Polyimide electrolyte membranes having fluorenyl and sulfopropoxy groups for high temperature PEFCs^x

A series of novel polyimide electrolyte copolymers (SPPI-X) containing fluorenyl and sulfopropoxy groups were synthesized. The copolymer composition (X: molar percentage of the fluorenyl groups) was set from 0-60 mol% so that the ion exchange capacity of the electrolyte membrane ranged from 1.3 to 2.9 meq g^-1. Tough and flexible membranes, with the typical polyimide brown colour, were obtained by casting from the polymer solution. The SPPI membranes are thermally stable up to 220 °C without glass transition, melting, and decomposition under a dry nitrogen atmosphere. Sulfopropoxy groups are effective at improving oxidative stability in Fenton's reagent since the main polymer chains are separated from sulfonic acid groups and are less suscep tible to hydrophilic attack by the oxidative radical species. A high mechanical strength, with 53 MPa of the maximum stress at break and 15% of strain, was confirmed for SPPI-30 membrane. The proton conductivity of the SPPI membranes was higher than 0.1 S cm^-1 at temperatures from 40 to 120 °C. The long term proton conductivity stability (ca. 1,000 h) was also confirmed at 120 °C, to prove the possible availability of the membrane materials for high temperature PEFC applications.