Anomalous Charge Transfer Behavior of an Iodide/Triiodide Redox Couple at an Ionic Liquid/Pt-Electrode Interface

Abstract

Several groups reported that a Grotthus-type bond exchange mechanism for the I–/I3 – couples contributes to charge transport and extraordinarily enhances bulk conduction in ionic liquids (ILs) beyond mass transport limitation. In the present work, we have studied interfacial charge transfer of the I–/I3 – couple at a Pt-electrode surface in one of such ILs demonstrating the extraordinary bulk property, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) added with EMImI and I2, and found that the exchange current density increased exponentially against the redox couple concentration. A closer examination revealed, in light of Marcus microscopic theory, concentration-dependent activation energy is a crucial factor for the anomalous increase in the exchange current density. Raman spectra of the IL suggest that the proximity of I3 – and I– should be increased at higher concentrations, resulting in the activation energy decrease that accelerates the charge transfer rate in the IL.

Grotthuss Transport of Iodide in EMIM/I3 Ionic Crystal

“…Although some solid-state studies exist, ,,,, most experimental studies that have empirically motivated the iodide Grotthuss exchange mechanism have been based on liquid-phase electrolytes. ,,− ,− A central question for the generality of our results to the liquid state concerns the proximity of I 3 – and I – anions in solution, since the EMIM/I 3 crystal structure confines iodide to be no more than 7 Å from the closest triiodide anion. However, from our analysis of the anion electronic structure (charge partitioning), we conclude that electrostatic interactions between I 3 – and I – anions are largely screened beyond 4–5 Å distances due to the ionic environment of EMIM/I 3 .…”

“…It has been speculated that Grotthuss-type exchange mechanism between triodide and iodide may contribute to the ion transport and conductivity. 3,4,13,17 In this article we provide molecular level evidence of such transport for iodide in ionic crystals.…”

“…The redox potential of iodide/triiodide is affected by the specific ion solvation free energies in the electrolyte, and the magnitude of this effect can be inferred from electrochemical potential measurements. However, the physics by which a specific electrolyte modulates the iodide transport and conductivity is much more difficult to determine, as evidenced by numerous experiments on different iodide-based electrolytes. ,,− …”

“…Additionally even though the PMIM/I/I 3 ionic liquid undergoes a liquid to nematic phase transition upon cooling, the conductivity remains largely unchanged, even though the viscosity increases by ∼6 orders of magnitude! Another example of nonintuitive conductivity/viscosity trends was observed for iodide ionic liquids based on long-chain cations, 1-undecyl-3-methylimidazolium (C 11 MIM) and 1-dodecyl-3-methylimidazolium (C 12 MIM). , C 12 MIM/I/I 3 forms thermotropic liquid crystals at room temperature whereas C 11 MIM/I/I 3 does not, and surprisingly it was found that the iodide diffusion was faster in the higher viscosity, liquid crystalline C 12 MIM/I/I 3 electrolyte. It has been speculated that Grotthuss-type exchange mechanism between triodide and iodide may contribute to the ion transport and conductivity. ,,, In this article we provide molecular level evidence of such transport for iodide in ionic crystals.…”