Mechanical and Transport Properties of Nafion: Effects of Temperature and Water Activity

Nafion is employed in Polymer Electrolyte Membrane (PEM) fuel cells, where it must function over a broad range of temperatures and water activities. The physical and transport properties of Nafion are strongly affected by both temperature and water activity. Below 50°C increased water activity plasticizes Nafion, but above 60°C increased water activity stiffens Nafion. Increased water activity results in slower stress relaxation of Nafion. At low water activity the proton conductivity of Nafion decreases with increasing temperature. These results are accounted for by clustering of the sulfonate groups. Clustering of sulfonate groups is energetically driven by hydrophobic-hydrophilic repulsive interactions. The entropy of mixing breaks apart the clusters at elevated temperature. Water absorption increases the volume fraction of the hydrophilic domains and induces clustering. A semi-quantitative regular solution model is introduced to account for the effects of temperature and solute activity on clustering. The results define processing conditions of temperature and solute activity to control the physical properties of Nafion in reproducible fashion.