Polymeric proton conducting membranes for medium temperature fuel cells

The availability of stable polymeric membranes with good proton conductivity at medium temperatures is very important for the development of methanol PEM fuel cells. In view of this application, a systematic investigation of the conductivity of Nafion 117 and sulfonated polyether ether ketone (S-PEEK) membranes was performed as a function of relative humidity (r.h.) in a wide range of temperature (80–160°C). The occurrence of swelling/softening phenomena at high r.h. values prevented conductivity determinations above certain temperatures. Nevertheless, when r.h. was maintained at values lower than 80%, measurements were possible up to 160°C. The results showed that Nafion is a better proton conductor than S-PEEK at low r.h. values, especially at temperatures lower than 120°C. The differences in conductivity were, however, leveled out with the increasing r.h. and temperature. While at 100°C and 35% r.h. the conductivity of S-PEEK 2.48 was about 30 times lower than the conductivity of Nafion, both membranes reached a comparable conductivity (4×10−2 S cm−1) at 160°C and 75% r.h. The effect of superacidity and crystallization of the polymers on the conductivity, as well as the possibility of using Nafion and S-PEEK membranes in medium temperature fuel cells, are discussed.