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.

Though Ca-O2 batteries show rising interest in the battery society due to their attractive energy density and availability of materials, the fundamental mechanisms of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in aprotic electrolytes are scarcely understood. This and preceding studies[1-3] aim at the investigation of the ORR mechanism especially on well-defined electrodes (single crystalline surfaces or those with adjusted roughness). Here we investigated the ORR in 0.1 M Ca(ClO4)2 in DMSO, as under convection for this system no deactivation is observed on Au and Pt electrodes. From earlier results[3, 4] we know, that on Au electrodes a slowly dissolving peroxide is formed, until a closed CaO2/CaO adsorbate layer is built up in a competing process, which inhibits further peroxide formation. The layer was characterized by XPS and AFM. An astonishing finding is that on Au electrodes of electrode roughness factor (fR) > 10 under convection, formation of superoxide is a diffusion limited process on top of the CaO2/CaO adsorbate layer (although peroxide formation is inhibited). In this study we also investigate the ORR/OER mechanism in the same system on Pt electrodes and compare the results to those, which we obtained from experiments on Au electrodes, as we found predominant superoxide formation on Pt electrodes in earlier studies[1]. This study shows, that also on Pt electrodes a CaO2/CaO adsorbate layer is the reason for predominant superoxide formation under convection. We found, that as one increases the electrode roughness, the ORR becomes more and more dominated by formation of adsorbed peroxide. Thus for low electrode roughness, we have to conclude that the in fact diffusion limited superoxide formation occurs through a CaO2/CaO adsorbate layer, which also on Pt electrodes inhibits further peroxide formation. As peroxide formation leads to almost complete electrode blocking in other M-O2 battery systems further leading to surface limited charges, one might think of utilizing the selective process inhibiting nature of the CaO2/CaO adsorbate layer to tackle those problems. Thus, in this study we will further show the influence of a CaO2/CaO adsorbate layer on Pt electrodes to the ORR in Li+ containing DMSO, as it is known, that Li2O2 effectively blocks the electrode. [1] P. Reinsberg, C. J. Bondue, H. Baltruschat, Journal of Physical Chemistry C 2016, 120, 22179. [2] P. Reinsberg, A. A. Abd-El-Latif, H. Baltruschat, Electrochimica Acta 2018, 273, 424. [3] P. P. Bawol, P. H. Reinsberg, A. Koellisch-Mirbach, C. J. Bondue, H. Baltruschat, ChemRxiv (Preprint) 2020. [4] A. Koellisch-Mirbach, I. Park, H. Baltruschat*, In preparation.

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in Ca 2 + containing dimethyl sulfoxide (DMSO) on atomically smooth Pt(111) and Pt(100) and rough Pt surfaces is reported. As demonstrated by cyclic voltammetry and XPS, the bromine adlayer used to protect Pt single crystals against ambient air and solvent vapor is desorbed in DMSO and does not affect the following measurements. 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 show, that on Pt electrodes the CaO 2 adsorbate layer formation determines the ORR product distribution. On Pt electrodes, calcium peroxide is formed on the clean electrode, whereas calcium superoxide is formed at the adsorbate covered electrode. We furthermore identified four key parameters, which strongly affect the ORR product distribution: 1) The electrode oxide interaction: A strong interaction increases superoxide contribution; 2) The alkaline earth metal oxide interaction: A strong interaction increases peroxide contribution; 3) The electrode surface area: A large electrode surface area increases peroxide contribution; 4) Electrode surface defects: Defects increase 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.