Danilczuk, Marek, Lukasz Lancucki, Shulamith Schlick, Steven J. Hamrock, and Gregory M. Haugen
Comparing Ex Situ and In Situ Membrane Degradation Processes in Fuel Cells: Deductions from Spin-Trapping ESR and 2D Spectral-Spatial FTIR
Marek Danilczuk,* Lukasz Lancucki,* Shulamith Schlick,* Steven J. Hamrock,# and Gregory M. Haugen#
*Department of Chemistry and Biochemistry, University of Detroit Mercy, 4001 W. McNichols Road, Detroit, MI 48221
#3M Fuel Cell Components Group, 3M Center, St. Paul, Minnesota 55144
Chemical degradation of proton exchange membranes used in fuel cells (PEMFC) has been studied by numerous experimental methods. These studies have attempted to identify the aggressive species as well as the attack sites in Nafion and other membranes, many of them perfluorinated. We have used model compounds and fluorinated membranes exposed to hydroxyl radicals, HO•, 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 Experiments with a fuel cell (FC) inserted in the resonator of the ESR spectrometer offer the ability to observe separately processes at anode and detection of 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 electrodes surface, and possible attack of the membrane by reactive H• or D• that do not occur in ex situ experiments, 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 formed 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 two degradation bands, C=O and C-H, appeared at the same depths from the cathode, 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. In conclusion: Hydroxyl radicals, HO•, are major aggressors that attack the PEMs in ex situ experiments. In an operating FC, however, hydrogen atoms can abstract a fluorine atom from the main chain and side chain of PEM, and initiate main chain and side chain scission, as well as further fragmentation.5
1. Roduner, E.; Schlick, S. In Advanced ESR Methods in Polymer Research, S. Schlick, Ed.; Wiley: Hoboken, NJ, 2006; Chapter 8, pp 197-228.
2. Danilczuk, M.; Coms, F.D.; Schlick, S. Fuel Cells 2008, 8(6), 436-452.
3. Danilczuk, M.; Coms, F.D.; Schlick, S. J. Phys. Chem. B 2009, 113, 8031-8042.
4. Danilczuk, M.; Schlick, S.; Coms, F.D. Macromolecules 2009, 42, 8943-8949.
5. Danilczuk, M.; Lancucki, L.; Schlick, S.; Hamrock, S.J.; Haugen, G.M. ACS Macro Letters 2012, 1, 280-285.