Transport of Ytterbium(III) in Bis(trifluoromethylsulfonyl)imide-Based Room-Temperature Ionic Liquids: Effects of Temperature, Viscosity, and Electrode Materials

Cyclic staircase voltammetry, rotating disk electrode voltammetry, controlled potential coulometry, and electrochemical impedance spectroscopy were used to probe the electrochemistry and mass transport of Yb 3+ in the six room temperature ionic liquids based on the bis(trifluoromethylsulfonyl)imide (Tf 2 N − ) anion with, 1-(1-butyl)−3-methylimidazolium (BuMeIm + ), 1-(1-butyl)−1-methylpyrrolidinium (BuMePyro + ), 1-(1-butyl)pyridinium (BuPy + ), 1-butyltrimethylammonium (BuMe 3 N + ), 1-ethyl-3-methylimidazolium (EtMeIm + ), and tri(1-butyl)methylammonium (Bu 3 MeN + ) cations. These investigations were carried out at glassy carbon as well as polycrystalline gold, platinum, and tungsten electrodes. The heterogeneous electron transfer rate of the quasireversible Yb 3+/2+ redox couple was found to depend strongly on the electrode materials with the fastest rate observed at gold and the slowest rate found at tungsten, but was independent of the physicochemical properties of the various ionic liquids, in particular, the absolute viscosity. However, the mass transport of Yb 3+ was dependent on the viscosity, and the temperature dependence of the diffusion coefficients was well represented by a Vogel-Tammann-Fulcher type expression for glass-forming ionic liquids. Analysis of the diffusion coefficient data with the Stokes-Einstein equation indicated that the solvodynamic radius of the diffusing Yb 3+ was constant and independent of the structure and properties of the ionic liquid cations. The solvodynamic radius of Yb 3+ was estimated from the “stick model” for the Stokes-Einstein equation. Application of the Random Closest Packing (RCP) model for spheres in consideration of the solvodynamic radius of the diffusing Yb 3+ and the ionic radii of Yb 3+ and Tf 2 N − , indicated that the former must diffuse in association with ∼5–6 of the anions.