Viologen Derivatives in Aqueous Organic Redox Flow Batteries: Progress and Perspectives

Aqueous organic redox flow batteries (AORFBs) are attracting increasing attention as intrinsically safe and scalable solutions for grid-level energy storage. Among various organic anolytes, viologens stand out for their tunable structures, two-electron redox behavior, and cost-effective synthesis from abundant precursors. This review comprehensively summarizes recent progress in viologen-based AORFBs, highlighting their core advantages and central role in defining system performance. The major challenges that currently limit practical application are critically analyzed, including molecular permeation, radical cation aggregation, two-electron transfer limitations, and alkalization-induced degradation. Strategies designed to address these limitations are then discussed, such as bipolar molecule design, conjugation extension, steric and size engineering, complexation, and substituent modification, emphasizing how tailored structural features can synergistically improve anolyte stability, solubility, and electrochemical performance. Furthermore, complementary in situ and ex situ characterization techniques have deepened understanding of redox mechanisms, degradation pathways, and aggregation states under operational conditions. Looking ahead, advancing viologen-based AORFBs will rely on designing stable, high-concentration electrolytes, achieving efficient two-electron cycling, and integrating artificial intelligence-guided molecular design to accelerate discovery. Together, these efforts aim to enable durable, high-energy-density systems and bridge the gap between laboratory research and commercial application.