X-ray Absorption and Solid-State NMR Spectroscopy of Fluorinated Proton Conducting Polymers

Abstract

19 F and 13 C solid-state nuclear magnetic resonance (SSNMR) spectra and near edge X-ray absorption fine structure (NEXAFS) spectra at the S 2p, C 1s, O 1s, and F 1s edges of three different types of perfluorosulfonic acid (PFSA) proton conducting polymers are reported. The NEXAFS spectra were recorded in a scanning transmission X-ray microscope (STXM). They are reported on quantitative intensity scales, suitable for use as reference standards for analytical spectromicroscopy studies. The NEXAFS spectral features are assigned using comparisons to the corresponding spectra of small molecule and polymer analogues, including C 1s and F 1s spectra of polytetrafluoroethylene (PTFE) recorded using STXM. The 19 F and 13 C SSNMR spectra are reported and analyzed. Within each type of spectroscopy, the spectra of the three types of PFSA are very similar. However, small but statistically significant differences were identified which give insights into how minor molecular structure differences are reflected in the SSNMR and NEXAFS spectra. The relative chemical sensitivity of SSNMR and NEXAFS for studies of PFSA materials is compared. Solution-state NMR diffusion profile analyses of two PFSA ionomer dispersions are also reported.

Calculating absorption dose when X-ray irradiation modifies material quantity and chemistry

“…6. Assignment of the fine-structure peaks is presented in Table S4 and has been discussed in detail elsewhere (Yan et al, 2018). The average NEXAFS spectrum of each pad was extracted and the spectra from the three edges appended to generate a single 280-720 eV spectrum for each pad.…”

Chemical and Morphological Origins of Improved Ion Conductivity in Perfluoro Ionene Chain Extended Ionomers

“…The strongest features observed are similar across all four ionomers, as shown in the Supporting Information, and consistent with previously reported spectra. 59 However, only PFICE-2 and PFICE-3 have a shoulder feature near 534 eV, a core-level absorption feature that has not been observed in PFSA materials. This is consistent with the simulated O K-edge results of PFICE-2 with six added water molecules (shown in the Supporting Information), suggesting that this shoulder feature originates from the bis(sulfonyl)imide groups.…”

“…NEXAFS spectroscopy probes transitions of core-level electrons to unoccupied states. 25 Previous work has utilized NEXAFS spectroscopy to probe chemical bonding of Nafion PFSAs in the soft X-ray regime 59 and near the sulfur K-edge. 60,61 Since the sulfur K-edge spectrum is sensitive to chemical environment around the sulfur atoms, it can distinguish between sulfur atoms in the sulfonic acid functional group and sulfur atoms in the bis(sulfonyl)imide groups.…”

Enhanced Performance of Ionomer Binder with Shorter Side‐Chains, Higher Dispersibility, and Lower Equivalent Weight

“…This technique involves averaging the orientation‐dependent interactions to zero on the time scale of the rotor revolution speed, which mimics molecular tumbling in the solution state. As a result, the CSA and the dipolar coupling interactions are mostly removed and well‐resolved solution‐state‐like spectra are obtained . The molecular motions and mobility changes in PFSA ionomers have been established by solid‐state 19 F NMR; as mentioned above, not only the side‐chains but also the main‐chains undergo fast motions strongly , .…”