(Invited) ORR in Ca²⁺ Containing DMSO on Pt and Au Electrodes – Reaction Mechanism Studied By Dems, AFM and XPS and Varied Surface Structure

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.