Membrane-Derived Fluorinated Radicals Detected by Electron Spin Resonance in UV-Irradiated Nafion and Dow Ionomers:  Effect of Counterions and H₂O₂

Abstract

Electron spin resonance (ESR) spectroscopy was used to detect and identify radicals formed by UV irradiation of Nafion and Dow perfluorinated membranes partially or fully neutralized by Cu(II), Fe(II), and Fe(III). This method allowed the monitoring of ESR signals from the paramagnetic counterions together with the appearance of membrane-derived radical species. The most surprising aspect of this study was the formation of membrane-derived radical species only in the neutralized membranes, and even in the absence of H2O2 in the case of Nafion/Cu(II) and Nafion/Fe(III). In Nafion/Cu(II), ESR spectra from radicals exhibiting hyperfine interactions with three equivalent 19F nuclei (the "quartet") and with four equivalent 19F nuclei (the "quintet") were detected. In Nafion/Fe(II) exposed to H2O2 solutions, the formation of Fe(III) was detected. Upon UV irradiation, strong signals from the chain-end radical ROCF2CF2* were detected first, followed by the appearance, upon annealing above 200 K, of the quartet signal observed in Nafion/Cu(II). In subsequent experiments with Nafion and Dow membranes neutralized by Fe(III), the ROCF2CF2* radicals were formed even in the absence of H2O2, indicating that the role of H2O2 is oxidation of Fe(II) to Fe(III); moreover, in these systems small amounts of the chain-end radicals were detected even without UV irradiation. This result validates the method used to form the radicals: the role of UV irradiation is to accelerate the formation of a signal that is produced, albeit slowly, even in the dark, and possibly during fuel cell operation. The major conclusion is that cations are involved in degradation processes; the point of attack appears to be at or near the pendant chain of the ionomer. Therefore when studying membrane stability, it is important to consider not only the formation of oxygen radicals, such as HO*, HOO*, and O2*-, that can attack the membrane but also the specific reactivity of counterions.

Degradation of a polymer exchange membrane fuel cell stack with Nafion® membranes of different thicknesses: Part I. In situ diagnosis

“…This can be explained by the high reactant concentration under idle conditions due to an extremely small operating current, resulting in a high reactant crossover, and then a high radical formation rate. The radicals produced are known to be the major cause for the chemical/electrochemical degradation of the membrane [30][31][32]. Fig.…”

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. Weak signals were detected even in the absence of H 2 O 2 if the membranes were neutralized with Fe͑III͒.…”

A Multiparadigm Modeling Investigation of Membrane Chemical Degradation in PEM Fuel Cells

“…[16][17][18][19] The most plausible origins of these ions are the degradation of iron containing end-plates which are used in the PEMFCs, and the oxidation of the pipes in the reactants management system. 20,21 Additionally, some debate still remains about the role of the precipitated Pt (arising from electrochemical dissolution mainly in the cathode CL) on catalyzing the H 2 O 2 decomposition.…”