A theory-based understanding of perfluorosulfonic acid (PFSA) class of ionomer aggregation in different solvents is crucial for transitioning from empirically guided fabrication of high-performance electrodes of fuel cells and other electrochemical devices. Considering Nafion as a model PFSA ionomer, herein, we report the details of its molecular assembly in various solvents (dielectric constant, ε_r = 2.38–109) using fully atomistic molecular dynamics simulation. In contrast to the long-held view of elongated aggregate structure of Nafion, our study shows that a variety of structures exist depending on solvent polarity. The work revealed the existence and clustering of ion pairs in Nafion dispersions, a phenomenon reported for the first time for PFSA ionomer dispersions. We introduce a new aggregation phase diagram for Nafion in different solvents: (a) type-I aggregate: aggregates with ionomer chains physically cross-linked via ionic clusters in low dielectric solvents [toluene, isopropyl alcohol (IPA), ethanol, glycerol]; (b) type-II aggregate: lamella like two-dimensional (2D)-aggregates self-assembled via weak hydrophobic interactions in intermediate dielectric solvents (formic acid); and (c) type III aggregate: tightly packed elongated aggregates formed through strong hydrophobic interactions in high dielectric solvents (water, formamide). Further, our fully atomistic simulations highlight the significant shortcomings of a prior coarse-grained approach wherein the solvent atoms are not explicitly modeled, resulting in an unrealistic aggregate structure in all solvents. With explicit consideration of the solvent molecules, we uncovered a new phenomenon of microphase segregation of solvents (isopropyl alcohol/water mixture) driven by PFSA’s surfactant-like hydrophilic/hydrophobic character.

中文翻译：

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Nafion分散体的原子MD研究：溶剂和抗衡离子在聚集体结构，离子簇和酸解离中的作用

基于理论的对不同溶剂中离聚物聚集的全氟磺酸（PFSA）类的理解，对于从经验导向的燃料电池和其他电化学装置高性能电极的制造过渡起着至关重要的作用。考虑到Nafion作为模型PFSA离聚物，在此，我们报告其在各种溶剂中的分子组装细节（介电常数，_εr= 2.38–109），使用完全原子分子动力学模拟。与长期以来对Nafion的细长聚集结构的看法相反，我们的研究表明，取决于溶剂极性，存在多种结构。这项工作揭示了Nafion分散体中离子对的存在和聚集，这是PFSA离聚物分散体首次报道的现象。我们为Nafion在不同溶剂中引入了一个新的聚集相图：（a）I型聚集体：在低介电溶剂[甲苯，异丙醇（IPA），乙醇，甘油]中具有通过离子簇物理交联的离聚物链的聚集体；（b）II型聚集体：类似薄层的二维（2D）聚集体，是在中间介电溶剂（甲酸）中通过弱疏水相互作用而自组装的；（c）III类骨料：通过在高介电溶剂（水，甲酰胺）中的强疏水作用形成的紧密堆积的细长聚集体。此外，我们的完全原子模拟突出了现有粗粒度方法的显着缺点，其中未对溶剂原子进行显式建模，导致所有溶剂中的聚集体结构都不切实际。通过明确考虑溶剂分子，我们发现了一种新的溶剂微相偏析现象（异丙醇/水混合物），该现象是由PFSA的表面活性剂般的亲水性/疏水性驱动的。我们的完全原子模拟强调了以前的粗粒度方法的显着缺点，其中未对溶剂原子进行显式建模，导致所有溶剂中的聚集体结构都不切实际。通过明确考虑溶剂分子，我们发现了一种新的溶剂微相偏析现象（异丙醇/水混合物），该现象是由PFSA的表面活性剂般的亲水性/疏水性驱动的。我们的完全原子模拟强调了以前的粗粒度方法的显着缺点，其中未对溶剂原子进行显式建模，导致所有溶剂中的聚集体结构都不切实际。通过明确考虑溶剂分子，我们发现了一种新的溶剂微相偏析现象（异丙醇/水混合物），该现象是由PFSA的表面活性剂般的亲水性/疏水性驱动的。