Polymeric proton conducting membranes for medium temperature fuel cells (110-160 degrees C)

The availability of stable polymeric membranes with good proton conductivit y 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-PEE K) membranes was performed as a function of relative humidity (r.h.) in a w ide range of temperature (80-160 degreesC). The occurrence of swelling/soft ening phenomena at high r.h. values prevented conductivity determinations a bove certain temperatures. Nevertheless, when r.h. was maintained at values lower than 80%, measurements were possible up to 160 degreesC. The results showed that Nafion is a better proton conductor than S-PEEK at low r.h. va lues, especially at temperatures lower than 120 degreesC. The differences i n conductivity were, however, leveled out with the increasing r.h. and temp erature. While at 100 degreesC and 35% r.h. the conductivity of S-PEEK 2.48 was about 30 times lower than the conductivity of Nafion, both membranes r eached a comparable conductivity (4 x 10(-2) S cm(-1)) at 160 degreesC 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 mem branes in medium temperature fuel cells, are discussed. (C) 2001 Elsevier S cience B.V. All rights reserved.