Thermally modified graphite electrodes for the positive side of the zinc-cerium redox flow battery

In this study, thermal modification of graphite electrodes for the positive side of the Zinc-Cerium redox flow battery is found to enhance their performance and stability by providing a larger and more chemically active surface area to carry out the Ce(III)/(IV) reaction. Prolonged calcination (>9.5 h) and firing temperatures greater than 600°C lead to a physical deterioration of the samples while at 300°C no effect on the cerium reaction within the investigated reaction temperature range (25-55°C) is evident. When treated at 600°C for 6 h, the milder oxidation conditions result in a porous graphite electrode (7.13 m² g^-1) with a low charge transfer resistance (0.14 mΩ cm-²) and negligible weight loss (W_TL = 6%). This thermally treated electrode is robust under repetitive cycling (300 cyclic voltammograms) with no degradation occurring within the time frame of the experiment (20 h). The quasi-reversible kinetics of the Ce(III)/(IV) reaction improve with elevated temperatures (k^o = 6.0 × 10^-3 cm s^-1, D = 8.5 × 10^-6 cm² s^-1 and j_o = 2.91 × 10^-3 A cm^-2) and are comparable to the present electrode of choice on this flow battery, the Pt-Ti mesh electrode.