Nafion Perfluorinated Membranes Treated in Fenton Media:  Radical Species Detected by ESR Spectroscopy

Abstract

ESR spectroscopy was used to detect and identify radicals in Nafion perfluorinated membranes exposed to the Fenton reagent based on Ti3+ (TiCl3 + H2O2). This method allowed monitoring the disappearance of the ESR signal from Ti3+ during reaction. The initially formed radical was HOO•, with magnetic parameters g 1 = 2.0261, g 2 = 2.0088, g 3 = 2.0031 (g iso = 2.0127), and a 1 = 5 G at 200 K. The disappearance of this radical above 220 K was accompanied by the appearance of a mixture of peroxy radicals TiOO• and superoxide radicals O2 -. The O2 - radicals were observed only in the dry Nafion after treatment and were stable up to 370 K; the g-tensor components are g 1 = 2.0190, g 2 = 2.0092, and g 3 = 2.0036 (g iso = 2.0106) at 300 K. In samples swollen by the hydrogen peroxide solution, signals from the TiOO• radicals predominated, with g-tensor components g 1 = 2.0228, g 2 = 2.0092, and g 3 = 2.0046 (g iso = 2.0122) at 200 K. A dynamical process lead to averaging of the g anisotropy of TiOO• radicals in the temperature range 220−320 K. The dynamics mechanism was identified with the “cubic jump” model: rotation of the O−O fragment about an axis equally inclined to the principal directions of the g tensor. A broad signal (line width ≈ 84 G, g = 2.0023) appeared in slightly dried samples after ≈14 days and increased in intensity after 92 days; this signal is thought to represent fluorinated alkyl radicals, formed by attack of oxygen radicals on the polymer chain. In the Fenton reagent (in the absence of Nafion) radicals are detected only for high concentrations of hydrogen peroxide, 30% w/v. In addition, the HOO• radicals formed are less stable, and the peroxy radicals TiOO• are the only species detected at and above 200 K. The presence of the Nafion membranes has important effects on both the type of radicals formed and their thermal stability.

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

“…[15]. The reactivity of a covalent bond to hydrogen with respect to abstraction by hydroxyl radical can be readily inferred from the strength of the bond to hydrogen as shown by the strong correlation between experimental bond strengths and rates of abstraction given in Figure 12 [9,11,12], again because of the much lower 14 N hfs compared to Á CH 2 COOH, 14.8 versus 16.0 G. It is reasonable to assume that the additional adduct in CF 2 HCOOH and in PFEESA is derived from Á OH reaction at the acid end group or corresponding anion. A recent kinetic study of the reaction of Á OH radicals with a homologous series of perfluorinated acids of the general formula F(CF 2 ) n COOH with n = 1-4 in the gas phase at 296 K has provided evidence for the activation of acid groups to Á OH attack by CF 2 or CF 3 groups [82].…”

“…Direct electron spin resonance (ESR) and spin trapping methods have been used recently by our group to detect and identify oxygen radicals as well as membrane-derived radical intermediates in Nafion upon exposure to oxygen radicals [14][15][16]. In these experiments, oxygen radicals were produced by two main methods, the Fenton [17] and the photo-Fenton reactions [18].…”

“…In these experiments, oxygen radicals were produced by two main methods, the Fenton [17] and the photo-Fenton reactions [18]. These reactions involve transition metal cations such as Fe(II), Fe(III) and Ti(III) that are known to produce hydroxyl ( Á OH), superoxide (O ÀÁ 2 ) and hydroperoxyl ( Á OOH) radicals [14].…”

Modeling and Simulation of the Degradation of Perfluorinated Ion-Exchange Membranes in PEM Fuel Cells

“…72,73 They detected the presence of the chain-end radicals R p − OCF 2 CF 2 ·. 72 As the irradiation time increased, the ESR signal intensity corresponding to these radicals increased, while the Fe͑III͒ intensity decreased.…”

Investigation of polymer electrolyte membrane chemical degradation and degradation mitigation using in situ fluorescence spectroscopy

“…However, detecting the presence of ROS within the PEM of an operating fuel cell is an extremely difficult proposition because free radical ROS (e.g., hydroxyl and hydroperoxyl radicals) have half-lives on the order of 10 −9 s (17). A survey of the literature suggests that ROS-induced PEM degradation has largely been studied through indirect methods, where the PEM degradation products were assayed from the effluent water emanating from the fuel cell (18,19) or the PEMs themselves were examined in postmortem studies (14,20). These ex situ approaches often required extensive sample preparation before analytical measurement, and did not yield real-time data.…”