The ORR in Ca2+ Containing DMSO: Reaction Mechanism, Electrode Surface Characterization and Redox Mediation

In this study we strengthen our fundamental understanding of the underlying reactions of a possible Ca-O₂ battery using a DMSO based electrolyte. Employing the rotating ring disc electrode, we find a transition from a mixed process of O₂^- and O₂^2- formation to an exclusive O₂^- formation at gold electrodes. We will show that in this system Ca-superoxide and Ca-peroxide are formed as soluble species. However, there is a strongly adsorbed layer of ORR products on the electrode surface which is blocking the electrode. Surprisingly the blockade is a partial blockade because the formation of superoxide can be maintained. During an anodic sweep the ORR product layer is stripped from the electrode surface. With X-ray photoelectron spectroscopy the deposited ORR products are 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. We will give evidences that the strongly attached layer on the electrocatalyst that is 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) is 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^- is reducing 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 will be discussed.