Membranes from Blended Ionomer/PVDF Nanofibers: I. PFSA/PVDF and PFIA/PVDF Fiber Spinning and Membrane Fabrication

Blends and composites of various perfluoroionomers and uncharged polymers, e.g., poly(vinylidene fluoride) (PVDF), have been widely studied as the proton conducting membrane material for direct methanol and hydrogen/air fuel cells. The general idea of blending was to decouple the mechanical and swelling properties of a membrane from its proton conductivity. Thus, for a hydrogen/air fuel cell, the goal was to fabricate a membrane with low sheet resistance, low areal swelling, and good mechanical properties. For a direct methanol fuel cell membrane, blended were made to lower methanol crossover methanol while maintaining a high proton conductivity. In 2008, Pintauro and co-workers showed how nanofiber electrospinning can be used to fabricate composite fuel cell membranes. The method was an alternative to traditional blending. These early membranes were made by electrospinning an ionomer fiber mat followed by the impregnation of an uncharged, reinforcing polymer into the inter-fiber voids; inverse structures where an uncharged polymer was electrospun and the voids were filled with an ionomer were also studied. Dual-fiber electrospinning was introduced in 2011 by Ballengee and Pintauro as a means to eliminate a separate interfiber void-filling impregnation step during nanofiber composite membrane fabrication. The ionomer and uncharged polymers were simultaneously electrospun as separate fibers that co-deposited as a well-mixed mat on the collector surface. Subsequent processing via hot-pressing, and either annealing or solvent vapor exposure, induced the flow of one of the polymer components into the interfiber void space while retaining the nanofiber morphology of the second polymer. Two distinct morphologies were produced from such dual fiber mats: (i) a network of ionomer nanofibers embedded in an uncharged reinforcing polymer matrix and (ii) an ionomer matrix reinforced by a network of uncharged nanofibers. Membranes were prepared via dual fiber electrospinning using various polymer combinations, including Nafion, Aquivion and 3M Company PFSA ionomers, and polyphenylsulfone, poly(vinylidene fluoride), and polyamide-imide as the uncharged components. We have recently developed a new electrospinning strategy for membrane preparation, were a solution of a binary polymer mixture is electrospun and then the single fiber mat is hot-pressed into a dense blended membrane. Here, we exploit the electrospinning process to insure blending of seemingly incompatible polymers (i.e., thorough mixing of the polymer components as the electrospinning solution emerges from the spinneret tip followed by rapid solvent evaporation and the “freezing in” of a blended morphology within a 100-500 nm diameter fiber. In this presentation the methods used to electrospin various single fibers blends of perfluoroionomers with Kynar HSV 900 PVDF will be presented, with a focus on Nafion perflourosulfonic acid ionomer and low equivalent weight PFIA ionomer from 3M Company. Procedures for converting the nanofiber mats into dense and defect-free membranes will be described. Preliminary characterization results will be summarized (water swelling, proton conductivity, and mechanical properties) and contrasted with dual fiber membranes and/or conventional solution cast blended films.