Interfaces in Metal-Oxygen Batteries

Abstract

This study deals to a large extent with the formation of adsorbates during the oxygen reduction reaction in organic electrolytes on gold and platinum surfaces. The mainly used methods are voltammetry (also at rotating ring disc electrodes), differential electrochemical mass spectrometry and atomic force microscopy. In the context of this work a novel low-volume electrochemical cell was developed, which shows several advantages over the frequently used H-cell. This cell was used for investigations on the applicability of hydrate melt electrolytes for metal-air batteries. Concerning these investigations it will be shown, that hydrate melt electrolytes are in fact inapplicable in this context. The operable electrochemical potential window of the electrolyte proved to be very small, other than expected from the reduced solvent activity. However, aprotic alternatives are proposed. The key part of this work deals with the oxygen reduction in DMSO. For Li⁺ it was found, that an adsorbate is formed during the oxygen reduction, which to large extent deactivates the electrode. This however was not the case for Ca²⁺ containing DMSO. In this case the formed CaO₂/CaO adsorbate only inhibits further peroxide formation (= main reaction product in this case). Superoxide can still be formed through or on top of the adsorbate on at least Au and Pt electrodes. The formation of the adsorbate in both cases is a competitive reaction to the formation of dissolving peroxide. Knowing this, a detailed mechanism is proposed for the oxygen reduction on Au and Pt electrodes in Ca²⁺ containing DMSO. Including older data, a correlation was found between the tendency of the oxide species formed during the ORR to bind to the electrode surface or to the alkaline earth metal cation (ion-pairing) and the contribution of dissolving peroxide to the ORR crent density. This allowed for proposing a more generalized ORR mechanism for all alkaline earth metal cations, concerning the ORR in DMSO for several electrode materials. Additionally it was found that 0.1 M LiCl + 0.1 M CaCl₂ completely dissolves in DMSO, though CaCl₂ is nearly insoluble in DMSO. This might indicate the formation of a [CaCl₃]^-complex. Furthermore, the ORR in this system hints to the presence of an autocatalytic reaction at the disc electrode in an RRDE experiment, which would have to be confirmed. For organic electrolytes an ordering perpendicular to the electrode surface, was revealed by recording force separation curves via AFM. In order to model such layers, a layer of sodium dodecylsulfonate (SDS*) was examined in comparison to the observations from literature made for the alkylsulfate and alkylthiolate systems. It will be shown that the flat adsorbed SDS*, on Au(111) electrodes, undergoes a phase transition to a condensed state as the electrode is charged positively.