Vibrational Spectroscopy in the Study of Composite and Nanostructured Materials for Electrochemistry

Our research group has recently demonstrated the ability to probe within composite membranes formed by sandwiching single layer graphene (SLG) between two Nafion layers (Nafion 211, 25 µm thickness). These graphene-sandwich membranes have shown high selectivity for proton transport across electrochemical cells [1,2]. Using a microscope equipped with an oil-immersion objective, the graphene layer is readily detected in confocal Raman microscopy depth-profiles (z-direction profiles) of the composite membranes. Spectra are sensitive to defects and stress within the buried graphene layer. This presentation will discuss the use of confocal Raman microscopy for structural characterization and spatial profiling in the development of composite polymer electrolyte membranes that contain a graphene layer as a barrier to ion transport. In a second application area, infrared spectroscopy is being adapted to investigate properties of Nafion ionomer particles in dilute dispersions and during transformation to gel and solid phases through the use of time-resolved attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. Since the properties of dispersing fluids can affect the mechanical strength of solution-cast Nafion thin films [3], it is of interest to gain insights into ionomer properties in the dispersion and during phase changes. A well-resolved peak associated with vibrational motions of the Nafion side chain -SO3 -end group is shown to be sensitive to gel-phase development. The persistence of hydration water in the vicinity of the -SO3 -group helps maintain a constant local refractive index that limits optical distortion in the band as the ionomer assembles into the phase-separated framework structure of the solid membrane. ATR FTIR measurements predict trends in evaporative water loss from the dispersion and entry into the gel-phase consistent with expectations of gravimetric measurements and earlier studies [3] of Nafion ionomer dispersion properties. 1. Bukola, S., et al. J. Am. Chem. Soc. 2018, 140, 1743. 2. Bukola, S., et al., ACS Appl. Nano Mater. 2019, 2, 964. 3. Kim, Y.S., et al., Macromolecules 2015, 48, 2161.