EXAFS and Moessbauer studies of iron neutralized Nafion ionomers

X-ray absorption spectroscopy was used together with Moessbauer 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 Moessbauer 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 A 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 A and a rather short Fe--O distance of 1.77 +- 0.02 A. The Fe--O--Fe bond angle is 155 +- 200