Molecular Simulation of Oxygen Solubility and Diffusivity in Ionomer on Pt Surface

Polymer electrolyte fuel cell (PEFC) is focused as a next-generation energy conversion system because of a high efficiency and a low environmental load. In the cathodic catalyst layer, there are Pt catalysts on supported carbon microparticles, and those particles are covered with ionomer films composed of polymer electrolytes, hydronium ions and water molecules. The ionomer has two properties for the water-generating reactions in the cathodic catalyst layer: the proton conductivity and the oxygen permeability. In particular, the dependence of the oxygen permeability on water content has not been clear.

In this study, we analyzed the oxygen permeability in the ionomer on the Pt surface using molecular dynamics simulations. At first, we constructed the system of the ionomer on the Pt surface, and then we set the gas oxygen molecules above the ionomer, and let the molecules permeate through the ionomer by the pressure gradient as shown in Fig. 1. Using this system, we calculated the oxygen solubility and the oxygen pathways in each region: the ionomer/gas interface, the bulk region, the ionomer/Pt interface. In general, the oxygen permeability is evaluated by the product of the oxygen solubility and the oxygen diffusivity. Thus, we calculated the oxygen solubility of each region in the ionomer at different water contents. As a result, the solubilities of all regions tend to decrease as water content increases. Moreover, we evaluated the oxygen pathways in the ionomer to evaluate the oxygen diffusivity, and the result shows that oxygen prefers the surface of hydrophilic regions to the hydrophobic regions. In experimental data, the diffusion coefficient of oxygen in a dry membrane is smaller than that of oxygen in bulk water. Therefore, the dependence of oxygen diffusivity on water content was estimated by calculating the ratio of the number of oxygen molecules in the hydrophilic region to that in the hydrophobic region.