Novel hybrid fluoro-carboxylated copolymers deposited by initiated chemical vapor deposition as protonic membranes

Copolymers of 1H, 1H, 2H, 2H,-perfluorodecyl acrylate and methacrylic acid are synthesized by initiated chemical vapor deposition (iCVD) to serve as proton exchange membranes for potential application in miniaturized fuel cells. The copolymerization of hydrophobic and hydrophilic monomers is difficult by conventional liquid-phase technique. We demonstrate that the use of iCVD is very promising for the synthesis of these membranes. Changing the monomer feed ratio we have a systematical control over the copolymer composition. The calculation of the copolymer ratios shows a moderate alternate composition of the copolymer which is advantageous to obtain highly conductive (acid functionalities) and stable (hydrophobic functionalities) membrane in the operating conditions of the fuel cells. The copolymers show a high degree of chain mobility when passing from dry to hydrated state as demonstrated by XPS analysis and by the high water contact angle hysteresis measured (>30°). An ionic conductivity of 70 mS/cm is obtained in conditions of highest percentage of hydrophobic functionalities. This conductivity value is comparable with the ionic conductivity of commercial Nafion^®.