Due to its theoretical high specific energy Li-O₂ systems have generated much interest within the electrochemical community [1]. Understanding the fundamentals of this system is import to improve it. Therefore, much effort has been put into characterizing the products and the mechanism of the underlying electrolyte systems and electrodes (also by ourselves [2-5]). Recently in the electrolyte, soluble redox mediators were proposed as additives to improve the sluggish kinetics of the oxygen reduction reaction (ORR) as well as the oxygen evolution reaction (OER)[6, 7]. Although the impact of this redox mediators is impressive, very little is known about the underlying mechanism.

Therefore, we focused in this study our research in the mechanism of redox mediation of the ORR. We applied rotating ring disc technique (RRDE) as well as the differential electrochemical mass spectrometry (DEMS) to get insights into the kinetics of the mediated ORR and into the amount of consumed electrons per oxygen. The DEMS experiments were performed in a thin layer geometry without convective transport within the cell. In addition, the DEMS setup is highly sensitive to the gases formed during ORR and OER. At higher electrode potentials, the amount of evolved CO₂ is reduced by adding a redox mediator to the electrolyte. We also observed that the amount of consumed oxygen is increased compared to the supporting electrolyte without redox mediator. In order to investigate the impact of the cationic charge density on the mechanism of the mediated ORR we also performed measurements in K⁺-, Mg²⁺-, Ca²⁺- containing electrolytes solutions. In our analysis of the data, we could calculate the share of current going into the mediation of the ORR. This analysis showed that for the investigated cations the amount of consumed electrons per oxygen molecule is 2 e^-/O₂. We found out, that the reaction speed of ORR could be influenced by changing the cation of the supporting electrolyte. The RRDE study revealed the generation of Li₂O₂ within the electrolyte volume through an analysis of the rotational dependency within the experiments.

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[2] P. Reinsberg, A. Weiss, P. P. Bawol and H. Baltruschat, J. Phys. Chem. C, 121, 7677 (2017).

[3] C. J. Bondue, P. P. Bawol, A. A. Abd-El-Latif, P. Reinsberg and H. Baltruschat, J. Phys. Chem. C, 121, 8864 (2017).

[4] C. Bondue, P. Reinsberg, A. A. Abd-El-Latif and H. Baltruschat, Phys. Chem. Chem. Phys., 17, 25593 (2015).

[5] H. M. A. Amin, C. Molls, P. P. Bawol and H. Baltruschat, Electrochim. Acta (2017).

[6] B. D. McCloskey and D. Addison, ACS Catalysis (2017).

[7] X. Gao, Y. Chen, L. Johnson and P. G. Bruce, Nat. Mater., 15, 882 (2016).