Electrochemical Formation and Characterization of Surface Blocking Layers on Gold and Platinum by Oxygen Reduction in Mg(ClO4)2 in DMSO

Metal-oxygen batteries employing non-aqueous electrolytes are a promising alternative to conventional lithium-ion batteries due to their high specific energies. One of the most appealing metals in such an application is magnesium, considering its price and its volumetric charge density. While previous reports already indicated the formation of a blocking layer on the oxygen-electrode surface during oxygen reduction, associated with a low reversibility, the reasons for this behavior are not yet understood. This work focuses on elucidating the composition as well as the electrochemical behavior of this blocking layer employing X-ray photoelectron spectroscopy, infrared spectroscopy and measurements with a rotating ring-disc electrode setup. The electrochemical measurements indicate that the blocking layer exhibits an effect similar to the valve effect, which is known from the oxidation of aluminum for example. Combining potentiodynamic and -static measurements, it could be shown that there is a current flow at cathodic potentials due to migration of ions within the layer. A further outcome of the electrochemical measurements is that the anodization of this surface layer proceeds in a self-accelerating manner. The spectroscopic analysis of the surface layer suggests the formation of magnesium peroxide on the surface as well as the presence of some sulfur and chloride containing compounds which must arise from the decomposition of the electrolyte. (C) The Author(s) 2018. Published by ECS.