Nafion and poly(phenyl sulfone) are simultaneously electrospun into a dual-fiber mat. Follow-on processing of the mat produces two distinct membrane structures: (1) Nafion reinforced by a poly(phenyl sulfone) nanofiber network and (2) Nafion nanofibers embedded in inert/uncharged poly(phenyl sulfone) nanofiber network. For structure 1, the Nafion component of the fiber mat is allowed to soften and flow to fill the PPSU interfiber void space without damaging the PPSU fiber structure (by use of a mat compression step followed by thermal annealing). For structure 2, the PPSU material in the mat is allowed to soften and flow into the void space between Nafion nanofibers without damaging the Nafion fiber structure (by mat compression, exposure to chloroform solvent vapor, and then thermal annealing). Both membrane structures exhibit similar volumetric/gravimetric water swelling and proton conductivity, where the conductivity scales linearly with Nafion volume fraction and the swelling is less than expected based on the relative amounts of Nafion. The in-plane liquid water swelling of membranes with Nafion reinforced by a poly(phenyl sulfone) nanofiber network is always less than that of the inverse structure. On the other hand, the mechanical properties of membranes with Nafion nanofibers embedded in poly(phenyl sulfone) are superior to membranes with the opposite structure. Compared to other fuel cell membranes, the nanofiber composite membranes exhibit very low in-plane water swelling and better mechanical properties, which translates into improved membrane/MEA longevity in a hydrogen/air open circuit voltage humidity cycling durability test with no loss in power production as compared to a Nafion 212 membrane.

There are a number of electrochemical and electromechanical applications where it is desirable to electrospin Nafion into nanofibers, including composite fuel cell membranes, sensors, and polymer-based actuators. Nafion and other perfluorosulfonic acid polymers, however, are notoriously difficult to electrospin and require the presence of a high molecular weight carrier polymer in the electrospinning solution. In this paper, we report on the morphology of electrospun Nafion nanofiber mats that are created using a low concentration (1-2 wt %) of poly(ethylene oxide) (PEO) as the carrier polymer. The effects of electrospinning conditions, i.e., air humidity, polymer solution solvent, carrier polymer molecular weight, electrospinning voltage, and electrospinning flow rate, on the quality of the electrospun mat (i.e., the presence=absence of unwanted bead or bead-on-fiber morphologies) and the mataveraged nanofiber diameter are presented and discussed. Bead-on-fiber structures are more prevalent when Nafion is electrospun at high humidity conditions and when the applied voltage is high. Ribbon-like morphologies form when a high molecular weight PEO carrier polymer is used. Nafion=PEO fiber diameter depends strongly on air humidity, solution solvent, carrier polymer molecular weight, and electrospinning flow rate, where the average diameter of well-formed Nafion=PEO nanofibers can be easily varied from 300 to 900 nm.

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A nanofiber composite membrane, composed of 72 vol% 660 equivalent weight (EW) perfluorosulfonic acid (PFSA, from 3 M Company) and 28 vol% polyphenylsulfone (PPSU), was fabricated and characterized. A newly developed dual fiber electrospinning method was utilized for membrane fabrication, where the two polymers were simultaneously electrospun into a single mat. Follow-on processing converted the mat into a fully dense and functional fuel cell ion-exchange membrane with polyphenylsulfone nanofibers embedded in the ionomer. The proton conductivity of the composite membrane was high, e.g., 93 mS/cm at 120°C, 50% relative humidity, compared to 37 mS/cm for commercial Nafion. The dimensional stability of the membrane upon liquid water uptake was excellent, with an in-plane (areal) swelling of only 5% at room temperature. A MEA containing a 660 EW nanofiber composite membrane with 3 M 825 EW PFSA ionomer as the catalyst binder had significantly higher power output in a hydrogen/air fuel cell than a MEA with a Nafion 211 membrane and Nafion PFSA binder. The power output of the nanofiber membrane MEA was relatively insensitive to changes in feed gas humidity, between 50% and 93% RH for cell temperatures of 80°C and 100°C.

A composite membrane, composed of 70 vol% 660 equivalent weight perfluorosulfonic acid (from 3M Company) and 30 vol% polyphenylsulfone, was fabricated and characterized. A newly developed dual fiber electrospinning method was utilized for membrane fabrication, where the two polymers were simultaneously electrospun into a dual-fiber mat. Follow-on processing converted the mat into a fully dense and functional fuel cell ion-exchange membrane with polyphenylsulfone nanofibers embedded in an ionomer matrix. The proton conductivity of the composite membrane was high, e.g., 0.070 S/cm at 80oC, 50% relative humidity. The dimensional stability of the membrane upon water uptake was excellent, with an in-plane (areal) swelling of only 5% in room temperature water.