这篇文章告诉我：燃料电池产物水中氟离子浓度越高，膜降级越快么？同一个材料可能是的，不同材料不正确。

读完你会知道：OCV测试膜寿命多样本的标准偏差有多大。

燃料电池开路运行空气侧尾排物质的离子色谱法分析与电解质膜寿命分布

Ion Chromatography and Combustion Ion Chromatography Analysis of Fuel Cell Effluent Water During Open Circuit Voltage

Michael A． Yandrasits

Sudha Marimannikkuppam

Matthew J． Lindell

Kyle A． Kalstabakken

Michael Kurkowski

Phuc Ha

Abstract

      Open circuit voltage tests were conducted on sixteen 3M Ionomer and eight Nafion™ NR211 membranes． Lifetime distributions were determined， and ion chromatography （IC） techniques were used to measure fluoride， sulfate， trifluoro acetate， and oxalate ions． Combustion Ion Chromatography （CIC） was used to determine the total organic fluoride （TOF） associated with water soluble membrane degradation fragments． The ratio of these products relative to the fluoride release rates were used to infer the likely degradation mechanism for each membrane． Peroxide attack at the sulfonic acid side chain was determined to be the least relevant reaction pathway while the long－proposed polymer chain end unzipping appears to be the dominant mechanism． Abstraction of the tertiary fluorine in the NR211 backbone and side chain is evident by organic fluoride release rates higher than can be explained by unzipping alone．

Cathode effluent water was collected approximately every other
day by routing the exit gas through a chilled stainless－steel vessel
and condensing the exhaust vapor to liquid water．

Figure 1． OCV potential for sixteen 3M 800EW ionomer membranes （solid lines） with fluoride （●）， sulfate （▲） and trifluoroacetate （♦） ion release rates．

主要产物是氟离子，硫酸根离子和三氟乙酸根，浓度分别为10＾2ug／cm2／day，0．5ug／cm2／day，0．3ug／cm2／day。

Figure 2． OCV potential for eight NR211 membranes （solid lines） with
fluoride （●）， sulfate （▲） and trifluoroacetate （♦） ion release rates．

氟离子，硫酸根离子和三氟乙酸根浓度分别为10＾2ug／cm2／day，0．5ug／cm2／day，10ug／cm2／day。

3M的膜OCV耐受性大约为NR211的两倍。

但是同一个工况为什么结果这么分散？

The same concept can be applied using a combustion ion chromatography method for measuring total organic fluoride （TOF） by splitting the sample into two analysis streams．The first stream detects inorganic fluoride ions by conventional IC methods， and the second stream combusts the sample thereby converting any organic carbon－fluorine bonds to carbon dioxide and fluoride ions． The total fluoride ions， which contains both total organic fluoride （TOF） and inorganic fluoride （IF）， are subsequently quantified by another IC measurement．

竟然还有草酸根。

把后面的机理拉过来

Scheme 5． Anticipated reactions for tetrafluoroethylene oxide resulting in oxalic acid．

Figure 4． Selected OCV， fluoride （● or ○）， and total organic fluoride （□）
data for three 3M 800EW ionomer membranes．

Figure 5． Selected OCV， fluoride （● or ○）， and total organic fluoride （□）
data for two Nafion™ NR211 membranes．

Scheme 1． Expected products for end group unzipping reaction originating at the carboxylic acid． 3M Ionomer shown above （1a） and NR211 shown below （1b）．

Scheme 2． Expected products for degradation reaction originating at the sidechain sulfonic acid． 3M Ionomer shown above （2a） and NR211 shown below （2b）．

One of the most useful measures of membrane degradation is the
fluoride release rate （FRR）． Fluorine atoms comprise over 65 weight
percent of typical PFSA ionomers and the most frequently invoked
“unzipping” mechanism proposed by Curtin and others suggests
much of the polymer will be degraded to fluoride and carbon dioxide in the presence of hydroxyl radicals．

Scheme 3． Expected products for degradation reaction originating at the tertiary fluorine or ether link． 3M Ionomer shown above （3a）， NR211 backbone linkage
only shown in the middle （3b）， and both NR211 linkages shown below （3c）．

Figure 6． Sulfate release rate vs fluoride release rate for 3M Ionomer samples （left） and NR211 （right）． Expected ratio of sulfate to fluoride ion for Scheme 2 shown with dashed line．

Figure 7． Organic fluoride vs inorganic fluoride ions for 3M Ionomer （left） and NR211 （right）． Expected ratio of organic to inorganic fluoride form Schemes 1 and 3 shown in dashed lines

实验结果支持降解路径1

Figure 8． Fluoride ions associated with TFA vs fluoride ions． Expected ratio for Scheme 3b shown with dashed line．

出现了几个问题，虽然路线是一样的，但是氟离子浓度和选用的材料相关，你还敢于文献数据用于你所用的不知道具体是什么结构的膜的降解浓度过程么？

3M的膜的降解产物浓度高但因为结构不同，并不能说明降解产物浓度高耐久性就差，需要具体问题具体分析。燃料电池产物水中氟离子浓度越高，膜降级越快么？同一个材料可能是的，不同材料不正确。

Scheme 4． Proposed degradation reaction from reference 13 resulting in tetrafluoroethylene oxide．

Conclusions

A series of 3M Ionomer and NR211 membranes without radical
scavenging additives or mechanical support were evaluated for
durability using the DOE OCV test． Lifetimes were fit to a
lognormal distribution as is typical for durability testing， with 3M Ionomer membranes having longer lifetimes despite higher fluoride
release rates．

插播：Several distributions were evaluated for these
data with normal， lognormal， and Weibull distributions all having
high correlation coefficients． The best fit was the lognormal
distribution with correlations of 0．987 and 0．984 for the 3M and
NR211 samples， respectively．

Ion chromatography and combustion ion chromatography analysis of cathode effluent water was used to infer relative
significance of three proposed degradation schemes． Sulfate ion
release rates were used to rule out meaningful degradation originating
at the terminal end of the sulfonic acid side chain． Total
organic fluoride methods were used to determine the amount of
fluorine containing， water soluble， membrane fragments． The ratio of
organic fluorine to inorganic fluoride is consistent with backbone
unzipping as the primary degradation pathway． The NR211 membranes， however， have a higher ratio implying degradation at the
backbone or side chain tertiary fluorine to be an important secondary degradation pathway． This conclusion is supported by the detection of trifluoro acetate in the NR211 membrane effluent water．
Interestingly， low levels of oxalate are detected in both the 3M
Ionomer and NR211 samples． The levels of this ion have no
correlation to the fluoride， or other ion， release rates and no
conclusions can be drawn from these data yet．

The combination of fluoride and organic fluorine values were
used to estimate membrane mass loss for both types of membrane．
The 3M Ionomer membranes lose at least 20％ of their mass while
NR211 is closer to 10％． These values represent inventories based on
cathode water only and therefore are likely to be underestimates as
degradation products are also apt to escape through the anode
effluent stream．

Lastly， it is well known that the conditions used in this study are
especially harsh favoring rapid membrane degradation． Cells under
normal operating conditions have orders of magnitude lower
degradation rates resulting in very low levels of fluoride ion release
and organic fluorine compounds．The techniques described in
this work can be applied to these non－accelerated operating conditions to better capture normal use degradation mechanisms and
validate the OCV or other accelerated methods．