Direct ESR and spin trapping methods for the detection and identification of radical fragments in Nafion membranes and model compounds exposed to oxygen radicals

Direct electron spin resonance (ESR) and spin trapping methods are used in our laboratory in order to identify radical fragments in fluorinated membranes (Nafion) used in fuel cells and in model compounds when exposed to reactive oxygen species. Oxygen radicals were generated by the Fenton reaction, by the photo-Fenton method, and by UV-irradiation of aqueous H₂O₂ solutions. Direct ESR detection led to the identification of fluorinated radical fragments in UV-irradiated Nafion neutralized by Cu(II), Fe(II), and Fe(III) cations and exposed to H₂O₂. In Nafion exposed to the Fenton reagent in the presence of DMPO (5,5-dimethyl-1-pyrroline) as the spin trap, radical adducts of hydroxyl and carbon-centered radicals (CCRs) were detected. A fluorinated model compound, perfluro-(2-ethoxyethane) sulfonic acid (CF₃CF₂OCF₂CF₂SO₃H, PFEESA), that mimics the side chain of Nafion was chosen for study, with the goal to shed light on the more complicated degradation processes in the polymeric membranes. Analysis of the spin adducts obtained from UV-irradiated PFEESA in the presence of H₂O₂ and MNP (2-methyl-2-nitrosopropane) as the spin trap suggested that the side chain of Nafion is a possible site of attack by oxygen radicals.