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Design and Development of Highly Sulfonated Polymers as Proton Exchange Membranes for High Temperature Fuel Cell Applications

  • Thuy D. Dang
  • Zongwu Bai
  • Mitra Yoonessi
Chapter

Abstract

A series of high molecular weight, highly sulfonated poly(arylenethioethersulfone) (SPTES) polymers were synthesized by polycondensation, which allowed controlled sulfonation of up to 100 mol %. The SPTES polymers were prepared via step growth polymerization of sulfonated aromatic difluorosulfone, aromatic difluorosulfone, and 4,4 ′-thiobisbenzenthiol in sulfolane solvent at the temperature up to 180 °C. The composition and incorporation of the sulfonated repeat unit into the polymers were confirmed by 1H nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. Solubility tests on the SPTES polymers confirmed that no cross-linking and probably no branching occurred during the polymerizations. The end-capping groups were introduced in the SPTES polymers to control the molecular weight distribution and reduce the water solubility of the polymers. Tough, ductile membranes formed via solvent-casting exhibited increased water absorption with increasing degrees of sulfonation. The polymerizations conducted with the introduction of end-capping groups resulted in a wide variation in polymer proton conductivity, which spanned a range of 100 –300 mS cm−1, measured at 65 °C and 85 % relative humidity. The measured proton conductivities at elevated temperatures and high relative humidities are up to three times higher than that of the state-of-the-art Nafion-H proton exchange membrane under nearly comparable conditions. The thermal and mechanical properties of the SPTES polymers were investigated by TGA, DMA, and tensile measurements. The SPTES polymers show high glass transition temperatures (Tg), ̃220 °C, depending on the degree of sulfonation in polymerization. SPTES-50 polymer shows a Tg of 223 °C, with high tensile modulus, high tensile strengths at break and at yield as well as elongation at break. Wide angle X-ray scattering of the polymers shows two broad scattering features centered at 4.5 Å and 3.3 Å, the latter peak being attributed to the presence of water molecules. The changes in the scattering features of the water in SPTES−70 membrane were examined as a function of drying time during an in situ drying experiment. The in situ small angle X-ray scattering from water swollen SPTES−70 membrane in a drying experiment exhibited a decrease in the water domain size morphology. AFM studies of SPTES−70 membrane in a humidity range (35 – 65 % RH) revealed an increased size of hydrophilic clusters with increasing humidity. SEM examination of cryofractured dry and swollen SPTES−70 membrane surface indicated a change from a smooth brittle fracture to a fractured surface with plastic deformation, verifying the plasticizing effects of the water molecules in the swollen membrane. Membrane electrode assemblies (MEAs), fabricated using SPTES-50 polymer as proton exchange membrane (PEM) incorporating conventional electrode application techniques, exhibit high proton mobility. The electrochemical performance of SPTES-50 membrane in the MEA was superior to that of Nafion. The SPTES polymers have been demonstrated to be promising candidates for high temperature PEM in fuel cell applications.

Keywords

Fuel Cell Proton Exchange Membrane Fuel Cell Membrane Electrode Assembly Fuel Cell System Cation Exchange Membrane 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer Science+Business Media, LLC 2009

Authors and Affiliations

  • Thuy D. Dang
  • Zongwu Bai
  • Mitra Yoonessi

There are no affiliations available

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