Molecular Dynamics Study of Oxygen Diffusivity in Catalyst Layer

Abstract

Proton exchange membrane fuel cell (PEMFC) is one of the promising systems for electricity generation. To achieve the high power density operation, the oxygen gas diffusion phenomenon in the porous media of PEMFCs is a key factor. Thus, gas transport in the porous media, i.e. gas diffusion layer (GDL), microporous layer (MPL), and catalyst layer (CL), has been investigated by experiments and simulations. Gas transport in such porous media is governed by both the molecular and Knudsen diffusion. In particular, the Knudsen diffusion becomes a predominant diffusion mechanism in CL because the characteristic pore size of CL is comparable to the mean free path of oxygen molecules, where gas molecules collide with solid surfaces more frequently than other gas molecules. In this case, the behavior of oxygen molecules on ionomer surfaces should be taken into account for the accurate analysis of gas transport in CL. Therefore, we have performed molecular dynamics (MD) simulations to investigate the scattering behavior of oxygen molecules on ionomer surfaces. Our previous study revealed that the scattering processes and energy accommodation strongly depend on the surface composition of ionomer film. While oxygen molecules that collide with Nafion chains tend to be reflected directly from the surface, they tend to be adsorbed on water molecules long enough and desorbed with better accommodation. The conventional scattering model does not fully describe such scattering behavior. The diffusion of adsorbed molecules on solid surfaces is called “surface diffusion”. This result suggests that both the Knudsen and surface diffusion coexist as the diffusion path in CL. Although some experiments show the contribution of the surface diffusion on the overall diffusion in nanopores, the impact of the surface diffusion on the overall gas transport in CL remains to be unclear. The elucidation of diffusion mechanism incorporating the surface diffusion is of great importance to analyze accurately the oxygen gas transport in CL. In this study, MD simulations have been performed to investigate the oxygen diffusivity in nanopore of CL. In order to quantitatively evaluate the contribution of the diffusion in gas phase and surface diffusion to the overall oxygen diffusivity, a simulation system of a slit nanopore composed of an ionomer thin film was constructed. The diffusion mechanism was classified into the gaseous diffusion and surface diffusion in the pore. The diffusion coefficients of oxygen molecules in each region were compared at different water contents and pore sizes. The overall oxygen diffusivity increases remarkably with an increase in the pore size. In contrast, at the same pore size, the oxygen diffusivity in the surface region decreases significantly as the water content increases. This result suggests that the oxygen diffusivity in the nanopore is affected by the adsorption and surface diffusion on the water layer. The elucidation of diffusion mechanisms in nanopore of CL allows us to construct a more accurate model of oxygen behavior on an ionomer surface compared to the conventional Knudsen diffusion and provides a quantitative estimation of the oxygen transport resistance.