The influence of operational parameters on the performance of an undivided zinc–cerium flow battery

An undivided zinc–cerium redox flow battery was studied under a wide range of operational conditions, including: (i) electrolyte composition; the concentrations of ([Zn2+], [Ce3+] and [H+]), (ii) current density (0–80 mA cm−2), (iii) electrolyte flow linear velocity (0.64–7.0 cm s−1) and (iv) temperature (20–60 °C). The charge efficiency increased at higher current densities and at higher electrolyte flow velocities. Unlike the divided zinc–cerium system, the charge–discharge performance decreased at higher temperature, since oxygen evolution became increasingly favored at the positive electrode. The use of a low acid concentration led to a poor conversion of Ce(III) to Ce(IV) ions during the discharge cycle. Mixed electrolytes containing methanesulfonate and sulfate anions have been evaluated at a high Ce(III) ion concentration, e.g. 0.4 mol dm−3. After charging the battery for 4 h, the conversion of Ce(III) to Ce(IV) ions became less efficient over time due to a greater fraction of the current being used in oxygen evolution. Critical aspects for improvements in the battery performance are considered.

► The performance of an undivided zinc–cerium flow battery under different conditions of temperature, concentration and electrolyte flow rate, was evaluated.
► Mixed electrolytes were considered; methanesulfonate and sulfate anions were tested.
► In a 30 min charge/discharge at 20 mA cm−2, charge and energy efficiencies were 82% and 72%, respectively.
► After 4 h charge, the conversion of Ce(III) to Ce(IV) became less efficient with the oxygen evolution reaction becoming more favored.
► After a larger number of cycles the carbon felt electrode was electro- and chemically-oxidized by Ce(IV) leading to degradation of the positive electrode.