Thermodynamic regulation of electrolyte to achieve air-tolerant viologen-based flow battery

Abstract

Aqueous organic redox flow battery (AORFB) is one promising grid-scale energy storage technology. However, the application is seriously hindered as AORFB cannot be stably operated in air, and the reason lies in the poor air tolerance of electroactive organics. Herein, we develop a thermodynamic regulation strategy through host-guest assembly to improve the air tolerance of the viologen-based anolyte. It is found that the reduced state of viologen forms a host-guest complex with the hydroxyethyl-β-cyclodextrin (HE-β-CD) molecule via hydrophobic effect, and the host HE-β-CD serves as a steric hindrance to kinetically shield the reduced viologen against oxygen attack. As demonstration, a flow battery fed with the viologen/HE-β-CD anolyte achieves a high Coulombic efficiency (∼99.9 %) and superior cyclability (∼97.73 % capacity retention rate over 300 cycles) when operated in air. This work provides a facile supramolecular method to tackle the most urgent issue of AORFB, which paves the way for its practical application.