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

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, we demonstrate the adaptability of a single-step, high-yield hydrothermal reaction for nine viologen chloride salts. New empirical insights are gleaned into fundamental structure-property relationships for multi-objective optimization. A new asymmetric Dex-DiOH-Vi derivative showcased the ability to achieve 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 exhibited 14-days 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.