Next Generation Anion Exchange Membranes Based on Perfluorinated Polymer Backbones

Perflourinated Anion Exchange Membrane (AEM) materials were first presented in 1987 by Toyo Soda Manufacturing Co., Ltd.  Many cation modifications have been reported, but chemistry was limited to a sulfonamide (-SO₂-NR-) linkage from the sulfonyl fluoride (-SO₂-F) precursor form.  In this project, we started with a sulfonyl fluoride precursor supplied by 3M company.  This is the same precursor used to fabricate the 3M perfluorinated sulfonic acid  (PFSA) proton exchange membrane and can be modified through several chemical approaches.  The 3M ionomer has a shorter side chain than Nafion^TM without the pendant trifluoromethyl (-CF₃) group.  The  3M ionomer provides higher degree of crystallinity, higher modulus, and higher alpha transition temperature (T_α) at a given equivalent weight (EW). The high electron withdrawing character of the perfluorosulfonic acid backbone and side chain is ideal for proton exchange membrane fuel cells, as it makes acids more acidic and can be used to form chemically stable covalent linkages.  However, for perfluoro (PF) anion exchange membrane (AEM), the high electron withdrawing character of the polymer is a challenge as it makes bases less basic and can result in lower stability of chemical bonds. Several Perfluorinated Quaternium ammonium  (PFQA) polymers have been fabricated by us.  We have fabricated several films using this chemistry and measured their ionic conductivity as a function of temperature and relative humidity. Additional insights in to the films behavior was obtained from in-situ SAXS.  An ionomer peak is observed in the SAXS analogous to that observed for PFSA films , but its behavior with increasing swelling is not always the same in a PFQA than for the corresponding PFSA.