| Popis: |
According to the US DOE’s multi-year Fuel Cell research, development, and demonstration plan, a 30,000 h lifetime, approximately four times that of the light-duty vehicles (LDV), is required for Heavy Duty Vehicle (HDV) applications [1]. Thus, highly accelerated stress tests are necessary in order to assess the viability to novel membrane concepts within a reasonable amount of time. Previously we have reported the development of a highly accelerated stress test (HAST) to generate local stressful conditions that are representative of those in automotive fuel cell stacks [2]. Using a 50-cm2 cell cycled between 0.05 and 1.2 A/cm2 with a low inlet RH in the co-flow configuration, the HAST creates a distribution of combined mechanical/chemical stressors in the membrane with the maximum chemical stress occurring near the gas inlets and the maximum mechanical stress near the outlets. Conducting HASTs using a current distribution measurement tool and a shorting/crossover diagnostic method to track the state of health of a state-of-the-art membrane containing both a mechanical support and a cerium-containing chemical stabilizing additive, the result shows that the membrane location with the most severe thinning coincides with that of the deepest membrane hydration cycling. Upon examination of the cerium redistribution patterns after the test, it was found that the severe humidity cycling generated by the HAST condition near the outlet region not only generated the highest membrane mechanical stress but also resulted in the strongest water flux, which may cause local depletion of the cerium added as chemical stabilizer. In this study, we have focused on determining the root cause of membrane failure in the region of hydration cycling. Two hypotheses were investigated. The first is that chemical degradation can be accelerated in the presence of mechanical stress. The chemical degradation leads to membrane thinning which, in turn, leads to fewer sulfonic acid sites in the membrane available for Ce cation exchange. Two synergistic pathways for membrane degradation by combined mechanical and chemical stressors have been summarized by Kusoglu and Weber [3]. Ex-situ hydrogen peroxide vapor cell tests conducted with a pressure bias across the membrane to create a tensile stress were conducted to see if mechanical stress has an impact on membrane chemical degradation rate [4]. The second hypothesis is that local depletion of the mobile Ce stabilizer in regions of high humidity gradients can lead to chemical degradation in those regions. We have shown previously that humidity gradients can cause cations to move with convective forces from wetter to drier regions within a perfluoro-sulfonic acid (PFSA) membrane [5]. To study this hypothesis, we conducted two types of tests. First, we repeated the HAST tests of the state-of-the-art membranes and stopped the tests prior to the onset on membrane thinning to see whether Ce depletion occurs before or after the onset of thinning. We also ran HAST tests on membranes which contained no Ce stabilizer to see if membrane failure still occurred in the region of strongest humidity cycles. In the absence of stabilizer, we expect chemical degradation to occur faster towards the cell inlets where, on average, the MEA is driest. We will show that the HAST test significantly accelerates membrane failure by increasing temperature and frequency of local deep humidity cycles without changing the primary failure mode or location of a state-of-the-art membrane. This work suggests that these HAST tests can be used to accelerate the development of membranes for HD fuel cell applications. DOE H2 Heavy Duty Truck Targets; https://www.hydrogen.energy.gov/pdfs/19006_hydrogen_class8_long_haul_truck_targets.pdf Y.-H. Lai et. al, J. Electrochem. Soc., 165 (6) (2018) F3100-F3103 Kusoglu and Weber, J. Phys. Chem. Lett., 6, 4547 (2015) Y.-H. Lai et. al, ECS Abstracts 2021; https://iopscience.iop.org/article/10.1149/MA2021-02381128mtgabs F.D. Coms et. al, ECS Abstracts 2021; https://iopscience.iop.org/article/10.1149/MA2021-02381130mtgabs |
