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In dynamic automotive operation, the fuel cell membrane is subjected to various chemical and mechanical stresses [1,2] that cause degradation. Lab scale membrane durability study typically uses accelerated stress testing (AST) [3], which simulates the stresses experienced by the membrane during dynamic automotive operation, but at elevated stress level to generate representative degradation modes in a shorter timeframe. Membrane visualization is important in degradation studies to identify the root cause of failure. Recently, four-dimensional (4D) in-situ visualization by X-ray computed tomography (XCT) [4–8] has facilitated more insight through non-invasive 3D imaging of the MEA. Previous 4D in-situ visualization studies on small scale MEAs have successfully tracked the membrane degradation process under pure chemical [6], pure mechanical [4], and combined chemo-mechanical ASTs [7]. However, the results of such ASTs are sensitive to a variety of parameters related to fuel cell design and operating conditions. For instance, when combined chemo-mechanical membrane stresses were imposed on a small scale MEA [7], the major failure mode observed through 4D in-situ XCT visualization was wide membrane cracks, mainly driven by mechanical stresses, but membrane thinning [9,10], which indicates chemical degradation, was not clearly observed. This failure mode was comparable to field tested or OCV RH cycled cells [11], where membrane cracks appeared without major membrane thinning, but differed substantially from the original AST findings under combined chemical and mechanical stresses where major membrane thinning and fluoride release was observed [9,10]. Therefore, the reasons behind such differences in membrane failure mode warrant further investigation. The objective of this work is to improve the understanding of the effect of various operating conditions on the combined chemo-mechanical membrane degradation mechanism and associated membrane durability in polymer electrolyte fuel cells. Small-scale fuel cells were subjected to a variable AST with alternating chemical and mechanical stress cycles and 4D in-situ XCT visualization [8]. Firstly, the root cause of mechanical stress dominating chemical stress in the previous work [7] was identified as RH being higher than the set point during the chemical phase due to heat loss, which reduced chemical stresses. Consequently, RH was selected as the target variable in the chemical phase to understand its impact on membrane degradation. Subsequent design mitigations were also made on the test hardware so that the cell temperature could be robustly controlled at elevated temperature to support accurate RH control. Meanwhile, the effects of gas flow rate and wet/dry phase duration during the mechanical RH cycling phase were also studied with the assistance of single frequency electrochemical impedance spectroscopy (EIS), which was used to continuously measure high frequency cell resistance (HFR) during RH cycling. Larger HFR swings between wet and dry phases were interpreted to represent larger amplitude of mechanical stress. It was found that reducing the cell RH during the chemical phase and maximizing the HFR swing during the mechanical phase can considerably affect the membrane failure mode and significantly reduce the test lifetime (8 cycles versus 32 cycles) compared to the previous study [7], as indicated in the attached figure. Analysis of selected planar and cross-sectional XCT images indicates that both membrane thinning and cracking were within the field of view investigated at EOL; therefore, the modified AST protocol was more efficient and chemo-mechanically balanced. Again comparing to the published results from Mukundan et al. [11], membrane failure mode in the present work after elevating chemical and mechanical stresses demonstrated combined degradation modes of both pure OCV and pure RH cycling ASTs, where membrane thinning and cracking appeared simultaneously. This result was also more consistent with COCV ASTs done by Lim et al. [9] and Sadeghi et al. [10] using larger scale technical cells. With reduced RH in chemical phase, membrane thinning became more significant. Although the membrane cracks were narrower and fewer in quantity compared to the previous work, they were formed much earlier. Future testing using this more robust and efficient chemo-mechanical degradation AST protocol on selected reinforced membranes is planned. Keywords: fuel cell; membrane durability; accelerated stress test; mechanical degradation; chemical degradation; X-ray computed tomography Acknowledgements: This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, Ballard Power Systems, and W.L. Gore & Associates. This research was undertaken, in part, thanks to funding from the Canada Research Chairs program. Figure 1 |
