燃料电池电位循环测试过程中原位成像观察阴极电极形貌的变化

4D in situ visualization of electrode morphology changes during accelerated degradation in fuel cells by X－ray computed tomography

Robin T．White
Alex Wu
Marina Najm
Francesco P． Orfino
Monica Dutta
Erik Kjeang

Abstract

A four－dimensional visualization approach， featuring three dimensions in space and one dimension in time， is proposed to study local electrode degradation effects during voltage cycling in fuel cells． Non－invasive in situ micro X－ray computed tomography （XCT） with a custom fuel cell fixture is utilized to track the same cathode catalyst layer domain throughout various degradation times from beginning－of－life （BOL） to end－of－life （EOL）． With this unique approach， new information regarding damage features and trends are revealed， including crack propagation and catalyst layer thinning being quantified by means of image processing and analysis methods． Degradation heterogeneities as a result of local environmental variations under land and channel are also explored， with a higher structural degradation rate under channels being observed． Density and compositional changes resulting from carbon corrosion and catalyst layer collapse and thinning are observed by changes in relative X－ray attenuation from BOL to EOL， which also indicate possible vulnerable regions where crack initiation and propagation may occur． Electrochemical diagnostics and morphological features observed by micro－XCT are correlated by additionally collecting effective catalyst surface area， double layer capacitance， and polarization curves prior to imaging at various stages of degradation．

表征方法：XCT、ECSA、双电层电容、极化曲线、EIS、HFR

加速工况：电位循环

the cell voltagewas cycled in H2／air from a lower potential of 0．6 V for 30 s， to an upper potential of 1．4 V for 60 s in a square wave pattern． The high upper potential was held for 60 s in order to accelerate degradation

EOL定义：An end－of－life （EOL） condition was established when
the cell voltage at 500 mA cm2 was less than half of that at BOL．

图像处理算法

极化曲线随电位循环的变化

催化剂ECSA和双电层电容随电位循环的变化

HFR随电位循环的变化

阴极催化剂层离子电阻随电位循环的变化

催化层和膜的厚度随电位循环的变化（数据进行了归一化处理），这里如果有SEM级别更细致的数据验证性会更好。

这个图有点意思。The area fraction calculation required a 2D top－down
view， which was achieved by applying a projection in the Z－direction
producing a representation of the cathode catalyst layer in 2D．

solid area fraction应该指的是除去空隙的固相实体的比率。一般我们说碳腐蚀指的是电极厚度减薄，没有能力区分脊部和流道部的厚度差异，而原位XCT测试告诉我们碳作为固相实体，随着电位循环在下降，而且流道内的碳腐蚀更厉害。

As can be seen in Fig． 5b， there is a considerable
difference between the solid area fraction values， indicating a
different level of corrosion of the CCL in regions under the channel
compared to that under the land． Although both areas showed a
significant decrease in area fraction from corrosion， the change
under the channel took place at a higher rate．

一般我们会将脊部或流道部气体扩散层内部的孔隙率认为是各自均一的，材料受压径向上还能有差异吗，对吧？就算你说不均一原位的条件下也没法精确测量。作者给出不同厚度层面上的气体扩散层孔隙率的分布状态。脊部的孔隙率从65％逐渐下降到55％，而流道部的孔隙率从80％逐渐下降到55％。

催化剂层中裂缝宽度随电位循环在增加。

催化剂层中裂缝单位长度的数密度随电位循环在增加。

Conclusions

Four－dimensional in situ visualization of cathode catalyst layer
degradation was achieved by the use of a custom small－scale fuel
cell fixture compatible with accelerated stress testing and nondestructive micro－XCT characterization at different points in time．
Upon cyclic voltage AST， a gradual progression in cathode catalyst
layer thinning up to 40％ was measured by quantitative XCT analysis． The thinning was correlated to a gradual increase in density by
collapse of the carbon support structure as observed by changes in
cathode catalyst layer relative attenuation of incident X－ray spectra
from beginning－of－life to end－of－life． The solid area fraction of the
cathode catalyst layer was calculated after the application of a
segmentation method using mixed Gaussian histogram fitting．
Analysis comparing specific regions under land and channel
showed a higher rate of degradation under the channel． This difference was attributed to the higher local reactions rates and higher
rates of water production and erosion under the channels． Finally，
identical－location tracking of individual cathode catalyst layer
cracks was demonstrated for the first time． Both formation of new
cracks and propagation／growth of existing cracks were observed．
Local variation in the relative attenuation of the X－ray beam suggests regions with high vulnerability to corrosion as shown by crack initiation and propagation． New insight into the crack formation and correlation with electrochemical measurements such as double layer capacitance and resistance is allowed by 4D in situ measurements． Most notably， the increasing morphological changes
observed during the later stages of the cathode catalyst layer
degradation process were in good agreement with the gradually
increasing losses in fuel cell performance as well as the distinct
decay trend in electrochemical double layer capacitance． This
demonstrates a clear correlation between conventional in situ diagnostics and the new XCT results： severe carbon corrosion，
responsible for losses in electrode capacitance and mass transport
performance， manifests itself in the form of catalyst layer thinning，
crack growth and overall compaction of the remaining porous solid
phase， and triggers eventual electrode failure． Future work to
investigate alternate degradation protocols such as start－up／
shutdown cycling and voltage cycling with different upper potential
limits is recommended to provide additional information about
specific degradation processes that occur during fuel cell operation
in the field．