Studies using microelectrodes of the Mg(II)/Mg couple in tetrahydrofuran and propylene carbonate

The deposition and dissolution of magnesium metal from various tetrahydrofuran and porpylene carbonate solutions has been investigated using microelectrode techniques. It is demostrated that from a solution of a Grignard reagent, C2H5MgBr (0.5 mol dm−3) in THF, it is possible both to electroplate magnesium onto copper and to redissolve it anodically with high coulombic efficiency. The equilibrium potential of the Mg/Mg(II) couple in this medium is estimated to be +850 mV vs Li/Li+ and the couple is also shown to be quite reversible with an exchange current density of 1 mA cm−2. In contrast we were unable to deposit Mg cathodically from solutions of MgBr2 and Mg(ClO4)2 in THF or PC even at 338 K and/or in the presence of a base lectrolyte (CH3)4NClO4 or LiAsF6; indeed, the presence of MgBr2 inhibits substantially the deposition of lithium metal. The addition of MgBr2 to the solution of Grignard reagent, however, enhances the rate of depostion of Mg metal. During long tome-scale experiments, magnesium metal also results from a chemical reaction between the electrodeposited lithium and magnesium ion in solution.