Dual-Platform Single-Molecule Redox-Targeting Reaction in Neutral Anthraquinone Flow Batteries

Aqueous organic redox flow batteries (AORFBs) represent a promising technology for large-scale energy storage due to their high abundance in nature, safety, cost-effectiveness, and flexibility in molecular design. However, AORFBs suffer from a low energy density and unsatisfactory stability. Herein, we report a pH-neutral and high-energy-density anthraquinone-based flow battery utilizing a redox-targeting (RT) concept. Interestingly, single-molecule redox-targeting (SMRT) reaction with two voltage plateaus is successfully designed between redox mediator (RM) anthraquinone-2,7-disulfonic acid disodium salt (2,7-AQDS) and solid material poly(2,5-dihydroxy-1,4-benzoquinone-3,6-methylene) (PDBM) harnessing proton-coupled electron transfer, combining the advantages of both SMRT and dual-RM RT systems. The neutral RT flow battery in this work exhibits impressive performance with a series of excellent results including a high volumetric capacity of 97.1 Ah L^-1 (based on the volume of anolyte, 14.22 times that of blank RFB), a coulombic efficiency of 99.99%, a capacity retention of 99.9% per cycle, and solid material utilization of 92.5%. Inspiringly, operando nuclear magnetic resonance and ultraviolet-visible spectroscopy are employed to dynamically monitor the solid-liquid interface between PDBM and AQDS during battery cycling, successfully demonstrating the two reversible redox-targeting reaction processes. This work develops a neutral, energy-dense aqueous organic redox flow battery, extensively elucidating the fundamental mechanism of the dual-plateau SMRT reaction, presenting a promising solution to large-scale long-duration energy storage.