Enhancement of Charge Transport Rates By Incorporation of Platinum Nanoparticles to Iodine/Iodide Ionic Liquid Redox-Conducting Electrolytes

The iodine/iodide redox system has so far been the most commonly and most successfully used as a charge relay (mediator) in Dye Sensitized Solar Cells (DSSCs) [1]. The semi-liquid or semi-solid iodine/iodide-based systems have probably led the most promising results so far in the area of practical DSSCs. More recently, the iodide/iodine redox couple has been considered together with ionic liquids. The resulting redox-conducting electrolytes have several advantages: high conductivity, low vapor pressure, high iodide/iodine 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, iodine or triiodide, if the charge transport mechanism is truly physical. If charge transport involves electron hoping between I^‑ and I, which seems to be operative as well, its effectiveness would require fast dissociation of iodine (I₂), or triiodide (I₃^-), molecule.  Indeed, both interfacial and bulk (self-exchange) electron transfers involving the iodine/iodide redox system are somewhat complicated and appear slower than one would expect. There is a need to develop means of inducing the I-I bond breaking in the I₃^- or I₂ molecule. It has also been established that platinum (e.g. when deposited on the counter electrode of DSSC) catalyzes interfacial electron transfers in the iodine/iodide redox system. Surface chemistry data provide clear evidence that iodine (or iodides) chemisorb readily on platinum as monoatomic iodine.  Strong interactions of Pt with iodine were reported and described; further, 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 concept of three-dimensional distribution of nanostructured platinum within the electrolyte phase) to enhance dynamics of iodine/iodide electron transfers (electron self-exchange) as an attempt to develop a new generation of charge iodine-based relays for DSSC. The diagnostic experiments have been performed using the electroanalytical methodology developed for solid-state electrochemical measurements in the absence of external liquid supporting electrolyte. They have included measurements using the planar three-electrode cell utilizing an ultramicrodisk electrode and two-electrode type sandwich configuration [2,3]. In the present case of the iodine/iodide system dispersed together with Pt nanoparticles within ionic liquid phase, mutual compensations between physical and effective (electron self-exchange type) diffusion mechanisms are postulated. Regardless the mechanism, the charge propagation rates expressed in diffusional terms have been found on 10^-6 cm² s^-1 level.

[1] M. Graetzel, Acc. Chem. Res., 42 (2009) 1788

[2] P.J. Kulesza, M.A. Malik in Wieckowski A (ed.), Interfacial Electrochemistry - Theory, Experiment and Applications, Solid-State Voltammetry 673, Marcel Dekker, New York (1999)

[3] P.J. Kulesza, J.A. Cox, Electroanalysis 10 (1998) 73