Improved Fuel Transport Selectivity in Polymer Electrolyte Membranes
- Author(s): Clark, Kyle Tyler
- Advisor(s): Devine, Thomas M
- Kerr, John B
- et al.
In polymer electrolyte membrane (PEM) fuel cells, fuel crossover through the membrane is a significant problem that contributes to reduction in cell efficiency and accelerated membrane degradation. The need for high water content in the membrane to produce acceptable conductivities leads to excessive fuel crossover while also limiting cell operating temperature to <100°C where platinum catalysts are easily contaminated by CO. This work focused on the study of transport functions in PEMs in an effort to reduce fuel crossover through the replacement of water with an alternative proton solvent.
The heterocycle imidazole was incorporated into Nafion® membranes as an alternative proton solvent because of its ability to form hydrogen bond networks similar to water, and its high proton conductivity (10-2 to 10-1 S/cm) at temperatures around 150°C. Imidazole must be covalently bonded to the membrane to prevent loss due to leeching by water, or sublimation at high temperatures. In this work, imidazole in the form of 4(5)-hydroxymethyl imidazole (ImOH) was imbibed into the perfluorosufonic acid membrane, Nafion®.
In this work, incorporation of ImOH into the membranes is shown improved the thermo-mechanical properties, as studied using dynamic mechanical analysis, while reducing water and methanol transport kinetics, as observed using dynamic vapor sorption and nuclear magnetic resonance. Study of the membrane using small angle x-ray scattering linked these results to changes in membrane morphology. Ex-situ study of methanol transport through the membrane was confirmed using in-situ electrochemical fuel crossover measurements on operational direct methanol fuel cells. The results showed that the use of ImOH as an immobilized proton solvent results in around a 50% reduction in methanol fuel crossover.