燃料电池气体扩散层微孔层中液态水的观测：形态、长大和聚集
Observation of water droplets inmicroporous layers for polymer electrolyte fuel cells by X－ray computednano－tomography
Satoshi YamaguchiSatoru KatoWataru YoshimuneDaigo SetoyamaAkihiko KatoYasutaka NagaiTakahisa SuzukiAkihisa TakeuchibKentaro Uesugi
An X－ray computed nano－tomography （nano－CT）system has been established at the BL33XU beamline of SPring－8． The opticalsystem consists of pseudo－Köhler illumination with a sector condenser zoneplate， an apodization Fresnel zone plate（A－FZP） as the objective lens， and aZernike phase plate． The imaging detector is a fiber－coupling type X－raycamera． The performance of the X－ray nano－CT system was confirmed by imaging anX－ray test chart． The system was subsequently applied to the observation of amicroporous layer for polymer electrolyte fuel cells and a simulatedmicroporous layer including liquid water． The nano－CT system， which can performa computed tomography measurement in lessthan 4 min， allowed visualization of a spherical water droplet produced inthe microporous layer． In the present study， the shape of water droplets in a nanoscale porous structure isinvestigated．
Figure 1 Schematic diagram of the Zernikephase－contrast X－ray microscopy optical system with pseudo－Ko¨ hlerillumination constructed at SPring－8 BL33XU
对这副图中的Zenike感兴趣的同仁可以从第二篇编辑文章中了解到这一出色的发明
Figure 2 Schematic diagram of thecondensation process of a simulated MPL． The length of the glass capillary wasabout 10 mm， and both ends of the glass capillary were open．
Figure 4 Nano－CT images of a cut MPL piece in darkphase－contrast mode． （a） Reconstructed CT image， and （b） 3D rendered image．
Figure 5 A 3D volume rendered image of the simulatedMPL measured by X－ray nano－CT． Solid materials that comprise the hydrophobicpore structure are represented in black． The blue color corresponds to liquidwater． Four spherical－shaped liquid water areas were observed in the pores
Figure 6 （a） 3D volume rendered image cropped arounda 8．15 um water droplet． （b） Pore size distribution obtained from the volume rendering image in（a）． The wet state indicates the distribution with a water droplet， while thedry state indicates that without a water droplet by removal after thebinarization process．
Figure 7 Orthogonal－cut images of the 3D volumerendered image in Fig． 6（a） without binarization． The water droplet appears asa circle in the center of each cross－sectional image． The water dropletincludes hydrophobic solid materials．
Figure 8 Schematic images of water droplet growthand aggregation model， which is the mechanism for the formation of liquid waterin a hydrophobic pore structure． （Step 1） Liquid water droplets less than 1 umin diameter condense at locations in the hydrophobic porous material． （Step 2）The water droplets grow with a spherical shape． If there are water dropletsnearby， then they possibly coalesce when they come into contact with eachother． （Step 3） The spherical shape of the liquid water droplets is stable， andthe droplets surround hydrophobic material．
Summary
An X－ray computed nano－tomography systemwas constructed at the BL33XU beamline of SPring－8． The effective pixel size with this system was 67．2 nm at 8 keV． The spatial resolution of the opticalsystem was higher than 200 nm and the measurement time for nano－CT was lessthan 4 min．
The particles that compose the MPL weresuccessfully visualized by the nano－CT system in Zernike phase－contrast mode．3D nano－CT imaging will provide information on the characteristics of porousstructures （e．g． pore size and tortuosity） to conduct computer simulations oftransport phenomena．
Condensed liquid water was observed in thepores of a model MPL． The water dropletshad a spherical shape， regardless of the structure of the pores of the hydrophobicporous material， and surrounded hydrophobic material． The results presentedhere provide information that will contribute to the improvement of thedrainage from the MPLs of PEFCs．
