Highly soluble viologen-PEG conjugates for aqueous organic redox flow batteries

Aqueous organic redox flow batteries (AORFBs) are a promising solution for the 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 to organic solvents. Furthermore, AORFBs provide a cost-effective alternative to aqueous vanadium-based redox flow batteries. However, designing molecules with adequate electrochemical properties that are also soluble in water and stable during long-term cycling remains a challenge. In this study, we investigate the impact of PEGylation as a potential solution to address these issues. A simple and efficient synthetic approach was used to prepare PEG-containing viologens, with different chain lengths, counterions and symmetries. Their physical and electrochemical properties, including solubility, viscosity, redox potential and heterogeneous electron transfer rate constant, were studied. The results demonstrate that PEG chains enhance solubility in water, and that the chloride counterion increases 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 high concentration (1 M) was a symmetric viologen containing two PEG units (12 days with 0.29% of capacity loss per day). Therefore, the PEGylation of organic molecules represents a valuable approach to improving the aqueous solubility of electrolytes for use in aqueous organic redox flow batteries.