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

Aqueous organic redox flow batteries (AORFBs) are an emerging grid energy storage 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, we demonstrate the adaptability of a single-step, high-yield hydrothermal reaction for viologen chloride salts, which have shown promise for pH neutral AORFB. Nine viologens, including five symmetric and four asymmetric, were synthesized in high purity for physiochemical and electrochemical characterization. New empirical insights are gleaned into fundamental structure-property relationships for multi-objective optimization. Ultimately, a new Dex-DiOH-Vi derivative showcased record viologen concentration of 2.5 M in an anolyte-limiting AORFB with 14-days of stable cycling performance. This work highlights the importance of designing efficient synthetic approaches of organic redox species for molecular engineering high-performance and sustainable flow battery electrolytes.