重载燃料电池膜加速耐久AMDT工况:工况中的渗氢、开路电压、极化性能、膜厚度、释放氟含量、膜形貌、铂带位置、膜力学特性的变化规律
Accelerated Membrane Durability Testing ofHeavy Duty Fuel Cells
Natalia Macauley
Alireza Sadeghi Alavijeh
Mark Watson
Joanna Kolodziej
Michael Lauritzen
Shanna Knights
Gary Wang
Erik Kjeang
Abstract
Regular durability testing of heavy dutyfuel cell systems for transit bus application requires several thousand hoursof operation, which is costly and time consuming. Alternatively, accelerateddurability tests are able to generate failure modes observed in field operationin a compressed time period, by applying enhanced levels of stress. Theobjective of the present work is to design and validate an accelerated membranedurability test (AMDT) for heavy duty fuel cells under bus related conditions. The proposed AMDT generates bus relevantmembrane failure modes in a few hundred hours, which is more than an order ofmagnitude faster than for regular duty cycle testing. Elevated voltage, temperature, and oxidant levels are used toaccelerate membrane chemical stress, while relative humidity (RH) cycling isused to induce mechanical stress. RH cycling is found to significantlyreduce membrane life-time compared to constant RH conditions. The role of aplatinum band in the membrane is investigated and membranes with Pt bandsdemonstrate a considerable life-time extension under AMDT conditions, withminimal membrane degradation. Overall, this research serves to establish abenchmark AMDT that can rapidly and reliably evaluate membrane stability undersimulated heavy duty fuel cell conditions.
The development of the AMDT for heavy dutyfuel cell vehicles is based on a thorough characterization of the Whistler HD6 dutycycle for key stressors, their levels, and occurrence.
endured temperatures between −12 ◦ C to 27◦ C
Table I. Summary of the AMDT runs and obtained life-times.
Figure 1. Hydrogen leak rate development during AMDT operation, showing slower leak growth at constant RH (green) and with PITM (red), compared to baseline (blue full). The initiation test (blue dashed) was stopped after leak initiation, in order to investigate the early stage of degradation.
Figure 2. Voltage fluctuations and voltage decay of the various AMDT runs: a) initiation; b) baseline; c) 90% RH; d) 100% RH; e) PITM-1; and f) PITM-2.
Figure 3. Open circuit cell voltage decay during AMDT operation, which is primarily a consequence of hydrogen leaks across the membranes.
Figure 4. Polarization curve decay of the various AMDT runs: a) initiation; b) baseline; c) 90% RH; d) 100% RH; e) PITM-1; and f) PITM-2.
Middle of Life (MOL)
Figure 5. Membrane thinning (thickness loss, in % of original thickness) measured by SEM at the end of life of the various AMDT runs.
Figure 6. Cumulative fluoride release during AMDT operation, obtained from conductivity measurements on the effluent water from the stacks.
Figure 7. Representative SEM images of the membrane damage induced by AMDT operation: a) baseline; b) initiation; c) constant RH; and d) PITM AMDT runs.
Figure 8. Pt concentration at the Pt band location in the membrane from inlet to outlet (at end of life) for the two AMDT runs with PITM.
Figure 9. Tensile stress–strain curves of BOL and AMDT degraded catalyst coated membranes at (a) room conditions and (b) fuel cell conditions. The origin of the curves is shifted from zero strain to higher values for clarity.
b的测量难度很大。
Figure 10. Tensile properties of BOL and AMDT degraded catalyst coated membranes at room(23 ◦ C,50%RH) and fuel cell (70 ◦ C,90%RH) conditions: (a) final strain; (b) elastic modulus; and (c) UTS.
ultimate tensile strength (UTS)
Conclusions
A baseline accelerated membrane durability test (AMDT) protocol was established for heavy duty fuel cell applications along with complementary experimental investigations to elucidate the respective roles of chemical and mechanical stressors in the overall membrane degradation mechanism and their impact on membrane life-time. The proposed baseline AMDT successfully accelerated membrane degradation using combined chemical-mechanical stress and significantly
reduced the time to failure compared to regular duty cycle operation, achieving membrane failure in less than 300 hours. The failure modes obtained with the AMDT were similar to those observed during field operation, comprising holes and cracks in the membrane accompanied by local thinning, which demonstrates the effectiveness of the test protocol. It was found that under baseline AMDT conditions, the majority of the chemical membrane degradation occurred during the leak initiation period, followed by a rapid growth of the leak rate caused primarily by mechanical degradation.
The effects of RH cycling and Pt in the membrane were evaluated in complementary AMDT runs. The effect of RH cycling was visible when the tests at constant RH lasted significantly longer than the baseline, confirming that RH cycling indeed accelerates membrane degradation. RH cycling was found to gradually decrease the strength of the membrane due to the imposed mechanical stress, which was also found to exacerbate the effect of chemical degradation. The AMDTs at constant RH exhibited slower leak growth than the baseline, allowing for longer operation of membranes with leaks. The membrane life-time at 90% RH was shorter than at 100% RH, proving that the level of chemical membrane degradation increases with reduced humidity. The largest hole sizes and densities were found in the RH cycled samples, which also exhibited rougher damage structure indicative of mechanical degradation.
Platinum band formation extended the membrane life-time and prevented decay in thickness and elongation which suggests mitigation of chemical degradation. As a result the AMDTs with PITM displayed up to a doubling of the effective life-time. The test with high PITM concentration resulted in the longest life-time, while the test with low PITM concentration was found to have areas with gaps in the Pt band that were prone to local chemical degradation and earlier failures. Tensile tests on AMDT degraded samples determined the deterioration of CCM toughness in samples subjected to chemical degradation, while in PITM samples where chemical degradation was controlled, the mechanical strength of the membrane was preserved.
From these results, it can be concluded that PITM and constant RH operation result in enhanced membrane life-time under heavy duty fuel cell conditions. The baseline AMDT protocol developed and demonstrated in this work is recommended for rapid and reliable testing of membrane durability for heavy duty fuel cell applications.
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