|M.Sc Student||Cohen Matan|
|Subject||Development of High Performance Fuel Cell Membranes Based|
on Electrospun Nafion Nanofibers
|Department||Department of Energy||Supervisor||Professor Viatcheslav Freger|
|Full Thesis text|
There currently is a growing global demand for highly efficient, renewable and non-CO2-producing energy sources. For the automobile industry, one such source is proton exchange membrane (PEM) fuel cells. The PEMs in fuel cells are typically made of Nafion, which is an ionomer with a perfluorinated backbone and side chains terminating in sulfonate groups. Bulk Nafion is isotropic, but composed of bundles of roughly parallel rod-shaped micelles, along which protons are conducted. These bundles can be aligned along a desired direction by utilizing their tendency to spontaneously align parallel to certain interfaces, or by mechanical stretching. Electrospinning utilizes both of these effects to produce highly-oriented Nafion nanofibers, and 400 nm diameter electrospun Nafion fibers were shown to have a 15-fold increase in directional conductivity relative to bulk Nafion, an effect which is assumed to be a result of bundle alignment. In this work, we explore preparation of composite proton exchange membranes, in which electrospun Nafion nanofibers are both confined within an inert matrix and aligned in a desired direction. Such confinement and alignment are expected to enhance membrane performance, as Nafion fibers will tend to swell along their main axis upon membrane hydration and promote conductivity and selectivity in the desired direction.
An electrospinning system was designed and built in-house. Electrospinning solution parameters and working conditions were optimized to obtain 350-500 nm diameter Nafion fibers. Using these fibers, two types of composite membranes were produced: Nafion in curable silicone matrix and co-electrospun Nafion/PVDF membranes. The former type was used to examine feasibility of using an electric field for aligning Nafion fibers in a solidifiable matrix. This approach did not show a detectable degree of alignment. Theoretical analysis showed that such alignment would require electric field strengths of at least 7.5∙106 V m-1, which wasn’t reached in our experiments. This approach was not pursued further, however, the required field strength may be achieved by using higher voltages and thinner membranes. The second type, Nafion/PVDF composites prepared by hot-pressing oriented electrospun mats, showed about a 3-fold higher conductivity along Nafion orientation compared to across it. We conclude that the latter technique can be useful for producing micro- and macro-aligned high-performance membranes.