DESIGN AND CHARACTERIZATION OF NAFION®/EX-SITU SILICA NANOCOMPOSITE MEMBRANES: EFFECTS OF PARTICLE SIZE AND SURFACE MODIFICATION

Abstract

This dissertation focuses on the preparation of new Nafion®/ ex-situ silica nanocomposites membranes and the impact of particle size of spherical silica particles on the nanocomposites' properties. To achieve acceptable power production, fuel cell polymer membranes are required with good proton conductivity, water retention, thermal and mechanical stability. However, to avoid poisoning of fuel cell electrocatalysts with CO or other fuel contaminants, they must be operated at temperatures (>100 °C). At these temperatures, fuel cell membranes dehydrate resulting in dramatic decreases in proton conductivity or complete failure as membranes crack due to volumetric stress from water loss. Even if fuel cell is kept in a humidified chamber, increasing temperature will eventually shut the cell down as Nafion®'s bicontinuous structure "dissolves" into a single poorly conducting phase at temperatures above the polymer's Tg.This research provides systematic studies of effects of silica particle size on properties of silica-Nafion® nanocomposites. Results of this study include new insights into requirements for reproducible particle syntheses, practical methods for avoiding silica particle floatation during Nafion® nanocomposite membranes preparation, and a summary of the influence of particle size and functionalization on Nafion® membrane properties. Stober particle syntheses showed high sensitive to ammonia concentration and we discovered that literature procedures' variability is likely due to researchers failure to actually measure ammonia concentration in their aqueous base (which can be 50% or more off). Homogeneous nanocomposite membranes, as determined by AFM and SEM, were successfully prepared using more viscous dispersions. It was observed that nanocomposites membranes with small particles (<50 nm) showed significant increases in proton conductivity at temperatures above 80 °C. Surface modification of the silica particles improved the proton conductivity at 80 °C. Enhancement on proton conductivity was more pronounced with small modified particles at temperatures < 80 °C but unmodified particles were better than modified particles at temperatures >80 °C. Small, unmodified particles led to enhanced thermal stability of the Nafion® ionic domain, however, surface modification did not result in any thermal stability enhancement. Contrary to the expected, mechanical properties of the Nafion® were degraded by adding the silica particles, especially with smaller particles (<50nm).