冠醚／铈配合物作为Nafion自由基猝灭剂对燃料电池膜耐氧化稳定性的影响

Enhancement of oxidative stability of PEM fuel cell by introduction of HO• radical scavenger in Nafion ionomer

Vo Dinh Cong Tinh

Dukjoon Kim

Abstract

The coordination complex of 18－crown－6 ether／cerium ions （CRE／Ce） is embedded in Nafion ionomer between catalyst and membrane layers in fabrication of membrane electrode assembly （MEA）． The anti－oxidation effects of CRE／Ce on ionomer and MEA are investigated not only by Fenton＇s test but also by FT－IR and XPS， ex－situ and in－situ fluoride ion （F−） emission rate， and cell performance measurements． While Ce3＋ plays an excellent role in scavenging HO• radicals， CRE alleviates migration of cerium ions out of the ionomer during cell operation． Both ex－situ and in－situ degradation rate tests show that the systems containing CRE／Ce emits much lower amount of fluoride and sulfonate （SO3−） ions than those without CRE／Ce． The open－circuit voltage （OCV） test at low humidity （30％） show that the cell containing the CRE／Ce is still alive even after 300 h operation， while other cells without it are terminated before it．

Fig． 1． （a） Self－regeneration mechanism of Ce3þ ions in the acidic medium and （b） degradation mechanism of Nafion structure by HO． radicals attack on C－O and C－S bonds．

图不清楚，但意思大家都理解了，和OH自由基和OOH自由基反应的是三价铈。

Table 1 IEC， hydration number at 40度， and mechanical properties of ionomers．

In this experiment， the amount of Ce3＋ ions were fixed at 10 wt ％ of ionomer in Nafion solution．

The thickness of the film was ～80 μm．

a mixture of 0．1g of Pt／C （40％）， 1 mL of DI water， 8．04 g of IPA， and 0．87 g of Nafion ionomer compound was sonically stirred for 30 min at room temperature to prepare a homogeneous solution．

没有支撑层，催化层中无铈

Fig． 2． TGA curves of the ionomer membranes．

Fig． 3． Temperature dependence of the water uptake of the ionomer
membranes．

Fig． 4． Cross－sectional SEM－EDS mapping of Ce－ionomer （a–c） and CRE／Ce－ionomer （d–f） membranes．

Fig． 5． The temperature dependence of the proton conductivity of ionomer membranes．

可以判断NafionXL中的铈不是直接添加三价铈，按照这种工艺实现的，或者10wt％ Ce含量太高了。

Table 2 The residual weight after treated in Fenton’s reagent at 80 度．

Fig． 6． The ions emission rate of Fenton’s reagent after different treatment time： （a） fluoride and （b） sulfonate group．

10 mL Fenton’s solution （3 wt％ H2O2， 4 ppm Fe2＋）80度处理后的残余溶液分析氟离子和磺酸根集团，The ionomer films （1＊3 cm2）

这个公式有些问题。

可控Nafion样品最终溶液的氟浓度0．035＊3＊24＊4＝10．08ppm（按公式）

或者10．08ug／10ml＝1ppm（按图像数据）。

Fig． 7． FT－IR spectra of ionomer membranes after different immersing time in Fenton’s reagent at 80度．

Fig． 8． （a） O 1s and （b） Ce 3d XPS spectra of ionomer membranes before and after treatment in Fenton’s reagent at 80 度．

这张图很有利的证明铈在膜表面的流失状态

Fig． 9． The fluoride emission rate of control MEA， Ce－MEA and CRE／Ce－MEA， （a） anode side， （b） cathode side and （c） both electrodes

Variation of voltage according to current （CV） or operation time
（OCV） was measured by a fuel cell station （SPPSN－300， CNL Energy，
Korea） at 90 度 and 30％ relative humidity （RH） at the various operation time．

并不像离线测试，在线测试只测试了氟离子浓度。

可控Nafion样品离线Fenton试剂96hr时0．035ugF－／cm2／hr

在线OCV测试0．0075ugF－／cm2／hr，100hr时0．0037ugF－／cm2／hr，在线大约是离线的1／10。

氢侧0．0095ugF－／cm2／hr，氧侧0．0057ugF－／cm2／hr，氢侧浓度是氧侧浓度的2倍。

the active area of 2．5＊2．5 cm2

在线OCV测试0．0075＊2．5＊2．5＊300＝14ppm（这里是累计浓度），这里只用用不正确的文献公式计算，因为根本不知道300hr OCV测试两侧总共收集到了多少液态水。

在作者的实验中添加铈对催化剂活化过电位影响很大。

Fig． 10． The cell performance of control MEA， Ce－MEA and CRE／Ce－MEA at （a） 0 h， （b） 120 h， （c） 240 h and （d） OCV test．

OCV降为0．65V以后就无法建立开路了。

Fig． 11． The images of the membrane－electrode assembly after OCV test （a） control MEA， （b） Ce－MEA and （c） CRE／Ce－MEA．

In this experiment， the amount of Ce3＋ ions were fixed at 10 wt ％ of ionomer in Nafion solution．我的理解是这篇文献使用了10wt％铈

含Keggin型多金属氧酸盐的新型电解质膜的OCV耐受性和自由基分解能力

中假设的10％mol Nafion H＋被取代，对应的铈的质量含量为0．42wt％Ce。这两个数据差距也太大了，重新读了一下原文，

Either 0．25 g of cerium （III） nitrate hexahydrate or 0．25 g of cerium
（III） nitrate hexahydrate／0．608 g CRE mixture was dissolved in 2 mL
DMSO to form the CRE／Ce coordination structure at room temperature．After stirring three days， these Ce3＋ ions or CRE／Ce solutions were dropped into the 50 g of Nafion ionomer solution under stirring of a magnetic bar for a day for the formation ionomer compound． In this experiment， the amount of Ce3＋ ions were fixed at 10 wt ％ of ionomer in Nafion solution．

M（cerium （III） nitrate hexahydrate）＝434．22g／mol

Ce量＝0．25g／434．22g／mol＊140g／mol＝0．08g

Nafion量＝50g＊0．05＝2．5g

Ce／Nafion＝0．08／（0．08＋2．5）＊100％＝3．1％

不是很理解哪里计算的不正确，但是这个含量也远大于含Keggin型多金属氧酸盐的新型电解质膜的OCV耐受性和自由基分解能力的计算用量。

文献中也有对商用膜中铈含量的测量数据，想知道么？可以给我留言，我悄悄告诉你。

Conclusion

As the weight loss and ex－situ fluoride emission rate of the Nafion
ionomer were significantly reduced by the presence of Ce3＋ during
Fenton’s test， its anti－oxidation ability was clearly proved， even though

the proton conductivity and water uptake were also negatively affected by it． Introduction of CRE along with Ce3＋ not only prolonged the antioxidation effect of Ce3＋ but also alleviated the reduction of proton conductivity and water uptake as CRE formed a coordination complex with Ce3＋ to prohibit its direct interaction with sulfonic acid groups of ionomer． FT－IR and XPS confirmed this Ce3＋ and CRE／Ce effect more profoundly， analyzing the chemical structure change during Fenton’s test． This HO． radical scavenging effect of Nafion ionomer was also found from the in－situ fluoride emission rate measured at the cathode and anode sites during cell operation． The ionomer containing Ce3＋ showed much lower fluoride emission rate than the control one in the first 150 h， but became similar to that of control one after 200 h owing to the mitigation of cerium ions toward outside．

这一点在文章中只是说，没有论证的结果。燃料电池膜电极加速测试中的元素成分迁移测量数据不支持这个结果。

The CRE／Ce containing MEA showed the lowest fluoride emission rate at both anode and cathode sides during 300 h． As the presence／absence of CRE／Ce3＋ in ionomer significantly affected the long term cell performance， the OCV test showed that the voltage of the control and Ce3＋ containing MEAs dropped much more quickly than CRE／Ce containing one for the same reason． From all of the experimental results， it was sure that the antioxidation enhancement of Nafion ionomer by addition of CRE／Ce played an important role in the eventual improvement of long term durability of MEA．