EPR investigation of HO/ radical initiated degradation reactions of sulfonated aromatics as model compounds for fuel cell proton conducting membranes
作者:Gerold Hübner、Emil Roduner
DOI:10.1039/a807129b
日期:——
In order to predict hydroxy radical initiated degradation of new proton conducting polymer membranes based on polystyrene, polyethersulfone, polyetheretherketone, or on polymers obtained by radiation grafting of styrene on different fluoropolymers, eight sulfonated aromatics were chosen as model compounds for EPR experiments, aiming at the identification of products of HO/ radical reactions with these polymers. Photolysis of H
2
O
2
was employed as the source of hydroxyl radicals. A detailed investigation of the pH profile was carried out for p-toluenesulfonic acid. Besides benzyl- and hydroxy-cyclohexadienyl radicals at lower pH values, phenoxyl radicals were identified, predominating in the pH range 10.5-13.0. A large number of new radicals give evidence of multiple hydroxylation of the aromatic rings, confirming reaction mechanisms proposed on the grounds of product analysis, but no evidence of dimerisation was found. The result as regards stability of organic proton exchange membranes for fuel cells is, that all unsaturated bonds and weakly bound atoms are subject to immediate attack by HO/. Ether links open by HO/ ipso addition. Strategies for the reduction of membrane degradation should focus on a minimisation of HO/ formation and of its access to the interior of the membrane.
为了预测基于聚苯乙烯、聚醚砜、聚醚醚酮或通过苯乙烯辐射接枝到不同含氟聚合物上获得的聚合物的新型质子传导聚合物膜的羟基自由基引发的降解,选择八种磺化芳族化合物作为 EPR 实验的模型化合物,旨在鉴定 H2O/自由基与这些聚合物的反应产物。 H的光解作用
2
氧
2
被用作羟基自由基的来源。对对甲苯磺酸的 pH 曲线进行了详细研究。除了较低 pH 值下的苄基和羟基环己二烯基自由基外,还鉴定出苯氧基自由基,主要存在于 10.5-13.0 pH 范围内。大量新自由基提供了芳环多重羟基化的证据,证实了基于产物分析提出的反应机制,但没有发现二聚化的证据。燃料电池用有机质子交换膜的稳定性的结果是,所有不饱和键和弱结合原子都会立即受到 H2O/ 的攻击。以太链接通过 HO/ ipso 加法打开。减少膜降解的策略应侧重于尽量减少 H2O2 的形成及其进入膜内部的机会。