Reflection Characteristics of Oxygen Molecule on Ionomer Surface

Abstract

The widespread utilization of polymer electrolyte fuel cells for transportation applications such as fuel cell vehicles requires high power density operation. However, as the current density increases, the diffusion polarization, which is attributed to the insufficient mass transport, degrades the cell performance. In particular, the oxygen transport resistance in catalyst layers (CLs) and micro porous layers (MPLs) at the cathode side is one of the major factors of the performance degradation. Therefore, the reduction in the oxygen transport resistance is important for the high power density operation. In the previous research, Kinefuchi et al. have investigated the oxygen diffusion resistance in MPLs and CLs using the direct simulation Monte Carlo (DSMC) method. They reported that the simulation well reproduces the experimental result in MPLs. However, the DSMC results underestimate the oxygen diffusion resistance in CLs. This discrepancy is caused by the drawback to a scattering model of gas molecules on surfaces, which plays a crucial role in DSMC analysis. In these calculations, the same scattering model is used for the scattering of oxygen molecules on surfaces regardless of the presence or absence of ionomer on micro porous carbons. An advanced scattering model that can reproduce the scattering and surface diffusion phenomena on ionomer surface, is required for more accurate analysis of the oxygen transport in CLs. The construction of the model needs the detailed understanding of the gas‒surface interaction. In this study, we have investigated the mechanics of oxygen scattering and surface diffusion on ionomer surface using molecular dynamics simulation. We modeled the solid surface in CLs as an ionomer film covering the carbon layers. The simulation domain was 51.1 × 44.2 × 100 Å3 in size. The ionomer film was composed of water molecules hydronium ions, and perfluorosulfonic acid molecules as typified by Nafion, water molecules, and hydronium ions. Oxygen molecules were directed to the ionomer surface and the trajectory calculations were performed for 20 ps. The initial translational energy and incident angle were the same for all the incident molecules, but the initial position and orientation were given randomly in order to obtain the sufficient statistics. If an oxygen molecule desorbed from the surface and reached the initial height, it was defined to be scattered. Firstly, we have evaluated the energy transfer between oxygen molecules and the ionomer surface by calculating between translational energy before and after the scattering. Oxygen molecules with low incident energy tend to receive energy when they leave the surface after the collisions. Next, we have evaluated the scattering phenomena of oxygen molecules on ionomer surface by examining the energy and angular probability distributions of scattered molecules. The energy distribution depends on the incident energy, and deviates from the distribution given by diffusive scattering model, that the incident molecules are assumed to accommodate completely with the surface. On the other hand, the scattering angle distribution is independent of the incident energy, and well-reproduced by the diffusive scattering model. These results suggest that oxygen molecules do not accommodate completely with the ionomer surface when they are reflected, however they are reflected following the diffusive scattering model because of the corrugation of the ionomer surface.