Viologen-based aqueous organic redox flow batteries: materials synthesis, properties, and cell performance

Aqueous organic redox flow batteries (AORFBs) are regarded as a promising solution for low-cost and reliable energy storage technology, contributing to large-scale integration of renewable energy sources. Among different organic redox materials, viologen molecules have received considerable attention as a negolyte in AORFBs due to their high solubility in water, reversible one/two-electron reduction, and easy functionalization. This review presents the recent progress of state-of-the-art viologen-based AORFBs and gives an overview of the structure–property–performance relationship. We summarize the synthetic strategies of viologens for AORFB application on the basis of three different classifications, i.e., conventional viologens, π-conjugated extended viologens, and polymeric viologens, with special emphasis on the synthesis procedure and final yield. Then, how the molecular engineering affects the viologen properties is discussed to address the limitations of current viologen-based AORFBs. Through presenting some representative examples, we further elaborate on the impacts of the properties of designed viologens on the battery performance when paired with an organic or inorganic posolyte. Finally, perspectives on the rational design of viologens, suitable posolytes, and custom-built membranes are provided for high-performance and long-life viologen-based AORFBs.