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(d[F⁻]/d[decomposed 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.     

Chemical degradation of proton conducting perflurosulfonic acid ionomer membranes studied by solid-state nuclear magnetic resonance spectroscopy

The degradation of two different types of perfluorinated polymer membranes, Nafion and Hyflon Ion, has been examined by solid-state ¹⁹F and ¹³C NMR spectroscopy. This spectroscopic technique is demonstrated to be a valuable tool for the study of the membrane structure and its alterations after in situ degradation in a fuel cell. The structural changes in different parts of the polymers are clearly distinguished, which provides unique insight into details of the degradation processes. The experimental NMR spectra prove that degradation mostly takes place within the polymer side chains, as reflected by the intensity losses of NMR signals associated with SO₃H, CF₃, OCF₂ and CF groups. The integral degree of degradation is found to decrease with increasing membrane thickness while for a given thickness, Hyflon Ion appears to degrade less than Nafion.     

Ex situ investigation of the proton exchange membrane chemical decomposition

The durability of Nafion^® polymer electrolyte membranes (PEMs) with potential application in PEM fuel cells has been investigated using accelerated durability tests to understand their degradation mechanism. After the attack by Fenton radicals, the Nafion^®111 membranes and the solution produced were collected for analysis. The existence of F ions in the solution indicated the chemical decomposition of the Nafion^® membranes during radical attacks. The F^- emission rate (FER) was about , corresponding to 0.024 wt% of F released from the membrane per hour. The NMR and FTIR spectrums demonstrated the polymer fragments mostly existed as whole side chains of the Nafion^® membrane. This result revealed that the degradation was originated from the decomposition of polymer main chain. Furthermore, the reflectance-FTIR revealed that the degradation of the PEMs was from the decomposition of the repeating units in the polymer main chains. With the increased loss of repeating units, small bubbles with the diameter of several microns started to form in Nafion^® membrane. These bubbles made the membrane vulnerable to hazards of gas crossover, which further led a catastrophic failure of the proton exchange membrane.