原位可视精确跟踪燃料电池电介质膜机械衰减的变化
Tracking the evolution of mechanical degradation in fuel cell membranes using 4D in situ visualization

Yadvinder Singh
Robin T．White
Marina Najm
Tylynn Haddow
Vivian Pan
Francesco P．Orfino
Monica Dutta
Erik Kjeang

Abstract

Mechanical degradation occurs in fuel cell membranes due to the dynamic environmental conditions of operational duty cycles， and is regarded as a critical determinant of fuel cell durability and lifetime． Imaging－based failure analysis is typically employed to characterize structural and morphological aspects of the degradation， and 3D visualization capability of X－ray computed tomography is effectively expanding the scope of this analysis． This work further leverages the additional non－destructive and non－invasive attributes of this visualization technique to capture 4D information pertaining to the evolution of mechanical degradation in fuel cell membranes． A custom fuel cell fixture is utilized to periodically track identical membrane locations during the course of its mechanical degradation， which is generated through an accelerated stress test． The predominant fatigue－driven membrane crack development process is found to proceed non－linearly in time and is spatially concentrated under the uncompressed channel regions． Membrane cracking location is shown to be strongly correlated with beginning－of－life MEA defects， namely， electrode cracks and delamination． In situ crack propagation rates are quantified and the presence of a ‘crack closure’ effect during mechanical membrane degradation is demonstrated． Unlike crack initiation， crack propagation in the membranes does not appear to be significantly influenced by electrode morphology．

这种研究难度很高。4D其中的1D指的是时间。

实验设备

成像的定义与图像处理

The applied RH cycle consisted of a 2 min wet phase （supersaturated
inlet gas at nearly 150％ RH） followed by a 2 min dry phase （dry inlet
gas at nearly 0％ RH）．

循环工况为干湿循环。

2000次循环后生成的膜与电极的缺陷，非线性增加。

图2中缺陷的定位，主要集中在流道区域，大多平行于流道。

缺陷随着时间的演变，从阴极的裂缝到膜到阳极的裂缝。

2000次循环后的缺陷统计

随着循环次数的增加，裂缝总长度增加，裂缝宽度轻微下降，而且还存在裂缝的消失现象。

2200次循环的10条裂缝在2000－2200次循环之间的长度增长速度，10条裂缝的延伸角度的统计。不得不佩服这一点做的很细致。

以为文章结束了，出现了一个实验CT中X射线对膜力学强度的影响。

Conclusions

X－ray computed tomography was utilized to develop a 4D in situ
visualization approach for studying the structural evolution of mechanical degradation in fuel cell membranes． This unique methodology
involves the use of a custom－designed X－ray transparent fixture，
which houses a small－scale single fuel cell and is compatible with
imaging on a laboratory－based X－ray computed tomography system．
Mechanical membrane degradation was applied on this cell using an
accelerated stress test protocol featuring controlled cycling of humidity under chemically inert conditions， and thereby generating fatigue－inducing cyclic mechanical stresses within the membrane． Three－dimensional images of identical locations within the MEA were captured at periodic intervals during the mechanical degradation process， which enabled detailed tracking of the structural and morphological features of the degradation mechanism and associated failure modes． Through thickness crack development was identified as the predominant damage feature within the membrane during its entire degradation process， leading to ultimate fuel cell failure via convective crossover leakage of reactant gases． This fatigue－driven crack development was found to proceed non－linearly in time with the degradation process， i．e．， both initiation and propagation accelerated dramatically during the later stages of degradation． The spatial distribution of crack development was also non－uniform and preferentially concentrated under the uncompressed channel regions of the MEA， which is attributed to locally elevated levels of tensile stress． The unique methodology of identical location tracking enabled root cause analysis for membrane cracks， which is demonstrated for the first time in this work． This approach revealed MEA defects present at the beginning of life as the original sites of crack formation． Specifically， electrode cracks and membrane— electrode delamination were found to be the main MEA fabrication defects capable of promoting localized membrane cracking in their vicinity， and their curtailment is accordingly suggested as a potential mitigation strategy against mechanical membrane degradation． Moreover， in situ crack propagation rates within the membrane plane were measured for the first time， with crack propagation found to proceed faster in the longer cracks． Interestingly， the membrane crack propagation was also accompanied by closure of certain adjoining cracks， which is possibly due to the accompanied stress redistributions and／or unrecovered plastic deformations during the drying phase of humidity cycling． Observations made during this work further suggest that while the electrode cracks play a prominent role in the initiation of membrane cracks through the stress concentration effect， the propagation of membrane cracks occurs fairly independently of electrode morphology．

The work presented herein leverages the non－destructive and noninvasive features of X－ray computed tomography to effectively capture
key characteristics relevant to the evolution of mechanical degradation
in fuel cell membranes in four dimensions， which is above and beyond
the three－dimensional nature of the XCT characterization technique
itself that already imparts an enhanced level of reliability and comprehensiveness to post－mortem failure analysis studies． These unique
capabilities demonstrate the effectiveness of XCT in uncovering important fundamental details of the membrane degradation processes，
and thereby offering valuable information for the development of mitigation strategies and durability improvement． In the opinion of the
authors， fuel cell durability research can benefit considerably by further employing this useful technique and the methodologies developed during this work to the study of other relevant evolutionary processes that are at play during fuel cell operation．