DFT - experimental IR spectroscopy of lithiated single ion conducting perfluorinated sulfonated ionomers: Ion induced polarization band broadening

Abstract

The impact of state of hydration and ion exchange on the exchange site local symmetry of Aquivion and Nafion perfluorinated sulfonated ionomers (PFSI) are probed by transmission IR spectroscopy.Hydrated PFSI-H membranes exhibit a pair of bands corresponding to a dissociated sulfonate exchange site with a local 3-fold axis of symmetry (C3V). C3V bands are supplanted by C1 bands (no local symmetry) corresponding to the sulfonic acid form of the exchange site. At intermediate states of hydration C3V and C1 bands co-exist. Hydrated PFSI-Li exhibits C3V bands. In contrast to PFSI-H, the PFSI-Li C3V bands persist throughout dehydration, with a final aggregate structure where each Li+ provides 1/3 of a charge per sulfonate oxygen with an overall C3V motif.The C3V band FWHM values progressively increase (hydrated PFSI-H<hydrated PFSI-Li<dehydrated PFSI-Li) in the case of Nafion and Aquivion.FWHM trends are explained in terms of electric field induced exchange site polarization. The smaller Stokes Einstein radius of aquated Li+ enables closer proximity to the exchange site and thus greater induced band broadening.The relevant protonic Stokes Einstein radius must be derived from a tracer diffusion coefficient (D*): An effective protonic diffusion coefficient (Dσ) obtained from conductivity measurements (9.31×10−5cm2/s) yields a non-physical Stokes Einstein radii of 0.26Å. A theoretically calculated D*, consistent with a group-1 diffusion coefficient trend analyses, yields an aquated proton Haven ratio (D*/Dσ) of 0.086, a value typical of solid state fast-ion conductors. The D* derived protonic Stokes Einstein radius of ~3.1Å (relative to Li+ 2.01Å) is consistent with the ion-to-exchange site proximity argument for the observed FWHM trends.