不同厚度的电解质膜燃料电池的OCV衰减速率差异的原位诊断

12年前的论文，本来是想闭眼睛想到结论，发现自己错了。

Degradation of a polymer exchange membrane fuel cell stack with Nafion® membranes of different thicknesses： Part I． In situ diagnosis

Xiao－Zi Yuan

Shengsheng Zhang

Haijiang Wang

Jinfeng Wu

Jian Colin Sun

Renate Hiesgen

K． Andreas Friedrich

Mathias Schulze

Andrea Haug

Abstract

The durability of polymer exchange membrane （PEM） fuel cells， under a wide range of operational conditions， has been attracting intensive attention， as durability is one of the largest barriers for commercialization of this promising technology． In the present work， membrane electrode assembly （MEA） degradation of a four－cell stack with Nafion membranes of different thicknesses， including N117， N115， NR212， and NR211， was carried out for 1000 h under idle conditions． By means of several on－line electrochemical measurements， the performance of the individual cells was analyzed at different times during the degradation process． The results indicate that the cells with thinner membranes have a lower open circuit voltage （OCV） due to the higher fuel crossover． Before degradation， the thickness of the membranes correlates with performance of the cell． However， with the advancement of degradation， the performance of cells with thinner membranes degraded much faster than those with thicker membranes， especially after 800 h of operation． The fast performance degradation for thinner membranes is evident by a dramatic increase in hydrogen crossover indicating membrane thinning or pinhole formation．

Table 1 Properties of Nafion® PFSA membranes．

Fig． 1． 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）．

Table 2 Degradation rate of individual cells with membranes of different thicknesses for each session

前200hr和后面的衰减速率不同

800hr－1000hr最薄的两种膜的电池出现快速衰减，但膜从25微米增厚到50微米并不能使耐OCV的时间延长，只是减缓了开路下降的衰减速率而已。但是这个结论不适用于更厚的电解质膜。

Fig． 2． OCV degradation trend for individual cells under idle conditions for 1000 h of degradation．

Fig． 3． Baseline polarization curves for each cell with membranes of different thicknesses before degradation （operating conditions： stoichiometry of H2／air ＝ 1．5／2．5 with a minimum flow rate of H2／air ＝ 1／2 slpm， cell temperature 70 ◦C and fully humidified on both sides）．

并不是只有电极可以找出传质极化随电流增加而增大，膜变厚也会在大电流造成显著的传质极化。

Fig． 4． Polarization curves for each cell with membranes of different thicknesses after 1000 h of degradation （operating conditions： stoichiometry of H2／air ＝ 1．5／2．5 with a minimum flow rate of H2／air ＝ 1／2 slpm， cell temperature 70 ◦C and fully humidified at both sides）．

最厚的两种膜几乎没有性能衰减。

Fig． 5． （a） Comparison of hydrogen crossover through MEAs with membranes of different thicknesses before degradation and （b） comparison of hydrogen crossover through the MEA with NR211 membrane before and after degradation．

This measurement confirms the dominant degradation mode switch from catalyst surface area loss to membrane decay．

800hr已经开始出现短路电流

Fig． 6． The effect of thickness on hydrogen crossover through membranes at different times of degradation．

Fig． 7． Comparison of HFR for cells with membranes of different thicknesses before and after every 200 h of degradation．

As degradation proceeds，the membranes have been fully humidified， and the structure change in the membrane becomes crucial， leading to a HFR increase with degradation time．

Fig． 8． Comparison of LFR for cells with membranes of different thicknesses before and after every 200 h of degradation．

薄膜中a decrease in charge transfer resistance with current due to the
increased driving force

Conclusions

Using a four－cell stack with Nafion membranes of different
thicknesses， an accelerated stress test under idle conditions was
carried out． The results show that the open circuit voltage （OCV）
of the individual cells decreases with decreasing the membrane
thickness， and it decreases much faster for thinner membranes，
especially after 800 h of operation． Before degradation， the thinner
the membrane， the better the cell performs． However， with the
progress of degradation， the cells with thinner membranes degrade
much faster with an average degradation rate of approximately
0．18mVh−1 for NR212 and 0．26mVh−1 for NR211， especially after
800 h， while cells with thicker membranes retain a slight degradation
rate throughout the test with an average degradation rate of
approximately 0．09mVh−1． The results indicate that it is the membrane degradation that accounts for the major source of the entire
cell performance degradation under idle conditions． The main reason for the drastic performance decay that occurred after 800 h for
thinner membranes is the dramatic increase in hydrogen crossover，
while the moderate degradation for the first 800 h may result from
the dominance of different degradation mechanisms， e．g．， catalyst
decay． Further post－analyses of the degraded MEA samples are currently being conducted to better understand the failure modes of
this degradation．