(PEFCE&E 21 Best Student Poster - Third Place) Molecular Dynamics Study of Proton Conductivity at an Interface between Nafion and Graphene Sheet

Abstract

In order to improve the power generation efficiency of polymer electrolyte fuel cells (PEFC), it is necessary to increase the flux of proton transport inside a proton exchange membrane (PEM) under high temperature and low humidity conditions. Nowadays, the Nafion membrane is used as a reference for all proton exchange membranes. This membrane developed by DuPont company demonstrates a high proton conductivity value, chemical stability and prolonged using in fuel cell conditions in comparison with other membranes. However proton transport properties are greatly reduced when the membranes are degraded. Researches have been actively conducted to solve the problem of proton transport reduction in PEM under high temperature and low humidity conditions by two approaches. One of the research focuses is to design alternative materials based on those engineering polymers, for example, polybenzimidazole, poly(ether sulfone), and poly(ether ketone). Another research focus is the introduction of additives. Among the many additives, carbon nanotubes (CNTs) which are next-generation materials with a high aspect ratio and excellent thermal durability and mechanical properties, have been attracting attention. Previous studies have reported that the PEEK (poly ether ether ketone ) / CNT composite membranes show higher durability than Nafion membranes. And the proton transport properties in Nafion / sulfonated CNT composite membranes are improved over the that of Nafion membranes. It is said that the proton transport property is improved by forming a proton path which is made by the influence of the structural characteristics of CNT. However, the proton transport mechanism at the interface between the carbon surface of CNT and the polymer has not been fully understood. The purpose of this study is to clarify the proton transport mechanism in the interface between the carbon surface region and the polymer region and to evaluate the effects of functional groups with carbon surface by using molecular dynamics (MD) simulations. The surface of a CNT was modeled with a graphene sheet (GS). The diameter of the CNTs used in the experiment is about 10-20 nm, which is beyond capability of our MD simulations, and thus the CNT surface was assumed to be a flat surface. The simulation system is shown in Fig. 1. GSs were placed at the top and bottom of the simulation box, and polymer, water molecules, and hydronium ions were placed between them. In order to evaluate the influence of distances between GSs on structural characteristics and transport property, we prepared calculation systems at the wall distances of 4, 6, 8 and 10 nm. We calculated the density distribution in the direction vertical to the GS and the self-diffusion coefficient of the protons at the GS surface region and the center region. Furthermore, a hydrophilic sulfonated GS was prepared by putting sulfo groups to the hydrophobic GS to investigate the effects of the hydrophilic and hydrophobic GS interface on proton transport. The results of the density distribution showed that the surface of pristine GS and sulfonated GS had water clusters, and that proton self-diffusion coefficient of pristine GS and sulfonated GS were larger than that of bulk Nafion. From these facts, it is considered that the diffusion by the grotthuss mechanism increased by improving the connectivity of the proton path on the GS surface and reducing the tortuosity. As a result, the proton transportability is considered to be improved. Figure 1