聚合物膜燃料电池膜中自由基淬灭添加剂在不同过程中的化学态

Clarifying the chemical state of additivesin membranes for polymer electrolyte fuel cells by X－ray absorption finestructure

Toshihiro Tanuma

Takanori Itoh

Abstract

Cerium and manganese compounds are used inthe membrane for polymer electrolyte fuel cells （PEFCs） as radical scavengersto mitigate chemical degradation of the membrane． The chemical states of ceriumand manganese in the membrane were investigated using a fluorescence X－rayabsorption fine structure （XAFS） technique． Membrane electrode assemblies（MEAs） were subjected to open circuit voltage （OCV） condition， under whichhydroxyl radicals attack the membrane； a shift in absorption energy in X－rayabsorption near edge structure （XANES） spectra was compared between Ce－ andMn－containing membranes before and after OCV testing． In the case of theCe－containing MEA， there was no significant difference in XANES spectra beforeand after OCV testing， whereas in the case of the Mn－containing MEA， there wasan obvious shift in XANES absorption energy after OCV testing， indicating thatMn atoms with higher valence state than 2＋ exist in the membrane after OCVtesting． This can be attributed to the difference in the rate of reduction；the reaction of Ce4＋ with ·OOH is much faster than that of Mn3＋ with ·OOH，leaving some of the Mn atoms with higher valence state． It was confirmedthat cerium and manganese redox couples reduced the attack from radicals，mitigating membrane degradation．

制备过程：

Powder of MnCO3 and Ce2（CO3）3 was added  to PFSA polymer dispersion （solid content 28．1 wt％， 910 equivalent weight  （EW）， dispersion in ethanol／water ＝ 6／4； Asahi Glass Co．， Ltd．） so that each cation  in the separate polymer dispersion became 5．5 mol／g－polymer （ca． 0．3 mol％），  then mixed with a magnetic stirrer until the carbonates were dissolved  completely．

Fig． 1． Chemical structure of the PFSA  polymer．

Fig． 2． Ce LIII－edge （top） and Mn K－edge  （bottom） XANES spectra （after background

subtraction） of a Ce／Mn containing  polymer electrolyte membrane and standard materials

（Std． 1－6）．

Fig． 3． Comparison of XANES absorption  energies before and after continuous MEA operation at 80 C， 100 ＆ 50％RH  for 150 h．

Table 1 Reactions involving HO．， HOO．， Ce  and Mn ions．

参考文献只到28：（，也不知道怎么弄的。

Fig． 4． Comparison of XANES absorption  energies before and after OCV testing．

Conclusions

The fluorescence XAFS technique was  applied to investigate the chemical state of Ce and Mn compounds that are  added to the polymer electrolyte membrane for fuel cells in order to improve durability  of the membrane． After continuous operation of the MEA， which uses membranes  containing Ce2（CO3）3 and MnCO3， at 0．2 A／cm2 for 150 h under both wet and dry  condition， little impact on the chemical state of Ce and Mn was observed．  When the MEAs being subjected to OCV condition for 150 h， under which  hydroxyl radicals attack the membrane， there was a difference in XANES absorption  energy level E0 before and after OCV testing of the Ce and Mn－containing  membranes． In the Mn K－edge XAFS spectra， absorption energy level E0  increased distinctly after OCV testing， whereas no difference was measured in  absorption energy level E0 for the Ce－containing membrane． This is most  likely due to the difference in the rate of reduction； the reaction rate of  Ce4＋ with ．OOH is over ten times faster than that of Mn3＋ with ．OOH， leaving some  of the Mn atoms with higher valence state and giving an increase in  absorption energy level E0．

The actual chemical states of Ce and  Mn in the polymer electrolyte membranes were directly observed using the  fluorescence XAFS technique for the first time．  It was confirmed that these additives within the membrane reduce the membrane  degradation rate through redox mechanism．

可以和丰田MIRAI 20万公里实车耐久测试后膜电极中铈的空间分布、价态原位分析的论证过程对比着看，但是没有引用关系。