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Using ESR spectroscopy to study radical intermediates in proton-exchange membranes exposed to oxygen radicals

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Direct ESR and spin-trapping experiments were used to study the behavior of Nafion, a perfluorinated ionomer membrane used in fuel cells, when exposed in the laboratory to oxygen radicals produced by Fenton and photo-Fenton reactions. DMPO (5,5-dimethyl-1-pyroline) was used as the spin trap. The results suggest that the two ESR methods provide complementary information on Nafion fragmentation. The presence of membrane-derived fragments was suggested indirectly by the presence of a broad signal (line width ≈ 84 G) after prolonged exposure of the membrane to the Fenton reagent based on Ti(III), and by the DMPO adduct of a carbon-centered radical in the spin-trapping experiments. The most convincing proof for the presence of perfluorinated radicals was obtained in Nafion membranes partially neutralized by Cu(II), Fe(II) and Fe(III) upon exposure to UV-irradiation in the presence or absence of H2O2 (photo-Fenton treatment). Identification of the chain-end radical RCF2CF2 with magnetic parameters different to those determined for the chainend detected in γ-irradiated Teflon, was taken as evidence for the attack of reactive oxygen radicals on the side-chain of the membrane. Additional support for this suggestion was the detection of the "quartet" ESR signal assigned to the CF3CO radical, and of the "quintet" ESR signal assigned to the radical centered at the intersection of the main and side chains. The limitations and advantages of each approach are discussed.

Affiliations: 1: Department of Chemistry and Biochemistry, University of Detroit Mercy, 4001 West McNichols, Detroit, MI 48221, USA


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