Molecular-Dynamics Studies on the Structures of Polymer Electrolyte Membranes and the Diffusion Mechanism of Protons and Small Molecules

نویسندگان

  • Tomonori Kawakami
  • Isamu Shigemoto
چکیده

We performed a series of molecular dynamics (MD) simulations on Nafion membranes which contain various quantities of water and methanol. The simulations gave a variety of nanoscale phase separated structures, such as cluster structures, channel structures, and cluster-channel structures. This is the first work that an atomistic MD simulation reproduced such nanoscale phase separations from random-phased amphipathic polymer solutions. The calculated cluster-channel structure is consistent with the experimental results of X-ray diffraction [1]. We also investigated the diffusion coefficients of water, methanol and proton in these membranes. Estimated diffusion coefficients of water in the membranes so much depend on the water contents that the largest value is about 10 times as large as the smallest one, and these values are in qualitatively good agreement with the NMR results [2]. For proton, we employed semi-classical MD approach by using empirical valence-bond (EVB) method [3]. EVB-MD approach can reproduce the hopping transfer of proton and estimate a reasonable value of diffusion coefficient of proton. Furthermore, our simulation indicated that water was rarely found in the hydrophobic domains of the Nafion membranes, whereas methanol penetrated into the hydrophilic / hydrophobic interfacial regions. We found that the methanol permeability around sulfonic groups is one-third of that in the water clusters. It means that methanol molecules are trapped by the hydrophilic groups of the membranes and the majority of methanol passes through the water clusters.

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تاریخ انتشار 2005