ELECTROCHEMICAL STUDIES OF SODIUM-CHLORIDE AS A LEWIS BUFFER FOR ROOM-TEMPERATURE CHLOROALUMINATE MOLTEN-SALTS

The reaction of equal molar quantities of 1-methyl-3-ethylimidazolium chloride (MEIC1) with AlCl3 results in the formation of a conductive ambient-temperature ionic liquid that, when investigated voltammetrically, exhibits 4.4V of electroinactivity between the reduction of MEI+ and the oxidation of AlCl4−. When unequal quantities of these reagents are used, however, the resulting melt has a much narrower electrochemical window due either to the formation of the acid Al2Cl7−(which reduces to Al at potentials that are nearly 2.0V positive of MEI+ reduction) or the base Cl− (which oxidizes to Cl2 at potentials that are 1.5V negative of AlCl4− oxidation). In this paper we investigate those equilibrium processes that control the chloroacidity in these ionic liquids. Specifically, we show that they undergo an acid exchange reaction AlCl4−(l) + AlCl4−(l) ⇌ Al2Cl7−(1) + Cl−(1) K = 1.0 × 10−16 that is similar in form and extent to the familiar autoprotolysis reaction in water. We are thereby able to compare Lewis acidity in these ionic liquids with Bronsted acidity in aqueous solution. This comparison has led to the discovery that NaCl acts as a Lewis acid-base buffer in MEICl/AlCl3 ionic liquids. Herein we provide electrochemical evidence that the presence of NaCl resists the deleterious effects of electrogenerated agents that ordinarily induce changes in chloroacidity and thereby narrow the electrochemical window. Because of this important development, we feel that it may now be possible to maintain the broad electrochemical window of the neutral melt even during the electrogeneration of a Lewis acid or base. © 1990, The Electrochemical Society, Inc. All rights reserved.