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°.