Phosphoric acid doped high temperature proton exchange membranes based on sulfonated polyetheretherketone incorporated with ionic liquids

A composite membrane was fabricated using a novel approach based on the ionic liquids 1-butyl-3-methylimidazolium chloride or 1-butyl-3-methylimidazolium hexafluorophosphate, sulfonated polyetheretherketone (SPEEK), and phosphoric acid. This proton conducting composite membrane shows promise for operation in high temperature proton exchange membrane fuel cells at working temperatures up to 160 °C without humidification. Proton conductivity at a level of 2.0 × 10^? 2 S/cm was achieved at 160 °C by the composite membrane with a molar ratio of 1:0.6:9 for SPEEK, 1-butyl-3-methylimidazolium (BMIM) cation and phosphoric acid, respectively. The sulfonation degree was 0.643 per polymer repeat unit with over 90% of the sulfate fixed anions forming a salt complex with BMIM cations. The tensile stress at break of the composite membrane was 15.5 MPa at room temperature, and it decreased from 4.1 to 1.9 MPa when the temperature increased from 110 to 160 °C, respectively.