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

Mg electrodes were studied in a variety of polar aprotic electrolyte soluti ons, using cyclic voltammetry (CV), impedance spectroscopy (EIS), surface s ensitive FTIR spectroscopy, element analysis by dispersive X-rays (EDAX), s canning electron microscopy (SEM), and electrochemical quartz crystal micro balance (EQCM) studies. The solutions included Mg, Li, Na, K and Bu4N+ salt solutions in acetonitrile (AN), propylene carbonate (PC) and tetrahydrofur an (THF). In addition, THF + RMgX (R = alkyl, X = Cl, Pr) solutions were st udied. This paper aims at providing a general description of the electroche mical 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 ClO4- and BF4- leads to the precipi tation of surface films. The impedance of Mg electrodes is very high, due t o these surface films (several orders of magnitude higher than that of Li e lectrodes in the same solutions). However, the above difference in the surf ace chemistry is clearly reflected by the electrode's impedance. Consequent ly, Mg dissolution in these solutions occurs via a breakdown of the surface films. However, it is possible to reduce the overpotential of Mg dissoluti on considerably by the presence of acidic species in solutions, which remov e part of the surface films chemically. Reversible Mg deposition and dissol ution are obtained in THF + RMgX solution due to the fact that in these sol utions, 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 a dsorption-desorption processes of the solution species. (C) 1999 Elsevier S cience S.A. All rights reserved.