Ionic liquids with fluorinated block-oligomer tails: Influence of self-assembly on transport properties

Abstract

A novel imidazolium iodide ionic liquid with an ω-perfluoroalkyl poly(ethylene glycol) (PEG) tail attached to the imidazolium ring has been synthesized for its potential incorporation as an electrolyte in dye-sensitized solar cells. The ionic liquid molecules, with block oligomer tails, self-assembled to form a solvent-free ionic gel, without the assistance of an external gelator or an immobilizing matrix. The solidification was evidently facilitated by the generation of ionic clusters due to electrostatic interactions, as well as microphase separation of the immiscible perfluoroalkyl and PEG segments of the cation. We report herein the synthesis and electrochemical properties of this block oligomer ionogel, along with the results of self-consistent mean field calculations probing the formation of nanostructures in the ionogel. Although properties such as high viscosity and high ionic conductivity appear incompatible, it is shown that a nano-structured fluid can support high iodide diffusion at low effective fluidity, and that the formation of an organic alloy, by simple blending of two imidazolium iodide salts, can produce significant conductivity enhancements without lowering the viscosity.