Fragmentation of Fluorinated Model Compounds Exposed to Oxygen Radicals: Spin Trapping ESR Experiments, and Implications for the Behavior of Proton Exchange Membranes Used in Fuel Cells

Danilcuzk, Marek, Andrew Perkowski, and Shulamith Schlick

Our group is involved in the study of fragmentation of polymeric membranes such as Nafion, the perfluorinated polymer that is an essential component of a fuel cell. Currently, the lifetimes of these membranes are below the requirements of the automotive industry. Our studies provide the information necessary that can lead to improved membrane stability.

Low-molecular-weight model compounds (MCs) for Nafion membranes used in fuel cells were exposed at 300 K to OH radicals produced by UV irradiation of aqueous H2O2 solutions. The MCs contained fluorinated and partially fluorinated groups terminated by sulfonic or carboxylic acid groups. The fragmentation process in the model compounds was studied by spin trapping ESR methods, using 5,5-dimethylpyrroline-N-oxide (DMPO) and 2-methyl-2-nitrosopropane (MNP) as the spin traps. The objective of these experiments was to assess the effect of the type of ionic groups (sulfonic or carboxylic) and of fluorine substitution on the spin adducts detected. DMPO experiments led to the detection of spin adducts of OH and of carbon-centered radicals (CCRs), and allowed the determination of the OH attack site on the ionic and/or on the protiated or fluorinated groups. Experiments with MNP as the spin trap were the most informative in terms of structural details for adducts obtained from each MC. The results allowed the identification of CCRs present as adducts, based on large hyperfine splittings from, and the number of, interacting 19F nuclei; in addition, oxygen-centered radicals (OCRs) as MNP adducts were also identified. To confirm the above assignments, control spin trapping experiments with CF3I and CF3CF2I were performed. After exposure to UV, these compounds generated CF3 and CF3CF2 radicals, which were trapped with DMPO or MNP spin traps. Taken together, the results deduced by spin trapping suggest that both sulfonic acid and acetic acid groups can be attacked by OH radicals, and confirm two possible degradation mechanisms in Nafion membranes: initiated at the backbone, and at the side chain.

Sites of attack by OH radicals in the model compounds, as suggested by experiments with DMPO and MNP as the spin traps