Assembling-induced redox property adjustment of Fe(III)/Fe(IV) electroredox couple-based commercial dye catholyte via bio-inspired multicoordination sphere construction strategy for stable aqueous redox flow batteries

Abstract

Redox flow batteries (RFBs) are capable of storing and integrating indirect clean energy. This is due to their scalability, safety, and reliability, as well as their independent decoupling of power and energy. Aqueous organic redox flow batteries (AORFBs) have redox-active materials that are composed of abundant elements, have tunable redox potential, greater solubility, and lower cost. Here We propose the use of an inexpensive commercial dye, naphthol green B (NGB), as a catholyte material for AORFBs. This is the first time that NGB has been utilized in the Fe(III)/Fe(IV) electroredox couple, demonstrating a high redox potential of 0.73 V vs. NHE under neutral conditions. Experimental characterizations and theoretical simulations indicate that hydroxypropyl-β-cyclodextrin (HP-β-CD) can serve as the secondary and outer coordination sphere of NGB, enhancing its electrochemical performance and stability, while also preventing transmembrane crossover. In the NGB/K3Fe(CN)6 test with HP-β-CD symmetric battery, NGB demonstrated excellent electrochemical stability. When used as anolyte with 1,1′-bis(3-sulfonatopropyl)-4,4′-bipyridinium (Spr)2V, the NGB/(Spr)2V AORFB with HP-β-CD demonstrated exceptional cycling performance at current rates ranging from 5 to 30 mA cm−2. It is worth noting that the NGB/(Spr)2V AORFB maintained over 99 % of its total capacity or experienced less than 0.00125 % capacity decay per cycle, with an average Coulombic efficiency of almost 100 % over 800 cycles at 10 mA cm−2. NGB is a novel and superior catholyte that offers multiple options for studying AORFBs and the potential for large-scale energy storage applications.