Molecular Dynamics Simulation of Scattering and Surface Diffusion of Oxygen Molecules on Ionomers in Catalyst Layers of PEFCs

Abstract

Polymer electrolyte fuel cells (PEFCs) are used in fuel cell vehicles and households stationary power sources because of their low operating temperatures, small size, and light weight. For further development of PEFCs, it is necessary to reduce the amount of platinum used in the catalyst layers. If the amount of platinum used in the catalyst layers is reduced, the current density per mass of platinum increases. The effect of diffusion polarization then becomes dominant in the overall voltage drop. One factor that causes diffusion polarization is the transport resistance of oxygen molecules in the catalyst layers. Therefore, an understanding of the transport of oxygen molecules in the catalyst layers is necessary to improve the performance of PEFCs under high current density conditions. The catalyst layers comprise porous structures by aggregation of ionomer-coated platinum-supported carbon. The Knudsen number is important for gas diffusion phenomena in porous structures. The Knudsen number is the ratio of the representative length of the system to the mean free path of gas molecules, and when this value is sufficiently small, the gas diffusion phenomenon can be regarded as molecular diffusion. When the Knudsen number is sufficiently large, the gas diffusion phenomenon becomes Knudsen diffusion, in which collisions with solid walls are more dominant than collisions between gas molecules. The Knudsen number is sufficiently large in the porous structure of the catalyst layer. Therefore, the effect of collision with solid surfaces is important in the oxygen transport in the catalyst layer. In previous studies, it has been clarified that the behavior of oxygen molecules incident on an ionomer thin film includes surface scattering, in which diffuse reflection occurs on the surface, and surface diffusion, in which oxygen molecules trapped on the surface move across the surface in a series of multiple collisions. However, the effect of surface diffusion on oxygen transport across the catalyst layer is not clear. The purpose of this study is to analyze the effect of oxygen molecules and ionomer conditions on surface diffusion using the MD simulation. We analyze the behavior of oxygen molecules impinging on ionomers in a computational system that reproduces the pores of the catalyst layer. The simulation system consists of polymer chains, solvent molecules (water molecules and hydronium ions), and oxygen molecules. Periodic boundary conditions were applied in the x-, y-, and z-directions. The behavior of oxygen molecules after they impact the ionomer wall such as their residence time during surface diffusion and the surface diffusion coefficient will be analyzed.AcknowledgmentsThis work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan, Grant number JPNP20003. It was performed on the Supercomputer system “AFI-NITY” at the Advanced Fluid Information Research Center, Institute of Fluid Science, Tohoku University. Figure 1