TOWARD AN EFFECTIVE DIRECT ETHANOL FUEL CELL THROUGH THE USE OF CATION EXCHANGED NAFION® MEMBRANES

Abstract

Proton exchange membrane fuel cells (PEMFCs), specifically direct ethanol fuel cells (DEFCs), are an attractive alternative to obtain electric energy for a large range of applications. Ethanol is a promising fuel choice as its non-toxicity and its availability from biomass resources advocate its use in a direct ethanol fuel cell. However, DEFCs face some major hurdles including sluggish kinetics at the anode due to poor catalysts and ethanol crossover through the present state-of-the-art proton exchange polymer membrane, Nafion®.

This thesis focuses on the development of a modified Nafion polymer electrolyte membrane with low ethanol crossover and retained proton conductivity. The first approach to these modified membranes involves using small monovalent and divalent cations and partially exchanging them into the Nafion membrane in place of protons. The resultant current voltage (IV) curves for the partially cation-exchanged Nafion membranes indicate lower parasitic currents and increased output current density. An optimal ion exchange level was found above which conductivity of the membrane is sacrificed in order to prevent ethanol crossover. Using a larger, cationic surfactant for the cation exchange provides another option for blocking ethanol crossover. For example, cetyl trimethyl ammonium cation-exchanged Nafion membranes block ethanol crossover with even more optimal results than the inorganic and small organic cations tested. The resultant current voltage (IV) curves using the partially exchanged Nafion membranes exhibit increased output current density at both 90C and 130C temperatures. SEM studies coupled with EDS indicate that the nitrogen atoms from the cetyl trimethyl ammonium cations can be seen throughout the membrane. Membrane contact time and concentration of the cetyl solution are key parameters to successfully infusing the membranes and achieving maximum blockage to ethanol transport. NMR diffusion experiments and XRD studies are utilized to gain insight into the mechanism behind blocking ethanol transport in the membrane.

Finally, this thesis presents data obtained when membranes modified to prevent ethanol crossover are combined with improved catalysts for the electrooxidation of ethanol. Membrane electrode assemblies (MEAs) made with the cetyl trimethyl ammonium partially exchanged Nafion membranes combined with a PtSnTiO2 catalyst for the anode provide IV curves with increased open circuit voltage and current densities when used in the DEFC. These results show promising results toward an effective direct ethanol fuel cell.