Electrospun Nanofiber Membranes and Electrodes for Fuel Cells R. Wycisk, J. Ballengee, J.W. Park, M. Brodt and P.N. Pintauro Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, TN, USA ABSTRACT Nanofiber electrospinning is a proven technique for fabrication of high performance hydrogen/air fuel cell membranes and electrodes. Herein, recent work is summarized on electrospun proton and hydroxide conducting polymeric membranes and low-Pt loading nanofibrous fuel cell cathodes. A dual-fiber electrospinning was utilized for fabrication of membranes from ionomers and reinforcing polymers. An electrospun membrane from polyphenylsulfone (PPSU) and a 3M ionomer (72 vol% 3M 660EW) showed a proton conductivity of 93 mS/cm at 120 o C and 50%RH. An MEA with this membrane gave high power output which was insensitive to hydrogen and air humidity in the 50-95%RH range. In a complimentary project, nanofiber cathodes composed of Pt/C powder embedded in a proton conducting binder were prepared. Very high power density, 906 mW/cm 2 at 80°C, was produced from MEAs with a Nafion 212 and a cathode electrospun from HiSpec™ 4000 catalyst, at a Pt loading of only 0.055 mg/cm 2 . Keywords: fuel cell, electrospinning, membrane-electrode assembly 1 INTRODUCTION Electrospinning is gaining popularity as a convenient and cost-effective technique for the fabrication of sub- micron diameter polymer fibers. During electrospinning, an electrostatic field causes a necking down of the polymer solultion or melt that is ejected from a spinnerette and then an accelearation of the extruded polymer jet as it travels towards a grounded collecting surface. During exposure to air, solvent is evaporated from the jet or the polymer melt cools and solid fibers are deposited on the collecting surface. Electrospinning was invented in the early 1900s, but little interest was seen until the work of Reneker and co- workers in the 1990s [1]. In 2008, Pintauro and co-workers [2] proposed utilization of electrospinning for the fabrication of nanocomposite fuel cell membranes as alternative to traditional membranes based on polymer blends and copolymers. These early membranes were fabricated by electrospinning an ionomer fiber mat followed by the impregnation of an uncharged, reinforcing polymer into the inter-fiber voids. Post-electrospinning processing steps were usually required to convert an electrospun mat into a dense and defect-free fuel cell membrane. Thses steps included mat compression, fiber welding, ionomer annealing, inert polymer impregnation, and a final hot acid/water treatment. The resultant nanofiber composite membrane morphology decoupled the proton conduction function of the ionomeric nanofibers from the mechanical support and swelling control functions of the uncharged polymer. Dual-fiber electrospinning was recently introduced to avoid a separate interfiber void-filling impregnation step during nanofiber composite membrane fabrication [3]. Now, the ionomer and the uncharged polymers are simultaneously electrospun as separate fibers that mix into a single mat. Subsequent processing, via hotpressing and either annealing or solvent vapor exposure, induces flow of one of the polymer components into the interfiber void space while retaining the nanofiber morphology of the second polymer. Another interesting application of polymer electrospinning is the fabrication of low Pt-loaded fuel cell electrodes for use in proton-exchange membrane (PEM) hydrogen/air fuel cells. This process was introduced by Zhang and Pintauro in 2011 [4] and its development is continuing. Very high power outputs are possible with the nanofiber morphology, due to high Pt cathode utilization, low oxygen mass transport resistance, and good water removal rates. Our recent work on electrospun fuel cell membranes and Pt electrodes will be discussed below. 2 DUAL-FIBER COMPOSITE MEMBRANES 2.1 Proton Conducting Membranes Fabrication and Characterization Two types of perfluorosulfonic acid polymers (PFSAs) were employed in the study: Nafion ® (EW 1100 g/eq.) and 3M660 (EW 660 g/eq.). PFSA’s micellar dispersions can not be electrospun directly and therefore a carrier polymer must be added to allow for spinnability. For example, a 400kDa MW polyethylene oxide (PEO) at a loading of 1wt% was used as the carrier for Nafion and a 1000kDa PEO was used as the carrier for 3M660 at 0.3wt%. A 2:1 n- propanol/water mixture was used as the solvent for the ionomer/PEO mixtures. Polyphenylsulfone (PPSU) served as the uncharged, reinforcing polymer which was electrospun from a separate spinnerette but simultaneously with the PFSA/PEO mixture. An NMP-acetone mixture (20 NSTI-Nanotech 2013, www.nsti.org, ISBN 978-1-4822-0584-8 Vol. 2, 2013 708