Abundant Organic Dye as an Anolyte for Aqueous Flow Battery with Multielectron Transfer

Organic redox flow batteries have the potential to surpass the challenges posed by inorganic electrolytes commonly used in flow batteries, thus achieving high performance and a sophisticated storage solution for the grid. Herein, we demonstrated a high performance aqueous organic redox flow battery (AORFB) utilizing a redox-active resource from nature, indigo carmine (5,5′-indigodisulfonic acid sodium salt) (IC-Na), as the anolyte. The 5,5′-indigodisulfonic acid (IC-H) is obtained through the substitution of sodium ions in IC-Na with protons (H⁺). The aqueous solubility of IC-H was increased dramatically from 0.035 to 0.760 M (1.52 M electron concentration) in protic solvents. The diffusion coefficients (IC-Na, 1.06 × 10^-5 cm² s ^-1 IC-H, 2.19 × 10^-5 cm² s ^-1) and reaction rate constants (IC-Na, 1.93 × 10^-4 cm s ^-1 IC-H, 1.86 × 10^-4 cm s^-1) of IC-Na and IC-H indicate rapid reaction kinetics. The highly soluble and affordable IC-H with fast redox kinetics was used as a sustainable anolyte by pairing with different catholytes. Moreover, a computational study was conducted which signifies the prospect of further improvements in solubility and voltage window by tuning the structure. Therefore, the environmentally benign and earth-abundant IC-H represents a promising choice for green and sustainable redox active anolyte of AORFB.