High conductivity perfluorosulfonic acid nanofiber composite fuel-cell membranes.

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 Scm 1 at 120 8C and 50% relative humidity (RH). 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 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. 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. 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, 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 mmolg 1 (i.e. , 33% more SO3H proton exchange groups per unit weight than commercially available 1100 EW Nafion). Data on electrospinning of PFSA polymers is sparse and limited to Nafion. Researchers have been unable to electrospin neat Nafion fibers from a Nafion/alcohol solution due to the polymer’s rod-like micellar morphology and the lack of sufficient chain entanglements. Electrospinning was only possible when a carrier polymer, poly(acrylic acid) or poly(ethylene oxide), was added to the Nafion solution at a high concentration (15–30 wt%). The carrier approach, utilizing poly(acrylic acid) (PAA), was adopted in the present study, with a focus on minimizing the PAA content of the PFSA nanofibers. Solutions of 3M PFSA/PAA in 1-propanol/water (2:1 w/w) solvent containing 5–15 wt% total polymer and 5–50 wt% PAA with respect to PFSA were electrospun. Scanning electron microscopy (SEM) images of the resultant mats and the average fiber diameters are shown in Figure 1. Electrospinnability was enhanced and the average fiber diameter increased as the PAA content and the total polymer concentration of the spinning solution increased. Electrosprayed droplets formed with a 5% solution containing 5% PAA due to insufficient number of polymer chain entanglements, whereas a 15% solution with 50% PAA could not be spun due to its very high viscosity. Only 5% PAA was needed to electrospin well-formed PFSA fibers when the total polymer concentration was 15 wt%. This is a significant drop in carrier concentration, as compared to all prior reports (due to our optimization of the electrospinning conditions). To further enhance proton conduction in the final nanofiberbased membrane, a 3-component system was electrospun, containing 60% 825 EW PFSA, 5% PAA, and 35% sulfonated octaphenyl polyhedral silsesquioxane (sPOSS, with an IEC of 4.8 mmolg ; molecular structure is shown in Figure 2). sPOSS was selected based on a prior report showing a positive influence of a similar compound on the proton conductivity of a non-fibrous direct methanol fuel-cell membrane. Electrospun PFSA/PAA (95:5) and PFSA/sPOSS/PAA (60:35:5) nanofiber mats were further processed by heating to 140 8C [a] Dr. W. Zhang, Prof. P. N. Pintauro Department of Chemical and Biomolecular Engineering Vanderbilt University, Nashville, TN 37235 (USA) Fax: (+1)615-343-7951 E-mail : [email protected] [b] Dr. J. Choi, Dr. R. Wycisk, Dr. K. M. Lee Department of Chemical Engineering Case Western Reserve University, Cleveland, OH 44106 (USA) [c] Prof. P. T. Mather Department of Biomedical and Chemical Engineering Syracuse University, Syracuse, NY 13244 (USA) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201000220.