Unlike ferrocene, bis(η -cyclopentadienyl)magnesium (magnesocene, MgCp ) is slightly dissociated in solvents, such as ethers, resulting in electrolyte solutions with low conductivity. MgCp /tetrahydrofuran solutions make possible reversible magnesium plating and stripping with low over-potentials for many cycles. The Mg deposits appear with a cauliflower-like morphology. IR and NMR spectroscopy confirm that the electrolyte is stable and not decomposed during prolonged cycling. The anodic stability limit is in the range of 1.5 V (at platinum) and 1.8 V versus Mg/Mg (at stainless steel), which may be sufficient for low-voltage cathode materials. MgCp is a first example of a completely new class of halide-free electrolytes, which may open up a new research direction for future magnesium metal and magnesium-ion batteries.

Aiming at a better understanding of the air electrode processes in Mg‐air batteries, we have investigated the activity of different electrode materials, viz., Pt, Au, glassy carbon (GC) and manganese (III)‐ and (IV)‐oxides (Mn2O3 and MnO2) for the electrocatalytic oxygen reduction (ORR) and oxygen evolution (OER) reactions in the ionic liquid 1‐butyl‐1‐methylpyrrolidinium bis (trifluoromethanesulfonyl) imide (BMP‐TFSI) and the influence of Mg2+ thereon. Employing planar model electrodes in a rotating ring disk electrode (RRDE) setup, we used cyclic voltammetry at different rotation rates to test the reversibility of these reactions and quantify the number of electrons transferred during the ORR. Reversible ORR and OER are observed for all electrodes in neat BMP‐TFSI electrolyte, while in the Mg2+ containing electrolyte (Mg‐TFSI2) the OER is strongly hindered for all electrode materials. This goes along with the rapid build‐up of a passivation layer during cycling, which increasingly inhibits also the ORR. The morphology and chemical composition of the passivation layer were characterized by scanning electron microscopy (SEM) and X‐ray photoelectron spectroscopy (XPS), indicating that MgO2 is the main product formed by the ORR.

Since the carbonate-solvents used in Li-ion batteries were incompatible for Li-O 2 electrochemistry, alternatives have been investigated recently. Dimethyl sulfoxide (DMSO) stands among the best solvents for the Li-O 2 system owing to its comparative stability against the superoxide and peroxide species formed during cell discharge. However, it has a drawback of instability towards metallic lithium. Recent reports suggest the use of concentrated electrolytes that prevent solvent decomposition via shielding from lithium-metal. Herein, LiTFSI/DMSO is chosen as salt/solvent configuration with 3 salt concentrations. The solution properties, namely ionic conductivity, Li-transference number, and viscosity are studied along with electrochemical properties such as Li-stripping/plating coulombic efficiency and symmetrical long-term cycling. It is shown that concentrated electrolytes (> 1 M) exhibit superior cycling stability and coulombic efficiency. Post cycling analysis on electrodes confirms solvent decomposition only in low concentrated electrolyte. Moreover, the presence of oxygen in concentrated solutions yields an even better cycling stability than oxygen free electrolyte.

The optical second harmonic (SH) response from the n-Si( 11 1) surface immersed in aqueous electrolyte solutions has been examined and is observed to be potential dependent. This potential dependence is observed for both the monohydrogen-terminated surface immersed in NH4F as well as for surfaces which are photoanodically oxidized in H2SO4. The potential dependence from the latter is screened in part by the presence of the insulating overlayer. Furthermore, a minimum is observed in the potential-dependent response which is shifted well anodic of the flatband potential. The SH observations are attributed to field effects within the space charge region of the semiconductor which are manifested in the higher order bulk SH response.Photocurrent transient analysis is utilized as a means of determining the flatband potential of these surfaces immersed in electrolyte solution. Experiments performed in UHV are discussed and compared to that observed in solution.

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.