Aqueous organic redox flow batteries using naphthoquinone and iodide maintaining pH of electrolytes desirably by adoption of carboxylic acid functionalized carbon nanotube catalyst

In this study, 2-hydroxy-1,4-naphthoquinone (NQ-OH) and potassium iodide (KI) are utilized as active materials for aqueous organic redox flow battery (AORFB). NQ-OH is one of the quinone derivatives and has excellent electrochemical properties, while the solubility of KI in water is high as 8.4 M. NQ-OH and KI are dissolved into potassium hydroxide (KOH) and potassium chloride (KCl), respectively, and used as anolyte and catholyte with open-circuit voltage of 1.15 V. The pH of KCl solution increases under different anolyte and catholyte conditions, and such pH changes induce undesirable irreversible reaction of KI. This pH imbalance between anolyte and catholyte is suppressed by applying the carboxylic acid functionalized carbon nanotube (CA-CNT) doped carbon felt (CA-CNT@CF) in positive electrode. The functional groups of CA-CNT react with hydroxide ions transported from anolyte, preventing the pH change of catholyte and keeping the pH balance between anolyte and catholyte. With that, the cycle stability of AORFB using CA-CNT@CF is twice better than that using pristine CF. Specifically, in AORFB using CA-CNT@CF operated under 40 mA cm⁻², discharge capacity of 4.83 Ah L⁻¹, utilization of 90.2%, columbic and energy efficiencies of 98.6% and 64.2% are recorded for 50 cycles, while no degradations are observed in anolyte. Since the performances of this AORFB using one membrane containing CA-CNT@CF are better than those of other conventional studies using dual membrane, this study can be a cornerstone to establish the simple and cost-effective AORFBs.