阴极催化层涂布缺陷的面积大小、在膜电极中的位置对膜COCV工况耐受性的影响

Impacts of electrode coating irregularities on polymer electrolyte membrane fuel cell lifetime using quasi in－situ infrared thermography and accelerated stress testing

Adam Phillips

Michael Ulsh

K． C． Neyerlin

Jason Porter

Guido Bender

Abstract

In－line quality control diagnostics for roll－to－roll （R2R） manufacturing techniques will play a key role in the future commercialization of the polymer electrolyte membrane fuel cell （PEMFC） used in automotive applications． These diagnostics monitor the fabrication of the membrane electrode assembly （MEA）， which detect and flag any non－uniformity that may potentially harm PEMFC performance and／or lifetime． This will require quantitative thresholds and a clear distinction between harmful defects and harmless coating irregularities． Thus， novel fuel cell hardware with quasi in－situ infrared （IR） thermography capabilities is utilized to understand how bare spots in the cathode electrode impact MEA lifetime． An accelerated stress test （AST） simulates chemical and mechanical degradation modes seen in vehicular operation． The actual open circuit voltage and rate of change of this voltage are used as in－situ indicators for MEA failure， enabling capture of the progression of failure point development． Bare spot coating irregularities located at the center of the electrode were found to have no impact on MEA lifetime when compared to a pristine MEA． However， MEA lifetime was found to be considerably shortened when these same irregularities are located at the cathode inlet and， especially， the anode inlet regions of the fuel cell．

加速工况：

During the AST the cell temperature was 80 C． Anode／cathode operating conditions were ambient pressures， i．e． 90／90 kPa in Golden， Colorado， 0．5／0．5 standard L／min H2／air gas flow rates， and 30／30 s duty cycle for dry and wet humidification， respectively．

Nafion NRE212开路状态干湿循环

For the CCMs， a SONOTEK ExactaCoat system with a 25 kHz Accumist
spray head was utilized to ultrasonically spray catalyst ink
onto Nafion NRE－212 membrane material．

Fig． 1 e Relative dew point temperatures of the anode during humidification cycling for （A） co－flow and （B） counter－flow
operation under AST operating conditions． The （C） HFR under AST operating conditions in a H2／N2 environment for co－flow
and counter－flow operation．

逆流状态HFR的波动更加滞后。

Fig． 2 e Schematic of IR hardware setup （not to scale）． （A） Fuel cell mode： Flow－field is in place for regular single cell fuel cell
operation with air and hydrogen． （B） IR thermography mode： Flow－field is removed， convective air flow and IR camera are
activated， small hydrogen flow is applied to anode．

Fig． 3 e （A） IR thermography mode． Thermal imaging shown （B） without and （C） with convective air flow． Blue colors
represent temperatures of the GDL． The higher indicated temperatures around the edge of the images （the white frame） are a
result of the difference in emissivity between the PTFE gasket and the GDL．

Fig． 4 e Schematic depicting the cathode catalyst layer．
Each MEA possessing a single coating irregularity had it at
either the inlet， the center， or the outlet． Flow
configurations shown for the co－flow （black arrows） and
counter－flow （red arrows） for the anode gas flow （dashed
arrows） and cathode gas flow （solid arrows）．

Our previous work reported on the effects that 100％
CL reduction cathode coating irregularities， herein referred to
as bare spots， had on initial PEMFC performance using a
segmented fuel cell

这里提到的不规则是指部分区域催化剂没有涂布。

Fig． 5 e （A） OCV and （B） the change in OCV as a function of AST operating time for a pristine MEA operated in co－flow
configuration．

Fig． 6 e （A） Hydrogen crossover limiting current density and （B） IR thermography capturing the onset of failure for a pristine
MEA operated in counter－flow configuration．

对比图5和图6，同向流51小时出现失效，逆向流121小时出现失效。

Fig． 7 e （A） Hydrogen crossover current densities and （B） failure point locations for pristine MEAs and MEAs possessing 0．5
and 1 cm2 centered coating irregularities operated in co－flow configuration．

同流0．5－1cm2阴极侧的缺陷对于膜的耐久性没有影响。但是缺陷位置和膜的失效位置具有相关性。

Fig． 8 e （A） Hydrogen crossover current densities and （B） failure point locations for pristine MEAs and MEAs possessing
0．5 cm2 inlet and center coating irregularities operated in co－flow configuration．

同流0．5cm2阴极侧的缺陷在入口侧影响比缺陷在中心对于膜的耐久性影响大。

Fig． 9 e （A） Hydrogen crossover current densities and （B） failure point locations for pristine MEAs and MEAs possessing
0．5 cm2 inlet and center coating irregularities operated in counter－flow configuration．

逆流0．5cm2阴极侧的缺陷在出口侧（缺陷在阳极入口侧）影响比缺陷在中心对于膜的耐久性影响大。

现象是这样，如果更深入解析为什么阳极入口这么容易发生耐久性失效问题，本质特征是否和在干湿循环－化学衰减循环联合工况下燃料电池电解质减薄与成分迁移的现象一致会更有意义。

又一次遗憾地发现因为实验工装夹具、实验条件不同对实验结论的影响，感兴趣的可以再回顾一下。

燃料电池阳极催化层缺陷在循环工况过程中的扩展

四类催化层缺陷对燃料电池局部膜降解失效的影响

Conclusions

Methodologies to understand the effects of as－manufactured
irregularities in MEA component materials are critical to
improve quality and reduce cost in PEMFC systems． In this
work， we demonstrated specialized equipment and protocols
to enable a determination of whether coating irregularities in
PEMFC electrodes impact the time to failure and location of
failure in an MEA． Degradation mechanisms present in MEAs
undergoing vehicular operation were simulated using a

combined chemical－mechanical AST． OCV－based metrics
were developed to enable the detection of the onset of MEA
failure． In conjunction with this protocol， a custom designed
fuel cell hardware paired with an IR camera was employed to
spatially detect the location of failure point development in
the MEAs． Finally， the hydrogen crossover as a function of AST
operating time was monitored and analyzed to enable a
quantitative comparison of the MEA lifetime between pristine
MEAs and MEAs with bare spots of various sizes and locations
in the cell． In exemplary studies under both co－flow and
counter－flow conditions， we found that bare spots located at
the center of the cathode electrode did not impact MEA lifetime
compared to that of pristine MEAs． However， for MEAs
with irregularities located at the inlet regions operated in a coflow
configuration， MEA lifetime was considerably shorter
than that of pristine MEAs． In addition， by operating MEAs in a
counter－flow configuration we observed that coating irregularities
located at the H2 inlet have the most significant impact
on MEA lifetime．

As we previously discussed， the impact of as manufactured
irregularities will depend on stack－ and
system－specific factors． The developments in the present work
aim to provide generally applicable methods and knowledge
that can assist stack and system integrators in understanding
quality requirements for MEA materials． Moving forward，
future studies focused on MEA failure will investigate the lifetime
impacts of less severe coating irregularities， such as
smaller sizes and loading reductions of less than 100％， i．e． thin
spots rather than bare spots． 如果有缺陷，至少需要小于0．5cm2，要一点载量比没载量好，这个地方留了一个悬念，如果局部位置的载量是标准正常载量的25％、50％、75％，对于实验结果有多大影响。

影响膜耐久性的所谓的入口区域有多大？占活性区的10％、20％还是多少？

Studies will focus on irregularities located at the inlet locations of the MEA， as conditions in those regions appear to be the most sensitive to the presence of electrode irregularities． Furthermore， methodologies to determine the impact of irregularities on degradation of performance over time， as opposed to those immediately proximal to
MEA failure， will also be presented in upcoming works．