(277d) Direct Methanol Fuel Cell Performance Enhancement with Segregated Sulfated Zirconia-Nafion Composite Membranes

Recent literature regarding composite membranes for direct methanol fuel cells (DMFCs) has focused upon achieving reduced methanol crossover while maintaining (or improving) proton conductivity. Researchers have been interested in membrane fillers such as silica or titania, and recent attention to filler functionality has resulted in inclusion of sulfated species that render the filler more acidic (electrophilic) than non-sulfated species. Another key parameter is filler dispersion (homogeneity). Dispersion is typically controlled by electrostatic stabilization of filler particles in recasting procedures and in-situ synthesis through sol-gel or related "ameliorated" techniques. We report upon work where non-homogeneous (or segregated) sulfated zirconia-Nafion composites are intentionally prepared to enhance electrode kinetics rather than reduce crossover effects. By using physical mixing and Nafion solution recasting procedures, composites with separate layers of Nafion and sulfated zirconia--with very little Nafion mixed into the sulfated zirconia layer--have been prepared and tested in DMFC single-cell builds. Results for such membranes show significant enhancement in power density. Power densities show enhancement of up to twofold over pure Nafion membranes. This enhancement has been further investigated utilizing half-cell experiments that show a shift in methanol electrooxidation onset to lower potentials, thus qualifying it as kinetic in nature. Results of these membranes will be presented, along with explanation as to possible mechanisms of enhancement. Furthermore, the practical benefits of such membranes will be discussed (due to apparent difficulty in preparing desired--i.e., homogeneous--composite catalyst layer microstructures with such kinetic promoters).