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 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, OHad). 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. Reported here is a new model capable of rationalizing, and ultimately understanding, pH-dependent electrocatalysis of the ORR in aqueous media, as well as in Li-O2 electrochemistry in organic solvents. The model is based on a bifunctional mechanism where effective catalysts must optimize the amount of surface surface coverage by HOad···H2O (alkaline), HOad···H3O+ (acid) and LiO2···H2O (organic solvents) complexes, which place water in a configurationally favorable position for proton transfer to O2- and HO2-, and bare metal sites, which facilitate electron transfer to intermediates. In aqueous systems the optimal balance of these sites is observed on Pt3Ni (111), which in alkaline solution exhibits the highest activity ever measured for electrocatalysis of the ORR. Furthermore, we observe that in organic solutions water acts simultaneously as a promoter in the production of Li2O2 and as a catalyst, regenerating itself through a sequence of steps that include the formation and recombination of H+ and OH- (Figure 1)

Water as promoter and catalyst in dioxygen electrochemistry at aqueous and organic electrified interfaces

249th National Meeting (2015)

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