(589c) Development, Structure and Properties of Novel Wholly Aromatic Pore-Filling Electrolyte Membrane for Pemfcs and Dmfcs

Polymer electrolyte membrane fuel cells (PEMFCs) have received much attention because of their potential application in portable electrical devices, automobiles, and stationary power devices. Especially, direct methanol fuel cells (DMFCs) are strong candidate of new portable devices. However, methanol crossover causes serious reduction of the DMFC performance. To solve this problem, we have developed a pore-filling electrolyte membrane. The pore-filling electrolyte membrane is composed of two materials: a porous substrate having pore sizes of sub micron or less, and a polymer electrolyte that fills the pores of the substrate. The filling polymer exhibits proton conductivity, and the porous substrate matrix mechanically prevents excess swelling of the filling polymer. Several benefits of pore-filling electrolyte membranes have been confirmed. Gel type filling polymer electrolytes were employed in previous reports. However, the reduction of methanol crossover is not enough. In this work, we chose aromatic materials for both substrate and filling polymer. Porous polyimide, chosen as substrate, is rigid and mechanically very strong to reduce swelling of filling polymer. Sufonated poly(arylene ether sulfone) (SPES), chosen as filling polymer, has hydrophobic part in the monomer unit, and can have phase aggregation morphology. Thus, we expected serious reduction of extra free water in the filling electrolyte and extremely low methanol crossover. As we expected, the pore-filling membrane showed excellent properties. For example, the membrane showed 300 times lower methanol crossover than Nafion117 with 30 wt% methanol feed solution at 25 ^oC, 10^-2 [S/cm] order proton conductivity at the same temperature, and high oxidative and thermal stability. The state of water inside the membrane is very important to show these kinds of phenomena, especially both extremely low methanol permeability and high proton conductivity. From the differential scanning calorimetry (DSC) analysis, no free water and less bound water were detected in the pore-filling membrane in contrast to SPES cast membranes. Furthermore, activation energy of proton conductivity was much higher than Nafion117 and SPES cast membrane. These results imply different structure and proton conducting mechanism in the pore-filling membrane from existing gel type polymer electrolyte membranes.