Kinetic study of Nafion degradation by Fenton reaction

To predict the durability of polymer electrolyte membranes in fuel cells, the degradation reactions of Nafion 117 films were studied as oxidation reactions with hydroxyl radicals as oxidation accelerators. The radical species were generated by the Fenton reaction between hydrogen peroxide (H₂O₂) and iron ions (Fe²⁺). The Nafion degradation kinetics were estimated by fluorine ion (F⁻) generation. The H₂O₂ and Nafion degradation reactions fit a pseudo-first-order rate constant. The values of the activation energy and frequency factor are 85 kJ mol⁻¹ and 3.97 × 10⁸ s⁻¹ for H₂O₂ decomposition in the presence of a Nafion film and 97 kJ mol⁻¹ and 9.88 × 10⁸ s⁻¹ for F⁻ generation. The Nafion surface morphology became rough after reaction for 12 h; small cracks, approximately 100 μm in length, were observed at temperatures below 60 °C. These cracks connected to make larger gaps of approximately 1 mm at temperatures above 70 °C. We also found a linear relationship between H₂O₂ consumption and F⁻ generation. The rate constant is temperature dependent and expressed as ln(dF⁻]/ddecomposed H₂O₂]) = −19.5 × 10³ K⁻¹ + 42.8. F⁻ generated and H₂O₂ consumed along with the Nafion degradation conditions can be predicted using this relation.