Two synergistic pathways for membrane degradation by combined mechanical and chemical stressors have been summarized by Kusoglu and Weber [1]. In a commonly observed pathway, mechanical stressors increase chemical degradation through the increasing gas crossover by mechanically induced macroscopic formation and growth of defects. In another proposed synergistic pathway, mechanical stress can directly enhance the chemical stressor at the nanoscale without increased gas crossover through defect formation. Up to date, very little evidence for the latter pathway have been reported in the literature, notably in works by Yoon et al. [2] and Kusoglu et al. [3] in ex-situ tests subjecting membrane to tension and compression, respectively, in Fenton’s solution. More recently, Robert et al. [4] reported enhanced fluoride emission rates of Nafion XL and NR-211 membranes by a 5 MPa cyclic compressive stress in a serpentine cell with H2O2 or Fenton’s solution.In this study, we have conducted experiments with a goal to correlate the membrane chemical degradation with various degrees of tensile stresses typically found in operating fuel cells [5]. To chemically degrade a membrane while applying tensile stresses, we utilized an ex-situ hydrogen peroxide vapor test that has been proven robust and useful in the investigation of the effect of temperature, RH, iron contamination, chemical mitigants, and chemical end chain modification on membrane chemical degradation [6-8]. Our experimental setup included four cells that can be tested in parallel under the same conditions. Each test cell is composed of a polysulfone plate on one side and a Teflon plate on the other. Both plates have a square 50 cm2 serpentine flowfield composing 2-mm wide channels and 2.125-mm wide lands. The channel depth is 1 mm. Membrane was compressed between the two flowfield plates with matching land/channel patterns. Tensile stress of the membrane across the channel was induced by a differential pressure from the N2 stream of higher pressure on the polysulfone side toward the Teflon side where a humidified gas stream of N2 and 30 ppm H2O2 flowed. The test cells were placed in a convection oven to maintain constant temperature during the test. The N2/H2O2 exhaust stream then flew into a chiller where the condensed solution was collected for subsequent fluoride concentration measurement.Two types of membranes were investigated in this study. One is the commercially available 25-µm Nafion® NR-211 without a chemical stabilizing additive and the other, a 12-µm ePTFE-supported PFSA membrane doped with cerium as a chemical stabilizer [9]. Tests were conducted with various constant differential pressures ranging from 0 to 100 kPa under 90°C and 30% RH. It should be noted that the NR-211 membrane ruptured in tests at and above 150 kPa, suggesting that our tests have covered a wide range of tensile stress from zero up to the mechanical strength limit of Nafion NR-211. Figure 1 shows the fluoride emission rate (FER) measured from 8 to 94 hours for both membranes under various pressures. It is seen that the FER of N211 increases with time but the 12-µm ePTFE-supported PFSA membrane is about 10 times lower while remaining relatively constant through the test. The lower FERs in the 12-µm PFSA membrane reflects the effectiveness of cerium as a chemical stabilizer. Importantly, there is no observable correlation between FER and applied differential pressures. In fact, all FERs for each membrane type are within the test-to-test variation from our experience with the H2O2 vapor test. The result in this study is contrary to those by Yoon, Kusoglu, and Robert, suggesting that more work on this subject is still required in the future. References Kusoglu and Weber, J. Phys. Chem. Lett., 6, 4547 (2015)Yoon et al., ECS Trans., 33(1), 907 (2010).Kusoglu et al., ECS Electrochem. Lett., 3, F33 (2014).Robert et al., J. Power Sources, 476, 228662 (2020).Lai et al., ASME J. Fuel Cell Sci. Tech., 6 (2), 0210021 (2009).Coms et al., ECS Transactions, 50 (2) 907-918 (2012)Xu et al., 220th ECS Meeting, Boston, MA, Oct. 13, 2011Coms et al., ECS Transactions, 64 (3) 389-402 (2014) https://www.hydrogen.energy.gov/pdfs/review18/fc156_kumaraguru_2018_o.pdf Figure 1
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Sep 16, 2024
Gidado M J + 5
Open Access