Molecular Analysis of Oxygen Permeability of Aromatic Hydrocarbon-Based Ionomers (芳香族炭化水素系イオノマーの酸素透過性の分子分析)

Abstract

Sulfonated PolyPhenylene BiPhenylene (SPP-BP) is a hydrocarbon-based polymer with high proton conductivity and can be synthesized easily and cost-effectively. The factors affecting the oxygen permeability of hydrocarbon-based ionomers are not known. In this study, oxygen permeability and local transport resistance of SPP-BP ionomer were analyzed in comparison with Nafion ionomer, which is a typical conventional Perfluorosulfonic acid (PFSA) ionomer. The oxygen permeability of SPP-BP is significantly better than that of Nafion when water content is low. However, as water content increases, the oxygen permeability drops to the same level as that of Nafion. Since oxygen permeability is considered to be highly dependent on the local structure of ionomer, ionomer was divided into three regions: ionomer/Pt interface, bulk region, and ionomer/gas interface based on density distribution. Local oxygen transport resistance of ionomer was analyzed from chemical potential. As a result, the ionomer/Pt interface is the rate-limiting structure for oxygen permeation for both ionomers. From the visualization of atomic structure of ionomer/Pt interface and trajectory lines of permeating oxygen, SPP-BP exhibits a decrease in the voids at the ionomer/Pt interface with increasing water content and a significant decrease in the trajectory line of oxygen permeating through the interface. On the other hand, Nafion had fewer voids at lower water contents than SPP-BP, and the number of voids and trajectory lines that reach Pt remained almost the same with increasing water content.

Keywords: molecular dynamics, polymer, Fuel cell