不同厚度的电解质膜燃料电池的OCV衰减速率差异的离线失效分析

Degradation of a PEM fuel cell stack with Nafion® membranes of different thicknesses． Part II： Ex situ diagnosis

Xiao－Zi Yuan

Shengsheng Zhang

Shuai Ban

Cheng Huang

Haijiang Wang
Vengatesan Singara

Michael Fowler

Mathias Schulze

Andrea Haug

K． Andreas Friedrich
Renate Hiesgen

Abstract

Part I of this study carried out membrane electrode assembly degradation of a four－cell stack with Nafion membranes of different thicknesses， including N117， N115， NR212， and NR211， for 1000 h under idle conditions． Through on－line electrochemical measurements it was found that as degradation advanced， cells with thinner membranes experienced much more rapid performance degradation than those with thicker membranes， especially after 800 h of operation， due to a dramatic increase in hydrogen crossover． In the present work we investigate the degradation mechanisms of this four－cell stack using several ex situ diagnostic tools， including scanning electron microscopy （SEM）， infrared （IR） imaging， ion chromatography （IC）， gas permeability measurement， contact angle measurement， and simulation． The results indicate that the drastic increase in hydrogen crossover is due to membrane thickness loss and pinhole formation．

Fig． 1． Chemical structure of Nafion． In this work， the equivalent weight is chosen to be about 1100 g of dry Nafion per mole of sulfonic acid groups， with x ＝ 6．5 and y ＝ m ＝ 1．

Fig． 2． Voltage degradation trends for individual cells under idle conditions for 1000 h （operating conditions： flow rate H2／air ＝ 1／2 slpm， cell temperature 70 ◦C and fully humidified on both sides）．

Fig． 3． Comparison of SEM images for NR211 samples before and after degradation， at different magnifications： （a） before degradation， at a magnification of 200×， （b） after degradation， at a magnification of 200×， （c） before degradation， at a magnification of 1．0k×， and （d） after degradation， at a magnification of 1．0k×．

Fig． 4． Comparison of SEM images for NR212 samples before and after degradation， at different magnifications： （a） before degradation， at a magnification of 180×， （b） after degradation， at a magnification of 180×， （c） before degradation， at a magnification of 1．0k×， and （d） after degradation， at a magnification of 1．0k×

Fig． 5． Comparison of SEM images for N115 samples before and after degradation， at different magnifications： （a） before degradation， at a magnification of 200×， （b） after degradation， at a magnification of 200×， （c） before degradation， at a magnification of 500×， and （d） after degradation， at a magnification of 500×．

Fig． 6． Comparison of SEM images for N117 samples before and after degradation， at different magnifications： （a） before degradation， at a magnification of 70×， （b） after degradation， at a magnification of 70×， （c） before degradation， at a magnification of 200×， and （d） after degradation， at a magnification of 200×．

Table 1 Membrane thickness change before and after degradation．

活化后受压后厚度稍有变化

薄电解质厚度降低的比例和厚电解质厚度降低的比例并不相同。

电解质厚度降低的绝对值也不相同。

NR212膜衰减至14微米已经无法有效工作，膜厚度减薄至32％，而目前先进的膜新状态仅为8－10微米。

Table 2 Catalyst layer thickness change before and after degradation．

催化剂变化不明显

Fig． 7． Comparison of IR images for fresh MEA samples with different membranes using 5％ H2 in N2： （a） fresh N117 MEA （average temp： 23．091 ◦C）， （b） fresh N115 MEA （average temp： 23．429 ◦C）， （c） fresh NR212 MEA （average temp： 23．724 ◦C）， and （d） fresh NR211 MEA （average temp： 24．063 ◦C）．

平均温度有轻微的差异。膜越薄平均温度越高。

Fig． 8． Comparison of IR images for N117 MEA samples before and after degradation under idle conditions using 5％ H2 in N2： （a） before degradation and （b） after degradation．

Fig． 9． Comparison of IR images for N115 MEA samples before and after degradation under idle conditions using 5％ H2 in N2： （a） before degradation and （b） after degradation．

Fig． 10． Comparison of IR images for NR212 MEA samples before and after degradation under idle conditions using 5％ H2 in N2： （a） before degradation and （b） after degradation．

Fig． 11． Comparison of IR images for NR211 MEA samples before and after degradation under idle conditions using 5％ H2 in N2： （a） before degradation and （b） after degradation．

Table 3 Overall comparison of fresh and degraded MEA permeability

Fig． 12． Pictures of bubbles after 1000 h of degradation under idle conditions using 5％ H2 in N2： （a） degraded NR211 and （b） degraded NR212．

截面SEM显示的是平均减薄，如果有透气位置的截面图像会更直观。电解质在均匀减薄的基础上出现某些位置的特异性减薄。

Fig． 13． Permeation of helium （a） through highly permeable MEAs and （b） through less permeable MEAs．

• non－permeable： no gas flow across the MEA until 50 psi
• less permeable： very low gas flow， indicated by bubble formation
at longer intervals
• highly permeable： considerable gas flow across the MEA， measured
by rotameter

因此使用了两种方法测试渗漏率

不均匀膜严格意义上不能使用渗透率进行估算。初步估算一下7微米膜5．22cm3．cm／cm2／s／cmHg，14微米膜2．16cm3．cm／cm2／s／cmHg，两个数据不一致说明膜分布针孔的密度还是不一样的，NR211针孔密度更高，这也好理解。

Table 4Comparison of contact angle change of GDLs．

Table 5Comparison of contact angle change of CLs．

Conclusions

Using a four－cell stack with Nafion membranes of different
thicknesses， an accelerated stress test under idle conditions for
1000 h was carried out． The results indicate that under these conditions， membrane degradation is the major source of the overall
cell performance degradation． The predominant reason for the
drastic performance decay that occurred after 800 h for thinner
membranes is the dramatic increase in hydrogen crossover， caused
by significant membrane thickness loss and pinhole formation．
Although the performance of thin membranes degrades much
faster than that of thick membranes， the thickness loss for the former is lower， as thin membranes have less material to degrade． The
mechanism of membrane thinning can be understood through the
simulation of Nafion weight loss via main chain unzipping． Apart
from membrane degradation， which is predominant in this accelerated
degradation test under idle conditions， other degradation
mechanisms also contribute to the total performance loss of the
stack – for example， catalyst degradation （thickness loss in the CLs）
and hydrophobicity loss in the GDL （contact angle decreases for
both GDLs）．