膜电极入口、中间和出口的人工针孔对燃料电池局部极化性能、电流密度、渗氢电流、电压、化学气氛、碳腐蚀、氟离子释放速率的影响

Effect of pinhole location on degradationin polymer electrolyte fuel cells

Merit Bodner

Christoph Hochenauer

Viktor Hacker

Abstract

This work analyses the impact of the location of pinholes in polymer electrolyte fuelcells on the degradation of the electrodes． Defects with a diameter of 0．45 mm were created in a 25 cm2membrane electrode assembly （MEA） of a fuel cell． The MEA was operated andcharacterised using a segmented single cell． The effects of the pinholes ondegradation were measured and evaluated． Defectsaffected the fuel cell behaviour during periods of hydrogen starvation， thusaccelerating the degradation process of the carbon support as well as the lossof active platinum catalyst surface area． Furthermore， the effects of theinduced pinholes on membrane degradation and performance decay were determined．Pinholes close to the anode inlet ingeneral have shown a more severe effect on the fuel cell operation parameters，such as open circuit voltage， performance， membrane resistance and hydrogencrossover， than pinholes at any other locations． Also， electrode degradation was accelerated． These effects were mainly due to locally increased temperatures．

Fig． 1． Locations of pinholes created by  MEA perforation of the 25 cm2 segmented cell， with the first set of pinholes  （1－3） near anode inlet， second set of pinholes （4，5） in the middle section，  and third set of pinholes （6，7） near the cathode inlet．

注意空气入口的位置

The MEA was perforated from the cathode  to the anode side with pinholes using a 0．45 mm needle． This diameter was  carefully chosen， considering two key aspects： enabling an observable effect  of the introduced defect on the fuel cell operation， without disabling the latter．

A single pinhole represents a damage of  0．16 mm2 or 0．006％ of the surface area of a cell．

这种针孔尺度和文献中实际遇到的针孔尺寸（1500hr耐久性测试增湿器串漏造成燃料电池电解质膜失效，探测针孔的泄露法、红外热成像和电压弛豫法对比和局限性75－200微米、COCV加速测试工况下的燃料电池膜降解：膜均匀减薄叠加形成针孔100微米）相当。

Table 1 Operation conditions

Starvation interval：To enable gas  analysis， nitrogen was added to the anode gas flow．

Table 2 Results from the electrochemical  characterisation．

1，1、2，1、2、3分别指的是针孔在1号位置之后，同时在1、2号位置之后，同时在1、2、3号位置之后。

Fig． 2． Polarisation Curves 1） a） with no  pinhole and with pinholes at locations 1， 1 and 2 and 1 to 3， b） with no  pinhole and with pinholes at the locations 6 and 6 and 7，

and current distribution at 400 mA／cm2 2）  a） without pinholes， b） after the first perforation at the anode inlet （row  3／column 10， position 1）， c） after the second perforation of the same segment  （position 2） and d） after the additional perforation （row 3／column 1，  position 3）， 3） a） without pinholes， b） after the first perforation （row 1／column  10， position 6） and c） after the additional perforation （row 1／column 1，  position 7） at the cathode inlet．

这些图对视力要求好高，如果有个数据标识在图上辨识度会好很多。

Fig． 3． Hydrogen diffusion current  distribution 1） a） without pinholes， b） after the first perforation at the  anode inlet （row 3／column 10， position 1）， c） after the second perforation of  the same segment （position 2） and d） after the additional perforation （row  3／column 1， position 3）

第三个孔没有造成位置3的局部渗氢电流的上升，反而让位置1、2的局部渗氢电流的上升。

2） a） without pinholes， b） after the  first perforation at position 6 （row 1／column

10） and c） after the additional  perforation （row 1／column 1， position 7） at the cathode inlet．

Table 3 Hydrogen content in the cathode  flow， oxygen content in the anode flow and summarised carbon emission for  both anode and cathode during polarisation measurement in dependence of the  pinhole location， all normalised for the lowest value．

The off－gas on either the anode or the  cathode sidewas analysed with an online gas analyser （ABB） during the fuel  cell operation． The content of CO and CO2 was analysed with two Uras 14 units  set for different concentration ranges； hydrogen， with a Caldos 17 unit； and  oxygen， with a Magnos 106 unit．

Table 4 Hydrogen content in the cathode  off－gas stream during hydrogen diffusion measurements in dependence of the  pinhole location， all normalised for the lowest value

Fig． 4． Potential over five starvation  cycles for 1） the reference MEA and the MEA with pinholes at the positions 1，  1 and 2， and 1， 2 and 3， 2） reference MEA and the MEA with pinholes at the  positions 4 and 4 and 5， 3） the reference MEA and the MEA with pinholes at  the positions 6 and 6 and 7．

只有位置6和位置7不会造成反极。

Fig． 5． Current distribution during fuel  starvation 1） a） without pinholes， b） after the first perforation at the  anode inlet （row 3／column 10， position 1）， c） after the second perforation of  the same segment （position 2） and d） after the additional perforation （row  3／column 1， position 3）， 2） a） without pinholes， b） after the first perforation  at position 4 （row 2／column 8） and c） after the additional perforation at  position 5 （row 2／column 3） at the cathode inlet and 3） a） without pinholes，  b） after the first perforation at position 6 （row 1／column 10） and c） after  the additional perforation （row 1／column 1， position 7） at the cathode inlet．

电流密度似乎不是哪里有孔哪里下降。

Fig． 6． Gas analysis of the anode off－gas  with a） no pinhole， b） pinholes in the positions 1， 2 and 3 and the cathode  off－gas with c） no pinhole and d） pinholes in the positions 1， 2 and 3．

这张数据带来很多新的认知，我一直以为如果有针孔，阳极侧氢会和阴极侧对流过来的氧气在催化剂表面迅速发生反应，而实验告诉我们不仅有氧，还会因为有氧产生了大于对比样的二氧化碳。

Table 5 Hydrogen content in the cathode  flow， oxygen content in the anode flow and summarised carbon emission for  both the anode and cathode during starvation， depending on the pinhole  location， all normalised for the lowest value

Fig． 7． Fluoride emission rate 1） without  pinholes and for the pinholes near the anode inlet （1－3）， in the middle  section （4， 5） and near the cathode inlet （6， 7） and 2） different fluoride  emission rates without pinholes and for the pinholes near the anode inlet （1－3），  in the middle section （4， 5） and near the cathode inlet （6， 7） for anode and  cathode．

The off－gas， that did not undergo gas  analysis， passed through a condensation trap． The condensed water was mixed  with TISAB II， as received from Thermo Scientific， at a ratio of 50：50 and  analysed in regards of fluoride ion content with a 9609BNWP fluoride ion selective  electrode from Thermo Scientific．

TISAB II Total ionic strength adjustment  buffer II solution总离子强度调节缓冲剂 II 溶液

TISAB通常与样品混合，因为它有助于维持恒定的离子强度和pH值。它主要用于消除metal－F络合物的络合。TISAB II 可以用氯化钠和冰醋酸、1，2－环己二胺二丁四乙酸和氢氧化钠制备。它含有高浓度的阳离子螯合剂，这种螯合剂可以水解氟化物离子对以形成氟化物离子。TISAB II 可用于使用牙科清漆中的氟离子敏感电极测定总氟浓度。它还可用于在存在Fe（III） 和Si的情况下测定氟化物。

Fig． 8． Computed tomography of a pinhole  at position 4．

Table 6 Summary of the effects of the  pinhole location on different electrochemical characteristics．

Table 7 Summary of the effect of the  pinhole location on different non－electrochemical characteristics．

Conclusion

The effect of pinholes at various locations  on the degradation of the membrane has been investigated． Furthermore， the  fuel cell was characterised and the impact of the induced defects on the chosen  parameter was observed．

During the operation of a defective fuel  cell， multiple different effects overlap． The crossover of hydrogen and oxygen，  temperature and current distribution， as well as the humidity do interact，  but a clear correlation was not always evident．

It can be concluded， that the effect  which pinholes have on the fuel cell operation is the strongest near the  anode inlet and the middle section． The higher hydrogen partial pressure  results in an increase of the local temperatures for defects in this area．  The unevenly increased temperature temporarily leads towards improved operation  parameters， but a sudden drop occurs after a critical point is reached and  the positive effect of the elevated temperature is exceeded by the negative  effects of the defects， such as carbon corrosion and performance decay．  Therefore， excessive crossover has to be avoided．

Many of the results support the conclusion  that one of the major effects of  membrane defects on fuel cell characteristics under operation conditions is  an uneven temperature distribution rather than an increased membrane degradation  rate due to hydrogen peroxide formation． The temperature is increased  locally near the area of  perforation． This accelerates the processes of carbon corrosion and platinum electrocatalyst agglomeration．

Contrary to what was expected， the increase of carbon corrosion was even  stronger affected by the potential than by high local temperatures． The  carbon oxidation rate is driven by cell reversal during starvation due to the  rising anode potential． When pinholes were introduced near the cathode inlet，  a small effect on fuel starvation was observed， while pinholes introduced in  the middle section showed an intermediate effect， and pinholes near the anode inlet induced a larger cell potential drop，  resulting in a higher carbon emission rate． The cell reversal is also  correlated to the loss of the cathode＇s active platinum surface area． Near  the anode inlet， pinholes clearly contribute to the degradation rate of the  carbon support and the Pt catalyst．

Under  the chosen conditions， defects near the anode inlet show a significant  acceleration of further degradation， whilst defects near the cathode inlet  have only a small effect．

从事燃料电池注定了你不一定出类拔萃，但必定与众不同。

这里专注科技阅读，内容硬，无翻译，偶尔吐槽表述上海一隅生活点滴。