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

Abstract

Polymer electrolyte fuel cells (PEFCs) are expected as a next-generation power sources due to their low emissions and high efficiencies. However, it is required to improve the power density at lower Pt loading. It is known that the losses of the power density are caused by activation losses, ohmic losses and mass transport losses, and that the mass transport losses are dominant at higher current density. Moreover, it is suggested that one of the causes of the mass transport losses is the shortage of oxygen in the cathode side of PEFC. In the cathodic catalyst layer, there are Pt catalysts on supported carbon microparticles, and those particles are covered with ionomer films which are composed of polymer electrolytes 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 water content dependence of oxygen permeation properties in ionomer on Pt surface using molecular dynamics simulations. Regarding the calculation system, Nafion was adopted to the polymer electrolyte in the ionomer, and water content, λ, was defined as the ratio of the number of sulfo groups in Nafion to the number of water molecules. We set λ = 3, 7, 11 and constructed a system of an equilibrium state of ionomer on Pt surface at each water content. Then oxygen molecules were inserted above the ionomer and permeate the ionomer along the thickness direction at steady state. Firstly, the density distributions were obtained to evaluate the structural properties of the ionomer. As a result, the region of the ionomer is divided into three region: ionomer/gas interface, bulk region, ionomer/Pt interface. Next, the number of permeated oxygen molecules through the ionomer was counted, and the oxygen permeability of the ionomer was estimated at different water content. As a result, the oxygen permeability decreases with increasing water content. Gas permeability is estimated using the permeation coefficient which is obtained by the product of the diffusion coefficient and solubility coefficient of gas molecules, indicating that the diffusivity and solubility are governing factors of the permeability. Therefore, the oxygen diffusivity and solubility was evaluated to discuss the oxygen permeation mechanism in the ionomer. The oxygen diffusion coefficient was obtained by the density distribution of the oxygen molecules along the thickness direction using the Fick’s law. As a result, the oxygen diffusivity in each region of the ionomer decreases at lower water content while slightly increases at higher water content, with the increase in water content. On the other hand, the oxygen solubility coefficient was calculated using the test-particle-insertion method, and the solubility in each region of the ionomer decreases with increasing water content. Finally, the oxygen permeability was evaluated using the diffusivity and solubility in each region. Consequently, the oxygen permeability decreases with increasing water content in each region, and the permeability in the ionomer/Pt interface is the smallest, which indicates that the oxygen dissolution in the ionomer/Pt interface is dominant in the oxygen permeation through the ionomer.