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首页> 外文期刊>The journal of physical chemistry, B. Condensed matter, materials, surfaces, interfaces & biophysical >Membrane-derived fluorinated radicals detected by electron spin resonance in UV-irradiated nafion and dow ionomers: Effect of counterions and H2O2
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Membrane-derived fluorinated radicals detected by electron spin resonance in UV-irradiated nafion and dow ionomers: Effect of counterions and H2O2

机译：通过电子自旋共振在紫外线辐照的母体和陶氏离聚物中检测到的膜衍生氟化自由基：抗衡离子和H2O2的作用

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Electron spin resonance (ESR) spectroscopy was used to detect and identify radicals formed by UV irradiation of Nafion and Dow perfluorinated membranes partially or fully neutralized by Cu(II), Fe(II), and Fe(III). This method allowed the monitoring of ESR signals from the paramagnetic counterions together with the appearance of membrane-derived radical species. The most surprising aspect of this study was the formation of membrane-derived radical species only in the neutralized membranes, and even in the absence of H2O2 in the case of Nafion/Cu(II) and Nafion/Fe(III). In Nafion/Cu(II), ESR spectra from radicals exhibiting hyperfine interactions with three equivalent F-19 nuclei (the "quartet") and with four equivalent F-19 nuclei (the "quintet") were detected. In Nafion/Fe(II) exposed to H2O2 solutions, the formation of Fe(III) was detected. Upon UV irradiation, strong signals from the chain-end radical ROCF2CF2* were detected first, followed by the appearance, upon annealing above 200 K, of the quartet signal observed in Nafion/Cu(II). In subsequent experiments with Nafion and Dow membranes neutralized by Fe(III), the ROCF2CF2* radicals were formed even in the absence of H2O2, indicating that the role of H2O2 is oxidation of Fe(II) to Fe(III); moreover, in these systems small amounts of the chain-end radicals were detected even without UV irradiation. This result validates the method used to form the radicals: the role of UV irradiation is to accelerate the formation of a signal that is produced, albeit slowly, even in the dark, and possibly during fuel cell operation. The major conclusion is that cations are involved in degradation processes; the point of attack appears to be at or near the pendant chain of the ionomer. Therefore when studying membrane stability, it is important to consider not only the formation of oxygen radicals, such as HOcenter dot, HOOcenter dot, and O-2(center dot-), that can attack the membrane but also the specific reactivity of counterions.
机译：电子自旋共振（ESR）光谱用于检测和鉴定由Nafion和Dow全氟化膜部分或完全被Cu（II），Fe（II）和Fe（III）中和的紫外线照射形成的自由基。这种方法可以监测来自顺磁抗衡离子的ESR信号以及膜衍生的自由基物种的出现。这项研究最令人惊讶的方面是仅在中和的膜中形成了膜衍生的自由基物种，对于Nafion / Cu（II）和Nafion / Fe（III），甚至在不存在H2O2的情况下也是如此。在Nafion / Cu（II）中，检测到自由基显示的ESR光谱与三个当量的F-19核（“四重态”）和四个当量的F-19核（“五重态”）发生超精细相互作用。在暴露于H2O2溶液的Nafion / Fe（II）中，检测到Fe（III）的形成。在紫外线照射下，首先检测到来自链端基团ROCF2CF2 *的强信号，然后在200 K以上退火后，在Nafion / Cu（II）中观察到四重信号的出现。在随后的用Fe（III）中和的Nafion和Dow膜实验中，即使不存在H2O2，也会形成ROCF2CF2 *自由基，这表明H2O2的作用是将Fe（II）氧化为Fe（III）。此外，在这些系统中，即使没有紫外线照射，也能检测到少量的链端基。该结果证实了用于形成自由基的方法：UV辐射的作用是加速生成的信号的形成，尽管该信号即使在黑暗中也很慢，甚至在燃料电池运行过程中也是如此。主要结论是阳离子参与降解过程。攻击点似乎在离聚物的侧链上或附近。因此，在研究膜稳定性时，不仅要考虑氧自由基的形成（例如HOcenter dot，HOOcenter dot和O-2（center dot-）），这些自由基会攻击膜，而且还应考虑抗衡离子的比反应性，这一点很重要。

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