A molecular dynamics study of a nafion polyelectrolyte membrane and the aqueous phase structure for proton transport
A molecular dynamics study of a nafion polyelectrolyte membrane and the aqueous phase structure for proton transport
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DOI：
10.1021/jp066388n
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发表时间：
2007-03-08
影响因子：
3.3
通讯作者：
Steele, William V.
中科院分区：
文献类型：
--
作者：
Cui, Shengting;Liu, Junwu;Steele, William V.
在水含量为5% ~ 20%的情况下，进行了水合Nafion的分子动力学模拟研究，以检验水合多电解质体系的结构和动力学。模拟结果表明，该体系形成了分离的疏水区域，主要由聚合物主链和亲水区域组成，且水分布不均匀。我们发现水的聚类与水的含量有很大的关系。在低含水量时，只形成孤立的小水团。随着水含量的增加，绝大多数水分子形成单一簇的可能性越来越大，这表明亲水性区域相互连接。我们用各种原子对相关函数来表征系统内形成的原子结构。水结构因子在簇间距离约为2.5 nm或更大的q值处出现峰值。随着含水量的增加，距离逐渐增大，这与散射实验的结果一致。我们发现水分子对水合氢离子的溶剂化程度是水含量的一个强函数。在5wt %时，大多数水合氢离子与不超过两个水分子水合，禁止结构扩散。随着水含量的增加，水合氢离子继续变得越来越水合，从而产生能够形成特征离子的结构，这是结构扩散的必要步骤。针对实验观察到的这些膜的电导率突然下降近5 wt %的事实，我们发现，在低含水量下，水合氢离子的局部结构和水纳米网络整体形态的不连接性质应该导致电导率差。
A molecular dynamics simulation study of hydrated Nafion at water contents ranging from 5 to 20 wt % was performed to examine the structure and dynamics of the hydrated polyelectrolyte system. The simulations show that the system forms segregated hydrophobic regions consisting primarily of the polymer backbone and hydrophilic regions with an inhomogeneous water distribution. We find that the water clustering strongly depends on the water content. At low water content, only isolated small water clusters are formed. As the water content increases, it becomes increasingly possible that a predominant majority of water molecules form a single cluster, suggesting that the hydrophilic regions become connected. We characterize the atomic structures formed within the system by various atomic pair correlation functions. The water structure factor shows a peak at q values corresponding to an intercluster distance about 2.5 nm and greater. With increasing water content, the distance moves to larger values, consistent with findings from scattering experiments. We find that the degree of solvation of hydronium ions by water molecules is a strong function of water content. At 5 wt %, a majority of the hydronium ions are hydrated by no more than two water molecules, prohibiting structural diffusion. As water content increases, the hydronium ions continue to become increasingly hydrated, resulting in structures capable of forming eigen ions, a necessary step in structural diffusion. Addressing the experimentally observed fact that conductivity in these membranes abruptly drops near 5 wt %, we find that both the local structure of the poorly hydrated hydronium ions and the disconnected nature of the global morphology of the water nanonetwork at low water content should contribute to poor conductivity.