Highly\nSoluble Viologen-PEG Conjugates for Aqueous\nOrganic Redox Flow Batteries

Aqueous\norganic redox flow batteries (AORFBs) are a promising solution\nfor large-scale storage of renewable energy. These systems store electricity\nvia electrochemically active organic molecules in an aqueous solvent,\nwhich offers superior safety and renewability compared with organic\nsolvents. AORFBs provide a cost-effective alternative to aqueous vanadium-based\nredox flow batteries. The design of molecules with optimal electrochemical\ncharacteristics, solubility in water, and stability during extended\ncycling remains a significant challenge. In this study, the impact\nof PEGylation was investigated as a potential solution to address\nthe aforementioned issues. Using a straightforward and effective synthetic\nmethodology, a series of PEG-containing viologens with varying chain\nlengths, counterions, and symmetries were synthesized. Their physical\nand electrochemical properties, including solubility, viscosity, redox\npotential, and heterogeneous electron transfer rate constant, were\nstudied. The results showed that PEG chains enhance solubility in\nwater and that the chloride counterion increased solubility by 25%\ncompared to the tosylate counterion. None of the studied modifications\nhad a significant impact on the electrochemical properties, demonstrating\nthe usefulness of <i>N</i>-substitution in tuning the solubility\nwithout compromising the use of the viologens as negolytes. Although\nasymmetric derivatives yielded higher solubilities (up to 2.7 M),\nthe most stable organic negolyte under cycling conditions at a high\nconcentration (1 M) was a symmetric viologen containing two PEG units\n(12 days with 0.29% capacity loss per day). The PEGylation of organic\nmolecules proved to be a valuable approach to improving the aqueous\nsolubility of electrolytes for use in aqueous organic redox flow batteries.