SYNTHESIS OF PERFLUOROPOLYMERS WITH THE CONJUGATE ACID OF 2,3,5,6-TETRAFLUOROANILINE AS A PROTON-EXCHANGE GROUP AND ALKALINE STABILITY OF ORGANIC CATIONS FOR FUEL CELL APPLICATIONS
Date of Award
Doctor of Philosophy
A perfluoro(2,3,5,6-tetrafluoroanilinium sulfate) (PF(TFA-H)) polymer was synthesized from NafionTM through a free-radical coupling reaction. The PF(TFA-H) polymer employs TFA-H as an alternative proton-exchange group instead of -SO3H. Compared to a similar recast NafionTM film, the PF(TFA-H) membrane exhibits better mechanical properties. Furthermore, PF(TFA-H) membrane was found to have better proton conductivity and fuel cell performance than the recast NafionTM at a temperature over 90 °C. In a gas phase test at 120 °C, PF(TFA-H) presents an ionic conductivity of 174 mS/cm while the recast NafionTM shows a value of 161 mS/cm. Due to the great proton conductivity, the PF(TFA-H) membrane provides a maximum power density of 365 mW/cm2 at a current density of 625 mA/cm2 in a 5 cm2 H2-air fuel cell at 120 °C. In the second part, various organic cations involving benzimidazoliums, quaternary phosphoniums, tetrakis(alkylamino)-phosphoniums and a H+ coordinated tetrahedral cage structure were prepared for the alkaline stability test. Benzimidazolium cations degraded through a ring-opening reaction at a strong base condition. Changing the C2 substituent to the large protecting group can prevent -OH from attacking C2 carbon to enhance the stability of the cation. With phenyl protecting groups, BnIm-Ph presents decent durability in 3 M NaOD/CD3OD/D2O solution at 80 °C leaving 95% of cation remaining after ten days. The quaternary phosphoniums decomposed via Cahors-Hofmann oxidation reaction in the alkaline solution. The proton-cage structure shows excellent stability in 1 M NaOD/CD3OD/D2O solution at 80 °C with 94% remaining after 150 days. However, it degraded via a ring-opening reaction at 3 M NaOD/ D2O solution at 140 °C. Tetrakis(alkylamino)phosphoniums are found to be the best alkaline stabled cations for the application of the anion-exchange membrane. No degradation was found for Tetrakis(alkylamino)phosphonium cations after 210 days in 1 M NaOD/CD3OD/D2O solution at 80 °C. Even in 3 M NaOD/ D2O solution at 140 °C, more than 98 % of cations remained after 70 days.
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