分子動力学法を用いた劣化高分子電解質膜内部における物質輸送特性の解析

Abstract

We have conducted molecular dynamics simulations of degraded Nafion membrane systems. The degraded Nafion molecules are made by breaking side chains randomly. Degradation level is controlled by changing the number of broken side chains. For side chain molecules of a degraded Nafion monomer and fragment molecules, the equilibrium bond lengths, the equilibrium angles and the partial charges have been defined originally by DFT calculations. In the simulation systems, there were four ten-unit degraded Nafion molecules and equimolar fragments generated by degradation, in various conditions, such as different water contents and degradation levels. Density, diffusion coefficient, and the number of water clusters have been evaluated. Our findings are, in the degraded Nafion membranes, water molecules and hydronium ions are more dispersed, indicating that Grotthus mechanism are suppressed, and the system becomes much fluidity owing to the existence of various fragments which facilitates Vehicle mechanism. There are two degradation variations of proton diffusion coefficient. In early degradation, the diffusion coefficient decreases because it is harder for the molecules to move freely in the system due to densification. After the maximum density, the diffusion coefficient increases due to fluidity of the system.