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