(Invited) Water As Promoter and Catalyst in Dioxygen Electrochemistry at Aqueous and Organic Electrified Interfaces

Water and di-oxygen are the most important and pervasive chemicals that influence all areas of science, from physics to biology to chemistry, and are particularly important in governing phenomena occurring at solid‑liquid interfaces in electrochemical environments. One property that sets aqueous-based electrochemical interfaces apart from all other interfaces is the unique arrangements of water molecules within the distinctive interactions between metal surfaces and hydrated ions. These interactions are complex, involving molecular and dissociative adsorption, as well as van der Waals forces both between water molecules and neighboring ions in the compact part of the double layer (i.e. solvation) and between water and covalently-bonded co-adsorbates (e.g. adsorbed hydroxyl species, OH_ad). Despite the enormous fundamental and technological significance in unfolding these complexities, previous work has treated the water molecule itself simply as a reactant needed to satisfy the stoichiometry of electrochemical reactions, rather than a vital hydrogen-/hydroxyl-donor molecule that can promote the rates of transformation of reaction intermediates to final products. The prime example of such an “incomplete” treatment of water is the oxygen reduction reaction (ORR) in “proton”-O₂ fuel cell systems and in lithium-O₂ battery systems. Current models guiding the development of new materials for these systems rely on a single descriptor - the binding energy of oxygen intermediates – that cannot explain many “anomalous” phenomena.

Here, we introduce a new model in which water itself plays a critical role in determining the reaction kinetics in dioxygen electrochemistry. The model is based on the formation of HO_ad···H₂O (aqueous solutions), and LiO₂···H₂O (organic solvents) complexes that place water in a configurationally favorable position for proton transfer to weakly adsorbed intermediates. We found that water plays a dual role, acting as a key promoter in both environments as well as a catalyst in Li-O₂ systems