Oxygen Transport Routes in Ionomer Film on Polyhedral Platinum Nanoparticles

Understanding the O₂ permeation phenomenon in the ionomer thin film on platinum (Pt) nanoparticles is vital to improve the electrocatalyst performance of proton exchange membrane fuel cells at a low Pt loading. In this study, the ionomer film structure, O₂ density distribution, transport fluxes, and permeation routes are investigated for carbon-supported polyhedral Pt nanoparticles (cube and tetrahedron) in the facet, edge, and corner regions. The molecular dynamic simulation takes into account the molecular interactions among the ionomer, Pt nanoparticles, carbon support, and O₂ molecules. The results show that a dense ionomer ultrathin layer with a tight arrangement of perfluorosulfonic acid is present on the Pt facets (namely region A). In the ionomer near the Pt edges and corners (namely region B), the structure is less dense due to the weaker Pt attraction, resulting in a higher O₂ density than that in region A. O₂ fluxes in the different regions show that approximately 90% of O₂ molecules reach the Pt cube and tetrahedron nanoparticles via their upper corner and edge regions. In the vicinity of Pt nanoparticles, O₂ permeation routes are inferred to penetrating region B to the Pt upper corners or edges instead of region A to the Pt facets.