Cross-linked poly(ether ether ketone) membranes with pendant sulfonic acid groups for fuel cell applications

A series of cross-linkable poly(ether ether ketone)s (PEEKs) with pendant sulfonic acid groups were synthesized and used to prepare polymer electrolyte membranes for fuel cells. These transparent and flexible membranes are insoluble in common organic solvents and demonstrated little swelling in hot water. Thermal and mechanical stability of the membranes were evaluated using a thermogravimetric analyzer and strain-stress test, respectively. The methanol permeability of the cross-linked PEEKs with pendant sulfonic acid group membranes is much lower than that of Nafion 117 membrane, by more than 10 times. The conductivity of the cross-linked PEEK membrane with a 5% degree of cross-linking is comparable to that of Nafion 117 at 50 °C (0.053 S cm^-1) and even better above 80 °C. This kind of membrane is expected to be a good alternative to Nafion 117 in polymer electrolyte fuel cell applications.