In CLs, Knudsen number of oxygen flows, which is defined as the ratio of the mean free path of gas molecules to the characteristic pore size, becomes of the order of 0.1‒1. It indicates that the oxygen gas transport mechanism falls in the transition regime of the Knudsen diffusion and the molecular diffusion. In this case, the collision between a gas molecule and a surface should be taken into account for the gas transport analysis because gas molecules collide with pore walls much more frequently than with other oxygen molecules. Therefore, the gas–surface interaction between ionomer surface and oxygen molecules plays a critical role for the accurate analysis of gas transport.
In previous studies, Kinefuchi et al. investigated the oxygen diffusion resistance in microporous layers (MPLs) and CLs using the direct simulation Monte Carlo (DSMC) method. They reported that the numerical simulations accurately reproduce the pressure dependence of diffusion resistance originating from the coexistence of the Knudsen and molecular diffusion mechanisms in MPLs. However, the overall oxygen diffusion resistance was underestimated in CLs. This discrepancy is attributable to the scattering model of oxygen molecules on the ionomer surface. Although the scattering model on the surface determines the gas flow in Knudsen diffusion regime, the diffuse reflection model, which is the most widely used model, is used in spite of different surface structure in MPLs and CLs. The construction of the scattering model of oxygen molecules on ionomer surface is of great importance to analyze accurately the oxygen diffusion resistance in CLs. In this study, molecular dynamics (MD) simulations have been performed for the analysis of oxygen scattering and surface diffusion phenomena on the ionomer surface. The solid surface in CLs was modeled as an ionomer film on a graphite surface. The ionomer film consists of polymer chains and solvent molecules, which are water molecules and hydronium ions. In all simulations, Nafion chain, which has the chemical structure with EW=1100, has been employed. Four polymer chains and 40 hydronium ions were placed on the graphite surface to ensure charge neutrality. The annealing process was applied in order to equilibrate the system. The initial position of oxygen molecules and the orientation to the ionomer surface were given randomly. 25 oxygen molecules were directed to the ionomer surface at the same time to obtain sufficient statistics. Trajectory calculations of oxygen molecules on the ionomer surface was carried out. The intermolecular interactions between oxygen molecules were not calculated to make the scattering phenomena of these molecules independent of each other. Oxygen molecules were given the incident temperature and incident angle as the initial condition. The incident temperature ranged every 150 K from 150 K to 600 K. The initial translational and rotational energies were determined according to the energy equipartition law. The bond length between the oxygen atoms in each molecule was fixed at the equilibrium length by the RATTLE method. The incident angle was set at 0°, 30° and 60° with respect to the surface normal. The trajectory calculations of 2000 molecules were performed until the scattered molecules moved away more than the cutoff length (12 Å) from the ionomer surface.
Two different scattering processes, which are the inelastic scattering (IS) and trapping desorption (TD) processes, were observed. It was found that the translational energy of reflected oxygen molecules in the IS process depends on the incident temperature, indicating that oxygen molecules were reflected retaining the effect of the incident energy. On the other hand, the translational energy distributions of the TD process are independent of the incident temperature and different from the Maxwell–Boltzmann distribution of the surface temperature. These scattering processes depend on the incident temperature and the water content. The dependence on the incident temperature suggests that oxygen molecules are more likely to reflect directly as the incident energy increases. Moreover, the TD process is likely to occur when oxygen molecules collide with solvent molecules in the ionomer surface.