含Keggin型多金属氧酸盐SiW12O405－的新型电解质膜的OCV耐受性和自由基分解能

Chemical Stability via Radical Decomposition Using Silicotungstic Acid Moieties for Polymer Electrolyte Fuel Cells

Andrew R． Motz

Mei－Chen Kuo

Guido Bender

Bryan S． Pivovar

Andrew M． Herring

Abstract

Chemical degradation of perfluorinated sulfonic acid membranes， such as Nafion， via radical attack， represents one of the current challenges of fuel cell durability． Here we report on a recent breakthrough in chemical durability that has been achieved through using covalently attached heteropoly acid （HPA） moieties as both the proton conducting acid and the radical decomposition catalyst． Exceptional chemical durability is reported for a thin （25 μm） film in an accelerated stress test that eventually had an open circuit voltage decay rate of 520 μV h−1， which was shown to be a result of the formation of an electrical short after thinning due to mechanical stresses． A mechanism is proposed using density functional theory in which the W atoms in the HPA reversibly change oxidation state from W（VI） to W（V） while decomposing radical species． Using rate constants found in the literature and realistic concentrations of scavenging species， it is hypothesized that the rate of radical decomposition can be ＞35x faster for HPA containing membranes than it is for Ce doped films． It is concluded that covalently tethered HPA should be considered as a next generation chemical stabilization strategy for polymer electrolyte fuel cells．

Figure 1． Structure of silicotungstic acid functionalized poly（vinylidene fluoride－co－hexafluoropropylene） （PolyHPA）

产物中m嵌段CF2－CH2不是CF2－CF2。

Figure 2． Fuel cell performance data at the beginning of the test at 80◦C with saturated inlet gases operating with H2／O2．

Figure 3． Cell voltage over time for the OCV hold test performed at 90◦C， 30％RH， under H2／Air， and an absolute pressure of 101．3 kPa． The HPA based film （－） had an initial voltage of 0．98 V and a final voltage of 0．72 V． Data for Nafion 211 （NRE－211） （红） and a sulfonated phenylated polyphenylene membrane （SPPB－H） （蓝） are shown for comparison．

A 50 cm2 square fuel cell was fabricated using Johnson Matthey ELE0162 gas diffusion electrodes （GDEs） with a geometric platinum loading of 0．35 mg Pt cm−2 for both the anode and cathode．

GDE内仍然采用Nafion体系。

The OCV decay rate of the PolyHPA （25 μm， 520 μV h−1， 0–500 h） is an improvement on that for Nafion N211 （25 μm， 2480 μV h−1， 0–100 h） and a chemically durable hydrocarbon membrane （33 μm，737 μV h−1， 0–300 h）．

Figure 4． Crossover current at 0．40 V for the LSV test at different points throughout the OCV hold．

Figure 5． Different elements of crossover during the OCV hold．

这是这种膜特有的特点，出现膜短路特性。

表5中列出了很多反应常数。没有从动力学方向思考自由基反应这个问题。

PFSA的反应常数是封端集团的反应常数

因此选取的是TFA的反应参数。

聚合物含有α－SiW12O405－基团，可以看做一种Keggin型多金属氧酸盐POM，

对于Dawson型多金属氧酸盐P2W18O626－而言，

作者表现出很强的文献挖掘能力。

The silicotungstic acid （HSiW） functionalized poly（vinylidene fluoride－co－hexafluoropropylene） （PolyHPA） membrane was synthesized as previously reported and contained 70 wt％ K8SiW11O39．

文中计算这个10％的H＋被三价铈取代，没有提供计算来源。也没有看别人对这个Nafion－211的分析。

Conclusions

The chemical stability of a 25 μm PolyHPA membrane was tested with an OCV accelerated stress test （H2， Air， 90◦C， 30％RH） and demonstrated outstanding chemical stability with a decay rate 520 μV h−1． The fuel cell suffered an electrical short after 200 h， resulting in increasing current density in the LSV crossover test， indicating mechanical support is needed for the practical use of this material．

The literature was reviewed for rate constants for reactions of HPAs with radicals and indicates that the rate constant for radical decomposition can be over an order of magnitude faster for HSiW than for Ce or Mn． DFT calculations were performed and thermochemistry arguments were used to develop two plausible reaction networks for radical decomposition and scavenger regeneration， with the most likely mechanism involving a ［SiW12O40−4］－H intermediate． The calculations were used to demonstrate how the higher concentration of HPA （ca． 0．7 M） could result in even more advantages for radical scavenging， as the overall rate depends on both concentration and rate constant． Future work should include mechanical support of thin membranes to avoid electrical shorting， incorporation of PolyHPA into the electrodes to reduce interfacial transport resistances arising from Nafion／PolyHPA interfaces， and studies on combined chemical and mechanical stress testing to probe the effects of migration． Lastly， the mechanism should be verified experimentally． The use of covalently tethered HSiW offers many benefits over traditional radical mitigation strategies and warrants further study．