Proton conducting membranes based on sulfonated aromatic polymers for PEM fuel cells: synthesis and properties

The proton exchange membrane is the heart of polymer electrolyte membrane (PEM) fuel cells. In order to obtain good fuel cells performances, the membranes should exhibit at temperatures above 100°C and low relative humidity morphological, hydrolytic and mechanical stability. In this thesis two different strategies for the synthesis of proton conducting membranes based on sulfonated aromatic polymers have been explored: hybrid organic-inorganic nanocomposites and formation of inter-chain links as a result of heat treatments. The use of hybrid materials allows to exploit the synergistic effect of the simultaneous presence of an organic component, the ionomer, and an inorganic component. In particular, a mixture of S-PEEK with high degree of sulfonation, as majority compound, and a silylated polymer based on PPSU as minority component was studied. The S-PEEK is used to ensure high conductivity, while the Si-PPSU ensures good mechanical stability (anchor phase). Hybrid nanocomposites based on S-PEEK with dispersed functionalized TiO2 were also studied. The second strategy followed, the synthesis of crosslinked polymers, was very positive. In particular it was observed for the first time, that it was possible to obtain inter-chain sulfone bonds using appropriate thermal treatments in the presence of DMSO as solvent casting. It was proved that the membranes obtained are able to withstand water up to 145°C without significant swelling and maintain mechanical stability and good conductivity. The membranes were characterized using several techniques, including: thermo-gravimetric analysis, static and dynamic mechanical measurements, water absorption measurements, either by immersion or in vapour phase, IR and NMR spectroscopy, dielectric spectroscopy and impedance analysis.