Synthesis of high Temperature proton-conducting polymer electrolyte membranes

Our synthetic approaches toward the development of high temperature proton-conducting polymer electrolyte membranes for the application in fuel cells are reviewed. Aromatic polyethers and polysulfides with acidic function are targeted. Oxidative polymerization or polycondensation of thioanisole derivatives gave sulfonium-containing precursor polymers, which were sulfonated to afford poly(phenylene sulfide) bearing up to two sulfonic acid groups per repeating unit. Poly(phenylene sulfide sulfonic acid) shows good thermal stability and high proton conductivity (>10^-2 Scm^-1) at 180°C and 100% RH. Composite materials of poly(phenylene sulfide sulfonic acid) with poly(ethylene oxide) have been prepared for non-aqueous proton conduction. Unlike the conventional proton migration as hydronium ions, bare protons could move through the composite membranes along with the segmental motion of poly(ethylene oxide). Novel aromatic polyether co-polymers composed of sulfonated tetraphenylphenylene (or hexaphenylbiphenylene), perfluorobiphenylene and hexafluoropentylene moieties have been synthesized. These polyether electrolytes are stable to hydrolysis and oxidation and show high proton conductivity. Analogs containing phosphonic acid and phosphinic acid have also been synthesized, of which electrolyte properties are investigated.