Kadirov Marsil K, Bosnjakovic Admira, Schlick Shulamith
Department of Chemistry and Biochemistry, University of Detroit Mercy, 4001 West McNichols, Detroit, Michigan 48221, USA.
J Phys Chem B. 2005 Apr 28;109(16):7664-70. doi: 10.1021/jp044987t.
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 19F nuclei (the "quartet") and with four equivalent 19F 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 HO*, HOO*, and O2*-, that can attack the membrane but also the specific reactivity of counterions.
电子自旋共振（ESR）光谱法用于检测和识别由紫外线照射部分或完全被铜（II）、铁（II）和铁（III）中和的Nafion和陶氏全氟化膜所形成的自由基。该方法能够监测顺磁性抗衡离子的ESR信号以及膜衍生自由基物种的出现。这项研究最令人惊讶的方面是仅在中和膜中形成了膜衍生自由基物种，甚至在Nafion/铜（II）和Nafion/铁（III）的情况下不存在过氧化氢时也是如此。在Nafion/铜（II）中，检测到了与三个等效的19F核（“四重态”）和四个等效的19F核（“五重态”）表现出超精细相互作用的自由基的ESR光谱。在暴露于过氧化氢溶液的Nafion/铁（II）中，检测到了铁（III）的形成。在紫外线照射下，首先检测到来自链端自由基ROCF2CF2的强信号，随后在高于200 K退火时，出现了在Nafion/铜（II）中观察到的四重态信号。在随后用铁（III）中和的Nafion和陶氏膜进行的实验中，即使不存在过氧化氢也形成了ROCF2CF2自由基，这表明过氧化氢的作用是将铁（II）氧化为铁（III）；此外，在这些系统中，即使没有紫外线照射也检测到了少量的链端自由基。这一结果验证了用于形成自由基的方法：紫外线照射的作用是加速一个信号的形成，即使在黑暗中，甚至可能在燃料电池运行期间，该信号也会缓慢产生。主要结论是阳离子参与了降解过程；攻击点似乎在离聚物的侧链处或其附近。因此，在研究膜的稳定性时，不仅要考虑能够攻击膜的氧自由基如HO*、HOO和O2-的形成，还要考虑抗衡离子的特定反应性。
