石墨双极板燃料电池加压恒流稳态1000hr耐久性测试传质区失效分析

Failure mechanism of PEM fuel cell underhigh back pressures operation

Chao Cai

Yan Rao

Yu Zhang

Fan Wu

Shang Li

Mu Pan

Abstract

Proton exchange membrane fuel cells（PEMFCs） are expected to function under relatively higher back pressures fortargeting higher outpower． Under this condition， the durability of fuel cellswill be a huge challenge for commercialization． In our study， a 1000－hdurability experiment was performed on a PEMFC to investigate the durabilityunder high back pressures． A semi－empirical fuel cell polarization curve modelwas used to separate the activation and concentration losses， and study theirchanges with testing time at different current densities． In addition， thecharge transfer resistance （Rct） related to oxygen reduction reaction （ORR） inthe catalyst layer was also investigated． Additionally， the mass transferresistance （Zd） was investigated using electrochemical impedancespectroscopy（EIS）． Moreover， the contact angle and energy－dispersive X－rayspectrum （EDX） of carbon paper surface were characterized． The resultsindicated that the increase in mass transfer resistance was the biggestcontributor to the loss of cell voltage with testing time． The decrease incontact angle of carbon paper surface implied that the weakening ofhydrophobicity contributed to an increase in the mass transfer resistance，which comes from the PTFE loss observed from EDX． This may result from theaggravated corrosion of carbon fiber or physical erosion induced byflooding in fuel cell under high back－pressure．

Fig． 1 e Schematic of the main componentsof PEMFC．

Fig． 2 e Physical graph of the individualcell．

Fig． 3 e Linear sweep voltammetry curvesfor different durability test time．

Fig． 4 e （a） Polarization curves and （b） iRcompensated I－V curves for various durability test time．

Fig． 5 e Variation in Tafel slope with thedurability test time．

Fig． 6 e Overpotential at different currentdensities （a）； A－Actual polarization curves at 0 h， B－Simulated polarizationcurves with no increase in the diffusion resistance in 1000 h， C－Actualpolarization curves after 1000 h （b）．

Table 1 e Changes in HFR with durabilitytest time．

Fig． 7 e Equivalent circuit．

Fig． 8 e （a） Electrochemical impedancespectroscopy for different durability test time； （b） Rct from equivalentcircuit modeling．

Fig． 9 e Contact angles of the fresh andaged GDLs．

Fig． 10 e Relative amount of C and F forcarbon paper surface （a） before the durability test； （b） anode after 1000 hlong durability test； （c） cathode after 1000 h long durability test．

这个表征不够敏感，耐久性前后原子分数差距都不到1％，接触角差异会变化那么多。

耐久性后阴极氟含量更低，而阴极接触角却略高。要么接触角各个区域差异很大，要么氟含量各个位置差异很大。

耐久性测试是恒流稳态测试The durability test wasperformed using the steady current method， for which the current density wasfixed at 800 mA／cm2．

气体扩散层commercial GDLs with micro－porouslayer （JNTG， 20－A3）

膜The membrane used in this MEA was a kindof Gore membrane with the thickness of 10 mm．

加压：The back pressures of cathode and anodewere maintained at 150 kPa（gauge pressure）．

比较常规的温度：The cell temperature was set to be75 C． The dew point temperatures of air and hydrogen （inlet gases） were set tobe 66．7 C．

Conclusions

In our study， a 1000－h long durabilityexperiment under high gas pressures was conducted on a PEMFC， and the reasonsfor the degradation of performance of fuel cell were analyzed using thesemi－empirical fuel cell polarization curve model， high－frequency resistance（HFR） method， electrochemical impedance spectroscopy， linear sweep voltammetry，contact angles and EDX analysis of GDL．

The results indicated that the main reasonfor the deterioration was the decline in water management capability of GDL，rather than the deterioration of the membrane and catalyst． When a fuel cellwas operated under high back－pressure， the water in it cannot be easily carriedaway by the tail gas． Therefore， flooding tends to form in the GDL of the fuelcell， which will aggravate the corrosion of carbon and PTFE loss， thusresulting in the decline of water management capability of GDL．

碳腐蚀通常指的是催化层的碳载体，文章未提供耐久性测试后的膜电极的截面图像和催化剂颗粒的变化，这个原因有些牵强。

不知道水管理的问题出在气体扩散层的基材层还是催化剂层。

不知道更换气体扩散层基材，失效现象是否相同。

This was confirmed by the EDX resultsbefore and after the durability test， and would greatly shorten the life offuel cell． More stable GDL should be developed to meet the needs of PEM fuelcell having high performance and long lifetime．