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Water Sorption and Percolation for Proton-Conducting Electrolyte Membranes for PEM Fuel Cells

The relationship between water sorption behavior and proton conduction in polymer electrolyte membranes based on sulfonated polyimide electrolyte membranes is studied from view points of polymer structure, ion exchange capacity, and percolation theory. The results indicate that the polymer chemical structure and ion exchange capacity show significant effects on water sorption and thus proton conductivity for various membranes. The density values of wet membranes decreased gradually with an increase in water uptake. Polymer electrolytes with flexible side-chain terminated with sulfonic acid group displayed smaller percolation threshold compared with main-chain-type polymer, indicating a better microphase-separation structure.