Water as a Promoter and Catalyst for Dioxygen Electrochemistry in Aqueous and Organic Media

Water and oxygen electrochemistry lies at the heart of interfacial processes controlling energy transformations in fuel cells, electrolyzers, and batteries. Here, by comparing results for the ORR obtained in alkaline aqueous media to those obtained in ultradry organic electrolytes with known amounts of H<sub>2</sub>O added intentionally, we propose a new rationale in which water itself plays an important role in determining the reaction kinetics. This effect derives from the formation of HO<sub>ad</sub>···H<sub>2</sub>O (aqueous solutions) and LiO<sub>2</sub>···H<sub>2</sub>O (organic solvents) complexes that place water in a configurationally favorable position for proton transfer to weakly adsorbed intermediates. We also find that, even at low concentrations (<10 ppm), water acts simultaneously as a promoter and as a <i>catalyst</i> in the production of Li<sub>2</sub>O<sub>2</sub>, regenerating itself through a sequence of steps that include the formation and recombination of H<sup>+</sup> and OH<sup>–</sup>. We conclude that, although the binding energy between metal surfaces and oxygen intermediates is an important descriptor in electrocatalysis, understanding the role of water as a proton-donor reactant may explain many anomalous features in electrocatalysis at metal–liquid interfaces.