(319b) Nano-Engineered Proton and Electron Conducting Materials for Fuel Cell Applications

We study the development of nano-engineered electrode/catalyst assembly for proton exchange membrane (PEM) fuel cells using nanofibers (diameter ~50-500 nm). In particular, this work focuses on controlling the nanoscale assembly of proton and electron conducting media within porous nanofiber mats. The aim is to develop catalyst supports (for PEM fuel cells) with well-defined triple phase boundary, necessary for efficient transport of electrons, protons and oxygen gas to the catalyst sites for the oxygen reduction reaction to take place at the anode.

Nanofibers are fabricated via a process called electrospinning that uses strong electric field to elongate a polymer solution or melt jet to form ultrathin fibers. Solutions of Nafion, the proton conducting ionomer in state of the art fuel cells, were not electrospinnable by themselves due to the formation of aggregates in solution owing to electrostatic interactions. Relaxation times and extensional viscosities of different Nafion solutions were measured to quantify the solution elasticity and to understand its correlation with Nafion electrospinnability. A methodology to enhance the extensional viscosity was developed that allowed us to successfully electrospin Nafion nanofibers. Scanning electron microscopy (SEM) was conducted to characterize the nanofiber diameter and morphology for different process parameters and solution compositions. Impedance spectroscopy has been conducted and Nafion nanofibers exhibit lower impedance or higher proton conductivity than bulk Nafion films. This work was followed by incorporation of electron conducting media within the Nafion nanofiber mats and transmission microscopy results exhibiting the nanoscale assembly of the two media will be presented.