Electrospun Ion-Conducting Composite Membrane with Buckling-Induced Anisotropic Through-Plane Conductivity

Abstract

Fuel cell (FC) is an attractive green alternative for today's fuel combustion systems. In common FCs, a polymer electrolyte membrane selectively conducts protons but blocks the passage of electrons and fuel. Nafion, the current benchmark membrane material, has a superior conductivity owing to unique morphology comprising randomly oriented elongated ionic nanochannels within its Teflon-like matrix. Channel orientation enhances Nafion conductivity, yet there has been no facile method to induce a stable alignment in the desired through-plane (TP) direction. Here, we report an approach based on dual electrospun Nafion-PVDF nanofiber composites that yields a stable TP alignment. It utilizes extreme thinness and strong inherent orientation within electrospun nanofibers, which is readily converted to TP alignment by plunging an electrospun nanofiber mat into a thin slit, resulting in nanofiber buckling and subsequent consolidation. Using TEM and SAXS, we demonstrate a pronounced and sustained TP ion channel orientation in prepared membranes, yielding a highly anisotropic swelling and conductivity exceeding that of bulk Nafion when normalized to Nafion content. The analysis also highlights the importance of PVDF as a stabilizing component, preserving orientation upon annealing, while a similarly prepared pure Nafion membrane loses anisotropy. The approach holds potential to advance the FC technology by overcoming current limitations of ionomeric membranes.

High-Performance Proton Exchange Membrane of Sulfonated Polystyrene-b-polyisobutylene-b-polystyrene

“…Nafion PEMs have high proton conductivity owing to the well-connected ionic pathways formed by their unique perfluorinated chemical structure . Actually, the existence of anisotropy between the in-plane and through-plane proton conductivity has been reported in Nafion membranes. − The in-plane proton conductivity of Nafion PEMs was measured to be in the range of 80–120 mS·cm –1 with a four-probe electrode − and the through-plane proton conductivity of Nafion PEMs was determined to be in the range of 20–50 mS·cm –1 by a two-probe electrode. − ,− The measurement results of the proton conductivity of Nafion PEMs by the four-probe electrode method are usually higher than those by the two-probe electrodes. − It has been reported that the PEM with high through-plane proton conductivity is profitable in preparing high-performance fuel cells. − However, Nafion PEMs still suffer from certain limitations including a complex synthetic process, high production cost, and high fuel permeability. , Nonperfluorinated hydrocarbon-based materials with rigid polymer chains, such as sulfonated poly(aryl ether ketone), , sulfonated poly(ether ether ketone), sulfonated poly(arylene ether sulfone), − sulfonated polybenzimidazole, sulfonated polystyrene, − etc., have been reported to be potentially applied to proton exchange membranes. In particular, polystyrene-based triblock copolymer thermoplastic elastomers containing middle soft rubbery segments, such as polystyrene- b -poly(ethylene- co -butylene)- b -polystyrene (SEBS) and polystyrene- b -polyisobutylene- b -polystyrene (SIBS) block copolymers, e...…”