Molecular dynamics studies on the structures of polymer electrolyte membranes and diffusion mechanism of protons and small molecules

Abstract

We performed a series of molecular dynamics (MD) simulations on Nafion® membranes containing various quantities of H2O and CH3OH. The simulations afforded diverse nanoscale phase-separated structures, such as clusters, channels, and cluster–channels. The calculated cluster–channel structure qualitatively agrees with the experimental results of X-ray diffraction studies. We also investigated the diffusion mechanisms for H2O, protons, CH3OH, H2, and O2 in these membranes. To reproduce the hopping transfer of protons, we employed a semi-classical MD approach using the empirical valence bond method. The estimated diffusion coefficients of H2O and proton in the membranes significantly depended on the H2O content, and these values showed qualitatively good agreement with the experimental results. The diffusion coefficient of proton in H2O-rich membranes was much larger than that of H2O, and the proton mainly formed H5O2+ complex. Furthermore, the simulation results indicate that the majority of CH3OH permeates through the H2O clusters, and the majority of H2 and O2 permeates through the hydrophobic region of the membrane.