Vibrational spectroscopy in the study of ionomer material properties

Date

2019-12

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Abstract

Proton exchange membrane (PEM) fuel cells are being adapted to power automobiles and buses. Interest stems from the pollutant-free nature of PEM fuel cell energy conversion processes. In striving to improve performance, one area of active research focuses on properties of the polymer electrolyte membrane separating the anode and cathode reactions within the fuel cell. Vibrational spectroscopy techniques, with their sensitivity toward polymer composition and structure, provide powerful tools for investigating fuel cell polymer electrolyte membranes. This dissertation explores novel measurement and data analysis strategies in the study of fluorinated fuel cell ionomer membrane materials through the use of vibrational spectroscopic techniques, including attenuated total reflection Fourier transform infrared (ATR FTIR) spectroscopy, confocal Raman microscopy and ATR FTIR microscopy. Using a set of perfluorinated sulfonic acid ionomer (PFSA) membrane standards, a linear calibration relationship was derived for predicting ionomer sulfonate content from area-normalized ATR FTIR spectra. The average number of backbone tetrafluoroethylene (TFE) units separating the ionizable-group containing side chains (m) in the standards was in the range of 7.2 - 2.1. When applied to independent spectral measurements, m was predicted with a standard error less than or equal to 0.3. In the study of orientational ordering in nanothin (70 nm - 8 nm) Nafion PFSA ionomer films, electromagnetic field calculations were performed to identify effects of optical dispersion distortion in ATR FTIR spectra of films supported on Si substrates. A spectral model was developed for Nafion thin films across the 1400 - 950 wavenumber region from frequency-dependent, isotropic optical constants derived from Kramers-Kronig analysis of ionomer transmission infrared spectra. The calculations reproduced overall polymer thickness-dependent changes in peak frequencies and band shapes observed in experimental spectra recorded with p- and s- polarized light. General trends were traceable to effects of anomalous dispersion and electric field enhancement within the nanoscale gap separating the Ge and Si phases. However, optical effects could not fully explain perturbations in spectra of the thinnest films, where molecular orientational ordering is expected to be strongest. Strategies for gleaning further molecular structural detail from vibrational spectra of ultrathin (<50 nm) ionomer films are discussed. Understanding of optical distortions in ATR FTIR spectra was applied to the interpretation of time-resolved measurements that followed changes as a dilute Nafion ionomer dispersion was transformed to gel and eventually solid film phases. Bands associated with ionomer framework C-F and C-C stretching vibrations were strongly affected by optical distortion, while the symmetric S-O stretching vibration of the Nafion side-chain end group displayed dilute analyte characteristics useful for estimation of the gel transition point from aqueous dispersions. In measurements on Nafion ionomers dispersed in ethanol, ATR FTIR spectra showed water enrichment occurs during evaporative solvent loss. A partial least squares (PLS) regression model built from a set of ATR FTIR spectra of water-ethanol mixtures was applied to determine the water mole fraction in the solvent during Nafion thin film development. A rapid increase in solvent water content was evident following transformation to the gel-phase. The results highlight the importance of considering water as a possible influence during ionomer film formation from low boiling point, non-aqueous dispersing fluids. Two additional studies explored spatial mapping of component composition within fluorinated ionomer membrane materials. Using a micro-ATR probe, ATR FTIR spectra were recorded from ~32 µm diameter circular regions at the surface of a laccase air-breathing biofuel cell cathode. Spectral features of enzyme, buffer anions and a carbon nanotube additive were identified. The spatial distribution of the components was heterogeneous. Finally, given the sampling depth of ATR FTIR measurements is limited to a few micrometers beneath a polymer membrane surface, confocal Raman microscopy was applied to probe more deeply within ionomer membranes. Single layer graphene (SLG) was detected at the interface separating Nafion-212 (25 µm thickness) membranes in a composite Nafion | SLG | Nafion sandwich structure. Raman spectra confirmed the buried SLG was free of defects following the hot-pressing procedure that was used to fabricate the composite membrane.

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Keywords

Vibrational spectroscopy, Ionomer materials

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