Physicochemical and electrochemical characterization of salt-in-water and water-in-salt potassium and lithium acetate electrolytes

Abstract

We report the physicochemical and electrochemical properties of various concentrations of potassium and lithium acetate (OAc) electrolytes from dilute to near saturation as well as mixed potassium/lithium acetate (32 m KOAc + 8 m LiOAc). The electrochemical stability window varied in the order of Pt < Au < highly ordered pyrolytic graphite (HOPG) < glassy carbon electrodes, and expanded when the salt concentration increased reaching 3.5 V in the dual cation water-in-salt electrolyte. This increase was due to the shift in the onset potential of the hydrogen evolution and oxygen evolution reactions to more negative and positive potential values, respectively. It is noteworthy that there is a larger shift in the onset potential of the oxygen evolution on the Pt electrode. The capacitance at the potential of zero charge of the HOPG electrode was found to be about 4 μF cm⁻² and almost independent of the electrolyte concentration while the potential of zero charge shifted to more negative values upon increasing the acetate salt concentration. Using ferricyanide and ferrocene methanol as redox probes, their redox potential, ion transport and electron transfer kinetics in acetate-based electrolytes were investigated. The apparent redox potential shifted in the opposite direction; to more positive and negative values for ferricyanide and ferrocene methanol redox systems, respectively. A significant decrease in the diffusion coefficient of ferricyanide ions, i.e. 4 orders of magnitude, was observed in the dual cation water-in-salt compared to a dilute single salt electrolyte. High viscosities might be partially responsible for the decrease of the heterogeneous rate constants obtained for the redox probes but other factors such as interactions with water, electrolyte ions and electrodes might be at play as well.