Proton Conductive Composite Membranes with Stable Anisotropic Though-Plane Conductivity: A New Preparation Approach

Abstract

In common PEMFCs, a polymer electrolyte membrane selectively conducts protons but blocks passage of electrons and reactants. Nafion, current benchmark membrane material, offers a superior conductivity owing to unique morphology comprising randomly oriented elongated ionic nanochannels within a Teflon-like matrix. Nanochannel orientation is a promising approach to enhance Nafion conductivity, yet there has been no facile method to induce a strong and stable alignment within membranes in the most desired through-plane (TP) direction.Here we report a novel and facile preparation approach based on dual electrospun Nafion-PVDF nanofiber composite that yields a stable TP alignment. The preparation starts from electrospinning of a mixed fiber mat, which is subsequently converted to a solid membrane by plunging the mat into a thin slit. Due to extreme thinness, the electrospun nanofiber readily buckle into a folded structure, thereby a strong inherent microscopic channel orientation within nanofibers is readily converted to macroscopic TP channel alignment. Ultimately, the folded nanofibers fuse, consolidating the membrane.A pronounced TP alignment of ion channels in the membranes, sustaining fusion and annealing, is demonstrated using TEM and SAXS. The analysis also highlights the importance of PVDF fiber as a stabilizing and confining component. Electrospun PVDF fibers within consolidated Nafion-PVDF composite preserve channel orientation upon annealing both in TP and in-plane (IP) aligned membranes, while pure Nafion membranes, similarly prepared from electrospun Nafion fibers, rapidly lose structural anisotropicity. Finally, the impedance measurements demonstrate highly anisotropic conductivity of both TP- and IP-aligned membranes, consistent with their anisotropic structure. Thus, measured TP conductivity of TP-aligned 46% Nafion composite is nearly three times its IP conductivity and about 70% that of Nafion 212, thereby the conductivity normalized to Nafion content exceeds that of pure Nafion. The approach, still leaving much room for optimization, holds potential to advance PEMFC technology by overcoming current limitations and fully utilizing advantageous features of Nafion membranes. Figure 1