Two low equivalent weight perfluorosulfonic acid (PFSA) polymers (825 EW and 733 EW) were successfully electrospun into nanofibers by adding as little as 0.3 wt% of high molecular weight poly(ethylene oxide) as a carrier polymer. The electrospun fiber morphology transitioned from cylindrical filaments to flat ribbons as the total concentration of PFSA + carrier in solution increased from 5 wt% to 30 wt%. PFSA nanofiber mats were transformed into defect-free dense membranes using a four-step procedure: (i) annealing the PFSA polymer during which time intersecting fibers were welded to one another at cross points (ii) mechanically compacting the mats to increase the volume fraction of nanofibers to $75%, (iii) imbibing an inert polymer, Norland Optical Adhesive (NOA) 63, into the mats (to fill entirely the void space between nanofibers) and then crosslinking the NOA with UV light, and (iv) removing the poly(ethylene oxide) carrier polymer by boiling the membrane in 1.0 M H 2 SO 4 and then in deionized water. The resulting membranes exhibited higher proton conductivities than that of commercial Nafion 212 membrane (0.16 S/cm at 80 C and 80% relative humidity and 0.048 S/cm at 80 C and 50% relative humidity for a membrane with 733 EW nanofibers), with low water swelling (liquid water swelling of 18% for membrane with high conductivity). The proton conductivity of both EW nanofiber composite membranes increased linearly with the PFSA nanofiber volume fraction, whereas gravimetric water swelling was less than expected, based on the volume fraction of ionomer. There was a significantly improvement in the mechanical properties of the nanofiber composite membranes, as compared to recast homogeneous PFSA films.
There is a need for polymeric hydrogen/air fuel-cell membranes that can efficiently conduct protons at moderate to high temperatures for wet and dry gas feeds. The US Department of Energy (DOE), for example, set an exceedingly stringent preliminary target for membrane conductivity, 0.10 S cm À1 at 120 8C and 50 % relative humidity (RH).[1] Herein, we describe the fabrication and basic properties of one membrane that exhibits outstanding proton conductivity over a wide range of humidity conditions at temperatures of 80 8C and 120 8C. The membrane is based on a nanofiber network composite design [2] with precise topological separation of the proton transporting and mechanically reinforcing polymer components. This desirable morphology is created via electrospinning, an electrostatic fiber processing technique that has been known for more than one hundred years and underwent a renaissance in the early 1990s, mainly due to the work of Reneker et al. [3,4] The use of electrospinning for membrane and porous filter fabrication is not yet widespread, but interest in this technique is growing. Nanofiber air filters with highly desirable retention characteristics have recently been commercialized. [5, 6] Electrospinning of ionic polymers, on the other hand, is quite new and the data on these systems are very scarce.The present implementation of electrospinning, leading to a functional proton conducting membrane, is unique and involves a sequence of four processing steps: 1) electrospinning a proton conductive blend containing a negatively charged polymer and a sulfonated molecular silica (silsesquioxane) to create a nanofiber mat, 2) welding of intersecting nanofibers to improve the connectivity of the protonic pathways, 3) compacting the mat to increase the volumetric density of the proton conductive fibers, and 4) impregnating the processed nanofiber network with an inert, hydrophobic (uncharged) polymer to fill the pores between fibers, reinforcing the membrane, and limiting ionomer swelling.The new ion-exchange membrane differs from alternative approaches, such as those based on block copolymers, [7,8] in that it combines two separate materials: one for proton conduction and the other as a reinforcement and for stabilizing ionomer swelling, which allows for better control of the nanostructure and properties. Thus, the submicron component (i.e., the "mixing" of the constituent electrospun nanofibers and inert, uncharged polymer matrix) produces a co-continuous morphology similar to that of a polymer blend at the point of phase inversion. Initial experiments focused on the highly charged 825 equivalent weight (EW) perfluorosulfonic acid (PFSA) polymer from 3M Corporation, with an ion-exchange capacity (IEC) of 1.21 mmol g À1 (i.e., 33 % more ÀSO 3 H proton exchange groups per unit weight than commercially available 1100 EW Nafion). Data on electrospinning of PFSA polymers is sparse and limited to Nafion. [9][10][11] Researchers have been unable to electrospin neat Nafion fibers from a Nafion/alcohol solution due to the ...
A class of nanofiber-based polymer/particle proton exchange membranes ͑PEMs͒ is described for use in H 2 /air fuel cells that operate at low humidity. The membranes were fabricated from electrospun nanofibers composed of sulfonated poly͑arylene ether sulfone͒ ͑sPAES͒ with sulfonated polyhedral oligomeric silsesquioxane ͑sPOSS͒ as a proton conductivity enhancer. The void space between nanofibers in an electrospun mat was filled with a mechanically robust and chemically stable UV-cross-linked polyurethane to create a gas impermeable membrane. Membranes with nanofibers composed of 2.1 mmol/g ion-exchange capacity sulfonated polysulfone with 40 wt % sPOSS and a nanofiber volume fraction of 0.70 exhibited a proton conductivity of 0.094 S/cm at 30°C and 80% relative humidity ͑RH͒, which was 2.4 times higher than that of Nafion 212 at the same conditions. The high proton conductivity was attributed to the high concentration of protogenic groups in the sPAES/sPOSS nanofibers and the ability of the nanofibers to hold water, where the equilibrium water-vapor uptake of the membrane was 3.8 times higher than that of commercial Nafion at 30°C and 80% RH.
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