Polysulfone ionomers for proton-conducting fuel cell membranes: sulfoalkylated polysulfones

Polysulfones (PSUs) carrying short pendant alkyl side-chains with terminal sulfonic acid units have been prepared and studied as proton-conducting membrane materials. The first step in the preparation involved quenching of lithiated PSU with SO2 gas, resulting in sulfinated PSU. In the second step, the lithium sulfinate units on the polymer were reacted with sodium 2-bromoethanesulfonate, sodium 3-bromopropanesulfonate, or 1,4-butane sultone to produce sulfoethylated, sulfopropylated, or sulfobutylated PSUs, respectively. Analysis by thermogravimetry showed that membranes based on the sulfoalkylated polymers were stable up to approximately 300 °C under N2 atmosphere. Calorimetry measurements revealed that the modified polymers absorbed large amounts of non-freezing water, corresponding to 11–14 mol H2O/mol SO3H under immersed conditions. The proton conductivity of a membrane based on a PSU carrying 0.9 sulfopropyl chains per repeating unit was measured to be 77 mS/cm at 70 °C under humidifying conditions.