Reduced Flow Battery Capacity Fade from Mixed Redox-Active Organics Beyond the Rule of Mixtures

Aqueous redox flow batteries using organic molecules offer a sustainable solution for long-duration energy storage, but their commercialization is challenged by capacity fade from molecular degradation. Here, we introduce a mixed-electrolyte strategy that enhances the stability of 2,6-dihydroxyanthraquinone (DHAQ) by enabling continuous in situ regeneration of redox-active species under standard operating conditions. By incorporating 0.1 M 4,4′-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate (DBEAQ) into a 0.1 M DHAQ electrolyte, the capacity fade rate is reduced from 4.7% to 0.9% per day, representing a 62% decrease with respect to the value of 2.35%/day expected from the rule of mixtures for a non-interacting mixture. Increasing the DBEAQ concentration to 0.2 M further decreases the fade rate to 0.43% per day, representing a 73% reduction relative to the expected value of 1.57%. Electrochemical and NMR analyses reveal that regeneration occurs via a two-step process: chemical oxidation of anthrone to its dimer, followed by electrochemical oxidation of the dimer back to DHAQ. This regeneration pathway is made possible by the mixed-electrolyte system, which shifts the concentration balance between DHA and dimer toward dimer formation, as DBEAQ acts as an oxidant due to its more positive redox potential. We demonstrate that this mixed-electrolyte strategy is not limited to DBEAQ, highlighting its broader applicability across the anthraquinone species. Moreover, the underlying regeneration mechanism may be extended to other anthraquinone derivatives that degrade through similar pathways, offering a promising framework to enhance electrolyte longevity and stability in organic redox flow batteries.