气体扩散层受压缩对液态水传输和微结构的影响［设计因素其十三］

Effect of compression on liquid watertransport and microstructure of PEMFC gas diffusion layers

A． Bazylak

D． Sinton

Z．－S． Liu

N． Djilali

Abstract

This work explores how the degradation ofthe gas diffusion layer （GDL） under compression contributes to the formation of preferential pathways forwater transport． Fluorescencemicroscopy is used to provide ex situ visualization of liquid water transport throughthe GDL placed beneath an optically transparent clamping plate． Transientimage data obtained with a CCD camera indicates that areas of compression inthe GDL coincide with preferential pathways for water transport andbreak－through． Preferential flow ofwater through the smaller pores resulting from GDL compression is contrary tothe expected behaviour in a hydrophobic medium， and this suggests a loss of hydrophobicity． Scanningelectron microscopy （SEM） is used to investigate the effect of compression onthe morphology of the GDL． These SEM images show that compressing the GDL causes the breakup of fibers and， indeed，deterioration of the hydrophobic coating．

Fig． 1． Schematic of fluorescence  microscopy and GDL clamping apparatus．

Table 1 Liquid breakthrough pressures

Fig． 2． Digital images from fluorescence  microscopy showing the evolution of water transport at （a） t＝4．9s， （b）  t＝5．2s， and （c） t＝6．5s． Images to the left have been inverted for clarity，  with the areas compressed by the O－ring shown as the area between the  concentric dashed circles． To the right of each image is the three－dimensional  rendering of the signal intensity．

Fig．3． Inverted fluorescence image at 5．2  s showing flow paths 1 and 2 shown in Fig． 4． The areas of the GDL that are  compressed exist between the concentric dashed circles．

Fig． 4． The liquid water surface height determined  via correlation with fluorescence intensity of paths 1 and 2， where path 1 appears  in the uncompressed GDL and path 2 appears in the compressed GDL．

A programmable syringe－pump （Harvard  Apparatus PHD 22／2000） was used to deliver the fluorescein dye via Teflon FEP  tubing （Upchurch Scientific， WA） from the syringe pump to the GDL at a rate  of 0．02mlmin −1 ．

Fig． 5． Inverted fluorescence images  showing the liquid water breakthrough location with respect to the  compression O－ring， where all trials except for trials 3 and 5 resulted in  breakthrough originating in the area above the compression O－ring． Red arrows  indicate where breakthrough coincided with the compression region． The blue  arrows indicate where excess fluid excited through the outlet hole，  post－breakthrough．

Fig． 6． SEM images of a Toray TGP－H－060  GDL with 10％ PTFE treatment before compression at （a） 200× magnification with  a hand－held 1cm×1cm sample （inset）， （b） 800× magnification， and （c） enlarged  image of （b） at 4000× magnification．

The Toray TGP－H－060 GDL is also a highly  porous media with more than 50％ of its pore sizes ranging from 30 to 40um，  with a thickness of 190um， and a mean fiber diameter of 8um．

Fig．7． SEM image of a Toray TGP－H－060 GDL  with 10％ PTFE treatment after it was compressed for 5min at 0．18MPa at 1000×  magnification with a 10kV beam voltage showing the damaged PTFE coating．

Fig． 8． SEM images of the GDL at 200x  magnification after it has been compressed for five minutes at （a） 0．18MPa，  （b） 0．36MPa， （c） 0．68MPa， and （d） 1．37MPa．

Conclusion

Compressing a PEMFC GDL has a large  influence on liquid water transport behaviour as well as on the GDL  microstructure morphology．Fluorescence microscopy was applied to provide ex situ  visualizations， showing that the compressed GDL provides preferential  pathways for water transport and breakthrough． SEM images clearly illustrating  the degradation of the GDL under varying compression pressures were  presented． The irreversible damage at  the surface of the GDL consists of fiber and PTFE coating breakage and  deformation， which locally produce greater proportions of hydrophillic to  hydrophobic surface area． Preferential pathways for water transport are a result of these localized  hydrophillic pathways in the bulk hydrophobic  GDL． The present work shows that compression alters liquid water transport and favors flow in the compressed areas of the GDL （under the land area）  due to both morphological changes  and possible loss of hydrophobicity． With respect to fuel cell operation， this liquid water transport behaviour may be beneficial as water can be  channeled to where transport is less  critical． Liquid water transport in  the fuel cell could be further controlled by pre－compressing the material in  strategic locations to facilitate removal of excess water in the assembled cell．

虽然是十五年前的一篇文献，但是实验仍然很新颖，不禁让我思考：

如果将疏水的气体扩散层进行预先压缩，是否影响气体扩散层的接触角？

再在O圈下进行液态水传输实验，是否通道1和通道2之间的差异会变小？

在不同温度条件下进行液态水传输实验会是什么结果？

气体扩散层中的液态水不是100％，对实测出来的液态厚度的结果有影响么？是累积总厚度还是包含了空隙的总厚度？

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