缓解干湿循环的机械因素造成燃料电池电极质膜失效的五种方法（其二）

Mitigation of Mechanical Membrane Degradation in Fuel Cells by Controlling Electrode Morphology： A 4D In Situ Structural Characterization

Yadvinder Singh

Robin T． White

Marina Najm

Alex Boswell

Francesco P． Orfino

Monica Dutta

Erik Kjeang

Abstract

Mechanical degradation is a critical mechanism responsible for the operational failure of fuel cell membranes． In addition to the membrane＇s intrinsic durability， component interactions play a crucial role in this degradation process． This work investigates the interaction and associated impact of electrode morphology on membrane failure under pure mechanical degradation conditions by utilizing 4D in situ visualization by X－ray computed tomography． Using periodic identical－location imaging， membrane damage progression is monitored and compared for electrodes with high and low initial crack density． Membrane fracture is found to be significantly curtailed through minimization of ab initio crack density in the cathode catalyst layer． Hydration－dehydration cycles， however， still introduce early electrode cracking which， as an intermediate step， exclusively governs the subsequent initiation and propagation of membrane cracks． Two distinct membrane failure mechanisms are identified that are characterized by： （i） permanent buckling deformation of the catalyst coated membrane； and （ii） direct membrane fracture from electrode cracks without buckling． The buckling phenomenon is found to be strongly influenced by the microstructure of the gas diffusion media and has a dominant contribution towards the overall frequency and scale of membrane fracture． Additionally， the effect of hydration on the in situ size and geometry of fracture features is demonstrated．

哪五种方法？我先卖个关子，这篇文章主要介绍第二种方法。

Figure 1． Identical location views of the membrane plane showing the distribution of membrane cracks at various stages of the RH cycling AST for MEAs fabricated with： （a） high； and （b） low crack density cathode catalyst layers， respectively． The planar views are taken near the middle of the membrane and digitally segmented， for visual clarity， into membrane （cyan） and void （black） phases． Locations under the gas flow channel and land regions are also indicated． Views of MEA cross－sections demonstrating the through－thickness penetration of selected membrane cracks are provided in the inset． Comparison of membrane crack coverage area between the high and low cathode crack density MEAs after 2000 AST cycles is shown in （c）． The values represent ratio of pixels occupied by void phase of the corresponding segmented images of membrane plane （ACL ＝ anode catalyst layer； M ＝ membrane； CCL ＝ cathode catalyst layer； GDL ＝gas diffusion layer．

而在四类催化层缺陷对燃料电池局部膜降解失效的影响中，

COCV和AMDT工况中，阴极裂缝和阳极裂缝对膜电极中膜的降解影响不大。说明工况不同，缺陷对于膜的降解影响可能会是不同的。

Figure 2． Planar views of the cathode catalyst layer： （a） identical locations of the low crack density cathode CL at progressive intervals during the RH cycling AST； and （b） high crack density cathode CL at beginning－of－life （BOL） and after 2000 RH cycles． Locations under the gas flow channel and land regions are also indicated． Evolution of cathode crack area fraction in the two examined MEA designs as a function of RH cycling is shown in （c）．

这张散点图需要注意纵轴的坐标是阴极裂缝面密度，和第1幅图不同。

Figure 3． Cross－sectional greyscale views of identical MEA locations inside the low cathode crack density specimen at various stages of RH cycling AST， showing the representative membrane failure mechanisms characterized by： （a） presence； and （b） absence of CCM buckling， respectively． Corresponding views from an additional dataset acquired after 2250 cycles under “wet” conditions are also shown．

在低电极裂缝密度条件下随着干湿循环两种不同的失效现象。

Figure 4． （a） Digitally segmented planar membrane view in the low cathode crack density MEA identifying the 30 largest （by means of coverage area） throughthickness membrane cracks in terms of their association with buckling （B） and non－buckling （N） failure mechanisms， respectively， after 2250 AST cycles． Categorized comparison of the membrane crack frequency and coverage relative to the total in－plane void area is provided in （b） and （c）， respectively．

第一种机理的数量更多，失效面积更大。

Figure 5． Overlaid image of planar view of low crack density cathode CL after 1000 RH cycles， which shows the formation of AST－induced cathode cracks， and outline of cracks （red） present in the membrane plane after 2250 cycles． Locations under the gas flow channel and land regions are also indicated．

阴极催化层裂缝的演变

Figure 6． Planar greyscale views of： （a） cathode GDL； and （b） anode GDL at the beginning－of－life （BOL） in the low cathode crack density specimen． The GDL images were acquired at a depth of about 50 μm from their interface with the CCM on either side． Buckling－induced membrane cracks present after 2250 AST cycles， as identified in，Fig． 4a are segmented and overlaid （red） on the BOL anode GDL plane in （b）． Locations under the gas flow channel and land regions are also indicated in （b）．

膜的裂缝位置和对应气体扩散层的空隙相关性

Conclusions

这篇论文的结论有些长。

The effect of electrode morphology on the durability and failure
processes of fuel cell membranes during pure mechanical degradation
was investigated by comparing two separate MEAs with
different ab initio cathode crack densities when subjected to an
RH cycling AST． Comprehensive degradation analysis was conducted
by utilizing XCT－based 4D in situ visualization， wherein
identical location， 3D microstructural characterization of these
MEAs was performed at periodical intervals． In both MEA designs，
membrane fracture was the predominant failure mode and the onset
of through－thickness membrane cracks developed around a similar
timeframe． 这个结果挺意外，但事实的确如此，做的更细致一些可能会稍有不同而已。

The scale of membrane damage in terms of the crack
coverage area， however， was significantly reduced with the use of
lower crack density cathode， thereby establishing this morphological
treatment as an effective strategy in mitigating the impact of pure
mechanical membrane degradation． Repeated expansion－contraction
cycles of the membrane still altered the original， relatively “defect
free” state of this electrode layer by introducing electrode cracks
formed mainly during the early stages of the applied AST． This early
stage electrode cracking was found to be exclusively responsible for
membrane fracture with both initiation and propagation of membrane
cracks occurring underneath these cycling－induced electrode
cracks． Root cause analysis of the electrode cracking and membrane
fracture process， however， showed their linkage to two distinct
failure mechanisms characterized by the respective presence or
absence of CCM buckling into void features within the anode GDL．
Supplementary failure investigations under hydrated conditions at
the end－of－test， which were uniquely afforded by the capabilities of
the adopted customized XCT imaging apparatus， revealed that the
buckling mechanism had a significantly higher impact on both the
frequency and coverage area of membrane fracture． Moreover， CCM
buckling occurred preferentially into the anode GDL wherein the
size of void regions was larger than for the cathode GDL and was
likely sufficient to overcome the critical buckling stresses generated under membrane compression during the AST hydration phase．这个结果很有趣。
These findings reaffirmed that the inter－component interaction effects influencing membrane durability extend beyond the immediately adjacent electrode layers， and defects／non－uniformities in the
GDL microstructure could strongly promote membrane failure．
Moreover， the inherent land－channel flow field configuration had a
supplementary influence on these interaction effects that preferentially
promoted both electrode－ and GDL－driven membrane failures
under the uncompressed channel regions． The present work also
showed that the in situ size and geometry of CCM／membrane
fracture features vary substantially between the dry and wet
conditions， and accordingly， indicated that fuel cell performance
and membrane lifetime could likely benefit from hydrated operation
even after the emergence of major membrane cracks．干湿循环避免是避免不了了，尤其在寒冷的地区需要冷启动时。

Our previous work involving the application of XCT－based 4D
in situ visualization approach had linked the development of
mechanically induced membrane cracks to ab initio electrode cracks in GDE－based MEAs． The present work， conducted on CCM based MEAs， has provided new insights on this topic by： （i）
demonstrating the favourable effect of minimizing the density of
ab initio electrode cracks on membrane mechanical degradation； （ii）
showing how the electrode crack network develops during RH
cycling despite the absence of ab initio cracks； and （iii） clarifying the
additional role of GDL microstructure on electrode and／or membrane
fracture． The insights gained during this work regarding the
fuel cell membrane’s in situ mechanical failure processes， as well as
their contributing factors， reaffirm the unique advantages generally
provided by non－invasive， 3D XCT imaging to membrane durability
studies． While the favourable effect offered by electrodes with a
lower ab initio crack footprint on membrane mechanical durability is
promising， further improvements could be realized by additional
material refinements that could allow sustainability of the electrode
layer’s structural robustness over extended durations of RH cycling．
These efforts could also benefit from a greater fundamental understanding of electrode layer mechanics， both characterization and
modelling， wherein their inherently porous and brittle material
character is accounted for． The present findings also emphasize the
importance of buckling phenomenon in membrane failure， and
accordingly， indicate a need for greater focus on its understanding
and mitigation．

你还想知道哪种缓解方法？欢迎给我留言，我优先给你安排。