Effect of Carboxylic Acid-Doped Carbon Nanotube Catalyst on the Performance of Aqueous Organic Redox Flow Battery Using the Modified Alloxazine and Ferrocyanide Redox Couple

Alloxazine and ferrocyanide are suggested as the redox couple for an aqueous organic redox flow battery (AORFB). Alloxazine is further modified by carboxylic acid (COOH) groups (alloxazine-COOH) to increase the aqueous solubility and to pursue a desirable shift in the redox potential. For obtaining a better AORFB performance, the overall redox reactivity of AORFB should be improved by the enhancement of the rate-determining reaction of the redox couple. A carboxylic acid-doped carbon nanotube (CA-CNT) catalyst is considered for increasing the reactivity. The utilization of CA-CNT allows for the induction of a better redox reactivity of alloxazine-COOH because of the role of COOH within alloxazine-COOH as a proton donor, the fortified hydrophilic attribute of alloxazine-COOH, and the increased number of active sites. With the assistance of these attributes, the mass transfer of aqueous alloxazine-COOH molecules can be promoted. However, CA-CNT does not have an effect on the increase of the redox reactivity of ferrocyanide because the redox reaction is not affected by the same influence of protons that the redox reactivity of alloxazine-COOH is affected by. Such a behavior is proven by measuring the electron transfer rate constant and diffusivity. With regard to AORFB full cell testing, when CA-CNT is used as a catalyst for the negative electrode, the performance of the AORFB increases. Specifically, the charge-discharge overpotential and infrared drop potential are improved. As a result, the voltage efficiency affected by the potentials increases to 64%. Furthermore, the discharging capacity reaches 26.7 A h·L^-1, and the state of charge attains 83% even after 30 cycles.