In this study, the oxygen reduction reaction (ORR) in Ca2+‐containing dimethyl sulfoxide (DMSO) at well‐ordered and rough electrode surfaces is compared by using cyclic voltammetry, differential electrochemical mass spectrometry, rotating ring disk electrode, and atomic force microscopy measurements. Slightly soluble CaO2 is the main product during early ORR on gold electrodes; after completion of a monolayer of CaO and/or CaO2, which is formed in parallel and in competition to the peroxide, only superoxide is formed. When the monolayer is completely closed on smooth annealed Au, no further reduction occurs, whereas on rough Au a defect‐rich layer allows for continuous formation of superoxide. CaO2 formed either via two subsequent 1 normale- transfer steps or by disproportionation of superoxide may be deposited on top of the CaO/CaO2 adsorbate layer. The slow dissolution of the peroxide particles is demonstrated by AFM. Whereas a smooth CaO/CaO2‐covered electrode shows severe deactivation and a CaO/CaO2‐covered rough electrode allows for diffusion‐limited superoxide formation, on single crystals peroxide formation is more pronounced. The reason is most likely the lack of nucleation sites for the blocking CaO/CaO2 layer. RRDE investigations showed sluggish reoxidation kinetics of the dissolved peroxide, which are most likely due to ion pairing with Ca2+. The apparent transfer coefficient is estimated by using variation of the electrode roughness, confirming the result of the usual Tafel analysis and indicating an equilibrated first 1 normale- transfer.

The electrodeposition of silver on Au(111) was investigated using lateral force microscopy (LFM) in Ag+ containing sulfuric acid. Friction force images show that adsorbed sulfate forms ()3×7R19.1∘ structure (θsulfate=0.2) on Au(111) prior to Ag underpotential deposition (UPD) and (3×3R30∘) structure (θsulfate=0.33 ) on a complete monolayer or bilayer of Ag. Variation of friction with normal load shows a non‐monotonous dependence, which is caused by increasing penetration of the tip into the sulfate adlayer. In addition, the friction force is influenced by the varying coverage and mobility of Ag atoms on the surface. Before Ag coverage reaches the critical value, the deposited silver atoms may be mobile enough to be dragged by the movement of AFM tip. Possible penetration of the tip into the UPD layer at very high loads is discussed as a model for self‐healing wear. However, when the coverage of Ag is close to 1, the deposited Ag atoms are tight enough to resist the influence of the AFM tip and the tip penetrates only into the sulfate adlayer.

In situ electrochemical lateral force microscopy (EC-LFM) has been employed to study the ordered structure of Li + containing tetraglyme (G4) in front of I-modified Au(111) and its influence on friction as function of normal load. Since the effect of water in aprotic electrolytes is a critical issue, the influence of water on the ordered structure and friction was also been investigated. Lateral force maps recorded at low normal load (F N < 30 nN) show that the adsorbed iodine forms a ffi ffi ffi 3 p � ffi ffi ffi 3 p À � R30 � structure (q I ¼ 0:33Þ independent of potential. With increasing normal load, observed atomic corrugations at both potentials (0.45 V and À 0.4 V) are in agreement with the Au(111) (1 � 1Þ structure while returning to a ffi ffi ffi 3 p � ffi ffi ffi 3 p À � R30 � structure with decreasing normal load. Thus we conclude that the AFM tip penetrates into the iodine adlayer without irreversible wear. Astonishingly, no clear friction increase was observed upon penetration into the iodine adlayer; also no corresponding step was found in force separation (FS) curves. On the other hand, FS curves for I-modified Au(111) in pure G4 solvent and Li + containing electrolyte clearly showed several steps suggesting that G4 molecules are forming up to five ordered layers. It is noteworthy that we observed two different push-through forces for the innermost layers. Considering the higher reproducibility of FS curves on I-modified Au(111) compared to bare Au(111) we assume that the low surface energy of the iodine monolayer leads to negligible interaction between G4 molecules and iodine adlayer, resulting in less perturbations of the structure by the solid phase and also an increase of push-through force. Charts of friction forces vs. normal load are found to be independent of applied potential and the concentration of water.

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.