Nanocomposite Semi-Solid Ionic-Liquid Electrolytes with Enhanced Charge-Transport and Redox Mediating Capabilities

Abstract

The triiodide/iodide redox system has so far been the most commonly and most successfully used as a charge relay (mediator) in dye sensitized solar cells (DSCs). The redox electrolyte plays a very important role in the DSC performance, and its usefulness largely depends on dynamics of both interfacial electron transfers and bulk charge propagation within the system. There has been also growing recent interest in room-temperature ionic liquids, especially in 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. Obviously, the triiodide/iodide redox couple has been considered together with ionic liquids. 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 mass transport coefficient of the triiodide/iodide redox couple, not only if the charge transport mechanism is predominantly physical at low concentrations but also when the Grotthus exchange mechanism is operative at high concentrations of the redox system. In general, both interfacial and bulk (self-exchange) electron transfers involving the triiodide/iodide redox system are somewhat complicated and appear slower than one would expect. Among kinetic limitations there is a need to break the I-I bond in the I3 - or I2 molecule; it has also been well-established that platinum (e.g. when deposited on the counter electrode) induces electron transfers within the iodine/iodide redox system. Strong interactions of Pt with iodide or iodine were reported and described. It is noteworthy that values of the iodine covalent radius (0.133 nm) and the Pt atomic radius (0.135 nm) are comparable. Formation of the monolayer type coverages of strongly adsorbed monoatomic iodine together with weakly bound electroactive iodine/iodide was also postulated. In the present work, we explore the interfacial (electrocatalytic) phenomena of nanostructured platinum (namely of Pt nanoparticles that are three-dimensionally distributed in the electrolyte phase at 2% weight level), and we utilize them to enhance triiodide/iodide electron transfers to develop more efficient charge relays for DSCs. Finally, to make the electrolyte more solid (non-fluid) and to improve the overall electron distribution within the redox-conducting electrolyte, we also introduce into our nanocomposite system multi-walled carbon nanotubes (CNTs), namely at 10% weight level, as supports for dispersed iodine-modified Pt nanoparticles. Using the microelectrode-based and sandwich-type electroanalytical methodologies of solid-state electrochemistry, we address here the charge transport dynamics within the semi-solid triiodide/iodide ionic-liquid electrolyte admixed with CNT-supported Pt nanostructures and comment on reasonably high power conversion efficiencies of DSCs utilizing such electrolytes.We acknowledge collaboration with M. Graetzel, S.M. Zakeeruddin and M. Marszalek of EPFL, Lausanne, Switzerland.