Use of voltage for recomposing degraded redox active molecules for flow battery applications

Abstract

Building energy storage systems from ubiquitous materials is the goal of researchers to lower the levelized costs of electricity storage to achieve a sustainable energy economy. Organic redox-active molecules are the better choice for achieving the dream provided they exhibit high solubility and stability/robustness during charge–discharge. Understanding how organic molecules degrade during the energy storage process is crucial to engineering structurally stable ones. In this study, we explain how 2,6-dihydroxyanthraquinone (2,6-DHAQ) undergoes capacity degradation due to dimer formation via a peroxo linkage. We coupled 2,6-DHAQ anolyte with ferrocyanide catholyte in a basic medium to achieve a flow battery. The dimer's oxidation potential is close to the redox potential of the ferro/ferricyanide, leading to quick polarization of the cell to zero volt during discharge. We broke the peroxo linkage using a high voltage reduction during charging. This allowed the cell to deliver its full capacity without slipping into any capacity imbalance between the anolyte and catholyte. Therefore, with this molecule, we could demonstrate a flow battery with 0.45 M (nearly saturated solution) 2,6-DHAQ reaching a capacity of ∼19 A h L⁻¹, which is otherwise difficult to reach owing to the capacity decay associated with dimer formation.