Jason Ballengee

Morphological Control of Electrospun Nafion Nanofiber Mats

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 mat-averaged 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.

Nanofiber-Based Composite Membranes for Fuel Cells

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.