250085 Ex-Situ Tests Based Correlations to Predict Chemical and Mechanical Durability of Polymer Electrolyte Membranes
Ex-situ Tests Based Correlations to Predict Chemical and Mechanical Durability of Polymer Electrolyte Membranes
R. Yadav, G. DiLeo, N. Dale, and K. Adjemian
Zero Emission – Research
Nissan Technical Center North America, Farmington Hills, MI-48331
Polymer electrolyte membrane (PEM) fuel cells are a high-potential technology to allow for zero-emission transportation. The durability of the PEM is a vital aspect that must be understood for the successful commercialization of PEM fuel cell vehicles. The ability to forecast the chemical and mechanical durability or lifetime of a membrane material in a fuel cell environment would be a valuable screening or development tool. This work proposes a chemical stability factor (CSF) and expands on a previously reported humidity stability factor (HSF), based on ex-situ testing, to predict the in-situ lifetime of PEM materials.
Chemical degradation of PEMs occurs through free-radical attacks (OH., OOH.) on weak and vulnerable sites (-COOH, side group) in the PEM[1-3]. These free-radicals are generated from H2O2 that can be formed through a 2-electron O2 reduction process at multiple locations in the membrane electrode assembly [4-5]. Open Circuit conditions (high potential, low RH) are favorable for the formation of free-radicals leading to severe chemical degradation of PEM. Variables such as temperature, O2 concentration, water activity (relative humidity), membrane thickness, polymer type (PFSA, hydrocarbon), and processing techniques (reinforcement or chemical stabilizers) affect the chemical durability of a membrane [6-7]. In automotive applications, PEMs are also exposed to numerous cycles of wet and dry conditions in which swelling and shrinking, stress, creep, and fatigue of the membrane lead to membrane cracking and performance loss [8-9].
In-situ evaluation of PEM chemical durability and mechanical durability is lengthy in time even with accelerated stress tests (i.e. OCV hold test: up to 4 weeks and dry/wet cycling: up to 10 weeks). Correlations based on quicker ex-situ tests to predict chemical durability and mechanical durability can help save significant in-situ testing times and resources. This work is aimed at developing such correlations defined as a chemical stability factor (CSF) for chemical durability and a humidity stability factor (HSF) for mechanical durability of PEMs. The CSF is a correlation based on oxygen permeation through the membrane and the fluoride emission from exposing the membrane to a solution of Fenton reagents. We have observed that the relative trend of PEM chemical durability obtained from OCV hold tests matches with the relative trend predicted by CSF. The correlation for HSF is based on the tensile strength of the membrane along with its swelling in liquid water. This factor has been previously reported by MacKinnon et al.  and in this work is applied to a wide variety of membranes. The relative trend of PEM mechanical durability from in-situ testing follows the relative trend predicted by HSF.
This work will discuss the fundamentals of these ex-situ-test-based correlations to predict in-situ chemical durability and mechanical durability. Results from OCV hold test hours, CSF, dry/wet cycling test, and HSF will be presented to show the reliability of these correlations.
We thank the Composite Materials and Structure Centers, Michigan State University for measurement of oxygen permeation rate across PEMs.
1. N. E. Cipollini, “Chemical aspects of membrane degradation,” ECS Transactions, 11(1), 1071-1082 (2007).
2. F. D. Coms, “The chemistry of fuel cell membrane chemical degradation,“ ECS Transactions, 16(2), 235-255 (2008).
3. A. Kabasawa, J. Saito, K. Miyatake, H. Uchida, and M. Watanabe,“ Effects of the decomposition products of sulfonated polyamide and Nafion membranes on the degradation and recovery of electrode,” Electrochem. Acta, 54, 2754-2760 (2009).
4. A. Ohma, S. Suga, S. Yamamoto, and K. Shinohara, “Membrane degradation behavior during open-circuit voltage hold test,“ J. Electrochem. Soc., 154(8), B757-B760 (2007).
5. V. O. Mittal, H. R. Kunz, and J. M. Fenton,“ Membrane degradation mechanisms in PEMFCs,” J. Electrochem. Soc., 154(7), B652-B656 (2007).
6. C. Chen and T. F. Fuller,“ The effect of humidity on the degradation of Nafion membrane,” Polym. Degrad. Stab., 94, 1436-1447 (2009).
7. V. A. Sethuraman, J. W. Weidner, A. T. Haug, and L. V. Protsailo, “Durability of perfluorosulfonic acid and hydrocarbon membranes: effect of humidity and temperature,” 155(2), B119-B124 (2008).
8. S. Vengatesan, M. W. Fowler, X. Z. Yuan, and H. Wang, “Diagnosis of MEA degradation under accelerated relative humidity cycling,” 196, 5045-5052 (2011).
9. J. Kang and J. Kim, “Membrane electrode assembly degradation by dry/wet gas on a PEM fuel cell,” 35, 13125-13130, (2010).
10. SM MacKinnon, TJ Fuller, FD Coms, MR Schoeneweiss, CS Gittleman, Y.–H Lai, R Jiang, and AM Brenner, “Membranes: Design and Characterization,” Fuel Cells – Proton Exchange Membrane Fuel Cells, Elsevier, 741-754 (2009)