The Oxygen Reduction Reaction 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-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.

Mechanistic Understanding of Oxygen Electrodes in Rechargeable Multivalent Metal‐Oxygen Batteries

“…In the above chemical mixing experiment (Figure 3e), OER was observed together with the formation of CO, CO 2 , SO 2 , H 2 O and CH 2 O as detected by mass spectrometer, [29b] which indicates that the disproportionation reaction is accompanied by the formation of 1 O 2 . 1 O 2 was shown to originate from superoxide disproportionation and attack electrolyte to evolve parasitic gases [9c] .…”

“…Reinsberg et al . reported using 2,5‐di‐tert‐butyl‐1,4‐benzoquinone (DBBQ) as a discharge mediator for Ca−O 2 batteries (Figure 3f) [29b] . The DBBQ mediator enhanced both ORR and OER currents.…”

“…The discharge products were oxidized back to O 2 at ∼2e − /O 2 at low overpotentials but increased to >3e − /O 2 at high overpotentials (Figure 3c), probably due to side reactions that occurred during oxidation of the discharge product or by‐products. XPS of the discharged cathode indicates that the discharge products contained O species of various chemical states, including superoxide, peroxide and oxide (Figure 3d), along with S species (attributed to the decomposition of DMSO by Pt or 1 O 2 ), and were covered by a carbon‐rich passivating layer, which can be attributed to CO 3 2− salts resulting from side reactions involving superoxide and 1 O 2 [9,29b] …”

Towards a Generalized ORR Mechanism in M2+‐Containing DMSO: Oxygen Reduction and Evolution in Ca2+‐Containing DMSO on Atomically Smooth and Rough Pt

“…The ORR mechanism on Au electrodes, which was described in detail in previous studies, is shown in Figure 8 for comparison. [23,24] In earlier studies on Au electrodes, [23,24] it was found that on a bare Au electrode in Ca(ClO 4 ) 2 containing DMSO, oxygen is reduced to peroxide, which dissolves into the bulk electrolyte. This peroxide can form a thick bulk layer on the electrode during continuous ORR, which slowly dissolves as no more peroxide is formed (as the potential is swept out of the ORR region).…”

“…As soon as the adsorbate layer is complete, exclusively superoxide is formed on top of this adsorbate layer [23] . Instructive is the comparison to the Li case: the difference of the theoretical standard potentials for peroxide and superoxide formation (Δ E 0 ) is larger than 2 V for Ca 2+ , while Δ E 0 is smaller than 0.5 V for Li + [24] . Thus, Li ‐ superoxide is formed at gold electrodes prior to peroxide (for kinetic reasons), but in presence of Ca 2+ peroxide is directly formed in DMSO.…”