On the electrochemical behavior of magnesium electrodes in polar aprotic electrolyte solutions

Abstract

Abstract Mg electrodes were studied in a variety of polar aprotic electrolyte solutions, using cyclic voltammetry (CV), impedance spectroscopy (EIS), surface sensitive FTIR spectroscopy, element analysis by dispersive X-rays (EDAX), scanning electron microscopy (SEM), and electrochemical quartz crystal microbalance (EQCM) studies. The solutions included Mg, Li, Na, K and Bu 4 N + salt solutions in acetonitrile (AN), propylene carbonate (PC) and tetrahydrofuran (THF). In addition, THF+RMgX (R=alkyl, X=Cl, Br) solutions were studied. This paper aims at providing a general description of the electrochemical behavior of Mg electrodes in different types of polar aprotic systems. It appears that Mg electrodes are spontaneously covered by surface films in most of the solutions studied. In AN and PC, solvent reduction seems to dominate surface film formation, while in THF, the solvent is inactive and, thus, reduction of salt anions such as ClO 4 − and BF 4 − leads to the precipitation of surface films. The impedance of Mg electrodes is very high, due to these surface films (several orders of magnitude higher than that of Li electrodes in the same solutions). However, the above difference in the surface chemistry is clearly reflected by the electrode’s impedance. Consequently, Mg dissolution in these solutions occurs via a breakdown of the surface films. However, it is possible to reduce the overpotential of Mg dissolution considerably by the presence of acidic species in solutions, which remove part of the surface films chemically. Reversible Mg deposition and dissolution are obtained in THF+RMgX solution due to the fact that in these solutions, irreversible formation of stable surface films on the Mg electrodes is avoided largely. However, EQCM studies showed that these processes are not just a simple two-electron transfer to Mg ions and are complicated by adsorption–desorption processes of the solution species.