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

Aqueous organic redox flow batteries offer a sustainable approach to long-duration energy storage but suffer from molecular degradation. Here, we present a mixed redox-active strategy that stabilizes 2,6-dihydroxyanthraquinone (DHAQ) by enabling in situ regeneration of redox-active species under standard operating conditions. By incorporating 0.1 M of 4,4′-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate (DBEAQ) into a 0.1 M DHAQ electrolyte, the fade rate is reduced from 4.7% to 0.9% per day, a 62% decrease relative to the 2.35%/day expected from a noninteracting mixture. Increasing DBEAQ concentration to 0.2 M further lowers fade to 0.43% per day, representing a 73% reduction relative to the expected value of 1.57%. Electrochemical and NMR data show that regeneration occurs via chemical oxidation of anthrone to a dimer, followed by electrochemical reoxidation to DHAQ. This approach is not limited to DBEAQ, suggesting broad applicability to other anthraquinones. The underlying regeneration mechanism offers a general framework for improving electrolyte stability in organic redox flow batteries.