Protic poly(diallylmethylammonium) poly(ionic liquid) proton exchange membranes with low fluorine content

Abstract

Proton exchange membranes (PEMs) are being studied as a key component to extend and improve green hydrogen technologies. One important issue nowadays is the need for reducing the amount of poly-fluoro alkyl substances (PFSA)s in materials employed in renewable energy devices due to their high toxicity. In the present work, we introduce a family of PFSA-free protic poly(ionic liquid)s based on cationic poly(diallylmethylammonium) (PolyDAMAH) and sulfonamides such as bis(trifluorosulfonyl)imide (TFSI) and bis(fluorosulfonyl)imide (FSI) as counter-anions. The synthesis was carried out in a three step procedure, first synthesizing the protic diallylammonium chloride monomer, then thermal radical polymerization and finally anion exchange of the chloride anion by the specified sulfonamide anions. The protic PolyDAMAH showed apparent molecular weights between 30 K and 40 K and glass transition values of 39 and 52 °C, respectively, for both homopolymers and high thermal stability up to 250 °C. From solid state NMR studies it was found that the FSI⁻ anion may allow a faster proton and anion mobility when compared to the TFSI⁻ anion in dry and humid states. To find the best match, blending protic poly(ionic liquid)s with TFSI⁻ and FSI⁻ anions improved the mechanical characteristics of the membranes, while maintaining low water uptake and high ionic conductivity. The optimized PolyDAMAH membranes were characterized by dielectric and mechanical relaxation measurements and showed more than six orders of magnitude decoupling of the ion dynamics from the mechanical relaxation. An ionic conductivity of 1.2 × 10⁻³ S cm⁻¹ at 100 °C and 75% relative humidity with the storage modulus higher than 2.2 × 10⁻⁵ Pa at 80 °C was obtained for the optimized blend, making this a promising material to be employed as proton exchange membranes for fuel cells at intermediate temperatures.