Direct methanol fuel cell membranes from polymer blends

Proton conducting membranes for a direct methanol fuel cell (DMFC) were prepared from the following polymer blends: Sulfonated poly [bis(phenoxy)phosphazene] (SPOP) - polybenzimidazole (PBI), Nafion - Teflon FEP (copolymer of tetrafluoroethylene and hexafluoropropylene), and Nafion - PBI. Films were made by either solution casting (for SPOP-PBI and Nafion-PBI) or melt processing (for Nafion-FEP). The resulting membranes (50-120 mum in thickness) were evaluated in terms of water swelling, proton conductivity, and methanol permeability. They were also fabricated in membrane-electrode assemblies and tested in a DMFC at 60°C. The proton conductivity and methanol permeability of the membranes were found to be dependent on the blend composition, e.g., ion-exchange capacity of SPOP and wt% of PBI (for SPOP-PBI blends), wt% FEP (for Nafion-FEP), and Nafion protonation degree and wt% PBI (for Nafion-PBI blends). The DMFC performance (maximum power density) with a SPOP-PBI membrane was comparable to that of a Nafion 117 at 1.0 M methanol, but the methanol crossover flux was 2.6 times lower than that of Nafion 117. With blended Nafion-FEP and Nafion-PBI membranes, the current-density/voltage behavior in a DMFC was similar to Nafion, with 1.4 and 1.5 times lower methanol crossover. Significant cost savings can be realized when using the blended films rather than Nafion 117 because they are thinner than Nafion 117 (by a factor of 3.0-3.5) and contain less of the expensive perfluorosulfonic acid polymer. With increasing methanol concentration (up to 10.0 M), the blended membranes exhibited superior DMFC performance and lower methanol crossover fluxes as compared to Nafion 117.