Electrode reaction and mass-transport mechanisms associated with the I⁻ / I₂ and H⁺ / H₂ redox couples in ionic liquid media
2017-03-01T02:55:07Z (GMT) by
Room temperature ionic liquids have received considerable attention over the past two decades as promising replacements for volatile molecular solvents in a range of applications. As the use of these neoteric solvents in electrochemical devices and in a growing range of processing technologies continues to expand, the limits to our knowledge of some of the fundamental underlying processes has become apparent. For this reason, the fundamentals of heterogeneous electron transfer reactions (i.e., thermodynamics, kinetics and mechanisms) and mass-transport have been investigated in ionic liquid media, focusing on the technologically relevant I⁻ / I₂ and H⁺ / H₂ redox systems. Initially, the utility of several voltammetric methods was evaluated for the purpose of quantifying the parameters associated with a range of electroactive species (i.e., diffusivity, stoichiometric number of electrons, and/or bulk concentration) in ionic liquid media. Conventional steady-state voltammetric methods (i.e., microelectrode or rotating disk electrode) are difficult to apply under highly viscous conditions and the peak currents of dc cyclic voltammograms are sensitive to heterogeneous kinetics/uncompensated resistance, limiting the application of these techniques for quantitative analysis in ionic liquid media. Semiintegral voltammetry and related convolutive techniques were shown to be powerful and also robust electroanalytical methods, which can be readily applied under highly viscous conditions to quantify the diffusivity of a range of electroactive species. Following the development of suitable electroanalytical methodology, the electrode reaction and mass-transport mechanisms associated with the I⁻ / I₂ redox couple was investigated in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. The iodide oxidation and iodine reduction processes were found to proceed in two steps on platinum, via a triiodide, I₃⁻, intermediate. These processes were simulated by combining a termolecular heterogeneous charge transfer reaction (<i>i.e.</i>, 2I⁻ ⇌ I₂ + 2e⁻) with a homogeneous process (i.e., triiodide formation, I⁻ + I₂ ⇌ I₃⁻). An analogous electrode reaction mechanism was observed on glassy carbon and boron-doped diamond electrodes, albeit with significant overpotentials when compared with the behaviour at platinum. Iodide oxidation on gold also proceeds in two steps, however the predominant intermediate species was found to be the diiodoaurate complex anion, [AuI₂]⁻, rather than triiodide. Finally, the diffusivity of iodide was found to be directly proportional to the concentration of triiodide present, showing up to a 50% enhancement under the conditions investigated. This enhancement in mass transport could not be explained by a decrease in solution viscosity (as predicted by the Stokes-Einstein equation), and was therefore attributed to electron-hopping and/or a Grotthuss-type bond-exchange reaction between iodide and triiodide. In the final body of work, proton transport, hydrogen evolution (at a platinum electrode) and equilibrium acidity (pKₐ was investigated in a broad range of ionic liquids. In bis(trifluoromethanesulfonyl)imide containing ionic liquids, when the conjugate acid of the anion (i.e., H[NTf₂]) is used as the proton source, proton diffusivity was found to be inversely proportional to medium viscosity, as predicted by the Stokes-Einstein equation. Furthermore, it was found that the ‘proton’ diffusion coefficient (derived electrochemically) is equal to the bis(trifluoromethanesulfonyl)imide anion self-diffusion coefficient (derived with <sup>19</sup>F NMR), indicating that the acid is not dissociated in this environment (i.e., the protons diffuse as H[NTf₂]). The hydrogen evolution reaction from both H[NTf₂] and a range of weak acids (sulfonamides, protonated amines, phenols, carboxylic acids or sulfonic acids) was simulated by combining the classical Volmer (rate determining step), Tafel and Heyrovsky reactions. Finally, a straightforward voltammetric method for calculating pKₐ values was devised and the equilibrium acidities (pKₐ) of twenty weak acids, covering eighteen orders of magnitude in acid strength (2.0 ≤ pKₐ ≤ 19.5) was quantified in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide.