尽管GDE被称为第一代膜电极技术，膜电极制备过程中的膜缺陷、缺陷对性能和耐久性的影响依然是一个令人感兴趣的话题。

表征技术：红外热成像、SEM

Visualization， understanding， and mitigation of process－induced－membrane irregularities in gas diffusion electrode－based polymer electrolyte membrane fuel cells

Min Wang
Samantha Medina
Josias Ochoa－Lozano
Scott Mauger
Svitlana Pylypenko
Michael Ulsh
Guido Bender

Abstract

Polymer electrolyte membrane fuel cells （PEMFC） show substantial promise for their application in electric vehicles． For large－scale manufacturing of PEMFCs， roll－to－roll coated gas－diffusion－electrodes （GDE） offer certain advantages over other production pathways． Procedures including hot pressing and coating an ionomer overlayer may be necessary for this manufacturing pathway to enable a suitable catalyst layer／membrane interface． The same procedures may potentially introduce membrane irregularities， especially when thin membranes are used． Limited understanding exists regarding if and to what extent such irregularities impact PEMFC performance and lifetime， and therefore be considered defects．

In this study， NREL＇s customized fuel cell hardware that enables quasi in－situ infrared （IR） thermography studies was utilized to visualize spatial hydrogen crossover and identify membrane irregularities that originated from the GDE－based MEA fabrication process． The structure of these membrane irregularities was investigated by scanning electron microscopy （SEM） and its impact on initial H2／air performance was determined． Accelerated stress testing （AST） revealed that these irregularities develop into failure point locations． These results were validated across many MEAs with identified process－induced membrane irregularities． By selecting specific gas diffusion media properties and by fine tuning the MEA hot pressing parameters， the formation of such membrane irregularities was mitigated．

膜：Nafion® 211 membranes from Ion Power （～25 mm） were used for MEA fabrication．

Fig． 1 e Top－down SEM images of the microporous layer of （a） 29BC and （b） H23C8 gas diffusion media； （c） Surface roughness of H23C8 and 29BC microporous layer measured by stylus profilometry．

Fig． 2 e Impact of hot pressing compression force on 29BC－based MEAs without ionomer overlayer on： （a） H2／air polarization
and high frequency resistance （HFR）； （b） OCV and hydrogen crossover limiting current density （iH2）； （cef） IR thermography of
29BC－based MEAs with different HP compression force and HP temperature of 125 C． Cross－sectional SEM images of 29BCbased
MEAs with 16 kg／cm2 HP compression force featuring （g） membrane pristine area with no irregularities and （h） area
with a PIM．

PIMs：process－induced morphology changes

Fig． 3 e STEM images of 29BC－based GDE with 0．023 mg Nafion／cm2 ionomer overlayer： （a） high－angle annular dark－field
（HAADF） image and corresponding elemental mapping of （b） platinum and （c） fluorine． Impact of GDM and presence of
ionomer overlayer for MEAs with 16 kg／cm2 and 125 C hot－pressing conditions on： （d） H2／air polarization and high
frequency resistance； （e） OCV and hydrogen crossover limiting current density （iH2）； （fei） and IR thermography of GDE－based
MEAs with different GDM and ionomer overlayer．

思路比较好理解，在GDE的催化层表面做一层缓冲层，容易和膜对接，减轻GDE表面的粗糙特性对膜的影响。红外热成像法差异不大，但极化曲线影响很大。在文章中只对阴极侧做了相应处理。

Fig． 4 e Top－down SEM images of the microporous layer of （a） 29BC and （b） H23C8 gas diffusion media； cross－sectional SEM
SEI images of （c） 29BC and （d） H23C8 gas diffusion media； cross－sectional SEM BSE images of PIMs in （e） 29BC－based MEA
（16 kg／cm2， 125 C） and （f） H23C8－based MEA （16 kg／cm2， 125 C）．

比较典型的两种膜缺陷。膜由25微米降至15微米、8微米。

Fig． 5 e IR thermography of H23C8－based MEAs with 0．023 mg Nafion／cm2 ionomer overlayer hot pressed at （aed） 125 C and
different compression forces； and （eeh） hot pressed at 14 kg／cm2 and different temperatures； and their respective （iej） OCVs
and hydrogen crossover limiting current densities．

Fig． 6 e Impact of fabrication conditions on performance of H23C8－based MEAs with 0．023 mg Nafion／cm2 cathode ionomer
overlayer： H2／air polarization curves and high frequency resistance measurements at different hot－pressing （a） compression
force and （b） temperature； OCV and cell voltage at 1．0 A／cm2 and 1．5 A／cm2 in air polarization curves of MEAs fabricated at
different （c） HP compression force and （d） HP temperature compared to expected baselines （dotted lines）．

重点来了，耐久性工况：开路干湿循环。

Throughout the AST， the cell was held at OCV at 80 C．
Anode／cathode operating conditions were ambient pressures，
500／500 sccm H2／air gas flow rates， and 30／30 s duty cycle for
dry and wet humidification （0／0％ and 100／100％ RH）， respectively．
The AST operation was manually stopped when the
OCV dropped below 0．9 V． As OCV drops below 0．9V， hydrogen
crossover current density （iH2） increases up to ～6 mA／cm2
based on theoritical calculations， indicating the onset of
membrane failure．

Fig． 7 e IR thermography of H23C8－based MEA （25 kg／cm2， 125 C） at （a） BOT and （b） EOT； IR thermography of H23C8－based
MEA （14 kg／cm2， 120 C） at （c） BOT and （d） EOT； （e）． OCV decay profile of H23C8－based MEA （25 kg／cm2， 125 C） and H23C8－
based MEA （14 kg／cm2， 120 C） during AST operation； （f， g）． Cross－sectional SEM BSE images of failure point of H23C8－based
MEA （14 kg／cm2， 120 C） at EOT under different magnifications．

初始膜缺陷的位置成为耐久性工况膜出现针孔的位置。针孔40微米状态，红外热成像可以很容易捕获。回顾一下：燃料电池车辆用电池失效分析中膜电极针孔的四种检测方法研究，红外热成像针孔的失效10－100微米。实际上初始膜电极中膜减薄也可以用红外热成像判断，只是热点温度没有针孔出现的时候那么明显，温差差2度左右，对标后可以用于膜缺陷的质量控制。

文章就结束了，如果耐久工况不是针对膜的，而是真实工况，是否在膜初始缺陷的位置出现破损更具有设计的指导意义。

Conclusion

Process－induced membrane irregularities were detected and
localized using NREL＇s quasi－in－situ IR thermography subsequent
to the fabrication of GDE－based MEAs． The structure
and formation mechanism of these membrane irregularities
were further studied in a large number of samples by crosssectional
SEM． Irregularities were observed to form near MPL
surface cracks due to uneven compression and membrane
deformation and at locations of GDL fiber penetration into the
membrane． The effect of fabrication conditions such as
addition of ionomer overlayer， MPL roughness， and hot
pressing compression force and temperature on membrane
irregularity development， hydrogen crossover， HFR and initial
cell performance were systematically investigated． The addition
of ionomer overlayer exhibited negligible effect on the
amount of membrane irregularities， but， consistent with our
prior studies， dramatically improved the performance by
facilitating the interfacial proton conduction between CL and
membrane． Under the same hot－pressing condition， MEAs
fabricated using GDM with a rougher MPL surface exhibited a
higher amount of PIMs． MEAs processed with more aggressive
hot－pressing conditions （higher compression force and／or HP
temperature） were observed to have a higher number of
membrane irregularities， higher hydrogen crossover and
lower open－circurt voltage． Despite this， these MEAs displayed
increased initial performance and decreased HFR due to better
contact between CL and membrane．

性能越好，缺陷越多，缺陷被掩盖。

Under accelerated testing， however， membrane irregularities were demonstrated to be seed points for failure， and to shorten MEA lifetime dramatically． We thus identify， at least within the scope
of this study， a conundrum： higher initial performance， or
longer lifetime？ We have clearly shown that， in the case of
prioritization of the latter， the selection of specific GDM
properties and HP fabrication conditions generally enabled
the mitigation of PIMs and extended AST lifetime． Certainly，
further study is warranted， especially given the extensive and
important new focus on heavy－duty fuel cell applications， for
which durability and extended lifetime are tantamount． We
further note that， while the effect of GDM morphology and
structure was explored herein， the impact of membrane
thickness and architecture， i．e． with and without reinforcement，
on PIM formation is clearly of interest， and is the topic
of a follow－on study．