A Fast and Precise Synthetic Tool for the Fabrication of Ion Conducting Polymers

Abstract

The rapidly growing fields of renewable energy technologies and future mobility demands a big step forward in storing the generated energy. Here, rechargeable redox-flow batteries (RFB) can serve as efficient and flexible storage device. The crucial element of the batteries are the ion conductive membranes separating the two electrolyte solutions. These membranes can either be anion- or cation-conductive. The backbone of already applied membrane polymers can vary from aromatic to fluorinated alkyl chains. However, the integration of ion conducting groups into a polymer chain can be challenging in multiple ways. An additional functional group can alter the polymer properties as it could lose its mechanical stability or processability. Another synthetic approach is originating from an already functionalized precursor. Polymerization of the modified precursors can lead to an undesired (low) molecular weight or high polydispersity index (PDI). In this study we report an extremely fast postfunctionalization method giving access to a broad library of ion exchange polymers based on polypentafluorostyrene (PPFS) within few minutes of reaction time. We chose postfunctionalization as the chain length and PDI can be maintained after modification. In this context, the pentafluorophenyl group of the polymer is important as it serves as platform for versatile functional polymers. The strongly electron-deficient ring in combination with the fluorine atom in para-position (4-F) makes fluoride to an excellent leaving group. This enables precise polymer modification with high yields, low synthetic effort and short reaction times. The functionalization could already be demonstrated for the introduction of proton conducting groups, such as sulfonic acid and phosphonic acid.1,2 Yet, membranes of pristine phosphonated or sulfonated PPFS tend to be brittle. Blended with a polybenzimidazole (PBI) they perform extremely good in vanadium redox flow batteries.3 These experiments served as starting points for design of our novel macromolecules in ion exchange membranes for fuel cells and redox-flow batteries. By introducing more flexible side chains by substitution of fluorine the mechanical ductility of the polymer increases. A broad variety of alkyl side chains could successfully be integrated into a phosphonated polymer. A library of varying phosphonic acid to alkyl chain ratios was investigated. Also the mechanical stability was tested with respect to the chain length. As the reaction is accurate with an NMR yield of >99% the amount of alkyl chain can be tuned precisely. Selected polymers were tested as membranes to prove their proton conductivity. Besides alkyl groups other functional groups could be successfully added to the broad library based on PPFS. With this type of reaction, perfluorinated, partially fluorinated or nonfluorinated alkyls, sulfonic acids and amines can be added to PPFS in an instantaneous reaction. In combination with the high thermal and chemical stability of PPFS this method provides an easy access to change the mechanical properties of desired polymers in a large scale and precise manner, paving the way to more cost-efficient materials.References(1) Atanasov, V.; Bürger, M.; Lyonnard, S.; Porcar, L.; Kerres, J. Solid State Ionics 2013, 252, 75–83. DOI: 10.1016/j.ssi.2013.06.010.(2) Atanasov, V.; Oleynikov, A.; Xia, J.; Lyonnard, S.; Kerres, J. Journal of Power Sources 2017, 343, 364–372. DOI: 10.1016/j.jpowsour.2017.01.085.(3) Bülbül, E.; Atanasov, V.; Mehlhorn, M.; Bürger, M.; Chromik, A.; Häring, T.; Kerres, J. Journal of Membrane Science 2019, 570-571, 194–203. DOI: 10.1016/j.memsci.2018.10.027.