Electrochemical Reaction Order of the Oxygen Reduction Reaction in Li⁺-Containing DMSO

Understanding the mechanism underlying the oxygen reduction and evolution in aprotic solvents is crucial for developing secondary metal–air batteries. Despite much scientific effort, the mechanism of the oxygen reduction in aprotic solvents in the presence of Li + ions is still not fully understood. In this work, rotating ring–disk electrode and differential electrochemical mass spectrometry experiments have been employed to investigate the influence of the oxygen partial pressure on the oxygen reduction and evolution reaction at gold, glassy carbon, and platinum electrodes. A further aim was to elucidate the different pathways leading to peroxide formation and to analyze their importance for the overall reaction. As expected, the electrochemical reaction order for the superoxide formation is unity. Despite that, the reaction order for the reaction path leading to peroxide is below unity, indicating the participation of an adsorption step. This is further indicated by the finding that the amount of peroxide deposited on the surface changes only very slightly upon increasing the oxygen concentration. While the oxygen reduction at glassy carbon and platinum takes place via the parallel formation of superoxide and peroxide, there is a distinct transition between superoxide and peroxide formation at the gold electrode. This transition occurs close to a potential where the rate of superoxide formation is mainly limited by diffusion. By investigating the rotation dependence of the collection efficiency, it can be shown that the peroxide formation at gold indeed proceeds via a direct reduction step without the formation of a soluble intermediate. This finding adds to the current mechanistic picture which only differentiates between the electrochemical and the chemical formation of lithium peroxide. For platinum and glassy carbon, this direct reduction without the formation of soluble intermediates cannot be noted. Based on these observations, a reaction scheme is presented including rate constants for the different reaction pathways.