Towards Understanding the Solvent‐Dynamic Control of the Transport and Heterogeneous Electron‐Transfer Processes in Ionic Liquids

Abstract

The impact of temperature-induced changes in solvent dynamics on the diffusion coefficient and standard rate constant k for heterogeneous electron transfer (ET) of ethylferrocene (EFc) in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF ]) is investigated. The results are analysed to understand the impact of solvent-dynamic control, solute-solvent interactions and solvent friction on the transport of redox probes and k . Concentration dependence of the diffusion coefficient of EFc in [BMIM][PF ] is observed. This is attributed to the solute-induced enhancement of the structural organisation of the ionic liquid (IL), which is supported by the concentration-dependent UV/Vis absorption and photoluminescence responses of EFc/[BMIM][PF ] solutions. Similar values of the activation energies for mass transport and ET and a linear relationship between the diffusion coefficient and the heterogeneous ET rate is observed. The ratio between the diffusion coefficient and the heterogeneous rate constant allows a characteristic length L , which is temperature-independent, to be introduced. The presented results clearly establish that mass transport and heterogeneous ET of redox probes are strongly correlated in ILs. It is proposed that the apparent kinetics of heterogeneous ET reactions in ILs can be explained in terms of their impact on thermal equilibration, energy dissipation and thermal excitation of redox-active probes.

Effect of Surface Oxygen on the Wettability and Electrochemical Properties of Boron-Doped Nanocrystalline Diamond Electrodes in Room-Temperature Ionic Liquids

“…With no dielectric solvent, the constituent ions do not exist as simple spherical charges. 6 Rather, the ions exist as ion pairs. From a fundamental electrochemical perspective, the use of RTILs allows one to investigate electron-transfer kinetics and redox reaction mechanisms of soluble redox systems in the absence of a dielectric solvent.…”

“…The short-and long-range organizations of the RTIL ions at an electrified interface, as well as their unique arrangement around a redox molecule, make the electrochemical properties of these novel media interesting to study. 6 The chemical and electrochemical stability of RTILs leads to a large voltammetric working potential window of 4−6 V, depending on the RTIL cation and anion. Electrochemical reactions in RTILs will be influenced by the arrangement of the ions at the electrified interface between the RTIL and the solid surface, and the RTIL's physicochemical properties (e.g., electrical conductivity and viscosity).…”

“…Coulombic interactions are the dominant force between the ions. With no dielectric solvent, the constituent ions do not exist as simple spherical charges . Rather, the ions exist as ion pairs.…”

“…Indeed, electron-transfer reactions in an RTIL will necessarily proceed differently than in a polar solvent/electrolyte system. The short- and long-range organizations of the RTIL ions at an electrified interface, as well as their unique arrangement around a redox molecule, make the electrochemical properties of these novel media interesting to study . The chemical and electrochemical stability of RTILs leads to a large voltammetric working potential window of 4–6 V, depending on the RTIL cation and anion.…”

Heterogeneous Rate Constants of the Electron-Transfer of Iron- and Ruthenium-bipyridine Complexes in Imidazolium-Based Ionic Liquids

“…17 Estimation of the heterogeneous electron-transfer rate constants for the ferrocene-ferrocenium system has been studied by Fawcett et al 18 and others. 19,20 Bard et al 21 reported that [Ru(bpy) 3 ] 2+/3+ has k s = 0.24 cm s −1 in DMF using a Pt electrode, yet Scott et al 6 found k s = 9.5 ± 3.9 cm s −1 (298 K) in DMF using a Pt electrode with 0.75 M LiCF 3 SO 3 as supporting electrolyte. Unfortunately, there is no temperature dependence given for both results.…”