Highly Soluble Viologen-PEG Conjugates for Aqueous Organic Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs) are a promising solution for large-scale storage of renewable energy. These systems store electricity via electrochemically active organic molecules in an aqueous solvent, which offers superior safety and renewability compared with organic solvents. AORFBs provide a cost-effective alternative to aqueous vanadium-based redox flow batteries. The design of molecules with optimal electrochemical characteristics, solubility in water, and stability during extended cycling remains a significant challenge. In this study, the impact of PEGylation was investigated as a potential solution to address the aforementioned issues. Using a straightforward and effective synthetic methodology, a series of PEG-containing viologens with varying chain lengths, counterions, and symmetries were synthesized. Their physical and electrochemical properties, including solubility, viscosity, redox potential, and heterogeneous electron transfer rate constant, were studied. The results showed that PEG chains enhance solubility in water and that the chloride counterion increased solubility by 25% compared to the tosylate counterion. None of the studied modifications had a significant impact on the electrochemical properties, demonstrating the usefulness of N-substitution in tuning the solubility without compromising the use of the viologens as negolytes. Although asymmetric derivatives yielded higher solubilities (up to 2.7 M), the most stable organic negolyte under cycling conditions at a high concentration (1 M) was a symmetric viologen containing two PEG units (12 days with 0.29% capacity loss per day). The PEGylation of organic molecules proved to be a valuable approach to improving the aqueous solubility of electrolytes for use in aqueous organic redox flow batteries.