Electrospinning Strategies for PFSA/PVDF Nanofiber Composite Membrane Fabrication

Abstract

Historically, the general strategy for developing new ion-exchange membranes focused almost exclusively on the synthesis of new polymers with a high IEC, where the ion-exchange groups are strongly dissociable. Unfortunately, the use of polymer chemistry alone has not been successful in achieving the performance goals of next-generation proton conducting membranes for fuel cells and redox flow batteries. For example, simply increasing the ion-exchange capacity of a polymer to improve conductivity has lead to polymers that are brittle when dry and swell excessively in water, with a loss in mechanical strength and counterion/coion selectivity. To overcome the problems with highly charged single polymer membrane systems, researchers have: (i) investigated block copolymers which self- assemble into separate phases, where one block provides selective transport pathways and the second block imparts mechanical strength, (ii) combined the desirable properties of two different polymers in a single membrane through blending, (iii) impregnated a functional polymer into a microporous inert support, where the support provides the requisite mechanical properties that the functional polymer lacks, and (iv) crosslinked the ionomer. The present paper will focus on blended ion exchange membranes, composed of perfluorosulfonic acid (PFSA) and polyvinylidene fluoride (PVDF), where nanofiber electrospinning is used to control/alter the nano/micro distribution of PFSA and PVDF. Three membrane fabrication strategies will be presented, using 1100 and 825 EW PFSA and Kynar PVDF: (1) dual fiber electrospinning, where the final membrane morphology is an ionomer matrix with an embedded network of uncharged PVDF reinforcing nanofibers, (2) electrospinning of submicron fibers composed of a PFSA/PVDF polymer blend, followed by fiber mat processing to create a dense and defect-free film, where the morphology is that of randomly oriented bundles of PFSA and PVDF nanofibrils, and (3) core-shell single fiber electrospinning, where the core is PVDF and the shell is a mixture of PFSA and PVDF, followed by compaction and densification to create a dense membrane, where the final membrane structure is similar to that of (1). The methods for fiber electrospinning and follow-on mat processing will be described. Physical property data will be presented for the different fabrication schemes and for different PFSA/PVDF membrane compositions. The use of selected membranes in hydrogen/air and DMFC fuel cells and H2/Br2 redox flow cells will be discussed.