Changes in the interfacial charge-transfer resistance of Mg metal electrodes, measured by dynamic electrochemical impedance spectroscopy

The interfacial electrochemical characteristics of unpassivated Mg metal electrodes have been studied using Staircase Galvano Electrochemical Impedance Spectroscopy (SGEIS) in THF/C6H5MgCl/AlCl3 solutions over a wide range of applied dc currents. The results shed light on the mechanism of electrochemical deposition and dissolution of Mg. We found that at the stationary condition (at the OCV, when no current flows) there is a very high interfacial charge-transfer resistance of around 40,000 Ωcm2. The impedance decreases to several hundred Ωcm2 when dynamic, though steady, processes of Mg deposition or dissolution take place. The alternating process through which impedance spectroscopy is measured is superimposed on this system. We show that the high impedance measured at the OCV with a low direct current is due to interfacial adsorption phenomena. However, the adsorption phenomena affect the interfacial charge transfer in different ways depending on the direction of the reaction. During steady-state deposition, the adsorbed layer leads to a large distance for electron tunneling, which translates to a high charge-transfer impedance across the interface. Application of relatively large steady-state negative currents leads to changes in the composition of the adsorbed layer and a significant reduction in the charge-transfer resistance. During the opposite process, dissolution, the high interfacial impedance measured is probably predominantly due to migration of Mg ions away from the Mg metal surface, across the adsorbed layer, to the bulk solution. At relatively high direct currents (and overpotentials >100 mV) the interfacial resistivity obeys Butler–Volmer kinetics, which are not complicated by adsorption phenomena.