Stability Investigations of [Emim]AlCl₄ Ionic Liquid

Abstract

1-Ethyl-3-methylimidazolium tetrachloroaluminate, abbreviated as [EMIm]AlCl 4 , is a widely-investigated ionic-liquid electrolyte for electrochemical processes and energy storage applications due to its wide electrochemical window, low melting point, low vapor pressure, and high conductivity. Thermal and chemical stability of [EMIm]AlCl 4 at room temperature and moderately elevated temperatures is an important factor of affecting its promising applications. The focus of this work is to systematically study the stability of [EMIm]AlCl 4 at elevated temperatures. Electrochemical studies of [EMIm]AlCl 4 were first performed by measuring its open circuit potentials and cyclic voltammograms at varying temperatures and holding time. The voltammograms shown in Figure 1(a) exhibit that [EMIm]AlCl 4 has a wide electrochemical window ranging from -1.6 to 1.2 V vs an Al reference electrode. The anodic and cathodic currents outside the window may correspond to the surface oxidation of the working electrode and decomposition of impurities present in the electrolyte. After the [EMIm]AlCl 4 electrolyte was heated at 110 o C for 48 hours, a white suspension layer was observed at the top layer of the electrolyte, which is attributed to the formation of alumina oxides. Considerable changes of the voltammograms for the heated [EMIm]AlCl 4 were shown in Figure 1(b), characteristic of the appearance of anodic and cathodic peaks in the electrochemical window and significantly increased currents. These voltammetric characters indicate the formation of electrochemical active species in the electrolyte through the decomposition of [EMIm]AlCl 4 . In this presentation, we will introduce our electrochemical and analytical results regarding thermal and electrochemical stability of the [EMIm]AlCl 4 electrolyte, and discuss the origins of chemical changes occurring during the thermal degradation of [EMIm]AlCl 4 . Figure 1