Alkaline aqueous redox flow batteries using 2,5-dihydroxy-1,4-benzoquinone and ferrocyanide adopting bismuth and carboxylic acid functionalized carbon nanotube catalyst

2,5-Dihyroxy-1,4-Benzoquinone (BQ-OH) and ferrocyanide (FeCN) are used as redox couple for alkaline aqueous redox flow battery (ARFB) due to the high solubility of BQ-OH in potassium hydroxide (KOH) electrolyte and low cost. Redox kinetics of BQ-OH that is slower than FeCN are enhanced by using bismuth and carboxylic acid functionalized carbon nanotube (Bi-CA-CNT) catalyst. With the catalyst, (a) nanostructure of CNT and (b) strong electrostatic interaction between positively charged bismuth and negatively charged BQ-OH increase both the active sites produced onto electrode and the amount of BQ-OH molecules attracted to electrode. Furthermore, the peak potential for BQ-OH reaction is reduced from 0.47 to 0.20 V. Even in ARFB single cell tests operated in 40 mA cm⁻², energy efficiency (EE) and capacity retention rate of ARFB including Bi-CA-CNT are higher than those of ARFB operated without catalyst, while when current density increases sequentially, ARFB including Bi-CA-CNT is well operated even at 120 mA cm⁻², while its discharging capacity measured at 100 mA cm⁻² (2.16 Ah L⁻¹) is higher than that measured without catalyst (1.13 Ah L⁻¹). Conclusively, electrode modification by Bi-CA-CNT promotes EE (from 73% without catalyst to 78% with the catalyst) and capacity utilization of ARFB (from capacity retention of 19.6% without catalyst to 42.6% with the catalyst at 100th cycle) with a better power density and discharge capacity.