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 O2and O2 2formation to an exclusive O2formation 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 2and 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 O2in presence of Ca 2+ 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 DBBQis reducing O2 to O2and O2 2whereas in the presence of DBBQ 2-O2 2is formed. The mechanism of the ORR with and without DBBQ will be discussed.

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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.

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.

We demonstrate via cyclic voltammetry, differential electrochemical mass spectrometry (DEMS) and rotating ring disk electrode (RRDE) investigations with variation of the electrode surface roughness and atomically surface structure, that the CaO/CaO2 adsorbate layer formation determines the ORR product distribution. We found that on Pt electrodes peroxide is formed on the clean electrode, whereas superoxide is formed at the adsorbate covered electrode. We furthermore identified four key parameters, which strongly affect the ORR product distribution. The electrode oxide interaction: A strong interaction shifts the product distribution to larger superoxide contribution. The alkaline earth metal oxide interaction: A strong interaction shifts the product distribution to larger peroxide contribution. The electrode surface area: A large electrode surface area delays the completion of the adsorbate layer and increases the peroxide contribution. Electrode surface defects: Defects allow for faster nucleation and thus foster the adsorbate formation, which finally leads to a larger superoxide contribution. Finally, reviewing earlier results of our group we provide a more general mechanism for the oxygen reduction alkaline earth metal cation containing DMSO, for a variety of electrode materials. [1] A. Koellisch-Mirbach, I. Park, M. Hegemann, E. Thome and H. Baltruschat, ChemSusChem, (2021). [2] P.P. Bawol, A. Koellisch-Mirbach, C.J. Bondue, H. Baltruschat and P.H. Reinsberg, ChemSusChem, 14 (2021) 428.