The Oxygen Reduction Reaction in Ca2+ -Containing DMSO: Reaction Mechanism, Electrode Surface Characterization, and Redox Mediation*.

In this study the fundamental understanding of the underlying reactions of a possible Ca-O2 battery using a DMSO-based electrolyte was strengthened. Employing the rotating ring disc electrode, a transition from a mixed process of O2 - and O2 2- formation to an exclusive O2 - formation at gold electrodes is observed. It is shown that in this system Ca-superoxide and Ca-peroxide are formed as soluble species. However, there is a strongly adsorbed layer of products of the oxygen reduction reaction (ORR) s on the electrode surface, which is blocking the electrode. Surprisingly the blockade is only a partial blockade for the formation of peroxide while the formation of superoxide is maintained. During an anodic sweep, the ORR product layer is stripped from the electrode surface. With X-ray photoelectron spectroscopy (XPS) the deposited ORR products were shown to be Ca(O2 )2 , CaO2 , and CaO as well as side-reaction products such as CO3 2- and other oxygen-containing carbon species. It is shown that the strongly attached layer on the electrocatalyst, that was partially blocking the electrode, could be adsorbed CaO. The disproportionation reaction of O2 - in presence of Ca2+ was demonstrated via mass spectrometry. Finally, the ORR mediated by 2,5-di-tert-1,4-benzoquinone (DBBQ) was investigated by differential electrochemical mass spectrometry (DEMS) and XPS. Similar products as without DBBQ are deposited on the electrode surface. The analysis of the DEMS experiments shows that DBBQ- reduces O2 to O2 - and O2 2- , whereas in the presence of DBBQ2- O2 2- is formed. The mechanism of the ORR with and without DBBQ is discussed.

K-O2 electrochemistry: achieving highly reversible peroxide formation.

Since the advent of the lithium-air battery, researchers have focused on understanding the underpinning mechanisms of the oxygen reduction and evolution reaction in aprotic solvents. In this work, the oxygen reaction in the presence of potassium ions in dimethyl sulfoxide was exploited as a model system to refine the present mechanistic picture of oxygen reduction in aprotic environments. In a combined approach utilizing differential electrochemical mass spectrometry in a generator-collector arrangement as well as classical electrochemical techniques, the reversible formation of insoluble peroxide as well as of slightly soluble superoxide is shown. As opposed to other peroxides in other non-aqueous metal-oxygen systems, potassium peroxide can be reoxidized to superoxide with an overpotential of as little as 100 mV. The investigation of the effect of the oxygen partial pressure between 0 and 1 atmosphere demonstrates how the precipitation of superoxide increases the oxidation overpotential of the peroxide and establishes a link between this work and other studies, in which the reversibility of the peroxide formation has not been identified.

Fast and Simultaneous Determination of Gas Diffusivities and Solubilities in Liquids Employing a Thin-Layer Cell Coupled to a Mass Spectrometer, Part II: Proof of Concept and Experimental Results.

A new method for simultaneously determining gas diffusivities and solubilities in liquids was presented and discussed in detail in Part I of this series. In this part of the series, the new measurement cell was employed to determine oxygen solubilities and diffusivities in 20 different dimethyl sulfoxide-based electrolytes. In addition, a comparison to values available in literature was made. From the temperature dependence of the diffusivity between 20 and 40 °C an activation barrier of 19 kJ mol-1 for the diffusion of oxygen in pure dimethyl sulfoxide was found. Moreover, qualitative agreement between Jones-Dole viscosity coefficients and the dependence of the diffusivity on the electrolyte concentration was confirmed. The temperature-dependent solubility measurements revealed an unexpected increase of the oxygen solubility for temperatures above 30 °C. While the oxygen solubility in the case of the alkali-perchlorates decreases with increasing electrolyte concentration, a pronounced salting-in effect for lithium bis(trifluoromethane)sulfonimide was observed.

Fast and Simultaneous Determination of Gas Diffusivities and Solubilities in Liquids Employing a Thin-Layer Cell Coupled to a Mass Spectrometer, Part I: Setup and Methodology.

Transport properties and solubilities of volatile species in liquid solutions are of high interest in different chemical, biological, and physical systems. In this work, a new approach for determining the diffusivity and solubility of gases in liquids simultaneously is presented. The method presented relies on the diffusion of a volatile species through a thin, liquid layer and the subsequent detection of the species using a mass spectrometer. Evaluation of the time development of the resulting transient yields the diffusion coefficient, while the concentration of the species in the liquid layer can be calculated from the steady-state value of the flux into the mass spectrometer. Apart from the geometry of the thin layer and the calibration constant of the mass spectrometer no additional or external data are required. Experimental results of the temperature-dependent solubility and diffusivity of oxygen in dimethyl sulfoxide are presented in our companion paper Part II and serve as a proof of concept.