3D visualization of membrane failures in fuel cells

Yadvinder Singha

Francesco P． Orfinoa

Monica Duttab

Erik Kjeanga

Abstract

Durability issues in fuel cells， due to chemical and mechanical degradation， are potential impediments in their commercialization． Hydrogen leak development across degraded fuel cell membranes is deemed a lifetime－limiting failure mode and potential safety issue that requires thorough characterization for devising effective mitigation strategies． The scope and depth of failure analysis has， however， been limited by the 2D nature of conventional imaging． In the present work， X－ray computed tomography is introduced as a novel， non－destructive technique for 3D failure analysis． Its capability to acquire true 3D images of membrane damage is demonstrated for the very first time． This approach has enabled unique and in－depth analysis resulting in novel findings regarding the membrane degradation mechanism； these are： significant， exclusive membrane fracture development independent of catalyst layers， localized thinning at crack sites， and demonstration of the critical impact of cracks on fuel cell durability． Evidence of crack initiation within the membrane is demonstrated， and a possible new failure mode different from typical mechanical crack development is identified． X－ray computed tomography is hereby established as a breakthrough approach for comprehensive 3D characterization and reliable failure analysis of fuel cell membranes， and could readily be extended to electrolyzers and flow batteries having similar structure．

Highlights

3D failure analysis approach is developed for fuel cell membranes．

Membrane crack development is possible without any influence from catalyst layers．

Gradual membrane degradation can introduce new fracture mechanisms．

Contribution of membrane cracks to gas leakage can be comparable to pinholes．

Delamination bolsters the impact of cracks in causing gas leakage through MEA．

题图看上去非常令人震撼。常规的2D截面分析能捕捉50微米内的这种裂缝非常困难。

分为两种类型，一种是仅仅膜有缺陷的，一种是催化层也有缺陷的。

第一种缺陷的比例很高，几乎50％。即使催化剂层和膜同时出现缺陷，阴极催化层缺陷和膜缺陷同时出现的概率更大。

裂缝的形态分布。

膜变薄和膜变薄的位置分布

Conclusions

This work demonstrated the novel application of laboratory－based
XCT for 3D failure analysis and detailed characterization of damage development in fuel cell membranes． In contrast toincumbent SEM techniques， the XCT approach enabled nondestructive acquisition of full 3D images of the membrane inside the MEA， without physical removal of other layers． The results obtained for a fuel cell subjected to combined chemical and mechanical membrane degradation revealed that the membrane damage was dominated by cracks． Roughly 50％ of the through thickness membrane cracks were found to be confined within the membrane without any structural connectivity with cracks in the‍‍ adjoining CLs． This observation evidences the presence of an independent crack development mechanism within the membrane
which is understood to be a newly discovered membrane failure
mode in addition to the previously known membrane crack
development influenced by the CL cracks． Furthermore， only 6％ of
the CL cracks were connected to membrane cracks， with the cathode
CL having four times higher crack connection with the membrane
than the anode CL． The hypothesis of the presence of multiple
membrane crack development mechanisms was further supported
by the identification of two distinct crack shapes， namely， Y－shaped
and I－shaped cracks with considerably different morphologies． The
peculiar Y－shaped cracks， which do not resemble the typical mechanical fracture， are likely to have formed after the membrane has
undergone a fundamental transformation in its material character
caused by combined chemical and mechanical degradation． The
membrane thinning was observed to be non－uniform across the
MEA with gas outlet regions experiencing 30％ higher average
thinning than inlet regions， with more frequent cracks identified in
the thinner regions． A very high post－mortem membrane crack
density of 750 cracks per cm2 was revealed， which is two orders of
magnitude higher than the previously reported membrane pinhole
density obtained by 2D SEM． Moreover， a significant number of
cracks had sizes comparable to pinholes， which suggests that cracks
can dominate the overall membrane failure by carrying high leak
rates． Delamination between the membrane and CLs was also found
to exacerbate the impact of certain cracks on membrane durability
by providing a connection between cracks at different locations，
thereby opening up an additional indirect path for the gas leakage
through the MEA．

In the area of fuel cell failure analysis， the 3D nature of imaging
utilized in this work allowed for investigations that were either
attempted for the first time or were carried out with a higher degree
of thoroughness compared to incumbent 2D imaging． This
establishes XCT as a breakthrough approach for comprehensive and
reliable failure analysis of fuel cell membranes． The non－destructive
and non－invasive nature of imaging enabled by XCT can be further
leveraged to investigate mechanisms for damage initiation and
growth and to explore interactions between internal MEA components without disassembling the layers． The XCT technique，
therefore， holds great promise to positively impact the future of
fuel cell research， and may also be applicable to other electrochemical
technologies such as electrolyzers and flow batteries that
make use of a similar MEA unit structure．

文中使用的膜是non－reinforced perfluorosulfonic acid （PFSA） ionomer membrane of Nafion NRE211 type

运行模式是The FFPs had co－flow parallel straight channels

加速工况为the cyclic open circuit voltage （COCV） AST protocol

In the area of fuel cell failure analysis， the 3D nature of imaging
utilized in this work allowed for investigations that were either
attempted for the first time or were carried out with a higher degree
of thoroughness compared to incumbent 2D imaging． This
establishes XCT as a breakthrough approach for comprehensive and
reliable failure analysis of fuel cell membranes． The non－destructive
and non－invasive nature of imaging enabled by XCT can be further
leveraged to investigate mechanisms for damage initiation and
growth and to explore interactions between internal MEA components
without disassembling the layers． The XCT technique，
therefore， holds great promise to positively impact the future of
fuel cell research， and may also be applicable to other electrochemical
technologies such as electrolyzers and flow batteries that
make use of a similar MEA unit structure．‍‍‍‍‍‍‍‍‍‍‍‍