Applications of some EPR methods to the investigation of the radical species produced by the reactions of hydroxyl radicals with PEFC-related fluorinated organic acids

Abstract

Fundamental information on the reactions of ˙OH radicals with perfluoroalkyl sulfonic acids and carboxylic acids is important for understanding the degradation of polymer electrolyte fuel cells (PEFCs). In the present research, the intermediate radicals produced by these reactions were detected and analyzed by means of three methods of electron paramagnetic resonance (EPR) spectroscopy. The conventional CW-EPR technique was applied to both frozen and flowing aqueous solution systems for detecting the reaction intermediates, while the time-resolved (TR) EPR technique was applied to the flowing solution system for analyzing spin dynamics parameters. The reactants tested were CF₃SO₃H, CHF₂CF₂SO₃H, CH₃SO₃H, CF₃COOH, CHF₂COOH, etc., and the ˙OH radical was generated from H₂O₂ by the irradiation of a UV laser. The radicals detected were ˙SO₃⁻, ˙CO₂⁻, ˙CF₃, ˙CF₂CF₂SO₃H, ˙CF₂COOH, etc. Based on the measurements of TR-EPR spectra, the dependences of the signal intensity on the time and magnetic field were analyzed, and then the longitudinal relaxation time (T₁) and the lifetime of these radical species were evaluated. The three EPR methods for detecting the intermediate radicals were compared to show the limitations of these techniques. Based on the detected radicals, the degradation mechanism reported for perfluoro acids was discussed.