EXAFS and Mössbauer studies of iron neutralized Nafion ionomers

Abstract

X-ray absorption spectroscopy was used together with Mössbauer spectroscopy to study the chemical state and the microstructure of iron neutralized Nafion membranes. The cation oxidation state deduced from the x-ray absorption-near-edge structure (XANES) of the ionomers agrees with that deduced from the Mössbauer isomer shift. Extended x-ray absorption fine structure (EXAFS) results confirm the existence of Fe(H2O)62+ octahedra, with a Fe–O distance of 2.11±0.01 Å in hydrated ferrous ionomers. As water content decreases below 6 wt. %, both coordination number and distances determined from EXAFS are found to decrease. The hydrated ferric Nafion salt contains many isolated Fe(H2O)63+ complexes which convert to a ferric dimer in vacuum-dried ferric salts which has an isomer shift of 0.58±0.02 mm/s relative to iron and a quadrupole splitting of 1.73±0.04 mm/s at 80 K. The dimer is shown to be [(H2O)5Fe–O–Fe–(H2O)5]4+, having an Fe–H2O distance of 2.05±0.02 Å and a rather short Fe–O distance of 1.77±0.02 Å. The Fe–O–Fe bond angle is 155±20°.

EXAFS and XANES studies of Zn2+ and Rb+ neutralized perfluorinated ionomers

“…These functions have been calculated theoretically by Teo and Lee [24]. In a previous paper [18,19] it was found that the theoretical t/and q~/are sufficiently accurate in predicting first row transition metal-oxygen distances if Eo is taken as a parameter of fit. Various techniques have been developed in order to extract information from normalizedx (k) spectra.…”

“…In previous papers [18,19] the EXAFS, XANES and M6ssbauer results for perfluorosulfonated ionomers neutralized with Ni 2+, Fe 2 § and Fe 3 § were described. In this contribution, the cation environment in zinc and rubidum perfluorosulfonated and peffluorocarboxylated ionomers is examined using EXAFS and XANES.…”

“…It is, however, possible to obtain complementary information from XANES to that extracted from EXAFS concerning the coordination geometry and the effective charge on the absorbing o4 % atom. For example in a previous paper XANES was used to determine the valence state of iron neutralized Nation membranes [19]. Certain types of symmetry are known to produce particular features in the near-edge spectrum.…”

Interplay between α-relaxation and morphology transition of perfluorosulfonate ionomer membranes

“…interactions between the water molecules and the Nafion ionic domains [17]. Increasing the water content promotes the dissociation of protons, thereby increasing the electrostatic repulsions between sulfonic acid groups [17,24,25]. Therefore, the displacements of a and b relaxations of Nafion to lower frequencies can be understood as a dilution process due to the higher water sorption capacity at high temperatures and relative humidity.…”

Magnetism, optical absorbance, and 19F NMR spectra of nafion films with self‐assembling paramagnetic networks

“…Various aspects of fundamental properties and applications of Nafion have been studied in the last few decades. In the early 1980s, Nafion doped with Fe 2+ and Fe 3+ ions was studied by Pan et al18, 19 They showed that Fe 2+ ions in Nafion have a stable divalent state and water molecules are strongly coordinated to these ions in an octahedral coordination. The first spin crossover system based on Nafion was synthesized by Hauser et al12 by treating Nafion in an aqueous solution of [Fe(2‐mephen) 3 ](ClO 4 ) 2 .…”