Effects of sulfonated side chains used in polysulfone based PEMs for VRFB separator

Polysulfone (PSU) based Proton Exchange Membranes (PEMs) were synthesized from different pre-sulfonated monomers and evaluated for use in a vanadium redox flow battery (VRFB). Two types of sulfonated monomers, containing either an allyl sulfonate or styrene sulfonate group, were synthesized by a Mizoroki-Heck reaction from a 4,4'-difluorodiphenyl sulfone derivative. These monomers were polymerized with 4,4'-(hexafluoroisopropylidene)diphenol to give partially fluorinated PSUs with controlled placement and content of sulfuric acid moieties. PEMs were then fabricated by solution casting and characterized against the membranes made from traditional backbone sulfonated PSU. The new PEMs revealed desirable physical properties and performance in a VRFB cell competitive with the Nafion membrane. Furthermore, each PEM with a different acid side-chain structure exhibited varying properties and performance depending on its acid content, providing a path to optimization.