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

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 DBBQ^2- O₂ ^2- is formed. The mechanism of the ORR with and without DBBQ is discussed.