燃料电池气体扩散层弯曲刚度各向异性对性能的影响：弯曲刚度、性能、HFR、流阻、厚度的差异分析［设计因素其六］

Influence of anisotropic bending stiffnessof gas diffusion layers on the electrochemical performances of polymer electrolytemembrane fuel cells

Kookil Han

Bo Ki Hong

Sae Hoon Kim

Byung Ki Ahn

Tae Won Lim

a b s t r a c t

The effects of gas diffusion layer’s（GDL’s） anisotropic bending stiffness on the electrochemical performancesof polymer electrolyte membrane fuel cells have been investigated for carbonfiber－felt and －paper GDLs． The bending stiffness values of all GDLs in the machinedirection are higher than those in the cross－machine direction． We haveprepared GDL sheet samples such that the machine direction of GDL roll isaligned with the major flow field direction of a metallic bipolar plate atangles of 0 （parallel： ‘0 GDL’） and 90 （perpendicular： ’90 GDL’）． The I－Vperformances of all the 5－cell stacks with 90 GDLs are higher than those with 0GDLs． All the 5－cell stacks with 90 GDLs show lower values of high－frequencyresistance （HFR） than those with 0 GDLs． However， the gas pressure differencesat both anode and cathode of 5－cell stacks with 90 GDLs appear to be similar toor slightly lower than those with 0 GDLs， making the lower HFR as a dominantfactor for the improved I－V performances． This may result from the reducedintrusion of 90 GDLs into gas channels than 0 GDLs as observed by lessthickness reduction under compression of 90 GDLs． A 45 GDL （skew alignment）also shows better performances than the 0 GDL．

Fig． 1 e A schematic representation ofcross－section of fuel cells that show GDL intrusion into gas channels of abipolar plate after cell compression： （a） before cell compression； （b）aftercell compression．

Table 1 e Key characteristics of the GDLsused in this study．

没有想到，MD／CMD两个方向的弯曲刚度竟然有这么大的差异，ratio说明纤维的排列偏离随机状态的程度。

Fig． 2 e A schematic illustration of GDLthickness measurement on a metallic bipolar plate as a function of compressionpressure．

这是一个很新颖的测试方法。设备来自专利技术KR Patent 10e0902316；2009

Fig． 3 e Surface morphology of macro－poroussubstrates of GDLs observed by SEM （magnification： 1003）： （a） GDL－F1； （b）GDLF2； （c） GDL－P1； （d） GDL－P2．

Fig． 4 e A schematic representation of GDLpreparation methods with two alignment angles of 0 and 90． The soliddouble－headed arrow in each GDL sample indicates the machine direction of GDLroll．

Fig． 5 e A schematic representation ofalignment of the GDL’s MD with the major flow field direction： （a） theta＝0（parallel alignment）； （b） theta＝90 （perpendicular alignment）．

Fig． 6 e Electrochemical I－V performancesof 5－cell stacks with GDLs having two alignment angles （0 and 90） at RH 50％／50％and 100％／100％ conditions： （a） GDL－F1； （b） GDL－F2； （c） GDL－P1； （d） GDL－P2．

湿态测试条件下性能GDL－F1：0．425V＠1．5A／cm2， GDL－F2：0．51V＠1．5A／cm2，GDL－P1：0．375V＠1．4A／cm2， GDL－P2：0．48V＠1．5A／cm2，

也就是GDL－F2＞ GDL－P2＞ GDL－F1＞ GDL－P1

MD方向Bendingstiffness的次序GDL－F2＞ GDL－P2＞ GDL－F1＞ GDL－P1，厚度也是这个次序，希望如果四个GDL厚度相同的情况下讨论Bending stiffness更有意义一些，或者同样的材料型号厚度不同来比较选材，或者同样的Bending stiffness不同材料型号的性能对比更令人感兴趣。

MD方向In－planeelectrical conductivity的次序GDL－F2＜GDL－P1＜GDL－P2＜GDL－F1

HFR的次序GDL－F2（76mohmcm2）＜GDL－P2（90mohmcm2）＜GDL－F1（100 mohmcm2）＜GDL－P1（110 mohmcm2＠1．4A／cm2）和MD方向In－plane electrical conductivity无明显的大小次序相关性。

IR free的性能GDL－F2：0．624V＠1．5A／cm2＞GDL－P2：0．615V＠1．5A／cm2＞GDL－F1：0．575V＠1．5A／cm2＞GDL－P1：0．529V＠1．4A／cm2仍然差异很大。

阳极侧流阻：GDL－F1 6kPa，GDL－F24KPa，GDL－P1 5kPa＠1．4 A／cm2，GDL－P27kPa

阴极侧流阻：GDL－F1 20kPa，GDL－F217kPa，GDL－P1 18＠1．4 A／cm2，GDL－P225kPa

压缩后的厚度＠2．4MPa计算：GDL－F1195．3＋－3．5 um，GDL－F2 330．3＋－0．6 um，GDL－P1 180．0＋－4．0 um， GDL－P2 220um

使用同一种碳纸，Bending stiffness最大的方向和流道方向垂直对性能有可能有一定帮助，但是并不能把Bending stiffness作为选择碳纸的依据，在这篇文章中碳纸厚度决定了Bendingstiffness的大小。

干态测试条件下：性能GDL－F1：0．375V＠1．5A／cm2， GDL－F2：0．48V＠1．5A／cm2，GDL－P1：0．35V＠1．4A／cm2， GDL－P2：0．44V＠1．5A／cm2

也就是GDL－F2＞ GDL－P2＞ GDL－F1＞ GDL－P1，操作条件的变化并未改变不同碳纸的性能序列。

0度CMD方向湿态测试条件下：性能GDL－F1：0．46V＠1．0A／cm2， GDL－F2：0．47V＠1．5A／cm2， GDL－P1：0．43V＠1．2A／cm2，0．52V＠1．0A／cm2，GDL－P2：0．46V＠1．5A／cm2

也就是GDL－F2＞＝ GDL－P2＞ GDL－P1 ＞ GDL－F1，序列有所改变，前两个的性能差距很小。

CMD方向Bendingstiffness的次序GDL－P2＞ GDL－F2＞ GDL－P1＞ GDL－F1。

为什么CMD方向Bendingstiffness GDL－P2这么好，性能却不高？

MD方向In－planeelectrical conductivity的次序GDL－P2＞ GDL－P1＞ GDL－F1＝GDL－F2

HFR的次序GDL－F2（80mohmcm2＠1．5A／cm2）＜GDL－P2（95mohmcm2＠1．5A／cm2）＜GDL－P1（110 mohmcm2＠1．2A／cm2）＜GDL－F1（120 mohmcm2＠1．0A／cm2）

IR free的性能：GDL－F2：0．59V＠1．5A／cm2，GDL－P2：0．6V＠1．5A／cm2

为什么CMD方向Bendingstiffness GDL－P2这么好，性能却不高？只能看出GDL－P2内阻和GDL－F2相比较大，具体原因不清楚。

因为每个电池运行的最大电流密度不一样，只比较＠1．5A／cm2的两个结果。

阳极侧流阻：GDL－F2 4KPa，GDL－P27kPa

阴极侧流阻：GDL－F2 18kPa，GDL－P226kPa

压缩后的厚度＠2．4MPa计算：GDL－F2320．7＋－3．1 um，GDL－P2 220um

GDL－F2 MD和CMD性能比较差异有些大，而GDL－P2 MD和CMD性能比较差异很小，如果是我选择我会选择GDL－P2，工艺稳定性会更好。

Fig． 7 e High frequency resistance of5－cell stacks with GDLs having two alignment angles （0 and 90） at RH 50％／50％and 100％／100％ conditions： （a） GDL－F1； （b） GDL－F2； （c） GDL－P1； （d） GDL－P2．

Fig． 8 e Hydrogen pressure difference atanode of 5－cell stacks with GDLs having two alignment angles （0 and 90） at RH 50％／50％and 100％／100％ conditions： （a） GDL－F1； （b） GDL－F2； （c） GDL－P1； （d） GDL－P2．

Fig． 9 e Air pressure difference at cathodeof 5－cell stacks with GDLs having two alignment angles （0 and 90） at RH 50％／50％and 100％／100％ conditions： （a） GDL－F1； （b） GDL－F2； （c） GDL－P1； （d） GDL－P2．

Fig． 10 e Thickness variation of GDLs onthe metallic bipolar plate as a function of compression pressure： （a） GDL－F1；（b） GDLF2；（c） GDL－P1； （d） GDL－P2．

The GDL specimens with two representativealignment angles of 0 and 90 were taken for this test and compressed twice to comparethe thickness hysteresis of the GDLs．

Bending stiffness两个方向的Ratio小到1．37时，在MD／CMD两个方向上压缩，碳纸的受流道压缩厚度差异就很小了。

The bending stiffness of GDL sample wasmeasured at a bending angle of 15 degree using a Taber Industries stiffnesstester （150－E V－5， Taber Industries， USA）

The ratios of land width／channel depth of anodeand cathode flow fields are 1．2 and 0．8， respectively， and the ratios of landwidth／channel width of anode and cathode flow fields are 0．6 and 0．5，respectively

Ratio 1．37而受压厚度差异不大可能和跨距有关，跨距越大两个方向的差异越明显。

图中的压强似乎不是名义压强，但是局部的最大接触压强9MPa也是挺大的值。一次性在脊下压缩到原始厚度的40％，一次性产生了35％的残余应变，不知道为什么要这么操作。

并不清楚装配到电池大致是在什么水平。

这里没有体现平板压缩的结果，不知道流道压缩和平板压缩的差异、流道内的嵌入高度能否通过流道结构仿真获得。

Fig． 11 e Performances of 5－cell stackswith GDL－F1 as a function of alignment angle at RH 50％／50％ and 100％／100％conditions： （a） cell voltage at 1000 mA cmL2； （b） HFR at 1000 mA cmL2．

Conclusions

We have investigated the effects of GDL’sanisotropy in bending stiffness on the electrochemical performances of PEMFCsfor carbon fiber－felt and －paper GDL samples under dry （RH 50％／50％） and wet （RH100％／100％） operating conditions． For a given GDL， the bending stiffness valueof the GDL in the machine direction is higher than that in the transverse orcross－machine direction． The degree of anisotropy in bending stiffness of feltGDLs is more pronounced than that of the paper GDLs． We have prepared GDL sheetsamples such that the machine direction of GDL roll is aligned with the major flowfield direction of a metallic bipolar plate at angles of 0 （parallel alignment）and 90 （perpendicular alignment）． For both carbon fiber－felt and －paper GDLshaving anisotropic bending stiffness， the 5－cell stacks with 90 GDLs showhigher I－V performances than those with 0 GDLs under given RH conditions． It isclearly observed that all the 5－cell stacks with 90 GDLs show lower HFR valuesthan those with 0 GDLs under given RH conditions at all current densities，reflecting

lower contact resistances in 90 GDLs cells．

Contrary to the results of a previousreport， the DP values of hydrogen at anode of the 5－cell stacks with 90 GDLsare substantially similar to those with 0 GDLs． The DP values of

air at cathode of the 5－cell stacks with 90GDLs appear to be similar to or slightly lower than those with 0 GDLs． But the differencesof DP values between 0 and 90 GDL stacks appear less pronounced and consistentthan those of HFR values． The thickness reduction of 90 GDLs on the metallicbipolar plate is typically less than that of 0 GDLs over all compression pressure，implying less intrusion of the 90 GDLs into gas channels of the bipolar plate．It is thought that the dominant factor for the improved I－V performances byapplying 90GDLs appears to be the lower electrical contact resistance ratherthan the lower gas pressure differences．

In the case of a skew alignment of 45 GDL，it is observed that the I－V performances of the 45 GDL stack are higher thanthose of 0 GDL stack， and the HFR values of the 45 GDL stack are lower thanthose of 0 one． This indicates a dependence of cell performances on thealignment angles of GDLs． It is concluded that the electrochemical I－V performancesof PEMFCs with GDLs whose higher bending stiffness directions （i．e．， machine）are perpendicular or skew to the major flow field direction may be higher thanthose with GDLs whose higher stiffness directions are in parallel with themajor flow field direction．