Enhancement of Electron Transfer Rates in Iodine/Iodide Ionic Liquid through Catalyzing the Chemical Step of I-I Bond Splitting: Development of Semi-Solid Redox Electrolyte Capable of Fast Charge Propagation

The ability of platinum nanostructures to induce splitting of I-I bond in the iodine molecule is explored here to enhance electron transfers in the iodine/iodide redox couple and thus to accelerate charge transport in the respective room-temperature 1,3-dialkylimidazolium cation based ionic liquid. There has been growing recent interest in room-temperature ionic liquids, especially those with 1,3-dialkylimidazolium cations due to their such important features as negligible vapor pressure, high ionic conductivity and thermal stability, fairly wide electrochemical window, and ability to dissolve organic and inorganic solutes. The resulting redox-conducting electrolytes have several advantages: high conductivity, low vapor pressure, high iodide concentration and good electrochemical stability. Among disadvantages is their high viscosity that certainly contributes to the low diffusion coefficient of the oxidized form of the redox couple if the charge transport mechanism is truly physical. It was established that platinum induces electron transfers within the iodine/iodide redox system. Strong interactions of Pt or Pd with iodide or iodine were also postulated.  Formation of the monolayer type coverages (on platinum or palladium) of strongly adsorbed monoatomic iodine together with weakly bound electroactive iodine/iodide was also postulated. By using solid-state voltammetric (both sandwich-type and microelectrode based) methodology, we demonstrate in the present work that effective (apparent) diffusion coefficients have significantly increased upon incorporation of the iodine-modified Pt or Pd nanoparticles and, in particular, the carbon-nanotube (multi-walled) supported iodine-modified Pt nanostructures. The Dye Sensitized Solar Cell utilizing the dye-covered TiO₂ and the iodine/iodide ionic liquid electrolyte admixed with the iodine-modifed Pt based nanostructures has yielded reasonably higher efficiencies and better physicochemical parameters in comparison to the analogous Pt-free system.

            In the present work, we explore the respective interfacial (electrocatalytic) phenomena on nanostructured platinum or palladium (namely Pt or Pd nanoparticles that three-dimensionally distributed in the electrolyte phase), and we utilize them to enhance iodine/iodide electron transfers and to develop systems capable of fast charge propagation. To further improve charge distribution within the redox-conducting electrolyte, we have also introduced multi-walled carbon nanotubes (as supports for dispersed iodine modified Pt nanoparticles) into our hybrid system.