The effect of plasma treated carbon felt on the performance of aqueous quinone-based redox flow batteries

4,5-dihydroxybenzene-1,3-disulfonic acid (Tiron) and anthraquinone-2,7-disulfonic acid (AQDS) are interesting redox couples for aqueous quinone-based redox flow batteries (QRFBs) because of their high solubility and good reaction reversibility in acidic condition. Tiron is rate-determining material due to its slow reaction kinetics. To improve that, Tiron is transformed into 2,4,5,6-tetrahydroxybenzene-1,3-disulfonic acid (TironA), which is effectively formed during the first cycle of QRFB. Once TironA is formed, a desirable two-electron redox reaction with stable and reproducible charge and discharge step of QRFB occurs, although the electron transfer of TironA is still lower than that of AQDS. To further facilitate that of TironA, oxygen (O₂) and nitrogen (N₂) plasma-treated carbon felt (CF) electrodes are suggested. Oxygen functional groups formed onto CF by O₂ plasma become the active sites for redox reaction of TironA. As the amount of oxygen functional groups formed increases, the redox reactivity of TironA is enhanced. In contrast, when N₂ plasma is used, pyrrolic N and quaternary N are mainly formed. With the formation, (i) hydrogen atoms within pyrrolic N act as proton donor and interact with oxygen groups of TironA and (ii) nitrogen cations within quaternary N interact with the negatively charged atoms of TironA by electrostatic interaction. Thus, both reaction kinetic of TironA and performance of QRFB increase. Regarding the performance of QRFB using N₂ plasma-treated CF, its energy efficiency (EE), discharging capacity, and state of charge are 62%, 17.3 Ahr·L⁻¹ and 64.5% for 50 cycle, which correspond to excellent achievements.