Conductivity and Water Uptake of Aromatic-Based Proton Exchange Membrane Electrolytes

Water uptake and proton conductivity as a function of temperature were determined for three aromatic-based, sulfonic acid-bearing polymers, plus the perfluoroalkyl sulfonic acid Nafion 117. Water uptake of submerged, equilibrated samples ranged from less than five water molecules per acid group for a high equivalent weight, sulfonated polyethersulfone to almost fifty waters per acid for a low equivalent weight, sulfonated polyetheretherketone. The most conductive aromatic-based polymer, sulfonated polyphenylquinoxaline (S-PPQ), had a room temperature conductivity of 9.8 × 10-3 S/cm, about an order of magnitude less than that of a perfluoroalkyl sulfonic acid under identical conditions. The slope of the S-PPQ Arrhenius conductivity plot was sufficiently steep that at 180°C, the proton conductivity, 1.3 × 10-1 S/cm, was only a factor of two lower than that of Nafion under similar conditions. The lower conductivity of the aromatic-based sulfonic acid polymers can be attributed to chain rigidity, lack of ion channels, and lower acidity.