Insight into oxygen diffusion mechanism in ionomer film on catalyst surface with varying perfluorosulfonic acid and water contents

Abstract

Classic molecular dynamics (MD) simulations were performed to explore the effects of perfluorosulfonic acid (PFSA) polymers and water content on the nanostructures of ionomer film on Pt/C surfaces and the corresponding oxygen diffusion mechanisms. It was found that the free voids of ionomers are primarily distributed in hydrophobic and interfacial regions, and the increased water content squeezes the backbones into confined regions and remarkably reduces the size of free voids. Oxygen molecules diffuse in the ionomer film by jumping from one free void to neighboring ones. Increasing the PFSA/water content promotes the thermal motion of the ionomer, resulting in the more frequent formation of the transient channels between neighboring free voids with higher oxygen diffusivity. Moreover, oxygen diffusion exhibits anisotropic behavior, where the through-plane diffusivity is lower than the in-plane diffusivity due to the varying structural characteristics of free voids and ionomer mobility in different directions. This is in contrast to the bulk Nafion, which is isotropic and features consistent structural characteristics in all directions and, therefore, the oxygen diffusion remains consistent across different directions. This work provides insight into the oxygen diffusion mechanisms in ionomer film on the surface of the catalyst, which is instructive for the design of novel ionomers with lower oxygen resistance.