Viologen Hydrothermal Synthesis and Structure–Property Relationships for Redox Flow Battery Optimization

Abstract Aqueous organic redox flow batteries (AORFBs) are an emerging technology for fire safe grid energy storage systems with sustainable material feedstocks. Yet, designing organic redox molecules with the desired solubility, viscosity, permeability, formal potential, kinetics, and stability while remaining synthetically scalable is challenging. Herein, the adaptability is demonstrated of a single‐step, high‐yield hydrothermal reaction for nine viologen chloride salts. New empirical insights are gleaned into fundamental structure–property relationships for multiobjective optimization. A new asymmetric Dex‐DiOH‐Vi derivative showcases an enhanced solubility of 2.7 m with minimal tradeoff in membrane permeability. With a record viologen cycling volumetric capacity (67 Ah L −1 anolyte theoretical), Dex‐DiOH‐Vi exhibits 14‐d of stable cycling performance in anolyte‐limiting AORFB with no crossover or chemical degradation. This work highlights the importance of designing efficient synthetic approaches of organic redox species for molecular engineering high‐performance flow battery electrolytes.