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

Abstract

In this study we strengthen our fundamental understanding of the underlying reactions of a possible Ca-O<sub>2</sub> battery using a DMSO based electrolyte. Employing the rotating ring disc electrode, we find a transition from a mixed process of O<sub>2</sub><sup>-</sup> and O<sub>2</sub><sup>2-</sup> formation to an exclusive O<sub>2</sub><sup>-</sup> 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(O<sub>2</sub>)<sub>2</sub>, CaO<sub>2</sub> and CaO as well as side reaction products such as CO<sub>3</sub><sup>2-</sup> 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 O<sub>2</sub><sup>-</sup> in presence of Ca<sup>2+</sup> 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<sup>-</sup> is reducing O2 to O<sub>2</sub><sup>-</sup> and O<sub>2</sub><sup>2-</sup> whereas in the presence of DBBQ<sup>2-</sup> O<sub>2</sub><sup>2-</sup> is formed. The mechanism of the ORR with and without DBBQ will be discussed.

Electrochemical Reduction of O2 in Ca2+‐Containing DMSO: Role of Roughness and Single Crystal Structure

“…This work strongly focuses on the ORR/OER mechanism in Ca 2+ ‐containing DMSO and the influence of the surface structure (single crystal orientation on the one hand and high roughness factors on the other). It supplements a previous study in which already a transition from mixed and formation to exclusively formation was observed on smooth electrodes [29] . The study further showed that the blocking thin film mainly consist of Ca(O 2 ) 2 , CaO 2 and CaO (identified by XPS) and is formed on Au and Pt electrodes.…”

“…Note that in the first sweeps up to 0.4 V oxidation of this adsorbate is slow and remains incomplete, whereas in subsequent sweeps the maximum current is at 0.25 V (peak potential). This indicates a nucleation and growth behavior for the free sites, as observed earlier for Ca 2+ electrolytes on smooth Au electrodes [29] and for Na + electrolytes on smooth Pt (but not on Au) [40]…”

“…However, in case of the lowest electrode roughness (Figure 7a), the decreasing reduction current indicates a transition from peroxide to superoxide formation. This transition from the 2 e − reduction to a 1 e − reduction had been found before and explained by the formation of a CaO/CaO 2 adsorbate layer, which had also been characterized by XPS [29] . The ORR is accompanied by an increase in ring current, which is astonishingly low as long as peroxide is the main product.…”

“…Therefore, we assume that the adsorbate layer formation is a slow reaction competitive to the formation of soluble peroxide. This experiment and the corresponding experiment on smooth gold [29] identify the adsorbate as reason for inhibition of peroxide formation, which persists after slow dissolution of the thicker CaO 2 bulk layer formed during ORR.…”

“…Concerning the decrease of peroxide formation rate during ORR at smooth electrodes and transition to the 1 electron superoxide formation, we had suggested that the electrode is slowly covered by an adsorbate (CaO 2 or CaO), which not only completely blocks peroxide formation, but also inhibits superoxide formation to some extent as it becomes a thin, closed layer [29] . To confirm this we conducted an RRDE experiment, where the Au electrode was first saturated with this CaO 2 /CaO adsorbate by holding the potential at −1.6 V during the scan for 5 min at 9 Hz rotation frequency.…”