Effect of Mixed Acid Media on the Positive Side of the Hybrid Zinc-Cerium Redox Flow Battery

• Small concentrations of hydrochloric acid (0.5 - 1 mol L−1) in 0.6 mol L−1 Cerium and 4 mol L−1 MSA favored the Ce3+/4+ reaction making it significantly faster and more reversible.
• The D values of the mixed acid media were larger than those of containing only MSA, especially for the oxidation reaction and increased with increasing temperatures.
• The EIS results showed that the solution resistance of the mixed electrolytes is considerably lower than unmixed electrolyte.
• The stability of the solutions containing 0.6 mol L−1 was found dependent on temperature and state of charge. Higher temperatures (>40 °C) and state of charge (>60% Ce4+) lead to precipitation.

The electrochemical activity of the Ce3+/4+reaction in the presence of various mixed acid electrolytes containing methanesulfonic acid (MSA) as the base electrolyte along with hydrochloric, nitric and sulfuric acid was investigated as a function of acid concentration and reaction temperature on a Pt disk electrode. Cyclic voltammetry in 0.6 mol L−1 cerium and 4 mol L−1 MSA with 0.5 - 1 mol L−1hydrochloric acid showed that the reversibility and kinetics of the Ce3+/4+reaction were significantly enhanced compared to that containing MSA only. Higher diffusion coefficients (D) and exchange current densities (jo) were also obtained for the mixed acid media containing sulfuric and nitric acid, but in this case the reversibility of the reaction was not that improved. Higher concentrations of hydrochloric, nitric and sulfuric acid (>1 mol L−1) made the oxidation of Ce3+ very hard to delineate from the cyclic voltammograms because of the side reactions occurring at such high potentials. The exchange current density (jo), obtained through polarization resistance and electrochemical impedance spectroscopy measurements (EIS), increased with temperature over the range 25 to 55 °C. The EIS results showed that the solution resistance of the mixed electrolytes is considerably lower than that of the unmixed electrolyte facilitating the charge transfer reaction. Nonetheless, solubility issues arose at high Ce4+ concentrations (state of charge > 60%) and elevated temperatures (>40 °C).