New Fluorinated Polymers Bearing Pendant Phosphonic Acid Groups. Proton Conducting Membranes for Fuel Cell

The synthesis and characterizations of original fluorinated copolymers bearing phosphonic acid side functions for novel membranes potentially involved in fuel cell applications are presented. These copolymers were obtained by chemical modifications of various poly(CTFE- alt -IEVE) and poly[(CTFE- alt -IEVE)- co -(CTFE-EVE)] terpolymers (where CTFE, IEVE, and EVE stand for chlorotrifluoroethylene, 2-iodoethyl vinyl ether, and ethyl vinyl ether) via the Arbuzov reaction. Then, the hydrolysis of the phosphonate moieties into phosphonic acid groups was carried out quantitatively in mild conditions, in the presence of bromotrimethylsilane. The ionic exchange capacities (IECs) determined by potentiometric titration were ranging from 2.9 to 6.8 mequiv/g. The thermal and electrochemical properties of the resulting membranes (processed by casting) were investigated. Whatever the phosphonic acid content, the main degradation started from 250 °C, showing a high thermo-oxidative stability of these copolymers. At 25 °C and 100% relative humidity (RH), the level of conductivity was found in the range 0.02−20 mS/cm and highly dependent on the IEC. Finally, moderate to good conductivity values (about 0.25 mS/cm) at higher temperature (120 °C) and lower RH (25%) were observed, which demonstrated a limited dependence toward both temperature and RH.