Synthesis and Properties of Poly(ether sulfone)s with Clustered Sulfonic Groups for PEMFC Applications under Various Relative Humidity

Novel sulfonated poly(ether sulfone) copolymers (S4PH-x-PSs) based on a new aromatic diol containing four phenyl substituents at the 2, 2 ', 6, and 6 ' positions of 4,4 '-diphenyl ether were synthesized. Sulfonation was found to occur exclusively on the 4 position of phenyl substituents by NMR spectroscopy. The ion exchange capacity (IEC) values can be controlled by adjusting the mole percent (x in S4PH-x-PS) of the new diol. The fully hydrated sulfonated poly(ether sulfone) copolymers had good proton conductivity in the range 0.004-0.110 S/cm. at room temperature. The surface morphology of S4PH-x-PSs and Nafion. 212 was investigated by atomic force microscopy (tapping-mode) and related to the percolation limit and proton conductivity. Single H-2/O-2 fuel cell based on S4PH-40-PS loaded with 0.25 mg/cm(2) catalyst (Pt/C) exhibited a peak power density of 462.6 mW/cm(2), which was close to that of Nafion 212 (533.5 mW/cm(2)) at 80 degrees C with 80% RH. Furthermore, fuel cell performance of S4PH-35-PS with various relative humidity was investigated. It was confirmed from polarization curves that the fuel cell performance of S4PH-35-PS was not as high as that of Nafion 212 under fully hydrated state due to higher interfacial resistance between S4PH-35-PS and electrodes. While under low relative humidity (53% RH) at 80 degrees C, fuel cells based on S4PH-35-PS showed higher peak power density (234.9 mW/cm(2)) than that (214.0 mW/cm(2)) of Nafion 212.