|Ph.D Student||Glouckhovski Robert|
|Subject||Development of a Novel Proton Exchange Membrane for PEM|
Fuel Cells and Water Electrolysis
|Department||Department of Chemical Engineering||Supervisors||Professor Yoed Tsur|
|Professor Viatcheslav Freger|
|Full Thesis text|
Nafion, a perfluorinated sulfonic acid polymer (PFSA), remains the main material of choice for Proton Exchange Membranes (PEMs). The high proton conductivity of Nafion is attributed to a random 3D network of elongated water channels formed by bundles of rod-like inverted micelles. Increasing the operational temperature of PEM-operated electrochemical devices, e.g. hydrogen-fed PEM fuel cells (PEMFCs), water electrolyzes (PEMWEs) and direct methanol fuel cells (DMFCs), from current 50-80 °C to above100 °C is highly desirable from the point of view of kinetics, catalyst utilization and heat management. However, the increased operational temperature in PEMFCs results in PEM dehydration and loss of conductivity, presumably due to the increasing tortuosity and eventual disintegration of the 3D water-channel network. On the other hand, the increase in operational temperature under full hydration in DMFCs and PEMWEs is limited by the PEM increasing methanol permeability through the same water channels and loss of mechanical stability. Confining Nafion in straight pores of nanometric size oriented normal to the membrane surface has a potential to overcome the aforementioned limitations.
This study demonstrated successful Nafion impregnation into straight pores of anodized aluminum oxide (AAO, pore diameter 200 nm) and polycarbonate track-etched (PCTE, pore diameter 600-1000 nm) membranes. In contrast with the previous studies, the problem of Nafion aggregation was overcome with a suitable choice of solvent, Nafion concentration and evaporation regime. The pore-filling Nafion demonstrated increased proton conductivity together with significantly reduced methanol and sodium chloride permeability. This effect was attributed to the lower tortuosity and better connectivity of water channels aligned by the pore walls, on one hand, and suppressed swelling, resulting in narrower water channels and stronger “salting-out” and Donnan exclusion effects, on the other hand. It could also be that the confinement encouraged formation of regions of closely packed sulfonic groups, which facilitate proton transfer but strongly exclude other solutes. The alignment of the pore-filling Nafion, however, could not be demonstrated by the small-angle x-ray scattering (SAXS), presumably due to the insufficient amount of Nafion in the experimental membranes. Also, no evidence of enhanced structural stability at elevated temperatures could be found.