Electrochemical Characterization of Salt-in-Water and Water-in-Salt Zinc Sulfate and Zinc Acetate Electrolytes

The electrochemical properties of Zn(OAc)(2) or ZnSO4 with water at different concentrations are investigated. The electrochemical stability window follows Pt < Au < glassy carbon electrodes, and expands with increasing concentration of electrolytes. The change in salt concentration does not significantly change the double layer capacitance, and the potential of zero charge of Pt, Au and glassy carbon electrodes are estimated to be 0.25-0.35 V-SCE, 0.05 V-SCE, and -0.20 V-SCE, respectively. With hydroquinone as the redox probe, the redox electrochemistry, ion transport and electron transport kinetics in these electrolytes are studied. The apparent redox potential of hydroquinone increases with the electrolyte concentration, and the diffusion coefficients of hydroquinone in Zn(OAc)(2) and ZnSO4 electrolytes decrease with the increase of electrolyte concentration. The electron transfer rate constants (k) between the electrode and hydroquinone in Zn(OAc)(2) and ZnSO4 electrolytes range in 1.28-1.46 cm s(-1) and 0.29-0.81 cm s(-1), respectively. The lower k in ZnSO4 electrolytes is related to the lower solvent reorganization energy, the interaction of electroactive ions with water, and the interaction of electrolyte cations.