燃料电池内部高分辨率液态水在多孔介质中的演变和传递（诊断其一）

High－resolution in－plane investigation of the water evolution and transport in PEM fuel cells

Christoph Hartnig

Ingo Manke

Robert Kuhn

Sebastian Kleinau

Jürgen Goebbels

John Banhart

Abstract

High－resolution synchrotron X－ray radiography is used to study the evolution of primary water clusters and the transport of liquid water from the catalyst layer through the gas diffusion layer （GDL） to the gas channels of a low temperature polymer electrolyte membrane （PEM） fuel cell． The liquid water content is quantified separately in the respective components； in the hydrophobic microporous layer （MPL） almost no liquid water can be observed． In the adjacent GDL， depending on the current density i0 water clusters are formed which lead to a diffusion barrier for the reactant gases． Water transport dynamics are explained and a recently proposed eruptive mechanism describing the transport from the GDL to the gas channels is imaged in a pseudo three－dimensional representation ［A． Bazylak， D． Sinton， Z．－S． Liu， N． Djilali， J． Power Sources 163 （2007） 784–792； S． Litster， D． Sinton， N． Djilali， J． Power Sources 154 （2006） 95–105； I． Manke， Ch． Hartnig， M． Grünerbel， W． Lehnert， N． Kardjilov， A． Haibel， A． Hilger， H． Riesemeier， J． Banhart， Appl． Phys． Lett． 90 （2007） 174105］． Based on a high temporal resolution the dynamics of the liquid water transport are observed； transient conditions resembling dynamic operation of the fuel cell are studied and an estimation of the time required to reach equilibrium conditions is given． The obtained spatial resolution of 3 μm is far below commonly used techniques such as neutron radiography or 1H NMR． Fundamental aspects of cluster formation in hydrophobic／hydrophilic porous materials as well as processes of multi－phase flow are addressed．

相关技术：

第一种方法：1H NMR

缺点：suffers thereby from the existence of metallic parts such as end plates

第二种方法：neutron radiography：

depending on the detector system spatial resolutions even below 30um have been obtained

the severely enhanced spatial resolution is counterbalanced by long image－to－image times in the range of several minutes．

第三种方法：synchrotron X－ray radiography

These limitations were overcome by means of synchrotron X－ray radiography．

The initial formation of small water agglomerations，the transport processes in the GDL and the transition from the GDL to the gas channel are visualized engaging a resolution down to 3um

Fig． 1． Fuel cell setup and viewing directions： 1 and 5 are the anodic resp． cathodic flow field， 2 and 4 the gas diffusion layers （GDL） and 3 is the membrane－electrodeassembly （MEA）． The cross－sectional （in plane） view allows for an investigation of the water transport from the catalyst layer to the gas channel．

Fig． 2． Normalized image of the cross－section of a PEM fuel cell． The image has been normalized with respect to an empty （water free） cell． Water agglomerates can be identified in this representation as bright spots； the schematic drawing of the cell in the lower part clarifies the respective components． The differentiation between MPL and GDL is demonstrated in the inset．

GDL层330－400um

MPL层50－60um

CCM层100um

Fig． 3． Liquid water formation as function of current density i0： （a）–（e）， （a）at i0 ＝ 250mAcm−2 hardly any liquid water is formed； （b）： larger values of i0（420mAcm−2） lead to initial water clusters on the cathode （white spots）； at i0 ＝500mAcm−2 water clusters appear at the anode； （c）–（e）： from i0 ＝ 500mAcm−2 onwardswater clusters are present to a large extent in both gas diffusion electrodes．Horizontal stripes are artifacts caused by thermal fluctuation of the monochromator setup； the black box depicts the area used to quantify thewater content as displayed in Fig． 5．

Fig． 4． Through－plane observations show clearly the initial spots of liquid water formation （bright spots） beneath the ribs of the flow field channel．

Fig． 5． Quantification and location of liquid water in the cathodic （C） and anodic （A） gas diffusion layer． Depending on the operating conditions， one or two diffusion barriers formed by liquid water can be detected． The water content of the MEA is shaded due to the low statistics caused by high absorption coefficients of platinum．

空气增湿，氢气不增湿。

但是：contrary to the processes on the cathode， already at low current densities（i0 ＝ 300–400mAcm−2） primary spots of liquid water accumulate in the GDL close to the MPL．

在较低电流密度区A second diffusion barrier close to the channel of the flow field caused by liquid water as observed at the cathode is not observed at the anode where no transport limitations due to the humidification exist．

Fig． 6． Dynamics of water formation in the gas diffusion layer at transient current densities： （a） visualization of the water distribution and （b） quantification of the water amount in the different layers． The jump in the current density from i0 ＝500 to i0 ＝600mAcm−2 is followed by an increase of the liquid water content in the anodic gas diffusion layer； the degree of filling on the cathode already reached a stationary point where only slight changes in the anodic water content can be observed．

Fig． 7． Eruptivewater transport fromthe gas diffusion media to the flowfield channel： （a） and （b） white spots denotewater agglomerates before and afterwater eruption； （c）differential image of the images （a） and （b）； blue： water clusters which were transported to the channel to form the droplet （red）． The interplay of the preferably hydrophobic
substrate and the water results in an eruptive behavior of the transport process （detailed operating conditions are given in the text）．

一个水滴体积约4nl。

Fig． 8． Cyclic water eruptions． The cyclic character of the eruptive water transport mechanism is determined by the amount of liquid water in the channel （detailed operating conditions are given in the text）．

Conclusion

We have investigated the cross－sectional transport of liquid
water in porous gas diffusion materials as employed in low temperature fuel cells by means of synchrotron X－ray radiography with
a spatial resolution of 3um and a time resolution of 5 s． The water
distribution in the GDL strongly depends on the water production
rate （the current density） and up to two different diffusion barriers
caused by liquid water were detected at high current densities．
The position of these diffusion barriers depends on the hydrophobic／
hydrophilic properties of the employed materials．
Transient conditions caused by changes of the current density
have been observed with regard on acquiring equilibrium
conditions in the water distribution； a delay of up to 12min is
needed to achieve a static condition at current densities exceeding
500mAcm−2．

The microscopic transport of liquid water is described by consecutive Haines jumps leading to a compact water cluster growth in the GDL． At the transition from the GDL to the gas channel， chokeoff
effects cause emptied pores which are filled gradually and lead
to a cyclic transport behavior． These results approve furthermore
transport theories used within the framework of percolation theory．
The presented finding might serve as basis to develop tailormade
materials with customized properties to remove excess
liquid water more efficiently． A uniform distribution of the liquid
water might finally lead to an increased performance and durability．
Modeling approaches of multi－phase flows can be adapted
based on these results and estimations on the amount of liquid
water involved in the overall two－phase water transport might be
deduced．

多孔介质渗流的显微观察表明，界面总是试图达到它的能量最小的状态，结果是弯液面形状的调节总是突然跃变的，这本身就说明流体并不是均匀地流过多孔介质，而是跃进式