|M.Sc Student||Odess Ariel Moshe|
|Subject||Nafion/PVDF Membranes with Enhanced through-Plane|
|Department||Department of Chemical Engineering||Supervisor||PROF. Viatcheslav Freger|
|Full Thesis text|
Fuel cell (FC) is the most attractive alternative for the inefficient and polluting combustion systems. In commonly used FCs, polymer electrolyte membranes (PEM) serve as a selective barrier between the cathode and anode compartments, conducting ions but blocking passage of electrons, fuel, and oxidant. High conductivity together with chemical, thermal and mechanical stability are desired for reliable and durable FC operation. Nafion, perfluoro-sulfonic acid material, is the benchmark PEM, owing to its highly stable Teflon-like backbone and superior conductivity, originating from the sulfonic groups forming conductive ionic microphase in the hydrated state. Structural studies show the conductive microphase in Nafion assumes elongated rod- or ribbon-like morphology and produces a well-connected network of elongated conductive nanochannels.
A possibility to enhance Nafion conductivity by aligning the nanochannels in a specific direction has been widely explored. In-plane membrane stretching was shown to produce a significant enhancement of in-plane conductivity, well correlated with SAXS analysis, showing channel alignment in anisotropic manner. A similar conductivity enhancement was achieved in nanofibers produced by electric field-driven stretching using electrospinning, which could be fused to produce nanofiber mats with enhanced in-plane conductivity. These studies demonstrated the large potential of channel and fiber alignment for improving membrane in-plane conductivity, yet through-plane conductivity, most desired for FC operations, remains a challenge. The alignment in Nafion is also not long-lasting and hydrated membranes eventually relax to isotropic state, while stable alignment presents another challenge.
The present study addressed these challenges by combining electrospun Nafion and PVDF nanofibers for confining and stabilizing channel arrangement in a composite Nafion-PVDF structure. In order to achieve the desired through-plane orientation both for fibers and nanochannels, a unique mechanical folding and compression was applied. The alignment in the new membrane as well as in the in-plane aligned Nafion-PVDF membrane was verified with X-ray diffraction and compared with that in commercial Nafion 115 membranes. SAXS analysis demonstrates nano-orientation of Nafion’s ionic channels within the composite membrane. In addition, SAXS results confirm the confining and stabilizing effect of the PVDF on the Nafion alignment. Micro and nano- orientation were investigated by SEM and TEM, which present fibers and ionic-channel orientation, respectively, even after mechanical compression. Ultimately, the influence of the orientation on conductivity was confirmed by measuring in-plane and through-plane conductivity in all membranes using impedance spectroscopy. The conductivity in the orientation direction for membranes with through-plane and in-plane aligned fibers was found to be 2.8 and 2.3 times higher than that of the perpendicular direction, correspondingly. Although the through-plane conductivity in 50:50 Nafion-PVDF composite membrane with through-plane orientation was about 20% lower than that of commercial Nafion 115, its conductivity normalized to Nafion content was found to be 40% higher than in Nafion 115. The results indicate that fiber orientation in Nafion-PVDF or similar composites may significantly enhance membrane conductivity, though further investigation and optimization of the PVDF/Nafion ratio and compression and heat treatment condition is required in order to fully understand its mechanism and potential.