PEGylated Imidazolium Ionic Liquid Electrolytes: Thermophysical and Electrochemical Properties

We report the synthesis and characterization of a series of imidazolium iodide ionic liquids (ILs) containing monomethoxy-terminated poly(ethylene glycol) (mPEG), and n-alkyl groups. These PEGylated ILs contain 7, 12, or 16 ethylene glycol units in the side chains, and are designed as potential electrolytes for energy conversion and storage devices such as dye-sensitized solar cells, supercapacitors, and Li ion batteries. The thermophysical (density, viscosity, and the temperatures of glass-transition, crystallization and melting) and electrochemical (nonfaradaic window, and capacitor leakage resistance) properties of the ILs, that are critical to these targeted applications, are studied using an array of techniques. 1-Alkyl-3-methylimidazolium iodide ILs are synthesized and characterized in parallel with the PEGylated ILs, to compare how the electrolyte properties of the two systems are affected by their detailed molecular structures, and especially by the ether oxygen atoms. The mPEG side chains show strong intramolecular interactions with the imidazolium ring, weakening Coulombic interactions between the imidazolium cation and the iodide anion. The PEGylated ILs, therefore, exhibit conductivities of the order of 0.1 mS cm⁻¹ (25 °C) despite their relatively high viscosities, and support a temperature-independent electrochemical window of about 2 V, demonstrating their suitability for extended temperature applications.