Charge/discharge cycles on Pt and Pt-Ir based electrodes for the positive side of the Zinc-Cerium hybrid redox flow battery

• The optimum rotation rate was found to be between 600 rpm and 900 rpm.
• The increase of temperature from 25 °C to 60 °C increased the voltage efficiency (ηV) for all the substrates examined.
• The majority of the coatings examined lost some of their activity on prolonged cycling (150 cycles).
• The substrates containing higher Pt coatings (optimum 10 g m−2) showed higher ηV values and cycling stability.
• The current densities that could be applied on the substrates tested were not greater than 25 mA cm−2.

In this study, the charge/discharge cycling behaviour of the Ce3+/Ce4+ redox couple in methanesulfonic acid medium on various Pt and Pt-Ir coated titania based substrates was investigated as a function of solution flow rate, temperature and charge/discharge current density using a rotating disk electrode. Although superior performances (in terms of the voltage efficiency, ηV) were obtained from the electrodes containing higher amounts of platinum, a deterioration of these electrodes with cycling was evident (after 150 cycles). At rotation rates between 600 rpm and 1200 rpm, high ηV (>90%) could be obtained as long as the current density j applied was below the mass transport limiting current density, jL. For j's above this value, the oxygen evolution reaction occurred with Ce3+ oxidation whereas the Ce4+ reduction was accompanied by oxygen reduction, resulting in lowered ηV's. Better stability towards cycling and higher ηV's were observed at 60 °C compared to 25 °C, especially for the etched Pt electrodes. The optimum loading for the Pt substrates was 3 g m−2 while for the case of the heat treated samples 10 g m−2 of Pt.