|Abstract||Magnesium oxygen batteries, having magnesium metal as anode and oxygen as cathode exhibit a high theoretical speciﬁc energy, and the required materials are cheaper and of higher abundance than those of state of the art lithium-ion-batteries. Unfortunately, there are severe challenges concerning the anode and the cathode of the battery. The key is to ﬁnd an electrolyte, which enables both magnesium stripping/plating and oxygen reduction/evolution. Up to now, most electrolytes with which magnesium metal can be electrochemically plated are highly reactive with oxygen or unstable at potentials prevalent at the oxygen electrode. Therefore, a two electrolyte system, one for each electrode, is the only feasible option as of now.
In this work, the electrolyte behavior at the oxygen electrode is investigated. Prelimi-
nary tests conﬁrmed the viability of the oxygen reduction in various electrolytes. The
subsequent analysis of the used electrode exhibited electrolyte decomposition products, but could not clarify the oxygen reduction mechanism, or the main discharge products, which are supposed to be MgO, MgO2 , or Mg(O2)2 . In order to analyze the immediate surface reaction product, a new cell setup was designed, using ﬂat working electrodes.
Using this setup, the electrochemical oxygen reduction reaction was investigated in two diﬀerent electrolytes, namely 0.5 M Mg(ClO4)2 in DMSO and 0.1 M Mg(TFSI)2 in BMP TFSI, chosen from analogs in the Li-O2 literature. It was shown that there is a soluble intermediate product, ideally Mg(O2)2 , which reacts further to form a passivating layer on the electrode surface. The ﬁnal reaction products were investigated with infrared spectroscopy, x-ray photoelectron spectroscopy and scanning electron microscopy. It was found that both electrolytes, despite diﬀerent properties and diﬀerent electrochemical behavior, exhibit the same oxygen reduction product - MgO2, agreeing with recent reports about other electrolytes. However, for the ﬁrst time, a quantitative proof for it was found in this work. Oxygen evolution, which is required for recharging a magnesium oxygen battery, was not observed in any of the investigated electrolytes.||dc.description.abstract