流道壁面亲疏水性、微孔层缺陷对燃料电池内部水分布的影响：毛细流动、相变流动
Impact of channel wall hydrophobicity onthrough－plane water distribution and flooding behavior in a polymer electrolytefuel cell
Ahmet TurhanSoowhan KimMarta HatzellMatthew M． Mench
Abstract
Through－plane liquid accumulation，distribution and transport inside polymer electrolyte fuel cell （PEFC）components were analyzed as a function of channel wall hydrophobicity with theuse of high－resolution neutron imaging． Neutronimages were taken with polytetrafluoroethylene （PTFE） coated and uncoated flowchannel walls． Anode to cathode liquid distribution was analyzed for eachcase at low and high current conditions over 20min of operation． The form and amount of liquid water insidethe channels and diffusion media （DM） were compared for hydrophobically coatedchannels and hydrophilic channels， and a primary liquid transport－floodingmechanism is suggested for each case． Thelocation and value of maximum water storage in DM at low and high current operationwere analyzed and slopes of water mass versus distance curve were calculated tocompare the significance of capillary liquid flow and phase－change－induced flowwithin the diffusion media． A significant effect of CL｜MPL and MPL｜DMinterfaces on liquid transport and flooding is found through the analysis ofmicro－porous layer （MPL） water content and saturation profile along the CL｜MPLand MPL｜DM interface region．
Fig． 2． Neutron images of tests with 0．2A／cm2 operation and （a） anode channel hydrophilic／cathode channel PTFE coatedand （b） anode channel PTFE coated／cathode channel hydrophilic．
Fig． 3． Through－plane liquid waterdistribution for 0．2 A／cm2 operation with （a）anode channel hydrophilic／cathodechannel PTFE coated and （b） anode channel PTFE coated／cathode channelhydrophilic．
Fig． 4． At every minute of operation， totalliquid water mass values inside the cathode DM and the cathode channel areplotted for （a） 0．2 A／cm2 and （b） 1．0 A／cm2．
Fig． 5． Neutron images of water build－up inDM and water discharge into channels at 0．2 A／cm2 operation for （a） anodechannel hydrophilic／cathode channel PTFE coated and （b） anode channel PTFEcoated／cathode channel hydrophilic．
Fig． 6． Through－plane liquid waterdistribution inside DM and MEA at 0．2 A／cm2 for （a） anode channelhydrophilic／cathode channel PTFE coated and （b） anode channel PTFEcoated／cathode channel hydrophilic．
Fig． 7． Through－plane liquid waterdistribution inside DM and MEA at 1．0 A／cm2 for（a） anode channelhydrophilic／cathode channel PTFE coated and （b） anode channel PTFEcoated／cathode channel hydrophilic．
Fig． 8． Schematic of liquid water transportand removal with coated channels and uncoated channels at （a） start－up， （b）after couple minutes of operation， （c） after significant liquid build－up and（d） real－time neutron images for each case．
Fig． 9． At every minute of operation， thetotal liquid water mass values in the cell components for anode channelhydrophilic／cathode channel PTFE coated case at （a） 0．2 A／cm2 and （b） 1．0A／cm2．
Fig． 12． （a） Isolated holes and connectedcracks on MPL surface ［62］ and （b） liquid water mass peaks around MPL｜DMinterface ［63］．
Fig． 13． Through－plane liquid saturationinside DM at 0．2 A／cm2 current density for （a） MPL porosity of 0．72 and （b） MPLporosity of 0．5． In both cases， there is a reverse jump at MPL｜DM．
Conclusions
In this study， the through－plane liquidstorage， transport and flooding mechanism inside a PEFC was investigated as afunction of channel wall hydrophobicity． The PTFE coating on the channel surfaceresulted in discrete droplets on the channel walls with a higher water removalfrequency． The uncoated channels have less water on average， and liquid forms afilm layer around the walls which promotes steadier operation， but is moredifficult to purge． The liquid removal from under the lands into the channelsvia land｜channel interface is found to be a significant water transport mechanismand the hydrophilic–hydrophobic nature of this interface effects the waterstorage in DM． Hydrophilic channel wallsenhance the liquid suction from under－the－land locations whereas PTFE coatedland｜channel interface suppress the liquid inside DM resulting more waterstorage in DM， as high as 15％ at high current condition． The PTFE coatedinterface also found to increase the liquid connectivity inDM－under－the－channel location． Based on these results， a DM－liquid transportmechanism is suggested depending on surface PTFE coating．
Ahighly non－linear behavior with a peak near the center of DM was found for thewater mass distribution regardless of channel wall surface energy， suggestingphase－change－induced （PCI） flow exists in DM as a significant source term． The steady－state water mass storage inside anode DM was determinedto increase with increasing current； however， significant difference in waterstorage was not observed inside the cathode DM． The peak water mass location in cathode DM also did not change withincreasing current． Based on this and the slopes of the water mass curves，it is suggested that after a criticalliquid saturation is reached in DM， liquid removal from the DM via capillaryflow dominates phase change－ induced flow．
Liquid water storage was observed in theMPL and the water mass curve through MPL｜CL and MPL｜DM interfaces was found to becontinuous． The liquid saturation curvesobtained using different MPL porosities suggest a reverse saturation jump atthe MPL｜DM interface which indicates MPL surface cracks and interface morphologyhave an important impact on liquid storage and distribution．
