Upcycling Plastic Waste into Sulfonated PET Ion Exchange Membranes for Redox Flow Batteries

The development of low-cost, sustainable, and fluorine-free proton exchange membranes (PEMs) is vital for enabling scalable vanadium redox flow battery (VRFB) technologies. In this work, we report the use of sulfonated polyethylene terephthalate (sPET) membranes, synthesized via chemical modification of recycled PET waste bottles, as a viable alternative to conventional Nafion. The sPET membrane demonstrated competitive electrochemical performance, with charge–discharge cycling confirming stable operation at moderate current densities and Coulombic efficiencies ranging from 66% to 80%. Electrochemical impedance spectroscopy revealed comparable charge transfer resistance to Nafion (31 vs 30 mΩ), especially under dynamic flow conditions. Despite slightly lower ionic conductivity and greater ohmic losses under stagnant conditions, the sPET membrane exhibited acceptable voltage and energy efficiencies (up to 59%), validating its applicability for energy storage systems. These results showcase sPET as a promising, sustainable, and cost-effective PEM derived from plastic waste, with strong potential for practical deployment in VRFB systems.