MPL层裂缝形状、间距、数量对燃料电池MPL－GDL界面和GDL内部液态水传输的影响
Liquid transport in gas diffusion layer of proton exchange membrane fuel cells： Effects of micro－porous layer cracks

Xin Shi
Daokuan Jiao
Zhiming Bao
Kui Jiao
Wenmiao Chen
Zhi Liu

Abstract

Micro porous layer （MPL） is a carbon layer （∼15 μm） that coated on the gas diffusion layer （GDL） to enhance the electrical conduction and membrane hydration of proton exchange membrane fuel cell （PEMFC）． However， the liquid transport behavior from MPL to GDL and its impact on water management remain unclear． Thus， a three－dimensional volume of fluid （VOF） model is developed to investigate the effects of MPL crack properties on liquid water saturation， liquid pathway formation， and the two－phase mass transport mechanism in GDL． Firstly， a stochastic orientation method is used to reconstruct the fibrous structure of the GDL． After that， the liquid water saturation calculated from the numerical results agrees well with the experimental data． With considering the full morphology of the overlap between MPL and GDL， it＇s found that this overlap determines the preferred liquid emerging port of both MPL and GDL． Three crack design shapes in MPL are proposed on the base of the similarity crack formation processes of soil mud． In addition， the effects of crack shape， distance between cracks， and crack number on liquid water transport from MPL to GDL are investigated． It is found that the liquid water saturation of GDL increases with crack number and the distance between cracks， while presents little correlation to the crack shape． Hopefully， these results can help the development of PEMFC models without reconstructing full MPL morphology．

Fig． 1 e （a） Surface topography of MPL obtained by conventional preparation methods （b） Schematic diagram of the
regularized design of the MPL crack shape．

Fig． 2 e Computational domain of the MPL／GDL coupling model．

Fig． 3 e Comparison of numerical results with X－ray experimental data  along the through－plane direction for GDL： （a）local porosity， （b） water saturation．

以前对这个图没有认知，读些不了解领域的知识还是挺有趣的。不理解GDL和MPL的厚度决定什么，既然0．4的位置液态水的饱和度已经接近于0，剩下0．6的厚度有必要留有那么大的余量么？减薄会影响什么？

Fig． 4 e Effect of MPL on the distribution of liquid water and oxygen in GDL： （a） distribution of liquid water saturation along
the through－plane direction， （b） distribution of liquid water in GDL with or without MPL．

Fig． 5 e Processes of liquid water intruding into the GDL． The Single means water forms a channel through only one crack，
the Double means water enters GDL through two cracks．

Fig． 6 e Effect of MPL overlap structure on water transport behavior： （a） water distribution at the interface of MPL and GDL， （b）
water distribution at each crack cross－section in sample 2， and（c） graph for the grid number in different MPL cracks．

Fig． 7 e Water saturation in samples with different MPL
crack shapes．

Fig． 8 e Influence of crack types on water transport in GDL （a） Temporal comparisons of 3 types of crack water saturation， （b）
comparisons of water saturation along the through－plane direction in GDL with 3 types of crack．

a和b的注释写反了。

数值变动的时间尺度ms级别。

Fig． 9 e Effect of porosity ε on GDL drainage stability： （a） comparison of average water saturation changes in 15 sets of
samples， （b） water transport processes in two samples with porosities of 0．7 and 0．8．

这个图比较典型，说明孔隙率对于排水稳定性并不是越大越好。

Fig． 10 e Effects of MPL crack spacings on water transport：
comparison of GDL water saturation in 6 sets of samples．

Fig． 11 e Liquid water distribution at the MPL／GDL interface
in MPL crack samples with different spacings．

这个图需要搭配图2看，计算域的边长是200um

Fig． 12 e （a） Schematic diagram of MPL with different number of cracks， （b） Effects of MPL crack number on liquid water
transport： variation of GDL water saturation with crack number， （c） Water distributions in sample A with different MPL crack
number．

Conclusion

In this study， a stochastic orientation method is used to reconstruct
the three－dimensional fibrous structure of gas diffusion
layer （GDL）． Based on the formation law of mud crack， the
controllability of cracks in MPL preparation process is discussed．
Three MPL crack shapes （Bar， T， Y－shape） are then proposed and
the overlap structure between MPL and GDL is considered． The
gas－liquid two－phase transport behavior is simulated by the
volume of fluid （VOF）method， and the transport mode of liquid
water in the GDL is analyzed． The effects of geometric parameters
such as crack shape， crack spacing，and numberof cracks on
the gas－liquid two－phase transport are also investigated． The
main conclusions are as follows：

1） The overlap between GDL fibers and MPL cracks has a great
influence on the formation of liquid removal pathway．
Liquid water tends to establish transport pathways in MPL
cracks with high structural integrity．
2） The time variation of liquid water saturation is calculated
to represent the stability of liquid removal pathway from
MPL to GDL． It is found that this stability can be promoted
by reducing the overall porosity of GDL．

只是0．7比0．8好，没有0．6、0．5等数据的支持。
3） The correlation of distance between MPL cracks to liquid
removal of GDL is investigated． The liquid water saturation
of GDL is found to increase with the distance between MPL
cracks．
4） The relationship between crack number and liquid
removal in GDL is investigated． With a constant crack area，
liquid water saturation of GDL is positively correlated with
the number of cracks．
These findings can provide a better understanding of the
liquid water behavior in GDL and MPL， and help PEMFC
model development with regard to the crack geometry and
distribution．