Introduction of Three-Dimensionally Distributed Catalytic Sites to Form Semi-Solid Redox Conducting Electrolytes: Enhancement of Electron Transfers within Iodine/Iodide Mediating System

Abstract

It is well known that performances of dye-sensitized solar cells(DSSCs) reach their best when the cells have liquid electrolytes. However, commercialization of these cells has been impeded owing to the technological problems related to hermitic sealing, precipitation of salts at low temperature, evaporation of liquids at high temperature, corrosion, and lack of long-term stability. Replacing liquid electrolytes in DSSCs with solid or quasi-solid electrolytes is expected to make the cells viable. The promising material to replace the liquid electrolytes are ionic liquids. They possess unique properties such as high chemical and thermal stability, relative non-flammability and wide electrochemical window. Among disadvantages of ionic liquids 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. To increase rates of electron hoping between I- and I3 -, the appropriate catalytic sites must be introduced to achieve fast dissociation of iodine (I2), or triiodide (I3 -), molecule. In other words, there is a need to develop means of inducing the I-I bond breaking and to significantly accelerate bulk electron transfers and overall charge propagation. 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. Surface chemistry data provide clear evidence that iodine chemisorbs readily on platinum, other noble metals, certain carbon or metal oxide nanostructures as monoatomic iodine. 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 using Pt [1] or Pd, which are three-dimensional distributed in the electrolyte phase (at 2% weight level) to enhance dynamics of the iodine/iodide electron self-exchange and develop a new generation of iodine-based ultra-fast charge 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. We have also characterized our arrangement by TEM images and zeta potential measurements. The obtained results have broader meaning than application in DSSCs. They provide means of fabricating systems capable of very fast charge propagation of importance in nanoelectronics, charge staorage and sensing. [1] I. A. Rutkowska, M. Marszalek, J. Orlowska, W. Ozimek, S. M. Zakeeruddin, P. J. Kulesza, M. Graetzel,ChemSusChem 8 (2015) 2560.