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．