Electrochemical methodology for determination of imidazolium ionic liquids (solids at room temperature) properties: influence of the temperature

Abstract

A set of six imidazolium ionic liquids (1a–b, 2a–c, 3), that were solids at room temperature, were characterized by electrical impedance spectroscopy to obtain information about their polarization resistance (R_p), conductivity (σ) and charge transfer activation energy (E_a). These experiments were performed at different temperatures in a glass micro-cell, equipped with three platinum electrodes. The observed conductivities were due to charge transfer processes of molecular oxygen at the electrode surface and mass transfer processes within the IL matrix. Higher temperatures resulted for all ionic liquids in increased conductivities. X-Ray diffraction of the ionic liquids 2a–c suggested that a higher degree of supramolecular two-dimensional organization, higher density, is related to an easier oxygen-electrode approximation, lower E_a. Two distinct temperatures ranges were observed. The larger conductivity increases in the higher temperature range were explained by melting (ILs 1–2) and fluxional behavior/reorientation phenomena of the ionic liquids and are due to enhanced oxygen diffusion (IL 3). In general, the understanding of imidazolium ionic liquid electrochemical properties could facilitate the development of new applications.