Plasma polymerized electrolyte membranes and electrodes for miniaturized fuel cells

Miniaturized fuel cells for portable systems like cellular phones, laptops, or other conventionally battery-driven devices, as well as long-term stationary monitoring electronics, have a potential market, especially for direct methanol fuel cells. However, design and fabrication technologies have to be adopted that allow the desired miniaturization of such a fuel cell. Thin film technologies like plasma polymerization and sputtering are suitable techniques for realizing membrane electrode assemblies only several microns in thickness that can be deposited on thin substrates (e.g., silicon wafers, porous foils, or others). Furthermore, plasma polymerized films exhibit a high degree of cross-linkage and are pinhole free even for films of only a few hundred nanometer in thickness, in contrast to conventionally polymerized films. In case of an electrolyte membrane these benefits yield a reduction of membrane resistance and a decreased methanol crossover. We have developed plasma polymerized electrolyte membranes using tetrafluoroethylene to generate the polymeric backbone of an ion-conductive membrane and vinylphosphonic acid to incorporate acid groups, which are responsible for the proton conductivity. Depending on the process parameters these films exhibit an ion conductivity in the range of 100 mS/cm to 200 mS/cm (at 80°C), determined by ac-impedance measurements. These films were optimized with respect to their use in direct methanol fuel cells to achieve a high ion conductivity and high thermal resistance. Porous graphite electrodes were fabricated using an acetylene plasma polymerization process. These films are combined with the plasma polymerized electrolyte membrane to form a thin film membrane electrode assembly.