Analysis of the Effect of Surface Diffusion on Effective Diffusivity of Oxygen in Catalyst Layer By Direct Simulation Monte Carlo

Abstract

Because in the catalyst layers (CLs) of polymer electrolyte fuel cells (PEFCs) platinum (Pt) is used as catalyst, we need to reduce the amount of Pt for cost reduction. In order to maintain the same output even when the amount of Pt is reduced, it is necessary to increase the current density per unit area of Pt surface. At high current density, mass transport is a major factor in voltage drop. Therefore, improving the oxygen transport properties in CLs will lead to cost reduction of PEFCs. Although the transport properties of oxygen in the porous structure of CLs have been studied, the effect of surface diffusion, which is the motion of an oxygen molecule on the ionomer thin film, on transport properties has not yet been clarified. The purpose of the present study is to analyze how the motion of an oxygen molecule on the ionomer thin film affects the oxygen transport properties in the CLs of PEFCs.We used the direct simulation Monte Carlo (DSMC) method. In this study, a computational system was constructed based on an actual catalyst layer sample. The three dimensional structure of the sample was obtained as sequential slice images using a focused ion beam-scanning electron microscope (FIB-SEM). In order to examine the effect of surface diffusion on the transport properties of oxygen molecules, the simulation was performed using a various conditions of the surface diffusion. The behavior of surface diffusion was determined by a surface diffusion coefficient (D s) and a time constant (τ).Figure 1 shows the results of the effective diffusion coefficient of oxygen in the CL. The red line shows the effective diffusion coefficient of oxygen in the calculation without considering the surface diffusion, which is 2.75×10-6 m2/s. The plot marks shows the each surface diffusion coefficient. As the surface diffusion coefficient increases, the effective diffusion coefficient of oxygen increases. When the surface diffusion coefficient is small, the increase in the time constant results in the decrease in the effective diffusion coefficient of oxygen. When surface diffusion coefficient is large, the increase in the time constant leads to the larger effective diffusion coefficient of oxygen. However, when the surface diffusion coefficient is large, as the time constant increases, the effective diffusion coefficient of oxygen has a peak value and then decreases. Although the surface diffusion coefficient becomes larger, the effective diffusion coefficient does not continue to increase. This trend occurs presumably due to the tortuosity of the three dimensional structure. When an oxygen molecule moves along the bended three dimensional structure, the distance of the motion of the oxygen molecule become larger than the straight-line distance between the start and end points. This is the reason why the effective diffusion coefficient does not continue to increase.Fig. 1: Effective diffusion coefficient of oxygen in the catalyst layer as a function of the time constant of the surface diffusion. The difference in plot marks shows the difference in the surface diffusion coefficient. The red line shows the effective diffusion coefficient of oxygen in the calculation without considering the surface diffusion. Figure 1