Composite S-PEEK membranes for medium temperature polymer electrolyte fuel cells

Sulphonated-PEEK polymers with two different sulphonation degrees (DS) were obtained by varying the sulphonation parameters. Ionomeric membranes were prepared as a reference. Composite membranes were obtained by mixing different percentage of 3-aminopropyl functionalised silica to the polymers dissolved in DMAc. The resulting membranes were characterised in terms of water uptake, IEC and proton conductivity in different conditions of temperature and relative humidity. The introduction of amino-functionalised silica in the lowest DS polymer slightly increases the water uptake, because it retains the water and in this case the hydrophilic property of the material is predominant. In the highest DS polymer the presence of SiO–NH2 decreases the water uptake and swelling, probably due to a major interaction between the amino groups of silica and the sulphonic groups of polymer. The proton conductivity of the highly sulphonated polymer does not change even if the swelling decreases, while in the poorly sulphonated polymer a slight increases of water retention increases the conductivity. A composite membrane was tested in PEFC at 120 °C reaching a maximum power density of 246 mW cm−2.