Poly(phenylene sulfonic acid)-expanded polytetrafluoroethylene composite membrane for low relative humidity operation in hydrogen fuel cells

A composite fuel cell membrane based on highly proton conductive, thermally crosslinkable poly(phenylene sulfonic acid) (cPPSA) copolymer is prepared by pore filling, to overcome the copolymer's poor mechanical characteristics. The membrane comprises a mechanically stabilizing scaffold from a commercial, expanded polytetrafluoroethylene (ePTFE) porous film, which is impregnated with cPPSA, followed by its thermal crosslinking. The membrane has a very high proton conductivity, 3.5 and 3.0 times higher that of a Nafion XL reference, at 80 °C and 40 %RH, and 90 %RH, respectively. Membrane-electrode assemblies fabricated with this membrane and commercial Pt/C-based electrodes (0.12 mgPt cm−2) are tested in hydrogen fuel cell with air and oxygen, at 80 °C. High power output and low gas crossover are observed. For example, with a 15 μm-thick cPPSA-ePTFE PEM, the maximum power density at 80 °C and 100 kPa backpressure, decreases by only 15%, from 1.00 W cm−2 to 0.85 W cm−2, when the RH of the hydrogen and air feeds are reduced from 100% to 30%. In comparison, a MEA with Nafion XL shows a significant, 52% loss, from 0.82 W cm−2 to 0.39 W cm−2, during similar reduction of fuel cell humidification.