Sulfonated poly(arylene ether sulfone) membrane containing sulfated zirconia for high-temperature operation of PEMFCs

Abstract

A new composite membrane that consists of sulfonated poly(arylene ether sulfone) (SPES) and a highly functionalized inorganic filler of sulfated zirconia (s-ZrO₂) was synthesized and evaluated for enhancement of water retention as well as proton conductivity at high temperatures above 120 °C in the absence of water. Since the SPES has less hydrophobic/hydrophilic difference between the polymer backbone and –SO₃H ionic groups, the proton channels are relatively small and not easily distributed, which results in no significant change at highly saturated conditions. Therefore, the effective interconnection between the –SO₃H ionic groups and bound water adsorbed on the surface of the s-ZrO₂ is the key in achieving a higher proton transfer for PEM fuel cells at high temperatures above 120 °C. Analysis by small angle X-ray scattering (SAXS) of recast membranes showed that SPES random copolymers with various s-ZrO₂ contents have a similar ionic cluster size and less pronounced phase separation regardless of the dry/swollen condition of the membrane. However, the phase images obtained by atomic force microscopy (AFM) showed that the connectivity of ionic groups varies, depending on the content of s-ZrO₂ in the polymer matrix. Larger and more interconnected hydrophilic regions were found with the SPES membrane when there was no significant particle aggregation in the hydrophilic moieties of the polymer matrix. In this study, the composite membrane with 5 wt% s-ZrO₂ had the highest proton conductivity at 120 °C due to better ionic interactions as well as the presence of bound water combined with the –SO₃H ionic groups.