Proton transport in proton exchange membranes

This work investigated several proton exchange membranes (PEMs): perfluorosulfonic acid-based polymers (Nafion®), sulfonated poly(ether ether ketone) (S-PEEK), radiation-grafted ethylenetetrafluoroethylene-grafted-poly(styrene sulfonic) acid (ETFE-g-PSSA), sulfonated trifluorostyrene-co-substituted trifluorostyrene (BAM®), sulfonated polystyrene-b-poly(ethylene-r-butylene)-b-polystyrene triblock copolymer (S-SEBS), and a series of novel photocurable polyelectrolytes. These polymer systems differ in their chemical structure, ion content, and morphology. Proton conductivity and water sorption behaviour as a function of ion content for the S-PEEK, ETFE-g-PSSA, BAM, and S-SEBS series have been investigated at room temperature under fully hydrated conditions. A detailed analysis of the data has shown that strong links exist between conductivity and acid concentration, and that a deeper understanding of these effects can be gained by examining proton mobility. Results indicate that variations in mobility appear as a consequence of the different chemical structures. The influence of water content was further investigated by evaluating the proton mobility of Nafion and each BAM membrane while equilibrated with water vapours of known relative humidities between 50 – 98% RH. The proton transport properties of BAM are highly susceptible to changes in relative humidity with the most dramatic effects being seen with the high ion content membranes. It is proposed that when these membranes lose water and shrink, they reorganize to form tortuous ion conductive pathways which retard proton movement. A series of se mi-interpenetrating network proton conducting membranes have been created by the photocuring of polymerizable polyelectrolyte liquids comprised of linear S-PEEK immersed in a solution of liquid monomers in a range of compositions. It has been shown that the relative composition of the components has a strong influence on mechanical properties, proton conductivity, and water sorption behaviour.