Re-thinking Fragmentation of Membranes in PEM Fuel Cells: Ex Situ vs In Situ Experiments, and Hydroxyl Radicals vs Hydrogen Atoms as Major Aggressors

Schlick, Shulamith, Marek Danilczuk, and Mariana Spulber

Attack of proton exchange membranes used in fuel cells (PEMFC) by radicals generated in the laboratory or during FC operation has been studied by numerous methods. These studies have attempted to identify the aggressive radicals as well as the attack sites in Nafion and other membranes, some of them perfluorinated. We have used model compounds and fluorinated membranes exposed to hydroxyl radicals, HO•, generated by UV-irradiation of hydrogen peroxide, and deduced that the polymer main chain as well as the side chain can be attacked by these aggressive HO• radicals; the resulting fragments were identified by spin trapping electron spin resonance (ESR).1,2

In situ experiments in a fuel cell (FC) inserted in the resonator of the ESR spectrometer offered the ability to observe separately processes at anode and cathode sides and to detect and identify the formation of HO• and HOO• radicals, H• and D• atoms, and radical fragments derived from the Nafion membrane.3,4 This study has demonstrated that in situ FC operation involves processes such as gas crossover, reactions at the catalyst surface, and possible attack of the membrane by reactive H• or D• that do not occur in ex situ experiments in the laboratory, thus implying different mechanistic pathways in the two types of experiments. Additional proof for the importance of membrane attack by hydrogen atoms was provided by our in-depth profiling by micro FTIR of cross-sections for Nafion 115 membranes in membrane-electrode-assemblies (MEAs) degraded during 52 h or 180 h at open circuit voltage (OCV) conditions.5 Corresponding 2D FTIR spectral-spatial maps indicated that C-H and C=O groups are generated during degradation. The highest band intensities for both groups appeared at a depth of 82 mm from the cathode in the MEA degraded for 180 h; the same bands were present but less intense at a depth of 22 mm from the cathode.

Degradation at these depths is most likely associated with the Pt band formed from the Pt catalyst dissolution and migration into the membrane, as described in the literature.6 The two degradation bands, C=O and C-H, appeared at the same depths from the cathode, 82 and 22 mm, suggesting that they are generated by a common mechanism or intermediate. This result was rationalized by a very important first reaction: Abstraction of a fluorine atom from the polymer main chain and side chain by hydrogen atoms, H•. This step is expected to cause main chain and side chain scission, and to generate RF–CF2• radicals that can react further with H2O2, H2O, and H2 to produce both –COOH and RCF2H groups.5

References

1.         Danilczuk, M.; Coms, F.D.; Schlick, S. Fuel Cells 2008, 8(6), 436-452.

2.         Spulber, M.; Schlick, S. J. Phys. Chem. B 2011, 115, 12415-12421.

3.         Danilczuk, M.; Coms, F.D.; Schlick, S. J. Phys. Chem. B 2009, 113, 8031-8042.

4.         Danilczuk, M.; Perkowski, A.J.; Schlick, S. Macromolecules 2010, 43, 3352-3358.

5.         Danilczuk, M.; Lancucki, L.; Schlick, S.; Hamrock, S.J.; Haugen, G.M. ACS Macro Letters 2012, 1, 280-285.

6.        (a) Péron, J.; Nedellec, Y.; Jones, D. J.; Roziére, J. J. Power Sources 2008, 185, 1209-217. (b) Inaba, M. ECS Transactions 2009, 25, 573-581. (c) Haugen, G.; Barta, S.; Emery, M.; Hamrock, S.; Yandrasits, M. In Fuel Cell Chemistry and Operation; A.M Herring, T.A. Zawodzinski, Jr., and S.J. Hamrock, Eds.; American Chemical Society: 2010; pp 137-151.