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

We analyzed the transport properties of oxygen molecules using the Monte Carlo method. We obtained the effective diffusion coefficient of oxygen molecules in a catalyst layer of polymer electrolyte fuel cells. In this calculation system, when oxygen molecules hit a wall, it diffuses on the ionomer surface for a period of time and then diffusely reflected. The simulation was performed using various conditions of the surface diffusion. The conditions are defined by the surface diffusion coefficient and the time constant that controls the residence time of oxygen molecules. When the surface diffusion coefficient is small, the increase in the time constant results in the decrease in the 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. This phenomenon occurs due to the three-dimensional bending structure of the catalyst layer. When an oxygen molecule moves along the bending three-dimensional structure, the distance along the ionomer surface becomes larger than the straight line segment between the start and end points.