Abstract
Concentrated aqueous electrolytes, called water-in-salt (WIS), have recently emerged as a new class of electrolytes. They permit to widen the electrochemical stability window of water, thermodynamically limited to 1.23 V. This is occurring because hydrogen reduction (HER) and oxygen oxidation (OER) reactions are shifted. They are currently attracting major interest for the trending development of aqueous rechargeable batteries.This thesis work focused on the factors causing the shift in the onset potential of the oxygen evolution reaction to more positive values with the increase of salt concentration. In order to reach this goal, theoretical and experimental studies were carried out. In a first part, the contribution of the decrease in the amount of free water on the shift of the water oxidation potential was investigated. For this, on one hand the effect of salt concentration on water activity was quantified. On the other hand, the impact of water activity on the Nernst potential of OER was determined. In a second part, the effect of water activity as well as the steric effect of anions on the OER potential was evaluated, using the Butler-Volmer equation. These two studies were realized by adapting the formalisms commonly used for conventional dilute electrolytes. In a third component, the chemical and electrochemical processes occurring in positive potentials were studied. All the work carried out has shown that water oxidation is not the only oxidation reaction occurring at a high positive potential. Moreover, it has been shown that the redox processes involved are complex and lead to the formation of a film on the surface of the positive electrode.