Title

Nanofiber-based composite structures for ion-exchange applications

Date of Completion

January 2012

Keywords

Chemistry, Polymer|Engineering, Chemical|Engineering, Materials Science

Degree

Ph.D.

Abstract

Fuel cells have gained tremendous importance in the recent years as an alternative technology to meet the energy demands. A key component of the proton exchange membrane fuel cell (PEMFC) is the membrane, which allows ion transport but separates fuel from oxidant. The objective of this work was to develop and study novel morphologies for PEM fuel cell applications, primarily composites using highly functionalized nanofibers. Highly sulfonated polystyrene (SPS) nanofibers (IEC ∼ 4.5 meq/g) were successfully electrospun and the process of electrospinning this difficult-to-spin material was studied in detail. The process and solution variables were adjusted to maximize the proportion of bead-free fibers in the non-woven mat. Beaded fibers and continuous bead-free fibers of SPS (500 kDa) in DMF could be spun at ∼ 2 Ce and 3.5 Ce, respectively, where C e is the entanglement concentration measured from solution viscosity measurements. The onset of formation of beaded fibers, as opposed to isolated beads, coincided with a sharp transition in the scaling of the storage modulus-concentration relationship. ^ For application in membranes, the high degree of sulfonation helps attain practical ion exchange capacities for the membrane; however, the fibers become very water soluble and hygroscopic. To improve stability of the fiber mats, SPS was co-spun with a high-molecular-weight polyethylene oxide (100 kDa). Addition of PEO to the polymer solution improved electrospinnability of the polyelectrolyte and a subsequent heat treatment improved stability of the fiber mats in water. The crosslinked films were swollen in D2O and both the gel and sol fractions were analyzed by NMR. The extracted material did not show any sulfonation peaks while the crosslinked swollen portion did, confirming that the heat treatment reduced the solubility of sulfonated polymer and tendency to leach out in water. In view of this observation, we have attempted to understand the chemistry of this apparent crosslinking reaction. ^ The crosslinked fiber mats were combined in varying proportions with polymer matrices to limit the swelling of membranes in water. The morphology and performance of these composite membranes were characterized. The crosslinked fiber mats had good in-plane conductivities, ∼ 0.1 S/cm at 25 °C and 98 % R.H. Finally, the highly conductive crosslinked fiber mats were mechanically cut by sonication to generate short fibers that can be dispersed and aligned in a polymer matrix to achieve bulk conductivity. ^