Electrospun and Cross-Linked Nanofiber Composite Anion Exchange Membranes

A unique cross-linked nanofiber composite morphology was employed to create highly conductive and mechanically robust anion exchange membranes (AEMs) for alkaline fuel cells and electrodialysis separations. Chloromethylated polysulfone (CMPSF), the precursor for a tetramethylammonium ionomer, and an inert reinforcing polymer, poly(phenylsulfone) (PPSU), were simultaneously electrospun from separate spinnerets into dual fiber mats. The mats were processed into dense and defect-free nanofiber composite anion exchange membranes by cross-linking a portion of the chloromethyl groups in the CMPSF fibers with an aliphatic diamine, softening the PPSU so that it flowed and filled the void space around the CMPSF fibers, and quaternizing the remaining chloromethyl groups of CMPSF. The final network of ionomer fibers embedded in a PPSU matrix had a very high ion exchange capacity (3.1 mmol/g) but was insoluble in water due to the presence of cross-links. Membranes after processing were mechanically strong (in both the wet and dry states) with a very high hydroxide ion conductivity and moderate water swelling. Thus, a membrane containing 65 wt % ionomer fibers had a OH- conductivity of 65 mS/cm in water at 23 degrees C, a stress at break of 14 MPa (for a water-equilibrated membrane at room temperature), and an equilibrium liquid water swelling of 144% at 23 degrees C. The AEM fabrication scheme is robust and can easily be extended to different base polymers, ion exchange groups, and cross-linking schemes.