Investigating the Kinetic Mechanisms of the Oxygen Reduction Reaction in a Nonaqueous Solvent

The high theoretical energy density of lithium–oxygen batteries brings the promise of higher performance than existing batteries, but several technological problems must be addressed before actual applications are made possible. Among the difficulties to be faced is the slow oxygen reduction reaction (ORR), which requires a suitable choice of catalysts and electrolytic solution. This can only be achieved if the kinetics and mechanism of this reaction are known in detail. In this study, we determined the rate constants for each elementary step of ORR for a platinum electrode in 0.1 mol·L^–1 LiClO₄/1,2-dimethoxyethane (DME), using a kinetic model in the frequency domain. We found that the energy storage capacity of lithium–air batteries can be increased by converting a large amount of lithium superoxide into lithium peroxide during the electrochemical step in comparison with chemical disproportionation. The mechanisms for ORR were supported by data from an electrochemical quartz crystal microbalance (EQCM): ORR could be distinguished from parasitic reactions induced by solvent degradation, and agglomerates of Li_xO₂ (1 ≤ x ≤ 2) were adsorbed on the electrode. The rate-limiting step for ORR was the electron transfer to the oxygen molecules strongly adsorbed onto platinum sites, particularly as a large amount of reaction product (Li₂O₂) adsorbed onto the electrode. Even though Pt sheets are likely to be impracticable for real applications due to their low surface area, they were useful in making it possible to determine the kinetics of ORR steps. This can now be employed to devise more involved electrodes, such as those containing dispersed Pt nanoparticles.