19F NMR Studies of 3M Ionomer Adsorption on Carbon Supports and Pt/C Catalysts

Wednesday, 4 October 2017: 15:00
National Harbor 3 (Gaylord National Resort and Convention Center)
N. M. Cantillo (The University of Tennessee - Knoxville), L. Li (University of Tennessee - Knoxville), J. Peng, G. A. Goenaga, and T. A. Zawodzinski Jr. (University of Tennessee, Knoxville, TN)
In this study, the adsorption properties of water-dissolved 3M 825 equivalent-weight (EW) ionomer and carbon (C) supports or carbon-supported platinum (Pt/C) catalysts were studied with 400 MHz 19F Nuclear Magnetic Resonance Spectroscopy (NMR). The supports and catalyst materials (with and without ionomer addition) were also analyzed with Brunauer-Emmet-Teller (BET) nitrogen adsorption-desorption, transmission electron microscopy (TEM) and X-ray diffraction (XRD).

3M ionomer/carbon/Pt interactions were also explored through differential scanning calorimetry (DSC). Using these methods, a series of samples with and without ionomer addition were analyzed to explore the different interactions between components (Pt/Carbon, and Carbon/ionomer).

Preliminary DSC results are shown in Figure 1 for the 3M ionomer, ionomer/Pt, ionomer/Vulcan-carbon and the catalyst layer (CL) samples, where the CL is composed of ionomer/Vulcan-carbon/Pt. The 3M ionomer and ionomer/Vulcan-carbon samples, on the other hand, exhibit a weak and broad endothermic peak between 50 and 120°C. This process has been associated with the combination of structural changes in ionic cluster, and water volatilization in PFSA ionomers.1Both experiments with Pt present showed an exothermic feature around 95°C and 135°C for the ionomer/Pt and the CL samples, respectively.

The adsorption behavior of 3M ionomer on the carbon and catalyst samples was explored using 19F NMR. Figure 2 shows an NMR spectrum of the 3M PFSA ionomer in water solution after base correction. This figure also includes the chemical shifts and peak assignments. The strongest peak at -76.55 ppm corresponds to the internal reference (trifluoro acetic acid, CF3COOH) in solution. The ratio of the most prominent 3M ionomer peak (-124.13 ppm) associated with the CF2 group in the backbone and the internal reference peak was used for the analytical study, a similar approach to previous studies of Nafion.2–4